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The growth and photosynthetic response of under-planted conifer seedlings to changes in understory light… Karakatsoulis, John 2004

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The growth and photosynthetic response of under-planted conifer seedlings to changes in understory light environment By  John Karakatsoulis B . S c , University of British Columbia, Vancouver, 1987.  A THESIS SUBMITTED I N PARTIAL F U L F I L L M E N T OF THE R E Q U I R E M E N T S FOR T H E D E G R E E OF DOCTOR OF P H I L O S O P H Y  IN  T H E F A C U L T Y O F G R A D U A T E STUDIES  Department of Forest Sciences  We accept this thesis as conforming to the required standard  The University of British Columbia April  2004  © John Karakatsoulis,2004  ii  ABSTRACT  The rapid invasion of non-crop vegetation following disturbance (i.e. clearcut forest harvesting) i n southwestern British Columbia was studied i n conjunction with the growth and physiological response of planted evergreen conifer seedlings. Primarily, this thesis investigated the changes i n understory light (photosynthetic active radiation — PAR) environment and the ability of Pseudotsuga menziesii (Mirb.) Franco (Douglas-fir), Thuja plicata Donn. (western redcedar), Tsuga heterophylla (Raf.) Sarge. (western hemlock), and Abies grandis (Dougl.) Forbes (grand fir) to photosynthesize and grow under low and changing light environments. The study was undertaken at the Malcolm Knapp Research Forest which is located i n the southwestern portion of British Columbia (49° 15' N and 122° 31' W) at an elevation of 140 m. The climate is described as wet, cool mesothermal with mild winters and dry, warm summers. Vegetation growth and invasion following clearcut logging was rapid. Within two years of harvesting the average vegetation height was 0.39 m and covered 48 % of the area. Within six years, vegetation height averaged 2.75 m and covered the entire site (100 %). Mid-summer light (PAR) levels within the vegetation plots (30 cm above the ground) declined from an average of 66 % to less than 4 % within four years. Net photosynthetic rates of planted understory Douglas-fir, grand fir, and western hemlock seedlings were generally less than 2.0 Limol n r s i n mid-summer. In contrast, 2  _1  planted seedlings of the same species growing in the open had mid-day photosynthetic rates at approximately 8.0 u.mol m ^ s . However, these static 1  iii  mid-summer measurements of understory PAR and the resultant photosynthetic response of conifer seedlings failed to account for the seasonality of light availability. Further studies of the seasonality of understory light availability within deciduous stands showed that PAR levels in early spring and late fall (period when the overstory canopy is leafless) went as high as 75 % of light levels in the open. The mid-day photosynthetic rates of the underplanted conifer seedlings were found to track the seasonality of understory light availability. Two distinctive photosynthetic peaks by the conifer species were shown to correspond with the fall and spring periods when understory light availability beneath deciduous vegetation was high due to leaf fall (in autumn) and prior to leaf flush in the spring. Conversely, the lowest photosynthetic rates were recorded during the summer months when the deciduous canopies were fully foliated, resulting in low levels of understory PAR, and during the winter months when air and/or soil temperatures were less than 5 °C. A further study was conducted to determine and quantify the contribution of understory sunflecks to overall photosynthetic photon flux density (PPFD) and to the CO2 uptake rates of understory western redcedar growing within stands of Alnus rubra Bong, (red alder) and Betula papyrifera Marsh, (paper birch). Western redcedar was capable of responding to changing understory PAR by closely tracking sunfleck activity. Sunfleck peaks between 200 and 445 u.mol m- s were closely correlated with 2  -1  photosynthetic peaks between 2 and 5 pmol m^s . Conversely, net 1  photosynthesis of western redcedar declined rapidly with the passing of sunflecks and remained very low (<0.5 pmol nrV ) during periods of diffuse 1  iv  light (PAR » 20 - 25 umol rrrV ). It was estimated that between 69 and 83 % 1  of CO2 fixed by understory western redcedar can be attributed to photosynthesis during periods of sunflecks and this may help to explain its presence i n understory environments within west coast ecosystems. The results presented i n this thesis have implications for forest management as it applies to silviculture systems and the ability to utilize certain conifer species in the understory of managed and unmanaged stands.  TABLE OF CONTENTS  ABSTRACT  ii  LIST O F T A B L E S  vii  LIST O F F I G U R E S  viii  LIST O F P L A T E S  xii  ACKNOWLEDGMENTS  xiii  Chapter 1. Introduction  1  Review  3  Research Objectives  9  Thesis Structure  9  Chapter 2. Vegetation development following clearcut logging: Effects on understory light availability, conifer seedling growth and photosynthesis... 12 Introduction  12  Study Area  16  Materials and Methods  18  Results and Discussion  24  Conclusions  52  Chapter 3. Seasonal trends in light availability, photosynthetic response and overall growth of understory Thuja plicata seedlings  54  Introduction  54  Study Area  56  Methods  57  Results and Discussion  64  Conclusions  89  vi  Chapter 4. Understory sunflecks and their effect on western redcedar seedling photosynthesis  91  Introduction  91  Methods  93  Results and Discussion  98  Conclusions  119  Chapter 5. Growth, biomass allocation and photosynthetic response of Douglas-fir, western hemlock and western redcedar seedlings grown under different light intensities 122 Introduction  122  Materials and Methods  123  Results  126  Discussion  140  Chapter 6. Summary and Conclusions Implications for Forest Management  144 152  References  155  Appendices  177  VII  LIST OF TABLES Table 2.1 Vegetation percent cover development by species on the control treatment plots 26 Table 2.2 plots  Vegetation mean height by species on the control treatment 27  Table 3.1 Pre-treatment biomass, biomass allocation, specific leaf area (SLA), leaf area ratio (LAR) and leaf weight ratio (LWR) 63 Table 3.2 Mean seasonal PAR levels within a red alder and paper birch stand and i n the open 72 Table 3.3 Changes i n mean height and stem diameter (at root collar) of western redcedar seedlings 83 Table 3.4 allocation)  Changes i n western redcedar seedling biomass (and biomass 84  Table 4.1 Total understory P P F D and P P F D due to sunflecks beneath the red alder and birch stands 102 Table 4.2  Estimated CO2 fixed by understory western redcedar due to  sunflecks beneath the red alder and birch stands Table 4.3 hour period  117  Estimated percent CO2 uptake of western redcedar over a one 120  Table 5.1 Changes in specific leaf area (SLA) and leaf area ratio (LAR)) of Douglas-fir, western redcedar and western hemlock seedlings grown under different light levels 136  viii  LIST OF FIGURES Figure 2.1 Diagrammatic representation of the treatment plots and seedling placement within one treatment plot Figure 2.2  19  Changes i n mean percent cover and height of vegetation growing  on the control plots  28  Figure 2.3 Changes i n mean stem diameter (at root collar) and stem height of planted red alder seedlings 32 Figure 2.4 Mid-summer changes in understory light availability (% of PAR in the open) over a four year period beneath the control (A), and alder (B) treatments 34 Figure 2.5 Correlation between understory PAR (as a % of PAR i n the open) and the sum of vegetation cover (control treatment) 35 Figure 2.6 Seasonal changes i n understory light availability (measured as % light i n the open) within the control (a) and alder (v) treatments 36 Figure 2.7 Mid-summer understory PAR and net photosynthetic rates (Pn) of Douglas-fir (DF), grand fir (GF), and western hemlock (WH) seedlings  38  Figure 2.8 Changes i n mid-day net photosynthesis of Douglas-fir, grand fir, and western hemlock growing within the alder (v) and control (a) treatments and i n the open (k) over a one-year period 40 Figure 2.9 Changes in mid-day air and soil (at 15 cm depth) temperatures within the alder and control treatments, and i n the open 41 Figure 2.10 Four years of growth response (stem diameter and stem height) of Douglas-fir seedlings growing within the eight treatments 43 Figure 2.11 Four years of growth response (stem diameter and stem height) of grand fir seedlings growing within the eight treatments 44  ix  Figure 2.12. Four years of growth response (stem diameter and stem height) of western hemlock seedlings growing within the eight treatments  45  Figure 2.13. Stem diameter and height growth response of Douglas-fir growing within the eight treatments  47  Figure 2.14. Stem diameter and height growth response of grand fir growing within the eight treatments  48  Figure 2.15". Stem diameter and height growth response of western hemlock growing within the eight treatments Figure 3.1  49  Seasonal trend i n understory light availability (measured as  percent P A R i n the open) within the alder (•) and birch (•) stands  68  Figure 3.2 Changes i n Leaf Area Index (A) and understory light availability (B) during autumn leaf senescence and leaf fall within the alder (o) and birch (•) stands  69  Figure 3.3 The relationship of understory P A R (Iz/Io: where Iz and Io are P A R levels i n the understory and i n the open, respectively) as a function of L e a f A r e a Index (LAI) within the alder (•) and birch (•) stands Figure 3.4  70  Number of days per month i n which atmospheric conditions  were classified as either sunny, cloudy, or a mixture of sun and cloud  73  Figure 3.5 Seasonal changes i n daily maximum and m i n i m u m air temperatures (A) and daily precipitation (B)  75  Figure 3.6 Seasonal changes i n (A) air and (B) soil temperature within the birch (•) and alder (•) stands and i n the open (A)  76  Figure 3.7 Seasonal changes i n mid-day leaf photosynthesis of understory western redcedar seedlings growing within birch (•) and alder (•) stands and i n the open (A)  77  Figure 4.1 The fluctuation i n understory light availability as measured every 10 seconds beneath the red alder (A), and birch (B) stands  99  Figure 4.2 Frequency distribution of sunfleck duration within the A) red alder, and B) birch stand 103 Figure 4.3 Changes i n understory light intensity within the red alder stand as a function of A) sunflecks, and B) the photosynthetic response of western redcedar 105 Figure 4.4 Understory air temperature (A) and stomatal conductance (B) of western redcedar during a period of sunfleck activity  109  Figure 4.5 (A) light saturation curve of western redcedar grown under lowlight greenhouse conditions (PAR « 45 u.mol rrr s ) i n the absence of sunflecks, and (B) the photosynthetic response curve of western redcedar growing beneath the red alder stand 110 2  1  Figure 4.6 Changes i n understory light intensity as a function of sunflecks (A) and the measured (•) and predicted (•) photosynthetic response of western redcedar (B) based on the greenhouse model 112 Figure 4.7 Changes i n understory light intensity as a function of sunflecks (A) and the measured (•) and predicted (•) photosynthetic response of western redcedar (B) based on the field model 113 Figure 4.8 Estimated photosynthetic response of western redcedar to sunfleck activity beneath the red alder stand using ; (A) the greenhouse model, and (B), the field model  114  Figure 4.9 Estimated photosynthetic response of western redcedar to sunfleck activity beneath the birch stand using ; (A) the greenhouse model, and (B), the field model 115  xi  Figure 5.1 Mean stem diameter (at root collar) of Douglas-fir, western redcedar, and western hemlock seedlings at the end of four months of growth under five light levels 127 Figure 5.2 Mean height of Douglas-fir, western redcedar, and western hemlock seedlings at the end of four months of growth under five light levels 129 Figure 5.3 Mean height:diameter ratio of Douglas-fir, western redcedar, and western hemlock seedlings at the end of four months of growth under five light levels 130 Figure 5.4 Mean total dry weight (shoots and roots) of Douglas-fir, western redcedar, and western hemlock seedlings at the end of four months of growth under five light levels 131 Figure 5.5 Changes i n biomass allocation (stem + branches, foliage, and roots) of Douglas-fir seedlings at the end of four months of growth under the five light levels 133 Figure 5.6 Changes i n the shoot:root ratio of Douglas-fir, western redcedar, and western hemlock seedlings at the end of four months of growth under five light levels 134 Figure 5.7 Mean leaf photosynthesis of Douglas-fir, western redcedar, and western hemlock seedlings at the end of four months of growth under five light levels 138 Figure 5.8 Photosynthesis - irradiance response curves for western redcedar (A), and Douglas-fir (B) grown under 45 % (•) and 3 % (•) full sunlight  139  Xll  LIST OF PLATES Plate 2.1 General view of the site following harvesting and site preparation  17  Plate 2.2 Aerial photograph of the experimental site five years following treatment establishment 29 Plate 3.1 canopies  Mid-summer view of the red alder (A) and the paper birch (B) 58  Plate 3.2 General mid-summer view of the understory vegetation within the red alder (A) and paper birch (B) stand 59 Plate 3.3 Red alder canopy in the spring prior to leaf flush (A), during leaf expansion i n the spring (B) and in mid-summer (C) 65 Plate 3.4 Paper birch canopy i n the spring prior to leaf flush (A), during leaf expansion i n the spring (B) and i n mid-summer (C) 66 Plate 4.1 General view of the red alder (A) and paper birch (B) understories and the placement of light sensors during sunfleck measurement  95  xiii  ACKNOWLEDGMENTS  I would like to thank the staff at the Malcolm Knapp Research Forest for their long-term support for this research. The field assistance of Marc Kimmins and Maureen O'shea-stone are also greatly appreciated. A n extended thank you goes out to the members of my committee: Dr. Robert Guy, Dr. Tony Kozak, Dr. Cindy Prescott and Dr. Peter Jolliffe. Your comments and suggestions, along with your moral support, have been much appreciated and have helped to keep me focused. I owe a dept of gratitude to my supervisor, Dr. Hamish Kimmins. Your constant support of my work and your tireless approach to the field of forest ecology has inspired me to always strive for knowledge. Thank you for believing i n me. Finally, I would like to thank my wife, Starr Webb, and my two sons, Julien and Alexis, for their support, compassion and just being there for me during this process. I dedicate this thesis to you.  1  CHAPTER 1. G E N E R A L INTRODUCTION: Review and Study Objectives Vegetation succession following disturbance can follow several different pathways (Pickett et al. 1987) which may include facilitation, 1  tolerance, and inhibition (Connell and Slatyer 1977). These pathways are not mutually exclusive, and after a period of facilitation, either tolerance or inhibition may be observed. The mechanism by which inhibition, or reduced growth rates i n the tolerance pathway, occur usually involves competition for light on moist sites, for moisture on dry sites, and for nutrients on infertile sites (or some combination thereof). While Connell and Slatyer's threepathyway model has been criticized for being too simplistic (Veblen 1992, Barnes et al. 1998 and references within, Kimmins 2004) it does help i n attempting to understand how forest succession, specifically secondary succession, proceeds within temperate ecosystems. Above-ground competition for light between evergreen and deciduous competitors has traditionally been assessed by "competition indices" (CI) measured i n the middle of the growing season when the deciduous species are fully foliated (Eis 1980, Wagner 1982, Howard and Newton 1984, Brand 1986, Cole and Newton 1987, Karakatsoulis et al. 1989, Wagner et al. 1989, Burton 1993, Biging and Dobberton 1995). These have had mixed success when applied to sites other than those for which they were developed (Brand 1986, Karakatsoulis et al. 1989, Burton 1993), often explaining less than 50 % of the growth of the evergreen trees.  Successional pathways refer to the patern of biotic and abiotic changes in an ecosystem over time (Kimmins 1997). 1  2  A major reason for this failure i n some climatic areas may be that midsummer CI's do not account for photosynthesis by evergreen plants during the deciduous species' leafless period (Emmingham and Waring 1977, Waring and Franklin 1979, Lassoie et al. 1983, 1985, Yoshie and Kawano 1986). In addition, correlations between growth and average light conditions are not always very high due to the confounding effects of other environmental factors, such as temperature, moisture and nutrients, which may exhibit temporal variations not related directly to competition. Yet a third reason may be that improved nutrition may raise light use efficiency of the conifer foliage (Field and Mooney 1986) so that the relative effects of light competition may be altered by variations i n nutrient availability both within and between sites. In forest management, light is an important factor i n vegetation dynamics following harvesting; in particular, the competition for light between evergreen crop seedlings and deciduous non-crop vegetation. On nutrient medium-to-rich and fresh-to-moist sites i n southwestern British Columbia, the establishment of conifers such as Pseudotsuga menziesii (Mirb.) Franco (Douglas-fir), Thuja plicata Donn. (western redcedar), Tsuga heterophylla (Raf.) Sarge. (western hemlock), and Abies grandis (Dougl.) Forbes (grand fir) is often hampered by fast growing deciduous pioneer tree species such as Alnus rubra Bong, (red alder) and Populus trichocarpa Torr. & Gray (black cottonwood), and shrub species such as Rubus spectabilis Pursh (salmonberry). These species quickly colonize clearcut areas and 2  frequently overtop planted conifer seedlings. Mid-summer light levels beneath these deciduous canopies may be reduced to a small fraction of full Clearcut is defined as an area of a forest in which all trees have been removed to a point at which the surrounding forest has little affect on soil processes or microclimate (Kimmins 1992). 2  3  sunlight, greatly reducing the photosynthetic response, and subsequently, the growth of understory conifer seedlings. However, static measurements of light beneath deciduous vegetation and of the photosynthetic response and the growth by understory species fail to account for the seasonal and temporal dynamics of understory light availability in the understory and the associated environmental changes. Considering the importance of understanding and being able to predict early secondary plant succession, theories and models of early succession are needed that explicitly account for not only species composition and growth, but also for the dynamic changes i n the understory environment (light and temperature) and the growth and physiological response (i.e. net photosynthesis) of crop species (evergreen conifers) growing within these environments.  Review Following overstory leaf expansion i n deciduous forests, mid-summer levels of photosynthetically active radiation (PAR) reaching the forest floor may be reduced to 1-10% of PAR in the open (Blackman and Rutter 1946, Anderson 1964, Canham et al. 1990, Ellsworth and Reich 1992). For example, Lassoie et al. (1983) reported light levels (PAR) of less than 100 u.mol n r s (approximately 7% of full sunlight) during the summer when 2  -1  the overstory of an eastern United States deciduous forest was fully foliated. DePamphilis and Neufield (1989) found similar seasonal trends i n light attenuation beneath a mixed oak-hickory forest i n southwestern United States, where mid-summer understory light levels were found to be less than  4  30 u.mol n r s- . Plants that grow i n this type of understory light 2  1  environment must be able to either maintain a net carbon gain during the summer months or photosynthesize during that portion of the year when the deciduous overstory is leafless (Chabot and Hicks 1982, Schulze 1982), or a combination of both. For example, Lassoie et al. (1983) found P A R levels of 800 p.mol n r s i n the fall, winter and spring (about eight times the mid2  1  summer levels) within a deciduous stand of trees. Because evergreens maintain most of their foliage all year long, evergreen plants i n the understory have the potential to photosynthesize during those periods when the overstory canopy is leafless and air and soil temperatures are above some minimal threshold. Parker (1961) measured positive CO2 uptake by Pinus sylvestris L., P. palustris M i l l . , and Chamaecyparis pisifera (S. & Z.) seedlings in northeastern United States during early November and early April, which is outside the normal growing season. A more detailed study by Yoshie and Kawano (1986) showed that net photosynthesis (Pn) by an evergreen understory plant, Pachysandra terminales, increased rapidly following spring snow melt and reached a maximum rate i n late April just prior to overstory leaf development. Following overstory leaf expansion, P n of this understory species declined, reaching a minimum in July when light intensity i n the understory was the lowest. A second increase i n P n was observed in early October when canopy foliage was senescing and understory light intensity was increasing. Juniperus virginiana L . (eastern redcedar), an evergreen coniferous tree found growing in eastern United States, was found to have a similar seasonal photosynthetic pattern when growing under a deciduous forest (Lassoie et al. 1983). Maximum photosynthetic rates were recorded i n the  5  early spring and early fall when the overstory canopy was leafless and understory light levels were usually above 800 u,mol n r s- (the apparent 2  1  saturation point for this species) on clear days. Minimum photosynthetic rates were correlated with mid-summer canopy closure and the resultant reduction i n average understory light availability (100 urnol n r s ). While 2  1  this level is relatively low compared to light intensity i n the open, it may not accurately represent the summer light conditions experienced by the understory plants found growing within mesothermal forests of British Columbia. For example, on the southwest coast of British Columbia, midsummer light levels beneath deciduous canopies of red alder and salmonberry can be as low as 25 u.mol n r s (Bigley 1988). For many plants, even for 2  _1  those classified as being "shade tolerant", this level of PAR represents the minimum light required to maintain a zero CO2 balance (i.e., their compensation point). The ability of understory plants to survive and grow i n such deeply shaded understory environments may reflect their ability to photosynthesize outside the summer period. However, there are no published reports to substantiate this theory within low- to mid-elevation west coast ecosystems. Unlike evergreen plants, temperate deciduous understory plants cannot take advantage of winter photosynthesis. They must rely on either early growth during the spring before the overstory canopy has leafed out or be adapted to grow under very low light regimes, or some combination thereof. Seasonal changes i n light availability beneath deciduous canopies can influence the distribution and growth of understory plants. Early work by Blackman and Rutter (1946) showed that the distribution of the annual Scilla non-scripta (bluebell) growing in a mixed deciduous forest (beech, ash  6  and sycamore) was a function of light levels during the early spring, prior to canopy leaf development, and not of mid-summer understory light levels. They concluded that bluebell is intolerant of deep shade, such as was found during the summer, and that it conducts all its annual growth in the early spring prior to overstory leaf expansion and thus avoids shade completely. DePamphilis and Neufield (1989) documented the phenology and ecophysiology of seedlings of the deciduous tree Aesculus sylvatica Bartr. (Georgia buckeye) growing i n the understory of oak-hickory forests i n the southeastern United States. A strong correlation was found between the phenology and growth of buckeye and the decline in understory light attenuation as a function of canopy leaf expansion. Leaf expansion of buckeye occurred between March and April when understory light levels were approximately 74% of full sunlight (1150 u.mol n r s ). The 2  1  commencement of leaf senescence by buckeye was correlated with the completion of the overstory i n late May when average light levels were less than 30 u.mol n r s . The phenology of buckeye suggests that it is intolerant 2  1  of shade, or a "shade avoider", and conducts most of its growth during the spring when both air and soil temperatures are above some threshold temperature and prior to overstory leaf expansion. In southwestern British Columbia, the deciduous shrub salmonberry is often found growing on moist, nutrient rich sites (Klinka et al. 1989) beneath the canopy of red alder and/or black cottonwood trees where it has only limited opportunity for spring and fall photosynthesis. Shade adapted shrubs like salmonberry must be able to maintain positive rates of CO2 uptake under extremely low average light levels, which may last from five to twelve  7  months i n temperate deciduous angiosperm and evergreen gymnosperm forests, respectively, or year-round beneath tropical forests. While early season growth is one adaptation of plants growing i n shaded understory environments, another is the utilization of sunflecks. Light levels beneath a canopy, whether deciduous or evergreen, are rarely constant throughout the day and are more often interrupted by periods of light which form sunflecks i n the understory environment. On clear days, 20 to 80% of the incoming light penetrating forest canopies and reaching the understory is i n the form of sunflecks (Bjorkman and Ludlow 1972, Pearcy and Calkin 1983, Pearcy 1983, 1987, 1990, Chazdon and Fetcher 1984, Chazdon et al. 1988, Chazdon and Pearcy 1991). Similarly, a large portion of CO2 uptake by understory plants has been attributed to photosynthesis during periods of sunflecks (Bjorkman et al. 1972, Pearcy and Calkin 1983, Pearcy 1987, 1990, Chazdon and Pearcy 1991, Pearcy et al. 1994, 1997). For example, Weber et al. (1985) attributed 35 % of mid-summer photosynthesis by Acer saccharum seedlings growing i n the understory of a mixed hardwood forest to sunflecks exceeding 50 |imol nv s- . Likewise, Pearcy (1987) 2  1  attributed 32 % of daily carbon gain by the understory plant Argyrodendron peralatum F. J . Muell. to the presence of sunflecks. Under a continuous evergreen coniferous canopy sunflecks may represent a substantial source of PAR for understory species. For example, on clear summer days, diffuse, mid-day understory light levels beneath late serai (> 200 years-old) western hemlock - western redcedar forests are usually less than 20 umol n r s , which represents less than 2 % of above 2  1  canopy P A R (Klinka et al. 1992). In contrast, understory P A R during periods of sunflecks may increase to 125 (imol n r s (8 % of above canopy PAR) or 2  1  8  higher. Understory western redcedar seedlings growing under these conditions have been found to increase their CO2 uptake from 0.77 (imol n r s (understory diffuse light conditions) to 3.84 u.mol n r s 2  -1  2  _1  during periods of sunfleck activity (Klinka et al. 1992, Karakatsoulis, unpublished data). Compared to seedlings growing in the open, understory photosynthetic rates represent 9 % (diffuse light) and 46 % (sunfleck) of the maximum rates obtained by open grown seedlings. Considering the seasonal and diurnal changes i n the understory environment, evergreen conifers may be the most shade adapted understory vascular plants i n temperate forests. They can respond to favourable light levels i n the early spring prior to overstory canopy development as well as during the early fall period when overstory deciduous trees are senescing and dropping their leaves. In addition, sunflecks may contribute significantly to the daily carbon gain by these plants. Under favourable temperature conditions, temperate conifers can also photosynthesize during the winter period (Helms 1965, Parker 1961, Pharis et al. 1970, Walker et al. 1972, Pisek et al. 1973). For example, Douglas-fir growing i n coastal Washington was shown to maintain positive CO2 uptake rates during the winter months (Helms, 1965) and this led to the hypothesis that up to 50% of annual carbon assimilation by temperate conifers growing i n western North America occurs during the period October—May (Emmingham and Waring 1977). Clearly, understanding the growth and photosynthetic characteristics of understory plants must incorporate more than a 'snap shot' evaluation of mid-summer light availability. We need to know the seasonal dynamics of competition for light, the potential and actual photosynthetic performance of evergreen crop trees, the seasonal changes in air and soil temperature and  9  the role of sunflecks in the understory i n relation to photosynthetic response of understory plants.  