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Lysimeter measurements of salal understory evapotranspiration and forest soil evaporation after salal.. Osberg, Peter Martin 1986

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LYSIMETER MEASUREMENTS OF SALAL DNDERSTORY EVAPOTRANSPIRAXION AND FOREST SOIL EVAPORATION AFTER SALAL REMOVAL IN A DOUGLAS—FIR PLANTATION  by PETER MARTIN OSBERG B.S.F., University of B r i t i s h Columbia, 1983  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES Department of S o i l Science We accept this thesis as conforming to the required standard  THE UNIVERSITY OF BRITISH COLUMBIA March 1986 ® Peter Martin Osberg, 1986  In p r e s e n t i n g  this  thesis i n partial  f u l f i l m e n t of the  r e q u i r e m e n t s f o r an a d v a n c e d d e g r e e a t t h e of B r i t i s h Columbia, I agree that it  freely  the L i b r a r y s h a l l  a v a i l a b l e f o r r e f e r e n c e and s t u d y .  agree that p e r m i s s i o n for  University  f o r extensive  s c h o l a r l y p u r p o s e s may  for  financial  shall  of  The U n i v e r s i t y o f B r i t i s h 2075 W e s b r o o k P l a c e V a n c o u v e r , Canada V6T 1W5 Date  1Q\  A r\\ ?  H  ;  19^4  Columbia  my  It is thesis  n o t be a l l o w e d w i t h o u t my  permission.  Department  thesis  be g r a n t e d by t h e h e a d o f  copying or p u b l i c a t i o n of this  gain  further  copying of t h i s  d e p a r t m e n t o r by h i s o r h e r r e p r e s e n t a t i v e s . understood that  I  make  written  ABSTRACT  Two weighing lysimeters were constructed i n a  23-year-old  Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) stand. (Gaultheria shallon Pursh.) understory was 30 x 40 m plot and one lysimeter was  E  s  (E ).  ranged between 0.08  and 0.34  removing s a l a l from the second lysimeter, which was s  was  0.34  - 0.41  the daily whole stand evapotranspiration. resistances ( r ) , c o  removed from a  For 8 consecutive  s  15-18% of the daily t o t a l stand evapotranspiration.  less dense tree canopy, E  salal  located at the center of this plot  to measure daily forest s o i l evaporation days, i n August 1984,  mechanically  The  mm  mm  d  - 1  which was  In July 1985,  after  positioned under a  d~* which was  17-21% of  Forest floor d i f f u s i v e  computed by rearranging the Penman-Monteith  equation, were found to range between 900 and 3500 s  m ^. -  The measured daily evapotranspiration rate from the ( E ) with s a l a l present was t  0.60  - 0.84  mm  d *, which was -  understory equivalent  to 30-42% of the daily stand evapotranspiration. The dependence on windspeed of the bulk aerodynamic resistance (s m ) -1  to vapour transport (r^) between the s a l a l canopy and measurement  height (0.3 m above the top of the s a l a l canopy) was r  = 25.2  u  0*554^ h w  found to be  u i s the mean windspeed (m s ) - 1  e r e  at measurement  A  height.  Approximately 70-75% of r ^ was  to the leaf boundary-layer resistance. foliage was  estimated  to be attributable  The evaporation rate from wet  found to be 5 times greater than the transpiration rate from  dry foliage exposed to similar meteorological conditions.  - iii -  A n a l y s i s of the r e l a t i v e importance of net a v a i l a b l e energy f l u x density and advective processes i n determining the latent heat f l u x density from the understory y i e l d e d a range of Q values between 0.15 and 0.22.  The vapour pressure d e f i c i t below the tree canopy was found to be  l a r g e l y determined by the vapour pressure d e f i c i t above the tree canopy, with very l i t t l e l o c a l adjustment.  - iv TABLE OF CONTENTS  Page ABSTRACT  i i  TABLE OF CONTENTS  iv  LIST OF FIGURES  vi  LIST OF TABLES  ix  NOTATION  xi  ACKNOWLEDGEMENTS  xvi  1.  INTRODUCTION  2.  THEORY  11  3.  METHODS  15  3.1  15  3.2  3.3  1  Site Description 3.1.1  Experimental  Site Location  15  3.1.2  Stand Structure  15  3.1.3  S o i l Description  16  3.1.4  Experimental  21  Plot Layout and Salal Removal  Measurements  24  3.2.1  Salal Leaf Area Index  24  3.2.2  Root Zone Soil Water Content and Potential  25  3.2.3  Meteorological Measurements  28  3.2.4  Forest S o i l Evaporation  30  3.2.5  Understory Evapotranspiration  34  Weighing Lysimeter  System  3.3.1  Design and Construction  3.3.2  Lysimeter C a l i b r a t i o n  35 ••  35 44  - v Page 4.  RESULTS AND DISCUSSION  50  4.1  Measured Forest Floor Evaporation Rates  50  4.2  Measured Understory Evapotranspiration Rates  59  4.3  Forest Floor Evaporation Rates Following Salal Removal...  67  4.4  The Dependence of the Bulk Aerodynamic Resistance to Vapour Flux on Windspeed and Leaf Area Index  70  Applying the Penman-Monteith Equation to the S a l a l Canopy  74  The Relative Importance of Net Radiation and Vapour Vapour Pressure D e f i c i t i n Determining the Latent Heat Flux Density from the Salal Canopy  81  4.5  4.6  5.  CONCLUSIONS  86  6.  REFERENCES  92  APPENDIX APPENDIX  I  S o i l Description  II Neutron Probe C a l i b r a t i o n  APPENDIX III  Theory of Counteracting Spring Balance  96 99 101  - vi LIST OF FIGURES  Figure 1.1  1.2  1.3  1.4  1.5  1.6  1.7  Page S o i l water retention (desorption) curves for the s o i l at the Dunsmuir Creek s i t e (1 MPa = 10 bars) V 10 cm, 0 40 cm, • 60 cm  22  Wet end s o i l water retention (desorption) curves for the s o i l at the Dunsmuir Creek s i t e (1 MPa = 10 bars) V 10 cm, 0 40 cm, • 60 cm  23  Cross-section through the lysimeter cylinder and the suspension tray; A = stainless s t e e l straps, B = stainless steel cylinder, C = stainless s t e e l pan with short side walls, D = stainless steel plate used to sever the bottom of the isolated s o i l column.  33  Side view of the lysimeter and weighing mechanism, showing the location of the main components; A = LVDT, B = top spring, C = suspension I beam, D = dash pot, E = support post, F = turnbuckle, G = lysimeter cylinder, H = steel retaining wall, I = sloping concrete floor and drain, J = concrete, K = 7.6 cm s t e e l channel guides, L = steel plates  36  Side view of the fulcrum assembly; A = 5 cm s t e e l angle iron, B = s t e e l web welded perpendicular to the suspension beam, C =2.54 cm thick soft steel top pivot block, D = hard tool steel b i t , 5 cm long, E = hard tool s t e e l b i t , 5 cm long, F = 2.54 cm thick soft s t e e l bottom pivot block, G = web of a short segment of I-beam, H = bottom plate of the suspension I beam, I = support post  38  End view of the fulcrum assembly; A = suspension I beam, B = top plate of the web welded across the suspension beam, C = 5 cm angle iron, D = s t e e l web, E = top and bottom pivot blocks containing the knife edge and bearing surface i n machined slots, F = short segment of I-beam with one plate removed, G = support platform, H = strengthening web, I = support post  39  End view of the spring attachment; A = 3.8 cm angle iron, B = 7.6 cm s t e e l channel guide; C = stainless s t e e l top spring, D = 3.8 cm angle iron positioned to r e s t r i c t v e r t i c a l movement i n the suspension beam, E = 3.8 cm angle iron, F = suspension beam, G = 1.27 cm threaded rod, H = 3.8 cm angle i r o n , I = stainless s t e e l bottom spring, J = 3.8 cm angle Iron  42  - vii -  Figure 1.8  Page Side view of the LVDT mounting arrangement and the position of the c a l i b r a t i o n micrometer; A = LVDT probe, B = 1.27 cm threaded rod, C = micrometer plunger  43  C a l i b r a t i o n data and equation r e l a t i n g the LVDT output to changes i n the mass of the lysimeter  45  Average apparent change i n lysimeter mass due to d i f f e r e n t i a l heating and expansion of the suspension beam  47  Apparent change i n lysimeter mass after completely shading the entire weighing mechanism  49  Diurnal v a r i a b i l i t y of the below canopy available energy flux density (A^) for the period August 8-15, 1984  51  Solar irradiance measured above the tree canopy (S ( )) and the available energy flux density below the tree canopy (A^( )) for August 8, 1984....  52  1.14  Same as for Figure 1.13 except for August 10, 1984  53  1.15  Daily t o t a l solar irradiance measured above the tree canopy ( S ( )) and daily average vapour pressure d e f i c i t measured below the canoy (D( )) for the period August 8-15, 1984  54  Daily below canopy available energy flux density expressed as equivalent water evaporated (A^/L ( )) and daily forest s o i l evaporation (E ( )) for the period August 8-15, 1984  55  Forest f l o o r d i f f u s i v e resistance ( r ) calculated from the rearranged Penman-Monteith equation, for the period August 8-15, 1984  58  Courses of available energy flux density measured above the tree canopy (A ( )) vapour pressure d e f i c i t and available energy flux density measured below the tree canopy (D( ) and Ab( ) ) , and the measured latent heat flux density from the understory (LE ( )) for June 26, 1985  62  1.9  1.10  1.11 1.12  1.13  t  t  1.16  s  1.17  1.18  c o  a  - viii  -  Figure 1.19  Page Cumulative understory evapotranspiration June 26, 1985  for 64  1.20  Same 'as for Figure 1.18  except for July 5, 1985  66  1.21  Bulk aerodynamic resistance to vapour transport (r^) from a wet s a l a l canopy to a height of 1.0 m calculated from equation (12) plotted against the mean windspeed (u) measured 0.3 m above the s a l a l canopy. Values of r ^ calculated from data collected with the lysimeter LAI=1 (o) and LAI=2 (A). The curve through the data represents the function r^ (s m ) = 25.2 u " , (u i n m s ) , and two lower curves represent the function -1  r = [F 184(0.035/u)°* ]/ 2 LAI (u i n m s ) , where the shelter factor equals 1.0 for LAI=1 ( ) and 1.5 for LAI =2 ( )  72  The natural logarithm of the bulk aerodynamic resistance to vapour transport (r^) calculated from equation (12) plotted against the natural logarithm of the mean windspeed (u)....  