Land and Food Systems, Faculty of
DSpace
UBCV
Humphreys, Elyn R.
Doctor of Philosophy - PhD
University of British Columbia
Forest harvesting and succession have a major impact on the dynamics of carbon exchange between forests and the atmosphere. This thesis examines the differences in stand-level fluxes of carbon dioxide (C02) and the biophysical factors, which affect respiration and photosynthesis in three different coastal Douglas-fir stands at different stages of development after harvesting. The eddy covariance technique was used to measure stand-level CO2 fluxes in Douglas-fir stands established in 2000, 1988, and 1948 on the east coast of Vancouver Island, British Columbia. A portable soil chamber system was used to measure CO2 efflux from the soil in each stand. One year of measurements (2002) was used to contrast net ecosystem production (NEP) between the three stands, while three consecutive years (September 2000 to September 2003) were used to examine interannual variability in NEP in the recently clearcut-harvested stand. Total annual NEP, ecosystem respiration (Re) and gross ecosystem production (GEP) increased with increasing stand age. In 2002, the 3-year-old stand was a large carbon (C) source (NEP = -610 g C m~2 y_ 1), the 14-year-old stand was a small C source (NEP = -130 g C m - 2 y_ 1), and the 53-year-old stand was a moderate C sink (NEP = 250 g C m~2 y- 1). Seasonal variations in both soil and ecosystem respiration were related to soil temperature, while within- and between-stand variability in soil respiration was related to the concentration of soil nitrogen. Differences in annual Re in these forests were better related to productivity than to soil temperature. Seasonal variations in NEP differed between stands and were related to differences in stand structural characteristics such as canopy roughness, leaf area, biomass, species composition and phenology. The effect of advection on NEP estimates was greatest in the 53-year-old stand and was also related to these structural characteristics as well as site characteristics. In the first three years after harvesting, leaf area increased with the growth of Douglas-fir seedlings, and pioneer and understory species in the youngest stand. Changes in the relationships between respiration and soil temperature and between GEP and radiation were linked to leaf area index. Results from this study emphasize the importance of stand structure and succession as well as site characteristics and weather variability on C exchange processes.
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Net ecosystem production of three coastal Douglas-fir stands at different stages of development after harvesting by Elyn R. Humphreys B.Sc , (Plant Biology) University of Guelph, 1997 M.Sc , (Soil Science) University of British Columbia, 1999 A THESIS S U B M I T T E D IN P A R T I A L F U L F I L L M E N T OF T H E R E Q U I R E M E N T S F O R T H E D E G R E E OF D O C T O R OF P H I L O S O P H Y in T H E F A C U L T Y OF G R A D U A T E STUDIES (Soil Sciences) We accept this thesis as conforming to the required standard T H E U N I V E R S I T Y OF BRITISH C O L U M B I A March 2004 ©Elyn R. Humphreys, 2004 11 Abstract Forest harvesting and succession have a major impact on the dynamics of carbon exchange between forests and the atmosphere. This thesis examines the differences in stand-level fluxes of carbon dioxide (C02) and the biophysical factors, which affect respiration and photosynthesis in three different coastal Douglas-fir stands at different stages of development after harvesting. The eddy covariance technique was used to measure stand-level C O 2 fluxes in Douglas-fir stands established in 2000, 1988, and 1948 on the east coast of Vancouver Island, British Columbia. A portable soil chamber system was used to measure C O 2 efflux from the soil in each stand. One year of measurements (2002) was used to contrast net ecosystem production (NEP) between the three stands, while three consecutive years (September 2000 to September 2003) were used to examine interannual variability in NEP in the recently clearcut-harvested stand. Total annual NEP, ecosystem respiration (Re) and gross ecosystem production (GEP) increased with increasing stand age. In 2002, the 3-year-old stand was a large carbon (C) source (NEP = -610 g C m~2 y _ 1 ) , the 14-year-old stand was a small C source (NEP = -130 g C m - 2 y _ 1 ) , and the 53-year-old stand was a moderate C sink (NEP = 250 g C m~2 y - 1 ) . Seasonal variations in both soil and ecosystem respiration were related to soil temperature, while within- and between-stand variability in soil respiration was related to the concentration of soil nitrogen. Differences in annual Re in these forests were better related to productivity than to soil temperature. Seasonal variations in NEP differed between stands and were related to differences in stand structural characteristics such as canopy roughness, leaf area, biomass, species com-position and phenology. The effect of advection on NEP estimates was greatest in the 53-year-old stand and was also related to these structural characteristics as well as site characteristics. In the first three years after harvesting, leaf area increased with the growth of Douglas-fir seedlings, and pioneer and understory species in the youngest stand. Changes in the relationships between respiration and soil temper-ature and between GEP and radiation were linked to leaf area index. Results from this study emphasize the importance of stand structure and succession as well as site characteristics and weather variability on C exchange processes. Ill Contents Abstract ii Contents iii List of Tables vi List of Figures viii List of Symbols xiv Acknowledgements xvi 1 Introduction 1 2 Comparison of ecosystem and soil respiration in three different-aged coastal Douglas-fir stands 11 2.1 Introduction 11 2.2 Methods 14 2.2.1 Stand descriptions 14 2.2.2 Ecosystem respiration measurements 19 2.2.3 Soil respiration measurements 24 2.2.4 Weather measurements 28 2.2.5 Analysis' 29 2.3 Results 32 2.3.1 Turbulent exchange of C O 2 at night 32 2.3.2 Weather 40 2.3.3 Relationships between respiration and environmental factors . 41 2.3.4 Within-stand spatial variations in respiration 46 2.3.5 Comparing soil and ecosystem respiration 52 2.3.6 Between-stand variations in annual respiration 55 2.4 Discussion 57 2.4.1 Influences of temperature and soil moisture on respiration . . 58 2.4.2 Factors influencing between- and within-stand variations in res-piration 59 2.4.3 Respiration scaling issues 62 iv 2.5 Conclusions 64 3 Measurements of atmospheric carbon dioxide fluxes in three Douglas-fir stands at different stages of development after harvesting 66 3.1 Introduction 66 3.2 Methods 67 3.3 Results 71 3.3.1 Seasonal weather and leaf phenology 71 3.3.2 Daily patterns of C O 2 exchange 71 3.3.3 Seasonal patterns and biophysical controls of C O 2 exchange . 75 3.3.4 Annual C O 2 exchange 83 3.4 Discussion 86 3.5 Conclusions 89 4 Net ecosystem production of a Douglas-fir stand for three years following clearcut harvesting 91 4.1 Introduction 91 4.2 Methods 93 4.2.1 Stand description 93 4.2.2 Eddy covariance measurements of fluxes 94 4.2.3 Comparison of fluxes from open- and closed-path IRGAs . . . 97 4.2.4 Energy budget closure 99 4.2.5 Weather measurements 100 4.2.6 Analysis and gap-filling 101 4.2.7 Stand measurements 102 4.3 Results 103 4.3.1 Stand growth 103 4.3.2 Weather 104 4.3.3 Diurnal, seasonal and interannual variations in N E P 104 4.3.4 Relationships between nighttime N E P (respiration) and envi-ronmental factors 109 4.3.5 Relationships between daytime N E P and environmental factors 111 4.3.6 Interannual variations in NEP, GEP, and Re 118 4.4 Discussion 119 4.5 Conclusions 124 5 Conclusions 126 References 131 A p p e n d i x 1: 141 Determination of the effective volume for the portable soil chamber system 141 A p p e n d i x 2: 144 Comparison of a LI-COR model LI-6400 soil CO2 efflux chamber system with a homemade chamber system 144 A p p e n d i x 3: 147 Comparison of CO2 and water vapour fluxes measured using open-path and closed-path infrared gas analyzers 147 vi List of Tables 1 Selected stand and site characteristics for the three Douglas-fir stands. 18 2 Selected soil characteristics for the three Douglas-fir stands. Values in parentheses indicate the standard deviation of the mean 19 3 Statistical characteristics of three groups of nighttime NEE measure-ments from the 53-y stand when u* exceeded u*tth- Values in brackets are the standard error (SE) of the mean 40 4 Weather conditions for the three Douglas-fir stands between May 1 and December 31, 2002. The standard deviation of the mean is in parentheses 41 5 Mean RS and RE measured between May 1 and December 31, 2002. Parameters A and B ± SE and the coefficient of determination, R 2, are from the regression of ln i^ against 5-cm TS (Eq. 6). Regression parameters Qiox and R\QX were calculated from A and B as described in the text. Significant differences in parameters between stands for either RE or RS (P < 0.05) are indicated by different superscripts and were determined for the original parameters A and B 43 6 Mean ± 1 SE Rwsi, soil temperature (TS), soil moisture (6V), organic matter (OM) content, depth of the LFH layer within the top 0-6 cm of the forest floor, and chemical characteristics (concentration of total C and N) for the stationary collars only (n = 10 stand -1) in the 3 stands. Significant differences between stands are indicated by differing superscripts 48 7 Spearman-rank correlation between Rwsi and soil characteristics for each of the three forest stands. Significant relationships (P < 0.05) are indicated by * 50 8 Spearman-rank correlation matrix for i ? i 0 s i and soil characteristics for the combined forest stands. Significant relationships (P < 0.05) are indicated by *. LFH depth is for the top 0 - 6 cm of forest floor only. 51 9 Annual Re and Rs (95% CI) using Eq. 6 with 5-cm TS and the param-eters listed in Table 5 53 10 Differences in RWs (//mol m - 2 s _ 1) and QiQs determined using mea-surements from soil collars located in areas representing the nighttime and daytime footprints. Significant differences (P < 0.05) within a stand are indicated by *. Degrees of freedom were 1 and 236, 214, and 234 for the 53-, 14, and 3-y stands, respectively 55 11 Parameters for Eq. 10 describing the relationship between daytime NEP and downwelling PAR for the four months shown in Fig. 21 for the three stands 75 vii 12 The annual relationship between Re and 5-cm Ts when > u*tth in the three Douglas-fir stands, where Re = RioQio ~ 1 0 ^ 1 0 . The number of half hours (n) from 2002 used to derive these parameters is also indicated 81 13 Annual and seasonal sums of NEP, G E P and R (values are rounded to the nearest 10) 84 14 Energy budget closure results for rain-free half hours for each year (September 1 to August 31) are indicated by the slope and intercept of the regression between H + XE and Ra. The number of half-hour measurements used in the linear least squares regression is indicated by n 100 15 Stand characteristics determined at the end of each growing season. Maximum leaf area index (LAI) includes all species growing on the site. Height and diameter values for the Douglas-fir seedlings are averages with 1 SE (standard error) of the mean in parentheses 103 16 Weather statistics during the three year study. Each year is taken from September 1 to August 31. Mean downwelling P A R is for the daytime only, while all other values are for 24-h. Standard errors of the means are in parentheses 105 17 The parameters for the exponential relationship (Eq. 13) between Re (//mol m - 2 s _ 1 ) and 2-cm Ts (°C) for each year. The coefficient of determination (R 2 ) and the number of samples (n) for each regression are also given. Re was obtained from nighttime N E P measurements um,th = 0.08 m s- 1 110 18 Parameters for the relationships between downwelling P A R and day-time N E P (Eq. 14) and G E P (Eq. 15) for the month of June in 2001, 2002, and 2003 113 19 Parameters for the relationship between daytime G E P and P A R (Eq. 15) for the growing season months of 2001, 2002, and 2003. Values of GEPQO are in //mol m - 2 s _ 1 115 20 Partial correlation coefficients for half-hour G E P and various biophys-ical variables when P A R was greater than 800 //mol m~ 2 s _ 1 116 21 Annual NEP, GEP, and Re between September 1, 2000 and August 31, 2003 118 22 Results from the comparisons of Fc and XE computed with open-path and closed-path LI-COR IRGAs in the 3- and 53-y stands 151 23 Transfer function (TF) characteristics for closed-path I R G A XE and Fc based on sensible heat cospectra ( C ^ f o r unstable daytime conditions (H > 100 W rn"2, XE > 80 W m~2) between July 4 and August 4, 2003 in the 3-y stand and between August 20 and September 5, 2003 in the 53-y stand 154 viii List of Figures 1 Accumulated tree bole volume (top panel) in coastal Douglas-fir for a site index of 35, representing the average tree height in m at 50 years of age. The bottom panel shows aboveground net primary production (ANPP) associated with the above increase in bole volume. Volume data were produced using TIPSY 3.0 (Table Interpolation Program for Stand Yields) (Mitchell et al., 2000) and estimates of C were calculated using an average wood density of 757 kg m - 3 (Cohen et al., 1985) with 50% C content 3 2 Hypothesized evolution of C O 2 fluxes during growth of a Douglas-fir stand (note, no units on graph). NEP is the difference between GEP and Re. These hypotheses are based on the relationship between ANPP and stand development (Fig. 1), the simulations of NEP in Janisch and Harmon (2002) and the hypotheses of Odum (1969) and Gorham et al. (1979) 4 3 Age classes for a 5 x 5 km area, which includes the 53-year-old (large footprint) and 3-year-old stands (small footprint) (ages in 2002). Map is used with permission from and constructed by Graham Stinson, Canadian Forest Service, Victoria based on 2000 inventory data pro-vided by Weyerhauser Co. and TimberWest Forest Corp. Non-forest in the legend includes forested wetlands 7 4 Locations of the three Douglas-fir stands on the east coast of Vancouver Island. Map is modified from Drewitt (2002) 8 5 Mean Re (nighttime NEE) versus mean (number of data points per bin = 100) for the three Douglas-fir stands measured between May 1 and December 31, 2002. Vertical error bars represent the 95% confi-dence interval for Re. The horizontal dashed line is the estimate of the average Re unaffected by = 6.3, 4.3, 3.7 //mol m - 2 s - 1 for the 53-, 14-, and 3-year old stands, respectively. The vertical dashed line represents the u*jth °f 0.32, 0.16, 0.08 m s _ 1 for the 53-, 14-, and 3-year old stands, respectively. See the text for more details 34 6 Nighttime half-hour vertical velocity in the instrument co-ordinates (w) normalized by half-hour cup wind speed (U) as a function of wind direc-tion for the three Douglas-fir stands when u* exceeded ut,th a n d when U was greater than 0.5 m s - 1. The solid line indicates the bin-averaged value (bin width = 30 degrees) for both daytime and nighttime half hours 36 7 Bin-averaged tilt angle as a function of wind direction in the three stands. The bin width is 30 degrees and includes all half hours with U greater than 0.5 m s _ 1 37 IX 8 Binned N E E measurements (n = 25) for all nighttime periods when u* > u*jh threshold against corrected vertical wind speed (w) for the 53-y, 14-y, and 3-y stands. The vertical bars are ± 1 SE of the mean. 38 9 Binned N E E residuals (n = 25) for all nighttime periods when u* > u^th threshold against corrected vertical wind speed (w) for the 53-y stand. The vertical bars are ± 1 SE of the mean 39 10 24-h mean 5-cm soil temperature, Ts, and 0-20-cm soil volumetric wa-ter content, 9V, for the three Douglas-fir stands between May 1 and December 31, 2002 42 11 Relationship between half-hour ecosystem (Re) and soil respiration (R3) on a log scale and 5-cm Ts for the 53-, 14-, and 3-y stands between May 1 and December 31, 2002. Also shown are the linear regressions (Eq. 6) for half-hourly Re and Rs (Table 5) 44 12 The relationships between Re and 5-cm Ts shown in Fig. 11 and Table 5 with binned values of Re and Rs for the three Douglas-fir stands. Each Re data point is the average of 30 half hours while each Rs symbol is the average of 15 measurements 45 13 The ratio of Rs to estimates of Rs calculated from Eq. 6 for each collar (20 collars per stand, 11 to 12 measurements per collar) between May 1 and December 31, 2002 against 0-12-cm volumetric water content (6V). 46 14 Distribution of R\QSi and Qios i values for the 3 stands (n = 20 stand - 1 ) between May 1 and December 31, 2002. One measure of Q ios i at the 14-y stand is not included and was = 8.3 47 15 The correlation between Riosi and total N from the 0-3 cm soil layer for the stationary collars (n = 10 stand - 1 ) in the three stands. Least squares regression resulted in i ? 1 0 s i = 0.54 N + 1.00 //mol m - 2 s _ 1 , R 2 = 0.53 52 16 Mean Rs for each sampling date vs. mean Re measured during the week bracketing each Rs sampling date. Vertical and horizontal lines are the 95% CI. The dashed line indicates the 1:1 line 53 17 Total annual modelled ecosystem respiration (Re) as a function of u*jttl for the three stands. The first data point shows annual Re estimated using Eq. 7 and parameters derived with 100% of the measurements. Each subsequent data point uses 50 measurements less than the pre-ceding data point. Vertical lines indicate the u*tth used in the general analysis for the three stands 56 18 Annual Rs (open symbols) and Re (closed symbols) for 2002 derived from function parameters in Table 5 versus annual gross ecosystem productivity (GEP) for the three Douglas-fir stands. Vertical bars indicate the upper and lower limits of the 95% CI for each annual estimate (see Table 9 for values) 57 X 19 2002 monthly total rainfall and average above-canopy air temperature for the 3-, 14-, and 53-y stands 72 20 Projected leaf area index (LAI) and weekly average daytime down-welling PAR for the 3-, 14-, and 53-y stands in 2002. Dashed lines represent interpolated measurements based on visual field observations. 73 21 Panels a) show selected diurnal cycles of half-hour measurements of NEP for the recently clearcut-harvested stand (3-y) (thin solid line), the pole-sapling stand (14-y) (dashed line), and the near-mature 53-y stand established (thick solid line). Data have been u* corrected and gap-filled in this panel. Panels b) show the relationships between non-gap filled NEP and downwelling PAR using data from the three days in a). Each data point is the average of daytime measurements of NEP for increasing levels of PAR grouped in widths of 100 //mol m~2 s _ 1. 74 22 Daily 24-h NEP in 2002 for the recently clearcut harvested stand (3-y), the pole-sapling stand established in 1988 (14-y), and the near-mature stand established in 1949 (53-y) 77 23 Average daytime NEP vs. downwelling PAR between January 1 and December 31, 2002. Each data point is the average daytime NEP (non-gap-filled) for 20 days with similar total PAR. Error bars are ± 1 SE. 78 24 The relationship between LAI and non-gap-filled NEP for downwelling PAR > 800 //mol m~2 s _ 1 for the three Douglas-fir stands for all of 2002. Each data point is the average of at least 45 half hour measurements of NEP after grouping by LAI bins of width 0.5. The error bars indicate ± 1 SE 79 25 Relationships between non-gap-filled NEP and PAR, D, and T under clear and cloudy sky conditions when PAR > 800 //mol m~2 s _ 1 for all of 2002. Each data point is the average of 200 half hours after sorting with PAR, D or T. The error bars indicate ± 1 SE 80 26 Relationships between non-gap-filled GEP and D when PAR > 1200 //mol m~2 s - 1 between June 15 and July 15 in the 53-y stand (thick solid line), the 14-y stand (dashed line), and the 3-y stand (thin solid line). Each data point is the average of 15 half hours. The error bars indicate ± 1 SE 82 27 Relationships between non-gap-filled GEP and T when PAR > 450 //mol m - 2 s _ 1 for March 2002 in the three Douglas-fir stands. Symbols are the same as in Fig. 26. Each data point is the average of 15 half hours. The error bars indicate ± 1 SE 82 28 Cumulative NEP in 2002 for the 3-, 14-, and 53-y stands 84 xi 29 Transfer function (TF) (panel a) and ensemble average cospectra for sensible heat (fCWT/cov(wT)) and for C O 2 (fCWSc/cov(wsc))(panel b) illustrating high frequency loss of w's'c for 47 nighttime half hours between May 4-9, 2003. The cut-off frequency (fco) of the T F is 0.73 Hz and TF parameters are a = 0.047, b = 0.994 98 30 Weather conditions between September 1, 2000 and August 31, 2003 including 7-day average daytime downwelling PAR (thick line) and diffuse PAR (thin line; top panel), 7-day average above-canopy air (T, thick line) and 2 cm soil temperature (Ts, thin line; 2nd panel), 7-day total rainfall (third panel), and 7-day average soil volumetric water content for the 0 to 20 cm depth (9V; bottom panel) 106 31 Average (7-day, 24-h) daily NEP (panel a) and half-hour NEP for daytime and nighttime periods (panel b) between September 1, 2000 and August 31, 2003. Panel c shows the measurements of LAI (A) and estimated LAI (dashed line) 107 32 Ensemble average of hourly NEP in June and March, 2001, 2002, and 2003. Hourly averages include only measured NEP. At night NEP measurements were excluded from the averages when < u+jh- Error bars are ± 1 standard error of the mean 108 33 Relationships between ecosystem respiration (Re) and 2-cm soil tem-perature (Ts) for all nighttime measurements made when it* exceeded for the 3 years (see Table 17). Each data point is the average of 100 measurements after sorting Re with Ts. Standard errors of the mean were about 1.1 //mol m~2 s - 1 but are not shown 109 34 Daytime NEP and GEP against downwelling PAR for the month of June in 2001, 2002 and 2003. Each data point is an average of 50 half-hour values. The light response of the half-hour measurements before averaging are described using Eq. 14 and Eq. 15 and are shown as solid lines (parameters appear in Table 18) 112 35 Average 3-day Rj, versus Ts (a) and Re versus Ts (b) sorted by 0-20 cm soil volumetric water content (9V) between September 1, 2002 and August 31, 2003 113 36 The relationships between daytime GEP and PAR (Eq. 15) for the growing season months of 2001, 2002 and 2003 (see Table 19). Each data point is an average of 50 half-hour values. Measures of LAI are listed for the end of each month if available (NA if not available). . . 114 37 The relationship between monthly GEPQQ and available measurements of LAI for all three years where, GEPQO = 3.9LAI + 0.7 //mol m - 2 s _ 1, R 2 = 0.75. Values of GEP^ and LAI are from Table 19 115 38 Annual NEP, GEP, and Re between September 1, 2000 and August 31, 2003 119 xii 39 Annual courses of seven-day average 24-h NEP, GEP and Re September 1, 2000 and August 31, 2003 120 40 Measured increases in C O 2 mixing ratio with time, dsc/dt during effec-tive volume tests where a known injection rate of C O 2 was introduced into the chamber system on a table top («)and on the soil ( o ) . The linear least square fit for the table tests, solid line, and the soil tests, dashed line, are also shown 143 41 Soil C 0 2 efflux measurements made by the homemade soil chamber system using an LI-800 IRGA compared with those made by LI-COR's model LI-6400 Soil C 0 2 Flux System. The least squares fit between the two systems is shown 145 42 Field setup of LI-7500/LI-6262 comparison in the 3-y stand in 2003. . 150 43 Half-hour XE and Fc measured with an open-path IRGA (LI-7500) and a closed-path IRGA (LI-6262) in the 3-y stand between July 4 and August 5, 2003. 1370 half hours were used in the analysis corresponding to rain-free and dew-free periods. The dashed line is the 1:1 line and the solid line indicates the fit of the linear least squares regressions, Fc>op = 1.08Fc>cp - 0.2 //mol m~ 2 s - 1 , R 2 = 0.95, n = 1370 and XE^ = l.\2XEcp + 0.4 W rn"2, R 2 = 0.98, n = 1370 152 44 Comparison of heat and water vapour mixing ratio cospectra (fCwx) and normalized cospectra (fCwx/w'x') (top panels) and transfer func-tions (TF) (bottom panels) for 46 daytime half-hours from July 4 -August 5, 2003 in the 3-y stand when H > 100 W m" 2 , XE > 80 W m - 2 and mean wind speed > 0.05 m s _ 1 153 45 The normalized ensemble average cospectra of sensible heat flux for the three Douglas-fir stands against non-dimensional frequency (Kaimal and Finnigan, 1994)(top panel) and frequency (Hz) (bottom panel). Non-dimensional frequencies include the measurement height(z), zero-plane displacement height id), and average horizontal wind speed (u). z was 3.5, 12, and 42 m and d was estimated to be 0.4, 4, and 23 m in the 3-, 14-, and 53-y stands, respectively. All half hours were taken from July 15 to 25, 2002 when H was between 180 and 200 W m~ 2 (average H was between 190 and 194 W m - 2 in all stands) where n = 9 for the 3-y stand, n = 15 for the 14-y stand, and n = 12 for the 53-y stand 155 xiii 46 The proportion of sensible heat flux attributable to frequencies be-yond 0.1, 0.5, and 1 Hz in the three Douglas-fir stands for a variety of wind speeds (u) and atmospheric stabilities, (z — d)/L where z is the measurement height, d is the zero-plane displacement, and L is the Monin Obukhov scaling length (Kaimal and Finnigan, 1994). Neg-ative (z — d)/L refers to unstable conditions (daytime periods with significant H and XE fluxes) while near-zero is neutral (generally over-cast or nighttime periods with high wind speeds) and positive is stable (nighttime periods with moderate wind speed) 156 47 The relationship between the correction factor for closed-path IRGA XE and half-hour average horizontal wind speed (it) for 46 daytime half-hours from July 4 - August 5, 2003 in the 3-y stand when H > 100W rn-2, XE > 80W m~2 and mean wind speed > 0.05 m s_1, CF = 0.07u +1.05, R 2 = 0.47 157 xiv List of Symbols Symbol Units Definition C g or mg g dry soil -1 carbon CF correction factor C 0 2 ppm carbon dioxide GEP g C m - 2 time - 1 or gross ecosystem production //mol C O 2 m - 2 s - 1 LAI leaf area index, m 2 projected leaf area m - 2 ground N mg g dry soil - 1 nitrogen concentration NEE //mol m - 2 s - 1 net ecosystem exchange of C O 2 NEP g C m - 2 time - 1 or net ecosystem production (= -NEE) //mol C 0 2 m - 2 s _ 1 PAR //mol m - 2 s _ 1 or MJ photosynthetically active radiation (photon m"2ri-l flux density or energy m - 2 ) A m - 2 area n m //mol mol - 1 cospectrum for C O 2 flux n m mmol mol - 1 cospectrum for latent heat CwT m °C cospectrum for sensible heat Fc //mol m - 2 s - 1 C 0 2 flux H W m"2 sensible heat flux L m Monin-Obukhov scaling length P kPa barometric pressure R J mor1 K - 1 universal gas constant (= 8.31) Re g C m - 2 time - 1 or ecosystem respiration //mol C 0 2 m - 2 s - 1 Rs g C m- 2 time - 1 or soil respiration //mol C O 2 m - 2 s - 1 Rio //mol C O 2 m - 2 s - 1 respiration at 10 °C Qw sensitivity of respiration to temperature T °C or K air temperature Ts °C soil temperature Tv °C or K sonic temperature U m s - 1 cup wind speed K m - 3 effective volume X V Symbol Units Definition cl m zero-plane displacement / Hz frequency f0 Hz cut-off frequency h m mean canopy height n sample number or non-dimensional fre-quency q g H 2 0 g - 1 specific humidity r g H 2 O g dry air - 1 water vapour mass mixing ratio sc //mol C O 2 mol dry C O 2 mole mixing ratio air - 1 sv mmol H 2 O mol dry water vapour mole mixing ratio air - 1 u m s _ 1 horizontal (streamwise) wind speed •u* m s _ 1 friction velocity u*tth m s _ 1 threshold friction velocity v m s - 1 lateral wind speed w m s - 1 vertical wind speed w m s _ 1 vertical wind speed in the instrument co-ordinates x m horizontal co-ordinate z m vertical co-ordinate or measurement height zm m measurement height zQ m roughness length a albedo or probability level AE W m"2 latent heat flux Xc //mol C O 2 mol - 1 C O 2 mole fraction Xv mmol H 2 O mol - 1 water vapour mole fraction pa mol dry air m~3 molar density of dry air pc mol C O 2 m - 3 C O 2 molar density pv mol H 2 O m - 3 water vapour molar density 6V m 3 m - 3 volumetric soil moisture xvi Acknowledgements Sincere thanks to my supervisor, Andy Black. This has been a wonderful pe-riod in my life and I owe much to him and his ability to create an exciting research environment and to attract so many interesting and capable people to the Biomet lab. Many colleagues and technicians were valuable contributors and have become great friends. These include Gord Drewitt, Ka i Morgenstern, David Gaumont-Guay, Zoran Nesic, Natascha Klujn, Tiebo Cai, Zhong L i , Rachpaul Jassal, T i m Griffis, Altaf Arain, Rick Ketler, Andrew Sauter, Armel Castellan, and all the Chads (Chad, Clancy, and David). Thanks to Altaf for all his hard work drawing up the initial proposal for this project. Special thanks to Gord, Zoran, and Rick who were there from the beginning to help me out with wiring, tower climbing, and bike commuting Vancouver-style. I had a great time hanging out in the forest and at Monfort house with Gilbert Ethier and all his field crew from U.Vic. I also had the pleasure to work with some of the best summer field assistants ever, including Lesley Dampier, Isla Myers-Smith, and Scott KrayenhofF. Tony Trofymow, Gurp Thandi, and Graham Stinson at the Canadian Forest Service, Pacific Forestry Centre very generously pro-vided assistance and information on stand characteristics. Dave Spittlehouse at the B C Ministry of Forests was a wonderful mentor and occasional field hand. Thanks also to my committee members, Mike Novak, Cindy Prescott, and T i m Oke for their support and valuable assistance. I appreciated the financial support of the Natural Sciences and Engineering Re-search Council of Canada (NSERC), Isaak Walton Kil lam Memorial Predoctoral Fel-lowship, Mary and David Macaree Fellowship, Leonard S. Klinck Memorial Fellow-ship, University of British Columbia Graduate Fellowship, Climate Change Funding Initiative in Agriculture, and teaching and research assistantships. Funding for the Vancouver Island research projects was through Forest Renewal British Columbia (1997 - 2000), an N S E R C Strategic Grant (2000 - 2003), the Canadian Foundation for Innovation, and finally, through funding for the Fluxnet Canada Research Network (NSERC, BIOCAP, Canadian Foundation for Climate and Atmospheric Sciences). Many thanks to our collaborators at Weyerhauser Co. and TimberWest Forest Corp. for their assistance and use of their land. Field support was provided by our able and friendly contractors, Fred Brufatto, A l Algaard, and Dwayne Young. I am grateful to my parents and all of my family for their unwavering support. XVII Thanks also to the folks at the Canadian Forest Service in Sault Ste. Marie, Rob Fleming and Deb Mossa who were originally responsible for setting me on this path. Finally, I am especially grateful to my husband, Mike Treberg and our side-kick, Maggie-the-dog. Going through this Ph.D. process together has definitely made it all worth while. Chapter 1. Introduction 1 Introduction 1 Disturbance and succession play a major role in determining forest structure and func-tion. Functional characteristics such as carbon, nutrient, and energy fluxes are closely linked to stand structural characteristics such as species composition, stand density, leaf area, and stand height. Carbon (C) flows into and out of forest ecosystems are primarily a function of the amount of carbon dioxide ( C O 2 ) fixed through photosyn-thesis by growing vegetation and the amount of C O 2 released through respiration by vegetation and decomposition processes. Other routes for C include methane fluxes and the movement of dissolved organic and inorganic C through the soil. The magni-tude and direction of the net exchange of C O 2 between forests and the atmosphere is expected to depend greatly on the forest's stage of development. In Canada, 1 million ha out of 247 million ha of commercial forestland are harvested on an annual basis. On average, another 2 to 3 million ha of forest are lost to fire, while 1 to 4 million ha are killed by insect damage annually (Kurz and Apps, 1999). When these natural and human-induced disturbances result in the initiation of secondary succession, dramatic changes in C 0 2 fluxes occur. Globally, about 120 Pg C y _ 1 (= 10 1 5 g C y " 1 ) is taken up in photosynthesis by vegetation while autotrophic and heterotrophic respiration and decomposition processes are estimated to release about the same amount. In comparison, C released through the combustion of fossil fuels and land-use change is about 8 Pg C y - 1 while the net uptake of C by the oceans is about 2 Pg C y - 1 (Schlesinger, 1997). Consequently, year-to-year variations in the net exchange of C by terrestrial ecosystems can have a large impact on global interannual variability of atmospheric C O 2 (Schimel et a l , 2001). A major effort to model terrestrial C budgets on regional, national and global scales is currently underway (Chen et al., 2000a,b; Kucharik et a l , 2000). Combined Chapter 