Research Objectives The objectives of this thesis were to describe and quantify: 1) the yearly and seasonal changes i n understory light levels beneath deciduous early successional vegetation, and beneath fully developed paper birch and red alder canopies; 2) the dynamic nature of understory sunflecks; 3) the photosynthetic responses of planted conifer seedlings of species varying i n shade tolerance to these environmental seasonal changes therein; and 4), the growth response of conifer seedlings varying i n shade tolerance grown under different light regimes within the above vegetation and under artificial shade. Two major hypotheses were tested: (i) post-disturbance successional pathways i n the Coastal Western Hemlock Drier Maritime (CWHdm) biogeoclimatic subzone on mesic-subhygric sites are tolerance rather than inhibition i n character because of non-growing season photosynthesis by mid and late serai evergreen tree species, and (ii) sunflecks contribute significantly to the daily CO2 uptake of understory plants.  Thesis Structure This thesis consists of six chapters. Chapters 2-5 describe four separate studies which address the above hypotheses. Chapter 2 addresses the questions: (i) how does the understory light environment change under several different plant community types on one site type during vegetation  10  development following clearcut logging on a me sic/sub hygric site i n the C W H d m subzone? (ii) does the photosynthetic response of understory conifer seedlings (Douglas-fir, western hemlock and grand fir) track the seasonality of understory light availability i n three different plant communities, and (iii) how does the growth of seedlings of these species vary with different light environments and different plant communities producing these light environments? Chapter 3 describes for two established deciduous tree stands (paper birch and red alder) and an open area: (i) the seasonality of light availability to seedlings, (ii) the photosynthetic response of western redcedar seedlings (a shade tolerant coniferous species), and (iii) the three-year growth and biomass allocation response of western redcedar seedlings under the above 3  conditions. The question of what proportion of understory P P F D can be attributed to sunflecks and whether sunflecks contribute significantly to the daily CO2 assimilation by understory western redcedar is addressed i n Chapter 4. The last experimental chapter (Chapter 5) presents a detailed investigation of the growth, biomass allocation and the photosynthetic response of Douglas-fir (a relatively shade intolerant species), western hemlock and western redcedar (two shade tolerant species) seedlings grown under controlled greenhouse conditions using five separate light levels. In chapter six (the final chapter), the results of the previous four chapters are summarized i n the context of interspecific competition and  The term biomass allocation is used to describe the allocation of carbohydrates to various plant components (i.e. roots, stem, foliage) which result in differences in weight (biomass) of the various plant components. 3  11  environmental change as a function of vegetation development following disturbance. The format of chapters 2-5 follows that of a journal manuscript with each chapter consisting of an introduction, methods, results and discussion, and a conclusion section. A n effort has been made to reduce redundancy between chapters, but some overlap was unavoidable, especially i n the introduction and discussion sections.  12  C H A P T E R 2. V E G E T A T I O N D E V E L O P M E N T F O L L O W I N G C L E A R C U T LOGGING: E F F E C T S ON UNDERSTORY LIGHT AVAILABILITY, CONIFER SEEDLING GROWTH, A N D PHOTOSYNTHESIS  INTRODUCTION  Vegetation succession and development following major disturbances such as wildfire or clearcut logging has been studied extensively i n various parts of North America (Isaac 1940, Franklin and Dyrness 1973, Long and Turner 1975, Wittinger et al. 1977, Henderson 1978, Irwin and Peek 1979, Stickney 1981, 1986, Alaback 1982, 1984, Hamilton and Yearsley 1988, Schoonmaker and McKee 1988, Messier and Kimmins 1991, Karakatsoulis and Kimmins 1993, Stuart et al. 1993, Leak and Smith 1996). These studies have focused on changes in vegetation percent cover, height, and/or changes in vegetation biomass over some period of time following disturbance. Many of them have attempted to correlate non-crop vegetation development with the growth (or more precisely the reduction in growth) of crop-trees (Radosevich et al. 1976, Wagner 1982, Radosevich 1984, Wagner and Radosevich 1989, Wagner et al. 1991, Christie 1994, Burton 1995, Harrington et al. 1995, Chang et al. 1996). The main approach has been to determine the competitive ability of non-crop vegetation by quantifying changes i n growth of crop tree species (i.e., stem diameter and stem height, biomass and biomass allocation) growing with and without non-crop vegetation. As well, the concept of resource limitation i n explaining plant competition has gained interest amongst plant ecologists (see: Tilman 1985, Wilson and Tilman  13  1991, Campbell and Grime 1992, Gleeson and Tilman 1992, Inouye and Tilman 1995, Riegel et al. 1995, Wilson and Tilman 1995). The three main resources plants compete for are: light (Van Gerwin et al. 1987, Johnasson 1989, Nilsson and Hallgren 1993, Reynolds and Pacala 1993, Shangshan et al. 1997, Lieffers et al. 1999, McCarthy 2001), nutrients (McMurtrie and Wolf 1983, Turkington et al. 1993, Belsky 1994), and water (Rink and V a n Sambeek 1987, Drivas and Everett 1988, Petersen et al. 1988, Welden et al. 1988, Barton 1993, Gordon and Rice 1993). Of these, competition for light has received the greatest interest, particularly i n the development of competition indices. However, the accuracy and reliability of using light availability to define competition has not always warranted the effort and expense of data collection: competition indices (or models) have generally explained less than 50 % of crop-tree growth (Brand 1986). For example, i n a study of Douglas-fir seedling growth as a function of non-crop vegetation cover and height, Karakatsoulis et al. (1989) reported less than a 5 % correlation between Douglas-fir stem height and stem diameter and a competition index developed specifically for this species (Wagner 1982). The failure of these light-generated competition indices to account for the growth of Douglas-fir seedlings i n the presence of non-crop vegetation may be due, i n part, to the limited information available on the dynamics of competition over time and changes i n the availability of resources both seasonally and over several years; specifically, the seasonal and yearly changes i n light availability within deciduous canopies and the response of conifer seedlings to these changes. In coastal areas of southwestern British Columbia, terrestrial ecosystems are generally dominated by evergreen coniferous forests  14  consisting mainly of Douglas-fir, western hemlock, western redcedar, grand fir, and Picea sitchensis (Bong.) Carr. (Sitka spruce) at lower elevations with Abies amabilis Dougl. ex Forbes (Pacific silver fir), Tsuga mertensiana (Bong.) Carr. (mountain hemlock), and Chamaecyparis nootkatensis (D. Don) Spach (yellow cedar) dominating at higher elevations. Following clearcut logging, these ecosystems are usually invaded and occupied by fast growing, early successional species such as Rubus spectabilis Pursh (salmonberry), R. parviflorus Nutt. (thimbleberry), Gaultheria shallon Pursh (salal), Pteridium aquilinum (L.) Kuhn (bracken fern), Vaccinium membranaceum Dougl. ex Hook, (black huckleberry), Rhododendron albiflorum Hook, (white-flowered rhododendron), Alnus rubra Bong, (red alder), and Populus trichocarpa Torr.& Gray ex Hook, (black cottonwood), to name just a few. The extent of plant invasion, species composition, and persistence will depend, i n part, on the degree of disturbance, the site condition (in terms of moisture and nutrient availability), and the abundance of seed rain or the presence of a seed or bud bank (Kimmins 1987). Traditionally, forest managers have attempted to bypass the early successional stage characterized by these species by planting, or promoting the natural establishment of, later successional coniferous crop trees usually within two years of harvesting the original stand. However, the success of these plantations has varied, with some failing due to the rapid invasion and growth of fast-growing non-crop species which quickly overtop and shade the slower growing conifer seedlings (see Walstad and Kuch 1987 and references therein, Caza 1991). On fresh-to-moist and nutrient medium-to-rich sites the overtopping of conifer seedlings by non-crop vegetation results i n low light availability  15  which reduces the rate of photosynthesis by conifer species and ultimately causes a reduction i n their growth. On nutrient poor and/or dry sites, noncrop vegetation also competes with planted conifers for available nutrients and moisture, further reducing crop-tree growth (Conard and Radosevich 1982, Carter et al. 1984, Lanini and Radosevich 1986, Price et al. 1986, Messier et al. 1988, Messier and Kimmins 1992, Christie 1994, Chang et al. 1996, Prescott 1996). Reports of mid-summer understory light levels have ranged from 1 - 8 % beneath early serai vegetation following clearcut logging (Bigley 1988, Karakatsoulis et al. 1989, Newton and Comeau 1990, Ellsworth and Reisch 1996) to 1 - 10% beneath mid to late serai forest stands (Blackman and Rutter 1946, Anderson 1964, Lassoie et al. 1983, DePamphilis and Neufield 1989, Canham et al. 1990, Ellsworth and Reich 1992, Klinka et al. 1992, Ellsworth and Reich 1996). However, there have been few studies that have recorded the change in the understory light environment as a function of early vegetation succession and how this change affects conifer seedling growth over periods greater than two years. In addition, earlier studies have suggested that the use of fertilizers at time of planting may increase crop tree growth during the early stages of plantation establishment and accelerate the eventual over-topping of competing, non-crop vegetation (Carlson 1981, Brockley 1988). The objective of this study was to: i) determine the change i n understory light environment as a function of early vegetation development following clearcut harvesting (secondary succession), ii) compare the changes in growth of conifer seedlings growing within different early serai vegetation types and i n the open, iii) determine whether slow release spot fertilization at  16  time of planting increases the growth of conifer seedlings, and iv) compare mid-summer and seasonal changes in photosynthetic rates of Douglas-fir, western hemlock and grand fir seedlings growing beneath two types of early successional vegetation and i n the open.  STUDY A R E A  The experimental area is located in the southeastern corner of the Malcom Knapp Research Forest (49° 15' N and 122° 31' W) and falls within the Pacific Ranges Drier Maritime Coastal Western Hemlock biogeoclimatic subzone (Krajina 1965, Klinka and Krajina 1986, Pojar et al. 1991), southwestern British Columbia. The climate is described as wet, cool mesothermal with mild winters and dry, warm summers (Klinka and Krajina 1986). Mean annual precipitation is 2140 mm with approximately 600 mm falling between April and September. The driest and wettest months are July and December, respectively, which have a mean monthly rainfall of 66 mm (July) and 332 mm (December) (Pojar and Meidinger 1991). The mean annual temperature is 9.2 °C with 16.8 °C and 1.4 °C representing the mean temperatures of the warmest and coldest month, respectively. The site is located at an elevation of 140 m, has a 5% slope with a western aspect and has a soil developed i n a gravely sandy loam outwash over an impervious layer at a depth of 1 - 2 m. The area (approximately 2.3 4  ha) was clearcut logged i n 1984-85 and the remaining stumps, slash, debris and large rocks were removed, leaving a relatively uniform area free of vegetation and debris (Plate 2.1).  The research area is adjacent to a 5 ha block which was harvested in 1982-83.  Plate 2.1. General view of the site following harvesting and site preparation.  18  MATERIALS AND METHODS Experimental Design In the spring of 1987, sixteen 20 x 25 m plots were established on the site i n a randomized complete block split-plot design consisting of two blocks, eight treatment plots per block and three conifer species per plot (Douglas-fir, grand fir, and western hemlock) (Fig. 2.1). These three species were chosen on the basis of their reported levels of shade tolerance: Douglas-fir, grand fir, and western hemlock are rated as shade intolerant, intermediate, and shade tolerant, respectively (Klinka et al. 1989). As well, all three species are important commercial species and are planted extensively throughout their natural range i n southwestern British Columbia. A fourth species, shade tolerant western redcedar, was originally planted with the above species but the extent of browsing by deer and the subsequent damage to seedlings forced me to remove this species from the study. Blocking was applied to remove variation i n down-slope soil moisture differences and i n proximity to the adjacent second growth forest. Within each plot, thirty-three seedlings of each of the three conifer species were planted i n a 2 x 2 m spacing regime. Each species was grouped i n three rows per plot and the order of grouping was randomly assigned for each plot (Fig. 2.1). A l l seedlings were provided by the British Columbia Ministry of Forests. Seedling age and stock type for each species are shown i n Appendix 1. Due to differences i n seedling age between species at time of planting, only differences i n growth between treatments for each species were compared over a four year period. However, an attempt is made to compare overall growth trends between the three species. The total number of seedlings planted was 1584 with each species represented by 528 seedlings.  19  No Veg. + Fertilizer  Shrubs + Fertilizer  Alder + Fertilizer  Alder  Control + Fertilizer  Shrubs  Control  No Veg.  Control + Fertilizer  No Veg. + Fertilizer  Alder  Alder + Fertilizer  Control  No Veg.  Shrubs + Fertilizer  Shrubs  GF  GF  GF  WH  WH  WH  DF  DF  DF  GF  GF  GF  WH  WH  WH  DF  DF  DF  GF  GF  GF  WH  WH  WH  DF  DF  DF  GF  GF  GF  WH  WH  WH  DF  DF  DF  GF  GF  GF  WH  WH  WH  DF  DF  DF  GF  GF  GF  WH  WH  WH  DF  DF  DF  GF  GF  GF  WH  WH  WH  DF  DF  DF  GF  GF  GF  WH  WH  WH  DF  DF  DF  GF  GF  GF  WH  WH  WH  DF  DF  DF  GF  GF  GF  WH  WH  WH  DF  DF  DF  GF  GF  GF  WH  WH  WH  DF  DF  DF  25 m  20 m  Fig. 2.1. Diagrammatic representation of the treatment plots and seedling placement within one treatment plot (GF = grand fir; W H = western hemlock; D F = Douglas-fir). Seedlings were planted 2 m apart with a 2 m buffer strip surrounding each plot.  20  Treatments Following the planting of conifer seedlings, the following eight treatments (four vegetation treatments x two fertilizer treatments) were randomly assigned to one of the eight plots within each of the two blocks: 1. Planting 2-year-old red alder seedlings beside each conifer seedling plus conifer fertilization (all planted conifer seedlings were fertilized at time of planting with forty grams of 14-14-14 (NPK) slow release fertilizer (Osmocote ®, Sierra Chemical Co., California)), 2. Planting red alder seedlings beside each conifer seedling, 3. Control plus fertilizer - all non-crop vegetation was allowed to develop naturally following fertilization at time of planting of each conifer seedling, 4. Control - all non-crop vegetation was allowed to develop naturally with the conifer seedlings, 5. Removal of all above-ground vegetation plus conifer fertilization, 6. Removal of all above-ground vegetation, 7. Removal of all deciduous trees and maintaining a shrub and herb community plus conifer fertilization, 8. Maintaining a shrub community (i.e., treatment #7 without the fertilization).  The "no above-ground non-crop vegetation" plots (treatments # 5 and 6) were maintained during the course of the study by cutting all non-crop vegetation at ground level using brush saws and hand clippers one to two times each growing season over the course of four years. The shrub plots were allowed to develop naturally i n the abscence of deciduous trees which were either cut down or allowed to develop to a maximum height of two meters (the choice of two meters corresponds with  21  the average maximum height attained by most west coast shrub species found growing on this type of site [Haeussler and Coates 1986]). The red alder treatments (treatment numbers 1 & 2) consisted of transplanting 2-year-old naturally established red alder seedlings from a nearby clearing to the study area. Seedlings were excavated with their entire root system virtually intact, placed in water-filled containers to reduce wilting, transported to the study site and planted immediately beside each conifer seedling. A l l vegetation, except the red alder and planted conifer seedlings, was cut at ground level once every year during mid-summer over the course of the study. The fertilizer treatments were applied by mixing 40 g of 14-14-14 (NPK) slow release fertilizer (Osmocote® Sierra Chemical Co.) into the soil at the base of each seedling soon after planting.  Measurements Vegetation Development The growth and development of non-crop vegetation was monitored by estimating percent cover and measuring height of vegetation i n late summer of 1987, 1989, 1990 and 1991. Sampling during the 1987 season was conducted by randomly sampling ten 1 x 1 m quadrats within each of the 16 plots prior to treatment establishment. Within each quadrat, percent cover of each species present was estimated visually. The average height of each species was measured to an accuracy of five centimeters using a one meter ruler and a telescopic measuring pole. Sampling i n 1989, 1990 and 1991 was limited to just the control plots. Quadrat number, size, and sampling method  22  remained the same for the 1989 measurements but slight modifications were made i n the subsequent two years (1990 and 1991). Because plant size (% cover and height) changed from the initial measuring period (1987), the original 1 x 1 m quadrat was no longer adequate. Therefore, quadrat size was increased to 5 m using a circular plot with a radius of 1.26 m. The 2  number of sample quadrats was reduced from ten to five for each plot measured.  Growth of Planted Conifer Seedlings and Red Alder Stem diameter (root collar) and height of all planted conifer seedlings were measured at the end of each growing season (1988, 1989, 1990 and 1991). A l l planted red alder trees were measured for stem height using a telescopic measuring pole and stem diameter using both mechanical and digital calipers to an accuracy of 0.1 mm at the end of the 1989, 1990, 1991 and 1998 growing seasons.  Changes in Understory Light Levels Light attenuation within the four vegetation treatments was measured during the mid-growing season i n 1989, 1990, 1991 and 1992. The sampling regime consisted of measuring photosynthetically active radiation (PAR) at 50 cm above the ground surface at ten randomly chosen points (sampling points coincided with planted conifer seedlings) using a Line Quantum Sensor (LICOR, Lincoln, Nebraska). Measurements were made between 11:30 A M and 1:30 P M on clear days. Measurements of incoming light levels  23  were made by periodically measuring PAR i n the open during the course of the sampling period.  Seasonal Changes in Soil Temperature Changes i n soil temperature (at 15 cm depth) were measured by inserting three alcohol thermometers in semi-open aluminum sleeves into the soil to a depth of 15 cm within each of three treatment plots (control, open and alder) during the seedling photosynthesis measurement period (July 1991 - June 1992). The choice of 15 cm depth corresponded with the average rooting depth of the planted conifer seedlings. The thermometers were allowed to equilibrate for a minimum of 30 minutes before each temperature reading.  Seedling Photosynthesis Seedling gas exchange measurements were made using a LICOR 6200 Portable Photosynthesis System (LICOR, Lincoln, Nebraska) consisting of a closed system infra-red gas analyzer connected to a microcomputer console and leaf measurement chamber. Measurements were made on five seedlings per species i n three of the eight treatments (alder + seedling fertilization, control + seedling fertilization, and open + seedling fertilization) between 11:00 A M and 1:00 P M on clear days or slightly overcast days every 4 - 2 0 days over a period of eleven months (July 1991 to June 1992). To maintain consistency i n sampling between seedlings growing in the open and those growing beneath the control and alder plots, a protocol was developed where a draw-down time of 30 seconds was used for each sample taken. The  24  sequence of measuring seedlings between and within treatments was changed for each measuring day to reduce diurnal bias. Net photosynthesis of Douglas-fir and grand fir seedlings was determined by clamping a one-litre chamber (LICOR 6000-12) over a portion of a one-year-old branch from the second whorl and recording the change i n CO2 concentration over a 30 second period. Measurements on hemlock seedlings were made by clamping the leaf chamber 10 cm from the tip of a lateral branch located mid-way up the stem. Two fans within the chamber maintained a minimum boundary layer resistance around the leaf sample. Chamber air and leaf temperature were monitored with chromel-constantan thermocouples. Chamber humidity and irradiance (directly above the chamber) were monitored with a Vaisala Humicap sensor and LICOR L i 190s-1 quantum sensor, respectively. Chamber humidity was maintained at or near ambient levels during each 30 s measurement. The length of time taken to complete one measurement and move to the next seedling was approximately 3 minutes. Therefore, the total number of seedlings that could be effectively measured during a two hour mid-day period was approximately 45.  RESULTS and DISCUSSION Vegetation Development and Changes in Understory Light Availability Vegetation Development Fifteen species (not including thallophytes) were found growing on the control plots from 1987 to 1991. Of these, the most ubiquitous were Alnus  25  rubra, Anaphalis  margaritacea,  Epilobium angustifolium,  munitum, Populus trichocarpa, Pteridium parviflorus,  Polystichum  aquilinum, Rubus leucodermis,  R. spectabilis, R. ursinus, Salix scouleriana, Solidago  R.  canadensis  and Spiraea douglasii (Table 2.1). A l l of these species were present i n 1987 and persisted, with different frequency, until 1991. The mean percent cover and height of vegetation growing i n 1987 on the control plots were 48 % and 0.39 m, respectively (Fig. 2.2). The percent cover of individual species (Table 2.1) ranged from < 1 to 14 % with the most abundant species represented by Populus trichocarpa (14 % cover), an early successional species which regenerates freely on moist, nutrient medium-rich sites (Klinka et al. 1989, DeBell 1990). There was little change i n total vegetation cover between 1987 and 1989, but the mean vegetation height had more than doubled (0.83 m) during the same period. The greatest changes i n species height were for the three tree species present; Alnus rubra, Populus trichocarpa,  and Salix scouleriana. Mean height for each of these three  species increased 7.8, 2.2, and 2.9 fold over this two year period, respectively (Table 2.2). By 1990, mean vegetation height (Fig. 2.2) had reached 1.69 m and had increased to 2.75 m by the following year (1991). In conjunction with height growth, mean vegetation cover had increased approximately twofold from 1989 to 1990 and by 1991 (6-years post-clearcutting) had reached 102 % (Fig. 2.2). Plate 2.2 shows an aerial photograph of the site five years following treatment establishment. The rapid development of vegetation cover on this site contrasts with other post-clearcut vegetation growth studies. Karakatsoulis and Kimmins (1993), studying ten years of vegetation development following clearcut logging within the same biogeoclimatic subzone, found approximately 60%  T A B L E 2.1 Vegetation percent cover development by species on the control treatment plots. Numbers in parenthesis represent one standard error of the mean; n = 80, 30, 20, and 20 for measurements taken in 1987, 89, 90 and 91, respectively.  Plant species  1987  a  Percent cover by year 1989 1990  1991  Alnus rubra Bong.  2 (0.4)  7 (3.6)  17 (0.2)  24 (0.9)  Anaphalis  margaritacea (L.) Benth. & Hook.  2 (0.8)  2 (0.8)  5 (0.6)  3 (0.9)  Epilobium  angustifolium L .  <1 (0.1)  <1 (0.1)  <1 (0.1)  <1 (0.2)  Polystichum munitum (Kaulfuss) Presl  <1 (0.1)  <1 (0.1)  <1 (0.1)  <1 (0.1)  Populus trichocarpa Torr. & Gray ex Hook.  14 (0.8)  23 (2.8)  33 (3.8)  46 (5.2)  Pteridium aquilinum (L.) K u h n i n Decken  1 (0.5)  2 (1.5)  4 (1.8)  4 (1.4)  Rubus leucodermis Willd.  3 (0.6)  <1 (0.3)  <1 (0.3)  <1 (0.3)  Rubus parviflorus  Nutt.  3 (0.6)  <1 (0.1)  4 (1.2)  8 (1.7)  Rubus spectabilis Pursh  5 (0.7)  2 (0.7)  4 (1-3)  8 (1.2)  Rubus ursinus Cham. & Schlecht.  1 (0.2)  <1 (0.1)  2 (0.9)  1 (0.7)  Salix scouleriana Barratt  1 (0.4)  1 (0.3)  6 (1.6)  3 (1.2)  <1 (0.1)  1 (0.7)  < i (o.i)  <1 (0.3)  1 (0.3)  <1 (0.2)  2 (0.9)  3(0.8)  Solidago canadensis L . Spiraea douglasii Hook.  a  Nomenclature follows Hitchcock and Cronquist, 1981.  T A B L E 2.2. Vegetation mean height by species on the control treatment plots. Numbers in parantheses represent one standard error of the mean; n = 80, 30, 20, and 20 for measurements taken i n 1987, 89, 90, and 91, respectively.  Plant s p e c i e s «  1987  M e a n height (m) by year 1989 1990  1991  Alnus rubra Bong.  0.32 (0.03)  2.50 (0 46)  3.50 (0.27)  4.82 (0.37)  Anaphalis  margaritacea (L.) Benth. & Hook.  0.47 (0.07)  0.60 (0 05)  0.86 (0.04)  0.68 (0.05)  Epilobium  angustifolium L .  0.15 (0.03)  0.15 (-)  0.20 (-)  0.95 (0.05)  Polystichum munitum (Kaulfuss) Presl  0.22 (0.07)  0.43 (- )  0.60 (-)  0.55 (-)  Populus trichocarpa Torr. & Gray ex Hook.  0.61 (0.06)  1.35 (0 10)  1.63 (0.10)  2.48 (0.16)  Pteridium aquilinum (L.) K u h n i n Decken  0.42 (0.07)  0.63 (0 04)  0.85 (0.08)  1.41 (0.16)  Rubus leucodermis Willd.  0.59 (0.05)  0.83 (0 14)  0.91 (0.07)  0.78 (0.23)  Rubus parviflorus  Nutt.  0.18 (0.01)  0.25 (0 03)  0.91 (0.07)  0.78 (0.23)  Rubus spectabilis Pursh  0.29 (0.02)  0.52 (0 05)  0.67 (0.06)  0.90 (0.07)  Rubus ursinus Cham. & Schlecht.  0.10 (0.01)  0.10 (-)  0.05 (0.01)  0.15 (0.03)  Salix scouleriana Barratt.  0.50 (0.06)  1.46 (0 27)  1.69 (0.11)  1.98 (0.13)  Solidago canadensis L .  0.60 (-)  0.64 (- )  1.70 (-)  0.70 (-)  Spiraea douglasii Hook.  0.52 (0.08)  0.69 (0 23)  1.11 (0.09)  1.20 (0.07)  a  Nomenclature follows Hitchcock and Cronquist, 1981.  