73  5  1.22  AII.l  the  - 1  - 1  Neutron probe count ratio ( c / c ) vs v o l . s o i l water content (9 ) at the Dunsmuir Creek s i t e . The equation of the repression line i s c/c = 0.348 + 4.619 9 ; r = 0.88, S = 0.11 or 6 = 0.217 c/c - 0.075 100 e v s s  V  2  s  V  - ix LIST OF TABLES  Table 1.1  1.2  1.3  1.4  1.5  Page Bulk density of the fine (< 2 mm) s o i l f r a c t i o n i n the cleared and uncleared plots at the Dunsmuir Creek site  17  S o i l texture analysis (%) for cleared and uncleared plots at the Dunsmuir Creek s i t e . The samples were taken from the s o i l excavated for the bulk density measurements l i s t e d i n Table 1.1  19  S o i l texture analysis (%) of cores used to obtain water retention curves for cleared and uncleared plots at the Dunsmuir Creek s i t e . The values are averages for two cores, except for the 60 cm depth value which was for one core. The cores were taken 1.5 m from the samples i n Table 1.2  20  Average stomatal resistance of the abaxial surface of s a l a l , growing i n and around the lysimeter, measured on 5 days during which the understory evapotranspiration was being measured by the weighing lysimeter  60  Daily understory evapotranspiration ( E ) for days when diurnal measurements were made; t o t a l p r e c i p i t a t i o n (P) on the lysimeter; daily available energy flux density measured above (A ) and below (Afc) the tree canopy; and daily average vapour pressure d e f i c i t (D) measured below the tree canopy  65  Daily average temperature (T) measured below the tree canopy; daily understory equilibrium evapotranspiration rate (E q); and the r a t i o of measured daily understory evapotranspiration to the daily understory equilibrium evapotranspiration rate (E^/E^)  68  Daily forest s o i l evaporation ( E ) following s a l a l removal, daily available energy flux density measured above (A ) and below (A^) the tree canopy, and daily average vapour pressure d e f i c i t (D) measured below the tree canopy  69  Hourly measured understory evapotranspiration ( E ) and calculated s a l a l t r a n s p i r a t i o n (E') for June 22 and 25, 1985  76  t  a  1.6  e  1.7  s  a  1.8  t  - x Table 1.9  1.10  Page The effect of a l t e r i n g the assumption about the amount of forest s o i l evaporation that occurred, while the s a l a l canopy was present, on the c a l c u l a t i o n of s a l a l transpiration  77  Hourly measured canopy resistances and computed c r i t i c a l canopy resistances for June 22 and 25, 1985  80  - xi -  NOTATION  A  available energy above the tree canopy ( i . e . , Rn-G)  (W m  )  a available energy below the tree canopy (W m C  volumetric  D  atmospheric vapour pressure  D e q  heat capacity of s o i l (J m~ deficit  °C  o  )  )  (kPa)  equilibrium atmospheric vapour pressure defined by equation (28)  deficit  (kPa)  atmospheric vapour pressure  deficit  just above layer i  D^_^  atmospheric vapour pressure  deficit  just above layer i-1  D  atmospheric vapour pressure  d e f i c i t at a reference  m  (kPa) (kPa)  height  within the well mixed portion of the planetary boundary layer (kPa) D  atmospheric vapour pressure d e f i c i t within the canopy at a level considered to be an e f f e c t i v e source height (kPa)  E^  transpiration rate of dry foliage (mm  E ^  equilibrium evapotranspiration  E'  calculated transpiration rate from the understory s a l a l period )  g  period )  rate (mm  -1  period ) -1  (mm  -1  E  g  E  E  forest s o i l evaporation  rate (mm  period ) -1  t o t a l understory evapotranspiration ( i . e . , vegetation transpiration + s o i l evaporation) (mm period ) w  F  evaporation  rate of intercepted water (mm  force of the bottom spring  period ) -1  (N)  DO  F  cw  force of the counterweights  (N)  downward force of the lysimeter (N) (mass of the lysimeter multiplied by the acceleration of gravity)  - xii F  shade factor  s  F  force of the top spring (N) J. O  G  s o i l heat flux density  (W m  )  G5  s o i l heat flux density  at the 5 cm depth (W m  Go  s o i l heat flux density  at the surface (W m  L  latent heat of vaporization  LA  leaf area (m )  LAI  measured leaf area of a small subsample (m )  LAI  leaf area index on a one-sided basis  )  )  of water (J k g ) - 1  2 2 2  (m  2  m  ) 9  LAI  i  leaf area index on a one-sided basis for layer i (m* m  2  )  LE  latent heat flux density  (W  LE^ ^  latent heat flux density  above layer i-1 (W m" )  LE!^  calculated  M  rate of change of s o i l heat storage In the s o i l above the heat flux plates (W m )  m ) -2  latent heat flux originating from layer i (W m  -2  ML  mass of leaves (g)  MT  mass of twigs (g)  M2  mass of larger foliage subsample (g)  P  p r e c i p i t a t i o n (mm p e r i o d )  Rn  net radiation flux density  (W m  net radiation flux density  above layer i (W m  Rn  - 1  i  "  -  2  )  —2  ) 2  Rn^ ^  net radiation flux density  S  solar irradiance  (W m  -  )  above layer i-1 (W m  -  )  )  - xiii a i r temperature (°C) a i r temperature at the beginning of a lysimeter measurement period (°C) LVDT output voltage when the lysimeter i s balanced (mV) LVDT output voltage (mV) depth of water i n the lysimeter  (mm)  water content of the fine s o i l f r a c t i o n ( i . e . , < 2 mm fraction) kg k g ) - 1  depth of water i n the lysimeter at the beginning of a measurement period (mm) depth of s o i l (m) 30 second count while the neutron probe i s positioned at a depth i n the s o i l s p e c i f i c heat of moist a i r (J k g  - 1  °C  - 1  )  30 second count while the neutron probe i s contained within the shield ( i . e . , standard counts) average s a l a l leaf diameter (m) distance between the point of attachment of the counterweights to the suspension beam and the fulcrum (m) distance between the point of attachment of the lysimeter to the suspension beam and the fulcrum (m) distance between the point of contact of the LVDT probe on the suspension beam and the fulcrum (m) distance between the point of attachment suspension beam and the fulcrum (m)  of the springs to the  volumetric stone fraction on a whole s o i l basis spring constant (g cm *) -  lysimeter c a l i b r a t i o n factor (mm mV *) -  ( i . e . , reciprocal of s e n s i t i v i t y  - xiv the temperature s e n s i t i v i t y of the modulus of r i g i d i t y  (°C-1)  radius of the lysimeter (m) bulk aerodynamic resistance to vapour transport from the s a l a l canopy to the meteorological sensors (s m - *) bulk aerodynamic resistance to vapour transport from layer i to the meteorological sensor height (s m - *) eddy d i f f u s i v e resistance above layer i (s m - )  to sensible and latent heat  flux  eddy d i f f u s i v e resistance to vapour transport from the surface to a reference height i n the w e l l mixed p o r t i o n of the planetary boundary-layer (s m - 1 ) boundary-layer resistance (s m" 1 )  to sensible and l a t e n t heat  flux  boundary-layer resistance to sensible and latent heat on a one-sided leaf area basis for layer i (s m - * )  flux  mean canopy resistance  to latent heat f l u x (s  mean canopy resistance  to latent heat f l u x for layer i (s  forest f l o o r d i f f u s i v e resistance c r i t i c a l canopy resistance  m - *)  to latent heat f l u x (s  m-1) m-1)  (s m - *)  leaf boundary-layer resistance side (s m - 1 )  to sensible heat f l u x on one  average canopy leaf boundary layer resistance f l u x (s m - *) leaf boundary-layer resistance side for layer I (s m - *)  to sensible  heat  to sensible heat f l u x on one  stomatal resistance to latent heat flux of one side of a hypostomatous leaf (s m - *) stomatal resistance i (s m _ 1 )  to latent heat f l u x of one side for layer  - XV  r V  r v a  r ^ V  -  boundary-layer resistance to latent heat flux of one side of a leaf (s m ) -1  average canopy boundary-layer resistance to latent heat f l u x (s m" ) boundary-layer resistance to latent heat flux of one side of a leaf for layer i (s m ) -1  s  slope of the saturated vapour pressure curve (kPa ° C  t u  time (s) mean windspeed (s m )  Xg  elongation of the bottom spring (cm)  ^VDT  displacement of the LVDT probe (cm)  X  elongation of the top and bottom springs (cm)  _ 1  )  -1  g  X^,  elongation of the top spring (cm)  y  psychrometric constant (kPa ° C  6^  term i n equation (6) to account for the net radiation and latent heat flux densities below layer i i n determining the latent heat flux from layer i (kPa) (defined by equation (7))  _ 1  )  q  9  v  whole s o i l volumetric water content (m  q  m~ )  p  density of moist a i r (kg m  p.  bulk density of the fine s o i l (< 2 mm) f r a c t i o n (kg m )  P  density of l i q u i d water (kg m  rs  Q  w  -  ) J  —3  )  weighting factor defined by equation (30)  q  - xvi ACKNOWLEDGEMENTS I am very thankful for the tremendous support and encouragement I received from my family and friends, most notably Marion Osberg, Peter Chapman, Lyle Osberg, Candace Morgan, Ian MacDonald, and John Robertson. I wish to express my sincere appreciation to Dr. T.A. Black, my academic supervisor, for unreservedly sharing his wealth of experience and knowledge with me.  I am also very grateful for his continuous  f i n a n c i a l support throughout the duration of my graduate studies. I wish to thank the other members of my academic Dr. T.M.  Ballard, Dr. M.D.  committee,  Novak, and Dr. D.L. Spittlehouse, for their  assistance i n problem d e f i n i t i o n and suggestions on sampling procedures and a n a l y t i c a l approaches. Don Giles and Dave Price deserve a special note of thanks for their s i g n i f i c a n t assistance i n conducting the f i e l d work, and for their much appreciated moral support when i t looked as though my iron a r t i s t r y would never swing. My a b i l i t y to conduct this research was greatly enhanced by the daily i n t e l l e c t u a l stimulation and friendship I received from Nigel Livingston, Bob Stathers, Doug Beames, and Dr. Frank K e l l i h e r . I wish to thank B.C. Forest Products Ltd. for their f i n a n c i a l support through the B.C.F.P. Graduate Student Fellowship i n S o i l Science. The f i n a n c i a l and material support, for this research, received from, MacMillan Bloedel Ltd., Woodland Services Division, the B.C. Ministry of Forests, the Natural Science and Engineering Research Council, and E l l e t t Copper and Brass Co. Ltd., was greatly appreciated.  - 1 -  1.  INTRODUCTION  The depletion of the old growth coniferous forests of B r i t i s h Columbia  and their subsequent  replacement with managed plantations has  shifted the r e s p o n s i b i l i t i e s of foresters from those associated with engineering and harvesting to the more b i o l o g i c a l problems of growing the next crop.  The old growth stands currently being harvested from  coastal forests contain very large timber volumes, per unit of land area, because the biomass has been accumulating for a long time. Managed plantations w i l l be harvested at an e a r l i e r stage of stand development and consequently the accumulated  timber volume at the time  of the next harvest w i l l be substantially reduced.  This reduction i n  timber volume per hectare w i l l create timber supply shortages i n some forest d i s t r i c t s i n the next ten to twenty years, assuming that the forest w i l l be managed to provide a balanced d i s t r i b u t i o n of maturing age classes (Anon 1980).  Private forest firms have already experienced  severe reductions i n the supply of certain timber species and  grades  which has resulted i n an i n a b i l i t y to maintain m i l l i n g operations at a profitable l e v e l .  Forest managers, i n an attempt to minimize current  and projected timber supply shortages, have recommended that s i l v i c u l t u r a l a c t i v i t i e s be Intensified and that the existing accumulated  biomass be more completely u t i l i z e d (Ainscough 1981).  The objective of intensive s i l v i c u l t u r e i s to grow the maximum volume, of the desired species, on the least number of stems, i n the  - 2 -  shortest period of time possible. operations  The success of s i l v i c u l t u r a l  i n improving stand productivity depends on their effect on  the s i t e energy regime, nutrient a v a i l a b i l i t y , moisture regime, and s o i l physical properties.  Controlling stand density, through precommercial  thinning or juvenile spacing and commercial thinning operations,  often  results i n increased growth rates f o r the remaining crop trees because the competition  between trees for available energy, nutrients, and water  is reduced (Daniel et a l . 1979; Smith 1962). Thinning  stands s h i f t s the growth capacity of the s i t e to fewer  trees which w i l l therefore reach harvestable time.  size i n a shorter period of  The possible benefits of reduced rotation age, reduced harvesting  costs, larger i n d i v i d u a l piece size, higher quality timber, better species mix, and opportunities f o r small yields midway through the rotation, have generated a lot of interest i n thinning second growth stands (Fries and Hagner 1970). Another strong incentive for forest firms considering investments in thinning juvenile stands i s i n Section 52 of the B r i t i s h Columbia Forest Act. This section provides allowable  for increases i n the current annual  cut i n response to increases i n forest productivity  achieved  through the application of more intensive s i l v i c u l t u r e (Forest Act 1979).  