1. Introduction 2 with atmospheric general circulation models, land-surface schemes are designed to model biosphere-atmosphere interactions (Verseghy, 2000). Some of the complemen-tary goals of these models are to predict the climatic consequences of rising atmo-spheric C O 2 concentrations (Bounoua et al., 1999), to investigate C-cycle and vege-tation responses to changes in climate and land-use, and to investigate fertilization effects of increasing atmospheric C 0 2 concentrations and nitrogen deposition (Thorn-ton et al., 2002). Both process- and inventory-based models are also being used to account for changes in terrestrial C stocks over periods of time such as the Kyoto commitment periods (Li et al., 2003). In all thorough understanding of forest structure and function at all stages of succession is critical. The forest industry and research communities have collected growth and yield data from which aboveground net primary production (ANPP) can be estimated (Mitchell and Cameron, 1985). Fig. 1 shows the accumulated biomass and A N P P resulting from the increment in total tree bole volume in a naturally generated second growth coastal Douglas-fir stand growing on land with a site index of 35 (predicted tree height in m at 50 years of age). Typical of forests throughout the world, A N P P (for marketable wood) is near-zero for a period after the original stand is removed, increases rapidly as stand growth reaches a maximum rate early in stand development and then gradually declines as the stand matures (Gower et al., 1996). However, growth curves only represent the C accumulated in the merchantable wood. They do not account for C accumulated within branches, leaves, or roots or for C released through decomposition processes or for the C cycling attributable to the growth of early successional or understory species. Also, the effects of variability in weather on annual A N P P are not accounted for. The development of micrometeorological techniques such as eddy covariance has enabled long-term monitoring of the exchange of C 0 2 between forests and the at-Chapter 1. Introduction 3 Stand Age (y) Figure 1: Accumulated tree bole volume (top panel) in coastal Douglas-fir for a site index of 35, representing the average tree height in m at 50 years of age. The bottom panel shows aboveground net primary production (ANPP) associated with the above increase in bole volume. Volume data were produced using T I P S Y 3.0 (Table Interpolation Program for Stand Yields) (Mitchell et al., 2000) and estimates of C were calculated using an average wood density of 757 kg m " 3 (Cohen et al., 1985) with 50% C content. mosphere (Baldocchi, 2003). By combining eddy covariance measurements of C O 2 fluxes with measured fluxes of methane and loss of dissolved organic and inorganic carbon, the net ecosystem production (NEP) of a forest stand can be determined. In the absence of a disturbance such as harvesting or fire which removes biomass from the land, N E P represents the net change in carbon stored within an ecosystem for a specified period of time. N E P is approximately equal to - N E E = G E P - Re, where N E E is the net ecosystem exchange of C 0 2 , G E P is the gross ecosystem production, and Re is ecosystem respiration composed of autotrophic (Ra) and heterotrophic res-piration (Rh). Positive N E P and G E P represent an uptake of C 0 2 by the ecosystem while positive Re and N E E represent a loss of C 0 2 from the ecosystem to the atmo-Chapter 1. Introduction 4 sphere. NEP is related to ANPP as NEP = ANPP + BNPP - Rh, where BNPP is the belowground portion of net primary production (NPP = GEP - Ra). Long-term eddy covariance measurements of NEP present the opportunity to assess the stand structural and environmental factors, which control interannual variability in NEP in forests at different stages of succession. Figure 2: Hypothesized evolution of C O 2 fluxes during growth of a Douglas-fir stand (note, no units on graph). NEP is the difference between GEP and Re. These hypotheses are based on the relationship between ANPP and stand development (Fig. 1), the simulations of NEP in Janisch and Harmon (2002) and the hypotheses of Odum (1969) and Gorham et al. (1979). Changes in NEP, Re, and GEP during forest succession are hypothesized to follow the general patterns shown in Fig. 2. Changes in the direction and magnitude of NEP are suggested to be closely linked to the structural status of these developing forests (Odum, 1969; Gorham et al., 1979; Janisch and Harmon, 2002). As reviewed by Landsberg and Gower (1997), the successional patterns of NEP are still speculative. Chapter 1. Introduction 5 After a stand-clearing disturbance such as clearcut-harvesting or intense fire, almost all of the leaf area of the stand is destroyed resulting in zero GEP, while heterotrophic respiration processes continue at similar, higher or lower rates. Consequently N E P is negative following the disturbance until a time when the C fixed through photosynthe-sis exceeds the loss of respired C C v The growth of planted or naturally regenerated seedlings along with other pioneer or understory species results in a gradual increase in leaf area and biomass (Fig. 1). Along with the increased G E P , rates of autotrophic respiration will also increase. In addition, the growth of annual and deciduous pioneer species could potentially result in increased heterotrophic respiration rates as a result of high quality litter inputs to the soil. Eventually leaf area and G E P level off and begin a period of relatively constant C assimilation. Re may continue to increase at this time as a result of autotrophic respiration while heterotrophic respiration rates may level, increase, or decrease depending on litter inputs, nutrient limitations, and changes in microclimate. N E P and A N P P then tend to decrease below their max-imum rates. This period of decreasing N E P may be due to increased Ra; however, there is evidence that it may be due to decreased G E P resulting from lower nutrient availability or greater stomatal limitations to C 0 2 uptake as forests age (Gower et al., 1996). At an old-growth stage of succession, N E P is predicted to be near zero (not shown in Fig. 2) when G E P balances Re. Only a few studies have measured N E P in similar forest types at different stages of succession (Amiro, 2001; Anthoni et al., 2002; Chen et a l , 2002; Rannik et al., 2002; Litvak et al., 2003). As expected, these studies have generally associated greater leaf area within the forest type with greater net C gains. However, these studies also show that forest physiology and N E P responses to drought and water vapour deficit differ between stands at different ages. In contrast, a relatively large body of work has found that annual N E P for forests at later successional stages varies considerably Chapter 1. Introduction 6 between forest types (Griffis et al., 2003), climates (Law et al., 2002) and between years (Barford et al., 2001; Carrara et al., 2003; Morgenstern et al., 2003). The Fluxnet community (groups carrying out research at sites with tower-based microm-eteorological measurements of N E P of terrestrial ecosystems) recently began to pair observations in older forests with observations in nearby recently disturbed forests (Amiro, 2001; Kowalski et al., 2003; Litvak et al., 2003). This work has shown that recently harvested or burnt forest stands can be net sources of C O 2 , while neighboring older stands are usually sinks for C O 2 . However, few measurements have been made for periods of a year and more, and consequently, little is known about the relative role that changes in stand structure, weather variability, and management practices have on seasonal and annual variations in N E P in very young forests. In 1997, a long-term C O 2 and water vapour flux monitoring project was initiated in a 47-year-old Douglas-fir stand on the east coast of Vancouver Island. A number of studies report on the biophysical factors which influence the seasonal and interannual exchange of carbon dioxide and water vapour in this stand (Drewitt et al., 2002; Humphreys et al., 2003; Morgenstern et al., 2003). Interannual variations in net C exchange have been linked primarily to differences in spring temperatures as they influence respiration. Consequently, year-to-year variations in N E P were large, as much as 100 g C m - 2 y - 1 . Measurements from this stand have been instrumental in the development and verification of a number of modelling efforts (e.g., E l Maayar et al., 2002). On the east coast of Vancouver Island, old-growth stands were harvested in the early 1900's and the resulting second growth stands are managed on 50- to 90-year harvest rotations (Spittlehouse, 2003). This disturbance, in addition to fire and insect damage, has resulted in a patchwork of stands mostly between 0 and 80 years of age with only small patches of old-growth left in parks and areas which are difficult to Chapter 1. Introduction 7 access (Fig. 3). To achieve a better understanding of C exchange processes in forested Figure 3: Age classes for a 5 x 5 km area, which includes the 53-year-old (large footprint) and 3-year-old stands (small footprint) (ages in 2002). Map is used with permission from and constructed by Graham Stinson, Canadian Forest Service, Victoria based on 2000 inventory data provided by Weyerhauser Co. and TimberWest Forest Corp. Non-forest in the legend includes forested wetlands. lands on Vancouver Island, there is a pressing need for additional data sets from forests which have not yet reached rotational-age maturity. In 2000 and 2001, C O 2 and water vapour flux measurements were initiated in a recently clearcut-harvested Douglas-fir stand and a 14-year-old stand, respectively. These three research sites now form the British Columbia Flux Station of the Fluxnet Canada Research Network established in 2002. A l l three forest stands are located on the east coast of Vancouver Island, B.C. between 170 and 300 m elevation within 13 km of Georgia Straight (Fig. 4). The oldest stand in the study was established in 1948 and is located only 3 km W N W of in 2000 1,000 Chapter 1. Introduction 8 a stand clearcut-harvested in winter 1999/2000 and planted in spring 2000. 48 km to the SE of these two research sites is the pole/sapling stand established in 1988. Vancouver Isla = 50 km Figure 4: Locat ions of the three Douglas-fir stands on the east coast of Vancouver Island. M a p is modified from Drewi t t (2002). The results from C O 2 flux measurement projects in unreplicated forest stands such as these are not meant to provide estimates of regional C budgets or definitively state the effects of disturbance on the C dynamics of Douglas-fir ecosystems in general. However these studies provide insight into C exchange processes and the biophysi-cal factors which affect them for these particular forest stands at different stages of development after harvesting. As mentioned earlier, results from this study will be invaluable for verifying modelling efforts and, in combination with similar research being carried out by Fluxnet groups throughout Canada and the world, these results can be used to identify patterns and make broader conclusions about C exchange processes (e.g. Valentini et al., 2000; Janssens et al., 2001; Law et al., 2002) and the effects of succession and disturbance. In order to improve our understanding of C exchange dynamics in forests at dif-Chapter 1. Introduction 9 ferent stages of development, this study examines the daily, seasonal, and interannual patterns of N E P in the three previously mentioned Douglas-fir stands. Chapter 2 presents an analysis of respiration processes in the three stands using nighttime eddy covariance (EC) and soil chamber measurements in 2002. The re-lationships between respiration and soil temperature, and how they differ between stands at the ecosystem and soil levels provide insight into the effects of stand struc-ture and site characteristics on respiration processes. Issues related to the validity and accuracy of E C measurements of C 0 2 fluxes when made at night during calm pe-riods and above sloping terrain are addressed. Soil chamber measurements illustrate the spatial variability of soil C 0 2 efflux within a stand and the biophysical factors responsible for this heterogeneity. Appendices 1 and 2 present some of the tests and development of the portable soil chamber system used in this study. Chapter 3 reports on the annual record of E C measurements of N E P in the three stands in 2002. The relationships between N E P and biophysical variables such as leaf area index, light, temperature, and water vapour deficit are contrasted between stands. Chapter 4 examines interannual and seasonal variations in N E P in the recently clearcut-harvested stand over a three-year period between September 2000 and Septem-ber 2003. The establishment and growth of pioneer species and the planted Douglas-fir seedlings is followed using measurements of leaf area index. Seasonal and annual variations in NEP, Re and G E P are related to these changes in stand structure. The degree to which interannual variations in N E P in this stand are related to variability in weather are investigated. In addition, this chapter and Appendix 3 examine the differences in C 0 2 fluxes measured using open- and closed-path infra-red gas analyz-ers. Chapter 5 summarizes the major results of this study, discusses the relevance of Chapter 1. Introduction 10 these results wi th respect to questions related to C exchange dynamics at different stages of forest succession, and identifies areas for future research in these three forest stands. Chapter 2. Respiration in three Douglas-fir stands 11 2 Comparison of ecosystem and soil respiration in three different-aged coastal Douglas-fir stands 2.1 Introduction Forest harvesting and subsequent forest regeneration impact above and belowground respiration rates in a variety of ways. Changes in stand structure such as biomass, leaf area, tree height and density, and species composition not only affect rates of autotrophic respiration but also result in changes in microclimate, soil carbon and nutrients, and soil microflora and fauna populations (Marshall, 2000), which directly impact heterotrophic respiration rates. The disturbance of soil through harvesting practices may result in the removal or volatilization of nutrients and changes to soil structure which can impact aeration, root penetration, and water retention and flow (Ballard, 2000). A l l of these factors regulate soil carbon pool dynamics in a complex way and ultimately play a major role in determining at what time the regenerating forest will become a sink for C O 2 . In a recent review of C O 2 exchange fluxes in European forests, Janssens et al. (2001) emphasized that gross ecosystem production (GEP) may be more important than mean soil or air temperatures in determining soil and ecosystem respiration of forests. Experiments preventing the allocation of photosynthates to roots have found an influence on rates of soil C 0 2 efflux within days, directly linking plant growth to respiration (Craine et al., 1999; Hogberg et al., 2001). Hysteresis in long-term soil respiration temperature-response functions gives indirect evidence that changes in phenology and microbial populations affect respiration processes (Drewitt et al., 2002). Net primary production has been found to correlate with microbial biomass (Zak et al., 1994) in addition to soil C 0 2 flux (Raich and Schlesinger, 1992; Janssens Chapter 2. Respiration in three Douglas-fir stands 12 et al., 2001), suggesting that ecosystems with greater productivity have greater detri-tal inputs to the soil, reducing nutrient limitations on decomposition processes. How-ever, seasonal and annual variability in soil and ecosystem respiration rates within a given ecosystem can be well described by seasonal variations in temperature and often soil moisture (eg. Pilegaard et al., 2001; Drewitt et al., 2002; Irvine and Law, 2002). In many landscape-scale dynamic carbon models, total forest respiration rates are estimated using a combination of temperature-dependent reaction rates ( Q i 0 models) and the supply of substrate materials to C pools of varying ease and speed of de-composition (Kurz and Apps, 1999; Harmon and Marks, 2002). Decomposition rates may also be modified by soil moisture conditions (Harmon and Marks, 2002). Some models incorporate the effects of nutrients such as nitrogen on growth and thus fur-ther link autotrophic and heterotrophic respiration to stand productivity (Law et al., 2001b). The effects of disturbances, such as harvesting, on decomposition rates have been incorporated into models by adjusting the litter inputs and by modifying the temperature response to increased rates of decay (Kurz and Apps, 1999). However, changes in microclimate with disturbance can be equally important. In general, sim-ulation models stress the importance of both temperature and productivity on the release of C O 2 to the atmosphere. Since forests at the landscape scale are a patchy mosaic of different aged stands, it is crucial to quantify C fluxes at the scale of a forest stand at all stages of development. This information plays an important role in the development and verification of simulation models. Furthermore, an improved understanding of the mechanisms which control C fluxes will enhance our ability to manage C pools throughout stand development. This study investigates respiration processes in three closely located coastal Douglas-fir stands at different stages of development. For an 8-month period in 2002, we used Chapter 2. Respiration in three Douglas-fir stands 13 both the eddy covariance technique above the forest canopy to examine nocturnal C O 2 losses from the ecosystem and soil chambers to quantify respiration rates from the forest floor. A n exponential relationship between respiration and temperature is used as a tool for investigating both the spatial and temporal variations in respiration processes. These relationships are used to estimate total annual C O 2 losses from each stand and examine the proportion of ecosystem respiration originating from the soil and the impact of microclimate and ecosystem productivity on annual respiration rates. Equating ecosystem respiration to nighttime eddy covariance measurements of C O 2 flux requires consideration of the influence of atmospheric turbulence and the theoretical assumptions on which the eddy covariance technique is based (Lee, 1998; Baldocchi et a l , 2000; Massman and Lee, 2002; Finnigan et al., 2003). The tendency to underestimate nighttime C 0 2 fluxes using eddy covariance has been widely ob-served (Black et a l , 1996; Goulden et al., 1997) and may be due to little or only sporadic mixing under stable atmospheric conditions, failure to accurately quantify C O 2 storage below the eddy covariance measurement height, and significant hori-zontal flux divergence resulting in the loss of C O 2 from the canopy during cold air drainage flow (eg. Baldocchi et al., 2000). Recent re-evaluation of the underlying assumptions in the analysis of surface fluxes has shown that low frequency contribu-tions to fluxes may be lost when inappropriate averaging times are used and when the co-ordinates are rotated so that mean vertical velocity is zero (Finnigan et al., 2003). These problems are often exacerbated in complex or sloping terrain as is the case in this study. The impact of these factors are expected to be minimized when nighttime eddy covariance measurements are made under well-mixed conditions. As a result, it is common to reject measurements made at night when the friction velocity (u*) is less than a minimum or threshold value (eg. Goulden et al., 1997). However, Chapter 2. Respiration in three Douglas-fir stands 14 despite using this criteria, the variations in the remaining C O 2 fluxes from one half hour to the next are often greater than what models or chamber measurements would predict for biological fluxes (Baldocchi et al., 2000). This emphasizes the importance of issues related to equating N E E to turbulent eddy fluxes. The objectives of this paper are to: 1) Investigate the turbulent and advective pro-cesses involved in nighttime C O 2 transport in the three stands; 2) Establish whether there are significant differences among stands in the coefficients of the empirical re-lationships between soil and ecosystem respiration rates and climate variables; 3) Examine the between- and within-stand spatial variability of soil respiration; 4) Com-pare annual estimates of soil respiration with ecosystem respiration with respect to spatial, temporal and methodological differences between the two components. 2.2 Methods 2.2.1 Stand descriptions The measurements were made in three different-aged coastal Douglas-fir stands on the east coast of Vancouver Island, BC, Canada between May 1 and December 31, 2002. In 2002, the stands were 53, 14, and 3 years old. For the most mature stand in this study, historical fire and regeneration maps indicated that much of the previous old-growth forest in the eastern half of the stand was logged and slash-burned in 1937 and the remainder logged and slash-burned in 1943 (Thandi and Trofymow, 2002). The current stand covered an area of 130 ha at 300 m above sea level (49°52'N, 125°20'W). Eighty percent of the trees within the stand were Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirbel) Franco), 17% were western redcedar (Thuja plicata Donn) and 3% were western hemlock (Tsuga heterophylla (Raf.) Sarg.). Stand density was 1100 trees h a - 1 , projected leaf area index (LAI) was 8.4, mean diameter at breast Chapter 2. Respiration in three Douglas-fir stands 15 height (DBH) was 29 cm, and average tree height was 35 m. Only a sparse understory was present. The sloping terrain (5-10 degrees) faced N E towards Georgia Straight 13 km away. About 3 km ESE of the 53-year-old stand, a 32-ha Douglas-fir stand established in 1940 was clearcut harvested in 2000, with only roadside debris piles burnt. In the early spring of 2000, the area was planted with 1-year-old seedlings (93% Douglas-fir, 7% western redcedar plug stock) at a density of 1500 trees h a - 1 . In 2002, the trees were 3 years old, on average 0.6 m tall with a mean basal diameter of 1.5 cm. L A I varied through the growing season from a low of 0.6 in Apri l to a maximum of 2.2 in June and July mainly with the growth of pioneer and understory species from the previous stand. Some of the more common species included bracken fern (Pteridium aquilinum), sword fern (Polystichum munitum), fireweed (Epilobium angustifolim), Oregon-grape (Mahonia nervosa), salal (Gaultheria shallon), twinflower (Linnaea bo-realis), bedstraw (Galium triflorum), bearberry (Arctostaphylos uva-ursi), various Rubus spp., and various asters, thistles, and grasses. The 3-year-old stand was lo-cated at 49°52'N, 125°17'W and at an elevation of 175 m on level terraced terrain. The 14-year-old, 110-ha stand was situated about 48 km SE of the 53-year-old stand (49°31'N, 124°54'W) at an elevation of 170 m on a 2 to 5 degree slope facing SE, 4 km from Georgia Straight. The trees were on average 5.8 m tall with a mean D B H of 7.5 cm and a density of 1240 stems h a - 1 . A dense understory of deciduous and evergreen species such as fireweed, sword and bracken fern, salal, Rubus spp., and red huckleberry ( Vaccinium parvifolium) resulted in a variable L A I for the whole stand with a maximum value of approximately 4.5 in late June. This area had been harvested in 1987, broadcast burned and planted in 1988 with 75% Douglas-fir, 21% western red cedar, and 4% grand fir. Herbicide was applied in 1992. Prior to the 1987 harvest, approximately 75% of the research area was a mixed second growth stand Chapter 2. Respiration in three Douglas-fir stands 16 of primarily Douglas-fir with western redcedar and western hemlock. The remainder of the area was primarily red alder (Alnus rubra Bong.). A n initial cut (and likely slash burn) of the original forest occurred in 1937-1938. The 53-year-old, 14-year-old, and 3-year-old stands are hereafter referred to as the 53-y, 14-y, and 3-y stands, respectively. A l l three stands were located within very similar ecosystems although differences in soil moisture and nutrient status, elevation and terrain were evident (Tables 1 and 2). Ecosystem classification was carried out using the biogeoclimatic ecosystem classification system, which incorporates climate, soil and vegetation characteristics (Meidinger and Pojar, 1991). A l l three stands were within the driest subzone of the Coastal Western Hemlock biogeoclimatic zone (CWHxm, very dry maritime Douglas-fir succeeding to coastal western hemlock) (Meidinger and Pojar, 1991). At an ele-vation of 300 m and furthest from Georgia Straight, the 53-y stand was within the Western variant (CWHxm2) while the 14-y and 3-y stands were within the Eastern variant (CWHxml) . Locally, site series associations indicate more specifically the typical climax plant communities expected for a given soil moisture and nutrient sta-tus within a subzone (Green and Klinka, 1994). The site series was determined in 2001 for eight, two, and two plots within the typical flux footprints of the 53-y, 14-y, and 3-y stands, respectively (Thandi and Trofymow, 2002) using surveys of both vegetation and environmental factors (Green and Klinka, 1994). In all three stands, considerable within-stand variability in soil nutrient and moisture regimes existed (Table 2). However, 96% of the area surveyed within the flux footprint in the 53-y stand (the area representing the source location for the tower measurements of C 0 2 fluxes, see Section 2.2.2), was characterized as site series 05 ( C W H - Sword fern) indi-cating an intermediate (slightly dry to fresh) soil moisture regime (SMR) and above average (rich to very rich) soil nutrient regime (SNR). In addition, other areas were Chapter 2. Respiration in three Douglas-fir stands 17 classified as 01 (intermediate SMR, average to below average SNR), 07 (moderate to very wet SMR, above average SNR) and 12 (wet SMR, above average SNR). In the 14-y stand, 95% of the area surveyed was also determined to be site series 05, with additional areas of 07. In the 3-y stand, 88% of the area surveyed was site series 05 with some additional 07 and 12. The soils of all three stands were humo-ferric podzols (Valentine et al., 1978) with similar concentrations of carbon and nitrogen measured within the L F H and mineral soil layers (Table 2). However, the depth of the L F H layer varied considerably between and within stands with the deepest average depth of L F H in the 14-y stand. In addition, soil texture was finer in the 14-y stand and there was a greater available water content. Further evidence of the similarity in site productivity characteristics between the three stands can be associated with the site indices (m) given to an area by the local foresters to indicate the expected height of 50-year-old trees. Both the 53-y and 3-y stands were given relatively high site indices of 35 m and 32 m, respectively. The 14-y stand was associated with a lower site index of 29, however, the size of the 14-year-old Douglas-fir trees correspond very well with the growth characteristics of coastal Douglas-fir in an area with a higher site index of 35 (Mitchell et al., 2000). Overall, these site and stand characteristics suggest that differences in C 0 2 fluxes between stands can be related to stand age and corresponding differences in stand structural characteristics (i.e., stand height, leaf area, species composition, litter characteristics) while bearing in mind potentially important differences in site location and soil texture and associated moisture dynamics. Chapter 2. Respiration in three Douglas-fir stands 18 Table 1: Selected stand and site characteristics for the three Douglas-fir stands. 53-y 14-y 3-y Stand characteristics: Current stand origin Stand height in 2002 (m) Measurement height (m) Stand density in 2002 (trees h a - 1 ) Douglas-fir (% of total trees) Site characteristics: Biogeoclimatic classification" Elevation (m) Slope (degrees) Aspect Soil series6 Terrain c 1949 35 43 1100 80 1988 5.8 12 1240 75 300 5 - 10 N E Quimper extensive sandy-silty morainal blanket overlying morainal veneer colluvium 2000 0.6 3 1500 93 CWHxm2/05 CWHxml/05 CWHxml/05 170 2 - 5 SE 60% Piggot, 40% Quimper sandy-silty morainal veneer-blanket &; 175 0 - 2 level Hayworth fluvial terrace "Biogeoclimatic (zone and subzone) information provided by the B C Ministry of Forests, (ftp:// ftp.for.gov.bc.ca/branches/research/external/ Ipublish/becmaps). Site series determined using on-site surveys Thandi and Trofymow (2002). See text for explanations. 6Jungen (1986) c Thandi and Trofymow (2002) Chapter 2. Respiration in three Douglas-fir stands 19 Table 2: Selected soil characteristics for the three Douglas-fir stands. Values in parentheses indicate the standard deviation of the mean. 53-y 14-y 3-y Soil mineral fraction (0 to 1 m): Texture gravelly loamy gravelly loam gravelly loamy sand sand - sand Coarse fragments" (m 3m~ 3) 0.33 0.29 0.43 Bulk density (kg m" 3 ) 1336 (252) 835 (174) 1335 (248) F C (m 3 m" 3 ) 0.21 0.35 0.19 W P (m 3 m" 3 ) 0.06 0.13 0.05 A W C 6 (mm) 150 220 140 Soil Mineral Fraction (0 to 6 cm): C c (mg g - 1 dry soil) 74 (41) 83 (3) 53 (28) N c (mg g " 1 dry soil) 2.2 (0.8) 2.9 (1.1) 1.8 (0.8) L F H (surface organic horizons): C c (mg g " 1 dry soil) 385 (178) 382 (125) 450 (129) N c (mg g - 1 dry soil) 8.1 (3.0) 6.4 (2.7) 5.2 (2.3) Average depth** (cm) 3.0 (2.1) 4.1 (3.3) 3.6 (4.8) "Coarse fragments refer to the fraction of materials with a diameter greater than 2 mm. 6 Available water content (AWC) is the volume of water in the soil between matric potential values of-0.01 (field capacity, FC) and -1.5 M P a (wilting point, W P ) derived from formulae in Saxton et al. (1986), calculated as (FC - W P ) x 1 m soil depth. c Carbon (C) and nitrogen (N) concentrations refer to total concentration of these elements measured using a L E C O induction furnace. Each value is an average of 8 - 1 4 samples. d Data collected by J .A. Trofymow. Each value is an average of 24 - 48 samples. 