28  120  r-  100 h w u  > 0  80  o  .o  «  60  w  > 40  Sum  o  ;  1  LI 1987  •„. • L I 1988  198?  I.I  I.I  1990  1991.  1990  1991  Year  69  1987  1988  1989 Year  Fig. 2.2. Changes i n sum of mean percent cover and height of vegetation growing on the control plots from 1987 - 1991. Bars represent one standard error of the mean, n = 10 for both vegetation cover and vegetation height for each year.  29  Plate 2.2 Aerial photograph of the experimental site five years following treatment establishment. The treatment plots are located at the left of the clearing.  30  cover on 6-year-old hygric, mesic and xeric clearcuts. Mean vegetation height on these sites was approximately 0.9, 0.8 and 0.6 m on hygric, mesic and xeric sites, respectively. Similarly, Messier and Kimmins (1991), studying post-clearcutting vegetation development on northern Vancouver Island, recorded 45 and 55% vegetation cover on 4- and 8-year-old clearcuts, respectively, and 75% cover on 15-year-old clearcuts. The dominant plant species on these sites was Gaultheria shallon (salal), an ericaceous evergreen shrub that rarely exceeds 1.2 m i n height when grown in the open (Clark 1976). The difference i n vegetation development following clearcut logging between the two above studies and this study may be due to differences i n the extent of soil disturbance caused by harvesting and site preparation. Vegetation invasion, establishment, species composition, and rate of growth following disturbances such as clearcut logging depend, i n part, on the presence of a seed or bud bank (vegetative propagules), the seed rain, the quality of the site (i.e. soil moisture and soil nutrient availability), the timing of the disturbance, and the extent to which the forest floor has been disturbed to create suitable seedbeds for early pioneer species (Kimmins 1987). Many fast-growing, early serai shrub and herbaceous species require exposed mineral soil for seed germination and high light levels for maximum growth (see Haeussler and Coates [1986] for a review of the autecological characteristics of the major herb, shrub and non-coniferous tree species of British Columbia). The degree of soil disturbance on my site was very high, with the majority of the forest floor having been removed during harvesting and site preparation, resulting i n approximately 100% mineral soil exposure (Plate 2.1). This high degree of mineral soil exposure, coupled with the site's  31  mesic-subhygric moisture regime and medium nutrient regime, produced ideal conditions for rapid vegetation invasion and growth. In contrast, the degree of soil disturbance on the sites reported by Karakatsoulis and Kimmins (1993) and Messier and Kimmins (1991) was minimal, resulting i n very little exposed mineral soil. This created an unsuitable seedbed for fastgrowing species such as red alder, cottonwood, and thimbleberry (to name just a few) and favoured slower growing and rhizomatous species, such as salal, which can regenerate from bud banks and grow on nutrient-poor and semi-decayed organic substrates (Klinka et al. 1989, Huffman et al. 1994). By the end of the 1991 growing season (6 years post-disturbance), the dominant plant species on the control plots were red alder (24 % cover) and black cottonwood (46 % cover) (Table 2.1) with a mean height of 4.82 and 2.48 m, respectively (Table 2.2). The rate of growth of naturally established red alder on the control plots and the planted red alder seedlings on the alder treatment plots was high, and matches the growth rate of red alder seedlings reported for similar sites i n the Pacific Northwest (Harrington and Curtis 1986). A t the end of the fifth growing season following the transplanting of 2 year-old red alder seedlings, mean stem height and stem diameter (at root collar) were 4.75 m and 28.0 mm, respectively (Fig. 2.3). This rapid early growth of red alder is the main reason why forest managers have tried to prevent this species from becoming established within slower growing conifer plantations or to remove (or control) it once established. The growth of red alder is greatest on nutrient medium-to-rich and moist low elevation sites within the Pacific Northwest of the United States and southwestern portions of British Columbia (Klinka et al. 1989). Within these areas, this species quickly invades and overtops conifer plantations (Walstad et al. 1987), which  32  20 16 1 ^ E oo  i  12 H 8 4 0  H  n 1989  1991  1993  1995  1997  Year  Fig. 2.3. Changes i n mean stem diameter (at root collar) and stem height of planted red alder seedlings over a ten-year period. Bars represent one standard error of the mean, n = 25 for both stem diameter and stem height measurements for each year.  33  often results i n reduced conifer growth and increased susceptibility to wind and snow damage due to the increase i n the stem height:diameter ratio of some species (most notably, Douglas-fir) and the reduction i n root growth. In cases of severe overtopping, conifer seedling mortality occurs.  Changes in Understory Light Availability Associated with this rapid increase in vegetation development within the control and alder plots was a steady decrease i n mid-summer understory light availability. The proportion of incoming light reaching the understory (at 50 cm above ground surface) beneath the control and alder plots i n midsummer was approximately 57 and 74%, respectively, i n 1989 (4 years postclearcutting), and declined steadily through to 1992 (Fig. 2.4). By 1992 (7 years post-clearcutting) mean mid-summer light levels beneath the control and alder plots had declined to 30 and 42 umol n r s- , respectively, which 2  1  represented approximately 2 % (control) and 3 % (alder) of the light available in the open. Remeasurement of these plots i n 1998 showed similar low light levels. This decrease i n understory light availability was strongly correlated with the increase in vegetation cover within the control treatment (Fig. 2.5). While mid-summer understory light levels declined steadily within the control and alder treatments over the four-year study period, the seasonal pattern of light availability was characterized by distinct lows and peaks. Light levels within the control treatment remained low during the summer period (< 5%) as a result of vegetation cover. However, with the commencement of autumnal leaf fall, the proportion of understory P A R increased from 4 to 37% of PAR in the open between mid- September and  34  Fig. 2.4. Mid-summer changes i n understory light availability (% of P A R i n the open) over a ten-year period beneath the control and alder treatments. Bars represent one standard error of the mean; n = 40.  35  Fig. 2.5 Correlation between understory PAR (as a % of PAR i n the open) and the sum of vegetation cover (control treatment). Model used to describe the line is: %PAR = 96.3 - 0.89 •(% cover). Bars represent one standard error of the mean, n = 10 for each each point.  36  Fig. 2.6. Seasonal changes i n understory light availability (measured as % light i n the open) within the control (A) and alder (•) treatments. Dates given as month/day. n = 5 for each sample point; standard error of the means are not shown.  37  mid-October. By mid-winter, understory light levels ranged from 45 - 70 % of P A R recorded i n the open (Fig. 2.6). With the spring flushing of leaf buds, understory light levels began to decline from approximately 60 % i n late March to less than 5 % by the end of April. A similar pattern was observed within the alder treatment. This seasonality i n understory light availability is characteristic of temperate deciduous vegetation and may be a significant factor for evergreen understory plants. The next step was to determine whether Douglas-fir, western hemlock and grand fir seedlings were capable of responding photosynthetically to these changes.  Conifer Seedling Photosynthesis Mid-summer net photosynthetic rates of Douglas-fir, grand fir and western hemlock seedlings growing within the control treatment averaged 0.6, 0.9 and 1.5 umol n r s , respectively, and corresponded with the very low 2  1  light environment the seedlings were growing i n (Fig. 2.7). Understory P A R levels within the control treatment were on average less than 30 umol n r s 2  -1  and represented < 2% of PAR levels found i n the open (« 1725 umol n r s ). 2  1  In contrast to P n rates of the three species growing i n the understory of the control treatment, seedlings growing i n the open were photosynthesizing at or near their maximum rates. The mean P n rate of open grown Douglas-fir was 8.4 umol n r s , which corresponds to P n rates reported for Douglas-fir 2  1  under saturating light conditions (Krueger and Ruth 1969, Hollinger 1987, Harrington et al. 1994). Similarly, mean Pn rates of open grown grand fir and western hemlock seedlings were 9.0 and 8.3 umol n r s , respectively. 2  1  38  10 rfc to  •3: its  :0 •Cb;n'trd.l • Alder' Open  CSJ: O  0 Control. •  Alder;. Open  Species  Fig. 2.7. Mid-summer understory PAR and net photosynthetic rates (Pn) of Douglas-fir (DF), grand fir (GF), and western hemlock (WH) seedlings growing within the control + fertilizer, alder + fertilizer, and open + fertilizer treatments. Bars represent one standard error of the mean, n = 5 and 10 for the P n and P A R measurements, respectively. Data for 1992, 7 years after clearcutting, 5 years after planting.  39  Mean light levels beneath the alder stand (39 umol n r s- ) were also low 2  1  during the 1992 mid-summer period, but were slightly higher than P A R levels found i n the control treatment, resulting in slightly higher P n rates for the three conifer species. Mean P n rates for the Douglas-fir, grand fir and western hemlock seedlings growing within the alder stand were 1.3, 1.6 and 2.1 umol m ^ s , respectively (Fig. 2.7), and represented approximately 15, 17 1  and 25 % of the mean photosynthetic rate attained by the open grown seedlings. While these Pn rates are typical of the low light conditions these species were growing in, it is still unclear whether the ability to maintain a positive CO2 uptake rate (albeit very low) under these low light conditions is sufficient to maintain perennial plant growth over several years. The next step was to determine whether the three conifer species were able to respond (photosynthetically) to seasonal changes i n understory light availability. Figure 2.8 shows the changes i n mid-day photosynthesis by understory Douglas-fir, grand fir and western hemlock seedlings over a one-year period. These changes corresponded to changes i n the understory light environment during the summer, fall and spring period (Fig. 2.6) and to changes i n air and soil temperatures during the winter months (Fig. 2.9). Two major P n peaks were evident. The first one occurred during the fall period when understory light levels were increasing as a result of overstory leaf senescence and leaf fall. The second occurred in the early spring prior to leaf flush and leaf expansion by red alder and shrubs and coincided with an increase i n air and soil temperatures (Fig. 2.9). Mid-winter P n rates by the three conifer species were depressed as a result of lower air and soil temperatures (Figs. 2.9). However, winter mid-  40  •%  S/10  9/19 10/29' jZ/8 1/17 , 1991 I .Date  2/26  4/6 1992  5/16  6/23  2/26  4/6: 1992  5/16. '6/25  Mr  %l  *?/l  8/10  8/10  9/19 10/29 1991  9/19: 10/29 1991  12/8  1/17 | Date  12/8:  1/17  1  2/26  4/6 1992  5/16  6/25  :Date  F i g . 2.8. C h a n g e s i n m i d - d a y n e t p h o t o s y n t h e s i s o f D o u g l a s - f i r , g r a n d f i r , a n d w e s t e r n h e m l o c k s e e d l i n g s g r o w i n g w i t h i n t h e a l d e r ( • ) a n d c o n t r o l (•) t r e a t m e n t s a n d i n t h e o p e n ( • ) over a one-year p e r i o d , n = 5 for e a c h d a t a p o i n t . E r r o r b a r s a r e n o t s h o w n . D a t e s are g i v e n as m o n t h /day.  41  %\  8/10:. S/IS 10/29 1991- "-  12/8  1/17 ;j  .2/26;  4/6 1992.  5/16  6/25  Date *.ALDER * € O N T R O L •QPEN  Fig. 2.9. Changes in mid-day air and soil temperatures (at 15 cm depth) within the alder and control treatments, and in the open, n = 15 for each data point. Error bars are not shown. Dates are given as month/day.  42  day air and soil temperatures remained above freezing during the study resulting i n positive, albeit low, CO2 uptake rates.  Conifer Seedling Growth The height and stem diameter of the three conifer species within the eight treatments showed similar growth trends after one year of growth (Figs. 2.10, 2.11 and 2.12; see also Appendices 2.1 - 2.9 for a summary of analysis of variance for the three species). However, by 1991, some treatment effects were evident. Douglas-fir stem diameter was significantly greater when fertilized at planting and grown in the open i n the absence of over-topping vegetation than i n other treatments (Fig 2.10 and Appendix 2.11). Similarly, after four years of growth, the stem diameter of western hemlock was greatest within the open + fertilized treatment (Fig. 2.12 and Appendix 2.13). Grand fir, on the other hand, did not exhibit similar results. The stem diameters of grand fir growing within the two alder and open stands were not significantly different from each other (Fig. 2.11 and Appendix 2.12). However, the lowest stem diameter growth for grand fir was found on seedlings growing within the unfertilized control treatment. There was no clear pattern between treatment and height growth of individual conifer species. Some of the individual seedlings growing i n the control, alder and shrub treatments were as tall as some of the seedlings growing i n the open. Tree seedlings grown under low light levels have sometimes been reported to grow as tall, if not taller, than comparable seedlings grown under high light levels (Atkinson 1984). Previous studies have shown that of the two most widely used non-destructive growth variables, stem height and stem diameter, height growth is least correlated  43  2.5  • 1988 • 1989 • 1990 • 1991  2  3  4  5  6  7  8  Treatment  Fig. 2 . 1 0 . Four years of growth response (stem diameter and stem height) of Douglas-fir seedlings growing within the eight treatments ( 1 = alder + fertilized; 2 = alder; 3 = control + fertilized; 4 = control; 5 = open + fertilized; 6 = open; 7 = shrub + fertilized; 8 = shrubs). Bars represent one standard error of the mean; n = 69 - 75 for each mean.  44  1.8 1.6  £  1  r  1.2  i  1  JC  |  11988 11989 • 1990 • 1991  4  0.8 0  6  53 o.4 0.2 0  2  3  4  5  6  Treatment  Fig. 2.11. Four years of growth response (stem diameter and stem height) of grand fir seedlings growing within the eight treatments (1 = alder + fertilized; 2 = alder; 3 - control + fertilized; 4 = control; 5 = open + fertilized; 6 = open; 7 = shrub + fertilized; 8 = shrubs). Bars represent one standard error of the mean; n = 69 - 7 5 for each mean.  45  Fig. 2.12. Four years of growth response (stem diameter and stem height) of western hemlock seedlings growing within the eight treatments (1 = alder + fertilized; 2 = alder; 3 = control + fertilized; 4 = control; 5 = open + fertilized; 6 = open; 7 = shrub + fertilized; 8 = shrubs). Bars represent one standard error of the mean; n = 69 - 75 for each mean.  46  with light treatment effects, especially at the seedling stage (Coates 1987, Wagner 1989, Caza 1991). This may be explained by the allocation of growth that plants exhibit when grown under different resource levels. Plants will generally allocate growth to that plant component which can reduce the effects of the most limiting factor (Iwasa and Roughgarden 1984, Reynolds and Pacala 1993, Canham et al. 1996). For example, plants grown under low light levels will tend to allocate a greater proportion of carbohydrate production to shoot growth than plants growing under high light levels (Brix 1967, Ledig et al. 1970, Thornley 1972, Chapin 1980, Atkinson 1984, Luken et al 1995). Likewise, plants growing under dry or nutrient poor conditions will tend to allocate more carbohydrate production to belowground components (Philpson and Coutts 1977, Ingestad and Lund 1979, Keyes and Grier 1981, Comeau 1986, Gleeson and Tilman 1992, Reynolds and Pacala 1993, Wilson and Tilman 1995, Canham et al. 1996). However, after 10 years of growth, there were distinct differences in both height and diameter growth between seedlings (of all three species) growing i n the open and those seedlings growing within the alder and control treatments (Fig. 2.13, 2.14 and 2.15). Open grown Douglas-fir had stem height and stem diameter growth values that were approximately 100% greater than seedlings growing within the alder and control treatments. As well, Douglas-fir mortality (not shown) was approximately 50% after 13 years within the alder and control treatments. Some of this mortality could be attributed to snow press and consequent stem breakage. Douglas-fir growing in the shade or under high densities tend to grow tall and spindly (Mitchell 2003) making them susceptible to stem breakage due to snow accumulation during the winter months.  47  ^ 10 E 8  • 1988  o •*-> 6 o  • • • •  E ro  Q  E  o  *•>  1  2  3  4  5  6  7  1989 1990 1991 1998  8  Treatment  _  10  §  8 • 1988  r-  6 E o CO  • 1989  4  • 1990  2 0  • 1991 3  4  5  6  7  8  11998  Treatment  Fig. 2.13. Stem diameter and height growth response of Douglas-fir growing within the eight treatments (1 = alder + fertilized; 2 = alder; 3 = control + fertilized; 4 = control; 5 = open + fertilized; 6 = open; 7 = shrub + fertilized; 8 = shrubs) following four years of growth and after ten years of growth within the alder +fertilized (1), control + fertilized (3) and open + fertilized (5) treatments Bars represent one standard error of the mean; n = 69 - 75 for each mean.  48  12 E  o  10  I  •  1988  •  1989  6  •  1990  4  •  1991  2  •  1998  • • • • •  1988 1989 1990 1991 1998  8  ro  CO  ifn 4  0 3  4  5  6  8  Treatment  8 7  I  6 5 4 3 2 1 0  I  •ill  1  2  3  4  5  6  ml  7  m\  i  8  Treatment  Fig. 2.14. Stem diameter and height growth response of grand fir growing within the eight treatments (1 = alder + fertilized; 2 = alder; 3 = control + fertilized; 4 = control; 5 = open + fertilized; 6 = open; 7 = shrub + fertilized; 8 = shrubs) following four years of growth and after ten years of growth within the alder +fertilized (1), control + fertilized (3) and open + fertilized (5) treatments Bars represent one standard error of the mean; n = 69 - 75 for each mean.  10 E  o  * E .ro 3 T3  H  1988  • 1989  6  • 1990  4  • 1991  E  • 1998  a  CO  1  2  3  4  5  6  7  8  Treatment  I  • 1988 • 1989 • 1990 • 1991 • 1998  i  1  i  2  i  3  rffi  i  4  5  6  if!  r  7  r  jfl  8  Treatment Fig. 2.15. Stem diameter and height growth response of western hemlock growing within the eight treatments (1 = alder + fertilized; 2 = alder; 3 = control + fertilized; 4 = control; 5 = open + fertilized; 6 = open; 7 = shrub + fertilized; 8 = shrubs) following four years of growth and after ten years of growth within the alder +fertilized (1), control + fertilized (3) and open + fertilized (5) treatments Bars represent one standard error of the mean; n 69 - 75 for each mean.  50  This morphological plasticity can be better illustrated by using heightrdiameter ratios as a better variable i n determining seedling growth response. Appendix 2.10, 2.11, and 2.12 show the height:diameter ratios for Douglas-fir, grand fir, and western hemlock seedlings, respectively, grown under the various treatments. For each species, seedlings grown i n the open (in the absence of light competition) had the lowest height:diameter ratios after four years of growth. Conversely, the highest ratios were recorded for seedlings growing within the control and shrub treatments which correspond with the very low light levels associated with these treatments. Seedlings grown within the alder treatment (both fertilized and unfertilized) had height:diameter ratios (after four years of growth) which were intermediate between seedlings grown in the open and within the control and shrub treatments, and which correspond to the slightly higher light availability recorded within the alder stands. Whether these trends i n height and diameter growth of the three species growing within the various treatments will continue cannot be assessed from this relatively short study period. However, we can speculate that Douglas-fir, which is classified as a shade intolerant species (Klinka et al. 1989, Hermann and Lavender 1990, Carter and Klinka 1992), will not be able to maintain a relatively high rate of height growth under the low light conditions found within the control and alder treatments due to the persistent overtopping by faster growing red alder (control and alder treatments), willow, and black cottonwood (control treatment). The allocation to height growth by understory Douglas-fir is at the expense of stem diameter growth which results in a tall, spindly tree susceptible to wind damage and snow press (Mitchell 2003) and eventually mortality.  51  The similar growth response of western hemlock and grand fir to that of Douglas-fir was unexpected. Because western hemlock and grand fir are classified as shade tolerant and shade tolerant to shade intolerant, respectively (Klinka et al. 1989, Packee 1990, Foiles et al. 1990, Carter and Klinka 1992, Klinka et al. 2000), we would expect to see a more balanced allocation between height and diameter growth (i.e. minimal etiolation) i n these two species compared to Douglas-fir. In other words, the "shade tolerance" of these two species should allow them to survive and grow within low light environments and eventually, given enough time, outgrow their less shade tolerant competitors. This concept of limited resource use, or resourceratio hypothesis, proposed by Tilman (1985) predicts that plants that can utilize a resource, such as light, at low levels will be better able to compete than those plants that require higher resource levels. However, the growth response of western hemlock and grand fir grown under low light levels during early plantation establishment i n this study is similar to the growth response of shade intolerant plants (such as Douglas-fir) and is consistent with Grime's (1973, 1979) theory that plants with the highest growth rates (this can be applied to height growth) will eventually outgrow their competitors. The growth results from this study show that all three species (Douglas-fir, western hemlock, and grand fir) respond similarly (after four years of growth) to low levels of light regardless of their shade tolerance classification. This concept of growth response to low light availability and resource allocation by conifer seedlings will be discussed further i n the following chapters.  52  CONCLUSIONS  The results from this study described the rapid invasion and establishment of non-crop vegetation following clearcut logging and intensive mechanical site preparation on nutrient medium to rich, and fresh to moist sites within the CWHdm subzone. Correlated with this rapid invasion was a linear decrease i n understory light availability over the four year study period. Light levels within the control and alder treatments (four years postclearcutting) varied from 57 to 74 %, respectively, of light levels i n the open, and from 2 (control) to 3 % (alder) of light levels i n the open within 7 years of harvesting. These low understory light levels resulted i n low mid-summer photosynthetic rates by planted Douglas-fir, grand fir, and western redcedar seedlings. However, mid-day photosynthetic rates by the above species were shown to be dynamic over the year, with two distinctive peaks occurring i n the fall and spring. This corresponded with changes i n understory light availability as a result of leaf senescence and leaf-fall i n the fall and winter and bud break and leaf expansion in the spring and early summer by the deciduous non-crop vegetation. This seasonality in understory light availability and the resultant photosynthetic response by evergreen conifer seedlings helps to explain the persistence and growth of evergreen conifers within early successional deciduous vegetation. For Douglas-fir and western hemlock, the greatest stem diameter growth i n the four competition treatments occurred with seedlings growing i n the open (absence of above-ground competition from early-seral vegetation). For Douglas-fir, the greatest mean stem diameter after four years of growth was achieved with fertilizing at time of planting and growing i n the open. However, the hypothesis that fertilizing conifer seedlings at time of planting  will help them to outgrow non-crop vegetation during plantation establishment was not substantiated i n this study. Fertilized seedlings of three species growing i n the control, alder and shrub treatments did not outgrow the deciduous vegetation and were overtopped by year four.  54  CHAPTER 3. SEASONAL TRENDS IN LIGHT AVAILABILITY, PHOTOSYNTHETIC RESPONSE AND O V E R A L L GROWTH OF UNDERSTORY THUJA  PLICATA  SEEDLINGS  INTRODUCTION As outlined i n Chapter 2, evergreen plants growing i n temperate climates with relatively mild winters can carry on photosynthesis outside the growing season as long as air and soil temperatures are above some critical level (Rutter 1957, Tranquillini 1964, Pollard and Wareing 1968, Emmingham and Waring 1972, Neilson et al. 1972, Bradbury and Macolm 1979). Evergreen understory plants growing i n temperate deciduous tree stands may be able to achieve significant annual photosynthesis by maintaining their photosynthetic activity during those periods when the overstory canopy is leafless (Lassoie et al. 1983, Yoshie and Kawano 1986, DePamphilis and Neufield 1989). Thuja plicata  Donn. (western redcedar) is a shade tolerant evergreen  gymnosperm tree species that grows i n cool mesothermal and cool temperate climates i n western North America (Klinka et al. 1989, Carter and Klinka 1992). Its geographical range lies between northern California (lat. 40° 10' N) and southern Alaska [lat. 56° 30' N (Minore 1990)]. Ecologically, western redcedar can tolerate a wide range of edaphic conditions, including nutrient poor soils and soils exhibiting soil moisture deficits during the growing season (Klinka et al. 1989, Minore 1990). However, the most productive growth of this species is found on nutrient rich sites (particularly with high nitrogen availability) with a fresh to moist moisture regime and a cool mesothermal climate (Klinka et al. 1989). On these sites, western redcedar  55  can be found growing both in the bverstory as a dominant tree species and as an understory seedling or sapling. It is also frequently associated with fast growing deciduous tree and shrub species such as Alnus rubra Bong, (red alder), Populus trichocarpa T. & G. (black cottonwood), Rubus spectabilis Pursh (salmonberry) and R. parviflorus Nutt. (thimbleberry). Within these plant communities, western redcedar is usually limited to growing i n the understory, at least during the early and middle stages of succession. The intensity of light i n the understory of these deciduous stands during the growing season can be as low as 2% of light conditions found i n the open (Bigley 1988). How, then, do understory plants such as western redcedar survive and grow i n these low light environments? To answer this question, four hypotheses were tested: 1) Western redcedar can maintain a positive CO2 balance at very low light intensities (i.e. the secondary foliage of this species is very shade tolerant); 2) Because western redcedar is an evergreen plant, it is capable of photosynthesizing during those periods when the overstory deciduous canopy is leafless and understory light levels are high (i.e. winter, spring and fall); 3) Western redcedar can utilize intermittent light spots (sunflecks) i n the understory to maintain a positive CO2 balance during the growing season. 