The immediate increase in allowable  investment does not have to be discounted  cut means that the return on  over long periods of time,  unlike investment i n planting, which i s considered  to be basic  s i l v i c u l t u r e , and i s not e l i g i b l e for Section 52 consideration. Thinning  i s an expensive s i l v i c u l t u r a l operation and the expense  can become prohibitive i f the stand must be entered  frequently.  - 3 -  Consequently,  the tendency has been for s i l v i c u l t u r i s t s  to prescribe a  juvenile spacing early i n the rotation followed by a single commercial thinning midway through the rotation, leaving just the number of stems desired at the culmination of the r o t a t i o n .  This sequence of  treatments  w i l l not decrease the f i n a l harvestable timber volume i f the residual stand density i s s u f f i c i e n t the s i t e .  to f u l l y u t i l i z e a l l available resources on  If the stand i s thinned to a density below that which  represents f u l l site occupancy then some of the productive capacity of the s i t e may  be shifted to understory shrubs, and the f i n a l harvestable  timber volume w i l l be  reduced.  Jackson et a l . (1983) i n an investigation of the causes of the poor growth response of Pinus radiata D. Don,  planted on deep sand, found  that thinning greatly reduced depletion of available s o i l water, but only for a period of two to three years, by which time f u l l canopy closure had been attained.  The application of f e r t i l i z e r to the stand  at the time of thinning increased volume production sixty to seventy percent over that of the control stand. thinning a young Douglas-fir (Pseudotsuga plantation, growing on a droughty  Black et a l . (1980) found that menziesii (Mirb.) Franco)  s i t e but not displaying any signs of  nutrient deficiency, resulted i n a poor growth response by the remaining crop trees.  S a l a l (Gaultheria shallon Pursh.), growing as an understory  shrub i n the thinned stand, was considered to be competing for the limited amount of available s o i l water.  Utilizing  a procedure which  combined water balance measurements of stand evapotranspiration with a simple vapour d i f f u s i o n model, which required periodic stomatal  - 4 -  conductance measurements for both the trees and the s a l a l , they estimated that as the f r a c t i o n of extractable s o i l water decreased  from  0.8 to 0.2 the f r a c t i o n of t o t a l transpiration originating from the s a l a l understory, i n the thinned stand, increased from forty percent to s i x t y - f i v e percent.  The transpiration rate of trees i n the thinned  stand was  found to be very similar to that of trees in a nearby  unthinned  stand.  Roberts et^ a l . (1980) measured transpiration from a 50 year old Scots pine (Pinus s y l v e s t r i s L.) plantation, with an understory of bracken fern (Pteridium aquilinum L . ) , and found that, for the summer period from June to July, the f r a c t i o n of t o t a l stand transpiration originating from the bracken fern increased from twenty-one percent to f i f t y - s e v e n percent. In a study comparing the transpiration from a stand of Scots pine with bracken fern i n the understory and a denser stand of Corsican pine (Pinus nigra var. maritima) Roberts et a l . (1982) found that the t o t a l stand transpiration was very similar but that the transpiration from the Corsican pine trees was  28% greater than from the Scots pine trees.  Further investigations by Roberts et_ a l . (1984) showed that the stomatal resistance of the bracken fern understory was of  less sensitive than that  the Scots pine trees to increases in vapour pressure d e f i c i t s , and  that this resulted i n an increase in the contribution, by bracken fern, to the t o t a l stand transpiration. Tan et a l . (1977) investigated the relationship of the stomatal resistance to vapour d i f f u s i o n exerted by Douglas-fir and s a l a l to s o i l water potential and atmospheric  vapour pressure d e f i c i t .  They showed  - 5 -  that, as the s o i l water potential decreased, the stomatal resistance of Douglas-fir increased more rapidly than did that of the s a l a l , i n response to increased vapour pressure Penman-Monteith equation,  deficits.  Using the  i n a growing season water balance model, to  p a r t i t i o n the t o t a l stand evapotranspiration  between the Douglas-fir  s a l a l canopies, Spittlehouse and Black (1982) estimated that  salal  transpired 33% of the available s o i l water over the growing season. stomatal resistance c h a r a c t e r i s t i c s for the two as functions of vapour pressure solar irradiance. d i f f u s i o n was  The  and  The  canopies were determined  d e f i c i t , root zone water p o t e n t i a l , and  leaf boundary-layer resistance to vapour  calculated using an equation for f l a t plates i n p a r a l l e l  turbulent flow.  Black and Spittlehouse  (1981) suggested that thinning  stands which experience periodic growing season water d e f i c i t s may water use from the overstory  shift  trees to the understory shrubs.  Stewart (1984) presented calculations which demonstrated that the transpiration from t a l l vegetation  is nearly twice as sensitive to  increases i n stomatal resistance as that from short vegetation.  He  suggested that, for short vegetation, a reduction in transpiration by stomatal closure w i l l result i n an increase i n leaf temperature, due the large aerodynamic resistance to sensible heat flux, consequently, the vapour pressure  and  gradient, from the substomatal  cavities of leaves to the a i r surrounding the leaves, o f f s e t t i n g the i n i t i a l reduction i n t r a n s p i r a t i o n . suggested that the turbulence  to  increases,  Stewart (1984) also  generated by t a l l , rough, forest canopies  r e s u l t s in a high rate of exchange between the forest and  the  - 6 -  atmosphere, and that as a result of this e f f e c t i v e mixing, understory vegetation i s l i k e l y to be exposed to atmospheric vapour pressure d e f i c i t s of similar magnitude to those that develop at the tree crown l e v e l , and w i l l therefore make a s i g n i f i c a n t contribution to the t o t a l stand transpiration. McNaughton and Jarvis (1983) considered the question of transpiration from understory vegetation and suggested that a dense tree canopy may  decouple the a i r above the understory vegetation from the  r e l a t i v e l y dry a i r above the tree canopy.  They suggested that this  could result i n the establishment of an equilibrium vapour pressure d e f i c i t , which would be determined  by the understory canopy resistance  to vapour d i f f u s i o n and the available energy flux density below the upper canopy; i n which case, the transpiration from the understory vegetation would tend to approach the equilibrium transpiration rate and would be determined  by the available  energy.  Zahner (1958) found that removing the hardwood understory from l o b l o l l y (Pinus taeda L.) and shortleaf (Pinus echinata M i l l . ) pine stands, growing on sandy, upland s o i l s , with a root zone depth of 2 metres, resulted i n a reduced rate of s o i l water depletion during the early part of the growing season and an increase i n the root zone volumetric water content of 0.04  m  3  m~ , 3  over stands where the  understory vegetation had not been removed. K e l l i h e r (1985) examined the effect of removing the s a l a l understory from a heavily thinned, 31-year-old Douglas-fir stand, on the root zone water balance.  The treatment was  confined to pairs of trees  - 7 -  with root zones isolated by trenching down to bedrock. the differences i n the root zone water content, season, were small.  He found that  throughout the growing  He used small lysimeters and porometer measurements  in combination with a vapour d i f f u s i o n model to estimate  the  contributions to t o t a l evapotranspiration by the Douglas-fir canopy, s a l a l understory,  and the forest f l o o r .  transpiration to be 0.5  to 1.0 mm  d  - 1  He estimated  the s a l a l  greater than the forest f l o o r  evaporation where s a l a l had been removed.  For modelling  the s a l a l  transpiration he used a value for the leaf boundary-layer resistance which was  determined from the relationship between leaf boundary-layer  resistance and windspeed developed by Spittlehouse and Black (1982), for a r t i f i c i a l s a l a l leaves, and he included a shelter factor of 2, following Thorn (1971), to take into consideration the effect of leaf overlap on the boundary-layer resistance to vapour d i f f u s i o n . Price et^ a l . (1985) measured the stomatal conductance and photosynthetic understory  rate of Douglas-fir trees growing on plots with the s a l a l  removed and present.  They found that removing s a l a l  increased both the stomatal conductance and the photosynthetic  capacity  of the Douglas-fir leaves.  They suggested that, while removing the  s a l a l resulted i n only very  small differences i n the root zone water  content, these small differences resulted i n s i g n i f i c a n t l y higher  soil  water potentials due to the steepness of the s o i l water retention curve, for  sandy loam s o i l , at low water  contents.  The physical p r i n c i p l e s underlying the micrometeorological of estimating evapotranspiration from vegetated  methods  surfaces have been  - 8 -  reviewed by Tanner (1968), Thorn (1975), and Garratt (1984).  The  combination methods, which are a l l based on the Penman-Monteith equation, are generally considered to be the most useful transpiration models currently available because they combine the physics and physiology of evaporation from vegetation with the transfer of water vapour i n the atmosphere (Jarvis et a l . 1981;  Stewart 1983).  The  combination method has been shown to be very sensitive to the values input for stomatal resistance, which for extensive vegetated surfaces must be considered as a mean canopy resistance, and this parameter has been found to be extremely variable, especially for tree canopies which present the added complication of requiring an estimate of the e f f e c t i v e leaf area index (Leverenz et^ a l . 1982; Milne et_ a l . 1985; Aston  1984)  actually contributing to transpiration. This i s not as serious a problem for s a l a l because Tan et_ a l . (1977) have shown that the stomatal resistance of s a l a l i s not as sensitive to changes i n vapour pressure d e f i c i t as i s Douglas-fir and this tends to confine the range of any v a r i a b i l i t y i n stomatal resistance during the day.  Furthermore,  Milne et a l . (1985) pointed out  that some of the v a r i a b i l i t y may have been due to the inherent d i f f i c u l t i e s i n measuring the stomatal resistance of conifer needles, whereas, porometer measurements on s a l a l leaves are easy to conduct. Shuttleworth (1978; 1979) developed a one-dimensional model of evapotranspiration from p a r t i a l l y wet forest canopies by applying the Penman-Monteith equation to d i s t i n c t layers within the canopy. (1985) modified the Shuttleworth multilayer model for use i n  Kelliher  - 9 -  hypostomatous canopies and used i t i n combination with a root zone water balance model to examine the effects of s a l a l understory removal on Douglas-fir transpiration rates.  