2.2.2 Ecosystem respiration measurements The eddy covariance (EC) technique was used to measure the half-hour fluxes of C 0 2 (Fc, //mol m - 2 s _ 1 ) above the canopy (positive Fc is an upward flux of C O 2 ) . Net ecosystem exchange (NEE, //mol m~ 2 s - 1 ) was calculated as the sum of Fc and the rate of change of C 0 2 storage in the air column below the height of the eddy flux Chapter 2. Respiration in three Douglas-fir stands 20 measurements (zm), N E E = Fc + zmp-ads-cldt. (1) where sc is half-hour average C O 2 mole mixing ratio (//mol mol dry a i r - 1 ) , t is time (s), and is the average mole density of dry air (mol dry air m~ 3). dsl/dt was calculated for each half hour using the difference in ~s~2 for the previous and following half hours. In the 53-y stand, a 4-level profile system was used to compute this storage term (see Morgenstern et al., 2003, for more details), while a single level at the E C height was used in the two younger stands. At night, measurements of N E E were interpreted as measurements of ecosystem respiration (i.e., Re = NEE) where positive values of N E E or Re indicated a loss of C 0 2 from the ecosystem. The E C instrumentation included three-axis sonic anemometer-thermometers and C O 2 / H 2 O infrared gas analysers (IRGA) installed on towers at 43, 12 and 3 m above the ground in the 53-, 14- and 3-y stands, respectively. Gi l l Instruments (Lymington, England) model R3 symmetric sonic anemometer-thermometers were used in the two younger stands while a symmetric model 1012R2 was used in the 53-y stand. In the 53-y stand, a model LI-6262 closed-path IRGA (LI-COR Inc., Lincoln, NE) was mounted on the tower in a temperature-controlled box. Air was drawn through a 4.2 m long heated sample tube and a 1 /xm German filter and the IRGA at 10 L m i n - 1 (Humphreys, 1999). A similar IRGA setup was used in the 3-y stand, except in this case, the IRGA housing and sampling tube were not temperature controlled and the flow rate was 6 to 8 L m i n - 1 through a 3 m long sample tube due to limited battery/solar power. The two E C IRGAs were operated in absolute mode by flowing a reference gas of nitrogen through the reference cell at 80 cm 3 m i n - 1 . A n open-path model LI-7500 IRGA (LI-COR Inc.) was used in the 14-y stand. It was tilted at 45° from vertical to permit drainage of water on the light source windows. In all 3 stands, Chapter 2. Respiration in three Douglas-fir stands 21 the point of gas sampling was within 30 cm of the center of the anemometer array. High frequency data was processed in the same way for all three stands. Half-hour Fc was computed as Fc = p~aW7c (2) where w is the vertical wind velocity, the primes indicate fluctuations from the half-hour mean, the overbar indicates the half-hour average, and w's'c is the covariance of w and sc. sc signals were adjusted for a delay of about 0.8 s in the 53-y stand and 1.2 s in the 3-y stand due to the time air takes to travel down the sample tube. Delay times were calculated by maximizing the cross-correlation of the air temperature measured by the sonic (Tv) and sc. The sonic anemometers were mounted vertically such that a 3-axis coordinate rotation of the wind vectors for each half hour was used to correct for the influence of the sloping terrain (Tanner and Thurtell, 1969). In the 53-y stand, analogue signals from the IRGA were measured at 125 Hz using a data acquisition system (model DAQBook/200, IOtech, Cleveland, OH) connected to a pc, digitally filtered and down-sampled to 20.83 Hz, the processing frequency matching the digital output from the Gil l 1012R2. In the 14-y stand, digital signals from the I R G A and sonic anemometer were transmitted to a low-power computer (SmallPC Computers, Oakville ON, Canada) at 20 Hz. In the 3-y stand, a model CR23X datalogger (CSI) logged analogue signals from the E C instruments at 8 Hz and output half-hour statistics. In addition, a low-power pc was periodically used to measure 20 Hz digital signals via RS-422 from the sonic anemometer and IRGA through the anemometer analog input unit. Although the 3-y stand had the most significant high-frequency component of all three stands (Appendix 3 ), the proportion of Fc attributable to frequencies greater than 4 Hz was less than 0.5% in the 3-y stand. In the 14-y stand, where the open path IRGA was used, instantaneous measure-Chapter 2. Respiration in three Douglas-fir stands 22 merits of sonic temperature (Tv) were converted to thermodynamic air temperature, T using an iterative process. First, the 20 Hz signal of water vapour density from the IRGA (pv, mol m~3) was approximately converted to mole fraction (xv, mol H 2 0 m o l - 1 moist air) with the 20 Hz Tv signal using, Xv = PvR= (3) where R is the universal gas constant (8.31 J m o l - 1 K - 1 ) and P is the half-hour average atmospheric pressure (kPa). Tv was then converted to air temperature (T, K) using, r = T „ / ( l + 0.32x„). (4) Then Eq. 3 was used to recompute Xv by replacing Tv with T. Differences in the estimates of T were usually negligible on the second iteration. High frequency C 0 2 density was then converted to mole mixing ratio using T and pv (i.e., sc = pc/(P/(RT) pv), thereby accounting for density fluctuations when computing Fc using Eq. 2. This method resulted in Fc values, which differed by less than 0 .2% of fluxes calculated using the W P L procedure (Webb et al., 1980) applied to the half-hour values of w'p'c. Using the procedure described in Aubinet et al. (2000), the average cut-off fre-quency (frequency where the transfer function reaches 71%) associated with high frequency losses of Fc due to sensor separation, differences in path averaging, and attenuation of fluctuations of sc through the sampling tube at night was 0.75 and 1.17 Hz for the 3- and 53-y stands, respectively. The resulting correction factor (CF) (Aubinet et al., 2000) for nighttime measurements of Fc was less than 1.08 and 1.02 in the 3- and 53-y stands, respectively for 90% of the time when u* > u^th (see Section Chapter 2. Respiration in three Douglas-fir stands 23 2.3.1 for In the 14-y stand, where an open-path IRGA was used, the C F asso-ciated only with sensor separation and differences in path averaging was determined using the daytime latent heat fluxes rather than F C . On average, the cutoff frequency was 1.58 Hz and C F was less than 1.02, 90% of the time in the 14-y stand. The pro-portion of FC attributable to frequencies greater than 1 Hz was about 3%, 1%, and 0.6% in the 3-, 14-, and 53-y stands, respectively, while for frequencies greater than 0.1 Hz, these proportions increased to 28%, 16%, and 6%, respectively. As a result, corrections to FC were applied only in the 3-y stand using the following relationship with half-hour average wind speed (u), C F = 0.05u + 1.00 //mol m~ 2 s _ 1 following Aubinet et al. (2000). See Chapter 4 and Appendix 3 for more details about transfer functions and correction factors for FC derived using closed-path I R G A signals. A l l IRGAs were calibrated with a zeroing gas (nitrogen) and a known C O 2 span gas (356 ppm, referenced to primary standards from the Canadian Greenhouse Gases Measurement Laboratory of the Meteorological Service of Canada in Downsview, ON) at regular intervals: nightly for the 53-y stand, approximately once per month for the 3-y stand, and once every 2 months for the 14-y stand. A comparison of FC computed with the open path LI-7500 IRGA and the closed path LI-6262 IRGA was conducted in the 53- and 3-y stands. There was excellent agreement in nighttime open-path (FCJOP) and closed-path {Fc<cp) C 0 2 flux during rain-free and dew-free conditions. Note that FCtcp was multiplied by C F for the tests done in the 3-y stand. The relationship was Fc<0p = 1.01F C ) C P - 0.3 //mol m~ 2 s - 1 , R 2 = 0.98, n = 318 for the tests in the 53-y stand and FCfop = 0.99FCJCP + 0.3 //mol m - 2 s _ 1 , R 2 = 0.92, n = 460 in the 3-y stand. Quality control procedures involved despiking and removing portions of the high frequency time series when calibrations were run before computing eddy statistics. Fluxes with statistics (min, max, variances) which did not fall within reasonable limits Chapter 2. Respiration in three Douglas-fir stands 24 (e.g., 5^ < 100 pmol m o l - 1 ) and/or occurred during instrument malfunction were removed. C O 2 fluxes were removed when the magnitude of Fc differed from FC!unear by more than 100% when Fc > 2 pmol m~2 s _ 1 , where FCi linear r& the C 0 2 flux calculated using the covariance of linearly detrended values of w and sc. Nighttime Fc which indicated C uptake by the stand were also removed from the analysis. In this chapter, all half hours with measurable rainfall were removed (about 8 - 10% of nighttime half hours). In the 14-y stand, periods when the windows of the open path IRGA were obscured due to rain or dew were also removed. Fluxes were not rejected on the basis of wind direction in all three stands since there were no large obstructions and there was sufficient area of homogeneous stand within the typical flux footprint (area which includes 80% of the total eddy flux). In the 53-y stand, the fetch was 400 m to the SW, about 700 to 800 m to the N E and surrounded on all sides by Douglas-fir stands ranging in age from 24 to 57 years (Humphreys, 1999). In the 14-y stand, there was between 400 and 700 m of fetch in all directions surrounded by both recently clearcut-harvested areas and mature forests. The 3-y stand was surrounded by 60-year-old forests in all directions with a 400 m fetch in both the N E and SW directions (prevailing wind directions) while a minimum fetch of 80 m to the N W and 120 m to the SE was still sufficient for flux measurements determined from footprint assessments (see Section 2.3.5). In total, these quality control procedures removed 32%, 40%, and 31% of half hour nighttime fluxes from the 53-, 14-, and 3-y stands, respectively. 2.2.3 Soil respiration measurements Measurements of C 0 2 flux from the soil were taken during the daytime every two to three weeks between May 15, 2002 and December 1, 2002. The measurements were made with a homemade closed-dynamic chamber system and an infrared gas analyzer Chapter 2. Respiration in three Douglas-fir stands 25 (model LI-800, LI-COR Inc.). The chamber headspace was made of 10 cm inner diameter P V C pipe union fitted with a P V C pipe cap as the top (volume 1426 cm 3 , surface area 75 cm 2). A temperature and relative humidity sensor (model HMP35CF, Campbell Scientific Inc. (CSI), Logan, UT, USA) was sealed into the top of the pipe cap to measure chamber air temperature and relative humidity. A diaphragm pump (model TD-4X2N, Brailsford Co., Rye, NY) cycled air through the chamber system at 2 L min" 1 and pulled sub-sampled air through the IRGA at 0.8 L min" 1. The 4 mm i.d. metal/plastic composite tubing (Type 1300, Synflex, Mantua, O H , USA), which returned air to the inside of the chamber, was perforated and .wound round the center of the chamber to cause mixing. Equal pressure was maintained between the chamber and the environment with 10 cm of 4 mm i.d. tubing set into the top of the pipe cap and pinched slightly at the top. A l l data were recorded and stored at 1 s intervals with a datalogger and storage module (models 21X and SM192, CSI). The opaque chamber inhibited photosynthetic uptake during a C O 2 flux measurement so that the measured flux represented Rs, respiration from the soil and forest floor materials including any respiration from small plants or moss. The soil C O 2 flux was calculated using where Ve is the effective volume (m 3), P/RT(1 + sv) is the density of dry air (mol dry air m - 3 ) calculated using the universal gas constant (R, 8.31 J m o l - 1 K _ 1 ) , the mean chamber air temperature (T, K ) , and the partial pressure of dry air ( P / ( l + sv), Pa) where sv is the mean water vapour mixing ratio (mol H 2 0 m o l - 1 dry air) within the chamber. dsc/dt is the linear increase in C O 2 mole mixing ratio (mol C O 2 mol" 1 dry air) during the 2 min long measurement (mean R 2 = 0.99) and A is the soil surface area Rs = VePdsJdt (5) RTA(1 + sv) Chapter 2. Respiration in three Douglas-fir stands 26 within the chamber (m 2). C O 2 concentration was converted from mole fraction (xc, mol C O 2 m o l - 1 moist air) to mole mixing ratio using the water vapour mole fraction, sc = Xc/(1 Xv), where Xv is the water vapour mole fraction, prior to determining dsc/dt. Ve was found to be 1471 cm 3 using a calibration procedure similar to that described by Drewitt et al. (2002). This calibration was carried out in the laboratory at the end of the season and involved determining the difference between two flux measurements, one immediately following the other, where the second measurement included a known rate of injection of C O 2 into the chamber. See Appendix 1 for more details. There was very good agreement between measurements made using this soil chamber system and using a model LI-6400-09 Soil C 0 2 Flux System (LI-COR Inc.), Rs = 1.05i?S)LI_coR 0.14/imol m~ 2 s _ 1 , R 2 = 0.96, n = 24 (Appendix 2 ). In each forest stand, 10 sampling plots were established. Since all three stands experienced different predominant daytime/nighttime wind directions between May to November when this study was carried out (see Section 2.3.1), five of the 10 plots were within the general daytime eddy flux footprint and the other five were within the nighttime flux footprint. The individual plots within the two groups were established in a random design in the 53-y stand and in a randomized grid design in the 14-y and 3-y stands. The difference in design was needed in order to have plots in the 53-y stand coincide with a pre-installed soil moisture measurement system located within 140 m radius of the eddy flux tower. Each group of plots in the 14- and 3-y stands was located within a radius of 30 and 60 m of the eddy flux towers, respectively. Within each plot, two P V C pipes (soil collars) that were 6 cm long with an inner diameter of 10 cm were set approximately 2.5 cm into the forest floor. One of the collars remained in place throughout the study while the second collar, known as the mobile collar, was moved three times within about 0.5 m of the stationary collar. Within 0.5 m of these two collars, a profile of three chromel-constantan thermocouples Chapter 2. Respiration in three Douglas-fir stands 27 was permanently installed to measure soil temperature, Ts at 2, 5, and 10 cm below the forest floor surface during soil respiration measurements. In addition, a fourth thermocouple was inserted near the collar being sampled to a depth of 2 cm below the forest floor surface. On each soil chamber sampling date, two replicates of Rs were obtained for each collar with a short 4 min period between measurements. Following these measurements, the volumetric water content of the soil, 9V was measured to a depth of 12 cm using an insertable soil water reflectometer probe (model Hydrosense, CSI). Additional measurements of 9V were obtained to a depth of 20 and 60 cm using two in-situ sets of time domain reflectometry (TDR) probes (Hook and Livingston, 1996) measured with a metallic time domain reflectometer (model 1052B, Tektronix, Beaverton, OR). In total, 11 sets of Rs measurements were made in the 14-y stand and 12 sets were made in both the 53- and 3-y stands for a total of 220 to 240 measurements of Rs. Soil physical and chemical characteristics for the stationary collars at each stand were sampled after the last measurement was taken at the end of November. The soil was cored (3 cm deep, 68.7 cm 3 volume) for the 0 to 3 cm depth including any litter material and for the 3 to 6 cm depth. These samples were analyzed for bulk density, fine fraction bulk density (< 2 mm), organic matter by mass (mg g _ 1 dry soil) using the loss on ignition method, and total carbon (C) and nitrogen (N) by mass (mg g - 1 dry soil) using a L E C O induction furnace. Measures of C and N for the mineral soil and L F H layers were determined from these cored samples when at least 70% by volume was mineral soil or when less than 30% was mineral soil, respectively. A l l samples were oven dried at 105° C for 24 h to remove hygroscopic water before these analyses were conducted. Chapter 2. Respiration in three Douglas-fir stands 28 2.2.4 Weather measurements In addition to monitoring soil moisture and temperature in each plot, several weather variables were measured continuously using dataloggers (models 23X, 21X, CR10X, CSI) in each stand. Among these variables were downwelling and upwelling shortwave radiation (W m - 2 ) measured using pyranometers (models CNR1 and CM5, Kipp & Zonen, Delft, Holland). Albedo (a) was calculated as the ratio of upwelling to down-welling shortwave radiation. Air temperature and relative humidity above the canopy were measured at the height of the eddy flux instrumentation (model HMP35CF, CSI) installed in an aspirated shield (model 076B radiation shield, Met-One Instru-ments, Grants Pass, Oregon, USA) in the two older stands and in an unventilated shield (R.M. Young 10-plate Gi l l Radiation Shield, CSI) in the 3-y stand. Rainfall was recorded with two tipping bucket rain gauges in each stand (model 2501, Sierra Misco, Berkeley, CA; Model 4000 Jarek Manufacturing Ltd. , Sooke, B C , Canada). A l l instruments were calibrated and checked against common standards. Soil temperature and moisture were also continuously measured at several depths at one location in the 53-y stand and two locations in the 14- and 3-y stands. Soil temperature measurements were made each half hour with chromel-constantan ther-mocouples inserted into the face of a soil pit at 2, 5, and 10 cm below the surface of the forest floor. To adjust these continuous measurements of Ts to reflect the spatial heterogeneity of each stand, the half-hour readings made concurrently with the man-ual reading of each stationary thermocouple were related using least squares linear regression. This adjustment (between 2% and 20%) was applied to the automated Ts (°C) measurements at 2, 5, and 10 cm and used for all of the following analyses which required these continuous measurements. Water reflectometer probes (model CS615, CSI) were installed at various angles Chapter 2. Respiration in three Douglas-fir stands 29 into the soil to measure the volumetric water content (9V, m 3 m - 3 ) of a surface L H / A horizon layer, two B horizon layers and a 4th layer extending between 70 and 100 cm below the surface. The first three layers varied slightly between stands but were approximately 2 to 5 cm, 10 to 20 cm, 30 to 50 cm. A n estimate of 9V for the layer between 0 and 20 cm was calculated using the weighted sum of the top two probes. Spatial heterogeneity was accounted for in a method similar to that for Ts using the manual readings of the 20-cm long T D R probes in the 10 sampling plots. These adjustments ranged from -20% to 40% in the three stands. L A I in the 3-y stand was measured using a relative frequency method (Wilson, 1959) on 10 permanently established 40-m long transects at 20 cm intervals (see Chapter 4 for more details). In the 14-y stand, an optical method (LAI-2000, LI-C O R Inc.) was used to measure LAI on two parallel 70-m long transects running SE and SW of the flux tower with measurements made at 8 permanent locations per transect. Measurements were made approximately once per month in the two younger stands. L A I for the 53-y stand was assumed to be a relatively constant value throughout the year (Humphreys et al., 2003). 2.2.5 Analysis The relationship between Re or Rs and Ts was analyzed using where Rx refers to either ecosystem (e) or soil (s) respiration. Eq. 6 can be written as (Fang and Moncrieff, 2001), ln R. :X = A + BTS (6) Rx — RIQXQ\QJ-TS 1 0 ) / 1 0 . (7) Chapter 2. Respiration in three Douglas-fir stands 30 The slope of Eq. 6 can be converted to the Qw value (Eq. 7), which is the increase in respiration for a 10°C increase from a reference temperature, Qiox = exp(lOB). The intercept of Eq. 6 can be converted to a reference respiration rate here taken to be at 10°C, RIQX = QWxexpA. The coefficient of determination, R 2 was reported for Eq. 6. The logarithmic transformation allowed the assumption of normality and ho-moscedasticity to be met for linear least squares regression. The 5-cm Ts consistently explained the greatest or next greatest amount of variation in R S at each stand. The uncertainty in annual estimates of R X derived from Eq. 6 were evaluated using the sum of all the upper and all the lower limits of the 95% confidence interval (CI) for each half-hour estimate of lni? x and then back transformed to R X . Soil chamber R S and E C R E measurements consist of repeated measurements on the same subjects (i.e., stationary collars and forest stands) and are therefore, serially dependent. To reduce non-randomness, the two replicates of R S for each collar were averaged unless the standard deviation was greater than 2 //mol m - 2 s - 1 . In the latter case, the replicate with the best coefficient of determination, R 2 was used to determine R S . There was no tendency for the second replicate of R S to be smaller or greater than the first. This indicated that the disturbance of setting a chamber on the collar did not measurably affect rates of C 0 2 flux. The independent variable of major interest, Ts, along with all other measures of climate is also uncontrolled and non-random. However, the potential measurement errors in Ts are assumed to be small enough to allow the use of least squares linear regression. Serial dependence precluded the use of standard statistical tests of significance (such as t and F tests based on standard distributions) (Zar, 1984). As recommended by Potvin et al. (1990), response curve coefficients (Eq. 6) were compared between stands rather than comparing original measurements. F tests for coincidental regressions were made following Zar (1984) and were followed by multiple comparisons among slopes and elevations. Differences Chapter 2. Respiration in three Douglas-fir stands 31 between stands in RioSi, the soil respiration rates at 10 °C for each collar i, and various soil characteristics were analyzed using a series of Kruskal-Wallis tests followed by nonparametric Tukey-type multiple comparisons (Zar, 1984). These nonparametric tests were used since much of the data did not meet the assumption of normality. To minimize the chance of making a Type I error when conducting multiple tests of the same hypothesis (i.e., no differences between forest stands) we used a non-independent sequential Bonferroni correction to the probability level (a = 0.05) at which an effect is considered statistically significant (Rice, 1990). The relationship between Rwsi and the soil characteristics within stands was examined using multiple Spearman-rank correlation as most of the bivariate populations were not normal (Zar, 1984). Statistical tests were conducted using J M P I N v.4 software (SAS Institute, Inc., 2000) or using factory- or self-coded routines in M A T L A B (The MathWorks, 1996). Tests were considered significant when P < 0.05. Although the 10 plots and single E C system in each of the three Douglas-fir stands may be considered pseudoreplicated with respect to stand age, statistical comparisons were made between stands in order to help describe measured responses and patterns with respect to those of statistical populations (Oksanen, 2001) at the scale of these particular stands. As such, our predictions are only strictly valid to these specific forest stand locations during this study period. However, future analyses, which incorporate results from multiple studies such as those conducted for Fluxnet forests (Janssens et al., 2001; Valentini et al., 2000), depend on the specific results of studies such as this one. Chapter 2. Respiration in three Douglas-fir stands 32 2.3 Results 2.3.1 Turbulent exchange of C 0 2 at night Eddy covariance measurements of Fc quantify the turbulent transfer of C 0 2 and are a function of both variations in atmospheric conditions and in the biological flux of C 0 2 from the forest stand. In all three stands, nighttime Fc was dependent on fric-tion velocity u* when conditions were calm. In the absence of advection, applying a storage correction to Fc when calculating N E E (Eq. 2) should correct the nighttime flux underestimation. Generally, Fc increased when the storage correction was ap-plied, especially for low it*. However, nighttime N E E (or Re) continued to increase with increasing u* (Fig. 5). A common method to determine the atmospheric condi-tions appropriate for the accurate measurement of turbulent C 0 2 fluxes is to select a threshold friction velocity (u*ttfl) at which Re are independent of «* assuming that the biological source strength of C 0 2 should not vary significantly with turbulence intensity (eg. Goulden et al., 1997). Sorting all available Re by u*, the mean Re of each sequential group of 100 measurements was plotted against the mean tt* for each group (Fig. 5). Although Re clearly levelled off for the 3-y stand, measurements for the two older stands continued to rise slightly as u* increased. To maintain consis-tency among the three stands, an estimate of the average Re unaffected by i i* was determined by sorting binned values of Re and averaging the highest 25% of these binned values. The u* associated with Re equal to 80% of this average Re unaffected by it* was designated as the u*>th (vertical dashed line in Fig. 5). u*tth w a s 0.32, 0.16, 0.08 m s - 1 for the 53-, 14-, and 3-y stands, respectively (Fig. 5) and was used to screen all Re examined throughout the study. In general, for a given horizontal wind speed (u), u* was greatest in the 53-y stand and least in the 3-y stand. The average u*/u for near neutral atmospheric Chapter 2. Respiration in three Douglas-fir stands 33 conditions was 0.23 in the 53-y stand, 0.14 in the 14-y stand, and 0.11 in the 3-y stand. Given a measurement height of 43, 12, and 3 m in the 53-, 14-, and 3-y stands, respectively, these u*/u ratios were used to estimate the roughness length (z0, m) of the three stands using the logarithmic wind profile (Arya, 1988). In order of decreasing stand age, zero-plane displacement (d, m) was set to approximately 2/3 of the stand height at 23, 4, and 0.4 m, respectively and z0 was 3.51, 0.46, and 0.07 m, respectively. Consequently, ut *,th, very large fluctuations in Fc from one half hour to the next, which were unlikely to represent real changes in biological respiration rates, still persisted in the 53-y stand. This suggested that advection processes were important in the nighttime exchange of C 0 2 in the 53-y stand due to the combination of sloping terrain and measuring fluxes above tall vegetation (Lee, 1998). Mass transport of C 0 2 can occur in extensive, relatively homogeneous forest stands when the terrain is not level and/or mesoscale circulations exist (Lee, 1998; Baldocchi et al., 2000; Massman and Lee, 2002). In the 53-y stand, nighttime winds flow down a slope which becomes less steep with distance downwind from the tower towards Georgia Straight. Wind streamlines are expected to converge and diverge over the changing surface and the associated terms in the conservation equation of sc, d(usc)/dx, are expected to become significant. Using the continuity equation and neglecting the divergence of horizontal eddy flux and horizontal advection (i.e., Ujdsc/dxj), Lee (1998) related these horizontal flow divergence/convergence terms to a vertical advection or mass-flow component associated with non-zero mean vertical velocity w(~s~l — (s^)), where (s^ ) is the mean C 0 2 mixing ratio between the ground and the eddy covariance measurement height where w and are measured. This Chapter 2. Respiration in three Douglas-fir stands 34 Figure 5: Mean Re (nighttime NEE) versus mean u* (number of data points per bin = 100) for the three Douglas-fir stands measured between May 1 and December 31, 2002. Vertical error bars represent the 95% confidence interval for Re. The horizontal dashed line is the estimate of the average Re unaffected by u* = 6.3, 4.3, 3.7 //mol m - 2 s _ 1 for the 53-, 14-, and 3-year old stands, respectively. The vertical dashed line represents the u^^h of 0.32, 0.16, 0.08 m s - 1 for the 53-, 14-, and 3-year old stands, respectively. See the text for more details. Chapter 2. Respiration in three Douglas-fir stands 35 term is added to the right hand side of Eq. 1 as follows: N E E = FC + zmp~ads-c/dt + w{Tc-{TJ). (8) Consequently, when w is non-zero and (s~c) is greater than s~d, N E E may be either over-estimated (positive w) or underestimated (negative w) by Eq. 1 when this advection or mass-flow term is neglected. Following Baldocchi et al. (2000), the mean measured vertical velocity (w) in the instrument co-ordinates (sonics were mounted vertically) normalized by horizontal (or cup) wind speed, U = \J(u2 + v2), is shown plotted against wind direction for all three Douglas-fir stands (Fig. 6). The true mean vertical velocity (w) is taken to be the vertical component of wind normal to the surface terrain while w includes the effects of topography, sensor orientation, and offsets due to the tower, sensor and other structures (Baldocchi et al., 2000; Lee, 1998). Both the 53- and the 14-y stands were situated on sloping terrain facing Georgia Straight. The sinusoidal trend of w/U with wind direction in these two stands reflects the topography and resultant complementary land/sea breeze and anabatic/katabatic wind flow. Fig. 7 shows the tilt angle (second coordinate rotation described by Tanner and Thurtell (1969)) associated with wind direction. This also effectively illustrates the differences in aspect and topographic slope (Table 1) between the three stands. In all three stands, nighttime winds generally come from the SW quadrant down the slope, while nighttime winds from the SE quadrant are often associated with winter low pressure systems. In the 3-y stand, the level terraces ran in the NE-SW directions with most changes in the elevation of the terrain occurring in the SE-NW directions. In addition, the adjacent tall forests as close as 80 m to the N W and 120 m to the SE funnelled winds almost exclusively to the N E and SW quadrants. Overall, the terrain was more Chapter 2. Respiration in three Douglas-Sr stands 36 Figure 6: Nighttime half-hour vertical velocity in the instrument co-ordinates (w) normal-ized by half-hour cup wind speed (U) as a function of wind direction for the three Douglas-fir stands when u* exceeded u*jh and when U was greater than 0.5 m s _ 1 . The solid line indi-cates the bin-averaged value (bin width = 30 degrees) for both daytime and nighttime half hours. level for the 3-y stand and w values were closer to zero (minimum and maximum w were -0.15 and 0.13 m s _ 1 in the 3-y stand, -0.36 and 0.52 m s - 1 in the 14-y stand, and -0.51 and 0.78 m s - 1 in the 53-y stand). Lee (1998) assumes that departures of Chapter 2. Respiration in three Douglas-fir stands 37 6 - 4 _ 6 i , , , , , , , 1 0 45 90 135 180 225 270 315 360 Wind direction Figure 7: Bin-averaged tilt angle as a function of wind direction in the three stands. The bin width is 30 degrees and includes all half hours with U greater than 0.5 m s - 1 . normalized w from the mean (solid line) shown in F ig . 6 indicate periods when w is non-zero and mass flow of C O 2 may become significant. The effects of flux divergence/convergence associated wi th non-zero w on measured N E E when exceeded u*ttri were investigated by examining both the measurements of nighttime N E E and the residuals of E q . 7 describing the relationship between nighttime N E E and Ts (i.e., NEEobserved - ~HEEpredicted)(see Table 5 and F i g . 9). For each stand, a regression of nighttime w against U was developed for wind directions, eh, in 1° increments using a subset of half hours for which half-hour wind direction was ± 5° of the increment value. Following Lee (1998), w was calculated as ib — a((f>) — b(4>)U. In general, w tends to remain relatively constant throughout the night or change in sign gradually. F i g . 8 shows that nighttime N E E varies wi th w in the 53-y stand but not in the two younger stands. F ig . 9 shows that observed N E E for Chapter 2. Respiration in three Douglas-fir stands 38 Figure 8: Binned NEE measurements (n — 25) for all nighttime periods when u* > u^jh threshold against corrected vertical wind speed (w) for the 53-y, 14-y, and 3-y stands. The vertical bars are ± 1 SE of the mean. the 53-y stand was greater than the predicted values more often when w was positive and less than the predicted value when w was negative. Since s^— (s^) is negative at night in the 53-y stand when respired C O 2 accumulates in the lower parts of the canopy (see Table 3 for examples), the mass flow term is positive when w is negative and addition of this term to observed N E E increases these nighttime N E E measurements bringing them closer to predicted values. With positive w, unrealistically large measured N E E are reduced (Table 3). While the examples in Table 3 indicate reasonable estimates of N E E with the addition of a mass-flow term, the magnitude of the mass flow term often tends to be too large and overcorrects N E E , especially for positive W. As discussed by Finnigan (1999) and Finnigan et al. (2003) advection is a two or three dimensional process and the neglected horizontal advection term may be important in nighttime C 0 2 transport in the 53-y stand. By excluding nighttime N E E measurements when it* < u*ittl and replacing them with estimates of Re using Eq. 7, there is the risk of double counting C 0 2 loss from the ecosystem if a sudden increase in turbulence (typically in the morning) results Chapter 2. Respiration in three Douglas-fir stands 39 _ 10 T CO <M I E o ^ .2 -0.1 0 0.1 0.2 w (m s~1) Figure 9: B inned N E E residuals (n = 25) for a l l nightt ime periods when u* > u*tth threshold against corrected vert ical w ind speed (w) for the 53-y stand. T h e vert ical bars are ± 1 S E of the mean. in a flush of C O 2 from the canopy and thereby compensates for the prior half hours where N E E was underestimated. However, no apparent flush of C O 2 was observed for a sudden increase in turbulence intensity in the 53-y stand. For example, the mean (± 1 SE) N E E measured in the first half hour after u* increased above u*jh was 7.7 ± 0.4 pmol m~2 s _ 1 (n = 232) compared to 7.1, ± 0.5 pmol m~ 2 s - 1 (n = 140) after an hour of well mixed conditions or 7.6 ± 1.2 pmol m - 2 s _ 1 (n = 51) after 4.5 hours of well mixed conditions. These results suggest that nighttime advective loss of C O 2 was not compensated by a morning flush of C O 2 and replacing nighttime measurements when u* < U*JH does not result in a double count of nighttime respiration. This analysis indicates that the E C technique, particularly in the 53-y stand, is missing an important advective component of nighttime C 0 2 exchange and the use Chapter 2. Respiration in three Douglas-fir stands 40 Table 3: Statistical characteristics of three groups of nighttime N E E measurements from the 53-y stand when tx* exceeded u * ^ . Values in brackets are the standard error (SE) of the mean. low N E E " well-modelled N E E high N E E n 25 58 22 N E E (/imol m " 2 s -1 ) w (m s _ 1 ) 0.6 (0.1) 4.7 (0.4) 29.9 (2.6) -0.053 (0.016) 0.001 (0.009) 0.050 (0.007) it* (m s _ 1 ) 0.53 (0.03) 0.63 (0.04) 0.56 (0.03) U (m s- 1 ) 3.1 (0.1) 3.5 (0.2) 2.5 (0.1) wind direction 217 (13) 209 (8) 239 (5) •Sc — (Airnol m o l - 1 ) -3.3 (0.7) -3.4 (0.4) -12.9 (1.5) pawfe- &)) (//mol m ~ 2 s _ 1 ) 3.8 (1.7) -1.2 (1.2) -22.2 (4.1) "Low N E E refers to periods when the relative N E E residuals, (NEEobserved - NEEpreciicted) I NEEprediCted were less than -0.8, well-modelled N E E refers to periods when the relative N E E residuals were ± 0.05, and high residual N E E periods refers to periods when the relative N E E residuals were greater than 4. of the Lee (1998) mass flow term cannot adequately correct for this. Half hours when u* > u*jh and when the residual N E E was very high or low were not removed from the dataset, however. This was partly because there was no objective way to l imit high fluxes from the dataset and because the relatively infrequent overestimates of N E E values wi l l offset some of the underestimates of N E E , which occur despite turbulent conditions. 