4) Western redcedar will perform better, and thus be more shade tolerant, under the high available nitrogen conditions of the alder stand than the birch. The objective of this chapter is to describe and interpret the seasonal changes i n the understory environment of two deciduous tree stands and the response of western redcedar seedlings to these changes. I examined: (i) the  56  seasonal patterns of photosynthetically active radiation (PAR; 400-700 nm) within red alder and Betula papyrifera Marsh, (paper birch) stands and i n the open; (ii) the photosynthetic response of western redcedar seedlings growing within the alder and birch stands and i n the open over a sixteen month period; and (iii) the overall growth and biomass allocation of the seedlings over a three-year-period. The question of whether western redcedar can utilize sunflecks will be addressed in Chapter 4.  STUDY AREA  This study was conducted within two single-species, even-aged deciduous tree stands located within the Malcom Knapp Research Forest (49°15' N and 122°31' W) in southwestern British Columbia . The climate is 5  described i n Chapter 2. The first stand consisted of red alder trees planted i n 1973 as part of a Nelder spacing (Nelder 1962) experiment. The soils had a sandy loam texture with a depth of > 1 m and a moder-mull humus form. Some gleying was evident at « 50 cm indicating a fluctuating water table during some period(s) of the year. In southwestern B.C., sites that are subject to a fluctuating water table will tend to have a low water table during the summer months (period of relatively low precipitation) and a high water table during the winter months (period of high precipitation). The second stand consisted of paper birch established in 1974 i n a 3 x 3 m spacing regime on a deep (> 1 m), moist, sandy loam soil with a moder-mull humus. Depth to the winter water table was estimated to be 40-60 cm (presence of  The choice of red alder and paper birch stands to conduct this underplanting experiment was based on the availability of even-aged, single species deciduous tree stands in close proximity of each other growing on similar sites. As well, the similarities or differences in western redcedar growth and physiological responses can be used to make better inferences with regards to other deciduous tree stands. 5  57  gleying within this range). Both stands are at approximately 145 m elevation and are located within 1 km of each other; both stands had closed canopies (Plate 3.1). The understory vegetation within the two stands consisted of a discontinuous layer of 1 - 1.5 m high salmonberry interspersed with thimbleberry and Spirea douglasii Hook, (hardhack). Plate 3.2 shows the understory vegetation within the alder and birch stands during midsummer.  METHODS Experimental Design In the spring of 1989, three 3 x 3 m plots (spaced approximately 0.1 m apart) were established i n the centre of the alder stand (this represented the most dense portion of the stand) and i n the birch stand. Plot location within the birch stand matched the light levels within the alder stand. In addition, two similar plots were established i n the open; one adjacent to the birch stand and the other adjacent to the alder stand. A l l plots were enclosed with 2.5 m high metal mesh fence to prevent deer from browsing on the planted western redcedar seedlings. A l l above-ground minor vegetation within each plot was clipped initially (early spring) and subsequently once every year (mid-summer) at ground level and removed from the plots. The soil within the open plots was similar to that in the adjacent understory plots. In April 1989, 99 western redcedar seedlings (one-year-old container grown) were planted i n each of the red alder and birch stands (33 seedlings  Plate 3.1. Mid-summer view of the (A) red alder, and (B) paper birch canopies.  Plate 3.2. General mid-summer view of the understory vegetation w i t h i n the (A) red alder, and (B) paper birch stand.  60  per plot) and 33 seedlings were planted i n each of the two open plots. Seedlings were planted at a spacing of 0.5 x 0.5 m to allow a maximum number of seedlings to be used while still insuring enough space between the seedlings to prevent intraspecific competition over the course of the study (three years).  Understory Light Availability Measurements of instantaneous PAR beneath the red alder and birch stands and i n the open were made over a period of sixteen months using a LICOR Line Quantum Sensor (LICOR Inc., Lincoln, Nebraska). P A R was measured once every 4-7 days from March - October and once every 7-14 days from November - March. Measurements were made between 11:30 A M and 12:30 P M on mostly clear days (some measurements were also made on overcast days) by placing the Line Quantum Sensor horizontally on 60 cm high wooden supports permanently located in the centre of each plot. Within each plot, two P A R readings were made; one with the sensor oriented northsouth and the second with the sensor oriented east-west. Each set of measurements (two per plot) took approximately one minute to obtain. The sequence of measurement between the alder and birch stands and i n the open was altered every measuring period to eliminate diurnal bias.  Canopy Leaf Area Index Leaf area index (LAI), canopy leaf area (projected) per unit of ground, of both the alder and birch stands was determined using a litter trap method (Madgwick and Olsen 1974, Burton et al. 1991). Ten 0.26 x 0.51 m (0.13 m ) 2  61  plastic trays with perforated bases were randomly placed on the forest floor in each of the two stands in mid-August 1990 prior to the commencement of autumn leaf fall. Any understory vegetation around the trays was clipped back 50 cm from the edge of the trays. Canopy leaf litter was collected once every 7-20 days from each tray commencing September 6, 1990 and ending November 18, 1990; this period corresponded with the beginning and end of autumn leaf fall by both species. Litter samples collected from each tray were placed i n separate bags, labeled and returned to the laboratory where they were analyzed for projected leaf area (LI-COR 3000 Leaf Area Meter), oven dried at 70 °C for 24 hours and weighed. Total L A I was calculated as the mean of the cumulative leaf area data obtained from the litter fall collections.  Leaf Photosynthesis Measurements of leaf photosynthesis were made on nine western redcedar seedlings per treatment per sampling day over the course of 16 months using a LI-COR 6200 portable gas exchange system (LICOR Inc., Lincoln, Nebraska) attached to a one litre leaf chamber (LI-COR 6000-12). Because of the difficulty of determining the age of western redcedar foliage due to its indeterminate growth pattern, measurements were made at a standard location (approximately 5 cm from the tip of a side branch) rather than attempting to measure any particular leaf cohort. Photosynthetic measurements were repeated every 4-7 days during the spring and fall and every 7-14 days during the summer and winter months. A l l measurements were made on dry foliage under ambient conditions during clear and/or cloudy, dry days between 11:00 A M and 1:00 P M . The area of the foliage that  62  was enclosed within the chamber was marked and later harvested for projected leaf area determination. These leaf area values were used to adjust the photosynthetic rates to a per m basis. 2  Along with leaf photosynthetic rates, instantaneous measurements of PAR, stomatal conductance, leaf temperature, chamber air temperature and relative humidity were simultaneously recorded by the LI-COR 6200.  Growth and Biomass Allocation Initial measurements of seedling height from ground to the terminal bud and stem diameter at root collar were made at time of planting (April 1989) and at the end of each subsequent growing season (1989, 90, 91) for all plots. In addition, forty western redcedar seedlings were randomly selected from the initial batch of seedlings and partitioned into foliage , stem and 6  roots, oven dried at 70 °C for 24 hours and weighed to determine pretreatment biomass and biomass allocation (Table 3.1). Projected leaf area of individual seedlings was determined on fresh foliage prior to drying by passing the foliage through a leaf area analyzer (LI-COR 3000 Leaf Area Meter). At the end of each growing season, fifteen seedlings were harvested (shoots and roots) from each of the red alder and birch stands and ten seedlings were harvested from the open grown plots. These were partitioned into foliage, stem and roots, oven dried and weighed. Projected leaf area was determined on fresh foliage prior to drying.  Foilage was determined by the presence of leaf scale.  63  Table 3.1. Pre-treatment biomass, biomass allocation, specific leaf area ( S L A ) , leaf area ratio ( L A R ) , and leaf weight ratio ( L W R ) of one-year-old container-grown western redcedar seedlings. Numbers within brackets represent one standard error of the mean, n represents the number of samples. 1  2  3  Pre-treatment (one-year-old seedlings)  Biomass Stem (g)  1.1 (0.1)  Foliage (g)  1.7 (0.1)  Roots (g)  1.7 (0.1)  Total (g)  4.5 (0.2)  Shoot: Root (g g" )  1.7(0.1)  1  Projected Leaf A r e a (cm ) 2  112.1 (4.8)  S L A l ( c m g' )  65.9 (1.5)  LAR  24.9 (0.9)  2  n  ( c m g" )  2  LWR  1  2  3  1  (g g" ) 1  0.4 (0.01) 40  Specific Leaf A r e a - Ratio of leaf area per leaf weight. L e a f A r e a Ratio - Ratio of leaf area per total plant weight. Leaf Weight Ratio - Ratio of leaf weight to total plant weight.  64  Soil Temperature and Macro-Climatic Data Soil temperature at 15 cm depth was recorded once every 4-14 days from November 28, 1989 to December 14, 1989 and from March 1, 1990 to November 25, 1990 within the red alder and birch stands and i n the open. Measurements were made by inserting three alcohol thermometers within semi-open metal sleeves into the soil to a depth of 15 cm i n each of the plots. A depth of 15 cm corresponded with the rooting depth of the planted western redcedar seedlings. The thermometers were allowed to equilibrate for a minimum of 30 minutes prior to recording the soil temperature. A l l measurements were made between 11:00 A M and 1:00 P M . Air temperature was recorded within the two stands and in the open on the same days, and measurements of daily minimum - maximum air temperatures and daily precipitation were obtained from a weather station located within one kilometer of the plots at approximately the same elevation.  RESULTS and DISCUSSION Understory Environment Seasonal Light Regime The seasonality of understory light availability beneath both alder and birch stands was directly correlated with canopy bud break and leaf expansion i n the spring and leaf senescence and leaf fall in the autumn. Plates 3.3 and 3.4 show the alder and birch canopies, respectively, during the spring prior to leaf flush, during leaf expansion, and i n mid-summer when both canopies were fully foliated. Understory diffuse mid-summer light levels ranged from 16 - 39 umol n r s- within the alder stand and from 9 - 3 7 2  1  Plate 3.3. Red alder canopy i n the (A) spring prior to leaf flush, (B) during leaf expansion i n the spring, and (C) i n mid-summer.  66  Plate 3.4. Paper birch canopy i n the (A) spring prior to leaf flush, (B) during leaf expansion i n the spring, and (C) in mid-summer.  67  umol n r s within the birch stand and represent approximately 2 % of the 2  -1  above-canopy PAR. These levels correspond to mid-summer light levels beneath red alder dominated stands i n southwestern B.C. (Bigley 1988) and beneath oak-hickory forests of southeastern United States (DePamphilis and Neufield 1989) but are substantially lower than mid-summer light levels (« 100 jamol n r s ) found beneath an eastern (United States) mixed 2  1  deciduous forest (Lassoie et al. 1983). Figure 3.1 shows the general seasonal pattern of understory light as a percentage of light i n the open. With the commencement of autumn and the associated senescence and leaf drop, the understory light levels beneath both canopies increased rapidly from 2 % to approximately 85 % of incoming light over a period of approximately 60 days. Figure 3.2 shows the estimated L A I and percent understory P A R of both the alder and birch stands, respectively, in late summer (August 15, 1990) and on subsequent days during the fall period. The mean L A I for the alder and birch stands was 3.7 and 5.9, respectively, prior to leaf fall and declined steadily until both stands were leafless (early November). The increase in PAR i n both understories was highly correlated with a decrease i n L A I (r values of 0.88 and 0.79 for alder 2  and birch stands, respectively) as a result of leaf fall (Fig. 3.3). Percent understory light availability remained high during the winter months when both alder and birch canopies were leafless, and only started to decline with the advent of canopy bud break and leaf expansion i n the spring. By early June, understory light levels had declined to approximately 2 % of light levels i n the open and remained low until the fall. Lassoie et al.  68  Fig. 3.1. Seasonal trend i n understory light availability (measured as midday percent P A R i n the open) within the alder (•), and birch (•) stands. Each point represents a mean of six measurements.  69  Fig. 3.2. Changes i n Leaf Area Index (A) and understory light availability (B) during autumn leaf senescence and leaf fall within the alder (O) and birch (•) stands. Bars represent one standard error of the mean, n = 10 (LAI) and 6 (PAR). Error bars are not shown for the P A R measurements.  70  0  .  1.  2.  3\  4  .  5  ••  6  Fig. 3.3. The relationship of understory PAR (Iz/Io: where Iz and lo are P A R levels i n the understory and i n the open, respectively) as a function of Leaf Area Index (LAI) within the alder (•) and birch (•) stands. The regression lines are represented by the general linear model; Ln(Iz/Io) = a + b-LAI.  71  (1983) and DePamphilis and Neufield (1989) reported similar seasonal trends in understory light attenuation beneath mixed deciduous forests of eastern and southeastern United States, respectively. While the increases and decreases of percent understory P A R were, i n general, predictable, actual understory PAR levels were not. Absolute P A R levels were as much a function of atmospheric conditions, i n particular the intensity and duration of cloud cover, as of overstory canopy conditions. Even though the proportion of understory light was relatively high during the fall and winter months, the actual PAR levels were low. Mid-day P A R levels during the winter months were as low as 187 umol n r s i n the open and 93 2  1  and 92 umol n r s beneath the birch and alder stands, respectively (Table 2  1  3.2). In southwestern B.C., winter months are characterized by cool temperatures, high precipitation and relatively long durations of cloud cover. During the course of this light study, the number of clear, sunny days per month during the fall-winter-spring period ranged from 2-7 days, while the number of overcast or cloudy days ranged from 10-25 days per month over the same period (Fig. 3.4). These prolonged periods of cloud cover during the non-growing period and the subsequent low PAR levels may limit net CO2 uptake by understory evergreen plants such as western redcedar regardless of canopy cover.  Air and Soil Temperatures Seasonal air temperatures were typical of southwestern B.C., with maximum temperatures occurring during the mid-summer months (July and August) and minimum temperatures occurring during the period November to March (Fig. 3.5). Maximum daily air temperatures rarely exceeded 30 °C  72  Table 3.2. Mean seasonal PAR levels within the red alder and paper birch stand and i n the open. Measurements were made between 11:00 A M and 1:00 P M using a Line Quantum SensOr (LICOR, Nebraska). Numbers i n parenthesis represent one standard error of the mean, n = 9 for each sample.  Light level (umol n r s ) 2  1  Date  Red alder  Paper birch  Open  July 22, 1989 Aug. 4, 1989 Aug. 26, 1989 Sept. 22, 1989 Oct. 2, 1989 Oct. 16, 1989 Nov. 7, 1989 Dec. 14, 1989 March 1, 1990 March 16, 1990 March 26, 1990 A p r i l 3, 1990 A p r i l 16, 1990 April 29, 1990 April 30, 1990 May 22, 1990 June 14, 1990 June 22, 1990 July 11, 1990 Aug. 14, 1990 Sept. 5, 1990 Sept. 14, 1990 Sept. 20, 1990 Sept. 28, 1990 Oct. 11, 1990 Nov. 2, 1990  31 (17) 39 (15) 23 (6) 38 (8) 68 (16) 148 (32) 92 (3) 183 (22) 573 (22) 406 (14) 857 (55) 721 (51) 435 (55) 178 (43) 130 (37) 82 (14) 18 (2) 41 (4) 16 (2) 30 (9) 62 (43) 69 (9) 123 (30) 75 (9) 44 (1) 122 (6)  22 (7) 16 (3) 9(2) 18 (4) 38 (9) 412 (51) 93 (3) 152 (13) 552 (71) 413 (16) 848 (73) 774 (79) 597 (63) 176 (52) 144 (41) 53 (8) 25 (1) 35 (4) 37 (1) 21 (2) 35 (12) 44 (15) 142 (55) 62 (14) 27 (1) 165 (2)  1636 (44) 1102 (181) 1363 (86) 1085 (117) 1190 (38) 664 (84) 187 (24) 178 (21) 876 (97) 868 (70) 1290 (45) 1351 (17) 1333 (35) 428 (24) 1019 (12) 398 (82) 1659 (14) 895 (64) 683 (69) 1483 (58) 1251 (26) 1165 (20) 1192 (12) 674 (71) 223 (23) 266 (26)  73  30  •* Sunny. ^ C l o u d y !  !  Mixed  •25^  2fr 15  10  0  J  J  A  S ;© TN- D J 1589 I  F  M A ,M  J J 1990  ;  A  .S' Q , N " ©  :Date:-  F i g . 3.4. Number of days per month i n which atmospheric conditions were classified as either sunny, cloudy, or a mixture of sun and cloud. Measurements were taken from the Malcolm Knapp Research Forest Climatic Station.  74  during the summer months and, except for two days during February, 1990, remained above 0 °C during the 16 month sampling period. This relatively mild seasonal temperature regime corresponds with the climate data reported by Waring and Franklin (1979) for the Pacific Northwest (PNW) of the United States and is associated with the hypothesis that up to 50 percent of C O 2 uptake by evergreen trees i n coastal Washington and Oregon occurs during the winter months when precipitation is high and air temperatures are above freezing (Emmingham and Waring 1977). However, this hypothesis does not take into account the minimum daily air temperatures during the winter period which may be low enough to limit net photosynthesis by evergreen plants. At the Malcolm Knapp Research Forest, daily minimum air temperatures were at or below 0 °C for several weeks during the winter period November-March with the lowest temperatures (-5 to -13 °C) occurring i n February (Fig. 3.5). Soil temperatures (at 15 cm) within both stands and i n the open followed the same pattern as air temperatures with the highest and lowest values recorded i n mid-summer and mid-winter, respectively (Fig. 3.6). Midsummer soil temperatures ranged from 14.5 - 17.0 °C i n the open and between 13.0 - 14.0 °C within the alder and birch stands. Winter soil temperatures dropped to less than 2 °C beneath both stands and i n the open, but remained above 0 °C during the recording period.  Leaf Photosynthesis (Seasonal) Photosynthetic rates of western redcedar seedlings growing within the alder and birch stands were just above the compensation point for most of the summer (Fig. 3.7). This was a reflection of the low light levels recorded  75  Fig. 3.5. Seasonal changes in daily maximum and minimum air temperatures (A) and daily precipitation (B). Data was collected from the Malcolm Knapp Research Forest Climatic Station.  76  F i g . 3.6. Seasonal changes i n mid-day (A) air and (B) soil temperature within the birch (•) and alder (•) stands and i n the open (A). E a c h air and soil measurement represent the mean of n = 9 and n = 3, respectively. Error bars are not shown.  77  Fig. 3.7. Seasonal changes i n mid-day leaf photosynthesis of understory western redcedar seedlings growing within birch (•) and alder (•) stands and in the open (A). Each data point represents the mean of n = 9. Error bars are not shown. Missing data between 12/28 and 2/26 represents a period of persistent wet weather i n which accurate measurements of leaf photosynthesis could not be attained.  78  under these stands during the summer months (1989 and 1990) when both alder and birch canopies were fully foliated and understory P A R was approximately 25 u,mol m s . However, even at these low light levels, there 2  _ 1  was still a small net carbon gain by the seedlings (< 1.5 umol n r s ). The 2  1  ability of western redcedar to photosynthesize at very low light levels helps to explain the shade tolerance of this species (Klinka et al. 1989, 1990, Minore 1990, Carter and Klinka 1992). Other field studies of this species' shade tolerance have shown similar results (Klinka et al. 1992). Positive rates of mid-summer net photosynthesis (0.8 pmol n r s ) have been recorded for 2  1  western redcedar seedlings planted beneath a mature western hemlock, Douglas-fir and western redcedar forest when understory light levels were approximately 16 umol m s . In comparison, maximum average rates of net -2  -1  photosynthesis by open-grown (outside the alder and birch stands) western redcedar seedlings during the summer months were « 7.0 umol n r s . 2  1  Klinka et al. (1992) recorded similar rates (« 8.0 umol m s ) of P n for open 2  1  grown western redcedar seedlings planted i n a recent clearcut within the same biogeoclimatic zone in southwestern B.C. The commencement of leaf senescence and leaf fall i n late September by both alder and birch resulted in an increase i n the amount of P A R reaching the forest floor and the planted western redcedar seedlings. By October 2, understory light levels under both the alder and birch stands had increased by approximately 75 and 300 %, respectively, over the mid-summer light levels. Correlated with this event was a rapid increase i n net photosynthesis by the understory western redcedar seedlings (Fig. 3.7). By mid-October, net photosynthetic rates of understory seedlings were approximately 150 % greater than mid-summer rates under both deciduous  79  stands. These higher rates were maintained throughout the fall and only started to decline with the onset of winter and the decline in both maximum and minimum daily air temperatures. By December 14, mid-day mean net photosynthetic rates of western redcedar seedlings growing i n the alder and birch understories and i n the open were 1.2, 0.7, and 1.1 umol m  - 2  s"-Ir-  respectively. Mid-day air temperatures at seedling height and the corresponding soil temperatures (15 cm depth) were 3.6, 3.5, and 3.3 °C (air temperature) and 4.8, 3.8, and 3.3 °C (soil temperature) for seedlings growing in the alder and birch understories and i n the open, respectively. Due to the persistent wet weather (Fig. 3.5) during the winter months (December 15 - February 28), gas exchange measurements and the associated environmental measurements could not be made during this period. However, it can be speculated that the prolonged periods of overcast conditions (Fig. 3.4) will have resulted i n low levels of absolute P A R which i n turn will have limited net photosynthesis by evergreen plants during the winter period regardless of air and soil temperatures. Under certain conditions, cloud cover can actually increase CO2 fixation by understory plants. Young and Smith (1983) reported an increase in net photosynthesis by the understory plant Arnica latifolia Bong, during cloudy periods as compared to sunny periods. This was due, i n part, to an increase i n diffuse light incident on the forest floor during cloudy conditions and a decrease i n air temperature and water loss by understory plants. In my study, however, low levels of humidity and high air temperatures were not limiting factors for understory plant photosynthesis during the winter months.  80  By early spring (March 1), understory light levels within the alder and birch stands had reached 573 and 552 umol nr s , which represented 65 and 2  1  63%, respectively, of PAR in the open. Mid-day air temperatures within the alder and birch stands and in the open had increased to 14.1, 16.8, and 16.3 ° C, respectively. Ordinarily, these conditions would be most suitable for early spring photosynthesis by most temperate plants. However, Pn of western redcedar seedlings growing in all three treatments remained very low with mean rates of 0.7, 0.4, and 0.4 umol nr s for the alder and birch stands, and 2  _1  in the open, respectively. These low photosynthetic rates may be due to the length of time required for the photosynthetic mechanism to recover following a frost event. Minimum air temperatures of-15 °C were recorded earlier in the week prior to an increase in the daytime temperature (Fig. 3.5A). Another reason may be due to low soil temperatures. Low photosynthetic rates were associated with low soil temperatures which were 1.7 and 1.3 °C within the alder and birch stands, respectively, and 0.8 °C in the open (Fig. 3.6). It appears that below some minimum soil temperature, net photosynthesis by plants is greatly reduced regardless of the air temperature. DeLucia (1986) and DeLucia and Smith (1987) found that the photosynthetic response of Engelmann spruce (Picea engelmannii Parry ex Engelm.) was greatly reduced at soil temperatures below 8 °C. Likewise, Lawrence and Oechel (1983) recorded a decline in Pn of seedlings from four taiga tree species when root temperatures were reduced from 25 to 5 °C while maintaining air temperature at 20 °C. While the exact mechanism linking cold soils and reduced rates of net photosynthesis is still unclear (see Gezelius and Hallen 1980, Oquist et al. 1980, Hallgren and Oquist 1990) it is speculated that cold soil temperatures  81  act to reduce water transport i n roots by increasing both the viscosity of water and root resistance, which in turn reduces stomatal conductance and CO2 uptake (see Kramer 1940, Larcher and Bauer 1981, Lopushinsky and Kaufmann 1984, Jurik et al. 1988). If this is the case, then seedlings, such as western redcedar, which have the majority of their roots i n the top 20 cm of soil, will be limited i n their ability to fix CO2 during the winter by low root temperature, regardless of air temperature. However, this may not be as limiting a factor for saplings and mature trees whose roots can penetrate to depths of 1 m (or more). At these depths, soil temperatures can be as high as 7 °C warmer during the winter months than temperatures near the soil surface [see Nobel (1991) and Brady and Weil (1996) for a review of soil temperatures as a function of soil depth]. Deep-rooted evergreen trees may therefore be less limited by fluctuations in surface soil temperatures and can potentially begin photosynthesizing as soon as air temperatures are above some critical point. This might account, i n part, for Emmingham and Waring's (1977) assertion that up to 50 % of CO2 uptake by (mature) evergreen trees growing i n the Pacific Northwest occurs outside the growing season (see also Harrington et al. 1994). With western redcedar seedlings, early spring P n was a direct function of soil temperature. From March 1 to April 3, soil temperatures beneath the birch and alder stands and i n the open increased from 1.3, 1.7 and 0.8 °C, respectively, to 6.8, 6.2 and 6.2 °C (Fig. 3.6). Correspondingly, net photosynthesis increased from 0.4 (birch), 0.7 (alder), and 0.4 umol m ^ s  1  (open) to 3.6 (birch), 4.0 (alder) and 5.2 umol n r s (open) over the same time 2  -1  period (Fig. 3.7). Understory light levels beneath both stands during this period were greater than 50 % of light levels i n the open (Fig. 3.1) with P A R  82  levels reaching 774 and 721 umol n r s beneath the birch and alder stands, 2  1  respectively, and 1351 umol n r s i n the open on April 3. By mid - April, P n 2  -1  of understory western redcedar began to decline steadily as both birch and alder canopies began to leaf out and expand, resulting i n a decrease i n understory PAR. This seasonality in understory PAR and the resultant photosynthetic response of understory western redcedar helps to explain the presence and persistence of this species within deciduous tree stands.  