The canopy resistance functions were  modelled using relationships proposed by Tan et a l . (1978) and Spittlehouse and Black (1982), and the aerodynamic resistance to turbulent  transport was estimated, following Thorn (1975), by assuming an  exponential  eddy d i f f u s i v i t y p r o f i l e from the top of the trees to the  s a l a l canopy. While some researchers  have questioned the v a l i d i t y of using  flux-gradient relationships to model the turbulent  transport of vapour  within plant canopies (Bradley et_ al_. 1983; Denmead 1984; Raupach and Legg 1984), Shuttleworth and Wallace (1985) have recently presented a one-dimensional model for estimating  evapotranspiration  from sparse  crops and s o i l , which i s e s s e n t i a l l y based on gradient-diffusion models of sensible and latent heat fluxes. evapotranspiration  They suggested that  models that incorporate  simple resistances to vapour  flux, such as, stomatal resistance, laminar boundary-layer resistance, and eddy d i f f u s i v e resistance, and that treat the canopy l i k e a large leaf, are a p r a c t i c a l compromise between physical rigour and the demands of f i e l d  applicability.  In recognition of the p o s s i b i l i t y that the forest floor evaporation from small plots cleared of s a l a l may have been depressed by the advection  of humid a i r from the surrounding s a l a l canopy onto the  treated plots Black et^ al_. (1984; 1985) i n i t i a t e d an investigation of the effects of s a l a l removal from a much larger plot (30 x 40 m) on the  - 10 -  s o i l water regime, t r e e t r a n s p i r a t i o n , and t r e e growth, of a 2 0 - y e a r - o l d Douglas-fir  plantation.  The experiment d e s c r i b e d  conducted i n c o n j u n c t i o n partitioning  the t o t a l  transpiration,  salal  The o b j e c t i v e s 1.  i n this thesis  was  w i t h t h i s l a r g e r p r o j e c t , with the i n t e n t i o n of  stand e v a p o t r a n s p i r a t i o n  between  t r a n s p i r a t i o n , and f o r e s t s o i l  tree  evaporation.  of t h i s study were t o :  design a lysimeter  which c o u l d  be used to measure the  evapotranspiration  of an u n d i s t u r b e d column of f o r e s t s o i l  salal, 2.  measure the e v a p o r a t i o n r a t e of the f o r e s t s o i l  after  salal  removal, and 3.  determine the r e l a t i o n s h i p between resistance  the aerodynamic  to vapour f l u x and windspeed f o r a s a l a l  transfer canopy.  and  THEORY  2.  The Penman-Monteith combination equation for the latent heat f l u x from an extensive, isothermal canopy of hypostomatous leaves i s (Kelliher  1985)  s(Rn - G) + LE =  TcZ Ha  s + y r ' va  D /r, p o 1 (1 + r /r  pc  c  (1)  ) va  where s i s the slope of the saturated vapour pressure curve (kPa °C~ ), o  Rn i s the net radiation flux density (W m  ), G i s the s o i l heat flux  2 —3 density (W m~ ), p i s the density of moist a i r (kg m ), Cp i s the s p e c i f i c heat of moist a i r (J k g constant (kPa ° C ) , D - 1  Q  ° C ) , y i s the psychrometric  - 1  - 1  i s the vapour pressure d e f i c i t at the exchange  l e v e l within the canopy (kPa), r ^  a  and r  v  a  are the average  boundary-layer resistances of a l l canopy leaves acting i n p a r a l l e l for sensible and latent heat transfer respectively (Thorn 1972; Shuttleworth 1978;  1979)  (s m ), -1  that i s  = r / 2 LAI  (2)  = r 12 LAI v  (3)  H  and  r  va  where r ^  a n  d r  v  are the boundary-layer resistance to sensible heat  and water vapour, respectively, on one side of a l e a f , and r  c  i s the  - 12 -  canopy or surface resistance given by r  = r /LAI + r /2 LAI  C  where r  S  s  (4)  V  i s the stomatal resistance of the side of the leaf with  stomata. Assuming s i m i l a r i t y between the boundary-layer resistance to sensible heat and water vapour transfer, i . e . , rg = r , then (1) can v  be written as  LE =  s +  Y  (1 + r / r c  H a  (5)  )  To use (5) f o r estimating the latent heat flux density from a plant canopy requires that the vapour pressure  d e f i c i t , D , be measured  within the canopy at a position considered water vapour.  If the vapour pressure  Q  to be the source l e v e l for  d e f i c i t i s measured at a height  above the canopy then extra resistances to vapour and sensible heat transport must be added i n series with the boundary-layer resistances. These extra resistances are due to the eddy d i f f u s i v e resistance of the a i r layer between the transpiring canopy and the meteorological  sensors.  Shuttleworth (1978; 1979) and Black et a l . (1970) showed that when the Penman-Monteith equation i s used to estimate the latent heat flux density from one layer of a multilayered canopy using  meteorological  measurements made at a height above the canopy of interest then the latent heat flux density originating from below the layer of interest  - 13 -  must be taken i n t o c o n s i d e r a t i on.  Using Shuttleworth's  theory, K e l l i h e r  the l a t e n t  layer  (1985) shows that  L  s(Rn^  - G)  i  -  i  E  +  '  s +  pc^  (D  f l u x d e n s i t y from  6j)/rt  -  ±  P ^ i ' (1 + r /r ,) ^ c T ^ 7  Y  Ai <> 6  A 7  where LE^ i s the d i f f e r e n c e between LE^ and radiation deficit  6  above the  =  ±  Ai  ts(Rn _ i  - G)(r  1  of l a y e r i and  V<  =  R 1  /2  ±  2  +  Y  If  =  r  s i  /  simplifies  L  E  I  .,  i  =  "  i  +  r  v i  r  heat  /  1  r 1  zero and  a  to s e n s i b l e and  i  ]/p  (>  c  7  p  latent  heat  ai  <> 8  (  (4) i s  2  (  f l u x d e n s i t y below the canopy of i n t e r e s t  L E ^ - i i s zero or very n e a r l y zero then  approximately  K  A  the s o i l  to Rn^-i and  T  L  LE _  i s d e f i n e d by  c  ci  the vapour p r e s s u r e  L A I ) ) + (s + )  r ^ i s the canopy r e s i s t a n c e , which from  r  i s the net  layer,  i s the bulk areodynamic r e s i s t a n c e  r  LE^_^, Rn^  f l u x d e n s i t y above the l a y e r ^ i s  transport  and  1979)  i can be w r i t t e n as  LE'  r^£  heat  (1978;  can be i g n o r e d .  9  )  i s equal  6i i s  Under these c o n d i t i o n s (6)  to  s(Rn. - G) + pc D./r.. i * pP . i A i 1  S  +  1  Y  (1  A  + r c iJ' r A , i ') A  1  (10) ( 1 0 )  - 14 -  When the canopy i s completely wet the canopy resistance becomes zero and (10) simplifies to  s(Rn LE' i  IT  - G) + pc D i  p  1  A  (11)  i  s+ y  Equation (11) can be rearranged to calculate the bulk aerodynamic resistance to water vapour transport as follows  pc ^  D.  LE^ (s + ) " s(Rn - G)  K  Y  '  Calder et a l . (1984) used this technique i n studies of r a i n f a l l interception losses from upland heather (Calluna vulgaris L. ). demonstrated that r ^ determined  They  from measurements made during  r a i n f a l l events were not s i g n i f i c a n t l y different from estimates obtained by a r t i f i c i a l l y wetting the foliage with plant sprayers.  - 15 -  3.  3.1  METHODS  Site Description  3.1.1  Experimental S i t e Location  The f i e l d experiment was conducted i n a Douglas-fir stand located in the Dunsmuir Creek watershed, approximately 30 km southwest of Nanaimo, B.C., and 5 km south of Mt. Hooker (40° 02' N, 124° 12' W). The s i t e i s located at an elevation of 450 m on a 2-5° slope with a northeasterly  aspect.  The B.C. Ministry of Forests' map of the biogeoclimatic units f o r the Nootka-Nanaimo area indicates that the Dunsmuir Creek watershed l i e s in the t r a n s i t i o n zone between the Nanaimo and Georgia Wetter Maritime Coastal Douglas-fir Variant, and the East Vancouver Island Drier Maritime Coastal Western Hemlock Variant (Anon 1979).  3.1.2 The  Stand Structure  stand was planted to Douglas-fir i n 1963, but substantial  natural regeneration has occurred so that the stand density varies considerably and i s as high as 5,000 stems per hectare i n places.  The  major conifer species i n the stand are Douglas-fir and western hemlock (Tsuga heterophylla (Raf.) Sarg.).  There i s also a minor component of  western red cedar (Thuja p l i c a t a Donn), western white pine  (Pinus  monticola  Dougl.), and lodgepole pine (Pinus contorta var. l a t i f o l i a  Dougl.).  The estimated  average height of the trees was 7-8 m, with the  exception of the western white pines which were much t a l l e r .  - 16 -  Red alder (Alnus rubra Bong.) and P a c i f i c willow (Salix lasiandra Benth.) were present around depressional areas with r e s t r i c t e d drainage, which remained moist even after extended  periods of warm dry weather.  The understory vegetation was predominantly  s a l a l , with bracken  fern present on moister s i t e s , and a small amount of d u l l Oregon-grape (Mahonia nervosa (Pursh) Nutt.) interspersed with the s a l a l . dominant moss growing i n the surrounding mature forest was (Hylocomium splendens  The  step moss  (Hedw.) B.S.G.).  Several large openings  i n the stand were found to be due to group  dying of trees infected by the root rot pathogen A m i l i a r i a mellea. Most of the western white pines displayed the symptoms of white pine b l i s t e r rust (Cronartium r i b i c o l a J.C. Fischer ex Rabh.) i n f e c t i o n and western g a l l rust (Endocronartium harknessi J.P. Moore) was  observed  on  several of the larger lodepole pines.  3.1.3  S o i l Description  On the basis of a f i e l d examination, by Mr. Paul Sanborn (Dept. of Soil Science, Univ. of B.C.), the s o i l was c l a s s i f i e d as a Duric Dystric Brunisol (Appendix I ) . The fine bulk density (Table 1.1) was determined by removing a l l the s o i l from holes of approximately 1 l i t e r i n volume extending from the 5 to 30 cm and 35 to 60 cm depths respectively. was  determined  The exact volume  by l i n i n g the hole with a p l a s t i c bag and f i l l i n g i t with  a measured volume of water.  The volume of fine s o i l p a r t i c l e s  was  obtained by subtracting the volume of the coarse fragments (> 2 mm  - 17 -  Table 1.1  Bulk density of the fine (< 2 mm) s o i l f r a c t i o n i n the the cleared and uncleared plots at the Dunsmuir Creek s i t e  Depth (cm)  Fine Bulk Density (kg m" ) 3  Uncleared Plot 15-25 40-50 60-70  770 950 890  Cleared Plot 10-20 40-50 70-80  880 860 1,150  - 18 -  diameter) from the measured hole volume. F i f t e e n s o i l samples were collected at several depths from 2 s o i l p i t s and sieved to remove the greater than 2 mm diameter f r a c t i o n .  The  hydrometer method was used to determine the percent sand, s i l t , and clay size p a r t i c l e s i n each sample.  The coarse fragment content i n a l l  samples was greater than 20% and therefore the s o i l was classed as a gravelly sandy loam to loamy sand (Tables 1.2 and 1.3). The volumetric stone fraction was determined by removing a l l of the material from a p i t measuring 0.5 m x 0.5 m x 0.7 m deep. the mm  p i t was calculated by determining the dry mass of the greater than 2 diameter and less than 2 mm diameter f r a c t i o n s ; dividing the former  by the measured density of the stones (2,520 kg m~ ) 3  l a t t e r by the average fine bulk density (900 kg m the  The volume of  two quotients.  -  and dividing the ), and then summing  The volumetric stone f r a c t i o n was found to be  approximately f i f t y - t h r e e percent. The s o i l water retention c h a r a c t e r i s t i c s of the s o i l were determined from desorption measurements made on undisturbed s o i l cores, obtained with 5.4  cm I.D. x 3 cm long brass cylinders.  collected from two s o i l p i t s .  Two  S o i l cores were  cores were taken from each of the 10  and 40 cm depths and one was taken from the 60 cm depth.  A l l desorption  measurements were made using a pressure membrane apparatus Moisture Equipment Corp., Santa Barbara, C a l i f . , USA).  (Soil  After each  e q u i l i b r a t i o n , the sample was weighed and then resaturated for 24 hours before applying the next pressure step. were averaged to give 3 retention curves.  Samples from a single depth  - 19 -  Table 1.2  S o i l texture analysis (%) for cleared and uncleared plots at the Dunsmuir Creek s i t e . The samples were taken from the s o i l excavated for the bulk density measurements l i s t e d i n Table 1.1  Depth (cm)  Sand  Silt  Clay  Texture  66.2 75.6  29.6 21.7  4.2 2.7  SL LS  69.9 72.8 73.4  26.0 23.9 25.4  4.1 3.3 1.2  SL SL/LS LS  Uncleared Plot 10 40 Cleared Plot 10 40 60  - 20 -  Table 1.3  S o i l texture analysisi (%) of cores used to obtain water retention curves for cleared and uncleared plots at the Dunsmuir Creek s i t e . The values are averages for two cores, except for the 60 cm depth value which was for one core. The cores were taken 1.5 m from the samples in Table 1 .2  De pth ( cm)  Sand  Silt  Clay  Texture  53.4 66.0 70.2  40.0 29.5 26.6  6.6 4.5 3.2  SL SL SL  71.8 76.0 64.0  25.0 21.4 32.6  3.2 2. 6 3.4  SL LS SL  Uncleared Plot 15 40 60 Cleared Plot 10 40 70  - 21 -  The volumetric water content for each sample was corrected by subtracting the volume of the coarse fragments the volume of the s o i l core. these samples ranged  (> 2 mm diameter) from  The bulk density of the fine f r a c t i o n i n  from 810 to 990 kg m~  3  (Table 1.3).  The s o i l water retention curves are shown i n Figures 1.1 and 1.2. Figure 1.1 shows the entire 0 to -1.5 MPa curves, while Figure 1.2 shows the wet end curves for the range 0 to -0.1 MPa.  These curves are  t y p i c a l of a coarse textured s o i l .  3.1.4  Experimental Plot Layout and S a l a l Removal  In 1983 a meteorological tower was erected i n approximately the center of the stand.  Wind d i r e c t i o n data, gathered in 1983, showed that  the p r e v a i l i n g daytime winds were from the east on fine days.  Changes  i n wind d i r e c t i o n generally moved through the south with westerlies accompanying storm fronts. topography measuring  Two areas within the stand, similar i n  and stand density, were selected for plot l o c t i o n s .  Plots  30 x 40 metres were then marked out i n these areas, with the  40 metre dimension oriented along the east-west axis. cleared of s a l a l was located to the northwest  The plot to be  of the meteorological  tower so that a i r passing over i t would not be picked up by sensors on the tower.  The untreated plot was located to the east of the tower so  that water balance measurements made on the untreated plot could be related to energy balance measurements made over the stand.  - 22 -  0 0.00  Figure 1.1 Figure  0.05  V  (m m" )  0.10  3  0.15  3  0.20 r  0.25  S o i l water retention (desorption)curves f o r the s o i l at ^ s i t e (1 MPa - 10 bars) V 10 cm, 0 40 cm, D  u  n  8  m  u  • 60 cm.  l  r  C  r  e  e  k  Figure 1.2  Wet end s o i l water retention (desorption) curves for the s o i l at the Dunsmuir Creek s i t e (1 MPa » 10 bars) & 10 era, 0 40 cm, O 60 cm.  - 24 -  In A p r i l 1984 one metre quadrat samples of the understory foliage were collected from ten locations within the plot to be cleared and from s i x locations around the outside edge of the untreated plot. Immediately  following the foliage sampling a l l of the understory  vegetation within the treatment plot was manually destroyed with sandvik clearing axes and c l i p p e r s .  New  foliage was observed to be sprouting  from s a l a l stolons by the middle of July.  At the end of the growing  season, October 1984, four, one metre quadrat samples of foliage were collected from the treated plot to obtain an estimate of the regrowth i n leaf area.  In the untreated plots, two, one metre quadrat measurements  of foliage were made to estimate the increase i n leaf area during the growing season.  In A p r i l 1985,  the herbicide Garlon 4a (Dow Chemicals),  was applied to the regrowth i n the cleared plot.  No s i g n i f i c a n t  regrowth during 1985 was observed after this application.  3.2  Measurements  3.2.1  S a l a l Leaf Area Index  The leaf area index (LAI) for a l l the understory vegetation collected i n the one metre quadrats was measured using the Licor Model LI 3000 portable leaf area meter, with the 3050A conveyor (Li-Cor Inc., Lincoln, NE, U.S.A.).  The meter was  attachment  calibrated between  samples using an aluminum disk of known area. Each quadrat sample was divided into two subsamples.  A l l the  leaves from the smaller subsample were stripped off the twigs and passed  - 25 -  through the meter.  A l l the leaves and twigs from the smaller subsample  and the larger subsample were then placed i n three separate paper bags and oven dried, at 70°C, to a constant mass.  The f i n a l dry mass of each  portion of the sample was measured and the leaf area of the sample was estimated using  LA = LAI (ML + MT + M2)/(ML + MT)  (13)  where LA i s the t o t a l leaf area of the sample, LAI i s the measured leaf area of the smaller subsample, ML and MT are the dry masses of the separated leaves and twigs from the smaller subsample, and M2 i s the dry mass of the larger subsample. Since the foliage samples were collected from one square metre quadrats, the calculated leaf area was a direct estimate of the LAI, on a projected leaf area basis, of the understory vegetation. of the subsampling  The  accuracy  procedure was checked by measuring the leaf area of  the entire f i r s t sample and comparing the measured leaf area with that estimated from Equation (13).  The estimated leaf area was  agree with the measured leaf area to within f i v e percent. understory LAI, within the stand, ranged from 0.5 for the plot area was  3.2.2  The  to 3, and the average  1.7.  Root Zonae S o l i Water Content and P o t e n t i a l  In A p r i l 1984, access tubes (48 mm each p l o t .  found to  five thin walled aluminum neutron moisture probe inside diameter) were i n s t a l l e d i n the s o i l within  The technique described by K e l l i h e r (1985) for producing  - 26 -  holes In the s o i l , into which the aluminum access tubes can be placed, was found to be impractical due to the extreme stoniness of the s o i l . To i n s t a l l a tube a trench 1 m long by 30 cm wide was dug down to the  compact t i l l .  The s o i l was separated according to depth i n t e r v a l  and covered to prevent evaporation.  A v e r t i c a l notch the same diameter  as the tube was made at one end of the trench and the tube was pushed firmly into the notch.  The depth of s o i l to the compact t i l l  was  measured, and at each 5 cm depth i n t e r v a l 2 s o i l samples were collected i n aluminum s o i l sampling cans for neutron hydroprobe c a l i b r a t i o n purposes.  The trench was then back f i l l e d and compacted layer by layer  to achieve as close to the o r i g i n a l density as possible.  The surface  organic layer, which had i n i t i a l l y been c a r e f u l l y scalped o f f , was replaced.  The neutron probe of a Campbell P a c i f i c Nuclear hydroprobe  (model CPN 503, Sacker S c i e n t i f i c Company, Edmonton, Alta.) was lowered into the aluminum access tube and a 30 second reading was taken at each 15 cm depth i n t e r v a l , beginning at 15 cm and continuing to the bottom of the  tube.  The s o i l samples were weighed, dried at 105°C to a constant  mass, and the volumetric s o i l water content was calculated using the following equation 8 = W v c where W  c  p.  fs  (1 - f ) / p  (14)  w  i s the water content of the fine f r a c t i o n of the gravimetric  samples on a dry mass basis, p f f r a c t i o n , and  s  i s the bulk density of the fine  i s the density of l i q u i d water.  - 27 -  Twenty more tubes were i n s t a l l e d , following the same procedure, into a contiguous clearcut  area.  Gravimetric s o i l samples from a l l the  trenches were used i n c a l i b r a t i n g the neutron hydroprobe. dried,  As the s o i l  s o i l pits were dug near selected c a l i b r a t i o n tubes and  gravimetric samples were collected from the 45 cm depth.  These samples  were treated as described above to obtain c a l i b r a t i o n data for dry soil.  The 45 cm depth was used to ensure that the surface could not  influence the hydroprobe measurements, even at very low s o i l water contents. The c a l i b r a t i o n of the neutron hydroprobe covered a range of volumetric water contents from 0.04  to 0.36  m  3  m  -3  (Appendix 2 ) .  The  linear equation found to f i t the data was  9 = 0.217 v where 8  (c/c ) - 0.075 s  (15)  i s the volumetric water content of the whole s o i l , c i s the  V  neutron count for a 30 second time i n t e r v a l with the probe i n the s o i l , and c  s  i s the neutron count for a 30 second time i n t e r v a l with the  probe i n the s h i e l d . Independent c a l i b r a t i o n of the same hydroprobe by K e l l i h e r (1985) and Giles  (1983) for s o i l s i n Courtenay, B.C.,  respectively,  and Mesachie lake,  B.C.,  yielded the following equations  9 = 0.224 (c/c ) - 0.009 v s  (16)  0  (17)  = 0.23  (c/c ) - 0.021  - 28 -  Equation  (15) was  based on almost twice the sample sizes of (16) and  (17), and the s i t e was  considerably stonier, however, a l l three  equations are very s i m i l a r . On August 2, 1984,  thermocouple psychrometers (model PCT-55, Wescor  Inc., Logan, Utah, U.S.A.) were i n s t a l l e d , i n pairs, at 15, 45, and cm depths, i n the cleared and uncleared plots.  75  The psychrometers were  placed i n 15 cm long holes, made with a screwdriver, in opposite faces of a carefully excavated  s o i l p i t . Loose s o i l was  tamped into the holes  to insure good contact between the ceramic cups and the soil.  surrounding  The thermocouple lead wires were wrapped around the p i t and  lead to the surface. density approaching  The s o i l p i t was  then  then r e f i l l e d and tamped to a  i t s o r i g i n a l condition.  The thermocouples were  cooled and measured using a Wescor model HR-33T dew  point  microvoltmeter  on both psychrometric and dewpoint modes.  3.2.3  Meteorological Measurements  A basic automated meteorological station was  maintained,  i n a more  recently planted area 300 metres from the research plots, during the growing seasons from 1983 to 1985.  The climate variables recorded on an  hourly basis were solar irradiance, a i r temperature, atmospheric humidity,  s o i l temperatures at 0, 20 and 50 cm, and windspeed (1 m  height).  Wind d i r e c t i o n was measured i n 1983,  l a t t e r half of the 1985 f i e l d season. Campbell S c i e n t i f i c CR21  1984  and during the  The data were logged on a  Micrologger and recorded on cassette tape.  At the research plots, meteorological measurements were made above the tree canopy in 1984 and 1985.  In 1984 meteorological measurements  - 29 -  were also made below the tree canopy within the cleared plot, and i n 1985 meteorologial measurements were made below the tree canopy i n the uncleared plot. The net radiation flux density above the stand was measured using a Swissteco ( S - l ) net radiometer.  In 1984, net radiation flux density  below the tree canopy i n the cleared plot, was measured with a Swissteco (S-l)  net radiometer.  In 1985, the net radiation flux density below the  tree canopy i n the uncleared plot was measured i n two locations.  At the  f i r s t , 2 m to the south of the weighing lysimeter a Swissteco (S-l) was used, while at the second, 30 cm d i r e c t l y above the lysimeter containing s a l a l , a Swissteco miniature (ME - l)net radiometer was used. The s o i l heat flux density was measured using a pair of s o i l heat flux plates (Middleton and Co. Pty, Ltd., South Melbourne, A u s t r a l i a ) positioned 5 cm below the s o i l surface.  