2.3.2 Weather Due to the relatively close geographic proximity of the three stands, the weather conditions characterized by mean 24-h downwelling shortwave radiation ( P A R , M J m - 2 d _ 1 ) , above-canopy air temperature (T, °C) , and rainfall were similar for the three stands between M a y 1 and December 31, 2002 (Table 4). However, the average diurnal amplitude of T was largest in the 3-y stand. Stand structural characteristics, Chapter 2. Respiration in three Douglas-fir stands 41 which affect stand microclimate, differed between stands. For example, leaf area Table 4: Weather conditions for the three Douglas-fir stands between May 1 and December 31, 2002. The standard deviation of the mean is in parentheses. 53-y 14-y 3-y Mean P A R (MJ m~2 13.8 (9.7) 14.3 (9.8) 13.6 (9.4) Mean T (°C) 11.1 (5.8) 11.9 (5.8) 11.4 (6.8) Mean diurnal T amplitude (°C) 6.1 (3.2) 6.8 (3.3) 10.0 (4.6) Mean 5-cm Ts (°C) 10.3 (3.4) 11.0 (3.3) 11.9 (5.8) Mean diurnal 5-cm T s amplitude (°C) 1.2 (0.6) 1.6 (0.8) 3.4 (1.9) Minimum 0-20 cm 9V (m 3m~ 3) 0.075 0.079 0.046 Total rain (mm) 657 721 683 index was greater (LAI = 8.4) and a was lower in the 53-y stand (0.09) than in the two younger stands. L A I was about 3.4 on average and a was 0.17 in the 14-y stand and L A I was about 1.4 and a was also 0.17 in the 3-y stand. As a result, despite similar above-canopy weather, 5-cm Ts and 0-20-cm 6V varied considerably between the three stands (Table 4, Fig. 10). Mean daily Ts and the amplitude of Ts were most extreme in the 3-y stand where there was little vegetation to intercept radiative energy. Greater L A I in the 14- and 53-y stands damped both diurnal and seasonal variations in Ts (Table. 4, Fig. 10). Soil moisture was greatest in the loam soil of the 14-y stand in spring and fall (Fig. 10). The rapidly draining sandy soils of the 3-y stand were associated with the lowest 9V of 0.046 m 3 m~ 3 in late August. No prolonged drought periods, where 9V was less than the wilting point (Table. 2), were observed in any of the three stands. 2.3.3 Relat ionships between respirat ion and environmental factors Fig. 11 shows the relationship between respiration and 5-cm Ts, derived as a linear regression of ln Rx against Ts (Eq. 6) for both Re and Rs. The proportion of variability Chapter 2. Respiration in three Douglas-fir stands 42 O1 1 1 1 1 1 1 1 May Jun Jul Aug Sep Oct Nov Dec Figure 10: 24-h mean 5-cm soil temperature, Ts, and 0-20-cm soil volumetric water content, 8V, for the three Douglas-fir stands between May 1 and December 31, 2002. in ln Rx explained by Ts was relatively low in all cases due to the large variability in Rs between collars and between half hours in the case of Re (Table 5). Respiration is directly compared between the three stands by plotting binned values of Rx (see Fig. 12 for explanation) against Ts along with the relationships derived for non-binned measurements (Fig. 12). If binned values are used instead to fit Eq. 6, between 78 and 95% of the variability in Rs (n = 14 to 16) and 71 to 92% of the variability in Re (n = 29 to 35) was explained by 5-cm T8. These higher R2 are within the range of 60-90% reported in the literature (Janssens et al., 2001) where sample sizes are generally much lower (between 10 and 50) and often represent an average of multiple measurements (e.g. Borken et al., 2002; Buchmann, 2000). The three temperature response curves for Re were significantly different (F = 164, Chapter 2. Respiration in three Douglas-fir stands 43 Table 5: Mean Rs and Re measured between May 1 and December 31, 2002. Parameters A and B± SE and the coefficient of determination, R 2 , are from the regression of lnRx against 5-cm Ts (Eq. 6). Regression parameters Qiox and .Riox were calculated from A and B as described in the text. Significant differences in parameters between stands for either Re or Rs (P < 0.05) are indicated by different superscripts and were determined for the original parameters A and B. Stand Mean R B± S E A± S E Q10 Rw R 2 n [imol ln(/imol m - 2 s _ 1 ) ln(^zmol m - 2 s - 1 ) /j,mo\ m _ 2 g _ ! o C _ ! m _ 2 s _ ! Ecosystem respiration (Re): 53-y 7.2 0.149 ± 0.006 0.10 ± 0.07 4.44 a 4.92 a 0.34 1048 14-y 4.8 0.130 ± 0.007 -0.08 ± 0.08 3.67 b 3.40 b 0.27 869 3-y 4.2 0.049 ± 0.004 0.55 ± 0.06 1.63c 2.83 c 0.10 1243 Soil respiration (Rs): 53-y 4.6 0.077 ± 0.008 0.46 ± 0.09 2.16 a ' b . 3.43 a 0.30 240 14-y 7.0 0.096 ± 0.008 0.56 ± 0.11 2.60 a 4.54 b 0.39 220 3-y 3.0 0.072 ± 0.005 -0.09 ± 0.08 2.04 b 1.88c 0.43 240 degrees of freedom (df) = 4, 3154, P < 0.0001). Furthermore, the three intercepts expressed as i ? i 0 e and slopes expressed as Qioe were also significantly different (F = 201, df = 2, 3148, and F = 113, df = 2, 3154, respectively, both P < 0.0001). Results from the pairwise multiple comparison tests for each parameter are shown in Table 5. Ri0e were greatest in the 53-y stand and lowest in the 3-y stand (Fig. 12). Re was most sensitive to 5-cm Ts in the 53-y stand where Qi0e was 4.5 followed by 3.1 in the 14-y stand and 1.6 in the 3-y stand. The average Re for a l l nighttime half hours where > u*ttrl was also greatest in the 53-y stand at 7.2 pmol m~2 s _ 1 compared to 4.8 pmol m~2 s _ 1 in the 14-y stand and 4.2 pmol m~2 s _ 1 in the 3-y stand. The three temperature response curves for Rs were also significantly different between stands (F = 110, df = 4, 694, P < 0.0001). A l l RWs were significantly Chapter 2. Respiration in three Douglas-fir stands 44 15 20 25 50 10 1 0.1 50 10 1 0.1 14-yf l 0 10 15 20 25 * r i r % ^ v . _ • - J e> 10 15 20 25 50 10 1 0.1 50 10 1 0.1 50 10 1 0.1 0 0 0 5-cm 7 s (°C) 53-y R 10 15 20 25 •». • • • 14-y R 10 15 20 25 3-y Rc 10 15 20 25 Figure 11: Relationship between half-hour ecosystem (Re) and soil respiration (Rs) on a log scale and 5-cm Ts for the 53-, 14-, and 3-y stands between May 1 and December 31, 2002. Also shown are the linear regressions (Eq. 6) for half-hourly Re and Rs (Table 5). different (F = 213, df = 2, 688, P < 0.0001), however, despite a range in Q10s of 2.0 to 2.6, the sensitivity of Rs to temperature was only significantly different between the 14- and 3-y stands (q = 3.7, df = 3, 694, P < 0.05). Mean Rs for the 14-y stand was 1.5 times greater than for the 53-y stand and 2.3 times greater than for the 3-y stand. The magnitude and temperature response of Re was similar to that of Rs in the 3-y stand (Figs. 11 and 12, Table 5). In contrast, Rs was larger than Re in the 14-y Chapter 2. Respiration in three Douglas-fir stands 45 Figure 12: The relationships between Re and 5-cm Ts shown i n F i g . 11 and Table 5 w i t h binned values of Re and Rs for the three Douglas-fir stands. E a c h Re data point is the average of 30 half hours while each Rs symbol is the average of 15 measurements. stand while Re was larger than Rs and more sensitive to Ts in the 53-y stand. Although the moisture status of the soil ranged from near-saturated to dry in all three stands during the measurement period (Fig. 10), non-linear models including the effects of 9V, LAI , and Ts on Re (eg. Suyker and Verma, 2001) resulted in either no or only marginal improvements in R2 when compared to Eq. 6. Rs normalized by the temperature response relationships derived for the individual collars in the three stands are plotted against 0-12-cm 9V in Fig. 13. No significant relationships could be found although there was a weak tendency for normalized Rs to increase with 9V when Chapter 2. Respiration in three Douglas-fir stands 46 2 1 _ 2 '6 r « 1 1 I I 1 I O n ° 53 - y R o Q ^ o BRH ^gCggS Q B g. o ° ° o T S ^ f o o f ^ o S o 0 o 8 DC i i i i • • • 14 -y B • • ° D o - o J ^ l ^ a o S * ° - _ ° -B_ _ n D _ _ • | g°D • • • Ho • • B • • a s D D ° • 1 1 1 1 o 3 - y A ^ " A A A A § A A A § A 0.1 0.2 0 - 12 cm 9^ (m 3m~ 3) 0.3 Figure 13: The ratio of Rs to estimates of Rs calculated from E q . 6 for each collar (20 collars per stand, 11 to 12 measurements per collar) between M a y 1 and December 31, 2002 against 0-12-cm volumetric water content (6V). 0V was less than 0.08 and 0.05 m 3 m~ 3 in the 53- and 3-y stands, respectively. As discussed by Drewitt et al. (2002), the poor relationships may have been due to the highly correlated nature of 6V and Ts (Fig. 10) and the small number of measurements at the very high and low range of 9V. 2.3.4 Within-stand spatial variations in respiration Within and between stand spatial variability in Rs is presented in terms of the pa-rameters Riosi and Qwsi determined for the individual collars (n = 20 stand - 1 ) (Fig. 14). The coefficients of variation (CV = s/x) for Ri0si were 0.41, 0.34 and 0.46 for the three stands in descending age and indicated considerable spatial heterogeneity. Rwsi were as low as 0.9 (3-y stand) and as high as 9.2 pmol m - 2 s _ 1(14-y stand). Chapter 2. Respiration in three Douglas-fir stands 47 CO CL E cd CO CD E 3 0_1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 flf0s.(umol m-2s-1) 70s/ Figure 14: Distribution of Riosi and Qiosi values for the 3 stands (n = 20 s tand - 1 ) between May 1 and December 31, 2002. One measure of Qio sj at the 14-y stand is not included and was = 8.3. We first investigated the impact of vertical temperature variations on Qi0s for each forest stand following Borken et al. (2002). As expected, QWs generally increased as deeper, more damped soil temperatures were used to develop the relationship between Rs and Ts. Ts at the 2-cm depth brought Q\Qs closest to 2.0 in the 14-y stand (Qios = 1.81, RWs = 4.19, R 2 = 0.32, n = 220) and in the 53-y stand (QWs = 1.95, RWs = 3.28, R 2 = 0.31, n = 240). In contrast, Qio« was near 2 when the 5-cm Ts was used for the 3-y stand (QWa = 2.04, R10a = 1.88, R 2 = 0.43, n = 240). Although there is no definitive QWs for all soils (Raich and Schlesinger, 1992), by assuming microbial and root activity respond similarly to temperature in all three stands, one interpretation of these results is that the active layer is predominantly within the shallow depths Chapter 2. Respiration in three Douglas-fir stands 48 in the two older stands while it may be deeper in the 3-y stand. This may be a consequence of the recent harvesting operation and disturbance to the forest floor in the 3-y stand. Table 6 illustrates the differences in mean it! 1 0 s j developed from the measurements collected from the soil collars, which were not moved in each of the three stands (n = 10). Table 6 also shows the differences in several soil characteristics measured at each of these collars in 2002. Table 6: Mean ± 1 SE RioSi, soil temperature (T s), soil moisture (0V), organic matter (OM) content, depth of the L F H layer within the top 0-6 cm of the forest floor, and chemical characteristics (concentration of total C and N) for the stationary collars only (n = 10 s tand - 1 ) in the 3 stands. Significant differences between stands are indicated by differing superscripts. Variable 53-y 14-y 3-y R10si (pmo\ m ~ 2 s _ 1 ) 3.7 (0.6) a 4.7 (0.7) a 2.0 (0.3) b 2-cm Ts (°C) 12.6 (0.1) a 14.8 (0.4) b 16.6 (0.6) b 13.1 (0.3) b 10-cm Ts (°C) 0-12-cm 6V ( m 3 m " 3 ) 11.4 (0.1) a 11.6 (0.3) a 12 (0.3) a 13 (1.6) a 9 (0.9) b 0-60-cm 9V ( m 3 m - 3 ) 0-3-cm N (mg g - 1 dry soil) 15 (1.0) a 18 (2.2) a 10 (0.5) b 4.3(0.9) 5.7 (0.9) 3.6 (0.7) 3-6-cm N (mg g - 1 dry soil) 2.3(0.4) 3.6 (0.6) 2.4 (0.7) 0-3-cm C (mg g - 1 dry soil) 166 (39) 267 (54) 242 (66) 3-6-cm C (mg g _ 1 dry soil) 75 (18) 177 (52) 156 (66) 0-3-cm O M (g g - 1 dry soil) 0.31 (0.23) 0.48 (0.09) 0.41 (0.11) 0-6-cm L F H depth (cm) 2.6 (0.4) 2.7 (0.7) 2.5 (0.7) Significant differences in all temperatures (P<0.001), 12-cm (P = 0.006) and 60-cm 8V (P <0.001), and Riosi (P = 0.0013) were found between the forest stands. In this analysis, only Riosi for the 3-y stand was significantly different from the other two stands, in contrast to the results shown in Table 5 where the measurements from both the stationary and mobile collars were used. O n average, the soil was warmer Chapter 2. Respiration in three Douglas-Gr stands 49 and drier in the 3-y stand than in the two older stands. Average Rwsi was also lower in the 3-y stand. In contrast, the 14-y stand had the highest average Rwsi value and greater 8V than the other two stands. Soil temperatures were lowest in the 53-y stand. Although not significantly different between stands, total N in the top 0-3 cm of soil/forest floor followed the trend in Riosi across stands. The concentration of total N was greatest in the upper 6 cm of the forest floor in the 14-y stand where Riosi was largest, followed by the 53- and 3-y stands. The concentration of total C was also greatest in the 14-y stand where the average RioSi was highest. Greater concentrations of C and N in the 14-y stand were related to the greater average depth of L F H since the average nutrient content of both the L F H and mineral soil fractions was similar in all three stands (Table 2). However, the trend in Riosi was not entirely reflected by total C, especially in the 3-y stand which had 242 mg C g - 1 dry soil in the surface layers, only 9% less than in the 14-y stand and 28% more than in the 53-y stand. The Spearman-rank correlations between i ? i 0 s i and a subset of soil biophysical characteristics including 2 and 5-cm Ts, 12 cm 9V, and total C, N , and C:N from the 0-3 cm soil layers are shown for each stand in Table 7. Other soil characteristics such as total organic matter and bulk density were measured (Table 2) but these were well represented by the concentration of total C for each stand (correlations generally > 0.90). Although the variables which were analyzed were also highly interdependent, only the correlations with Riosi are shown in Table 7. A l l stands were then combined for analysis. The complete Spearman-rank correlation matrix from this analysis is shown in Table 8. Although RWsi was related to different variables in different stands, the general relationship between the soil variables as indicated in Table 8 was consistent between stands. In all three stands, there was a positive correlation between i ? 1 0 s i and the concentration of total N in the 0-3 cm soil layer Chapter 2. Respiration in three Douglas-fir stands 50 (Table 7, F i g . 15). This correlation between Riosi and total N was significant in the 53-y stand while it was not significant in the 14-y nor 3-y stands. In the 3-y stand, only the abiotic characteristics significantly correlated wi th i? io s i , suggesting areas which were generally warmer and drier had higher respiration rates. The negative correlation between RioSi and 9V observed in al l stands may indicate limitations to diffusive transport of 0 2 to roots and microbes and/or simply describe the impact of cooler temperatures associated wi th wetter locations on overall autotrophic and heterotrophic processes (Table 8). The C : N ratio, often used an indicator of substrate quality, was not well correlated wi th Riosi for any of the stands. Table 7: Spearman-rank correlation between Riosi and soil characteristics for each of the three forest stands. Significant relationships (P < 0.05) are indicated by *. Soil Depth(cm) 5 3 ^ l T y 3 ^ characteristic T -1 s 2 cm 0.478 -0.139 0.806* T 5 cm 0.273 -0.042 0.527 9V 12 cm -0.321 -0.284 -0.758* C 0-3 cm 0.418 0.297 0.236 N 0-3 cm 0.721* 0.358 0.571 C : N 0-3 cm 0.115 0.224 0.285 L F H depth 0-6 cm 0.740* 0.465 0.025 Chapter 2. Respiration in three Douglas-fir stands 51 Table 8: Spearman-rank correlation matrix for i?io s; and soil characteristics for the com-bined forest stands. Significant relationships (P < 0.05) are indicated by *. L F H depth is for the top 0 - 6 cm of forest floor only. RlOsi T T -L S @v C N C:N 2 cm 5 cm 0-12 cm 0-3 cm 0-3 cm 0-3 cm RlOsi 1 Ts 2 cm -0.108 1 Ts 5 cm -0.212 0.877* 1 6V 12 cm 0.087 -0.753* -0.724* 1 C 0-3 cm 0.382* 0.255 0.207 0.020 1 N 0-3 cm 0.578* 0.152 0.031 0.045 0.753* 1 C:N 0-3 cm 0.193 0.360 0.381* -0.224 0.802* 0.350 1 L F H depth 0.354 0.077 0.159 -0.009 0.676* 0.481* 0.520* Chapter 2. Respiration in three Douglas-fir stands 52 10 • A A 3 -y 0. 0 5 10 Soi l nitrogen (mg g _ 1 dry soil) 15 Figure 15: The correlation between Riosi and total N from the 0-3 cm soil layer for the stationary collars (n = 10 s tand - 1 ) in the three stands. Least squares regression resulted in R10si = 0.54 N + 1.00 //mol m " 2 s _ 1 , R 2 = 0.53. 2.3 .5 C o m p a r i n g s o i l a n d e c o s y s t e m r e s p i r a t i o n Comparisons between Re and Rs must be interpreted carefully as these two measures of respiration do not represent equivalent biological and meteorological phenomena and do not capture the same spatial or temporal variations in respiration. Fig. 16 shows the average Rs for each stand on each sampling date compared with the average Re, where u* > u * ^ , for the week surrounding the Rs sampling date. On average Rs was 54%, 120%, and 69% of Re estimates in the 53-, 14-, and 3-y stands, respectively (Fig. 16). The 95% confidence intervals on both the measures of Re and Rs in the 14-y stand indicate that Re generally coincides with Rs. However, Rs is much lower than Re in the 53-y stand and slightly lower in the 3-y stand. Chapter 2. Respiration in three Douglas-fir stands 53 — 15 0 5 10 15 0 5 10 15 0 5 10 15 R (u\mol m 2 s 1) Figure 16: M e a n Rs for each sampling date vs. mean Re measured dur ing the week brack-eting each Rs sampling date. Ver t ica l and horizontal lines are the 95% C I . T h e dashed line indicates the 1:1 line. The estimates of annual Re and Rs derived from E q . 6 using half-hourly 5-cm Ts and the parameters in Table 5 are shown in Table 9. Annua l Rs was 67%, 130%, and 69% of annual Re in the 53-, 14-, and 3-y stands, respectively. Al though these results are reasonable for the 53-y stand where its expected that considerable C O 2 would be respired from the large amount of aboveground biomass, the ratio of Rs to Re greater than 1 in the 14-y stand and well below 1 in the 3-y stand cast doubt on the representativeness of one or both of these measures of respiration. Table 9: Annual Re and Rs (95% CI) using Eq. 6 with 5-cm Ts and the parameters listed in Table 5. Re Rs (g C m ~ 2 y" ') (g C m "2 y " 1 ) 53-y 1710 (1600 - 1830) 1170 (1080 - 1280) 14-y 1290 (1210 - 1370) 1680 (1530 - 1840) 3-y 1090 (1020 - 1160) 750 (690 - 820) In each stand, the 20 soil collars covered only 0.15 m 2 of ground surface while flux Chapter 2. Respiration in three Douglas-fir stands 54 footprints could extend hundreds of meters from the tower in the upwind direction depending on atmospheric stability and wind speed. Strong katabatic/anabatic flow and land/sea breeze circulations established relatively distinct daytime and nighttime wind directions as shown in Fig. 6. Since the E C data in this study were restricted to nighttime periods which met u^th (Fig. 5), there was a relatively constant source location and size. A three-dimensional backward Lagrangian footprint model (Kljun et al., 2002) was used to determine the smallest possible area that includes 80% of the total flux or concentration footprint. This analysis was run for a number of stable and near-neutral conditions observed to occur with u* values greater than the determined for each stand. For example, in the 53-y stand, the footprint extended from 170 to 360 m in the upwind direction, corresponding to an area of 1.2 to 3.3 ha. The footprint areas were much smaller for the two younger stands. Eighty percent of the total flux included an area of approximately 0.2 to 1.9 ha or 65 to 320 m upwind in the 14-y stand and 0.06 to 0.08 ha or 42 to 62 m upwind in the 3-y stand. In each stand, 10 collars (five plots) were within the typical daytime footprint while another 10 collars were within the nighttime footprint. In the 53-y stand, the Rs response to Ts was not significantly different between the two groups (Table 10). Although there is a very large difference in area represented by the chamber and E C technique, this analysis suggests no particular bias exists from the E C measurements sampling only the nighttime footprint. However, there may still be differences in aboveground biomass characteristics between the two footprint areas. In the 14- and 3-y stands there was a significant difference between the two groups in RWs response to Ts but not in Qios (Table 10). For these two stands, a potential bias might exist in the annual Re estimates based on the source strength of the nighttime footprint area. Fig. 17 illustrates the effect of varying u*tth on the estimate of annual Re in 2002 Chapter 2. Respiration in three Douglas-fir stands 55 derived from Eq. 6 with 5-cm Ts for each forest stand. The tendency is for annual Re to increase with u^^h a s fewer calm period measurements are included in the derivation of i? ioe and Q i o e - Annual Re tends to level off near the u*tth actually chosen since half-hourly Re is generally independent of u* beyond the threshold values. Table 10: Differences in Rws (//mol m - 2 s _ 1 ) and Q\QS determined using measurements from soil collars located in areas representing the nighttime and daytime footprints. Significant differences (P < 0.05) w i th in a stand are indicated by *. Degrees of freedom were 1 and 236, 214, and 234 for the 53-, 14, and 3-y stands, respectively. 53-y 14-y 3--y Q l O s -RlOs Q l O s RlQs Q l O s night 3.6 2.3 4.2* 2.4 1.5* 2.0 day 3.3 2.0 5.0* 2.6 2.4* 2.0 F 2.92 0.82 7.3 0.43 29.5 0.002 P 0.09 0.37 0.007 0.51 < 0.001 0.96 n 120 120 110 110 120 120 2.3.6 Between-stand variations in annual respiration Annual estimates of Re increased with increasing stand age, varying by more than 600 g C m - 2 between the oldest and youngest stands in this study (Table 9). At a global scale (Raich and Schlesinger, 1992), annual respiration rates tend to increase with annual temperature. However, in this study, there was no relationship between annual Re and mean annual air temperatures since air temperature was nearly the same in all three stands (Table 4). A negative correlation between annual Re and mean annual Ts was observed since mean Ts decreased with increasing stand age. However, as shown in Fig. 18, annual estimates of Re were positively correlated with annual gross ecosystem productivity (GEP) where Re = 0.41 G E P + 870 g C m - 2 . However, the derivation of G E P itself depends on Re (i.e., G E P = N E P + Re) (see Chapter 2. Respiration in three Douglas-fir stands 56 Figure 17: To ta l annual modelled ecosystem respiration (Re) as a function of u*jh f ° r the three stands. The first data point shows annual Re estimated using E q . 7 and parameters derived w i t h 100% of the measurements. E a c h subsequent data point uses 50 measurements less than the preceding data point. Ver t ica l lines indicate the u*jh used i n the general analysis for the three stands. Chapter 3 for more details on the calculation of annual G E P ) . To remove some of this dependence between variables, Re was compared with daytime NEP. The general pattern remained the same with measured daytime NEP, Re = 0.51 N E P d a y + 1090 g C m - 2 . Annual Rs and G E P were lowest in the 3-y stand, but annual Rs was greatest in the 14-y stand which had an intermediate GEP. Although annual Re did not increase with mean Ts, differences in microclimate between the three stands did play a role in moderating the differences in annual estimates of Re. For example, the annual Re for the 3-y stand would be 100 g C m - 2 less if the cooler and more damped Ts from the 53-y stand was used with Eq. 7 and the parameters in Table 5. In contrast, annual Re for the 53-y stand would be Chapter 2. Respiration in three Douglas-fir stands 57 CM I o o "ca 2000 1500 £ 1000 Q . CO CD DC m 500 CO c c < Annual GEP (g C m Figure 18: Annual Rs (open symbols) and Re (closed symbols) for 2002 derived from function parameters in Table 5 versus annual gross ecosystem productivity (GEP) for the three Douglas-fir stands. Vertical bars indicate the upper and lower limits of the 95% CI for each annual estimate (see Table 9 for values). 880 g C m - 2 greater if the warmer Ts from the 3-y stand wi th much greater diurnal amplitude was used to derive the estimate. 2.4 Discussion The temperature responses of Re and Rs varied considerably between the three Douglas-fir stands. These differences were related to biophysical factors such as the amount of biomass and the concentration of total N in the forest floor. Addi t iona l re-lationships between microclimate and forest productivity were investigated by scaling up Re and Rs spatially and temporally wi th the temperature response functions. Chapter 2. Respiration in three Douglas-fir stands 58 2.4.1 Influences of temperature and soil moisture o n respirat ion In al l three Douglas-fir stands, soil temperature was the environmental factor which best correlated wi th seasonal variations in Re and Rs while the effects of 9V and L A I were less important. Longdoz et al. (2000) also found that 8V d id not significantly affect Rs in a mixed Douglas-fir and beech forest in the Belgian Ardennes where 9V remained greater than the wil t ing point soil water content. This is in contrast wi th a dramatic Rs response to rainfall during Mediterranean summers in coppice oak and ponderosa pine forests (Irvine and Law, 2002; Rey et al . , 2002). Occasionally, the effects of soil moisture may be clear when comparing subsequent years. For example, Pypker and Fredeen (2002a) found that soil respiration was not correlated wi th soil moisture in a given year while between years, both Ri0s and QWs were larger in the wetter year while productivity and temperature remained the same. Annua l values of <2io and Rw for the relationship between Re and 5-cm Ts in the 53-y stand have been shown to increase with wetter years, but these results are again confounded by the correlation between cool temperatures and rainfall (Morgenstern et al. , 2003). Interannual variability in annual Rs or Re in other older or mature forest stands have generally been found to be largely dependent on annual environmental controls such as soil moisture (Irvine and Law, 2002) and temperature or growing season length (Barford et al. , 2001; Pilegaard et al. , 2001; Morgenstern et al . , 2003) rather than forest productivity. More long-term monitoring studies of C O 2 exchange in forests recovering from a disturbance such as harvesting are necessary to determine if and when changes in stand structure and biomass are more important in controlling annual Rs or Re than abiotic factors (see Chapter 4 ). QWs for al l three stands varied from 2.0 to 2.6 when derived wi th 5-cm Ts, falling in the center of the range of values that have been reported in the literature (eg. Chapter 2. Respiration in three Douglas-fir stands 59 Raich and Schlesinger, 1992). At the ecosystem level, Q i O e increased with stand age from 1.6 in the 3-y stand, 3.2 in the 14-y stand and 4.5 in the 53-y stand. Other tall dense forest stands have also been associated with high QWe due to the difficulty in characterizing the temperature response of respiring sources distributed through the soil profile and through a tall canopy with a single temperature. For example, <3ioe was 4.3 for a 25-m tall aspen forest (Griffis et al., 2003) and 3.0 for a 25-m tall beech forest (Pilegaard et al., 2001). However, this observation of increasing QiQe with increasing stand age also emphasizes the increasing importance of aboveground vegetation as a source of respired C O 2 , located away from the 5-cm soil depth. 2.4.2 Factors influencing between- and within-stand variations in respi-ration Given the close geographic proximity of the stands and similar weather, our study emphasized that mean annual temperature was not the driving factor differentiating the rates of respiration at the soil and ecosystem levels between stands. The mean soil temperature in the 3-y stand was greatest by almost 2 °C but annual Re and Rs were only 64% of the annual values for the 53-y stand. Raich and Schlesinger (1992) reviewed a large number of soil respiration studies and found that on a global scale, annual soil respiration increased with increasing mean annual air temperature. However, considerable scatter was associated with this relationship (R 2 = 0.42). For example, at a mean annual air temperature between 6 and 7°C, annual soil respira-tion estimates for various sites around the world ranged from less than 100 to 1000 g C m ~ 2 y - 1 . Evidence that site-to-site variability was linked to other factors asso-ciated with vegetation growth was shown with a strong positive correlation between mean annual soil respiration and mean net primary productivity (NPP) of different vegetation biomes (Raich and Schlesinger, 1992) and grassland systems (Raich and Chapter 2. Respiration in three Douglas-fir stands 60 Tufekcioglu, 2000). A number of studies examining C O 2 fluxes from multiple sites within close prox-imity of one another have confirmed that the site-to-site variability in Rs and Re tends not to relate to mean soil or air temperature. Relationships between Rs and Ts varied between four neighbouring Norway spruce stands ranging in age from 47 to 146 years (Buchmann, 2000). There was little variability in annual soil respiration in six forest stands in Germany despite differences in mean soil temperatures (Borken et al., 2002). For ecosystem level C O 2 losses, Anthoni et al. (2002) found that annual Re and G E P were greater in a 250- compared with a 14-year-old ponderosa pine stand in Oregon under the same climatic conditions. On a continental scale, the ecosystem respiration from the Euroflux forests increased with increasing latitude while mean annual air temperature generally decreased (Valentini et al., 2000). Both annual soil and ecosystem respiration from the Euroflux forests related better to G E P than to mean air temperature or precipitation despite a range in temperature of 1 to 15.3 °C and in precipitation of 530 to 1400 mm (Janssens et al., 2001). In Janssens et al. (2001), annual Rs was on average 55% and Re was 80% of GEP. A l l forests except one in that review were C sinks and were generally composed of trees over 25 years of age. In this study, the ratio of Rs to G E P was similar for the 53-y stand at 59% but much higher at 154%, and 195% for the 14- and 3-y Douglas-fir stands, respectively (Fig. 18). When annual Re was compared to G E P , the ratios changed to 86%, 107%, and 240%, for the 53-, 14-, and 3-y stands, respectively . These ratios show that the 53-y stand was an annual sink for C, the 14-y stand was near equilibrium (Re = G E P ) and the 3-y stand was respiring almost 2.5 times as much C as it fixed, nearly all from the soil. For undisturbed forest stands over the age of 25 in the Janssens et al. (2001) review, annual Rs was 63% of Re. Lavigne et al. (1997) found that in six Canadian Chapter 2. Respiration in three Douglas-fir stands 61 coniferous boreal stands, four of which were mature while two were sapling-stage, soil respiration was between 48 and 71% of Re where Re was determined by component fluxes rather than by E C measurements. In close agreement with these results, the ratio for the 53-y stand in this study was 68%. In contrast, soil respiration has been found to make up nearly 100% of Re at a disturbed forest in Sweden where the soil had been drained (Janssens et al., 2001). Although the relationships between Rs and Ts varied between stands in this study, we cannot comment on how harvesting disturbance would have affected respiration in these particular stands without pre-harvest measurements or extensive replication. However, in the two younger stands, measurements and temperature-response curves for Rs and Re were more similar than for the 53-y stand (Fig. 12), suggesting that soil respiration still dominates ecosystem respiration in these young stands. More productive forests are often associated with greater rooting density, higher rates of litter input and fine root turnover as well as warm, well-watered, nutrient-rich soils, the same biotic and abiotic conditions which would be associated with high rates of heterotrophic respiration. Across all three stands, RiQs at the stand level and for individual collars was positively correlated with soil N concentration (Fig. 15). Greater concentrations of C and N were associated with greater depths of the L F H layer. On average, the 14-y stand had the greatest depth of L F H and concentrations of C and N . This was potentially related to the stage of forest development. A dense understory of deciduous and coniferous species grew between the open canopy of Douglas-fir trees. In contrast, the 53-y stand had only a sparse understory of shade-tolerant species below the closed-canopy of Douglas-fir, while the deciduous species were still just beginning to become established in the 3-y stand. Although not quantified, these differences in canopy structure would be expected to result in different litterfall rates and litter types. The 14-y stand may have had the greatest Chapter 2. Respiration in three Douglas-fir stands 62 litterfall rates and the greatest amount of deciduous litter. In addition, the presence of N-fixing alder in the 14-y stand before harvesting may have played a role in increasing the N content of the soil and forest floor. The influence of differing soil texture and consequent available water content could influence between-stand differences in Re and Rs. However, during the study period, weekly average 8V was between F C and W P . Tree growth and microbial activity tend to be l imited by soil moisture only during drought or water-logging. However, differences in soil texture may be important in long-term growth and development of the stand. Consequently, the relationships between stand age and Re or Rs may differ significantly between the three stands examined in this study. 