Growth and Biomass Allocation After three years of growth, stem height and stem diameter of western redcedar seedlings growing beneath both the red alder and paper birch stands were significantly lower than for seedlings grown i n the open (Table 3.3). Stem heights of open grown seedlings were on average 12.4 and 18.0 cm greater than seedlings growing beneath the alder and birch stands, respectively. Stem diameter of open grown seedlings was 1.8 cm greater than the alder-grown seedlings and 2.7 cm greater than the birch-grown seedlings. Total seedling dry weight (shoots and roots) and the dry weight of the various seedling components (stem + branches, foliage and roots) were significantly greater i n open grown seedlings i n comparison to seedlings grown beneath the two stands in all three years of the study (Table 3.4). By the end of the third growing season, total mean dry weights of open grown seedlings were on average 1.7 and 2.8 times greater than seedlings growing beneath the red alder and paper birch stands, respectively. Differences i n total biomass and biomass allocation between seedlings grown beneath the alder and birch stands were not significant for the first two years of growth.  83  Table 3.3. Changes i n mean height and stem diameter (at root collar) of western redcedar seedlings growing within the alder and birch stands and i n the open over a three year period. Numbers within brackets represent one standard error of the mean. Values i n the same row followed by the same letter are not significantly different at p < 0.05 (Tukey test), n represents the number of samples.  Alder  Birch  Open  47.3 (0.7) 3 . 6 ° (0.1) 13.3 (0.2) 58  45.8 (1.0) 3.5 0(0.1) 13.3 a (0.4) 53  46.5 (1.1) 3.7 « (0.1) 12.8 (0.3) 54  0.506 0.199 0.340  53.5 a (0.7) 3.9 a (0.1) 13.7 «(0.2) 58  49.6 (0.8) 3.5 ° (0.1) 14.3 a (0.3) 53  55.6 (1.1) 5.0 (0.1) 11.3 (0.3) 36  0.000 0.000 0.000  6 1 . 3 ° (1.1) 4 . 8 ° (0.1) 12.9 (0.2) 42  54.8 & (1.1) 4.0 (0.1) 13.9 (0.3) 38  69.9 (1.7) 6.6 (0.3) 10.8 (0.3) 23  0.000 0.000 0.000  63.0 « (1.6) 5.9 (0.2) 10.9 (0.3) 27  57.4 a (1.7) 5.0 (0.2) 11.5 (0.3) 23  75.4 (2.5) 7.7 c (0.4) 10.0 0 (0.4) 14  0.000 0.000 0.007  P  Initial Height and Diameter Height (cm) Stem Diameter (mm) Height: Diameter (cm mm"l) n  a  a  a  a  a  End of 1st Growing Season Height (cm) Stem Diameter (mm) Height: Diameter (cm mm"l) n  b  a  c  b  End of 2nd Growing Season Height (cm) Stem Diameter (mm) Height: Diameter (cm mm" ) n 1  a  b  b  c  c  c  End of 3rd Growing Season Height (cm) Stem Diameter (mm) Height: Diameter (cm m m ' ) n 1  a  a  b  b  b  b  84  Table 3.4. Changes i n western redcedar seedling biomass (and biomass allocation), leaf area, specific leaf area (SLA), leaf area ratio (LAR), and leaf weight ratio (LWR) growing beneath the alder and birch stands, and i n the open over a three year period. Numbers within brackets represent one standard error of the mean. Values i n the same row followed by the same letter are not significantly different at p < 0.05 (Tukey test), p and n represent the level of significance and sample number, respectively. Alder  Birch  Open  P  End of 1st Growing Season Biomass Stem (g) Foliage (g) Roots (g) Total (g) Shoot: Root (g g-1) Leaf area (cm ) 2  1.7 0(0.1) 2.3 « (0.2)  1.5 (0.1) 2.2 0(0.1)  2.4 (0.2)  0.000  3.6 (0.3)  0.000  1.5° (0.1) 5.5 « (0.4)  1.6 (0.1)  2.8 (0.3)  0.000  5.3 a (0.2)  8.8 (0.7)  0.000 0.137  a  a  2.5 a (0.2) 2 . 7 ° (0.1) 135.5 (9.5) 117.7 (6.5) a  b  a  b  b  b  b  b  b  2.2 « (0.1)  159.8 0 (14.8) 79.9 a (7.4)  S L A (cm g-1)  67.8  L A R (cm g" )  24.2 « (0.6)  21.9  L W R (g g-1) n  0.42 15  0.42 a (0.01) 0.41 a (0.01) 15 10  2  2  1  a  (4.8) (0.01)  58.8 (3.2) b  a  (0.7)  18.0  b  (0.9)  0.023 0.023 0.000 0.759  End of 2nd Growing Season Biomass Stem (g)  4.0 (0.4)  2.8 a (0.3)  8.7 (1.2)  0.000  Foliage (g)  5.4 « (0.7)  8.6 (1.6)  0.016  Roots (g)  2.3 a (0.3)  4.8 a (0.5) 2.0 a (0.2)  5.3 (0.6)  0.000  Total (g)  11.7 a (1.3)  9.6 (1.0)  22.6 (2.2)  0.000  4.4 a (0.3)  3.7 (0.2)  a  Shoot: Root (g g"l) Leaf Area (cm ) 2  S L A (cm g-1) 2  L A R (cm g-1) 2  L W R (g g-1) n  a  b  b  b  b  3.5 « (0.5)  0.072  a  232.4 a (28.6) 1 9 0 . 7 ° (23.1) 256.3 (31.8) 44.1 « (1.5) 39.3 a (1.6) 32.8 (2.3) 19.9 a (0.9) 19.4 a (1.1) 11.5 (1.0)  0.263  b  0.000  a  b  0.45 15  a  (0.02)  0.49 15  a  (0.01)  0.37  b  (0.05)  0.000 0.005  10 cont.  85  ...cont.  Alder  Birch  Open  E n d o f 3rd G r o w i n g S e a s o n Biomass 3.6  b  (0.3)  12.1 c(1.3)  0.000  4.6  b  (0.4)  9.9 « (1.6)  0.001  3.9 « (0.4)  2.1 b (0.1)  6.3 c (0.7)  0.000  16.9 « (1.5)  10.3 b (0.7)  Stem (g)  6.0  a  Foliage (g)  7.0  a  Roots (g) Total (g) Shoot: Root (g g-1) Leaf A r e a (cm ) 2  (0.5) b  (0.7)  3.5 « (0.2) 398.3  a  4.0  (41.7) 294.5  a  (0.3) (23.6)  a  S L A ( c m g-1)  56.6 « (1.0)  64.6  L A R ( c m g-1)  23.2 a (0.7)  28.6 b (0.6)  2  2  L W R (g g-1) n  0.41 a (0.01) 15  b  (1.2)  0.44 a (0.01) 15  28.3  c  3.5  a  (3.3)  0.000  (0.2)  0.171  443.2 a (67.0)  0.054  (1.6)  0.000  15.2 c (0.7)  0.000  45.5  c  0.34 & (0.02) 10  0.000  86  However, by year three, total dry weight of western redcedar seedlings grown beneath the red alder stand was significantly greater than for seedlings grown beneath the birch stand (Table 3.4). These differences may be associated with slight differences i n light availability beneath the two stands during the growing season. Table 3.2 shows the mean light levels beneath the two stands and i n the open over a one-year-growing period. While midsummer light levels beneath both stands were very low compared to light levels i n the open, light levels beneath the alder stand were higher (although not statistically significant) than those found beneath the birch stand. Whether these differences are substantial enough to result i n greater biomass of western redcedar seedlings growing beneath the red alder stand is not clear from this study and requires further investigation. A n alternative reason for the difference i n growth between the two treatments may be due to the differences i n nutrient availability of the soils beneath the two stands. It has been widely reported that nutrient availability, and specifically nitrogen, is relatively high beneath nitrogen fixing alder stands (Johnsrud 1979, Binkley 1981, 1983, 1984, Cote and Camire 1984, Huss-Danell et al. 1992, Van Miegroet et al. 1992, Clein and Schimel 1995). If nitrogen availability was indeed higher within the alder stand than within the birch stand it may account for the difference i n understory seedling biomass. There is strong evidence to show that most plants respond positively to an increase i n nitrogen availability; specifically, net photosynthesis (and subsequently the increase i n total growth) is positively correlated to increases i n nitrogen (Bjorkman and Holmgren 1963, Mooney et al. 1978, Field 1981, Gulmon and Chu 1981, Pearcy et al. 1982, Field and Mooney  87  1983, Seemann et al. 1987, Sinclair and Horie 1989, Marshall and Vos 1991, Vapaavuori et al. 1995, Tan and Hogan 1995). However, these studies were conducted under saturating light conditions and not under the low light levels that are typically found i n understory environments. Other studies of net photosynthesis by plants grown under high nitrogen levels and low light intensities have shown little increases in net photosynthesis when compared to plants grown under low nitrogen levels and the same low light intensities (Gulmon and Chu 1981, Thompson et al. 1988, Walters and Reich 1996). Bjorkman (1981) suggests that plants adapted to grow under shade conditions decrease their nitrogen content [expressed as nitrogen per leaf area (g n r )] and construction costs by manufacturing thin leaves with a high specific leaf area (SLA). Indeed, while overall mean leaf area did not differ significantly after three years of growth between western redcedar seedlings grown beneath the alder and birch stands and i n the open, there was a significant difference i n their S L A after the third growing season. Seedlings grown beneath the birch stand had on average significantly higher S L A values than both open grown seedlings and seedlings grown beneath the alder stand (Table 3.4). This plasticity i n leaf area, or more specifically SLA, as a function of different light levels is well documented (see Fitter and Hay 1987, Fitter 1989 and references therein) and may be a better morphological parameter by which to determine plant responses to subtle differences i n light regimes than overall growth measurements such as height and stem diameter (Carter and Klinka 1992, Klinka et al. 1992, Wang et al. 1994). The mean leaf weight ratios (LWR) of western redcedar did not differ significantly between the two deciduous stands and remained relatively constant over the three-year study. However, by the end of the second  88  growing season, open grown seedlings had mean L W R significantly lower than seedlings grown beneath the two stands (Table 3.4). This trend continued through to the end of the third growing season at which point 34 % of the total weight of the open grown seedlings was allocated to foliage compared to 41 and 44 % allocated to alder and birch grown seedlings, respectively (Table 3.4). The reason for this plasticity i n biomass allocation to foliage as a function of light levels is not well documented. Fitter and Hay (1987) suggest that the L W R of a species remains relatively constant over a range of light levels and only changes for shade intolerant plants which display etiolation (i.e., low L W R when grown under low light conditions). This concept is inconsistent with my findings which show that the L W R of western redcedar grown in the open was significantly lower than western redcedar grown i n the shade. Walters and Reich (1996) found similar results with various hardwood tree seedlings grown under low light and high nitrogen availability. However, if we look at the shoot:root ratios of western redcedar seedlings we see that there is no significant difference between the three treatments i n all three years (Table 3.4), which fits with Fitter and Hay's (1987) concept of morphological constancy of shade tolerant plants across a gradient of light availability. For example, plants that are not adapted to grow i n the shade (i.e., shade intolerant plants) will tend to allocate more of their assimilates to those plant components that can best respond to the stress (Wilson 1988, Gleeson and Tilman 1992, Reynolds and Pacala 1993, Wilson and Tilman 1995). Brix (1967) showed that Douglas-fir (a relatively shade intolerant species) seedlings grown under low light conditions (« 68 umol n r s ) had a shootrroot ratio twice as high as seedlings grown at higher 2  1  89  light levels (« 270 umol n r s ). These changes in shoot:root ratios of 2  :1  Douglas-fir and the relative unresponsiveness i n shootrroot ratio of western redcedar growing i n my study within low-light understory environments and in the open may be related to differences i n their ability to compete under low-light conditions. Under these conditions, shade intolerant Douglas-fir will allocate a higher proportion of its growth to the shoot, favouring a faster growing stem component which may outgrow its shade-casting competitors (see Chapter 5 ) . This fits with Grime's (1973, 1979) theory that species with the highest rate of growth will eventually out-compete slower growing species. In other words, Douglas-fir's "strategy" (of carbon allocation) is to try to outgrow its competitors by overtopping them and thereby alleviating lowlight stress. Conversely, Tilman (1984, 1989) predicts that species that are able to maintain active growth (or photosynthesize) at low levels of some resource (i.e., light) will result i n those species persisting under low-resource conditions and eventually out-competing their neighbours. Tilman's theory is more applicable to western redcedar which is able to maintain a positive CO2 uptake under low light levels. Under these conditions, western redcedar maintains a steady growth rate resulting in a more balanced shoot:root ratio than shade intolerant plants such as Douglas-fir. A more detailed study of biomass allocation of conifer seedlings grown under different light levels is presented i n Chapter 5 .  CONCLUSIONS  Western redcedar is a shade tolerant species capable of maintaining positive CO2 uptake rates under very low understory light levels (< 40 umol m s ) . In addition to its shade tolerance, this species can 2  1  90  respond to seasonal changes in understory light availability as a function of seasonal changes i n overstory leaf area. In southwestern British Columbia, red alder and paper birch trees commence bud break and leaf expansion i n mid-April; both species begin autumnal leaf drop by mid- to late-September and are leafless by late-October. At the beginning and end of the growing season, air and soil temperatures are high enough to allow understory plants such as western redcedar to photosynthesize. Favourable temperatures, coupled with relatively high understory PAR values result i n significant levels of P n by understory evergreens during these two seasons. This seasonality of understory light availability and the ability of evergreen species such as shade tolerant western redcedar to physiologically respond to these changes has implications for forest and natural resource management. In particular, this species can be used for planting i n selectively harvested or partially cut stands i n low- to mid-elevation sites i n southwestern British Columbia. As well, western redcedar can be used as an inter-crop species planted with faster growing red alder (a nitrogen fixing tree), paper birch, and on moist sites, black cottonwood.  91  C H A P T E R 4. U N D E R S T O R Y S U N F L E C K S A N D T H E I R E F F E C T O N WESTERN REDCEDAR SEEDLING PHOTOSYNTHESIS  INTRODUCTION  The previous two chapters described the seasonal changes i n the understory light environment of early serai vegetation, and of mid-seral birch and alder stands, from overstory leaf emergence and expansion during the spring and summer period to leaf senescence and leaf fall in the autumn. Understory plants such as seedlings of western redcedar, Douglas-fir, grand fir, and western hemlock were shown to respond photosynthetically to the seasonality of understory light availability, with the highest P n rates recorded i n the spring and fall when the overstory was leafless and air and soil temperatures were warm enough to permit photosynthesis. Conversely, when mid-summer air and soil temperatures were high, understory light levels were at their lowest, resulting i n very low rates of seedling P n by all four species: generally less than 2.0 umol n r s . However, these 2  1  measurements reflect P n rates under relatively stable diffuse light conditions. They ignore any rapid (seconds) to intermediate (minutes) changes of light availability that may occur during the day as a result of sunflecks. Understory light levels are rarely stable for more than short durations during the daytime periods and are more often dynamic and may represent the norm under cloudless conditions. As a result, understory plants are usually subjected to a diurnal light environment that is constantly changing  92  over time scales of seconds to minutes as a result of sunflecks (Bjorkman and Ludlow 1972, Pearcy and Calkin 1983, Pearcy 1983, 1987, Chazdon and Fetcher 1984, Pearcy et al 1985, 1994, Chazdon 1988, Chazdon et al. 1988, Pearcy 1990, Chazdon and Pearcy 1991, Pfitsch and Pearcy 1992). The proportion of understory light in the form of sunflecks can range up to 80 % of the total light received at any one point i n an understory environment (Bjorkman and Ludlow 1972, Pearcy and Calkin 1983, Chazdon and Fetcher 1984, Pearcy 1983, 1987, Chazdon et al. 1988, Messier et al. 1998) and sunflecks may be the major factor determining the daily CO2 uptake by understory plants (Bjorkman et al. 1972, Pearcy and Calkin 1983, Pearcy 1987). Most models of plant photosynthesis ignore this small scale spatial and temporal variability, treating the light environment as a static system (see H a l l 1979, Farquhar et al. 1980, Marshall and Biscoe 1980, Gutschick 1984, Host et al. 1990). This omission of what is an important component of the understory environment may significantly reduce the accuracy of these models. Various models have been used to predict CO2 uptake i n fluctuating light environments. Early predictions used steady-state photosynthetic response curves to predict CO2 uptake as a function of some mean light level. These models tended to overestimate CO2 uptake due to their integration of light over a time period characterized by diffuse light punctuated by shortterm, rapid, and high increases i n PAR (see Gross 1982 for a review of this method). Other models have been based on laboratory studies where length, periodicity, and intensity of both diffuse light and lightflecks (a term used to denote an artificial source of light) have been controlled, resulting i n reliable estimates of CO2 uptake by plants growing under controlled, laboratory  93  conditions (Pearcy et al. 1987, Stoop et al. 1990). However, these models have rarely been tested under field conditions: we do not know if they provide an accurate representation of CO2 uptake by understory plants exposed to sunflecks. This chapter reports a study that was designed to: (i) document the intensity and duration of sunflecks beneath red alder and paper birch stands (from Chapter 3); (ii) measure and model the photosynthetic response of understory western redcedar to natural sunfleck activity; and (iii) determine the overall contribution of sunflecks to CO2 uptake by understory western redcedar.  METHODS Sunfleck Determination The proportion of photosynthetically active radiation (PAR) i n the understory due to sunflecks was determined on two cloudless days (July 3 & 30) by mounting a LI-COR point sensor (LI-190SA Quantum Sensor, L I COR, Inc., Lincoln Nebraska) on fixed supports 60 cm above the soil surface within the birch and red alder stands (Chapter 3). PAR i n the open on both days was approximately 1400 pmol n r s . Sensors were attached to data 2  1  loggers and instantaneous measurements of PAR were recorded every 10 seconds from 11:11 A M to 4:38 P M (July 3) i n the birch stand, and from 10:13 A M to 4:00 P M (July 30) i n the alder stand. The choice of a 10 second interval was based on initial observations that sunfleck residency time i n both stands was greater than (or equal to) 10 seconds. Chazdon and Fetcher (1984) argued that a sampling interval should incorporate the minimum sunfleck duration to minimize any error when calculating integrated  94  measurements of PAR. Plate 4.1 shows the placement of the two sensors i n both understories.  Leaf Photosynthesis During Sunflecks Direct Measurements Measurements of leaf photosynthesis of western redcedar during periods of sunfleck activity were made over a period of 2.2 hours (2:06 P M 4:18 PM) within the understory of the red alder stand on a clear (PAR « 1400 umol m ^ s ) , windless day (June 18, 1992). Due to the lack of wind above 1  and within the alder canopy, the duration of sunflecks within the understory was a result of the Earth's rotational movement and not of leaf movement. Therefore, the residency time of any one sunfleck on a single point (i.e., a seedling) was longer than would occur during a wind event which would result i n sunfleck residency on any one point lasting only a few seconds or less (Pearcy 1983). The longer sunfleck duration allowed the use of the LI-COR 6200 (closed system) gas analyzer (LICOR Inc., Lincoln, Nebraska) to measure the net effect of the sunfleck on photosynthesis. Measurements of P n using the closed system were made using a draw-down time of 30 seconds for each measurement. This time period ensured a constant light environment with P A R either as a sunfleck or as diffuse light during each measurement. However, care must be taken i n choosing the appropriate type of gas analyzer to use during understory conditions i n which sunfleck residency time at any one point is of relatively short duration (< 30 s) such as is found under persistent windy conditions. Under the latter conditions, a closed  P l a t e 4.1. G e n e r a l v i e w of t h e (A) r e d a l d e r , a n d (B) p a p e r b i r c h u n d e r s t o r i e s a n d t h e p l a c e m e n t of l i g h t s e n s o r s d u r i n g s u n f l e c k m e a s u r e m e n t s .  96  system such as the LI-COR 6200 would be inadequate for accurate measurements of leaf photosynthesis (see Field et al 1991, Leverenz and Hallgren 1991, and Long and Hallgren 1993). A more appropriate approach would be to utilize an open differential system such as the commercially available LCA-4 portable gas analyzer (Analytical Development Co., Hoddesdon, UK) or the LI-6400 Portable Photosynthesis System (LI-COR, Inc., Lincoln, Nebraska). Thirty-one measurements were made, one every 60 - 240 seconds, either under diffuse light or under a sunfleck environment over the course of 2.2 hours. A t the end of each 30 second recording, PAR, air (within the chamber) and leaf temperature, and stomatal conductance were simultaneously recorded along with leaf Pn. Relative humidity within the chamber during each measurement was maintained at approximately 45 %, the ambient humidity beneath the stand. A one-litre LI-COR chamber (LI COR 6000-12) was clamped over the same portion of western redcedar foliage for each measurement; this portion was later clipped for determination of projected leaf area (LI-COR 3000 Leaf Area Meter). Measurements of net photosynthesis and stomatal conductance were adjusted to projected leaf area.  Modeling Net CO2 Uptake in a Dynamic Light Environment Two approaches were taken to determine CO2 uptake by the western redcedar seedlings. The first was to develop a photosynthetic light response curve (model) for western redcedar seedlings grown under controlled light conditions i n the greenhouse. The second was to develop a similar response curve for field conditions.  97  The greenhouse model was developed using data collected from western redcedar seedlings grown at low light intensities (« 45 umol n r s ) 2  -1  under permanently shaded conditions for four months (see Chapter 5). The use of shade screen within the greenhouse in addition to the opaque glass of the greenhouse itself prevented any sunflecks from reaching the seedlings during the experiment. At the end of the four month growing period, seedlings were placed under total darkness for six hours prior to being subjected to a step increase i n light intensity. Using a 300-W tungsten halogen lamp, light intensity was increased from 0 to 1000 umol n r s i n 2  _1  steps of 0, 50, 100, 200, 350, 500, and 1000 umol m ^ s by placing the lamp 1  at different distances from the seedling. Net photosynthesis was measured using a LI-COR 6200 gas analyzer. Each seedling was allowed to acclimate for twenty minutes at each light level prior to measuring leaf net photosynthesis. A n external fan was used to reduce heat build-up caused by the lamp by circulating air around the outside of the leaf chamber. A i r temperature within the chamber during the measurement period was maintained at 21- 23 °C. The field model was developed using the photosynthetic response to sunfleck data of western redcedar growing in the alder understory on June 18, 1992. The equation used to estimate P n as a function of light intensity for both the greenhouse and field data was a modified version of Blackman's empirical optima and limiting factors model (Blackman 1905, Mitscherlich 1913 i n Fitter 1989):  Y = b + A ( l - e-cX)  equation 1  98  Where;  Y = the photosynthetic rate (umol n r s ), b = a constant, e = 2.718, 2  _1  A = the maximum photosynthetic rate attainable (umol n r s ) c = a coefficient describing the steepness of the curve, X = the level of PAR (umol n r s ). 2  2  1  1  RESULTS and DISCUSSION Understory Sunflecks Daily light availability beneath both birch and alder stands was greatly increased by the presence of sunflecks in the understory. Figure 4.1 shows the fluctuating light intensities at a single point beneath both stands over a period of 5.47 hours (birch) and 5.78 hours (alder). Diffuse light under the stands during the recording period was approximately 20 umol n r s for 2  _1  alder and 24 umolm"2-s-l for birch, with maximum light intensities during sunflecks reaching > 800 umol n r s and > 500 umol o r s beneath the alder 2  1  2  _1  and birch stands, respectively. These peak values correspond with studies i n red fir (Abies magnifica A. Murr.) forests of California (Ustin et al. 1984) and a Costa Rican rainforest (Chazdon 1986) where maximum sunfleck values of P A R reached 624 and 500 umol n r s , respectively. Sunfleck intensity 2  1  exceeding 1300 umol n r s has been recorded within a 200 + year-old 2  _1  western redcedar dominated forest i n southwestern British Columbia (Karakatsoulis, unpublished data) on a clear, windless, summer day. However, the average sunfleck intensity in this stand was 126 umol n r s . 2  _1  Total photosynthetic photon flux density (PPFD), as measured by point sensors, beneath the two stands over the separate recording periods was 1,127,860 umol-m- for birch and 911,850 umol-m- for alder (Table 4.1). 2  2  99  1000 800 s  600  5  400  — i «  200 0 3000  6000  1000 r-i  m  600  •&  40j0  i  12000  15000  18000  21000  18.000  21000:  B:  800  k  "o  9000  200  1  0  ;o.  3000^  eooo  9000;  mpoo  1*00.0  Time (Seconds)  Fig. 4.1. The fluctuation i n understory light availability as measured every 10 seconds beneath the red alder (A), and paper birch (B) stands. Measurements were made on clear days i n mid-summer using point quantum sensors positioned 60 cm above the ground surface.  100  Total P A R was partitioned into diffuse light and light as sunflecks, where sunflecks were classified as light intensity at twice the level of diffuse light (Pearcy 1990). In this study, 40 umol m ^ s was used as the minimum P A R 1  to be classified as a sunfleck. Other studies have used different values to determine the minimum sunfleck intensity (see Pearcy 1983, Canham et al. 1990, 1994) beneath various forest types. However, it is generally agreed that the minimum value of light intensity that can be classified as a sunfleck should correspond to some observable change i n the subject of study (Chazdon 1988), or to an increase in light intensity that differentiates it from background diffuse light (Pearcy 1990). In the case of western redcedar growing beneath birch and alder canopies, light intensity of 40 umol n r V  1  was high enough to elicit a significant change i n leaf photosynthesis. The proportion of understory P P F D due to sunflecks was calculated using a modified version of Gross' (1982) model: A-BC P P F D due to sunflecks  =  equation 2  DB + (A - BC) Where; A is the uncorrected amount of P P F D due to sunflecks (umol-m- ) and is estimated as: 2  Total P P F D (umol-m- ) - Cumulative P P F D < 40 umol-m , 2  2  B is the mean diffuse PAR, C is the total accumulated sunfleck time (seconds), D is the total time (seconds).  Using equation 2, the corrected amount of PAR due to sunflecks was calculated to be 501518 umol n r beneath the alder stand and 2  101  654159 umol-nr beneath the birch stand. These values represent 55 and 58 2  % of understory P P F D , respectively (Table 4.1), which is i n agreement with sunfleck studies within both tropical (Bjorkman and Ludlow 1972, Pearcy 1983, Pearcy and Calkin 1983, Chazdon and Fetcher 1984, Chazdon et al. 1988) and temperate forests (Pfitsch and Pearcy 1989). However, even with the added contribution of sunflecks to the total PAR received i n the understory during a sunny day, the proportion of incoming light received by the understory was still extremely small. Using an incoming light intensity of 1400 umol m^S" for both sites over the period of study, P P F D received by 1  the understory (including sunflecks) as a proportion of P P F D received i n the open was approximately 3 and 4% within the alder and birch stands, respectively (Table 4.1). The residency time of sunflecks beneath both stands ranged from 10 seconds to 7.3 minutes beneath the alder stand, and between 10 seconds and 11.7 minutes beneath the birch stand. The mean residency time of sunflecks beneath both alder and birch stands was 37 and 69 seconds, respectively (Table 4.1), with approximately 80 % of sunflecks beneath both stands lasting less than 90 seconds (Fig. 4.2). These findings are similar to sunfleck duration reported beneath a closed canopy Australian rainforest (Bjorkman and Ludlow 1972) i n which the majority of recorded sunflecks was less than 120 seconds long. Mynenei and Impens (1985), using a geometrical model to determine the residency time of a sunfleck due to the Earth's rotation alone (in the absence of wind), predicted 50 seconds as an average period over which light intensity was stable at any particular point. Conversely, Pearcy (1983) found that 60 % of the sunflecks beneath a Hawaiian rainforest were less than 30 seconds long when measured over two consecutive summers. This relatively short sunfleck duration was attributed to rapid leaf canopy  102  Table 4.1. Total understory P P F D and P P F D due to sunflecks beneath the red alder and birch stands over a period of 5.79 and 5.47 hours, respectively. Sunfleck is defined as P A R > 40 umol n r V . 1  Alder understory  Duration (seconds) Mean diffuse P A R (umol n r V ) 1  Total P P F D (umol nr ) Cumulative P P F D > 40 umol m V Cumulative P P F D < 40 umol m ^ s 1  2  1 1  P P F D due to sunflecks (corrected)^ Percent P P F D due to sunflecks Estimated^ P P F D i n the open (umol nr ) 2  Birch understory  20844  19692  20  24  911850 590070 321780  1127860 814110 313750  501518  654159  55  58  29181600  27568800  Estimated understory P P F D as a percentage of P P F D i n the open  3  4  Mean sunfleck duration (seconds)  37  69  Minimum sunfleck duration (seconds)  10  10  Maximum sunfleck duration (seconds)  440  700  4600  6380  Total time i n which sunflecks were present (seconds)  1 Photosynthetic photon flux density. 2 From equation 2 3 P A R i n the open was measured as 1400 umol m- s . 2  -1  103  0.8 -i 0.7 O' ;  *3 . :  d  0.6  ft.  03  0.4 • o  0J -  •ft cr cu  300  450  600  900  1200  0:8 4  B  :a:  0.6  -4  ft':  0.4.4 w  0.2 4 .'ft  .© :*-< fe'  3QQ  600  Time (seconds):  Fig. 4.2. Frequency distribution of sunfleck duration within the A) red alder, and B) paper birch stand.  104  movement caused by the characteristic windy conditions of the area (see also Tang et al. 1988). The total time in which sunflecks were present beneath the two stands i n my study was 77 and 106 minutes beneath the alder and birch stands, respectively, and represented 22% (alder) and 32% (birch) of the total recording period (Table 4.1). This investigation revealed the dynamic nature of the understory light environment i n the study stands. The next step was to determine whether understory western redcedar was capable of responding photosynthetically to this changing light environment, and whether sunflecks contributed significantly to the daily CO2 uptake by western redcedar.  Leaf Photosynthesis Due To Sunflecks Direct Measurements The variation i n CO2 uptake by understory western redcedar during the 2.2 hour study period was closely related to the changing light intensities caused by the passing of sunflecks. A total of 31 measurements were made over this period and are numbered sequentially i n Fig. 4.3. Three distinctive P n peaks that corresponded to an increase in PAR due to sunflecks were recorded. P n measurements 1 - 3 show initial low rates of CO2 fixation which correspond to low levels of PAR (< 23 umol nr s- ). PAR measurement 2  1  4 represented the commencement of the first sunfleck (45 umol m^s ) with 1  increasing intensity peaking at 254 umol m s 2  _1  before returning to pre-  sunfleck levels. P n tracked changing PAR closely, with pre-sunfleck P n rates of 0.1 - 0.3 umol m^s- increasing to 2.9 umol nr s- with the initiation of the 1  2  1  first sunfleck (measurement 4). This was followed by a  F i g . 4.3. Changes i n understory light intensity within the red alder stand a function of A) sunflecks, and B) the photosynthetic response of western redcedar. Numbers represent every fifth sample.  106  further increase i n P n to 4.2 umol nr s as sunfleck intensity peaked. The 2  1  pattern of P n decline was also closely correlated to the decrease i n PAR as the sunfleck passed and light intensity returned to diffuse levels. This close relationship between P n and PAR during the passage of a sunfleck was repeated for both the subsequent two sunflecks (measurements 22 and 25) (Fig. 4.3). Of interest is the rapid increase in Pn from measurement 3 to 4. The time duration between these two measurements was 4 minutes i n which time P A R increased 2.1-fold from 21 to 45 umol n r V while the corresponding 1  rate of P n increased 10-fold from 0.3 to 3.0 umol m^s ). This rapid and high 1  increase i n P n following a relatively modest increase i n P A R may be linked to a pre-induction sunfleck period in which the leafs photosynthetic apparatus has been previously "primed" allowing it to respond quickly to a fluctuating light environment (Gross 1982, Chazdon and Pearcy 1986, Chazdon 1988, Pearcy 1990). Plants placed i n low light (at or near their compensation point) show a slow but gradual increase i n their photosynthetic response following rapid increases i n light (Chazdon and Pearcy 1986, Pearcy 1988). This has been demonstrated with Alocasia macrorrhiza, a subtropical understory plant, i n which it took approximately 35 minutes for the plant to reach steady state (Pn) following a rapid increase of PAR from 10 to 400 umol n v V (Chazdon and Pearcy et al. 1990). In contrast, subjecting the same plant to periods of increased light (lightflecks) during a low light period primed, or induced, the plant to a state i n which further lightflecks of even 20 second duration were matched by a rapid and high rate of P n (Chazdon and Pearcy 1986). More recently, Pearcy et al. (1997) suggest that this induction period is a function of ribulose bisphosphate (RuBP) regeneration.  1  107  It can be speculated that the rapid and high response rate of P n by western redcedar to sunflecks during the 2.2 hour sampling period can be attributed to an induction period which began earlier i n the day following the first sunfleck. Indeed, test measurements (not shown) prior to the sampling period showed similar rapid and high P n response i n conjunction with sunfleck activity. However, care must be taken i n determining the induction period. It has been demonstrated that plants can only maintain a fully induced state if they are subjected to some minimum frequency of sunfleck activity. With Alocacia macrorrhiza, complete induction loss was observed when the plant was subjected to low-light levels for a period of greater than 60 minutes (Chazdon and Pearcy 1986). Pearcy et al. (1997) suggest that a period of 5 - 10 minutes of low light may be sufficient to move a plant from it's induced state to an uninduced state. In the case of western redcedar, there was no observable loss of induction during the sampling period even though there was a period of 54 minutes of low light between the first and second major sunfleck. Chazdon and Pearcy (1986) speculate that shade tolerant plants, or plants adapted to grow i n understory environments, can maintain a longer period of induction following a return to low-light levels than can less shade tolerant plants. See also Chen and Klinka (1997). The role of stomatal conductance during periods of sunfleck activity has been suggested as a mechanism restricting CO2 uptake by understory plants (Wong et al. 1978, van Caemmerer and Farquhar 1981, Farquhar and Sharkey 1982, Gross et al. 1991). However, there is evidence to suggest that the stomatal conductance of shade tolerant understory plants is relatively constant during sunfleck activity (Knapp and Smith 1990), assuming all other parameters are constant (i.e. humidity, air temperature, soil moisture).  108  Knapp and Smith (1990) have proposed two major categories of plants: trackers and non-trackers. Tracker plants have stomata which are sensitive to fluctuating light levels and respond to fluctuations i n light availability, while non-tracker plants have stomata which are relatively unresponsive to sunfleck activity. With understory western redcedar, stomatal conductance rates fluctuated around a mean of 0.05 mol n r V (Fig. 4.4) during the 1  recording period, but the value was not correlated to changes i n light intensity, indicating a non-tracker response.  Modeling Net CO2 Uptake in a Dynamic Light Environment In the previous two sections it was shown that sunfleck activity and sunfleck duration beneath red alder and birch stands are substantial and contribute significantly to the total P P F D received by understory plants during clear, windless, summer days. In addition, photosynthetic response of understory western redcedar during sunfleck periods was rapid and intense. Maximum rates of P n due to high light intensity sunflecks were approximately 4-10 times greater than P n rates under background diffuse light conditions (which were usually at or near the compensation point). However, what remained to be determined was the total net CO2 uptake by western redcedar growing i n the understory under dynamic light conditions and what proportion of that total CO2 uptake could be attributed to sunfleck activity. Figure 4.5 shows the photosynthetic light saturation curves and their associated models for the greenhouse and field data, respectively. These two models were then used to estimate P n as a function of changing light  109  Fig. 4.4. Understory air temperature (A) and stomatal conductance (B) of western redcedar during a period of sunfleck activity. Measurement times coincide with P A R and photosynthetic measurements (Fig. 4.3).  110  Fig. 4.5. (A) Light saturation curve of western redcedar grown under low light greenhouse conditions (PAR « 45 umol m V ) in the absence of sunflecks, and (B), the photosynthetic response curve of western redcedar growing beneath the red alder stand during varying sunfleck intensities. Curves i n A and B are described by the models: 1  Pn = -0.273 + 4.804(1 - xp(-0-003-PAR)) conditions), e  ; R  = .99, n = 7 (greenhouse  2  0  and Pn = -1.313 + 5.503 (1 - exp(- - 15-PAR)) 0  0  ; R  2  = .89, n = 20 (field conditions). 0  Ill  intensity due to sunflecks using the PAR data from the Pn-sunfleck study. These i n turn were compared to the actual P n response values as measured by the LI-COR 6200 gas system (Figs. 4.6 and 4.7). The greenhouse model tended to underestimate Pn at higher sunfleck intensities i n comparison to the actual recorded P n rates (Fig. 4.6). Estimated P n rates at low light intensities were also lower than the actual rates. In comparison, estimated Pn rates using the field model tracked very close to the actual data (Fig. 4.7). The difference in the two models may be attributed to the light environment during the growth of the greenhouse seedlings vs. the field grown seedlings. The greenhouse seedlings were grown under artificially shaded conditions (PAR « 45 umol m- s ) in the absence of any sunflecks. 2  _1  Therefore, these seedlings were never subjected to an induction period. This lack of priming of the photosynthetic apparatus may have limited the seedling's P n response to any step increase i n light intensity. Recall that the seedlings were kept i n darkness for 6 hours prior to the first light increase and that the seedlings were "acclimated" for 20 minutes at each light level before the photosynthetic rate was recorded. This 20 minute acclimation period may not have been long enough to fully prime the seedling for the next step increase (Chazdon and Pearcy 1986, Pearcy et al. 1997). In comparison, the field grown seedlings were subjected to the natural fluctuations i n the understory light environment, and were therefore fully induced and physiologically responsive to succeeding sunflecks. Even though the greenhouse model underestimated CO2 uptake i n response to sunflecks, it was still of interest to compare both models when applied to the birch and alder understory light data. Figures 4.8 and 4.9 show the estimated rates of P n as predicted by the two models i n response to  112  Fig. 4.6. Changes i n understory light intensity as a function of sunflecks (A) and the measured (•) and predicted (•) photosynthetic response of western redcedar (B) based on the greenhouse model: P n = -0.273 + 4.804(l-exp(-0-003-PAR)) R = 0.99. 2  ;  113  Fig. 4.7. Changes i n understory light intensity as a function of sunflecks (A) and the measured (•) and predicted (•) photosynthetic response of western redcedar (B) based on the field model: P n = -1.313 + 5.503(l-exp(-00 5PAR)) R2 = .89. 1  ;  0  114  0  3000  6000  9000  12000  15000  1&0G0  21000  - :. 15000  ./  ...21000,  Time (seconds)  I ;  o  -  '  aooo:  •., '-;- ;;v • 60Q0. 9000 :  v  12000  J  isooo  ;|:  Time (seconds}  Fig. 4.8. Estimated photosynthetic response of western redcedar to sunfleck activity beneath the red alder stand using; (A) the greenhouse model, and (B), the field model. Both models were applied to the sunfleck data i n Fig. 4.1A.  115  Fig. 4.9. Estimated photosynthetic response of western redcedar to sunfleck activity beneath the paper birch stand using: (A) the greenhouse model, and (B) the field model. Both models were applied to the sunfleck data i n Fig. 4.1B.  116  sunfleck activity beneath the two stands. Both models tracked sunfleck activity beneath both stands closely with peaks i n P n corresponding with the associated sunfleck peaks. The rapid decrease i n PAR from sunfleck back to diffuse light was also followed rapidly by a decline i n Pn. The main difference i n estimated Pn response by the two models was in their prediction of P n rate as a function of both low and high light intensity. The field model predicted a much steeper increase i n P n at low light levels (between 0 and 200 umol m - V ) which i n turn estimated higher P n rates at lower light levels 1  than the greenhouse model. In addition, the saturation point (Pn), using the field model, was reached at approximately 300 umol-nr^-s ! vs. 800 umol nr -  2  s as predicted by the greenhouse model. 1  Estimates of total CO2 fixed by western redcedar using the two models are shown i n Table 4.2. For both alder and birch stands, estimates of total CO2 fixed by understory western redcedar seedlings using the field model were approximately 63 % greater than estimates based on the greenhouse model. The proportion of CO2 fixed due to sunflecks was determined using Gross' equation (Gross 1982): % CO2 fixed due to sunflecks =  A - BC  equation 3  DC + (A-BC)  Where; A = the amount of CO2 fixed over the recording period at P A R > 40 umol m^s B = the total sunfleck duration (seconds), C = P n rate at the mean diffuse PAR level; Cfield 0-113 umol n r s - for the alder stand and 0.351 umol m^s for the birch stand. 2  =  1  -1  0.007 umol m- s for the alder stand and 0.061 umol nr s for the birch stand. D = the total sampling period (seconds). ^greenhouse 2  1  =  2  _1  1  117  Table 4.2. Estimated CO2 fixed by understory western redcedar due to sunflecks beneath the red alder and birch stands over a period of 5.79 and 5.47 hours, respectively. Sunfleck is defined as PAR > 40 umol m - V . 1  Alder understory  Birch understory  Total estimated CO2 fixed (|_imol nr ) by western redcedar using: 2  greenhouse model field model  4912 13214  7820 21606  4854 11793  7103 17468  Estimated CO2 fixed (u.mol nr ) due to sunflecks using: 2  greenhouse model field model  Percent! CO2 fixed due to sunflecks using: greenhouse model field model  1 Estimated using equation 3.  97 83  85 69  118  Based on the above equation, the estimated proportion of CO2 fixed due to sunfleck activity was large. The field model estimated that 83 % (alder stand) and 69 % (birch stand) of the total CO2 fixed by western redcedar was due to sunflecks. The greenhouse model estimated 97 and 85 % of the total CO2 fixed within the alder and birch stand, respectively, are due to sunflecks. The higher values estimated by the greenhouse model are attributed to its lower estimates of P n at low light levels: P n 0.007 and 0.061 umol m s 2  1  at  P A R levels of 20 and 24 umol m V (mean diffuse PAR) within the alder and 1  birch stands, respectively. In comparison, estimated P n rates using the field model were 0.113 and 0.351 umol m ^ s for PAR levels of 20 and 24 1  umol m- s , respectively. Therefore, at low light levels, the greenhouse model 2  _1  tended to estimate lower P n rates than the field model. The result was that a much greater proportion of CO2 uptake was attributed to sunflecks by the greenhouse model compared to the field model. The above proportions of CO2 uptake due to sunflecks using field data are some of the highest reported (see Pearcy and Calkin 1983, Pearcy 1987) and are probably attributable to the low levels of diffuse light beneath the alder and birch stands during the measurement period. The importance of sunflecks in CO2 assimilation by understory plants will vary depending on the background diffuse light levels, sunfleck duration and intensity, and in the ability of understory plants to utilize sunflecks by increasing their photosynthetic rate. The importance diminishes as the background diffuse light environment increases. For example, Gross (1982) estimated that sunflecks contributed < 10 % of the carbon uptake by Fragaria  virginiana  when subjected to one hour of low light (48 umol m^s ) punctuated by a 2 1  119  minute long sunfleck (1200 umol n r V ) even though P P F D due to sunflecks 1  accounted for 44% of the total P P F D . The importance of diffuse light i n estimating the sunfleck contribution to CO2 uptake by understory western redcedar can be illustrated by the following example using the field model for net photosynthesis (Pn = -1.313 + 5.503(1 - e -0.015PAR)  an  < i Gross' (1982) equation:  % CO2 uptake (function of diffuse light)  =  A IB +A  Equation 4  Where A is the amount of C 0 fixed during sunflecks minus the amount of 2  CO2 fixed i n the absence of sunflecks over the same duration and B is the amount of CO2 fixed over the total time period due to diffuse light. Using a time period of one hour, diffuse light level as either 20, 40 or 60 umol m- s , 2  _1  and a sunfleck intensity of either 100 or 400 umol n r V with a duration of 18 1  minutes, an estimation of CO2 uptake due to sunflecks can be made (Table 4.3). Estimations of the contribution of sunflecks to CO2 uptake diminished considerably with an increase i n background diffuse light.  CONCLUSIONS  Mid-summer light levels beneath the closed paper birch and red alder stands are some of the lowest recorded. However, understory sunfleck intensity and duration contribute significantly to the overall P P F D received beneath both birch and alder canopies, accounting for 55 - 59% of the total  120  Table 4.3. Estimated percent CO2 uptake of western redcedar over a one hour period with sunfleck intensity of 100 and 400 umol m ^ s and duration of 18 minutes. Diffuse background light is at 20, 40 and 60 umol n r V . P n and percent uptake are estimated using the field model (Pn = -1.313 + 5.503(1 - e -0.015-PAR) n d equation 4, respectively. 1  1  a  Diffuse Light Level (umol n r V ) 1  Sunfleck Intensity (umol m^s ) -1  20  40  60  Percent CO2 Uptake Due to Sunflecks 100  88  32  13  400  92  44  26  121  P P F D recorded. Western redcedar seedlings have the photosynthetic capability to utilize understory sunflecks by increasing their photosynthetic rates during sunfleck periods. It was estimated that between 69 and 83 % of CO2 fixed by understory western redcedar can be attributed to photosynthesis during periods of sunflecks and this may help to explain its presence i n understory environments within west coast ecosystems. The importance of the sunfleck contribution to C O 2 uptake by understory plants is greater i n those light environments where diffuse light levels are very low.  122  C H A P T E R 5. G R O W T H , B I O M A S S A L L O C A T I O N A N D PHOTOSYNTHETIC RESPONSE O F DOUGLAS-FIR, W E S T E R N H E M L O C K AND WESTERN REDCEDAR GROWN UNDER D I F F E R E N T LIGHT INTENSITIES  INTRODUCTION  The previous chapters (Chapters 2, 3, and 4) showed the dynamic nature of the understory light environment during the early stages of succession, both within mixed herb and shrub environments (Chapter 2) and beneath the early and later stages of deciduous tree stand development (Chapters 2 and 3). Mid-summer daytime light levels beneath closed deciduous canopies were shown to fluctuate from low diffuse light levels of < 20 umol n r V to > 600 umol n r V as a function of sunfleck activity 1  1  (Chapter 4). Conifer seedlings planted within these environments were shown to respond photosynthetically to both the seasonality of understory light availability due to the seasonal changes i n canopy leaf area (Chapters 2 and 3) and to the daily variations i n understory light availability due to sunfleck activity (Chapter 4). These field studies helped to establish a better understanding of understory environments within the study ecosystems and how plants growing within these environments can survive and grow. However, field studies, while essential for our understanding of plant dynamics under "natural" conditions, do not always permit us to isolate and manipulate certain variables i n the absence of other confounding variables. Some may argue that it is impossible, or at the very least highly unlikely, to obtain  123  meaningful results from field studies due to their inherent heterogeneity. While this may be true under some circumstances, most field researchers recognize the natural variation in nature and attempt to minimize it by choosing appropriate research sites and experimental designs. Nevertheless, it is important to compliment field studies with more controlled laboratory and/or greenhouse studies (or vice versa) to better understand plantenvironment interactions and responses. The following study was established to determine the photosynthetic and growth response of Pseudotsuga menziesii  (Mirb.) Franco (Douglas-fir), Thuja plicata Donn.  western redcedar), and Tsuga heterophylla (Raf.) Sarg. (western hemlock) seedlings grown under different levels of artificial shade within controlled greenhouse conditions. Specifically, I investigated: i) the growth (stem height and stem diameter) and biomass allocation of the above species grown for 4 months under 3, 6, 12, 24, and 45 % full sunlight, and ii) the photosynthetic response of the three species grown under the five different light levels.  MATERIALS AND METHODS  Five light regimes were established within a greenhouse at the University of British Columbia, Vancouver, BC. Light intensity was varied by constructing 1.0m x 1.5m x 1.5m frame enclosures covered with layers of grey nylon screen mesh. Light intensities (PAR) of 3, 6, 12, 24 and 45% of full sunlight (approximately 48, 81, 178, 293, and 663 umol m V , 1  respectively) were achieved by using 4, 3, 2, 1 and 0 layers of nylon mesh, respectively. The 45% light treatment represented the ambient light level within the greenhouse. Each light treatment was replicated twice.  124  In the spring of 1990, 360 one-year-old container grown Pseudotsuga menziesii  (Mirb.) Franco (Douglas-fir), Tsuga heterophylla (Raf.) Sarg.  (western hemlock) and Thuja plicata Donn. (western redcedar) seedlings were acquired from the B.C. Ministry of Forests. Stock type and seedlot information is shown i n Appendix 3.1. Twenty seedlings of each species were selected (based on shoot and root uniformity) and planted individually into four litre pots containing two parts sterilized soil, one part vermiculite, one part perlite and ten grams 14-14-14 (NPK) slow release fertilizer (Osmocote®, Sierra Chemical Co., California). Potted seedlings of each species were randomly placed within each of the five light treatments and maintained for four months. Seedlings were watered on a regular basis and were maintained at field capacity (i.e. excess moisture was allowed to drain from the pots). In addition, thirty seedlings per species were used to determine pre-treatment biomass allocation and leaf area parameters.  Seedling Growth A l l potted seedlings were measured for stem height and stem diameter (at root collar) at the beginning of the study. A n additional thirty seedlings of each species were partitioned into stem + branches, foliage, and roots, oven dried at 70 °C for 24 hours and weighed. Projected leaf area was determined on each foliage sample prior to drying using a LICOR Leaf Area Meter (LICOR Inc., Lincoln, Nebraska). These data were used to determine pretreatment seedling stem height and stem diameter, biomass, biomass allocation, and leaf area parameters. At the end of four months of growth under the five light treatments, all seedlings were measured for stem height and stem diameter, partitioned into  125  stem + branches, foliage, and roots (soil particles were gently washed from the root mass), oven dried at 70 °C for 24 hours and weighed. Total leaf area of each seedling was determined using a LICOR Leaf Area Meter on fresh foliage.  Photosynthesis Leaf photosynthesis was measured on six conifer seedlings (three from each of two replicates) per species per light treatment using a closed system C 0 gas analyzer (LICOR 6200 Portable Gas Exchange System, LICOR Inc., 2  Lincoln, Nebraska). Photosynthetic measurements were made between 11:30 A M and 2:00 P M i n mid-September. The ambient light level within the greenhouse during the measurement period was approximately 700 umol nr 2  s  _1  and represented approximately 45 % of the light intensity found outside  the greenhouse. Individual measurements were made by enclosing a one litre sealed cuvette over a portion of a seedling side branch and recording the change i n CO2 concentration within the cuvette over a 30 second period (details of this technique are given i n the previous chapters). At the end of each sampling period, that portion of the seedling branch that was enclosed within the cuvette was excised and measured for projected leaf area. The leaf area was used to adjust the photosynthetic measurement accordingly.  Light Reaction Curves Prior to harvesting, one seedling per species grown under each of the five light treatments was selected and used to develop light reaction curves.  126  Seedlings were kept i n the dark for six hours and then subjected to increasing irradiance i n steps of 0, 50, 100, 200, 350, 500 and 1000 umol nr s2  1  by using a 300-W tungsten halogen lamp placed at different distances from  the cuvette-encased foliage. Each seedling was allowed to acclimate for 20 minutes at each light level prior to measuring leaf net photosynthesis. A i r temperature during each measurement was maintained at approximately 24 °C.  RESULTS  Growth and Biomass Allocation Stem Diameter and Stem Height A l l three species showed a positive increase i n stem diameter growth as a function of increased light availability (Fig. 5.1). However, there were differences i n the pattern of diameter growth between Douglas-fir, western redcedar, and western hemlock. Douglas-fir stem diameter growth showed an almost linear response from 3 % up to 24 % full sunlight and achieved maximum growth (8.2 mm) under the 45 % light regime. In contrast, western hemlock and western redcedar seedlings displayed a more curvilinear response to increasing light, although maximum stem diameter growth also occurred at the 45 % light treatment (Fig. 5.1). Western redcedar and western hemlock seedlings achieved maximum height growth of 38.3 and 36.1 cm, respectively, when grown under the highest light treatment (Appendix 3.2). Both species displayed an overall positive height increase i n response to increasing light. However, there was no significant difference i n height growth between the 24 and 45 % full sun  127  Dougl^-fir 6  6  -1  :0  J  ipo.  200  1  L.  .300  400  ioo «oo  700:  W^stern*edcedar 8:  S:  •ft-  100  200  ,  •9  -100'  ;;300  400  500  «00 : 700  ^  Western heMock  2pS '36<f 400 . 500  =600 700  P-AX.(jiin6l#.:% > l  Fig. 5.1. Mean stem diameter (at root collar) of Douglas-fir, western redcedar, and western hemlock seedlings at the end of four months of growth under the five light levels. Bars represent one standard error of the mean; n = 20. Note differences i n the Y-axis scale.  