2.4.3 R e s p i r a t i o n scaling issues Estimates of scaled up Rs have been found to exceed E C estimates of Re in other studies (eg. Drewitt et al . , 2002; Lavigne et a l , 1997). Differences in the area rep-resented by the two techniques may be one of the main reasons for this discrepancy. For example, to accurately characterize the soil respiration processes of a forest stand requires adequate sampling of an unknown degree of heterogeneity. Considerable spa-t ia l variability in the temperature response of Rs was found in a l l three stands. Other studies have reported similarly large ranges of Riosi (Longdoz et al . , 2000) as well as C V in the range of 0.3 and 0.4 in ponderosa pine forests (Law et al. , 2001a; X u and Q i , 2001). Given this degree of variability, it was not surprising that differences in the temperature response of Rs were observed between this study and one conducted in 2000 in the 53-y stand (Drewitt et al . , 2002). In 2000, Drewitt et al . (2002) measured nighttime Rs wi th automated, clear plexiglass soil chambers at six locations no more than 5 m apart, each covering 0.19 m 2 of forest floor surface area south of the eddy flux tower. The 2000 study estimated that Rs accounted for 117% of the annual Re. Chapter 2. Respiration in three Douglas-fir stands 63 The mean i?io si (-Rio determined for individual soil chamber locations) ± 1 SE was 6.13 ± 0.73 pmol m - 2 for six chambers in the 2000 study and 3.64 ± 0.34 pmol m~2 for 20 chambers in the 2002 study and were significantly different between study years (Kruskal-Wallis x2 = 8.1815, P = 0.0042). Although the distribution of R10si was normal in 2000, a strong negative skew was observed in 2002 (Fig. 14). However, the minimum and maximum values of Riosi in 2000, 3.99 and 8.29 pmol m~ 2 s - 1 , respec-tively, did fall inside the range of values observed in 2002 (2.58 and 8.48 pmol m~2). It is impossible to determine which, if either, study accurately represented the forest soil respiration without knowledge of the population variance and distribution. Using the C V observed for the 20 values of Riosi determined for each soil collar monitored in 2002 and assuming a normal distribution (although this was not observed), 13 to 68 collars would be required to get a confidence interval of approximately ± 25% to ± 10% of the mean in subsequent studies in the 53-y stand with 95% confidence (Krebs, 1999). This suggests that the 20 collars deployed in the 2002 study were adequate to represent our best estimate of the population variance. As in many other EC-based studies of C O 2 fluxes, we examined measurements made when u* exceeded u*,^ so that problems of insufficient turbulence, drainage flows, and storage calculations would be minimized (Goulden et al., 1997; Black et al., 1996). In the 3-y stand, u* as low as 0.08 m s - 1 were sufficient to support turbulent fluxes of C O 2 measurable at the sensor height while needed to be greater than 0.16 and 0.32 m s - 1 in the 14- and 53-y stands, respectively. Presumably with greater stand height and aerodynamic roughness, wind flow is more likely to decouple from the sources of respired C O 2 and increase the potential for C O 2 to escape undetected. However, even when exceeded u*ith, both over and underestimation of nighttime N E E were partly attributed to vertical advection of C O 2 in the 53-y stand. In agree-ment with Lee (1998), this was not the case for the two younger stands with shorter, Chapter 2. Respiration in three Douglas-fir stands 64 less dense vegetation in which smaller values of non-zero w were observed. Although a mass flow term was found to reduce very large nighttime N E E or increase very low nighttime N E E , this term often over corrected and resulted in unrealistic nighttime N E E estimates. Aubinet et al. (2003) found that both vertical and horizontal advec-tion terms were necessary to achieve reasonable N E E estimates in a tall mixed forest on a uniform 3% slope in Vielsalm, Belgium, suggesting that advection is a two or three dimensional process. The shortcomings of the E C technique limit our ability to describe environmental controls on ecosystem respiration processes. Other methods of estimating Re include the laborious process of scaling component fluxes measured with chamber techniques (Lavigne et al., 1997) and extrapolating daytime E C measurements using the intercept of hyperbolic light-response curves (Suyker and Verma, 2001; Griffis et al., 2003). The latter option makes use of the fact that E C measurements tend to be far more successful during the day when convective conditions exist. These methods could be useful in verifying the magnitude of daily Re and in shedding light on which methodology causes annual Rs to exceed Re, as it did in the 14-y stand. 2.5 Conclusions This study examined respiration processes and turbulent transport of C 0 2 in three closely located Douglas-fir stands at different stages of development after harvesting. We used both E C and chamber techniques to measure C O 2 fluxes in these three stands over an 8-month period. Without the confounding effects of differences in weather between the three stands, the influence of stand structure and site characteristics on microclimate, C O 2 transport processes, and respiration processes could be examined. Evidence for vertical advection of C O 2 in the 53-y stand emphasized that E C Chapter 2. Respiration in three Douglas-fir stands 65 measurements must be interpreted carefully, especially on sites wi th ta l l , dense vege-tation on less than ideal terrain. We also observed that momentum absorption by the rough canopy of the 53-y stand resulted in higher average u* than in the two younger stands. However, lower values of u*jh were needed in the younger stands for C O 2 fluxes to equal the C O 2 respired by the forest stands. Examining the relationship of both Re and Rs to temperature was a useful way to characterize statistically the differences in the respiration processes between the three stands. Efforts to parti t ion Re into above- and below-ground components were confounded by the differences in the E C and chamber systems representation of spatial and temporal variations in Re and Rs. O n average, Rs was 54%, 120%, and 69% of Re in the 53-, 14-, and 3-y stands, respectively. Annua l estimates of Re increased from 1090 g C m - 2 y - 1 in the 3-y stand to 1290 g C m - 2 y " 1 in the 14-y stand to 1710 g C m - 2 y - 1 in the 53-y stand. Most importantly, this study emphasized that annual Re and Rs were better correlated wi th stand productivity ( G E P and daytime N E P ) and related soil characteristics such as soil N content than wi th temperature. Mean annual T was very similar at al l the stands due to their close proximity while Ts was influenced by stand structure (annual Ts decreased wi th increasing stand age) and resulted in a negative correlation wi th annual Re. To better account for and predict the consequences of disturbances such as harvesting on the release of C O 2 from the landscape, more work is needed into scaling efforts and the effects of advection on flux measurements. Chapter 3. NEP in three Douglas-fir stands 66 3 Measurements of atmospheric carbon dioxide fluxes in three Douglas-fir stands at different stages of development after harvesting 3.1 Introduction Forests play a significant role in the global carbon cycle. Climate, variability in weather, species, age, and current and past disturbances are some of the major fac-tors which control stand-level C dynamics. The development of instrumentation to monitor long-term C O 2 and energy exchange between forests and the atmosphere has provided considerable information on the differences in C and water dynamics in forests in varying climates around the world (Baldocchi, 2003). More recently, multi-year studies have shown that variability in weather can have a dramatic impact on the net exchange of C O 2 at a particular stand (Barford et al., 2001; Pilegaard et al., 2001; Carrara et al., 2003; Morgenstern et al., 2003). To date, much of this work has focused on near-mature or late-successional forest ecosystems. Currently, efforts are being made to quantify the C dynamics of forests at different stages of development after natural or human-induced disturbances such as harvesting and fire (Schulze et al., 1999; Amiro, 2001; Law et al., 2001b; Chen et al., 2002; Litvak et al., 2003). Given the same weather conditions, these studies emphasize that the bio-physical factors which regulate net ecosystem production (NEP) in young developing forests and mature and/or late-successional forests may differ greatly. Although there is considerable knowledge of how successional processes in managed and unmanaged forests influence stand composition and structure and long-term aboveground stem-wood biomass production (ANPP), there is uncertainty regarding the magnitude and direction of seasonal and annual N E P and the biophysical factors which control N E P Chapter 3. NEP in three Douglas-fir stands 67 in developing forests. Here, we examine one year of N E P measurements in three closely located coastal Douglas-fir stands (Pseudotsuga menziesii ssp. menziesii) at different stand devel-opment stages after harvesting. Similar weather, climate, and site characteristics at each research site enabled differences in stand structure and composition to be re-lated to differences in diurnal and seasonal fluxes of carbon dioxide. The relationships between daytime N E P and biophysical factors such as light, diffuse light, air satu-ration deficit and temperature were contrasted between the three stands. Estimates of annual N E P , gross ecosystem production ( G E P ) and total ecosystem respiration (Re) were compared between stands to examine the potential spatial variability of C exchange rates in forested lands. 3.2 Methods The eddy covariance technique was used to measure half-hour fluxes of C O 2 above three different-aged Douglas-fir stands located on the east coast of Vancouver Island, B C . This chapter reports on the measurements made between January 1 and Decem-ber 31, 2002. The oldest stand was established in 1949 and was 53 years old in 2002. This stand was about 35 m ta l l and composed of 80% Douglas-fir at a density of 1100 trees h a - 1 . Al though Douglas-fir on Vancouver Island can reach ages of 300 to 500 years, harvesting rotations are about 40 to 80 years in this region making the 53-y stand near maturity. The youngest stand of the study was located 3 k m to the E S E of the 53-y stand. It consisted of a 32-ha area which had been clearcut-harvested in the winter of 1999/2000 and planted wi th 1-year-old Douglas-fir seedlings (1500 trees h a - 1 ) in the spring of 2000. In 2002, the trees were 3 years old and about 60 cm tal l . Forty-eight k m S E of these two stands is a pole-sapling stand harvested and planted Chapter 3. NEP in three Douglas-fir stands 68 with Douglas-fir in 1988. In 2002, the stand was 14 years old, on average 5.8 m tall with a stand density of 1240 trees h a - 1 . The 53-year-old, 14-year-old, and 3-year-old stands are hereafter referred to as the 53-y, 14-y, and 3-y stands, respectively. A detailed description of the three sites can be found in Chapter 2 . The eddy covariance instrumentation consisted of a Gi l l R2 or R3 sonic anemometer-thermometer and an infra-red gas analyzer (IRGA). A n open-path I R G A (model LI-7500, LI-COR) was operated in the 14-y stand, while closed-path IRGAs were run in the 53- and 3-y stands. A correction factor was applied only to the closed-path Fc val-ues measured at the 3-y stand to account for high frequency flux loss due to damping of C 0 2 fluctuations within the sampling tube (see Chapter 4 for more details). The open-path IRGA was unable to make measurements in wet weather; however, after applying corrections to the open-path fluxes, the agreement with a closed-path IRGA was within 4% during dry conditions. Comparisons of the C 0 2 fluxes (F c ) measured using open- and closed-path IRGAs are discussed in detail in Chapter 2 , Chapter 4 , and Appendix 3 . The rate of change in storage of C 0 2 in the air column below the E C instrumentation was calculated using four levels of C 0 2 measurements in the 53-y stand and one level at the E C height in the 14- and 3-y stands as described in Chapter 2 . Net ecosystem exchange of C 0 2 (NEE) was then computed as the sum of Fc and this storage term. Net ecosystem production (NEP) is to a very good approx-imation assumed to equal - N E E . Positive values of N E P mean net C uptake by the stand and negative values mean net loss of C. C loss as methane fluxes are negligible from well-drained forest soils, while leaching of dissolved organic or inorganic C below the root zone usually accounts for less than 6 g C m~ 2 y - 1 in forests (Landsberg and Gower, 1997). In each stand, nighttime measurements of N E P (= —Re) correlated with friction velocity up to threshold values (u*,^ ) of 0.08, 0.16, and 0.32 m s - 1 for the 3-, 14-, and 53-y stands, respectively (Chapter 2 ). N E P measurements made Chapter 3. NEP in three Douglas-fir stands 69 when it* < u*jh were assumed to underestimate real ecosystem respiration rates and were excluded from the data set. These measurements were replaced with estimates of Re derived from the exponential relationship between half-hour soil temperature and nighttime N E P when u* > u*>th during the entire year (see below for these functions). Further details of the eddy covariance instrumentation, setup, and data processing are given in Chapter 2 and Chapter 4 . Continuous measurements of weather variables were also made in each forest stand. These included measurements of downwelling photosynthetically active ra-diation (PAR, pmol m - 2 s - 1 ) , above-canopy air and soil temperature (T and Ts, respectively), air saturation deficit (D), rainfall, and volumetric water content (9V). Details of the sensors used in these measurements are given in Chapter 2 and Chapter 4 . To differentiate clear from cloudy days, downwelling shortwave radiation in W m~ 2 for completely clear sky conditions was estimated following Stull (1988) and then multiplied by 2 to convert approximately to pmol m~ 2 s - 1 . These values agreed well with measurements of P A R on clear days. After removing half-hour fluxes of poor quality (see Chapter 2 ), 84, 73, and 92% of all N E P measurements remained for the 3-, 14-, and 53-y stands, respectively. Nighttime measurements were also excluded from the analysis when u* < u^th- This removed between 50% to 65% of all nighttime half hours. Gaps in the data sets were filled using a common procedure for all three stands. First, an exponential relationship between half-hour nighttime soil temperature and nighttime N E P was determined using half hours from the entire year. Daytime Re values were estimated using this relationship with daytime Ts. G E P values were obtained by adding measured values of daytime N E P to these calculated daytime Re values. A rectangular parabolic Chapter 3. NEP in three Douglas-fir stands 70 relationship between G E P and downwelling P A R , _ aGEPppPAR G E P ~ c tPAR + GEPoo ( 9 ) where GEPQO is the asymptote and a is the effective quantum yield, was evaluated for June 15 to July 15. This relationship was used to estimate G E P for all daytime half hours. These estimates of G E P were then adjusted to the seasonal variations in G E P by finding the slope of the linear regression forced through zero between the estimates and measurements of G E P (independent variable) for 100 consecutive half-hour measurements moving through the year in increments of 20 half hours (see for example, Falge et al., 2001). Missing daytime half-hour N E P values were filled using the difference between the estimated G E P and Re while missing nighttime half hours were filled using —Re. Only half hours with good quality, measured N E P were used to analyze the effects of environmental variables on N E P and G E P . Measurements of leaf area index (LAI, m 2 projected leaf area m - 2 ground) were made every 3-4 weeks throughout the growing season in the two younger stands. In the 14-y stand, this was accomplished by making measurements with an optical sensor (LAI-2000'Plant Canopy Analyzer, LI-COR Inc.) at 16 locations on two parallel 70-m long transects running SE and SW of the tower. In the 3-y stand, a relative frequency method was used on ten 40 m long transects, described in detail in Chapter 4 . In the 53-y stand, a single L A I value (8.4) was estimated using relationships between leaf area and D B H discussed by Barclay and Trofymow (2000). This L A I was equal to the value measured using the LAI-2000 (4.0) corrected for branch and leaf clumping using a clumping index of 0.48 (i.e., corrected L A I = 4.0/0.48). Chapter 3. NEP in three Douglas-fir stands 71 3.3 Results 3.3.1 Seasonal weather and leaf phenology The average weather for all three stands was very similar and illustrates the typi-cal coastal conditions of wet, cool winters and warm, dry summers (Figs. 19 and 20). Small differences in air temperature were likely attributable to differences in how energy is partitioned in the three stands and to differences in the height at which the temperature measurement was taken (8 m above the stand in the 53-y stand, 6 m above the stand in the 14-y stand, and 3 m above the ground in the 3-y stand). Differences in rainfall in part reflected the differences in elevation of each site. Differences in average weekly P A R were generally small except for a few weeks when mesoscale variations in sky conditions resulted from events such as convective showers or orographic uplift. L A I was greatest at 8.4 in the 53-y stand and was assumed to not vary seasonally. In contrast, L A I in the two younger stands increased through the growing season until a peak value was reached by late June or early July. Maximum L A I was approximately 4.8 in the 14-y stand and 2.2 in the 3-y stand (Fig. 20). 3.3.2 D a i l y patterns of C 0 2 exchange Selected diurnal courses of N E P for the three stands are shown in Fig. 21a. Regardless of the time of year, daytime N E P in the 3-y stand was always less than the two older stands (Fig. 21). Daytime net uptake of C was observed throughout the year in the 14- and 53-y stands, while this only occurred in mid-summer in the 3-y stand (3rd panel of Fig. 21). Despite the smaller Douglas-fir trees with less leaf area in the 14-y stand, N E P in June 2002 was similar to rates in the 53-y stand. At night, N E P corrected for low conditions was generally more negative in the 53-y stand than in Chapter 3. NEP in three Douglas-fir stands 72 Figure 19: 2002 monthly total rainfall and average above-canopy air temperature for the 3-, 14-, and 53-y stands. the two younger stands. The relationship between N E P and downwelling P A R can be expressed using rectangular hyperbolic relationship, N E P = Q N E P Q C P A R a P A R + N E P ^ RH (10) where NEPoo is the asymptotic value of NEP, a is the effective quantum yield, and Rd is the intercept representing the average respiration rates during the day. Eq. 10 was used to produce the curves in Fig. 21b for the half-hour N E P measurements shown in Fig. 21a. Table 11 lists the parameters for Eq. 10 for all the daytime N E P measurements in the months Feb, April , June and October. In the recently harvested stand, P A R explained less than 10 % of the variation in half-hour N E P except for Chapter 3. NEP in three Douglas-fir stands 73 J F M A M J J A S O N D 2002 Figure 20: Projected leaf area index (LAI) and weekly average daytime downwelling P A R for the 3-, 14-, and 53-y stands in 2002. Dashed lines represent interpolated measurements based on visual field observations. the summer when R 2 was 0.42 (June). In contrast, in the other two stands, half-hour variations in daytime N E P were strongly related to P A R year-round. The similarity in N E P between the 14- and 53-y stands (Fig. 21a) was also reflected in the relationships between N E P and P A R for June 2002. June was also the month when the proportion of variation in half-hour N E P was least explained by P A R in the 53-year-old stand ( R 2 = 0.48). In F ig . 21a, the daytime N E P measured in the 53-y stand significantly decreased over the three consecutive days selected in June. Each day in this series was associated wi th decreasing cloudiness, increasing temperature and increasing air saturation deficit. The relative impact of these environmental variables on daily average N E P in the three stands are explored below. Chapter 3. NEP in three Douglas-fir stands 74 a) 20 0 -20 20 ^ 0 i w 04 E -20 o E 3 20 0_ LU 2 0 -20 20 0 -20 1 1 — ^/^-^A-^V i V °" ^^y—^ 2/18 2/19 2/20 .A i i /v - v 4/1 4/2 4/3 J» »J\.i /A/\ i i t 6/19 6/20 6/21 1000 2000 10/5 10/6 2002 10/7 1000 2000 1000 2000 1000 2000 PAR (pmol m 2 s 1) Figure 21: Panels a) show selected diurnal cycles of half-hour measurements of N E P for the recently clearcut-harvested stand (3-y) (thin solid line), the pole-sapling stand (14-y) (dashed line), and the near-mature 53-y stand established (thick solid line). Data have been u* corrected and gap-filled in this panel. Panels b) show the relationships between non-gap filled N E P and downwelling P A R using data from the three days in a). Each data point is the average of daytime measurements of N E P for increasing levels of P A R grouped in widths of 100 /xmol m - 2 s _ 1 . Chapter 3. NEP in three Douglas-fir stands 75 Table 11: Parameters for Eq. 10 describing the relationship between daytime N E P and downwelling P A R for the four months shown in Fig. 21 for the three stands. a N E P o o (//mol m - 2 s _ 1 ) Rd (/miol m~ 2 s - 1 ) R 2 3-y stand: Feb 0.02 0.8 1.4 0.06 April 0.01 1.0 2.1 0.07 June 0.07 7.9 5.3 0.42 Oct 0.01 1.5 2.3 0.11 14-y stand: Feb 0.04 4.7 1.6 0.56 April 0.03 8.2 2.4 0.43 June 0.07 17.6 6.3 0.51 Oct 0.05 9.7 3.9 0.49 53-y stand: Feb 0.07 10.3 1.8 0.57 Apri l 0.05 19.0 2.8 0.71 June 0.08 20.3 6.9 0.48 Oct 0.06 18.3 2.9 0.63 3.3.3 Seasonal patterns and biophysical controls of CO2 exchange Seasonal patterns of daily N E P differed significantly between the three stands (Fig. 22). In the 53-y stand, the maximum net uptake of C occurred in May when temper-atures were cool but light levels were relatively high and daylengths were relatively long. In contrast, maximum N E P in both the 14- and 3-y stands occurred in June and July when L A I was greatest (Fig. 20). N E P was most negative in the two young stands during early fall (Sept) when temperatures were still warm and the vegetation was senescing (decreasing LAI). Despite the daytime net uptake of C in mid-summer in the 3-y stand (Fig. 21), this stand was a year-round source of atmospheric C O 2 on a daily-basis. In the 14-y stand, daily N E P was positive during the summer between Chapter 3. NEP in three Douglas-fir stands 76 A p r i l and August. In contrast, daily net C uptake could occur year-round in the 53-y stand even in late fall and winter when temporary sunny conditions increased photosynthetic uptake in relation to relatively low Re as a result of cool temperatures. Dai ly N E P in the 53-y stand could also be negative throughout the year. For example, on July 21, 22, and 23 and A u g 12 and 13, N E P indicated relatively large daily C losses from all three stands and particularly the 53-y stand (Fig. 22). These days were associated with clear skies, very warm temperatures, and high D. The daily average daytime N E P was correlated wi th downwelling P A R in al l three stands wi th increasing sensitivity for P A R less than 600 //mol m - 2 s - 1 wi th increasing stand age (Fig. 23). In the 53-y stand, N E P increased unti l the average daytime P A R reached about 900 //mol m ~ 2 s _ 1 beyond which N E P decreased wi th increasing P A R . The results above, the seasonality of the relationship between N E P and P A R in the two younger stands, the low R 2 values in the 53-y stand (Table 11), and the summer days wi th negative N E P suggest that environmental factors other than P A R have a significant role in regulating N E P in al l three stands. However, the functional relationships between N E P and P A R , L A I , temperature, D , and soil moisture are difficult to determine as these variables are highly correlated wi th one another (Figs. 19 and 20). To remove some of the effects of P A R , half-hour N E P were selected when P A R was greater than 800 /tmol m - 2 s _ 1 . A s expected, seasonal variations in the average daytime N E P positively relate to L A I in the two younger stands where seasonal variations in leaf phenology occurred (Fig. 24). L A I is also clearly a major factor in explaining the differences in magnitude in N E P between the three stands. Aga in using only N E P measured when P A R was greater than 800 pmol m ~ 2 s - 1 , F ig . 25 shows the differences in average N E P during days which were mostly clear (daytime average P A R / p o t e n t i a l P A R > 0.8) and days which were at least partly Chapter 3. NEP in three Douglas-fir stands 77 Figure 22: Daily 24-h N E P in 2002 for the recently clearcut harvested stand (3-y), the pole-sapling stand established in 1988 (14-y), and the near-mature stand established in 1949 (53-y). Chapter 3. NEP in three Douglas-fir stands 78 10 _2' i i i i i i 0 200 400 600 800 1000 1200 Mean daytime PAR ihmol rrf2 s_1) Figure 23: Average daytime N E P vs. downwelling P A R between January 1 and December 31, 2002. Each data point is the average daytime N E P (non-gap-filled) for 20 days with similar total P A R . Error bars are ± 1 SE. overcast (daytime average PAR/potential PAR < 0.8). Having limited PAR to high light conditions, there was only a weak relationship between NEP and PAR for either clear or overcast conditions. However, the average NEP for overcast days was always greater than those for clear days by 0.5, 2.2, and 3.3 /xmol m~2 s - 1 in the 3-, 14-, and 53-y stands, respectively. Not only were partly overcast conditions associated with more diffuse light and lower average daytime PAR, these days were also associated with lower T, and D (Fig. 25). In all three stands, NEP increased with increasing D up to a threshold value beyond which NEP decreased with increasing D. The response to D was greatest in the 53-y stand where the average clear-sky NEP decreased from 12 to 7 pmol m - 2 s _ 1 when average D increased from 0.7 to 1.4 kPa. The threshold D, where NEP began to decrease, increased with decreasing stand age and in all stands. Chapter 3. NEP in three Douglas-fir stands 79 I o E o o CO rr 'w <C CO £ o LU 3 12 r 10-8 o 5 3 - y • 14_y A 3 - y co CO CU > < LAI Figure 24: The relationship between L A I and non-gap-filled N E P for downwelling P A R > 800 (xmol m ~ 2 s - 1 for the three Douglas-fir stands for all of 2002. Each data point is the average of at least 45 half hour measurements of N E P after grouping by L A I bins of width 0.5. The error bars indicate ± 1 SE. Maximum D also increased with decreasing stand age. N E P generally increased with increasing T for overcast conditions. During clear conditions, N E P decreased significantly with increasing T beyond a temperature of about 17°C, 18°C, and 19°C in the 53-, 14-, and 3-y stands, respectively. Overall, at least partly overcast sky conditions were associated with greater daytime N E P associated with either lower D or T or both. The relative impact of T on rates of respiration or photosynthesis and D on the photosynthetic uptake of C O 2 (GEP) is difficult to isolate using N E P measurements. The positive exponential relationship between nighttime N E P and Ts illustrates the dependence of respiration on temperature (Table 12 and see Chapter 2 ). These relationships were used to obtain G E P from daytime measurements of NEP. Chapter 3. NEP in three Douglas-fir stands 80 I O E CL LU 800 1000 1200 1400 1600 1800 PAR (urnol rrf2 s"1) 1 1.5 D (kPa) 2.5 Figure 25: Relationships between non-gap-filled N E P and P A R , D, and T under clear and cloudy sky conditions when P A R > 800 //mol m - 2 s _ 1 for all of 2002. Each data point is the average of 200 half hours after sorting with P A R , D or T. The error bars indicate ± 1 SE. Chapter 3. NEP in three Douglas-fir stands 81 Table 12: The annual relationship between Re and 5-cm Ts when u* > u^^h i n the three Douglas-fir stands, where Re — RwQ^w 1 0 ^ 1 0 . The number of half hours (n) from 2002 used to derive these parameters is also indicated. Rw pmol m~ 2 s _ 1 Qw R 2 n 53-y 4.7 5.1 0.47 2089 14-y 3.5 4.0 0.52 1364 3-y 2.5 2.1 0.27 2130 However, E C measurements are generally unable to resolve seasonal variations in the relationship between Re and Ts, which soil chamber measurements suggest are significant (Drewitt et al., 2002). Consequently, the seasonal and diurnal effects of photoinhibition, microbial and vegetation biomass dynamics and acclimation on respiration processes are forced into the G E P term. Keeping these limitations in mind, Fig. 26 shows the relationships between G E P and D when P A R > 1200 pmol m~ 2 s - 1 between June 15 and July 15, 2002, when L A I was relatively constant at the two younger stands. Again, by limiting the G E P data to periods of high light, little dependence on P A R remained. In the two older stands, there was a strong decrease in G E P with increasing D, with the 53-y stand having the more negative response. In the 3-y stand, G E P showed only a slight negative response to D. These results indicate that the reductions in N E P with high D shown in Fig. 25 are in part, a function of decreased photosynthetic uptake. Low air temperatures were also observed to have an influence on photosynthetic uptake of C O 2 . March was the coldest month of 2002 (Fig. 19). During this month, when P A R was greater than 450 pmol m~ 2 s - 1 , G E P were insensitive to P A R and D, but positively correlated with T, particularly at temperatures below 7°C (Fig. 27). For T below -3°C in the 53-y stand, G E P was near zero and no longer varied Chapter 3. NEP in three Douglas-fir stands 82 30r 25-0) -CJ 1 20-E o E 15-0-LU C3 10-5 0^ 0 o 53-y • 14-y A 3-y 0.5 1.5 D(kPa) 2.5 Figure 26: Relationships between non-gap-filled G E P and D when P A R > 1200 fxmol m - 2 s _ 1 between June 15 and July 15 in the 53-y stand (thick solid line), the 14-y stand (dashed line), and the 3-y stand (thin solid line). Each data point is the average of 15 half hours. The error bars indicate ± 1 SE. 14, 0 U • • -- 5 0 5 10 T(°C) Figure 27: Relationships between non-gap-filled G E P and T when P A R > 450 u-mol m ~ 2 s _ 1 for March 2002 in the three Douglas-fir stands. Symbols are the same as in Fig. 26. Each data point is the average of 15 half hours. The error bars indicate ± 1 SE. wi th temperature. For T above 7°C, G E P was relatively constant in al l three stands. Unlike any of the other N E P or G E P relationships wi th environmental variables Chapter 3. NEP in three Douglas-fir stands 83 discussed above, the data shown in Fig. 27 roughly collapsed to a single general trend when divided by LAI . For example, for all the measurements made between -1 and 1°C, the average G E P normalized by L A I of 0.7, 3, and 8.4 were 0.6, 0.8, and 0.7 pmol m - 2 leaf area s - 1 , while between 7 and 7.5°C, normalized average G E P was 1.4, 1.8, and 1.2 pmol m - 2 s - 1 for the three stands in ascending age, respectively. This suggests that at these cool temperatures with low angle P A R and low D, the differences in physiological response to the environment and the effects of canopy depth are minimized compared to periods of warm temperatures, high P A R and D. 3.3.4 Annual C 0 2 exchange Cumulative sums of N E P for 2002 for the three stands are shown in Fig. 28 and Table 13. In this year, the 53-y stand was a sink of 250 g C m~ 2 y - 1 , the 14-y stand was a moderate source of -130 g C m - 2 y - 1 , and the 3-y stand was a large source of -610 g C m - 2 y _ 1 . Estimates of total annual Re and G E P increased with increasing stand age. As expected, both Re and G E P were greater in the growing season months (April-Sept) than in the cooler months (Jan - Mar, Oct - Dec) in all stands. Slightly more C was lost from the 3-y stand during the growing season months compared to the winter or cooler periods of the year. In contrast, the 14-y stand acted as a small C sink during the growing season and a source during the cool months. The 53-y stand was near neutral (i.e., N E P « 0) during the cool months and a large C sink during the growing season (Table 13). The uncertainty in these annual N E P sums stems from the random and systematic errors incorporated in the analysis. Following Morgenstern et al. (2003), a random error of 20% on each of the half-hour measurements of N E P resulted in an error of only 2, 3, and 5 g C m - 2 y - 1 in the annual N E P of the 3-, 14-, and 53-y stands, respectively. A random error of 10 pmol m~ 2 s - 1 for each half hour still resulted in Chapter 3. NEP in three Douglas-Gr stands 84 Net c loss J F M A M J J A S O N D Figure 28: Cumulative N E P in 2002 for the 3-, 14-, and 53-y stands. Table 13: Annual and seasonal sums of N E P , G E P and R (values are rounded to the nearest 10). 