128  light treatments for western hemlock seedlings. In contrast, Douglas-fir height growth responded linearly to increasing light and reached maximum height growth under the 12 % light treatment (Fig. 5.2). Seedlings grown under the two highest light levels did not have significantly greater height growth (Appendix 3.2). The effect of increased light levels on height:diameter ratios (cm/mm) was variable amongst the three species. Douglas-fir height:diameter decreased steadily from 9.1 cm/mm at 3 % light to 5.0 cm/mm at 45 % light (Fig. 5.3). Conversely, there were no significant differences for western redcedar seedlings grown under the five light treatments (Appendix 3.2). There was no clear pattern observed with the western hemlock seedlings (Fig. 5.3). The only significant difference in height:diameter ratios occurred between seedlings grown under the 6 and 45 % light treatments (Appendix 3.2).  Biomass and Biomass Allocation Overall total seedling biomass (shoots and roots) accumulation increased as a function of increased light availability for all three species (Fig. 5.4). However, Douglas-fir and western hemlock showed a much stronger response to increasing light than western redcedar. Total seedling dry biomass for Douglas-fir and western hemlock grown at the 45 % light treatment was 10.2 and 8.0 times, respectively, greater than the pretreatment seedlings (Appendix 3.3). In contrast, mean total biomass of western redcedar seedlings grown at 45 % light was only 4.1 times greater than the pre-treatment seedlings (Appendix 4.3).  129  4$  Dougl^-fir 40  35  -'•  30  0.  too  ;  •»  200  ••••  • ••' ••  300  400  500  $00  700  ••40.  Western redcedkt  •:tr £ 30  IS'  0  _i 100 200  ' • J 300 400  >  i  a..  SCO- «0Q: 700:  Western hemlock  35-'  '8 •  [  20;;  0  100  200  300  400  500  SOOi  700  Fig. 5.2. Mean height of Douglas-fir, western redcedar, and western hemlock seedlings after four months of growth under the five light levels. Bars represent one standard error of the mean; n = 20. Note differences i n the Yaxis scale.  130  to Douglas-fir a a  a. .  4  100  0  12  :  200  300) -4M, 500  500  700  r  Western redcedar  H is a a  0  ;;t00  200  300 .400; 500  600  .700;  ST  W^steriiihemlock  ,•» 7  ii—J  0  10C  1 — i — t — i — t — J  200  300  400  500  600  .r^'(^mpl;m- s-^)'' 2  700  .  ."  Fig. 5.3. M e a n height:diameter ratio of Douglas-fir, western redcedar, and western hemlock seedlings after four months of growth under the five light levels. B a r s represent one standard error of the mean; n = 20. Note differences i n the Y-axis scale.  131  218 r :  n.&. -  D0Uglas -fir :  • 4J  f i t , ::«» .-  4*; o.o; 0.,  100  200  300  400  500  600 JOO  y s  Western redcedar  r  if  I; I  9  :  100 :200  _J  -t.  300,  400  L.  500 <M0. WO: :  14  'ii  Western hemlock  *>  -*  0  "I00 200 ;  300 400  500  «00 700  Fig. 5.4. M e a n total dry weight (shoots and roots) of Douglas-fir seedlings, western redcedar, and western hemlock seedlings after four months of growth under the five light levels. Bars represent one standard error of the mean; n = 20. Note differences i n the Y-axis scale.  132  Western redcedar and western hemlock showed no distinctive pattern in biomass allocation to roots, stem + branches, and foliage as a percentage of total seedling biomass along an increasing light gradient (Appendix 3.3). In contrast, there was a distinctive shift in biomass allocation by Douglas-fir. Percent allocation to foliage changed from a mean value of 39.7 % at 3 % light down to 27.1 % for seedlings grown under the 45 % light treatment. Conversely, allocation to stem + branches and roots increased with increasing light availability (Fig. 5.5 and Appendix 3.3). The proportion of Douglas-fir seedling shoot dry mass to root dry mass (S:R) declined as a function of increasing light (Fig. 5.6). Seedlings grown under the 3 % light treatment had a S:R of 2.5 which was significantly higher than seedlings grown under the highest light treatment (S:R = 1.9) (Appendix 3.3). Similarly, the lowest S:R value for western hemlock was recorded for seedlings grown under the 45 % light treatment. However, there were no significant differences between seedlings grown under the 3, 6, 12, and 24 % light treatments (Fig. 5.6 and Appendix 3.3). Unlike Douglas-fir and western hemlock, the S:R of western redcedar seedlings did not show any clear response under the five light treatments. For example, while values of western redcedar seedlings grown under the 45 % light treatment were significantly lower (S:R = 2.6) than for seedlings grown under the 12 and 24 % light treatments (S:R = 3.5 and 3.1, respectively), they did not differ significantly from seedlings grown at the 3 and 6 % light treatment (Fig. 5.6 and Appendix 3.3).  133  Fig. 5.5. Changes i n biomass allocation (stem + branches, foliage, and roots) of Douglas-fir seedlings at the end of four months of growth under the five light levels. Lines are defined by the following models: % stem + branches = 8.8 + 29.7(1 - exp(-0.03 * PAR)); R = 0.99 % roots = 27.4 + 9.3(1 - exp(-0.002 * PAR)); R = 0.97 % foliage = 9.8 + 17.7(1 + exp(-0.009 * PAR)); R = 0.98 2  2  2  P A R is measured i n umol m ^ s  4  Regression lines are based on data presented i n Appendix 3.3  134  ©buglas-fif  ISO:!  215*  •S ;232 -  :X<M  >l.*4  •tsv,  100  '200  300  400  500  600  700:  Weste;rn ;redcedar :  352;,  JJ; 3J24 ; 136-  Tit  X40::L-r-r-i0' 100  200  300:  400:  500  S0O  700  Wesfera^emlMk  •3.40:  3.0S  J? 2.76  Z44  112 t'30  0  -V 100  , 200  300  400 . 500: ;S00;  700  Fig. 5.6. Changes i n the shoot:root ratio of Douglas-fir, western redcedar, and western hemlock seedlings at the end of four months of growth under the five light levels. Bars represent one standard error of the mean; n = 20.  135  Leaf Area Parameters Specific leaf area (SLA) of western hemlock seedlings varied between the five light treatments but there was no clear response to increasing light levels. For example, the only significant difference occurred between seedlings grown under the 3 % light treatment (SLA = 67.7 cm /g) and 12 % 2  light treatment (SLA = 74.3 cm /g) but there was no significant difference 2  between seedlings grown under the lowest and highest light treatments (Table 5.1). Conversely, the S L A of Douglas-fir seedlings declined as light levels increased, with the highest and lowest S L A values recorded under the 3 and 45 % light treatments, respectively (Table 5.1). Similarly, western redcedar seedlings grown under the 45 % light treatment had a significantly lower S L A (57.4 cm /g) than seedlings grown under the 3, 6, 12 , and 24 % light 2  treatments. There were no significant differences between seedlings grown under the latter four light levels (Table 5.1). Similar patterns were recorded with regards to changes in leaf area ratio (LAR) for the three species (Table 5.1). The L A R of Douglas-fir declined as a function of increasing light with the highest L A R recorded under the 3 % light treatment (LAR = 15.7 cm /g) and the lowest under the 45 % light 2  treatment ( L A R = 9.0 cm /g). No clear L A R response was observed i n 2  western hemlock seedlings grown under the five light treatments. Seedlings grown under the highest and lowest light treatments showed similar LAR's. The only significant difference occurred between seedlings grown at the 45 % light treatment and seedlings grown under the 12 and 24 % light treatment.  T A B L E 5.1. Changes i n specific leaf area (SLA) and leaf area ratio (LAR) of Douglas-fir, western redcedar and western hemlock seedlings grown under different light levels. Means within each row followed by the same letter do not differ by Duncan's H S D multiple comparisons test at P = 0.05. Numbers in parentheses represent one standard error of the mean. Analysis of variance tables are shown in Appendix 3.7, 3.8 and 3.9.  Percent Light Pre-treatment  3  6  12  24  45  Douglas-fir SLA  1  LAR2  (cm /g)  38.3  (0.7)  39.7b (1.6)  36.1 (1.3)  38.0 (1.5)  (  11.2  (0.4)  15.7a (0.8)  12.9  11.7 (0.5)  18  18  2  C f f l  n  2/g)  30  3  ab  b  (0.6)  ab  bc  18  32.5  (1.5)  32.7  9.5cd(0.5)  9.0  a  18  a  d  (1.1) (0.5)  18  Western redcedar S L A (cm /g)  57.5a (1.0)  68.7a (2.1)  74.3  L A R (cm /g)  29.3  (0.5)  30.6  31.3  30  18  2  2  n  a  (1.5)  a  a  (1.8)  66.7  a  (2,2)  68.7  (1.3)  29.6  a  (1.2)  30.1  19  18  a  a  (2.5)  57.4  b  (1.2)  (1.2)  24. l  b  (0.7)  a  (2.3)  18  18  Western hemlock S L A (cm /g)  75.9  (2.4)  67.7  (1.4)  80.5 (3.2)  87.6  L A R (cm /g)  34.2  (1.3)  33.4 (2.3)  36.5 (1.6) 11  2  2  n  30  a  ab  8  ab  ab  Projected leaf area (cm ) per dry leaf weight (g). 2  Projected leaf area (cm ) per total dry weight (g) of seedling. 2  Number of samples.  (2.6)  77.9 (2.4)  74.3  39.2 (1.7)  36.4 (1.2)  30.8  13  17  18  b  b  ab  b  C  (1.4)  137  As with SLA, the L A R of western redcedar seedlings was unaffected by an increase i n light intensity from 3 to 24 % full light and only significantly decreased at the highest light level (Table 5.1)  Leaf Photosynthesis Seedling leaf photosynthesis of all three species responded asymptotically to an increase i n light availability (Fig. 5.7). The lowest photosynthetic rates (« 1.0 umol m V ) for all three species were recorded 1  under the 3 % light treatment (PAR « 48 umol m^s ). Maximum 1  photosynthetic rates for Douglas-fir (11.1 umol n v V ) and western redcedar 1  (7.2 umol n r V ) seedlings were recorded under the 45 % light treatment 1  (PAR « 663 umol m"2 s" ). However, the mean maximum photosynthetic rate 1  for western hemlock (6.4 umol nr s- ) seedlings was achieved by the 24 % full 2  1  P A R treatment (PAR « 293 umol n r V ) with no significant increase i n P n at 1  the 45 % light treatment. The light levels at which maximum net photosynthesis was achieved by Douglas-fir, western redcedar, and western hemlock are i n accordance with other studies (Leverenz 1981, Major 1990, Grossnickle and Arnott 1992). Photosynthesis - irradiance response curves for Douglas-fir and western redcedar seedlings (response curves were not obtained for western hemlock) grown under the highest and lowest light treatments (45 and 3 % of full light, respectively) are shown i n Fig. 5.8. Douglas-fir seedlings grown under the 3 % light treatment displayed a photosynthesis - irradiance response curve typical of a "shade tolerant" plant (i.e. low compensation and saturation points). Indeed, the compensation point for the low light seedling was approximately 35 umol m ^ s lower than for the high light seedling. 1  138  12 Dougias-fir  10  100  200  300  400  500  600  700  ;  100  200;  300  400  500  S00  Western Tedceciar  700  •r  Western hemlock  6 S\  4  3' • r. (i 0  100  200  300  400:  500  ;soo  700  PATilCixinolTn^s- ) 1  Fig. 5.7. Mean leaf photosynthesis of Douglas-fir, western redcedar, and western hemlock seedlings grown under the five light levels. Bars represent one standard error of the mean; n = 6 for each data point.  139  Western redcedar  S3,  •pah  m mtmtL  -.ta a co  c . ~>  • iPii;= -0.91??* ?.553'*(1 ^exp(.0.QO43>P&^  ;  .'•i'Q .0  200;  400  600  800  1000  B  Douglas-fir  200  400  600:  100:  .1:000.  .PAR' (umolsm^s^i  Fig. 5.8. Net Photosynthesis (Pn) - irradiance response curves for (A) western redcedar, and (B) Douglas-fir grown under 45 (•) and 3 % (•) full sunlight.  140  Western redcedar showed a similar response but with lower compensation and saturation points recorded for the low light seedling.  DISCUSSION The pattern of height growth as a function of light intensity for Douglas-fir, western redcedar, and western hemlock is similar to other studies of both conifer (gymnosperm) and angiosperm light-growth response (Shirley 1945, Fairbairn and Neustein 1970, Atkinson 1984, Carter and Klinka 1992). For example, early work by Shirley (1945) showed that maximum height growth of red pine, white pine, jack pine and white spruce seedlings was achieved under 45 % of full sunlight. Similarly, Fairbairn and Neustein (1970) observed an increase i n stem height of various conifer species when grown at 6, 12, 25, and 50 % full sunlight but found very little further height increase from 50 to 100 % light intensity. However, the response of Douglas-fir height i n my study showed that maximum height growth was achieved at 12 % full sunlight. This corresponds with the field results presented i n Chapter 2 which shows high Douglas-fir height growth response under the heavy shade of the red alder and control treatments. The high height growth of Douglas-fir grown under low light conditions can be interpreted as a plastic morphological response to resource availability. In other words, Douglas-fir responds to low light conditions by allocating a greater percentage of its growth resources to stem elongation to sustain its height growth. Luken et al. (1995) found similar results with Lonicera maackii  when grown under 5, 25 and 100 % full sunlight - the  greatest mean relative stem growth was found under the 25 % light regime. This strategy is i n agreement with Grime's theory (1973, 1979) of resource  141  allocation (i.e., species with rapid growth rates can out compete those species which have slower growth rates). This type of growth can be observed i n secondary succession, especially within forested ecosystems which have been clearcut harvested. Under these conditions, especially in cool mesothermal climates typical of southwest British Columbia, newly created clearcuts are rapidly invaded by fast growing pioneer species such as fireweed, salmonberry, thimbleberry, red alder, and cottonwood (the presence and vigour of these species will depend on site conditions - i.e. moisture and nutrient availability, appropriate substrate, etc.). Douglas-fir, whether naturally or artificially (planted) established, must be able to compete for light with these early pioneer species (i.e. outgrow them by putting on rapid height growth) or be overtopped by them. Therefore, overtopped Douglas-fir will allocate more growth to stem elongation to avoid heavy shade. The result is a tall but spindly tree (very small stem diameter) which may not survive. But how does Grime's theory apply to western redcedar and western hemlock? According to his theory, western redcedar and western hemlock are less competitive than Douglas-fir at 12 % of full light. Height growth increments of western redcedar and western hemlock seedlings grown at the 12 % light treatment are approximately 45 % less than Douglas-fir (height increment is not shown but can be estimated by subtracting the pretreatment height growth from the treatment height growth, Appendix 4.2). Therefore, using the results from this study and applying Grime's theory we can conclude that under low light conditions, Douglas-fir is better able to compete than western redcedar and western hemlock because of Douglas-fir's superior height growth under low light environments. However, i n southwestern British Columbia, western redcedar and western hemlock  142  seedlings and saplings can be found in great abundance beneath mixed conifer and mixed deciduous stands; often growing in light environments approximating 2 % of light levels in the open. What we rarely find beneath these stands are Douglas-fir seedlings, even though Douglas-fir may be present in the overstory. This absence of Douglas-fir in the understory has led to the classification of this species as shade intolerant. However, it has also been suggested that the absence of this species in the understory may also be a function of seedbed conditions, i.e., the lack of exposed mineral soil (Klinka personal communication). Obviously, we must look for different theories of plant strategies under low light conditions. Tilman proposed an alternative theory for plant growth strategy: the resource ratio hypothesis (Tilman 1985, see also Tilman 1989, Wilson and Tilman 1991, 1995, Inouye and Tilman 1995). This asserts that plants which can lower their resource requirements, such as light, can out-compete those plants that cannot. Therefore, long-lived perennial plants such as western redcedar and western hemlock which can maintain positive rates of CO2 uptake under low light conditions, as well as maintain a somewhat balanced allocation of carbohydrates (i.e. biomass) to stems + branches, foliage, and roots, are better able to survive and compete in the short-term and eventually dominate over the long-term. Canham et al. (1996) have suggested that while plant growth is limited by low light availability beneath dense forest canopies plant growth may also be limited by the availabilty of other resources such as water and nutrients (see also Latham 1992, Bazzaz and Miao 1993). However, under very low light levels (< 3 % of full sunlight) light is the dominant resource that plants compete for - other resources such as moisture and nutrients have minimal  143  affect on plant growth at low light levels (Pacala et al. 1994, Canham et al. 1996).  144  CHAPTER 6. SUMMARY AND CONCLUSIONS In this final chapter, the results of the previous sections are discussed in the context of; 1) vegetation dynamics following clearcut logging on low elevation sites within the CWHdm subzone; 2) the resultant change i n understory light availability during secondary succession; 3) the seasonality of light availability and the photosynthetic response of underplanted conifer seedlings within early- and mid-seral plant communities; 4) the growth response of conifer seedlings to light competition under both natural and artificial conditions; and 5), the dynamic nature of understory sunfleck activity and the photosynthetic response of western redcedar. Vegetation invasion and growth following clearcut logging and mechanical site preparation on a fresh to moist and nutrient medium to medium-rich site within the CWHdm subzone was shown to be very rapid. Within two years of harvesting, percent vegetation cover averaged 48 % with a mean height of 0.39 m. Within 6-years of harvesting percent cover had increased to 102 % with a mean height of 2.75 m and was dominated by red alder and black cottonwood in the overstory and by various shrub and herb species i n the understory (Table 2.1). This rapid invasion and growth by early successional species was attributed to the nutrient and moisture status of the site and to the nature and extent of disturbance. The clearcut logging method resulted i n an open area free of any shade-casting trees which allowed fast growing, shade intolerant plants such as red alder and black cottonwood to quickly invade. In addition, the intensive nature of the site preparation (removal of most of the organic debris) resulted i n a site composed mostly of exposed mineral soil which produced a favourable seed bed environment for early pioneer species.  145  As a result of the vegetation growth over the four year period, there was a steady decline in mid-summer understory light availability within the control and alder treatments. Mid-summer light levels within the control and alder treatments declined from 57 % and 74 % of light levels i n the open, respectively, four years following harvesting, to 2 % (control) and 3 % (alder) at seven-years-post harvesting. These low light levels were correlated with low mid-day photosynthetic rates by underplanted conifer seedlings. Net photosynthetic rates of Douglas-fir, grand fir, and western hemlock seedlings growing within the control and alder treatments were generally less than 2.0 umol m  - 2  s"l i n mid-summer. In contrast, planted seedlings of the same  species growing i n the open had mid-day photosynthetic rates at approximately 8.0 umol n v V , four times the rate of the underplanted 1  seedlings. However, these static measurements, while important i n explaining average mid-day understory light availability and the resultant photosynthetic response of understory conifer seedlings during the growing season, fail to account for the seasonality of light availability. Because of the dominance of deciduous vegetation on the control plots and the presence of red alder within the alder treatment, the seasonality of understory light availability was mostly a function of the presence or absence of canopy foliage. During the late fall and through to the early spring period, the deciduous canopies were leafless, allowing a large portion of incoming light to penetrate into the understory. On clear days, light levels within the control and alder treatments were as high as 75 % of light levels i n the open (Fig. 2.6). Understory light levels remained high during the early spring and only started to decline with the onset of canopy bud break and leaf expansion (late March). By early April, understory light levels had declined to  146  approximately 20 % of light i n the open. By June, canopy leaf growth within both the control and alder treatments was sufficient to reduce understory light levels to less than 5 %. Light levels remained low during the summer months and only started to increase i n September-October with the commencement of leaf fall. This resulted i n a pattern of understory light availability that included two distinctive peaks of high light availability during the winter - spring and fall - winter periods separated by a "trough" of low understory light availability during the summer period. The mid-day photosynthetic responses of underplanted conifer seedlings were found to track the seasonality of understory light availability. Two distinctive photosynthetic peaks by the conifer species were shown to correspond with the fall and spring period when understory light availability within the control and alder treatments were high due to leaf fall (in autumn) and prior to leaf flush i n the spring (Fig. 2.8). Conversely, the lowest photosynthetic rates were recorded during the summer months when the deciduous canopies were fully foliated, resulting i n low levels of understory PAR, and during the winter months when air and/or soil temperatures were less than 5 °C. Similar trends of understory light availability and photosynthetic response were found with western redcedar seedlings planted within red alder and birch stands (Chapter 3). Mid-summer levels of understory PAR beneath both stands were approximately 2 % of light i n the open and ranged from 16 - 39 and 9 - 37 umol n r V within the alder and birch stands, 1  respectively. Recorded rates of net photosynthesis by western redcedar seedlings growing within these two stands during the summer months were generally less than 1.5 umol m V . Similar to Douglas-fir, grand fir, and 1  147  western hemlock seedlings, western redcedar showed a seasonal photosynthetic response with the highest mid-day rates occurring i n the fall and spring when both the alder and birch canopies were leafless and air and soil temperatures were high enough to prevent the inhibition of seedling photosynthesis (Figs. 3.6 and 3.7). The above results (as outlined i n chapters 2 and 3) show the seasonal changes i n understory light availability beneath early successional deciduous vegetation and established stands of red alder and birch, and the resultant photosynthetic response of evergreen coniferous seedlings growing within these types of vegetation. It is proposed that mid-summer, static measurements of understory light availability and the photosynthetic response of conifers is inadequate i n explaining their "shade tolerance" or i n explaining the dominance of conifers within temperate forests of southwestern British Columbia. In addition, previous studies of light competition have failed to account for the dynamic nature of understory sunflecks and their significant contribution to overall P P F D and to the CO2 uptake rates of understory plants. As outlined i n Chapter 4, the contribution of sunflecks to the daily P P F D within closed canopy red alder and birch stands was large. It was estimated that 55 % and 58 % of total P P F D during the period of deciduous leaf cover was due to sunflecks beneath red alder and birch stands, respectively (Table 4.1). Also, it was shown that western redcedar was capable of responding to changing understory P A R by closely tracking sunfleck activity (Fig. 4.3). Sunfleck peaks between 200 and 445 umol m V were closely correlated with photosynthetic peaks between 2 and 5 umol nr s- . Conversely, net photosynthesis of western redcedar declined 2  1  1  148  rapidly with the passing of sunflecks and remained very low (<0.5 umol m^s- ) during periods of diffuse light (PAR « 20 - 25 umol m^s ). 1  1  This rapid photosynthetic response of western redcedar to sunfleck activity can be attributed to a pre-induction period in which previous exposure to sunflecks "primed" the photosynthetic apparatus resulting i n a quick response (increase i n net photosynthesis) to subsequent sunflecks. While this hypothesis was not directly tested in this study, it can be speculated that the length of the induced period (i.e. the length of time that a plant remains i n a photosynthetically-induced condition following a return to diffuse light) may be used as a meaningful test of a plant's shade tolerance (see Pearcy et al. 1997, H u l l 2002). It can be seen from this study that trying to use established theories of plant-plant competition to explain the establishment and growth of evergreen trees is problematical. Many of these theories were developed for herbaceous plants and their application to trees may be questionable. What can be shown, however, is that trees, especially evergreen trees, are capable of utilizing different strategies to "deal" with a limiting and ever changing light resource. These strategies include: - Rapid height growth, therefore alleviating or avoiding light competition by outgrowing neighboring plants (Grime's theory of resource allocation). This is evident with Douglas-fir, a long-lived shade intolerant pioneer species (within cool mesothermal climates). - Low compensation points (i.e. the ability to maintain a positive CO2 uptake balance under low light conditions - Tilman's theory of resource ratio hypothesis).  149  - The morphological (i.e. evergreeness) and physiological (photosynthetic response) ability to utilize light availability over a 12-month period (the concept of the seasonality of photosynthesis). - The rapid utilization of sunflecks to maintain a positive CO2 balance during mid-summer periods of low understory light availability. How do these results contribute to the debate over the two opposing theories about plant-plant competition advanced by Tilman and Grime? The results of this study suggest that both theories are useful i n explaining plant competition - but, only under specific conditions and for specific species. The resource allocation theory put forward by Grime is most effective i n explaining plant growth following major disturbances, such as clearcut harvesting, on sites where nutrients and moisture availability are not limiting and where many pioneer plant species establish at the same time. Those species that allocate their resources to rapid height growth and leaf area development can outgrow slower growing and lower stature plants and eventually dominate the site, at least through the early and mid-stages of succession. However, allocation to rapid height growth is not an advantage for understory plants regenerating and growing within established forest stands which may have canopy heights of 10 - 70 m. Shade intolerant plants cannot effectively utilize the low light availability to maintain prolonged height growth and will eventually die. Indeed, Douglas-fir seedlings growing within the control and alder treatments (chapter 2) maintained rapid height growth for the first few years of establishment but eventually died (data not shown) due to low light availability. This phenomenon of etiolation can be seen with other forest plant species such as Gaultheria shallon (Messier and Mitchell 1994), an evergreen shrub found growing at low- to mid-elevations  150  on the west coast of British Columbia on recently disturbed sites and within forested sites of varying canopy closure. During mid- to late-successional stages, Tilman's resource ratio hypothesis is more appropriate i n explaining plant growth under low light availability. In other words, plants such as "shade tolerant" western redcedar are both morphologically (evergreen nature and a more balanced allocation to above- and below-ground components) and physiologically (ability to photosynthesize at low light levels and to utilize changing light availability both seasonally and diurnally) adapted to establish and grow under low light conditions found within both deciduous and evergreen forest stands. From this discussion, I conclude that neither Tilman's or Grime's theories, on their own, provide a complete explanation for all plant-plant interactions. They can be best viewed in the context of plant succession where both theories are appropriate but at different stages and under different types (i.e. primary and secondary) of succession. But can we go beyond the above two contrasting theories of plant plant interactions (or competition)? At the beginning of this thesis I presented the concept (put forward by Connell and Slatyer [1977]) of the three pathway model of plant succession: facilitation, tolerance, and inhibition. Vegetation (crop tree species) establishment following some disturbance can either be facilitated by the establishment and presence of other vegetation (i.e. on dry, south-facing slopes certain species may require the presence of other species to ameliorate site conditions), inhibited by faster growing vegetation which are more effective at competing for resources, or crop tree seedlings can tolerate the presence of other vegetation and  151  eventually, given enough time, come to dominate a site. This last point is the basis for the monoclimax (or climatic climax) theory of plant (forest) succession put forward by Clements (1916). . While Connell and Slatyer (1977) provide a relatively simple approach in explaining successional pathways it is not an appropriate basis by which to explain all aspects of plant succession. Some plant species may be inhibited while others are facilitated by the same species (see Kimmins [1997] for a review of successional theory). Whether we choose to subscribe to one or any of these theories of plant - plant interactions, or methods of plant succession, to explain our observations of ecological processes may not be overly rewarding. The pursuit of a unifying theory of plant - plant interaction, while helping us to better understand natural systems, may be viewed as a quest for the "Holy Grail" - natural systems are too varied and complex to allow us the luxury of just one theory to explain natural diversity At the beginning of this thesis I proposed two general hypothesis: (i) post-disturbance pathways i n the CWHdm biogeclimatic subzone on mesic subhygric sites are tolerance rather inhibition i n character because of nongrowing season photosynthesis by midand late serai evergreen tree species, and (ii) sunflecks contribute significantly to the daily CO2 uptake of understory plants, specifically western redcedar. The results of my research lead me to accept the second hypothesis but more research, and specifically long-term research, is needed before I can fully accept or reject the first hypothesis. What I have attempted to show with this thesis is that plants grow i n dynamic environments - ever changing - both diurnally, seasonally, and over  152  s e v e r a l y e a r s - e s p e c i a l l y w i t h r e g a r d s to t h e l i g h t e n v i r o n m e n t . O u r o b s e r v a t i o n s o f u n d e r s t o r y p l a n t s a n d t h e i r role i n p l a n t s u c c e s s i o n , e s p e c i a l l y w i t h r e g a r d s to m e s o t h e r m a l c o n i f e r o u s forests o f s o u t h w e s t e r n B r i t i s h C o l u m b i a , c a n be e x p l a i n e d , i n p a r t , b y t h e i r a b i l i t y to u t i l i z e understory light both seasonally (seasonality of light a v a i l a b i l i t y beneath d e c i d u o u s v e g e t a t i o n ) a n d over p e r i o d s o f seconds to h o u r s ( c h a n g e s i n l i g h t availability v i a sunflecks i n the understory).  Implications for Forest Management Forest practices throughout the world, a n d p a r t i c u l a r i n B r i t i s h C o l u m b i a , h a v e c h a n g e d s i g n i f i c a n t l y over t h e l a s t t w o decades, m a i n l y d u e to p u b l i c p r e s s u r e . F o r e s t m a n g e r s h a v e b e e n a s k e d , a n d i n some cases l e g i s l a t e d (e.g. t h e F o r e s t P r a c t i c e s C o d e o f B r i t i s h C o l u m b i a ) , t o r e e v a l u a t e p r e s e n t forest p r a c t i c e s a n d to propose n e w a n d i n n o v a t i v e s i l v i c u l t u r a l s y s t e m s t h a t b e s t m e e t society's c o n c e r n s a b o u t p a s t forest p r a c t i c e s , s p e c i f i c a l l y a l t e r n a t i v e s t o l a r g e c l e a r c u t h a r v e s t i n g o f forest s t a n d s . U p u n t i l r e c e n t l y , t h e m a j o r i t y o f forests i n B r i t i s h C o l u m b i a w e r e harvested u s i n g the clearcut method; a system that removes most i f not a l l of t h e m e r c h a n t a b l e t i m b e r f r o m a site ( d u r i n g one p e r i o d ) l e a v i n g a n a r e a d e v o i d o f s t a n d i n g trees. T h i s m e t h o d h a s b e e n v i e w e d as t h e m o s t e c o n o m i c a l w a y o f h a r v e s t i n g t r e e s a n d t h e m o s t effective s y s t e m for r e g e n e r a t i n g a n e w crop o f trees. I n m o s t e c o s y s t e m s t h i s h a s b e e n a n efficient a n d e c o n o m i c a l m e t h o d o f h a r v e s t i n g a n d e n s u r i n g efficient regeneration (specifically v i a planting). A l t e r n a t i v e s to the clearcut system have also been used i n B r i t i s h C o l u m b i a , especially i n parts of the s o u t h e r n interior. These alternative systems have included; single-tree selection,  153  which results i n uneven aged stands; group-selection, which creates a series of small openings i n a forest stand (this allows several trees i n a group to reach maturity at the same time); and, strip-selection, a method of harvesting trees along long, narrow strips. Other methods that are also utilized are shelterwood systems which maintain a portion of the existing stand during the seedling establishment stage. Within each of the above silvicultural systems the objective is to extract timber while still maintaining some of the structural characteristics and ecological attributes of the pre-disturbed (harvested) forest. However, regardless of which silvicultural system is used (excluding clearcutting), regeneration of understory seedlings, whether due to natural regeneration or by planting, will have to occur under a partial canopy (see Messier et al. 1999). The success of regeneration within these stands will depend, i n part, on the species' ability to maintain a positive carbon balance under some level of reduced light availability (see Coates and Burton 1999, Drever and Lertzman 1999, McCarthy 2001). Within dry interior forests, partial shade can increase the survival rate of understory seedlings by ameliorating extreme daytime temperatures. However, the success of regenerating crop species after partial removal of the existing stand within west coast ecosystems is still unclear. The level of canopy opening via partial cutting required to regenerate species i n the understory will depend, primarily, on the "shade tolerance" of a species (Leifers et al. 1999, Brandeis et al. 2001, Kneeshaw et al. 2002 ) and on the spatial distribution of the overstory canopy (Battaglia et al. 2002, Brown and Parker 1994, Horn 1971). My study suggests that if this technique of forest stand harvesting is used i n mesothermal ecosystems then only certain species can be successfully used  154  based on their ability to grow and maintain themselves i n low light understory environments and eventually achieve rotation age. Unfortunately, this may preclude the utilization of such commercially important species such as Douglas-fir, a species that is best adapted to grow in the absence of light competition. However, with careful planning and an understanding of each commercial species' autecology (silvics), specifically its ability to grow under reduced light levels, sound management plans can be introduced which minimize the use of traditional clearcut methods and incorporate various silvicultural systems which utilize partial removal of existing forest stands.  155  REFERENCES Alaback, P. 1982. Dynamics of understory biomass in Sitka spruce-western hemlock forests of southeast Alaska. Ecology 63: 1932-1948. 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Effect of cloudcover on photosynthesis and transpiration i n the subalpine understory species Arnica latifolia. Ecology 64: 681-687. Zhang, S., Hennessey, T . C , and Heinemann, R.A. 1997. Acclimation of Loblooy pine (Pinus taeda) foliage to light intensity as related to leaf nitrogen availability. Can. J . For. Res. 27: 1032-1040.  177  APPENDICES Appendix 1. Seedling age and stock type of Douglas-fir, western hemlock, and grand fir (from Chapter 2).  Species  Seedlot/Elevation  Age & Stock Type  Douglas-fir  1293/300 m  2 + 0 Bare root  Western hemlock  18784/60 m  1 + 0 P S B 211  Grand fir  3637/Unknown  3 + 0 Bare root  178  Appendix 2.1 Analysis of variance of Douglas-fir stem diameter (mm) growth for the 1988, 89, 90, and 91 growing seasons. Analysis was conducted using SYSTAT® (SYSTAT, Inc. Evaston, IL.) 1988  Source  Sum of Squares  DF  Mean Square  F-Ratio  P  Block Treatment Error  9.004 22.811 1484.618  1 7 570  9.004 3.259 2.605  3.457 1.251  0.063 0.273  Source  Sum of Squares  DF  Mean Square  F-Ratio  P  Block Treatment Error  3.426 1288.682 2643.936  1 7 383  3.426 184.097 6.903  0.496 26.668  0.482 0.000  Source  Sum of Squares  DF  Mean Square  F-Ratio  P  Block Treatment Error  62.768 2939.992 6258.510  1 7 476  62.768 419.999 13.148  4.774 31.944  0.029 0.000  Source  Sum of Squares  DF  Mean Square  F-Ratio  Block Treatment Error  10.920 6977.901 12808.689  1 7 495  10.920 996.843 25.876  0.422 38.524  1989  1990  1991  0.516 0.000  179  Appendix 2.2. Analysis of variance of Douglas-fir height growth (m) for the 1988, 89, 90, and 91 growing seasons. Analysis was conducted using SYSTAT® (SYSTAT, Inc. Evaston, IL.) 1988  Source  Sum of Squares  DF  Mean Square  F-Ratio  Block Treatment Error  0.000 0.364 4.042  1 7 351  0.000 0.052 0.012  0.033 4.517  Source  Sum of Squares  DF  Mean Square  F-Ratio  Block Treatment Error  0.012 3.397 15.968  1 7 383  0.012 0.485 0.042  0.278 11.638  Source  Sum of Squares  DF  Mean Square  F-Ratio  Block Treatment Error  0.896 8.529 31.744  1 7 476  0.896 1.218 0.067  13.434 18.270  Source  Sum of Squares  DF  Mean Square  F-Ratio  Block Treatment Error  0.418 16.345 63.333  1 7 495  0.418 2.335 0.128  3.263 18.250  0.855 0.000  1989  0.598 0.000  1990  0.000 0.000  1991  0.071 0.000  180  Appendix 2.3. Analysis of variance of Douglas-fir heightrdiameter ratio (cm:mm) for the 1988, 89, 90, and 91 growing seasons. Analysis was conducted using SYSTAT® (SYSTAT, Inc. Evaston, IL.) 1988 Source  Sum of Squares  DF  Mean Square  F-Ratio  P  Block Treatment Error  0.009 122.304 1940.852  1 7 351  0.009 17.472 5.529  0.002 3.160  0.968 0.003  Source  Sum of Squares  DF  Mean Square  F-Ratio  P  Block Treatment Error  2.209 147.911 367.258  1 7 383  2.209 21.130 0.959  2.303 22.036  0.130 0.000  Source  Sum of Squares  DF  Mean Square  F-Ratio  P  Block Treatment Error  4.240 450.175 703.337  1 7 476  4.240 64.311 1.478  2.870 43.524  0.091 0.000  Source  Sum of Squares  DF  Mean Square  F-Ratio  P  Block Treatment Error  36.432 777.186 1180.835  1 7 495  36.432 111.027 2.386  15.272 46.542  0.000 0.000  1989  1990  1991  181  Appendix 2.4. Analysis of variance of Grand fir stem diameter (mm) growth for the 1988, 89, 90, and 91 growing seasons. Analysis was conducted using SYSTAT® (SYSTAT, Inc. Evaston, IL.) 1988 Source  Sum of Squares  DF  Mean Square  F-Ratio  P  Block Treatment Error  16.456 42.483 2578.710  1 7 510  16.456 6.069 5.056  3.255 1.200  0.072 0.301  Source  Sum of Squares  DF  Mean Square  F-Ratio  P  Block Treatment Error  22.298 1147.791 6049.677  1 7 506  22.298 163.970 11.956  1.865 13.715  0.173 0.000  Source  Sum of Squares  DF  Mean Square  F-Ratio  P  Block Treatment Error  24.126 2148.972 8113.977  1 7 510  24.126 306.996 15.910  1.516 19.296  0.219 0.000  Source  Sum of Squares  DF  Mean Square  F-Ratio  Block Treatment Error  30.632 4656.757 13666.701  1 7 511  30.632 665.251 26.745  1.145 24.874  1989  1990  1991  0.285 0.000  182  Appendix 2.5. Analysis of variance of Grand fir height growth (m) for the 1988, 89, 90, and 91 growing seasons. Analysis was conducted using SYSTAT® (SYSTAT, Inc. Evaston, IL.) 1988 Source  Sum of Squares  DF  Mean Square  F-Ratio  P  Block Treatment Error  0.009 0.243 8.190  1 7 510  0.009 0.035 0.016  0.546 2.160  0.460 0.036  Source  Sum of Squares  DF  Mean Square  F-Ratio  P  Block Treatment Error  0.001 0.426 12.124  1 7 506  0.001 0.061 0.024  0.054 2.543  0.817 0.014  Source  Sum of Squares  DF  Mean Square  F-Ratio  P  Block Treatment Error  0.210 2.430 23.779  1 7 510  0.210 0.347 0.047  4.498 7.445  0.034 0.000  Source  Sum of Squares  DF  Mean Square  F-Ratio  Block Treatment Error  0.177 4.934 49.122  1 7 511  0.177 0.705 0.096  1.843 7.332  1989  1990  1991  0.175 0.000  183  Appendix 2.6. Analysis of variance of Grand fir height:diameter ratio (cm:mm) for the 1988, 89, 90, and 91 growing seasons. Analysis was conducted using SYSTAT® (SYSTAT, Inc. Evaston, IL.) 1988 Source  Sum of Squares  DF  Mean Square  F-Ratio  P  Block Treatment Error  0.461 20.865 488.394  1 7 510  0.461 2.981 0.958  0.482 3.113  0.488 0.003  Source  Sum of Squares  DF  Mean Square  F-Ratio  P  Block Treatment Error  1.070 74.099 368.272  1 7 506  1.070 10.586 0.728  1.471 14.544  0.226 0.000  Source  Sum of Squares  DF  Mean Square  F-Ratio  P  Block Treatment Error  0.964 163.523 408.331  1 7 510  0.964 23.360 0.801  1.204 29.177  0.273 0.000  Source  Sum of Squares  DF  Mean Square  F-Ratio  P  Block Treatment Error  0.000 328.740 611.605  1 7 511  0.000 46.963 1.197  0.000 39.238  0.999 0.000  1989  1990  1991  184  Appendix 2.7. Analysis of variance of Western hemlock stem diameter (mm) growth for the 1988, 89, 90, and 91 growing seasons. Analysis was conducted using SYSTAT® (SYSTAT, Inc. Evaston, IL.) 1988 Source  Sum of Squares  DF  Mean Square  F-Ratio  P  Block Treatment Error  32.359 56.791 1069.658  1 7 485  32.359 8.113 2.205  14.672 3.679  0.000 0.001  Source  Sum of Squares  DF  Mean Square  F-Ratio  p  Block Treatment Error  18.752 712.947 3286.134  1 7 446  18.752 101.850 7.368  2.545 13.823  0.111 0.000  Source  Sum of Squares  DF  Mean Square  F-Ratio P  Block Treatment Error  72.545 1184.024 5314.633  1 7 468  72.545 169.146 11.356  6.388 14.895  0.012 0.000  Source  Sum of Squares  DF  Mean Square  F-Ratio  p  Block Treatment Error  18.791 2587.493 9006.979  1 7 481  18.791 369.642 18.726  1.004 19.740  0.317 0.000  1989  1990  1991  185  Appendix 2.8. Analysis of variance of Western hemlock height growth (m) for the 1988, 89, 90, and 91 growing seasons. Analysis was conducted using SYSTAT® (SYSTAT, Inc. Evaston, IL.)  1988 Source  Sum of Squares  DF  Mean Square  F-Ratio  Block Treatment Error  0.014 0.630 7.271  1  7 485  0.014 0.090 0.015  0.908 6.006  Source  Sum of Squares  DF  Mean Square  F-Ratio  Block Treatment Error  0.000 4.361 19.535  1 7 446  0.000 0.623 0.044  0.010 14.224  Source  Sum of Squares  DF  Mean Square  F-Ratio  Block Treatment Error  0.000 9.183 45.000  1 7 468  0.000 1.312 0.096  0.000 13.644  1991 Source  Sum of Squares  DF  Mean Square  F-Ratio  Block Treatment Error  0.052 12.765 80.425  1 7 481  0.052 1.824 0.167  0.309 10.907  0.341 0.000  1989  0.919 0.000  1990  0.995 0.000  0.578 0.000  186  Appendix 2.9. Analysis of variance of Western hemlock height:diameter ratio (cm:mm) for the 1988, 89, 90, and 91 growing seasons. Analysis was conducted using SYSTAT® (SYSTAT, Inc. Evaston, IL.) 1988 Source  Sum of Squares  DF  Mean Square  F-Ratio  P  Block Treatment Error  28.597 130.080 1182.405  1 7 485  28.597 18.583 2.438  11.730 7.622  0.001 0.000  Source  Sum of Squares  DF  Mean Square  F-Ratio  P  Block Treatment Error  11.859 235.304 1015.882  1 7 446  11.859 33.615 2.278  5.206 14.758  0.023 0.000  Source  Sum of Squares  DF  Mean Square  F-Ratio  P  Block Treatment Error  42.127 347.845 1672.657  1 7 468  42.127 49.692 3.574  11.787 13.904  0.001 0.000  Source  Sum of Squares  DF  Mean Square  F-Ratio  Block Treatment Error  15.299 624.828 2016.602  1 7 481  15.299 89.261 4.193  3.649 21.291  1989  1990  1991  0.057 0.000  Appendix 2.10. M e a n stem diameter at root collar (Dia), stem height (Ht) and height:diameter ratio (H:D) of planted Douglas-fir seedlings at the end of each of the four growing seasons (1988, 89, 90 and 91). Means within each column followed by the same letter are not significantly different from each other at p < 0.05 (Tukey test). Sample number for each mean ranged from 69 - 75.  1988  1989  1990  1991  Treatment*  Dia (mm)  Ht (m)  H:D (cm: mm)  Dia (mm)  Ht (m)  H:D (cm:mm)  Dia (mm)  Ht (m)  H:D (cm: mm)  Dia (mm)  Ht (m)  H:D (cm: mm)  1  7.4a  0.55a6  8.1 a*  15.6a  0.96a  6.2a6  19.3a  1.41a  7.4a  23.0a  1.966  8.7c  2  7.3a  0.48a6  7.3a6  13.16  0.776  5.9a  15.96  1.136  7.2a  19.16  1.62c  8.7c  3  7.0a  0.54a6  1.9ab  12.66  0.94a  7.4c  14.76a*  1.29a  9.06  17.76c  1.73c  10.16  4  7.3a  0.5 la6  13ab  12.16  0.806  6.66  14.3a"  1.136  1.9ad  15.8c  1.41a  9.1c  5  7.3a  0.56a  8.2a6  17.8c  0.95a  5.4a  22.4c  1.35a  6.1c  28.3a"  1.796c  6.4a  6  7.4a  0.486  6.8a  13.86  0.756  5.4a  17.6a6  1.036  5.9c  23.4a  1.48ac  6.5a  7  6.8a  0.54a6  8.66  14.06  0.826  5.9a  17.06  1.38a  8.2a"  20.6a6  1.856c  9.2c  8  7.1a  OASab  l.Oab  12!46  0.716  5.7a  15.36a"  1.146  1.6ad  17.8c  1.56ac  9.0c  Treatment 1 = red alder + fertilizer; 2 = red alder; 3 = control + fertilizer; 4 = control; 5 = no vegetation + fertilizer; 6 = no vegetation; 7 = shrub + fertilizer; 8 = shrub.  oo  Appendix 2.11. M e a n stem diameter at root collar (Dia), stem height (Ht) and height:diameter ratio (H:D) of planted Grand fir seedlings at the end of each of the four growing seasons (1988, 89, 90 and 91). Means within each column followed by the same letter are not significantly different from each other at p < 0.05 (Tukey test). Number of samples for each mean ranged from 69 - 75.  1988  1989  1990  1991  Treatment*  Dia (mm)  Ht (m)  H:D (cm: mm)  Dia (mm)  Ht (m)  H:D (cm:mm)  Dia (mm)  Ht (m)  H:D (cm:mm)  Dia (mm)  Ht (m)  H:D (cm: mm)  1  11.7a  0.54ab  4.6ab  18.2a  0.74a  4.1a  20.9a  1.04ace  5.0a  23.2bc  1.40a  6.1a  2  11.4a  0.54ab  4Mb  15.76  0.686  4.5a  18.06  0.936c  5.3a  21.36c  1.30a  6.2a  3  11.6a  0.56ab  4.9ab  15.26  0.74a  5.06  17.16  1.06e  6:26  18.86  1.36a  7.36  4  10.8a  Q.55ab  5.1a  13.2c  0.67a  5.16  14.6c  0.876  6.06c  15.7a  1.106  7.16  5  11.1a  0.55a6  5.\ab  17.5a  0.72a  4.2a  21.0a  l.OOace  4.9a  25.4c  1.30a  5.2c  6  11.4a  0.52ab  4.6ab  15.86  0.67a  4.3a  19.0a6  0.906a"  4.8a  23.6c  1.166  5.1c  7  11.3a  0.51a  4.6b  15.96  0.74a  4.76  17.26  1.06e  6.26  19.26  1.37a  7.26  8  11.7a  0.586  4.9ab  14.3c  0.72a  5.16  16.66c  5.7c  18.56  1.226  6.6a  0.94acd  Treatment 1 = red alder + fertilizer; 2 = red alder; 3 = control + fertilizer; 4 = control; 5 = no vegetation + fertilizer; 6 = no vegetation; 7 = shrub + fertilizer; 8 = shrub.  oo oo  Appendix 2.12. Mean stem diameter at root collar (Dia), stem height (Ht) and height:diameter ratio (H:D) of planted Western hemlock seedlings at the end of each of the four growing seasons (1988, 89, 90 and 91). Means within each column followed by the same letter are not significantly different from each other at p < 0.05 (Tukey test). Sample number for each mean ranged from 69 - 75. 1988  1989  1990  1991  Treatment*  Dia (mm)  Ht (m)  H:D (cm: mm)  Dia (mm)  Ht (m)  H:D (cm: mm)  Dia (mm)  Ht (m)  H:D (cm:mm)  Dia (mm)  Ht (m)  H:D (cm:mm)  1  6.96  0.53a  7.8ac  10.4ac  0.75a  7.4aa"  11.36  0.89ac  8.1a  12.5a  1.096  8.9c  2  5.8a  0.456  7.9ac  8.16  0.586  7.3a  9.06c  0.736c  8.2a  10.5ac  0.95a  9.0c  3  6.46  0.56ac  8.96  9.4a  0.84a  9.06  9.96c  1.0a  10.56  10.8ac  1.26  11.26  4  6.0a  0.51a6c  8.76  7.76  0.66a  8.56  8.66c  0.79c  9.16c  9.4c  0.95a  9.9ca"  5  6.5b  0.486  7.5ac  11.2c  0.76a  l.Oad  13.5a  1.05a  8.1a  17.16  1.316  7.9a  6  6.4b  0.466  7.2c  9.2a  0.596  6.5ac  10.06  0.72c  7.5a  11.lea  0.80ac  7.3a  7  6.4b  0.51a6c  8.1a6  9.5a  0.77a  8.16a"  10.56  1.0a  9.66  11.7a  1.216  10.46a"  8  6.0a  7.86  7.8ac  7.46  0.566  l.lad  8.0c  0.676c  8.4ac  9.0c  0.89a  9.8c  •Treatment 1 = red alder + fertilizer; 2 = red alder; 3 = control + fertilizer; 4 = control; 5 = no vegetation + fertilizer; 6 = no vegetation; 7 = shrub + fertilizer; 8 = shrub.  oo  190  Appendix 3.1. Seedling age and stock type of Douglas-fir, western hemlock, and western redcedar (from Chapter 5).  Species  Seedlot  Age & Stock Type  Douglas-fir  6421  1 + 0 P S B 313B  Western hemlock  18784  1 + 0 P S B 313B  Western redcedar  Unknown  1 + 0 PSB 313A  Appendix 3.2 Four month mean stem height and diameter growth response of Douglas-fir, western recedar and western hemlock seedlings grown under five light levels. Means i n each row followed by the same letter do not differ by Tukey's H S D multiple comaprisons test at P = 0.05. Numbers i n parentheses represent one standard error of the mean. Analysis of variance tables are shown in Appendix 3.4, 3.5 and 3.6.  Percent Light Pre-treatment  3  6  12  24  45  Douglas-fir Height (cm)  21.9 (0.2)  31.8  a  (1.2)  3 5 . 7 (1.0) a  43.2 (1.2) b  41.9  b  (0.9)  4 0 . 9 (1.7) b  Diameter (mm)  3.2 (0.1)  3 . 5 (0.1)  4.3  (0.1)  5.7  b  (0.2)  6.8 (0.3)  8.2  d  (0.3)  H : D l (cm:mm)  6.9 (0.1)  9 . 1 (0.3)  8.3 (0.2)  7.7  b  (0.2)  6.3C (0.2)  5.0  d  (0.2)  N  a  a  99  19  21.8 (0.4)  2 7 . 1 (1.1)  a  ab  20  20  C  20  20  Western redcedar Height (cm) Diameter (mm)  2.5 (0.1)  H : D (cm:mm)  8.7 (0.2)  a  2.6  (0.1)  32.5 (1.0) b  b  34.3 (1.0) bc  38.3 (1.0) C  (1.2)  3.5  b  (0.1)  3.5  b  (0.1)  4.2  10.6 (0.3)  10.7 (0.5)  9.7  a  (0.4)  9.9  a  (0.3)  9.3  102  19  21  19.4 (0.3)  2 3 . 3 (1.4)  29.9 (1.1)  Diameter (mm)  2.7 (0.1)  2.9 (0.2)  3.6 (0.2)  4.4  H : D (cm:mm)  7.3 (0.2)  8.2 (0.5)  8 . 4 (0.3)  N  a  a  3.2  33.8 (1.0)  b  a  20  20  C  a  (0.1) (0.3)  20  Western hemlock Height (cm)  N  1  101  Height:Diameter ratio  a  a  ab  10  b  ab  a  13  33.2 c(l.l)  36.1C (1.5)  (0.2)  4.8 (0.1)  5.4C (0.2)  7.2 (0.3)  7.0 (0.3)  6.9  30.8 (1.1) b  b  ab  16  b  bc  ab  19  21  b  (0.4)  Appendix 3.3. Changes i n percent biomass allocation (stem + branches, foliage and roots) of Douglas-fir, western redcedar and western hemlock seedlings grown under different light levels. Means within each row followed by the same letter do not differ by Tukey's H S D multiple comparisons test at P = 0.05. Numbers in parentheses represent one standard error of the mean. Percent Light^ Pre-treatment^  6  Douglas-fir  12  24  45  Percent of total seedling weight^  Stem + branches  35.0  (0.8)  31.6a (0.8)  35.9  b  (0.5)  38.7 (1.0)  38. l  Foliage  29.1  (0.7)  39.7a (0.8)  35.6  b  (0.6)  31.3 (0.6)  29 7C (0.6)  Roots  35.9  (1.0)  28.7  28.6  a  (0.6)  30.0 (1.0)  32. 2at>(l.l) 3 4 . 4 ( l . l )  1.8  (0.1)  2.6  Stem + branches  18.4  (0.5)  30.8  (1.2)  32.0  Foliage  51.1  (0.5)  44.2a (1.2)  41.8  Roots  30.5  (0.7)  25.0ab(o.8) 3.iab (0.1)  S:R  a  a  (l.i) (0.2)  2.5a (0.1)  b  C  a  2.4  a  b  (0.1)  (1.0)  b  38.5  (1.0)  b  2 7 . i d (0.6) b  2 2a  b  (0.1)  2.0  b  (0.1)  Western redcedar  S.R  2.3 (0.1)  a  (0.9)  32.3a (0.9)  31. 8  (1.1)  44.7a (0.8)  43. 5  26.2 (0.8)  23.0 (0.8)  24. 7at>(0.7) 28.2C  2.9a (0.1)  3.5 (0.2)  3. l  2 8 . 9 (1.0)  a  a  bc  a  b  a  (0.8)  29.9a (1.5)  a  (0.6)  41.9a (0.8)  b  (0.1)  2.6  28. 7  a  (0.9)  25.8  a  (0.7)  (0.1)  Western hemlock ( )  23.9  (0.7)  27.1  Foliage  44.9  (0.8)  48.ia (1.3)  45.3ab(0.7)  44.7 (0.7)  46. 5  a  (1.3)  41.1b (1.6)  Roots  31.2  (1.0)  24.9a (1.3)  27.5 b(i.2)  26.4 (1.3)  24. 9  a  (1.5)  33.1  b  (1.9)  2.7a (0.2)  2.8 (0.2)  3. Qa (0.3)  2.0  b  (0.2)  S:R  2.2(0.1)  3.0  a  a  (1.3)  27.3a  Stem + branches  (0.2)  L 0  a  a  ab  a  a  a  (1.0)  1 N = 30 for each species. 2 N = 20 for each species per light treatment. 3 The sum of the three seedling components per species per treatment may not add up to 100% due to rounding error.  193  Appendix 3.4. Analysis of variance of stem height, stem diameter and height:diameter ratio of Douglas-fir seedlings (greenhouse study) growing under the five different light levels. Analysis was conducted using SYSTAT® (SYSTAT, Inc. Evaston, IL.)  Stem Height Source  Sum of Squares  DF  Mean Square  Between Treatments  1805.2  4  451.3  Within Treatments  2838.5  94  30.2  DF  Mean Square  F-Ratio  P  14.9  0.000  F-Ratio  P  84.7  0.000  F-Ratio  P  Stem Diamter Source  Sum of Squares  Between Treatments  275.2  4  68.8  Within Treatments  76.4  94  0.8  HeightrDiamter Ratio Source  Sum of Squares  DF  Mean Square  Between Treatments  204.2  4  51.1  Within Treatments  103.5  94  1.1  46.4  0.000  194  Appendix 3.5. Analysis of variance of stem height, stem diameter and height:diameter ratio of western redcedar seedlings (greenhouse study) growing under the five different light levels. Analysis was conducted using SYSTAT® (SYSTAT, Inc. Evaston, IL.)  Stem Height Source  Sum of Squares  DF  Mean Square  Between Treatments  1261.1  4  315.3  Within Treatments  2035.7  95  21.4  F-Ratio  P  14.7  0.000  F-Ratio  P  19.1  0.000  F-Ratio  P  Stem Diamter Source  Sum of Squares  DF  Mean Square  Between Treatments  27.3  4  6.8  Within Treatments  33.9  95  0.4  Height:Diamter Ratio Source  Sum of Squares  DF  Mean Square  Between Treatments  29.3  4  7.3  Within Treatments  231.4  95  2.4  46.4  0.022  195  Appendix 3.6. Analysis of variance of stem height, stem diameter and height:diameter ratio of western hemlock seedlings (greenhouse study) growing under the five different light levels. Analysis was conducted using SYSTAT® (SYSTAT, Inc. Evaston, IL.)  Stem Height Source  Sum of Squares  DF  Mean Square  Between Treatments  1218.8  4  304.7  Within Treatments  2099.9  74  28.4  F-Ratio  P  10.7  0.000  F-Ratio  P  22.4  0.000  F-Ratio  P  Stem Diamter Source  Sum of Squares  DF  Mean Square  Between Treatments  54.2  4  13.5  Within Treatments  44.8  74  0.6  DF  Mean Square  HeightrDiamter Ratio Source  Sum of Squares  Between Treatments  29.3  4  7.3  Within Treatments  231.4  95  2.4  46.4  0.022  196  Appendix 3.7. Analysis of variance of specific leaf area (SLA), and leaf area ratio (LAR) of Douglas-fir seedlings (greenhouse study) growing under the f i v e different light levels. Analysis was conducted using SYSTAT® (SYSTAT, Inc. Evaston, IL.)  Specific Leaf Area (SLA) Source  Sum of Squares  DF  Mean Square  Between Treatments  701.8  4  175.5  Within Treatments  2912.4  83  35.1  DF  Mean Square  F-Ratio 5..0  P 0.001  Leaf Area Ratio (LAR) Source  Sum of Squares  Between Treatments  511.0  4  127.7  Within Treatments  472.4  82  5.8  F-Ratio  P  22.2  0.000  197  Appendix 3.8. Analysis of variance of specific leaf area (SLA), and leaf area ratio (LAR) of wester redcedar seedlings (greenhouse study) growing under the five different light levels. Analysis was conducted using SYSTAT® (SYSTAT, Inc. Evaston, IL.)  Specific Leaf Area (SLA) Source  Sum of Squares  DF  Mean Square  Between Treatments  2781.6  4  695.4  Within Treatments  6274.7  86  73.0  DF  Mean Square  F-Ratio 9.5  P 0.000  Leaf Area Ratio (LAR) Source Between Treatments Within Treatments  Sum of Squares 587.0  4  146.8  2085.5  84  24.8  F-Ratio 5.9  P 0.000  198  Appendix 3.9. Analysis of variance of specific leaf area (SLA), and leaf area ratio (LAR) of wester hemlock seedlings (greenhouse study) growing under the five different light levels. Analysis was conducted using SYSTAT® (SYSTAT, Inc. Evaston, IL.)  Specific Leaf Area (SLA) Source  Sum of Squares  DF  Mean Square  Between Treatments  2355.2  4  588.8  Within Treatments  5924.1  62  95.6  DF  Mean Square  F-Ratio 6.2  P 0.000  Leaf Area Ratio (LAR) Source Between Treatments Within Treatments  Sum of Squares 605.9  4  151.5  1869.5  60  31.2  F-Ratio 4.9  P 0.002  

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