3-y 14-y (g C m"2 y"1) 53-y Annual NEP -610 -130 250 GEP 430 1210 1960 Re 1040 1340 1710 Summer (May - Sept, inclusive) NEP -330 50 270 GEP 390 990 1540 Re 720 940 1270 Winter (Jan - April and Oct - Dec, inclusive) NEP -280 -180 -20 GEP 40 220 420 Re 320 400 440 Chapter 3. NEP in three Douglas-fir stands 85 a relatively small error of 30 g C m - 2 y " 1 for all three stands. More substantial were the systematic biases associated with the data processing decisions. As discussed earlier, chamber measurements of soil C O 2 efflux suggest that significant hysteresis exists in the soil component of the annual Re — Ts relationships (Drewitt et al., 2002). Although this functional pattern is not resolvable with E C measurements, gap-filling can also be done using an annual Re — Ts relationship ad-justed to seasonal shifts in respiration using a moving window method similar to the method described earlier for gap-filling GEP. This method can result in significantly higher annual Re compared with estimates obtained using a constant annual Re — Ts relationship. The selection of u*.tttl, and the application of an energy budget closure (ebc) correction (Black et al., 2000) can also significantly alter annual estimates of NEP. The average ebc, derived as the linear least squares regression of all half hour measurements of sensible and latent heat fluxes against available energy (Humphreys et al., 2003), was 0.80, 0.90, and 0.91 in the 53-, 14-, and 3-y stands, respectively. Reducing u^th by 50% to 0.16, 0.08, and 0.04 for the 53-, 14-, and 3-y stands, respec-tively resulted in larger net C uptake (or less net C loss) in all three stands (350, 0, and -550 g C m~ 2 y - 1 , respectively). Using the regular u^th values and applying an ebc correction of 1.25, 1.11, and 1.10 increased the absolute magnitude of C exchange in the three stands to 310, -150, and -670 g C m - 2 y - 1 in the 53-, 14-, and 3-y stands, respectively. In contrast, using a moving-window estimate of Re, the regular u+^h val-ues, and no ebc correction reduced annual N E P to 110, -180, and -680 g C m - 2 y - 1 in the 53-, 14-, and 3-y stands, respectively. Morgenstern et al. (2003) also found that these uncertainties modified annual values greatly while interannual differences remained similar. Here, the general nature of the differences between stands remained the same while the average range in N E P was about ± 100 g C m - 2 y - 1 for the three stands. Chapter 3. NEP in three Douglas-fir stands 86 3.4 Discussion Diurnal, seasonal and annual C 0 2 exchange differed greatly between the three Douglas-fir stands. With similar weather in all three stands, differences in C exchange dynam-ics could, in part, be associated with differences in stand structural characteristics such as species composition and age, phenology and leaf area, and associated physio-logical characteristics. During 2002, the 3-y and 14-y stands were an annual C source while the 53-y stand was a C sink. One of the most important factors controlling N E P in all three stands was the magnitude and seasonal variations in LAI . A relatively constant and high L A I in the 53-y stand enabled daily N E P to be positive at any time of the year, particularly in early spring when light levels were high and temper-atures were low. In contrast, the canopy of the 3-y stand was dominated by pioneer species, many of which were deciduous. In this stand, half-hour measurements of N E P were not positive until mid-summer when L A I was at its maximum. However, even maximum L A I was only about 2.2 and consequently, daytime photosynthetic uptake of C O 2 did not compensate for the loss of C O 2 by respiration and decompo-sition processes on a 24-h basis. The species composition and phenology of the 14-y stand was a mix of the two above situations. The 5.8 m-tall Douglas-fir combined with the dense understory of deciduous and evergreen species led to a relatively high year-round L A I of at least 2 to 3 and a mid-summer maximum of 5. Positive daytime N E P was observed year-round, but positive N E P on a 24-h basis generally occurred between late March and early August. A number of other paired-tower C O 2 flux studies have found that between forests with different LAI , the forests with greater L A I have greater annual or daily growing season N E P (Amiro, 2001; Anthoni et al., 2002; Rannik et al., 2002; Litvak et al., 2003). Exceptions generally involved old-growth or late-successional forests such as Chapter 3. NEP in three Douglas-fir stands 87 450-year-old Douglas-fir (Chen et al., 2002), 200-year-old Siberian pine (Schulze et al., 1999), and 70-year-old black spruce (Litvak et al., 2003), which had higher L A I than younger neighboring forests, but had lower daily NEP. Theoretically, a forest stand may be C neutral on an annual basis at two very different stages of succession; early in stand development when the rapid accumulation of biomass momentarily balances respiration losses of C O 2 (see Fig. 2 in Chapter 1 ) and much later when ecosystem respiration balances the slower growth and C sequestration of mature old trees. In various forest ecosystems, this early carbon balance compensation point is estimated to be between 11 and 19 years in black spruce stands regenerating from stand-replacing fires in Manitoba (Litvak et al., 2003), about 14 years in harvested Siberian pine stands (Schulze et al., 1999), and about 14 years for simulations in Douglas-fir (Janisch and Harmon, 2002). In this study, the 14-y stand was nearing C neutrality. In 2002, the net C loss from this stand was 130 g C m~ 2 y - 1 . A cooler fall and winter without significant frost damage and a slightly cooler, cloudier summer could reduce both respired losses of C O 2 and physiological limitations to C O 2 uptake and result in a positive or near-zero N E P in this stand. However, a hotter summer with significant drought and early senescence could cause the stand to be an even larger C source. Control or elimination of understory species would likely result in lower G E P but may also result in lower Re and could also significantly impact the C budget of the stand. In the past few years, the 53-y stand has been a relatively large C sink with an-nual N E P between 270 and 420 g C m~ 2 y " 1 (Morgenstern et al., 2003). Although the magnitude of half-hour N E P in the 53-y stand was greater than the two younger stands, the C exchange processes were generally more sensitive to environmental vari-ables. This has led to interannual variations in N E P as large as 100 g C m~ 2 y - 1 (Morgenstern et al., 2003). Compared with past weather conditions (between 1998 Chapter 3. NEP in three Douglas-fir stands 88 and 2001) most of 2002 was sunnier and warmer than normal except for March when the average T was only 1.5° compared to an average of 4.1° for 1998 to 2001 (Mor-genstern et al. , 2003). The cold period in March reduced N E P in that month by inhibit ing photosynthesis, while reduced N E P throughout the rest of the year was a result of higher temperatures and higher rates of respiration combined wi th higher D and reduced photosynthetic uptake of C O 2 . Relationships between growing season N E P and environmental variables such as PAR, T, and D were similar in form for all three stands but again, the magnitude of N E P and its sensitivity and to these variables increased wi th increasing stand age. A l -though the influence of diffuse light on N E P was difficult to isolate, cloudy conditions were associated wi th greater N E P in al l three stands. However, as found elsewhere, the interdependence of environmental variables such as partly overcast conditions be-ing associated wi th lower T and D result in complex but often complementary control of N E P (Gu et al. , 2002). Despite the uncertainty in using an annual relationship be-tween nighttime N E P and Ts to estimate G E P and Re from N E P , these components of C exchange provided insight into the relationships between N E P and environmental variables. A s expected, on an annual basis both G E P and Re correlated positively wi th L A I or stand age. However, the greater sensitivity of G E P to high D and Re to Ts in the 53-y stand occasionally resulted in daytime N E P which was lower or similar to N E P in the 14-y stand in mid-summer. Lower rates of photosynthesis wi th high D have been observed in older coniferous stands when compared to younger neigh-boring stands at both the canopy and leaf-levels (Chen et al . , 2002; Hubbard et al. , 2002). Differences in canopy composition may also be partly responsible for this re-sponse. Herbaceous annuals and perennials tend to have greater maximum rates of C assimilation than coniferous and hardwood trees (Wullschleger, 1993). The plant community in the 14-y stand was a mix of Douglas-fir and understory species such Chapter 3. NEP in three Douglas-fir stands 89 as huckleberry, salal, bracken fern, and fireweed. The fireweed was found to have higher stomatal conductance and light-saturated assimilation rates twice those of the Douglas-fir trees (Tiebo Ca i , unpublished data). Soil moisture effects on N E P were not observed in any of the three stands in 2002. This year was not a particularly dry year and the dry summer season tends to result in a gradual decline in soil moisture, which correlates well wi th temperature, L A I , and P A R . 3.5 Conclusions 1. In three Douglas-fir stands at different development stages, greater leaf area was associated wi th greater daily, seasonal and annual N E P . Annually, the 53-y stand was a sink for C , the 14-y stand was a weak to moderate source, and the 3-y stand was a large source. 2. Differences in stand composition and phenology resulted in different seasonal patterns of N E P wi th maximum N E P in M a y in the 53-y stand and in June /Ju ly in the two younger stands. 3. The general form of the relationships between N E P and environmental variables such as P A R , D, and T were similar in al l three stands. However, the sensitivity to these variables was greatest in the 53-y stand and least in the 3-y stand. The results described here show that three forest stands at different development stages have very different magnitudes and temporal patterns of N E P . This has major implications for estimating the regional C budget of forested lands. Not only is N E P affected by stand age, site characteristics and disturbance history, but also interannual variations in P A R , T , and D can play an important role in affecting N E P in forests by influencing vegetation growth and leaf-emergence, respiration rates and physiological Chapter 3. NEP in three Douglas-fir stands 90 constraints to C 0 2 uptake. Continued monitoring in these three stands over time wi l l be crucial in order to evaluate the impact of interannual weather variability on the source/sink strength of each stand. This w i l l also allow us to monitor the consequences of future changes in stand structure on C exchange dynamics wi thin these three stands as they age. Chapter 4. Three years of NEP in a clearcut 91 4 Net ecosystem production of a Douglas-fir stand for three years following clearcut harvesting 4.1 Introduction Forest harvesting has a major impact on the nature of carbon dioxide ( C O 2 ) exchange between the land surface and the atmosphere. With the removal of the aboveground biomass, the amount of C O 2 taken up through photosynthesis is significantly reduced while the remaining belowground biomass and slash decompose, releasing C O 2 . Con-sequently, harvested lands tend to act as net sources of C O 2 . The duration and magnitude of this carbon (C) loss depends on the balance between photosynthetic uptake and respiration loss of C and the biophysical factors which affect these pro-cesses. In the years following harvesting, the growth of planted and/or pioneer species gradually modifies the microclimate and C balance. Currently about 1.02 x 106 ha of Canada's 247 x 106 ha of commercial, forest land are harvested annually. In British Columbia, 193 x 103 ha are harvested each year with 30 to 40 x 103 ha in coastal forests. As a whole, these productive coastal forests cover only 9 x l O 6 ha but have the largest C content per unit area in Canada at 375 x 103 kg C h a - 1 (Kurz and Apps, 1999). It is estimated that 86 x 10 1 2 kg of C are stored in the vegetation and soils of Canada's forests. Consequently, disturbance through harvesting, fire, and in-sect damage has the potential to significantly affect regional and national C budgets (Kurz and Apps, 1999). Only recently have research studies measured stand-level C O 2 fluxes in early suc-cessional forests (Amiro, 2001; Pypker and Fredeen, 2002a,b; Rannik et a l , 2002; Kowalski et al., 2003). These stands were all less than 6 years old and were measured or estimated to be C sources on an annual basis although they occasionally behaved Chapter 4. Three years of NEP in a clearcut 92 as C sinks on a daily or seasonal basis. However, the magnitude of this annual source strength is uncertain in the majority of these studies since measurements were not made in winter. There is clearly a need for year-round, long-term measurements of C O 2 fluxes above disturbed forests in order to quantify the annual net C exchange and identify the biophysical factors which determine seasonal and interannual variations. A n increasing number of multi-year studies in late-successional forest ecosystems have shown interannual variations in weather to affect annual C exchange. For exam-ple, warmer spring temperatures increased photosynthetic uptake as a result of earlier leaf emergence and consequently, increased the annual C sequestration of boreal aspen (Chen et al., 1999), while warmer spring and summer temperatures enhanced respira-tion rates and decreased the annual C sequestration of coastal coniferous Douglas-fir (Morgenstern et al., 2003) and in a Danish beech forest (Pilegaard et al., 2001). Soil temperatures and the volume of unfrozen soil controlled seasonal and interan-nual variations in respiration, while photosynthesis remained relatively constant in an old black spruce stand (Goulden et al., 1998). Over a 9-year period in a northern hardwood forest (Barford et al., 2001), drought increased annual C sequestration by depressing respiration rates while cool and cloudy summer weather decreased pho-tosynthesis and reduced annual C sequestration. That study also provided evidence that both growth rates and respiration depended on the C fixed in previous years suggesting that changes in canopy structure and function can be important on time scales of less than a decade. In recently harvested stands, the re-establishment of vegetation and decomposition of debris and underground biomass from the previous stand may exert a stronger influence on annual net C exchange than weather. In this study, net ecosystem exchange of C O 2 was measured for three years in a recently clearcut-harvested Douglas-fir stand (Pseudotsuga menziesii ssp. menziesii) on the east coast of Vancouver Island using the eddy covariance method. During this Chapter 4. Three years of NEP in a clearcut 93 period, interannual variations in weather and rapid changes in vegetation occurred with the growth of planted Douglas-fir seedlings, understory species from the previous stand, and pioneer species. This provided the opportunity to investigate the impact of varying weather, microclimate, and revegetation on respiration and photosynthesis processes after harvesting. 4.2 Methods 4.2.1 Stand description The recently clearcut-harvested Douglas-fir stand was located on the east coast of Vancouver Island, British Columbia (49°52.330'N, 125°17.537'W) in a 32-ha area that had been clearcut-harvested in winter 1999/2000 and planted with 1-year-old seedlings, 93% Douglas-fir and 7% western redcedar (Thuja plicata), at a density of 1600 stems h a - 1 in March 2000. The previous stand was unmanaged Douglas-fir established between 1932 and 1941. The stand was mechanically felled and hoe-chucked to the roadside, typically leaving behind more than 90% of the tops and limbs behind on the harvested area. Full length logs were then processed at the roadside leaving behind small diameter tops, bark, and trim ends. The harvested volume was 535 m 3 h a - 1 . Roadside debris piles were burnt in November 2000. No other management has been practiced since planting. The stand was located in the C W H x m l biogeoclimatic zone (very dry maritime Coastal Western Hemlock), 175 m above sea level on a gravelly fluvial level terrace with 0% to 2% slope. In the top 20 cm of the soil profile, the texture was loamy sand with 14% coarse fragments (> 2 mm in diameter) and about 8% organic matter. Below 20 cm to a depth of 1 m, the soil texture was sand with about 48% coarse frag-ments and generally less than 4% organic matter. The organic C and total nitrogen Chapter 4. Three years of NEP in a clearcut 94 in the surface was extremely variable and could be as much as 410 mg C g - 1 and 10 mg N g - 1 . Below 20 cm, there was generally less than 80 mg C g _ 1 dry soil and 2 mg N g _ 1 dry soil. The soil was very well drained with an available water content of 140 mm. A site index of 32, the expected height in m of 50-year-old coastal Douglas-fir, indicates the site was relatively productive for tree growth. 4.2.2 Eddy covariance measurements of fluxes A continuous record of fluxes, weather, and microclimate variables began on Septem-ber 1, 2000 and continued for three years. The eddy covariance (EC) technique was used to measure half-hour fluxes of C 0 2 (Fc, pmol m - 2 s _ 1 ) 3 m above the ground. From August 2000 until December 2001, the E C instrumentation consisted of a three-dimensional sonic anemometer (model CSAT3, Campbell Scientific Inc.) and an open-path C 0 2 / H 2 0 infrared gas analyzer (IRGA) (model LI-7500, LI-COR Inc., Lincoln, NE) installed on a boom 2 m from a triangular lattice tower 10-m tall. However both instruments did not function properly during wet conditions and were replaced with a Gi l l Instruments model R3 symmetric sonic anemometer-thermometer (Lymington, England) and a model LI-6262 closed-path IRGA (LI-COR) in January 2002. The LI-6262 was housed within an insulated box and air was sampled through a 3-m long insulated tube and filter at an unregulated flow of 6 to 9 L m i n - 1 , providing turbulent flow in the tube. The LI-6262 was run in absolute mode by flowing scrubbed nitrogen gas through the reference cell at 80 cm 3 m i n - 1 . Both sonic-IRGA configurations were designed to reduce the need for sensor-separation corrections by having the IRGA within 30 cm below the center of the sonic transducer array (Kristensen et al., 1997). E C fluxes were computed from the covariance of vertical wind velocity (w, m s - 1 ) and the appropriate scalar value or horizontal wind speed following procedures described by Chen et al. (1999). High frequency C 0 2 and H 2 0 mole fractions (xc and mmol Chapter 4. Three years of NEP in a clearcut 95 m o l - 1 moist air, respectively) measured by the closed-path I R G A were converted to mole mixing ratios (sc and sv, mmol m o l - 1 dry air, respectively). Air temperature, (T, K) was computed from sonic temperature, Tv and water vapour mole fraction, Xv using T = Tv/(1 + 0.32x„/1000). When the open-path I R G A was in operation, Fc and latent heat flux were computed from the covariance of w and the density of C O 2 and H 2 0 (pc and pv, mol m - 3 ) . The W P L procedure was then applied to remove the effects of air density fluctuations (Webb et al., 1980). The I R G A signals were adjusted for a delay resulting from the travel time of air down the sample tube. The delay was determined by maximizing the cross-correlation of fluctuations of Tv and sc or sv and was about 1.1 and 1.3 s for sc and sv, respectively. A 3-axis coordinate rotation of the wind vectors and covariances was applied (Tanner and Thurtell, 1969). Throughout the study, a CR23X datalogger (Campbell Scientific Inc. (CSI), Logan, UT) logged analogue signals at 8 Hz and output half-hour statistics. Beginning in February 2002, 20 Hz digital signals from the Gi l l R3 sonic anemometer were transmitted and stored on a low-power computer (SmallPC Computers, Oakville ON, Canada). Analogue signals of Xc and Xv, sample cell temperature and pressure from the LI-6262 IRGA were also input to the anemometer's analogue-to-digital converter. Fluxes calculated with 20 Hz vs. 8 Hz signals differed by less than 0.5% at night when stable or neutral atmospheric conditions existed and when the high frequency contribution to fluxes becomes more important (Kaimal and Finnigan, 1994). Net ecosystem exchange (NEE, pmol m - 2 s - 1 ) was calculated as the sum of Fc and the rate of change of C 0 2 storage in the air column below the height of the eddy flux measurements (zm), N E E = FC + z^oX/dt. (11) where is half-hour average C 0 2 mole mixing ratio (pmol mol dry a i r - 1 ) , t is time (s), Chapter 4. Three years of NEP in a clearcut 96 and ~p~a is the average mole density of dry air (mol dry air m - 3 ) . ds~c/dt was calculated for each half hour using the difference in for the previous and following half hours. sc was also measured 50 cm above the forest floor for 10 months in 2001 using a model LI-800 IRGA (LI-COR). Storage change was less than ± 3 pmo\ m - 2 s _ 1 at all times. Storage change calculated using the C O 2 measurements at the E C height and 50-cm height was only 5% greater on average than the calculation using only the measurement at the E C height (R 2 = 0.79). Positive N E E values indicate C O 2 release to the atmosphere. A l l IRGAs were calibrated with zero gas (nitrogen) and a known C O 2 span gas once per month. Calibration for water vapour was conducted a few times per year in the laboratory using a dew point generator (model LI-610, LI-COR) and monitored in the field by comparing measurements from a nearby humidity sensor (model HMP45C, CSI). Quality control procedures involved despiking and removing short calibration sec-tions from high frequency signals before computing eddy statistics when high fre-quency data were available. Fluxes associated with statistics (min, max, variances) which did not fall within reasonable limits (e.g., < 100 pmol m o l - 1 ) and/or oc-curred during instrument malfunction were removed. Measurements made during rain or snowfall were also excluded when the LI-7500 I R G A or CSAT3 anemometer were deployed. Fluxes were also excluded when, during wet conditions, temperature statistics from the sonic anemometer did not closely match those made using a 25 pm. fine-wire chromel-constantan thermocouple. Fluxes were not rejected on the basis of wind direction since there were no large obstructions and there was sufficient area of homogeneous stand within the typical flux footprint (area which includes 80% of the total eddy flux, see Chapter 2 ). 400 m of fetch was available in both the N E and SW directions (prevailing wind directions) while a minimum fetch of 80 m to the N W and Chapter 4. Three years of NEP in a clearcut 97 120 m to the SE was still sufficient for flux measurements. These procedures removed 33% of N E E measurements over the three-year period. 4.2.3 C o m p a r i s o n of fluxes from open- and closed-path I R G A s A n on-site comparison of Fc and latent heat flux (XE) measured using the open-and closed-path IRGAs was conducted between July 4 and August 5, 2003. During rain-free and dew-free conditions, all daytime and nighttime Fc derived from the open-path I R G A were larger than those from the closed-path IRGA: FCt0p = 1.08FC ! C p — 0.2 pmol m~2 s - 1 , R 2 = 0.95, n = 1370 and for latent heat flux, XE^ = 1.12XEcp + 0.4 W m~ 2 , R 2 = 0.98, n = 1370. On average, Fc>op was 2% greater than FCyCp at night and 6% greater during the day. The differences were greater for XE where XECj0p was 10% greater than XEc>cp at night and during the day. A comparison of the cospectrum for sensible heat (CWT) with the cospectrum for C O 2 measured using the closed-path IRGA (CWSC) (Kaimal and Finnigan, 1994) is shown in Fig. 29. CWT is assumed to represent the spectral energy distribution associated with the turbulent transfer of a scalar quantity with the least number of sensor effects (Moore, 1986). Lower C W S C in the high frequency range suggested that the smaller fluxes measured with the closed-path IRGA were a result of reduced high frequency response likely due to imperfect turbulent flow within the sampling tube and/or interactions between the C O 2 and H 2 0 molecules and the walls of the sampling tube. Following a similar procedure to the one described in Aubinet et al. (2000), transfer functions were developed using the ratios of C W S C to CWT for a few days each month in order to characterize the high frequency flux losses due to sensor separation and attenuation of fluctuations of sc down the sampling tube. Since the largest Fc occurred at night, nighttime periods were selected when the sensible heat flux was more negative than -25 W m - 2 , Fc was greater than 1 pmol m~ 2 s - 1 and u* Chapter 4. Three years of NEP in a clearcut 98 was greater than the threshold friction velocity (u*th) of 0.08 m s - 1 (see Chapter 2 ). Fig. 29 illustrates an example of the transfer function developed for 47 nighttime half hours between May 4 and May 9, 2003. The transfer function (TF) was expressed as, T F ( / ) = e x p ( - ( / / a ) 6 ) (12) where / is the frequency (Hz), and a and b are coefficients fit to the ratio of fCWSc/w's'c to normalized JCWTJW'T' . Over the 21 months the closed-path I R G A was in opera-tion, the cut-off frequency (fco, i.e., / where T F is 2 - 0 - 5 = 0.71) was on average 0.75 Hz. In general, the proportion of FC attributable to frequencies greater than 0.1 and 1 Hz were 27% and 2%, respectively. 0.5 a) 0.4 0.3 ^ 0.2 O 0.1 1 tC /covlws) ws v c c fCwl/cov(wT) b) 10" ' (H z ) 10 u Figure 29: Transfer function (TF) (panel a) and ensemble average cospectra for sensible heat (fCWT/cov(wT)) and for C O 2 (fCWSc/cov(wsc)) (panel b) illustrating high frequency loss of w's'c for 47 nighttime half hours between May 4-9, 2003. The cut-off frequency (/ c o) of the T F is 0.73 Hz and T F parameters are a = 0.047, b = 0.994. Chapter 4. Three years of NEP in a clearcut 99 A correction factor (CF) was calculated as the ratio of w'T' to w'T' degraded by the transfer function described above (Aubinet et al., 2000). Small changes in the transfer function and fco from one period of the year to the next were not related to changes in sensor setup or in air flow rates through the sampling tube. Throughout the study, the C F applied to closed-path measurements of FC was related to half-hour wind speed (u, m s _ 1 ) using a T F with fco of 0.75 Hz as follows, C F = 0.05u + 1.00. On average, this increased the magnitude of FC by 4% but could be as large as 25% for the maximum observed wind speed of 4.9 m s _ 1 . The relationship between FC measured with the open and closed-path IRGAs was improved with the application of the C F to FC>CP, where F C T 0 P = 1.04FC ] C p - 0.2 pmol m~2 s _ 1 , R 2 = 0.93, n = 1385. At night, F C J O P was 99% of F C I C P and during the day, it was 102%. 4.2.4 Energy budget closure Energy budget closure was used to check that the E C flux measurements remained consistent throughout the three-year period. Sensible (H) and latent (XE) heat fluxes were measured using the E C technique and available energy (Ra) was calculated as the difference between net radiation and the sum of soil heat flux and the rates of change of energy storage in the biomass and air column (for further details on these calculation procedures, see Humphreys et al., 2003). XE measurements made with the closed-path IRGA were adjusted using a C F , where C F = 0.07u + 1.01 where u is in m s _ 1 . The slope of the regression for rain-free half-hour H and XE against Ra was very similar each year at about 90% (Table 14). These results suggested that there were no significant changes in the quality of the E C flux measurements throughout the three years. Chapter 4. Three years of NEP in a clearcut 100 Table 14: Energy budget closure results for rain-free half hours for each year (September 1 to August 31) are indicated by the slope and intercept of the regression between H + XE and Ra. The number of half-hour measurements used in the linear least squares regression is indicated by n. Year Slope Intercept R 2 n (W rn" 2) 2000-2001 0.91 -0.8 0.87 11675 2001-2002 0.89 -3.5 0.91 12821 2002-2003 0.89 -7.3 0.93 13225 4.2.5 Weather measurements Air temperature and relative humidity were measured using a shielded thermistor and humidity sensor (model HMP45C, CSI) 3 m above the ground. Air saturation deficit (D, kPa) was computed using the average half-hour water vapour mole fraction mea-sured using the IRGA, air temperature and barometric pressure (model PTA427A, Vaisala Inc., London, ON). Radiation components, including total and diffuse down-welling photosynthetically active radiation (PAR, pmol m - 2 s _ 1 ) , were measured 8 m above the ground using quantum sensors (model LI-190SA, LI-COR and model BF2 Sunshine Sensor, Delta-T Devices Ltd., Cambridge, U K ) . Soil temperature (Ts, °C) was measured continuously with a single profile of copper-constantan thermocouples buried 2, 5, 10, 20, 50, and 100 cm below the soil surface. Manual measurements from 30 additional thermocouples (10 each at 2, 5, and 10 cm below the surface), were made every 2-3 weeks between May and December 2002. These measurements were used to linearly adjust the continuous Ts measurements made at the top three depths of the profile to provide an estimate of the average Ts for the site. The average adjustment to 2, 5, and 10 cm T"s was a reduction of 20%, 5%, and 2%, respectively. Rainfall was measured using two tipping-bucket raingauges (model 2501, Sierra Misco, Berkeley, Chapter 4. Three years of NEP in a clearcut 101 C A ; Model 4000 Jarek Manufacturing Ltd., Sooke, B C , Canada). Soil volumetric water content (6V, m 3m~ 3) was measured continuously using six water reflectometers (model CS615, CSI) at 4 depths (2x 3-5 cm, 2x 10-20 cm, l x 45-60 cm, and l x 70-100 cm). As with Ts, 10 manually measured sets of time domain reflectometry (TDR)-type moisture probes gave a spatial representation of the average 0-20 cm and 0-60 cm 6V (for further details, see Chapter 2 ). These 10 replicates were used to reduce the values continually measured by the reflectometers by about 25%. 4.2.6 Analysis and gap-filling Net ecosystem production (NEP, pmo\ m~ 2 s - 1 ) was taken as the negative of N E E assuming negligible C losses from fluxes of methane and the drainage of dissolved organic and inorganic C from the soil. N E P is approximately equal to the difference between gross ecosystem photosynthesis (GEP) and ecosystem respiration (Re), i.e., N E P « - N E E = G E P - Re, where positive N E P and G E P indicate uptake of C 0 2 by the ecosystem and positive Re indicates loss of C 0 2 from the ecosystem. Nighttime measurements made under calm conditions were removed from all analyses. These periods were selected as those half hours when the measured friction velocity (u*) was less than a threshold value (u*ith) of 0.08 m s _ 1 (Chapter 2 ). G E P was calculated from daytime N E P measurements using estimates of daytime Re derived from the annual relationship developed between nighttime N E P (= —Re) and 2-cm Ts (all nighttime half hours during the specified year that meet u*jh requirements). Short gaps in the N E P record of 2 h or less were filled using linear interpolation. Large gaps were filled with the annual relationship between Re and Ts for nighttime periods and with the difference between estimated G E P and Re for daytime periods. G E P was estimated using a rectangular parabolic relationship between G E P and P A R developed for June and July and adjusted for the rest of the year by regressing Chapter 4. Three years of NEP in a clearcut 102 measurements against model estimates for a window of time 100 consecutive half-hours wide, moved in an increment of 20 half hours at a time. Non-linear and linear least squares regression were used to evaluate the relation-ships between C exchange and biophysical variables. Significance and significant differences between linear regression models were determined following Zar (1984). Functional regression was used to evaluate the relationships between fluxes and mod-elled results, where neither variable needs to be considered known (or error-free). A l l relationships were developed using unfilled data. Estimates of the random error for annual sums of G E P , Re, and N E P were de-scribed using the random error of a sum associated with a 20% measurement error on each half-hour flux measurement (Morgenstern et al., 2003). The magnitude of systematic biases resulting from the methods used to reject and replace half-hour measurements were explored by Morgenstern et al. (2003). They found that the absolute value of the annual sums significantly differed among methodologies, while interannual differences remained relatively constant. 4.2.7 Stand measurements Leaf area index (LAI) was measured using a relative frequency method where a metal probe (5 mm in diameter) was vertically inserted through the foliage at 20-cm intervals over 10 permanently established 40-m long transects in 2001, 2002, and 2003. In 2000, the vegetation was small enough to use a grid of 25 points, evenly distributed, over a 50 cm by 50 cm frame every 5 m along the same 10 transects, which extended 50 m that year. L A I was computed as the ratio of the number of probe contacts with vegetation to the total number of probe insertions (20/transect). These measures of L A I represent the projected area of foliage per area of ground (m 2 m~ 2). However, when the foliage is at any angle other than parallel to the ground, this method Chapter 4. Three years of NEP in a clearcut 103 underestimates LAI (Wilson, 1959). Since LAI was only intermittently measured, a daily value of LAI was estimated by interpolating measurements and adjusting values to reflect digital images of the vegetation cover recorded throughout the study. Percent cover was estimated using the ratio of the number of probe insertions which contacted at least one leaf to the total number of probe insertions. Douglas-fir height and basal stem diameter were measured at the end of each growing season using six 3-m-wide, 100-m-long transects. 4.3 Results 4.3.1 Stand growth The vegetation changed considerably during the three-year period (Table 15). In the fall of 2000, LAI was 0.2 and the only vegetation was small 1-year-old seedlings and sparse understory vegetation left from the previous 60-year-old stand harvested only 8 months earlier. By the end of the 2003 growing season, the 3-year-old seedlings were Table 15: Stand characteristics determined at the end of each growing season. Maximum leaf area index (LAI) includes all species growing on the site. Height and diameter values for the Douglas-fir seedlings are averages with 1 SE (standard error) of the mean in parentheses. 2000 2001 2002 2003 Douglas-fir height (cm) N/A 40 (2.0) 62 (2.0) 88 (3.2) Douglas-fir basal diameter (cm) N/A 1.02 (0.05) 1.53 (0.04) 1.91 (0.07) Maximum LAI 0.24 (0.02) 0.76 (0.13) 2.17 (0.11) 2.53 (0.13) Maximum percent cover (%) 22 (2.0) 45 (5.5) 86 (2.4) 77 (2.1) on average 0.9 m tall with some trees as tall as 1.75 m and an abundance of pioneer species resulted in an LAI of 2.5 (Table 15). Some of the more common pioneer species were bracken fern (Pteridium aquilinum), fireweed (Epilobium angustifolim), Chapter 4. Three years of NEP in a clearcut 104 and various asters and grasses. Other species which increased in abundance during the three years had persisted from the previous stand and included Oregon-grape (Mahonia nervosa), salal (Gaultheria shallon), sword fern (Polystichum munitum), twinflower (Linnaea borealis), bearberry (Arctostaphylos uva-ursi), bedstraw (Galium triflorum), and various Rubus spp. 4.3.2 Weather Weather variables such as downwelling PAR, diffuse P A R , T, and rainfall followed the same general seasonal trends but varied in magnitude between years (Table 16, Fig. 30). Consequently, Ts and 8V also differed between years. 2002-2003 was the warmest year due to an exceptionally warm winter (October 2002 to February 2003). The lowest annual rainfall occurred in 2000-2001 due to a drier than normal win-ter. However, the summer of that year (June - August 2001, inclusive) was cooler, cloudier and wetter than the other two summers with an average daytime P A R of 610 pmol m~ 2 s - 1 , total rain of 167 mm and an average 8V of 0.12 m 3 m - 3 . In contrast, only 35 mm of rain fell during the same period in 2003, resulting in the driest soil conditions of the study period with an average 8V of 0.07 m 3 m~ 3 . 4.3.3 Diurnal, seasonal and interannual variations in N E P Fig. 31a and b shows the seasonal variations in weekly average N E P for 24-h, daytime, and nighttime periods for the three years. Each year, the greatest rates of 24-h C O 2 uptake (positive NEP) were observed in June or July when L A I was greatest (Fig. 31c). The maximum half-hour daytime N E P (7-day average) increased from near zero at 0.5 pmol m - 2 s - 1 in 2001, to 1.2 pmol n T 2 s" 1 in 2002, and to 2.4 pmol m~ 2 s - 1 in 2003 (Fig. 31b). The magnitude of nighttime N E P followed the trends in Ts and T (Fig. 30) with the most negative values generally in August or September. In Chapter 4. Three years of NEP in a clearcut 105 Table 16: Weather statistics during the three year study. Each year is taken from September 1 to August 31. Mean downwelling PAR is for the daytime only, while all other values are for 24-h. Standard errors of the means are in parentheses. Mean T Mean Mean PAR Total rain Mean 9V 2-cm T3 (°C) (°C) (/rniol m~ 2 s~x) (mm) m 3 m~ 3 Annual: 2000-01 8.2 (0.05) 8.7 (0.06) 460 (5) 973 0.168 (0.0003) 2001-02 8.4 (0.06) 8.6 (0.06) 504 (5) 1224 0.160 (0.0004) 2002-03 9.3 (0.05) 8.9 (0.05) 505 (5) 1293 0.144 (0.0004) Summer (June - Aug, inclusive): 2000-01 15.1 (0.07) 17.2 (0.06) 610 (9) 167 0.118 (0.0007) 2001-02 16.8 (0.08) 17.6 (0.06) 728 (10) 88 0.091 (0.0006) 2002-03 17.1 (0.08) 17.9 (0.06) 740 (10) 35 0.071 (0.0004) Winter (Oct - Feb, inclusive): 2000-01 3.5 (0.05) 2.7 (0.04) 226 (5) 584 0.191 (0.0003) 2001-02 3.5 (0.05) 2.9 (0.04) 231 (5) 916 0.202 (0.0002) 2002-03 4.8 (0.05) 3.2 (0.04) 246 (5) 843 0.176 (0.0004) the winter of 2002-03, NEP was more negative at night than previous years while daytime NEP was less negative than previous years. This was also the winter when air temperatures were unusually warm (Fig. 30) and LAI estimates remained at about 0.5 (Fig. 31c). Fig. 32 shows the ensemble average of the diurnal cycle of C O 2 exchange in June and in March of each year. In June, the largest net uptake of C O 2 (or least net loss of CO2 ) occurred around 9 am PST in all three years. This coincided with relatively high PAR, while Ts was still low. As Ts increased to its peak value around 3 or 4 pm PST, NEP decreased. Diurnal cycles of NEP in June were clearly affected by the increase in vegetation growing on the site between 2001 and 2003. NEP was negative throughout the day in June 2001 when LAI was about 1. In contrast, daytime NEP values in Chapter 4. Three years of NEP in a clearcut 106 ^ 1 0 0 0 2 0 o o 1 0 h-0 1 5 0 E~ E, 1 0 0 c 'co 5 0 DC 0 0 . 2 i E CO 0.1 E, CD 0 S N J M M J S N J M M J S N J M M J 2 0 0 0 2 0 0 1 2 0 0 2 2 0 0 3 Figure 30: Weather conditions between September 1, 2000 and August 31, 2003 including 7-day average daytime downwelling PAR (thick line) and diffuse PAR (thin line; top panel), 7-day average above-canopy air (T, thick line) and 2 cm soil temperature (Ts, thin line; 2nd panel), 7-day total rainfall (third panel), and 7-day average soil volumetric water content for the 0 to 20 cm depth (8V; bottom panel). June 2002 and 2003 were as large as 2.5 and 4 pmol m~2 s'1 (Fig. 32) when LAI was about 2.2 and 2.5, respectively. At night, NEP was increasingly more negative in each subsequent year likely due in part to increasing vegetation and increasing Ts. Chapter 4. Three years of NEP in a clearcut 107 I E 3 LU CO I o E LU —3 2 0 -2 -4 -6 Day -Night -A B )-o A C) 2 < ^ A 1 • _ _ • ^ ' x _ _ " S N J M M J S N J M M J S N J M M J 2000 2001 2002 2003 Figure 31: Average (7-day, 24-h) daily NEP (panel a) and half-hour NEP for daytime and nighttime periods (panel b) between September 1, 2000 and August 31, 2003. Panel c shows the measurements of LAI (A) and estimated LAI (dashed line). On average nighttime NEP was -2.8 //mol m 2 s 1 in June 2001, -4.8 //mol m 2 s 1 in June 2002, and -6.2 /xmol m"2 s"1 in June 2003 (Fig. 32). No net uptake of C 0 2 occurred in March in any year. At night, more negative NEP in 2001 and 2003 appeared to be due to the higher Ts in those two years. Chapter 4. Three years of NEP in a clearcut 108 Figure 32: Ensemble average of hourly N E P in June and March, 2001, 2002, and 2003. Hourly averages include only measured N E P . At night N E P measurements were excluded from the averages when u* < u*jh- Error bars are ± 1 standard error of the mean. In 2001, NEP became more negative (i.e., Re increased) during the daytime as Ts increased. The daytime loss of C O 2 in 2002 and 2003 was less than in 2001 and was not as strongly related to Ts. This suggests that C O 2 uptake by the available evergreen vegetation such as Douglas-fir, salal, and Oregon-grape was occurring in the early spring of 2002 and 2003, partly offsetting the increase in respiration expected to occur with increasing Ts during the daytime. Chapter 4. Three years of NEP in a clearcut 109 4.3.4 Relationships between nighttime N E P (respiration) and environ-mental factors To quantify the relationship between Re and Ts, nighttime measurements of N E P (= —Re), which met the u * ^ criterion were related to 2, 5, and 10-cm Ts for each year. Fig. 33 and Table 17 show the results for the 2-cm Ts. The response of Re to Ts o E zi. 10 2 - c m T (°C) 20 Figure 33: Relationships between ecosystem respiration (Re) and 2-cm soil temperature (Ts) for all nighttime measurements made when u* exceeded u*jh for the 3 years (see Table 17). Each data point is the average of 100 measurements after sorting Re with Ts. Standard errors of the mean were about 1.1 //mol m~ 2 s _ 1 but are not shown. was assessed using ln i ? e = A + BTS. The results of this analysis are shown in Table 17 using Re — RwQ (T s-10)/10 10 (13) where Qio = exp(105) and Re at 10°C, Rw — Q i o e x p A The logarithmic trans-formation allowed the assumption of normality and homoscedasticity to be met for linear least squares regression (see Chapter 2 for more details). Chapter 4. Three years of NEP in a clearcut 110 The R 2 for each year was relatively low but the relationships were significant (P < 0.001). R 2 values varied less than 0.01 wi th the use of different Ts; however, both Rio and Qio increased wi th decreasing depth of Ts. For example, in 2001-02, the 10-cm Ts resulted in an Rw of 2.1 pmol m~2 s _ 1 and a Qi0 of 2.0 while the 2-cm Ts resulted in an Rio of 2.5 pmol m ~ 2 s - 1 and a Qw of 2.2. In addition, the relationships developed using Ts at the 2-cm depth accounted for the temperature-induced variations in daytime N E P better than the other relationships using the other depths (see next section). Table 17: The parameters for the exponential relationship (Eq. 13) between Re (fiimol m ~ 2 s _ 1 ) and 2-cm Ts (°C) for each year. The coefficient of determination (R 2 ) and the number of samples (n) for each regression are also given. Re was obtained from nighttime N E P measurements u+jh — 0.08 m s _ 1 . Year Rw pmol m ~ 2 s _ 1 Qw R 2 n 2000-01 2.1 1.7 0.13 1317 2001-02 2.5 2.2 0.28 1596 2002-03 3.2 1.8 0.15 1622 The relationships between Re and Ts significantly differed among the three years (F = 55, df = 4, 4529, P < 0.0001). However, al l the Qw values were near 2.0 and in 2000-01 and 2002-03, the Qw values were not significantly different (P < 0.05). The Rio values increased each year from 2.1 to 2.5 to 3.2 pmol m~2 s _ 1 . Al though 2002-03 had the highest Re values, this year also had the most prolonged and severe drought of the three years. This resulted in a noticeable decrease in Re wi th 6V less than 0.07 m 3 m - 3 . However, this trend was more noticeable using estimates of daytime respiration rather than using Re (see next section, F i g . 35). Chapter 4. Three years of NEP in a clearcut 111 4.3.5 Relationships between daytime N E P and environmental factors The main biophysical factors which have been shown to affect daytime N E P include P A R (diffuse and direct), temperature, D, 9V, and, in the case of deciduous or mixed forests, L A I (e.g. Law et al., 2002). Fig. 32 showed that the daytime variations in N E P in June 2001, 2002, and 2003 appeared to relate to both P A R and Ts, while differences between years reflected differences in LAI . Fig. 34 illustrates the relationships between daytime N E P and P A R and G E P and P A R for the same time periods. Rectangular parabolic light response functions were used to parameterize the two relationships, N E P = - Rd (14) a P A R + NEPoo v 1 _ Q G E P Q Q P A R G E P " a P A R + GEPQQ ( 1 5 ) where a is the apparent quantum yield for either N E P or G E P , NEPQQ and GEPQO are the asymptotic values of N E P and GEP, respectively, and Rd is the intercept of the N E P - P A R relationship which estimates the average daytime respiration (Suyker and Verma, 2001). G E P was calculated as N E P - Re, where Re was estimated us-ing the annual temperature relationships shown in Table 17 in order to remove the dependency of N E P on Ts through its effects on Re. N E P and G E P measured in June 2001, 2002, and 2003 were clearly related to PAR; however, the strength of this relationship improved each year (as seen by the increasing R 2 ) (Table 18). P A R explained a greater proportion of the variability in G E P than in N E P for all three years. Rd increased each year from 3.0 to 4.4 to 4.8 pmol m~ 2 s - 1 (Table 18). The increase in R^ may be related to the increase in Rio observed each year (Table 17). It may have also been related to the increase in average daytime 2-cm Ts measured Chapter 4. Three years of NEP in a clearcut 112 Figure 34: Daytime N E P and G E P against downwelling P A R for the month of June in 2001, 2002 and 2003. Each data point is an average of 50 half-hour values. The light response of the half-hour measurements before averaging are described using Eq. 14 and Eq. 15 and are shown as solid lines (parameters appear in Table 18). Following Suyker and Verma (2001), Rd was determined for consecutive three-day periods throughout the three years. Rd was generally lower than the average measured nighttime Re. The linear regression between Rd and Re as the independent variable resulted in Rd — 0.5i? e + 1.2 pmol m ~ 2 s - 1 , R 2 = 0.32, n = 268 for periods when E q . 14 could be fit to the data. These calculated estimates of respiration are less influenced by low atmospheric turbulence. Consequently, the relationships between Rd and Ts were similar to those found for Re, but clearly showed the influence of late summer drought on Rd, particularly in July and August of 2003 (Fig. 35). The increase in the fit and magnitude of the parameters in both the relationships Chapter 4. Three years of NEP in a clearcut 113 Table 18: Parameters for the relationships between downwelling P A R and daytime N E P (Eq. 14) and G E P (Eq. 15) for the month of June in 2001, 2002, and 2003. 2001 2002 2003 N E P - P A R relationship: a 0.019 0.058 0.077 N E P ^ 3.0 7.3 9.5 Rd 3.1 4.7 5.8 R 2 0.23 0.44 0.53 G E P - P A R relationship: a 0.009 0.036 0.038 G E P ^ 3.5 8.9 10.5 R 2 0.44 0.73 0.78 <M I o E DC 2 1 0 o e > 0.1 m 3 m " 3 . • 0.01 < 9 < 0.1 m 3 rn"3 * e < 0.07 m 3 m 3 V B B O O • O o ° % • • 0 0 % 0 * * c o * 0 0 ° o o ° ° o o 0 o o ° u ,*» * • 0 ° o » V ci Co n * o o 00CL a * * 8 J> ° ° c o c® w O • * " o 0 g o o 10 20 2-cm 7 s(°C) co CNJ I E o E «_* DC • a ° o Be 0 * • • ~ B O B • * ** -& D ** O O % * * O • 0 0 * * 8 %a * * • 0 % °csS 0 * " V 0 ° * ^ % ° 0 0 0 0 0 10 20 Figure 35: Average 3-day Rd versus Ts (a) and Re versus Ts (b) sorted by 0-20 cm soil volumetric water content (6V) between September 1, 2002 and August 31, 2003. Chapter 4. Three years of NEP in a clearcut 114 between PAR and NEP and GEP (Fig. 34 and Table 18) were related to changes in LAI. In June 2002, LAI was between 1.8 and 2.2, only slightly lower than LAI in June 2003 (1.9 and 2.5), while in June 2001, LAI was approximately 1.0. Figure 36: The relationships between daytime G E P and P A R (Eq. 15) for the growing season months of 2001, 2002 and 2003 (see Table 19). Each data point is an average of 50 half-hour values. Measures of L A I are listed for the end of each month if available (NA if not available). Variations in the relationship between GEP and PAR during the growing period of each year are shown in Fig. 36 and Table 19. GEPQO increased with increasing LAI in all three years (Fig. 37). In 2001, Eq. 15 did not fit variations in GEP (R2 < 0.01) until May. In the following two years, Eq. 15 fit GEP well year-round (not shown) with monthly R 2 between 0.05 and 0.86. For a given month, both GEPQO and a were generally greater than the preceding year, particularly in the spring. In August of each year, senescence of the deciduous vegetation resulted in lower G E P Q Q but greater Chapter 4. Three years of NEP in a clearcut 115 Table 19: Parameters for the relationship between daytime GEP and PAR (Eq. 15) for the growing season months of 2001, 2002, and 2003. Values of GEPoc are in /xmol m~2 s_ 1. 2001 2002 2003 a GEPoo R 2 a GEPoo R 2 a GEPoc R 2 March 0.007 0.9 0.34 0.020 2.6 0.75 April 0.006 2.1 0.50 0.017 4.1 0.82 May 0.010 1.0 0.11 0.013 4.7 0.60 0.026 6.9 0.84 June 0.009 3.5 0.45 0.034 9.0 0.73 0.037 10.5 0.78 July 0.036 5.2 0.76 0.030 10.8 0.77 0.054 9.4 0.86 August 0.023 4.2 0.57 0.063 7.2 0.82 0.088 6.7 0.82 OL. This decrease in LAI between July and August was greatest in 2003, likely due to much greater moisture stress that year (Fig. 30). As a result, August 2003 GEP and LAI were lower than August 2002 GEP and LAI (Fig. 36). C 0 2 uptake occurred in late winter/early spring in 2002 and 2003 as shown by the weak relationships between GEP and PAR for March (Fig. 36). 12 0 n • • i 0 1 2 3 LAI Figure 37: The relationship between monthly GEPoo and available measurements of LAI for all three years where, GEPoo = 3.9LAI + 0.7 //mol m"2 s"1, R 2 = 0 75 Values of GEPoo and LAI are from Table 19. Chapter 4. Three years of NEP in a clearcut 116 Table 20: Partial correlation coefficients for half-hour G E P and various biophysical variables when P A R was greater than 800 fimol m ~ 2 s _ 1 . GEP in PAR T PAR D LAI 9V (total) (diffuse June 2002 0.05 0.20 0.42 -0.08 0.41 -0.43 July 2002 0.20 0.37 0.56 -0.23 0.04 -0.29 June 2003 0.35 0.08 0.46 -0.21 -0.13 -0.48 July 2003 0.01 0.10 0.38 -0.07 0.45 -0.29 Day-to-day variability in NEP was largely attributable to seasonal variations in LAI and daily variations in PAR and Ts. With much of the effect of temperature removed, day-to-day variability in GEP was mostly dependent on LAI and PAR (Fig. 36). Any effects of diffuse PAR, D, and 6V were difficult to discern as they were highly correlated to each other and to LAI at this site (Figs. 30 and 31). However, a noticeable decrease in GEP with PAR greater than 1000 pmol m~2 s _ 1 was observed in June and. July 2003. By excluding measurements of GEP when PAR was less than 800 pmol m~2 s _ 1, the effect of PAR on GEP was limited and allowed relationships between GEP and other environmental variables to be examined. The partial correlation coefficients between half-hour GEP and a number of environmental variables for June and July in 2002 and 2003 are shown in Table 20. Although partial correlation analysis indicates the correlation between a pair of variables while holding all the others constant (Zar, 1984), any non-linearity in the other variables can influence the results. Table 20 shows for PAR > 800 pmol m~2 s"1, GEP did not correlate better with any particular environmental factor. In addition, in July 2002 and June 2003, GEP was still well correlated with PAR. In all cases, GEP was positively and relatively Chapter 4. Three years of NEP in a clearcut 117 strongly correlated with diffuse PAR, while GEP negatively correlated with D to a variable extent. A strong positive correlation between GEP and LAI was observed in June 2002 and July 2003. This correlation appeared to relate to the increase in GEP in June 2002 when vegetation was growing and leafing-out. In contrast, this correlation between GEP and LAI related to the decrease in GEP in July 2003 when the vegetation began to senesce early in July 2003 after a prolonged drought (Fig. 30). The strong negative partial correlation with 6V suggested that greater GEP correlated with decreasing soil moisture. Through the summer, and particularly in June, an increase in T, LAI, maximum PAR, and D coincided with a steady decrease in 6V and as a result, the negative partial correlation between GEP and 9V could be an artifact of time and increasing LAI and maximum PAR. A simple empirical model of NEP accounting only for the effects of LAI, PAR and Ts was developed using, •— LAI aGEP PAR T W O where a was 0.05, GEP^ was 11.2 pmol m"2 s"1 (R2 = 0.90, n = 1289 from June 15 to July 15, 2003 when L A I m a x was 2.5), and Rio and Qio are given in Table 17 for each of the three years. Using half-hour measurements of PAR, 2-cm Ts and LAI (Fig. 31), Eq. 16 produced half-hour estimates of net ecosystem production (NEP) for all three years, where NEP = 0.57NEP -0.4 pmo\ m~2 s _ 1, R 2 = 0.36, n = 24762. Much better agreement was found between the daily average NEP and the estimated value, NEP = 0.89NEP + 0 ,umol m"2 s _ 1, R 2 = 0.33, n = 516, where n is the number of days with at least 10 half hours of which met all the quality criteria (see methods section). The relatively low R 2 likely reflects the half-hourly and daily variation in NEP, which can be attributed to the effects of additional environmental variables Chapter 4. Three years of NEP in a clearcut 118 such as D as well as the uncertainty in estimating LAI between measurements, and the influence of turbulence on EC flux measurements. 4.3.6 Interannual variations in NEP, GEP, and Re Annual sums of NEP, GEP, and Re are shown in Fig. 38 and Table 21. In all three years, the stand was a large source of atmospheric C with annual values of NEP of -620 g C m~2 in the first year, -520 g C m~2 in the second year, and -600 g C m~2 in the third year. In each year since harvesting, both annual losses of respired C (Re) and uptake of C (GEP) increased (Fig. 38 and Fig. 39). However, the relative increase in GEP and Re was variable during the three years, resulting in a maximum difference in annual NEP of 100 g C m - 2 y _ 1 , which was between the first and second years. For example, although LAI and GEP reached higher values in 2002-03 than in the preceding years, a late summer drought resulted in early senescence and a decline in GEP (Fig. 39). In that same year, the combination of an exceptionally warm winter and higher Re for a given temperature than in previous years (Table 17) was associated with higher winter rates of respiration. Consequently, more C was lost in 2002-03 despite greater vegetation growth than in 2001-02. Table 21: Annual NEP, GEP, and Re between September 1, 2000 and August 31, 2003. Year NEP (g C m-2) GEP (g C m"2) Re (g C m"2) 2000-01 -620 220 840 2001-02 -520 530 1050 2002-03 -600 640 1240 The choice of gap-filling strategy had a great effect on the magnitude of NEP in Chapter 4. Three years of NEP in a clearcut 119 _ 1200 E o a> cu O) c CO x: o x LU o 900 600 300 -300 -600 2000-01 2001-02 2002-03 Figure 38: Annual N E P , G E P , and Re between September 1, 2000 and August 31, 2003. each year, but did not significantly affect the interannual differences. The error bars shown in F i g . 38 refer to two other estimates of annual N E P , G E P , and Re when 1) was decreased by half and 2) when a moving window method was used to adjust the annual relationship between Re and Ts (Chapter 3 ), similar to that for estimating seasonal variations in G E P . In contrast, the random error associated wi th a 20% error on each half-hour measurement and gap-filled estimate of N E P is less than 5 g C m ~ 2 y _ 1 . 4.4 Discussion This study demonstrates the dynamic nature of C O 2 exchange between a regenerating Douglas-fir stand and the atmosphere. As expected, the stand was a large source of atmospheric C O 2 during the first three years after harvesting. The continuous E C measurements of C O 2 fluxes provided evidence that the regrowth of vegetation and interannual variations in weather both influenced annual N E P at this stage of stand Chapter 4. Three years of NEP in a clearcut 120 Figure 39: Annual courses of seven-day average 24-h NEP, GEP and Re September 1, 2000 and August 31, 2003. development. Of the few studies which have examined NEP in recently disturbed forests, most have focused on the growing season months. From May to the end of August, the aver-age NEP in this 1 to 3-year-old Douglas-fir stand was -1.6 , -1.4, and -1.3 g C m - 2 d _ 1 for 2001, 2002, and 2003, respectively indicating a net loss of C in all three years. Chapter 4. Three years of NEP in a clearcut 121 However, the year-round range in daily NEP varied from a small net uptake of C (0.8 g C m - 2 d _ 1 ) in June 2003 to a high net loss of C in August 2001 (-4.9 g C m~2 d"1). The average summer NEP of a 1-year-old boreal aspen stand (-1.6 g C m~2 d - 1 ) on a harvested site and a 1-year-old boreal jack pine stand (-0.8 g C m~2 d - 1 ) on a burned site (Amiro, 2001) were similar to the results obtained here. In contrast, greater average net losses of C were observed in a 5-year-old stand of Scots pine in southern Finland (-2.5 to -4.0 g C m~2 d _ 1) (Rannik et al, 2002). In northern B.C., average NEP was 0.3 g C m - 2 d _ 1 5 years after clearcut-harvesting, while in the sixth year, the sub-boreal white spruce and lodgepole pine stand became a C source with an average NEP of-0.94 g C m - 2 d _ 1 (Pypker and Fredeen, 2002a). Annually, our 1 to 3-year-old Douglas-fir stand lost between 500 to 620 g C m~2. This was about twice the annual loss from a 1-year-old French pine stand (200 to 340 g C m - 2 y_1)(Kowalski et al., 2003). Annual Re in this pine stand was about 1000 g C m~2 and was very similar to the values obtained for our 1 to 3-year-old Douglas-fir stand (800 and 1200 g C m~2). However, with very rapid regrowth in the pine forest, GEP had reached about 730 g C m - 2 y _ 1 within the first year and a half after harvesting, while the Douglas-fir stand only reached 640 g C m~2 in the third year after harvesting. These numbers indicate that site-to-site variability in NEP in recently disturbed forests can be large. Differences between sites relate to the complex interactions between site microclimate, nutritional status, debris and management, belowground and above-ground respiration processes, and the nature and speed with which the vegetation returns. Interannual differences in NEP as large as 110 g C m~2 y - 1 were observed in this study. As expected annual NEP was most negative in the first year immediately after harvesting (-620 g C m - 2 ) , but it was the second year (-520 g C m - 2 ) and not the third year (-600 g C m~2), which had the least negative annual NEP. Pypker Chapter 4. Three years of NEP in a clearcut 122 and Fredeen (2002a) observed differences in total growing season NEP in a mixed white pine and lodgepole pine stand 5 and 6 years after harvesting. They found that accumulated plant biomass and photosynthetic uptake of C were similar between years, while soil respiration rates were greater in the second year, possibly due to increased soil moisture. In the current study, the total annual Re also increased each year despite noticeable drought conditions during the summer of 2003. Greater Re was in part a result of greater Ts in each subsequent year, but was also a result of greater rates of Re for a given Ts. Since the increase in Qio was not significant between 2000-01 and 2002-03, there was no strong evidence that the source of respired C 0 2 shifted towards the aboveground biomass or to deeper or shallower soil depths with changes in rooting patterns. However, the increase in Rw over the three years indicated that either greater rates of decomposition or greater rates of maintenance and growth respiration were occurring in each subsequent year. Weber (1990) measured soil C 0 2 efflux in an immature aspen stand and found a temporary decline in soil C 0 2 evolution compared to control plots for two years after harvesting and burning. By the third year, soil C 0 2 evolution had returned to pre-treatment rates. This trend for soil C 0 2 efflux to increase in the early years after harvesting was also suggested in a jackpine chronosequence study by Striegl and Wickland (2001). Further, these authors and others (Amiro, 2001) have identified a trend in boreal systems in which soil respiration rates decline immediately after harvesting or fire, then increase to levels which exceed pre-harvest rates, followed by a gradual decline to mature-forest rates. At the ecosystem level, this pattern would be expected to be further enhanced with the addition of aboveground autotrophic respiration. In 2002, annual Re in the 3-year-old Douglas-fir stand was 68% of the Re in a nearby near-mature Douglas-fir stand (Chapter 3 ), suggesting a correlation between stand structural characteristics such as above and belowground biomass and Chapter 4. Three years of NEP in a clearcut 123 leaf area with stand functional characteristics such annual Re. In addition to changes in annual Re, annual GEP increased significantly in each year as the planted Douglas-fir seedlings, understory, and pioneer plants became es-tablished. The increase in vegetation was observed as a measurable increase in LAI each year. Growth of Douglas-fir seedlings, growth of species which were understory plants in the previous stand, and establishment of pioneer species all contributed to the increase in GEP. In late June 2003, 89% of the LAI was attributed to species other than Douglas-fir. Any vegetation management practices which affect the es-tablishment of plants on harvested stands such as planting, herbicide, or brushing, would have a large effect on annual GEP. Whether reduced vegetation would result in a similar reduction in Re warrants further investigation. The long-term effects of the growth of understory and pioneer species on the long-term C sequestration of this stand is unknown. Most models suggest that beyond a decade or two, growth of understory and pioneer species results in no net C uptake or loss (Li et al., 2003). However, at the seedling stage, competition and/or facilitation by the understory and pioneer species could either decrease or increase the growth of the Douglas-fir seedlings and ultimately, affect the longterm sequestration of C by the stand. As discussed earlier, interannual variations in weather have caused large interan-nual variations in NEP in late-successional forest ecosystems (Pilegaard et al., 2001; Barford et al., 2001; Carrara et al., 2003; Morgenstern et al., 2003). Interannual varia-tions in NEP are more difficult to attribute to weather in ecosystems where vegetation biomass and LAI varies seasonally and interannually largely as a function of weather. In this young Douglas-fir stand, respiration was the dominant C cycle process and depended primarily on temperature and to a small degree on soil moisture. PAR and LAI regulated daytime NEP year-round, providing evidence for photosynthetic uptake of C 0 2 even when daily NEP was still negative. By partitioning daytime NEP Chapter 4. Three years of NEP in a clearcut 124 into GEP and thereby removing the effects of temperature, PAR explained a greater proportion of the variability in GEP than it did for NEP. A significant drought in the summer of 2003 reduced the correlation between GEP and PAR somewhat and appeared to induce early senescence, reducing the magnitude of both GEP and Rd (and Re). Suyker and Verma (2001) presented C O 2 flux measurements from a na-tive tallgrass prairie which also illustrated the negative impacts of drought stress on both C uptake through photosynthesis and on C loss through respiration. The other noticeable weather anomaly was the warm winter of 2002-03. However, by applying the 2002-03 relationship between Re and Ts to Ts from Nov - Jan 2000-01, the total Re for that period only decreased by 12 g C m~2. Consequently, with more ever-green vegetation on site during the winter of 2002-03, the increased rates of C O 2 loss were mostly related to additional decomposition and autotrophic respiration. The influence of other biophysical variables on daytime NEP such as diffuse PAR and D followed the expected trends (Gu et al., 2002), but were not strong. This may have been a result of the correlation between D and diffuse PAR with other variables such as PAR, LAI, and Ts. Or, a mix of annual and perennial, deciduous and evergreen species with different physiological thresholds and responses may have smoothed out the response of NEP to high D or diffuse PAR. 4.5 Conclusions The annual NEP measured in a coastal Douglas-fir stand during the first three years after clearcut-harvesting ranged from -500 to -620 g C m - 2 y _ 1 . This is, to date, among the largest net emissions of atmospheric C recorded at a flux tower site (Law et al., 2002). During the first three years after harvesting, the net C lost to the atmosphere was as much as two times the net C gain measured in a nearby 35-Chapter 4. Three years of NEP in a clearcut 125 m-tall, 50-year-old Douglas-fir stand, where annual NEP was between 270 and 420 g C m - 2 y"1 (Morgenstern et al., 2003). The light response relationship for GEP varied seasonally and between years, with maximum GEPoo doubling between the first and third years as maximum LAI increased from 1 to 2.5. Annual GEP also increased from 220 g C m~2 y"1 in 2000-01 to 640 g C m~2 y"1 in 2002-03. Larger values of Rio in each year after harvesting also linked respiration rates to stand productivity. Despite greater LAI and GEP in the third year, warmer temperatures, drought and early senescence of the deciduous pioneer species resulted in more negative NEP than the previous year. Net atmospheric C O 2 exchange in this very young stand was related to growth of Douglas-fir, growth and establishment of understory and pioneer species, and interannual weather variability. Chapter 5. Conclusions 5 C o n c l u s i o n s 126 This study presents results from the first few years of concurrent long-term measure-ments of N E P in three Douglas-fir stands at different stages of development after harvesting. The 50 km proximity of the stands minimized the confounding effects of weather and enabled differences in stand structural characteristics such as stand biomass and leaf area, height, roughness, and species composition to be related to differences in microclimate and C O 2 fluxes between the forests and the atmosphere. The major findings of this study are summarized below. 1. There was evidence of advection of C O 2 in the 53-year-old stand, but not in the two younger stands. The combination of tall vegetation and sloping ter-rain produced the vertical gradients in C O 2 and air-flow patterns necessary for advection to reduce and enhance eddy fluxes of C 0 2 . 2. Momentum absorption by the rough, 35-m-tall canopy of the oldest stand re-sulted in a higher friction velocity (u„) for a given windspeed measured at the height of the eddy covariance instrumentation when compared with the two younger stands. The threshold friction velocity (u*ith) associated with sufficient turbulence for E C measurements to accurately measure the loss of C O 2 respired from the soil and the canopy increased with increasing stand height and aero-dynamic roughness in the three stands. The value of u*ittl was 0.08 m s _ 1 in the 3-year-old stand, 0.16 m s _ 1 in the 14-year-old stand, and 0.32 m s _ 1 in the 53-year-old stand. 3. In 2002, the 53-year-old stand was a moderate C sink (250 g C m~ 2 y - 1 ) , the 14-year-old stand was a small C source (-130 g C m - 2 y _ 1 ) and the 3-year-old stand was a large C source (-610 g C m~ 2 y - 1 ) . As expected, the magnitude Chapter 5. Conclusions 127 of NEP was proportional to the leaf area index (LAI) of the stand. Seasonal patterns of NEP were also very different between stands with days showing net C uptake throughout the year in the 53-year-old stand and maximum C uptake in spring. NEP followed the patterns in PAR and LAI in the 14-year-old stand with greatest uptake in June and July, and negative NEP during the fall and winter. In contrast, daily NEP were negative year-round in the 3-year-old stand with the least amount of C lost in June when LAI was maximum and the greatest amount of C lost in August and September, when temperatures were warm and the deciduous species were senescent. 4. Total annual ecosystem respiration (Re) was greatest in the oldest stand and least in the youngest stand, emphasizing that site-to-site differences in average or annual Re are not only a function of mean annual temperature but also of stand structure and productivity. Efforts to partition Re between soil and above-ground sources were confounded by the differences in the eddy covariance (EC) and chamber system's representation of spatial and temporal variations of C O 2 fluxes. By deriving Rw values from temperature response curves, the considerable spatial variability of Rs within a stand and between stands was found to correlate with the concentration of N within the soil/forest floor. 5. Variations in the diurnal and seasonal patterns of daytime NEP in all three stands in 2002 were largely a function of PAR and temperature. Seasonal variations in LAI played a major role controlling C exchange dynamics in the two younger stands where deciduous annual and perennial pioneer and understory species comprised a significant portion of the stand vegetation. In the two older stands, high air saturation deficit (D) was important in limiting daytime uptake of C. In general, the form of the response of daytime NEP to PAR, Chapter 5. Conclusions 128 D, and temperature under clear and cloudy conditions was similar between all three stands. However, the sensitivity of NEP to these environmental variables was always greatest in the 53-year-old stand. 6. Three years of C O 2 flux measurements in the clearcut stand emphasized the importance of vegetation growth on C exchange dynamics. Both GEP and Rio increased in each subsequent year after harvesting linking production and respiration processes. Annual NEP, however, did not monotonically increase as drought and warm temperatures in the third year resulted in greater C loss than in the second year. The results listed above agree with the general hypotheses regarding how C ex-change dynamics differ at different stages of succession (Chapter 1 ). GEP correlated well with the leaf area available for C assimilation by photosynthesis. Re also increased with increasing leaf area or biomass. Although this study attempted to control for weather, site characteristics such as elevation, soil nutrients, soil texture and avail-able water content, and harvesting and management practices varied slightly between stands. These factors have been shown in models to have a large role in determining succession and how C exchange processes change over time after disturbance (Thorn-ton et al, 2002). In the interest of accounting for and predicting the consequences of disturbances such as harvesting on regional C budgets, the data gathered in this and other similar monitoring studies will need to be used in the development and validation of remote sensing indices, and process and inventory-based models. The findings of this study indicate a number of areas which merit continued or additional research. First, the C exchange measurements in the youngest stand are some of the first multi-year observations made in a recently disturbed forest. These re-sults emphasize that a disturbed site can be an extremely large source of C and that Chapter 5. Conclusions 129 feedback effects between weather and vegetation are important in determining the magnitude of annual NEP. However, the impact of pioneer species on the long-term accumulation of C as well as on annual N E P is not known. Continued monitoring of N E P in this stand would be valuable while conducting chamber-based experiments controlling the abundance of pioneer and understory vegetation. In addition, con-tinued monitoring will provide data to investigate the factors which determine when leaf-level physiological responses, such as stomatal constraints on photosynthesis, will be observed in ecosystem-level C O 2 fluxes. Replicated experiments designed to mea-sure N E P in recently clearcut-harvested sites paired with undisturbed sites would enable testing of hypotheses regarding the nature of C loss after harvesting. Another area of study for further investigation is the impact of advection on C O 2 fluxes, especially in the 53-year-old stand. Evidence presented here suggested that without knowledge of all the terms in the conservation of mass equation for C O 2 , the sum of E C measurements of turbulent C O 2 flux and changes in storage from a sin-gle profile system can either underestimate or overestimate the biological source/sink strength. In Drewitt (2002), observations of continuous downslope flow beneath the canopy during both the day and night support the case for drainage losses of C O 2 . Techniques other than E C measurements could be used to quantify ecosystem C exchange and constrain E C flux measurements (Baldocchi, 2003). Chamber measure-ments of component C O 2 fluxes would require extensive sampling due to the high degree of heterogeneity of the forest floor (Chapter 2 ) and very likely, also of the vegetation. However, measurement campaigns could be carried out over shorter time periods. The biometric method of determining changes in C stocks over time (e.g. Barford et al., 2001) is being done by the Canadian Forest Service over a period of 5 years in the three stands. However, the effectiveness of this method will also be limited by the heterogeneity of the stand, but may provide a baseline for 10 or 20 year Chapter 5. Conclusions 130 resampling. Mesoscale modelling is currently underway to help elucidate the nature of air-flow above these stands (Kanda et al., 2004). Finally experiments designed to directly investigate advection processes within the stand such as those by Aubinet et al. (2003) will be important in providing confidence in EC-derived fluxes. Finally, EC measurements of nighttime NEP were used in this study to develop temperature-response curves and partition daytime measurements into GEP and Re. The small interannual differences in the relationship between Re and soil temperature in the youngest stand emphasized that changes in autotrophic and heterotrophic processes can be significant and are expected to also vary seasonally. Soil chamber measurements of soil C 0 2 efflux in the 53-year-old stand have also confirmed that seasonal variations in respiration processes exist (Drewitt et al., 2002). However, the difficulty in measuring turbulent fluxes during calm, stable conditions at night limits our ability to resolve seasonal patterns. There is a need for experiments designed to quantify the extent of acclimation in root and above-ground autotrophic respiration rates, seasonal dynamics and interactions between vegetation growth and autotrophic respiration, potential impacts of photoinhibition, and finally interactions between seasonal patterns in litter inputs and decomposition rates. For example, a major component of the Fluxnet Canada Research Network is the measurements of the stable isotope composition of C 0 2 respired by the ecosystem. These measurements can provide information about the source organic material and the effects of environmental conditions on photosynthesis (Farquhar et al., 1989). An improved understanding of these interactions and processes will improve confidence in partitioned estimates of GEP and further our understanding of C cycling in forest ecosystems. References 131 References Amiro, B., 2001. Paired-tower measurements of carbon and energy fluxes following disturbance in the boreal forest. Global Change Biology 7, 253-268. Anthoni, P. M., Unsworth, M. H., Law, B. E., Irvine, J., Baldocchi, D. D., Van Tuyl, S., Moore, D., 2002. Seasonal differences in carbon and water vapor exchange in young and old-growth ponderosa pine ecosystems. Agricultural and Forest Meteo-rology 111, 203-222. Arya, S. P., 1988. Introduction to Micrometeorology. 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Appendix 1 141 Appendix 1 : Determination of the effective volume for the portable soil chamber system The soil chamber system in this study was built in-house and required that we deter-mine the influence of leaks and adsorption of C O 2 on the total volume of the system. With the effects of leaks and adsorption, this total volume is expressed as the effective volume of the chamber system, Ve. Leaks and other losses of C O 2 occur through air escaping from the sides of the collar, pressure equalization tube, and through adsorp-tion of C 0 2 and water vapour molecules on the system materials. Although Ve will vary over time and from collar to collar due to differences in the height of the forest floor surface within the collars, in humidity levels and associated C O 2 adsorption rates, or in pressure equalization rates, we used a constant Ve for all flux calculations. The geometric volume of the chamber system was determined using the following measurements: System Component Volume Tubing and optical bench ~ 60 cm 3 Chamber 1426 cm 3 Humidity sensor (HMP) -23 cm 3 Total 1463 cm 3 To determine the effective volume, the chamber system was calibrated in the lab using a modification of the method described by Drewitt et al. (2002). The calibration involves injecting C O 2 at a known rate (Iknown m o l C O 2 s"1) into the return line of the chamber system set on a collar sealed onto a table surface. The measured rate of C O 2 concentration increase (dsc/dt, mol C O 2 mol" 1 dry air s"1) was related to Ve as Appendix 1 142 follows, T r ^knownR'-^ (1'7\ e ~ dsc/dtP/(l + sw) [ ' where R the universal gas constant, P the barometric pressure, and T the mean tem-perature (K) and sw the mean water vapour mixing ratio in the chamber headspace during the measurement. These tests were repeated for a collar set into a bucket of soil in the lab in order to simulate field conditions. In these cases, dsc/dt was mea-sured once wi th no C O 2 injected followed by a measurement wi th a known injection of C O 2 . In, E q . 17, the difference of these two slopes replaced dsc/dt. We injected 10% (by volume) C O 2 wi th the balance in dry air at 9 different flow rates from 0.25 c m 3 min" 1 to 7.75 c m 3 min" 1 for 200 s to 50 s, respectively to obtain dsc/dt from the last 40 s of each measurement. A small positive offset (0.2 / /mol mol" 1 s"1) in the relationship between dsc/dt and hnown indicated that there was a small error in the flow of C O 2 established within the chamber system, most noticeable at very low flow rates below 1 c m 3 min" 1 . To minimize this error, the slope of the least squares fit between dsc/dt and Iknown for flow rates greater than 1 c m 3 min" 1 was used to calculate Ve from E q . 17 (Fig. 40). The effective volume, Ve for the table tests was 0.001480 m 3 (n = 6, R 2 = 0.9996) while it was 0.001471 m 3 (n = 6, R 2 = 0.9955) for the soil tests. Tests on the soil surface resulted in slightly smaller Ve likely due to the soil surface being slightly raised within the collar area. Since the calibrations on the soil surface are more representative of field conditions, we used the soil test Ve for al l calculations. Appendix 1 143 Figure 40: Measured increases in C O 2 mixing ratio with time, dsc/dt during effective volume tests where a known injection rate of C O 2 was introduced into the chamber system on a table top («)and on the soil (•). The linear least square fit for the table tests, solid line, and the soil tests, dashed line, are also shown. Appendix 2 144 Appendix 2: Comparison of a L I - C O R model LI -6400 soil CO2 efflux chamber system with a home-made chamber system Soil C O 2 efflux measurements made by our homemade soil chamber system using an LI-800 IRGA were compared with those made by LI-COR's model LI-6400 Soil C 0 2 Flux System with a model LI-6400-09 soil chamber attachment. Nine collars were installed in the same manner as at our research sites, in the forest near our laboratory two days prior to making measurements. On Nov 24, 2002 the homemade chamber was used to make measurements lasting 2 min on six of the nine collars following the C 0 2 efflux measurement protocol described in this paper with the exception that three repetitions were made on each collar. On Nov 25, three of the collars were measured with two repetitions each. C O 2 fluxes were computed as usual with the normal volume of 0.001471 m 3 and a surface area of 0.007543 m 2. Between repetitions with the homemade system, the LI-6400 was used to measure C O 2 fluxes according to the measurement procedure outlined in the 6400-09 Soil C O 2 Flux Chamber Instruction Manual (Publ. No. 9710-119, Oct 1997). The chamber head was set near the collar to determine ambient C O 2 concentration. This value was input as the target with a delta of 15 ppm on Nov 24 and 10 ppm on Nov 25 due to the smaller fluxes being measured. The LI-6400 specifically minimizes the impact of accumulated C O 2 within the chamber on C 0 2 diffusion from the soil surface along a concentration gradient. Once the chamber is placed on the collar, air within the headspace is scrubbed for C 0 2 (and water vapour if desired) to a value of approximately 20 ppm below the ambient or target C O 2 concentration. As C O 2 diffuses into the headspace, data are recorded and intermediate C 0 2 fluxes are computed. The intermediate fluxes Appendix 2 145 recorded when the C O 2 concentration is between target ± the delta value are regressed against C 0 2 concentration. The final C O 2 flux is that which corresponds to the target C O 2 concentration. During our tests, R S ranged from 0.7 to 3.3 pmol m - 2 s _ 1 and repetitions were not combined (Fig. 41). The homemade soil chamber system (HS) agreed very well with the LI-6400 system (LS) where, RS!HS = 1-05 RS,LS - 0.1 pmo\ m~ 2 s-\ R 2 = 0.96, n = 24. LI-6400 C0 2 flux (Limol rrf2 s"1) Figure 41: Soil C O 2 efflux measurements made by the homemade soil chamber system using an LI-800 I R G A compared with those made by LI -COR's model LI-6400 Soil C 0 2 Flux System. The least squares fit between the two systems is shown. The difference between the two systems was largest when measuring low fluxes. This was more likely due to the increasing importance of accurately accounting for leaks and small differences in effective volume when measuring fluxes of low magnitude than how the two system differ in their manipulation of the concentration gradient. As a result, our tests confirmed that for fluxes in this range, diffusion of C O 2 was not inhibited in our homemade soil chamber system. Other characteristics of the homemade system such as the use of an effective volume of 0.001471 m 3 , the short equilization pressure tube and the correction for water vapour flux was at least as Appendix 2 146 successful as that used in the LI-6400 system. Appendix 3 147 Appendix 3: Comparison of CO2 and water vapour fluxes measured using open-path and closed-path infrared gas analyzers Fluxes of C O 2 (Fc) and water vapour (XE) were measured in the Douglas-fir stands using either an open-path or closed-path infrared gas analyzer (IRGA) (models LI-7500 & LI-6262, LICOR Inc, Lincoln, Nebraska, USA). The open-path analyzer mea-sures in-situ C O 2 and water vapour density requiring the additional measurement of concurrent high frequency temperature or sensible heat flux to remove the effects of density fluctuations on C O 2 and H 2 O concentrations (Webb et al., 1980). With a closed-path IRGA, air is drawn through a tube (sometimes heated) before passing through the sample cell. This has the advantage that temperature fluctuations in the air stream are damped and only water vapour fluctuations need to be removed from the measurement of C O 2 and H 2 O mole mixing ratios. In addition, flux mea-surements may be made during rain or snowfall as long as the sonic anemometer continues to function normally. The disadvantage is that despite efforts to achieve turbulence within the sample tube, attenuation of C O 2 and H 2 O fluctuations tends to occur due to radial variation in the velocity profile and interactions between the water and C O 2 molecules with each other and with the tubing walls (Leuning and Judd, 1996). This tends to reduce the covariance between w and C O 2 or H 2 0 resulting in a loss of flux. The magnitude of this loss depends on the proportion of flux attributed to frequencies above which concentration fluctuations are damped. The magnitude of this error depends on atmospheric stability, surface characteristics and height above the surface as well as sample tube length, cleanliness and air flow rate. Flux loss can also occur with the open-path IRGA due to spatial averaging over the path-length Appendix 3 148 of the sensor and due to increased lateral separation between the sonic anemometer and the bulkier open-path I R G A (Leuning and Judd, 1996). Theoretical and exper-imental transfer functions have been developed to correct for frequency losses due to sensor separation, path-length averaging and attenuation of concentration fluctua-tions wi thin the tubing (Leuning and Judd, 1996; Moore, 1986). The analysis which follows examines the magnitude of these flux losses and ultimately, the comparability of C O 2 and H 2 0 fluxes measured in the three Douglas-fir stands using open- and closed-path analyzers. Converting high frequency readings of C 0 2 and H 2 0 to mole mixing ratios ac-counts for the effect of fluctuating air density, which depends on the temperature and water vapour in the air. Alternatively, half-hour kinematic fluxes of sensible (H) and latent heat (XE) can be applied to Fc and XE using the W P L procedure (Webb et al. , 1980). However, without a measure of high frequency thermodynamic temperature (T) from a fine-wire thermocouple, for example, sonic temperature (Tv) measured by the sonic anemometer-thermometer must be converted to T using a measure of the water content of the air (Kaimal and Finnigan, 1994) for either the half-hour or high frequency method, where q is specific humidity (g H 2 0 g _ 1 moist air), pv and p are the molar densities of water vapour and air (mol m~ 3 ) , respectively, and Xv is the mole fraction of water vapour (mol m o l - 1 moist air). When an open-path I R G A is used, pv is known but T = Tv/(l + O.blq) = Tv/(1 + 0 .32p„/p) = Tv/(1 + 0.32Xv) (18) p = P/RT (19) Appendix 3 149 is unknown without a measure of T and P, the mean barometric pressure. Barometric pressure can be measured on site but T and p must be solved either mathematically or numerically using an iterative procedure. The iterative procedure is described in Chapter 2 . First , the 20 Hz signal of pv was approximately converted to mole fraction (Xv, rnol H 2 0 m o l - 1 moist air) wi th the 20 Hz Tv signal using, X v = PvR= (20) where R is the universal gas constant (8.31 J m o l - 1 K _ 1 ) and P is the half-hour average atmospheric pressure (kPa). Tv was then converted to air temperature (T, K ) using E q . 18. Then E q . 20 was used to recompute Xv by replacing Tv wi th T. Differences in the estimates of T were usually negligible on the second iteration. Alternatively, following a derivation outlined by K a i Morgenstern (unpublished), E q . 18 can be substituted into E q . 19 giving, ^ 7 ^ 1 + 0 - 3 2 T ) - ( 2 1) Klv p B y mult iplying the above equation by p, a quadratic equation can then be solved for P, _ '-mt^+r™^^ (22) where the negative root of the solution to the quadratic equation is ignored. This alternative method can then be used to compute T from E q . 19 and explains why the iterative procedure converges so quickly. Mole mixing ratios for C 0 2 (s c) and H 2 0 (sv) can then be calculated as the ratio of pc or pv to p — pv. In general, there was less than a 0.5 % difference between the W P L procedure and the high-frequency conversion procedure when computing half-hour Fc and XE. Appendix 3 150 To compare flux measurements using the open-path and closed-path IRGAs, an LI-7500 was setup alongside the LI-6262 operating in the 53- and 3-y stands (Fig. 42). The LI-7500 was placed below the sonic array by no more than 30 cm to minimize flux loss due to sensor separation (Kristensen et al., 1997). Measurements in the 3-y stand were made for 1 month between July 4 and August 4, 2003. In the 53-y stand, 17 days of measurements were made between August 20 and September 5, 2003. Closed-path IRGA fluxes were calculated as described in Chapter 2 and Chapter 4 . Figure 42: Field setup of LI-7500/LI-6262 comparison in the 3-y stand in 2003. As expected, fluxes computed with the open-path I R G A were generally greater than those computed wi th the closed-path I R G A in the tests in both stands (larger slope and/or offset) (Table 22, F i g . 43). However, the percent difference was greater for both XE and Fc in the 3-y stand than in the 53-y stand. To illustrate the loss in XE when a closed-path I R G A is used rather than an Appendix 3 151 Table 22: Results from the comparisons of Fc and XE computed with open-path and closed-path LI-COR IRGAs in the 3- and 53-y stands. Location & Date Flux Slope Offset n R 2 3-y XE (W rn"2) 1.12 0.4 1370 0.95 04/07-05/08, 2003 Fc (/imol m~ 1.08 -0.2 1370 0.95 53-y XE (W m-2) 1.0 1.9 686 0.98 20/08-05/09, 2003 Fc (pmo\ m~ 2 s- 1 ) 1.04 -0.4 686 0.98 open-path IRGA, Fig. 44 shows the ensemble average cospectra (Cwx) for 46 unstable daytime half hours in the 3-y stand where wx refers to either sensible heat flux (H, wT) or XE for the open-path IRGA (wsv>op) or the closed-path IRGA (wsVtCp. The associated transfer functions (TF) were expressed as, TF(/)=exp(-(//a) b) (23) where / is the frequency (Hz), and a and b are coefficients fit to the ratio of fCWSv/w's'v to fCwT/WT'. The TF in the left panel of Fig. 44 was fit to the ratio of fCWSvcp to fCWSvop. The TF in the lower half of Fig. 44 can be characterized by their cut-off freqencies, (/co, i.e., / where TF is 2 - 0 5 = 0.71). For the TF shown in the lower center of Fig. 44 ( fCWSvJu7s^p/fCwT/^), fco = 0.28Hz, a = 0.57, b = 1.53. Similarly, for the TF shown in the lower left panel ( fCWSvcp/w's'vcp/' fCWSvop/w's'ocp) fco = 0.21Hz, a = 0.54, b = 1.10. And for the TF shown in the lower right corner ( fCWSvoJ^^/fCwT/u7V), fco = 1.04Hz, a = 3.00, b = 1.01. What becomes clear is that there is greater high-frequency flux loss for the closed-path w's'v when compared to either the cospectra for open-path w's'v or for w'T'. It also indicates that although CWSvop is not identical to C W T , there is only very minor flux loss in the high-frequency range which is likely attributable to the effects of sensor separation Appendix 3 152 XE (Wrrf2) Figure 43: Half-hour XE and Fc measured with an open-path I R G A (LI-7500) and a closed-path I R G A (LI-6262) in the 3-y stand between July 4 and August 5, 2003. 1370 half hours were used in the analysis corresponding to rain-free and dew-free periods. The dashed line is the 1:1 line and the solid line indicates the fit of the linear least squares regressions, FCt0p = 1.08F c, c p - 0.2 /Limol m " 2 s _ 1 , R 2 = 0.95, n = 1370 and A-E o p = 1.12XEcp + 0.4 W m - 2 , R 2 = 0.98, n = 1370. and slight differences in the averaging path-length and frequency response of the open-path IRGA compared to the 3D sonic anemometer. This also means that using CWT as a surrogate for the 'ideal' cospectrum shape of C W S V , is justified. The TF characteristics for closed-path IRGA XE and FC based on CWT for both the Appendix 3 153 0.015 0.01 0.005 Figure 44: Comparison of heat and water vapour mixing ratio cospectra (fCwx) and nor-malized cospectra (fCwx/w'x') (top panels) and transfer functions (TF) (bottom panels) for 46 daytime half-hours from July 4 - August 5, 2003 in the 3-y stand when H > 100 W m - 2 , XE > 80 W m - 2 and mean wind speed > 0.05 m s _ 1 . 3-y and 53-y stands are summarized in Table 23. fco tends to be of lower frequency for XE than Fc indicating potentially more high-frequency flux loss for XE. This is likely due to the adsorption of H 2 0 molecules onto the sides of the tubing walls and within the filter, especially when these pathways are not clean. However, the impact of high-frequency attenuation of sc or sv wi thin the sampling tube on XE or Fc depends on the proportion of the flux, which lies beyond fco. Cospectra of sensible heat are used to illustrate the effects of canopy roughness, measurement height, wind speed and stability on the distribution of turbulent energy. Appendix 3 154 Table 23: Transfer function (TF) characteristics for closed-path I R G A XE and Fc based on sensible heat cospectra (CWT)ior unstable daytime conditions (H > 100 W m - 2 , XE > 80 W m~ 2) between July 4 and August 4, 2003 in the 3-y stand and between August 20 and September 5, 2003 in the 53-y stand. Location Flux a b fco (Hz) n 3-y XE 0.57 1.53 0.28 46 Fc 1.35 1.15 0.54 46 53-y XE 0.76 1.29 0.33 23 F 1 c 2.65 0.54 0.37 23 Fig. 45 shows how the normalized fCwx vary between stands largely as a function of wind speed and measurement height (bottom panel). All three cospectra were for an average H of about 192 W m~2, the average wind speed (u) was 1.6, 1.8, and 1.7 m s _ 1 and the median atmospheric stability ((z — d)/L, see below for explanation) was -0.6, -1.6, and -0.5 in the 3-, 14-, and 53-y stands, respectively. One of the major differences between cospectra was the location of the peak of the normalized fCwr-It was at a much lower frequency of about 0.009 Hz in the 53-y stand, about 0.015 Hz in the 14-y stand, and 0.05 Hz in the 3-y stand. These differences were minimized once proximity to the canopy surface and wind speed were accounted for (top panel). Peak fCWT was near n of about 0.1 in all three stands, similar to that found for the Kansas data over flat uniform terrain (Kaimal and Finnigan, 1994). This shows that the turbulent transfer of heat is spread across a wide range of eddy sizes, however, the 3-y stand is dominated by eddies that are smaller than in the 14- and 53-y stands. In all three Douglas-fir stands, C W T shifted towards higher frequencies with higher wind speeds. C W T also shifted towards higher frequencies in the 14-y and 53-y stands when more neutral or stable atmospheric conditions existed. However, the proportion of flux attributable to frequencies beyond 0.1, 0.5, and 1 Hz varied considerably Appendix 3 155 '(Hz) Figure 45: The normalized ensemble average cospectra of sensible heat flux for the three Douglas-fir stands against non-dimensional frequency (Kaimal and Finnigan, 1994) (top panel) and frequency (Hz) (bottom panel). Non-dimensional frequencies include the mea-surement height(z), zero-plane displacement height (d), and average horizontal wind speed (u). z was 3.5, 12, and 42 m and d was estimated to be 0.4, 4, and 23 m in the 3-, 14-, and 53-y stands, respectively. A l l half hours were taken from July 15 to 25, 2002 when H was between 180 and 200 W m - 2 (average H was between 190 and 194 W m - 2 in all stands) where n = 9 for the 3-y stand, n = 15 for the 14-y stand, and n = 12 for the 53-y stand. between stands. This is shown in F i g . 46 using measurements from July 2002 in al l three stands. Clearly, attenuation of sc and sv would be expected to have the greatest impact on fluxes measured in the 3-y stand where a large proportion of the turbulent flux was associated wi th higher frequencies. B y developing a relationship between T F correction factors (the ratio of w'T' to w'T' degraded by the transfer function described above (Aubinet et al . , 2000)) and wind speed, the variable effects of high frequency attenuation on XE and Fc can be Appendix 3 156 0.5 0.4 0.3 0.2 0.1 0 3-y 0 0.5 0.4 0.3 0.2 0.1 0. -2 -1 o o<*b° 0 0 • A A A A A A D A • 0.5 0.4 0.3 0.2 0.1 0 o o o o o o QRp* D • m • ^ A -2 -1 0 (z-co//. 0.5 0.4 0.3 0.2 0.1 0 of =0.1 Hz of =0.5 Hz M= 1 Hz o o o o • • • D P S o ° o o -2 -1 0 Figure 46: The proportion of sensible heat flux attributable to frequencies beyond 0.1, 0.5, and 1 Hz in the three Douglas-fir stands for a variety of wind speeds (u) and atmospheric stabilities, (z — d)/L where z is the measurement height, d is the zero-plane displacement, and L is the Monin Obukhov scaling length (Kaimal and Finnigan, 1994). Negative (z—d)/L refers to unstable conditions (daytime periods with significant H and XE fluxes) while near-zero is neutral (generally overcast or nighttime periods with high wind speeds) and positive is stable (nighttime periods with moderate wind speed). accounted for (Fig. 47). In general, the relationship between CF and u varied relatively little throughout the years in each stand. On average for XE, CF = 0.07w + 1.01 while for Fc, CF = 0.05u + 1.00 in the 3-y stand. In the 53-y stand, CF = 0.006u+ 1.0 for Fc and CF Appendix 3 157 1.35 -i i i i i i i i 0 0.5 1 1.5 2 2.5 3 u (m s_ 1) Figure 47: The relationship between the correction factor for closed-path I R G A XE and half-hour average horizontal wind speed (u) for 46 daytime half-hours from July 4 - August 5, 2003 in the 3-y stand when H > 100W m - 2 , XE > 80W m ~ 2 and mean wind speed > 0.05 m s - \ C F = 0.07u +1.05, R 2 = 0.47. = 0.03w + 1.0 for XE. A t the time of analysis, no C F was determined for the 14-y stand since an open-path I R G A was st i l l the only type of I R G A deployed. In conclusion, this analysis shows that wi th the use of transfer functions and wind speed adjusted correction factors, fluxes computed wi th open and closed-path I R G A measurements can be defensibly compared.
Thesis/Dissertation
eng
Soil Science
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Graduate
Net ecosystem production of three coastal Douglas-fir stands at different stages of development after harvesting
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