A pair of integrating  thermometers with 5 temperature diodes spaced at 1 cm intervals was placed i n the s o i l , near the heat flux plates, to measure the change i n the mean s o i l temperature for the 0-5 cm depth.  Gravimetric s o i l  samples from the 0-5 cm layer were p e r i o d i c a l l y collected and the volumetric water content was determined, as this has a strong influence on the volumetric heat capacity of the s o i l . organic layer was found to be 0.18 kg m~ . 3  energy stored (W m ) -2  M = C(AT/At) Z  The bulk density of the The rate of change of the  i n the 0-5 cm layer was calculated using (18)  where C i s the volumetric heat capacity of the s o i l sample, AT i s the change i n temperature, At i s the time i n t e r v a l (usually 30 min.), and Z  - 30 -  i s the depth of the s o i l heat flux plates.  The s o i l heat flux (W m  )  at the surface was computed from G  0  = G  + M  5  (19)  where G5 i s the s o i l heat flux density measured at 5 cm with the s o i l heat flux plates. The windspeed was measured above and below the stand with sensitive Casella ( C F . Casella and Company, Ltd., London, England) cup anemometers.  The anemometer below the stand was positioned  approximately 30 cm above the s a l a l canopy adjacent to the lysimeter.  A  hotwire anemometer (Wilh, Lambrecht KG, Gottingen, West Germany) was used occasionally to measure low windspeeds above the s a l a l canopy. Air temperature uncleared plot, was  and humidity, below the tree canopy i n the measured with an FD300 s i l i c o n diode  psychrometer,  ventilated at 3-4 m s , and positioned approximately 30 cm above the - 1  s a l a l canopy.  The s i l i c o n diode psychrometer was  during the course of the experiment,  repeatedly calibrated,  with an Assmann  psychrometer.  A l l meteorological data were e l e c t r o n i c a l l y recorded using a Campbell S c i e n t i f i c model CR-7  data logger, printed out by a Campbell  S c i e n t i f i c CR56 printer, and recorded on cassette tapes.  A continuous  record of the below tree canopy net radiation flux density, measured by the miniature net radiometer above the lysimeter, was  obtained with a  Soltec (Model VP-6723S) chart recorder.  3.2.4  Forest S o i l Evaporation  Daily t o t a l evaporation from an undisturbed column of forest  soil  was measured i n August 1984 using a weighing lysimeter (described i n  - 31 -  Section 3.3).  A location near the centre of the cleared plot  selected for the lysimeter, surrounding s a l a l canopy.  in order to minimize advectlon from the A s t e e l lined well (1 m x 0.65  m wall thickness) with a sloping concrete floor and  damaging the area surrounding the lysimeter  The  lysimeter  wall.  The  skyline was  s o i l core was  cylinder, 0.6  was  In order to avoid  location a skyline of  suspended from the skyline  1.58 was  and  also used to bring the s o i l core to the  obtained by positioning a stainless s t e e l  meters inside diameter by 0.75  wall thickness,  x 0.003  i n s t a l l e d over the work area and a l l s o i l  carefully loaded into a container removed.  m I.D.  drainage hole,  prepared i n advance of the s o i l core being excavated.  cm diameter wire rope was  was  meters deep and 0.48  over a portion of the forest f l o o r .  A 0.48  cm  cm  thick  stainless steel plate with a s l i g h t l y larger diameter than the cylinder was  placed on top of the cylinder.  Twenty 25 kg s t e e l discs were then  stacked on the cylinder to provide a downward force. dug  A trench was  around the bottom edge of the cylinder and as s o i l was  cylinder slowly s l i d down, i s o l a t i n g an undisturbed core. cylinder was was  down the f u l l 0.75  replaced.  removed the When the  m the weights were removed and  c a r e f u l l y placed on the top edge of the cylinder and  cribbing  the weights  This allowed the cylinder to be forced down another 2.5  that the cylinder was  below the surface  level.  then  Webbed ducting  tape  cm  so was  wrapped around the top edge of the cylinder to hold the organic layer i n place.  Positioning the top edge of the cylinder below the  minimized the radiation load on the lysimeter walls and conduction through the walls to the s o i l inside.  surface  the heat  - 32 -  When the cylinder had been forced down to i t s f i n a l depth the weights and cribbing were removed and the stainless s t e e l plate, that had supported the weights, was used to sever the bottom of the s o i l core.  This was accomplished by a combination of excavating under the  s o i l core and forcing the stainless s t e e l plate under with hydraulic jacks. Wire rope (0.48 cm diameter) was positioned under the stainless s t e e l plate at the bottom of the isolated s o i l core and the sample was hoisted into the a i r , suspended  on the skyline.  The sample was  lowered  onto a tray which was s l i g h t l y larger diameter than the cylinder.  The  tray had short walls 8 cm i n height, and 5 cm wide stainless s t e e l straps which were welded to the bottom of the tray and ran up the side of  the cylinder walls and protruded 10 cm above the s o i l surface (Figure  1.3).  The isolated s o i l core was  straps.  then suspended  from these three metal  The gap between the cylinder and the support tray walls was  sealed with ducting tape.  The support tray was watertight.  The lysimeter sample was moved to the lysimeter well on the skyline and lowered into position. beam by three 0.48  The sample was suspended  cm diameter wire ropes.  from the lysimeter  Each of the suspension wires  had a 15 cm turnbuckle spliced into i t so that the s o i l core could be l e v e l l e d i n the w e l l .  This was important because i t f a c i l i t a t e d keeping  the width of the annulus uniform. lysimeter area.  The annular area was  17% of the  -  Figure  1.3  33  -  Cross-section through the lysimeter cylinder and the suspension tray; A • stainless steel straps, B • stainless s t e e l cylinder, C • stainless steel pan with short side walls, D » stainless steel plate used to sever the bottom of the Isolated s o i l column.  - 34 -  3.2.5  Understory Evapotranspiration  The understory evapotranspiration July 1985,  rate was  measured during June and  using a second lysimeter constructed  the uncleared  plot.  i n s t a l l a t i o n was cleared plot.  the previous  The method of lysimeter construction  similar to that described  and  for the lysimeter i n the  A few modifications were made to improve the  performance (described in Section 3.3),  summer i n  lysimeters  and i n the technique for  obtaining the s o i l plus s a l a l community sample. The stainless steel cylinder was which was  a b i t clumped.  A clump was  positioned over a s a l a l community chosen i n order to minimize damage  to the roots of plants which might be growing close to the cylinder walls.  Cribbing was  weights were loaded  b u i l t up on top of the cylinder and 455 kg of steel onto the cribbing.  s o i l core the sample was The  sample was  After i s o l a t i n g the s a l a l  and  well watered and allowed to drain for one  day.  moved into the lysimeter well, suspended from the  lysimeter beam, and l e f t over the winter.  The  t o t a l mass of the  soil,  lysimeter cylinder and suspension tray, with the s o i l column f u l l y recharged with moisture was In the spring of 1985,  approximately 400  kg.  stomatal resistance measurements were  conducted on s a l a l plants growing within the lysimeter and on plants surrounding the lysimeter.  The measurements were made with a Ll-Cor,  Model LI-1600 steady state porometer. Understory evapotranspiration was 21 to July 11.  The  measured continuously  change i n lysimeter mass was  data logger and a continuous trace was  recorded  from June  by the  CR7  obtained with the Soltec chart  - 35 -  recorder.  The stomatal resistance of s a l a l plants growing i n and around  the lysimeter was measured p e r i o d i c a l l y . On July 8 a l l of the s a l a l i n the lysimeter was  clipped and the  bare s o i l evaporation rate was measured for 3 days.  3.3  Weighing Lysimeter System  3.3.1  Design and  Construction  In order to minimize  the amount of excavation necessary to i n s t a l l  the lysimeter and i t s weighing mechanism an above ground balance type lysimeter was  constructed (Figure 1.4).  was used as the suspension beam.  A 15 cm x 15 cm s t e e l I-beam  A 15 cm O.D.,  thick walled, s t e e l  support post, was used to elevate the suspension beam above the s a l a l canopy.  The support post was  positioned in a hole dug down to the  compact t i l l and then back f i l l e d with concrete to ensure that i t was rigid. Tubular extensions, approximately 0.40  m long, were welded onto  both ends of the suspension beam, i n order to reduce shading by the beam of the lysimeter.  The extensions were constructed of 4.5 cm O.D.  with a 3.2 mm wall thickness. ted  pipe  The extensions were braced by triangula-  s t e e l strapping (3.8 cm wide x 0.64  cm thick), welded to both sides  of the extension tubes and the top plate of the I-beam (see Figure 1.4). This proved to be a r i g i d arrangement and i t achieved a substantial reduction i n the portion of the lysimeter's sky view which was by the suspension beam.  occupied  To further reduce the shading effect of the  - 36 -  Figure 1.4  Side view of the lysimeter and weighing mechanism, showing the location of the main components; A = LVDT, B = top spring, C = suspension I beam, D = dash pot, E = support post, F = turnbuckle, G = lysimeter cylinder, H = steel retaining wall, I = sloping concrete floor and drain, J = concrete, K = 7.6 cm steel channel guides, L = steel plates.  - 37 -  weighing mechanism, a l l of the above ground components were positioned north of the lysimeter well. The lysimeter sample and the counterweights were suspended  from the  support beam by 3 wire ropes which were fastened to eye-bolts that were located near the ends of the extensions.  The ring of the eye-bolt was  welded so that i t could not open up under heavy loading.  A l l the wire  ropes were run through wire rope thimbles wherever they had to take a sharp bend, and the ends were secured with 2 wire rope clamps to prevent slipping. The suspension beam pivoted on 2 knife edges which were constructed of high speed tool s t e e l blanks embedded i n right angle grooves at 45° to the horizontal, machined i n 2.54 cm thick blocks of soft s t e e l (Figure 1.5). The knife edges bore on another set of high speed tool s t e e l blanks which were also embedded i n soft s t e e l .  The bearing blanks  were set i n slots which were s l i g h t l y deeper than the blank, thereby creating a small l i p which prevented the knife edges from s l i d i n g o f f the polished surfaces.  The combination of the high speed tool s t e e l  embedded i n the soft s t e e l blocks w i l l be referred to as top and bottom pivot blocks, for the knife edge and bearing surface components respectively, throughout the rest of the text. The top pivot blocks were bolted onto a steel web which had been welded at a right angle to the suspension beam, at i t ' s midpoint (Figure 1.6).  The top pivot blocks were bolted with a single 1.90 cm bolt  positioned at the centre of the block so that when the suspension beam was lowered down onto the knife edges the pivot blocks could rotate,  - 38 -  •F  3  Figure 1.5  Side view of the fulcrum assembly; A = 5 cm steel angle iron, B = steel web welded perpendicular to the suspension beam, C = 2.54 cm thick soft steel top pivot block, D = hard tool steel b i t , 5 cm long, E = hard tool s t e e l b i t , 5 cm long, F = 2.54 cm thick soft s t e e l bottom pivot block, G = web of a short segment of I-beam, H = bottom plate of the suspension I beam, I = support post.  - 39 -  Figure 1.6  End view of the fulcrum assembly; A = suspension I beam, B = top plate of the web welded across the suspension beam, C = 5 cm angle iron, D = steel web, E = top and bottom pivot blocks containing the knife edge and bearing surface i n machined s l o t s , F = short segment of I-beam with one plate removed, G = support platform, H = strengthening web, I = support post.  - 40 -  and the weight would be evenly distributed across the 5 cm length of each knife edge.  The knife edges were separated  by 0.35 m i n order to  provide l a t e r a l s t a b i l i t y to the support beam, and were positioned at the midline of the I-beam.  This arrangement was found to be superior to  an e a r l i e r arrangement where b a l l bearing pivots were positioned below the bottom plate of the suspension beam. The bottom pivot blocks, containing the hard bearing surfaces, were bolted to the web of short lengths of 15 x 15 cm I-beam, from which one plate had been removed to form a T.  The f l a t surface of each T rested  on a s t e e l support platform which was welded to the top of the s t e e l support post.  A web was welded under the s t e e l support platform to give  i t added strength.  The top and bottom pivot blocks could be moved  around, to a limited extent, on the support platform. adjustment of the lysimeter container within the w e l l .  This f a c i l i t a t e d Once the  lysimeter was positioned so that i t could not come i n contact with the well walls the bottom pivot blocks were firmly fixed i n place on the support platform with large metal C type clamps. A simple dash pot f i l l e d  with heavy weight o i l for damping out  v i b r a t i o n due to gusts of wind was fastened onto the suspension beam approximately  0.80 m from the fulcrum, on the counterweight end (see  Figure 1.4). The counterweights were 25 kg and 10 kg c i r c u l a r metal plates.  For fine adjustments numerous smaller metal plates of known  mass were a v a i l a b l e .  Standard weights were used for c a l i b r a t i o n  purposes. A matched pair of stainless s t e e l springs (1.27 cm I.D. x 16.83 cm long, 1.59 mm wire thickness, 107 c o i l s ) was attached to the top and  - 41 -  bottom of the suspension beam at a point 0.98 m from the fulcrum (Figure 1.7).  With the suspension beam held i n a l e v e l position each spring was  tensioned equally.  G i f f o r d e_t a l . (1982) used matched pairs of springs  to damp wind induced o s c i l l a t i o n s , and they stated that matched pairs of springs reduced  the temperature s e n s i t i v i t y of the spring balance  apparatus (Appendix I I I ) . V e r t i c a l changes i n the position of the suspension beam, due to changes i n the mass of the lysimeter sample, were detected using DC l i n e a r voltage d i f f e r e n t i a l transformers  (DC-DC LVDT Model D2/100A, RDP  Electronics Ltd., Wolverhampton, U.K.).  The LVDT was held i n a clamping  arrangement fabricated from 2.54 cm thick soft steel i n order to embed i t i n a large thermal mass which would not fluctuate rapidly i n temperature (Figure 1.8). clamping  A micrometer was also fastened to the  arrangement so that the voltage at a known and repeatable  displacement  could be monitored at different temperatures.  technique was useful i n demonstrating  This  the s t a b i l i t y of the LVDT output  voltage. The voltage supply f o r the LVDT was provided by a 12 V automotive battery.  A voltage regulator (LM317, 180 mA) provided a stable 6 V  supply to the LVDT (50 mA). every 2 seconds. sampling  The output voltage was logged by the CR7  The maximum, minimum, and mean voltage for each  i n t e r v a l was recorded on cassette tape.  The output voltage was  also continuously recorded with the Soltec chart recorder. Angle iron cross pieces, which r e s t r i c t e d the range of v e r t i c a l movement of the suspension beam, were bolted on the outer spring  - 42 -  Figure 1.7  End view of the spring attachment; A • 3.8 cm angle iron, B • 7.6 cm steel channel guide; C » stainless Bteel top spring, D • 3.8 cm angle iron positioned to restrict vertical movement in the suspension beam, E • 3.8 cm angle iron, F • suspension beam, G • 1.27 cm threaded rod, H = 3.8 cm angle iron, I - stainless steel bottom spring, J • 3.8 cm angle iron.  Figure 1.8  Side view of the LVDT mounting arrangement and the position of the c a l i b r a t i o n micrometer; A = LVDT probe, B - 1.27 cm threaded rod, C = micrometer plunger.  - 44 -  supports.  These cross pieces acted as protection for the LVDT by  preventing the probe from being pushed beyond i t s designed range ( F i g . 1.7). The lysimeter was the beam was  operated as close to neutral as possible; i . e . ,  maintained  near l e v e l .  This was  standard weights from the counterweights  3.3.2  achieved by removing  as the lysimeter lost mass.  Lysimeter C a l i b r a t i o n  C a l i b r a t i o n of the lysimeter was  accomplished  by adding a series of  5, 10, 20, and 50 gram standard weights to the surface of the lysimeter a f t e r i t had been sealed with 0.1 mm  plastic.  The LVDT output  was  continuously recorded on a chart recorder and recorded for one minute sampling  intervals on the CR7 data logger.  A l l the weights were then  sequentially removed In reverse order as a check on hysteresis.  During  the course of the experiment, 5, 10, and 20 gram weights were frequently added and removed from the lysimeter and the change i n voltage recorded.  This provided a check on possible changes i n the lysimeter  c a l i b r a t i o n factor. The c a l i b r a t i o n data presented i n Figure 1.9 of the change i n voltage ouput (mV) (grams).  for a change i n lysimeter mass  The s e n s i t i v i t y of the lysimeter apparatus which i s given by  the slope of the c a l i b r a t i o n curve was standard error of estimate was resolution at 1.7 grams.  13.57  of water.  found to be 7.6 mV g - 1 .  The  mV which placed a lower l i m i t on  The surface area of the lysimeter was  and therefore the loss of 1.7 grams was mm  are presented i n terms  0.28  equivalent to the loss of  A resolution of approximately  2 grams was  m*  0.006  - 45 -  1 0 0 0 8 0 0  Y  0 20 40 60 80 100 1 2 0 MASS (g)  Figure 1.9  Calibration data and equation r e l a t i n g the LVDT output to changes i n the mass of the lysimeter.  - 46 -  observed i n the f i e l d by repeatedly adding 1 and 2 gram weights to the lysimeter. To test the temperature s e n s i t i v i t y of the LVDT electronics the sensor was repeatedly cooled to 4°C and then allowed to warm to 24°C, the voltage change over this range i n temperature was  4 mV or 0.2 mV/°C  which was equivalent to 0.0001 mm/°C. Apparent  increases i n lysimeter weight during the morning were  i n i t i a l l y thought to be due to condensation forming on the bottom of the lysimeter.  The lysimeter was pulled from the well at a time when the  apparent weight gain was greatest and checked for condensation, however no condensation was observed to be present.  The apparent weight gains  were ultimately attributed to d i f f e r e n t i a l heating of the suspension beam which resulted from the irregular pattern of direct solar irradiance below the tree canopy.  Painting the beam white reduced the  apparent weight gain but was not t o t a l l y e f f e c t i v e .  A shade was erected  over the weighing mechanism and this resulted i n a s i g n i f i c a n t improvement. At  the end of the experimental period the lysimeter was sealed with  black polyethylene and the zero d r i f t was monitored for 7 days.  Figure  1.10 shows the average apparent change i n lysimeter mass due to d i f f e r e n t i a l heating of the beam.  The shade was not completely  e f f e c t i v e at 1600 PST when the sun was at a low angle and managed to shine on the counterweight end of the beam.  Hourly measurements of  evapotranspiration were corrected for apparent weight gains and losses due to d i f f e r e n t i a l heating of the suspension beam.  - 47 -  0.04  E E cc  0. LU Q LU  i  D O  LU  6  Figure  1.10  PST  12  (h)  18  Average apparent change i n l y s i m e t e r mass due t o d i f f e r e n t i a l h e a t i n g and expansion of the suspension beam.  - 48 -  On a daily basis the lysimeter zero was very stable.  Figure 1.11  shows the apparent change i n lysimeter weight after completely shading the entire apparatus. The following calculation shows that maintaining a uniform radiative heat load on the suspension beam i s important to the measurement of hourly evapotranspiration. suspension beam increases i n temperature  If the lysimeter end of the 1°C and the thermal c o e f f i c i e n t  of linear expansion of s t e e l i s 12 x 10~ ° C , then the apparent 6  - 1  increase i n the mass of the lysimeter would be approximately 5 g, (1°C x 12 x 1 0  - 6  °C  - 1  x 400 kg), simply due to the expansion of the beam on one  side of the fulcrum.  - 49 -  0.010 E E  I  0.005 V  0.000 UJ Q CJ  -0.005 [  -0.010 6  12  18  24  PST ( h ) Figure 1.11 Apparent change i n lysimeter mass after completely shading the entire weighing mechanism.  - 50 -  4.  4.1  RESULTS AND  Measured Forest Floor Evaporation  DISCUSSION  Rates  The diurnal v a r i a b i l i t y of available energy flux density ( i . e . Rn G) below the tree canopy (A^) for the  days, August 8-15,  which forest s o i l evaporation  measured with the lysimeter, i s  shown i n Figure 1.12.  ( E ) was s  1984,  during  The i n t e r a c t i o n between the tree canopy and  the  temporal v a r i a b i l i t y of the above f orest solar irradiance (S^) i n determining A^ i s p a r t i c u l a r l y evident for August 8 and 1.13  and  1.14).  While the t o t a l S  equal, 24.55 and 24.33 MJ m~  2  1.91  MJ m"  2  explanation  d  - 1  d  - 1  t  for these 2 days was  10  (Figures  approximately  respectively, the 24 hr t o t a l A  on August 8 and 1.30  MJ m~  2  d~  for this is that, on August 10, S  when direct solar radiation could penetrate  t  on August 10.  1  was  low during  was  b  The periods  through gaps i n the tree  canopy above the lysimeter, and high during periods when the tree canopy e f f e c t i v e l y intercepted the bulk of the solar r a d i a t i o n . Over the eight day period, the daily t o t a l S 9.52  and 24.55 MJ m  -2  d , - 1  t  ranged between  and the daily average vapour pressure  above the canopy ranged between 0.40  and 0.90  kPa  (Figure 1.15).  deficit The  tipping bucket rain gauge, located at the meteorological station, recorded PST  3.7  mm  of p r e c i p i t a t i o n on August 5 and  on August 13, 0.9 mm The 24 hr total E  (Figure 1.16).  s  of p r e c i p i t a t i o n was was  6, and from 0200 - 0300  recorded.  found to range between 0.08  The daily evaporation  and 0.34  rates were large during  the  nm  d"  1  - 51 -  125  —  1  1  '  '  '  '  r  r  96  * 120  * 144  * 168  100 -  75  _25  I-  1  0  Figure 1.12  I  * 24  * 48  * 72  1  TIME (h)  1  192  Diurnal v a r i a b i l i t y of the below canopy available energy flux density (Afc) for the period August 8-15, 1984.  - 52 -  0  Figure 1.13  6  PST  12  (h)  18  24  Solar i r r a d i a n c e measured above the tree canopy (St( )) and the a v a i l a b l e energy flux density below the tree canopy (A^ ( )) for August 8, 1984.  - 53 -  0  Figure  1.1 A  6  Same as f o r F i g u r e  PST  1.13  12  (h)  18  except f o r August  24  10,  1984.  -  54  -  • 0.8 "D CM  i  E  0.6 - *  — J  4-"  CrO  • 0.4  0.2  Figure 1.15  Daily t o t a l solar irradiance measured above the tree canopy (St( )) and d a i l y average vapour pressure d e f i c i t measured below the canopy (D( )) for the period August 8-15, 1984.  - 55  -  0.8  1  0.8  I  07  07 —  E E  *~  <  • 0.6  0.6 0J5  0.4  0.4  |  0.3 \  • 03 co UJ  02  -I  0.1  0.2 0.1  0.0  0.0 8  8 1 0  1 1 1 2 1 3 1 4 1 6  AUGUST  Figure 1.16  Daily below canopy available energy flux density expressed as equivalent water evaporated (Afo/L ( )) and daily forest s o i l evaporation ( E ( )) for the period August 8-15, 1984. s  - 56 -  f i r s t three measurement days, with 47-65% of A^, going into latent heat flux.  While  and the average atmospheric vapour pressure d e f i c i t  (D) were similar on August 9 and 11, E almost 50%.  s  on August 11 was less by  The higher rate of evaporation on August 9 was l i k e l y due  to the evaporation of water which i n f i l t r a t e d  the surface of the organic  layer during the p r e c i p i t a t i o n events recorded on August 5 and 6. increased A  and D maintained the evaporation rate on August 10 even  b  though the surface resistance to vapour d i f f u s i o n ( r as  The  c 0  ) was  increasing  the surface organic layer dried. A similar pattern of events was observed on August 13 where the  increased evaporation rate appeared to be due to the evaporation of a small amount of water which accumulated on the surface of the organic layer during the p r e c i p i t a t i o n event recorded i n the early morning. following day D remained constant, A reduced by 70%.  b  decreased by 15%, and E  s  The  was  An Increase i n A^ and D on August 15 to levels  comparable to those observed on August 12 resulted i n similar values for E  s  on these two days. The average volumetric s o i l water content to a depth of 0.75 m was  measured on August 5 and 17, and was found to have decreased from 0.10 to 0.08 m  3  m  -3  (Black et a l . 1985).  The water balance analysis for this  period indicated that the average daily evapotranspiration from the stand where s a l a l had been removed was between 1.28 and 1.50 mm of water.  The average 24 hr E  s  during the measurement period was 0.23  mm  of water, which was 15-18% of the whole stand evapotranspiration. Gravimetric samples of the 0-50 mm organic layer, collected on August 5 and 17, showed that the volumetric water content of the surface  - 57 -  3  3  decreased from 0.08 to 0.04 m  m  , which was equivalent to the  evaporation of 2 mm of water.  The t o t a l evaporation recorded by the  lysimeter for the 8 day measurement period was 1.85 mm of water. For  similar s o i l moisture conditions but lower Afo due to higher  stand density Plamondon (1972) found that E  s >  i n a Douglas-fir stand  with no understory at the U.B.C. Research Forest at Haney, ranged from 0.1 - 0.15 mm d  - 1  , which was 33-50% of A^.  At higher s o i l moisture  contents he found that 80-100% of A^ went into latent heat flux, which resulted i n maximum 24 hr E  of 0.3 mm d  s  - 1  .  K e l l i h e r (1985) measured  E , i n plots with understory removed i n a thinned Douglas-fir stand s  near Courtenay, B.C., which ranged from 0.6 mm d water content was high, down to 0.1 mm d  - 1  when the root zone  - 1  at low s o i l water content.  Figure 1.17 shows the change i n the forest f l o o r d i f f u s i v e resistance ( r  c o  ) from August 8 to 15. Values of r  c o  , which i s  considered to be the resistance to vapour d i f f u s i o n r e s u l t i n g from the presence of a thin, dry, surface layer of organic matter, were calculated by rewriting (10) as pc r  co  =  D  - IlY  (R +  - G)  ' A ^ - T i —  -  l  )  -  1  ]  (  The bulk aerodynamic resistance (r^) was assumed to be 100 s  2  0  )  m . _1  Decreasing the estimate of r ^ by 50% resulted i n a 10% decrease i n the computed values for r 800-3400 s m  -1  c o  .  K e l l i h e r (1985) found r  for a similar forest f l o o r .  pinus radiata stand found r  c  o  c  0  ranged from  Denmead (1984) working i n a  to range between 120 and 1430 s  m . -1  - 58 -  4000 i  J"*  E  '  i  1  i  i  i  i  '  '  '  3000  CO  2000 -  1000  Figure 1.17  1  8  '  9  '  '  10  '  11  12  AUGUST  13  14  15  Forest floor d i f f u s i v e resistance ( r ) calculated from the rearranged Penman-Monteith equation, for the period August 8-15, 198A. c o  - 59 -  4.2  Measured Understory Evapotranspiration Rates The  leaf area index (LAI, projected area) of the s a l a l growing i n  the lysimeter was  found to be 1.0.  varied between 0.5 and 3.0.  New  the lysimeter, i n A p r i l 1985, been damaged i n the process summer.  The  The LAI of the s a l a l within the plot  leaves were observed on the s a l a l i n  which indicated that the plants had  of i s o l a t i n g the s o i l column the  not  previous  stomatal resistance to vapour d i f f u s i o n ( r ) for s a l a l s  plants growing within and around the lysimeter was  found to be very  similar for the period during which l y s i m e t r i c measurements of understory  evapotranspiration were made (Table 1.4).  This  provided  further evidence that the roots of the s a l a l plants i n the lysimeter were not excessively damaged when the s o i l column was  i s o l a t e d , and  indicated that the s o i l moisture conditions they experienced,  during  the  course of the experiment, were not s i g n i f i c a n t l y different from the moisture conditions i n the s o i l surrounding  the lysimeter.  The average volumetric s o i l water content decreased from 0.11 The  to 0.08  m  m  of the plot area  during the period June 26 to July 9.  s o i l water potential on July 11, measured with the  soil  psychrometers, indicated that the s o i l water potential ranged from MPa  at the 15 cm depth to -0.4  MPa  at the 75 cm depth.  -0.5  Interestingly  the s o i l moisture potential i n the plot where s a l a l had been removed found to be the same as where s a l a l was p r e c i p i t a t i o n had been recorded since the unsaturated was  very low,  present.  No  significant  for several days prior to June 26,  hydraulic conductivity of the coarse textured  i t can be assumed that there was  was  and soil  no s i g n i f i c a n t drainage  - 60 -  Table 1.4  Average stomatal resistance of the abaxial surface of s a l a l , growing i n and around the lysimeter, measured on 5 days during which the understory evapotranspiration was being measured by the weighing lysimeter  Average Stomatal Resistance (s m Date  Time (PST)  S a l a l i n Lysimeter  )  Salal Outside Lysimeter  June 21  1500  770  670  22  1500  610  510  25  1030  820  840  26  1330  920  900  July 08  1030  1,390  1,340  - 61 -  from the plot, and that the change i n volumetric s o i l water content due to evapotranspiration.  was  The t o t a l stand evapotranspiration, for the  period June 26 - July 9, was  24 mm  of water or approximately 2 mm  d~*.  The tree canopy above the lysimeter containing s a l a l was much less dense than the canopy above the lysimeter i n the cleared plot. 1.18  shows the t y p i c a l (June 26) diurnal trend i n above (A ) and below a  tree canopy available energy flux density for fine days. was  Figure  18.5 MJ m  2 24 MJ m~  2 —d1  and A^ was  5.5 MJ m  The 24 hr  A  a  2d 1 , whereas, for a day with  1 d~  —2  S  t  (this i s estimated  to correspond to a 24hrA  a  of  1  16-18 MJ m d~ ) measured above the cleared plot,the 24 hr A^ —2 1 only 1.7 MJ m d~ . From Figure 1.18  was  i t can be seen that the peak i n the  evapotranspiration rate (0.11 mm hr~ coincided with the peak i n D.  1 equivalent to LE = 70 W m — 2)  In the morning, while D was  low, a period  of high Ajj only s l i g h t l y increased the evapotranspiration rate above the night time rate; however, as D increased between 0900 PST and PST  the evapotranspiration rate continued  1200  to increase despite a dramatic  reduction i n A^. From 0900 - 1200  PST  the latent heat flux density equalled and even  exceeded A^» which required that there be a sensible heat flux from the a i r to the evapotranspiring surfaces. was  Since A],, for this period,  close to zero the supply of r e l a t i v e l y warm a i r must have come from  above, which suggests that the a i r surrounding coupled  to the atmosphere above the stand.  the s a l a l canopy was  well  A comparison of D above and  below the tree canopy showed that they were v i r t u a l l y the same. A cumulative plot of evapotranspiration for June 26 i s shown i n  - 62 -  600  04  E  5  H 15  400  H1  200  ^  •*  0.5 IQ  0 0 i CM  i  i  400  • 400  E  E  200  200  <  •  1  - -  5  *..*--  *  12  8  PST  Figure 1.18  16  (h)  20  24  Courses of available energy flux density measured above the tree canopy (Aa( )) vapour pressure d e f i c i t and available energy flux density measured below the tree canopy (D( ) and A^( ) ) , and the measured latent heat flux density from the understory (LE( )) f o r June 26, 1985.  - 63 -  Figure 1.19.  The 24 hr t o t a l evapotranspiration for days when diurnal  measurements were made are presented  i n Table 1.5,  together with  depth of p r e c i p i t a t i o n on the lysimeter, 24 hr t o t a l A  a  the 24 hr average D measured above the s a l a l canopy.  The 24 hr t o t a l  evapotranspiration varied from 0.63 mm;  however,there was  - 0.84  and A^,  the and  mm with a mean value of  0.70  no s i g n i f i c a n t decline i n the daily t o t a l  evapotranspiration during the 17 day measurement period. The  lysimeter recorded  0.14  mm  (2200 PST) and July 5 (0500 PST). in A , a  A, D  of p r e c i p i t a t i o n between July 4 Figure 1.20  shows the diurnal trend  below tree canopy D, and the corresponding  trend i n  latent heat flux density (LE) from the understory  for July 5.  negative LE indicated for the periods 0000 - 0100  PST and 0500 - 0600  PST  indicate p r e c i p i t a t i o n on the lysimeter.  The  An increase i n D between  0700 and 0800 PST appeared to result i n a rapid evaporation  of the  p r e c i p i t a t i o n intercepted by the s a l a l foliage and the surface of the forest f l o o r organic l i t t e r layer. and was  The LE for this period exceeded  much greater than the LE which was  time of the  normally  A  D  observed at that  day.  After the intercepted water was  evaporated the  rate dropped to a l e v e l approaching that which was  evapotranspiration normally  observed  for the morning period, however, A^ and D during the early afternoon were s u b s t a n t i a l l y lower than on previous days and therefore the evapotranspiration rate was  also reduced.  An improvement i n the weather  i n the later afternoon resulted i n a peak in A^ between 1500  and  1600  PST but D remained low and as a result the evapotranspiration rate did not  increase.  - 64 -  1  0.8  h  0  6  12  PST Figure 1.19  18  24  (h)  Cumulative understory evapotranspiration for June 26, 1985.  - 65 -  Table 1.5  Daily (24 hour) understory evapotranspiration ( E ) for days when diurnal measurements were made; t o t a l p r e c i p i t a t i o n (P) on the lysimeter; daily available energy flux density measured above (A ) and below (A^) the tree canopy; and daily average vapour pressure d e f i c i t (D) measured below the tree canopy t  a  Date  t (mm) E  P  A  (mm)  (MJ  b (MJ m~ ) A  a m" ) 2  2  D (kPa)  June 21  0.70  -  19.0  7.7  0.92  22  0.71  -  15.9  6.5  0.60  25  0.68  -  19.7  5.7  0.78  26  0.73  -  18.5  5.5  0.82  July 04  0.84  0.04  6.2  1.19  05  0.63  0.10  10.6  4.9  0.52  06  0.60  -  17.4  5.3  0.85  07  0.71  -  17.6  6.2  1.22  -  Figure 1.20  Same as for Figure 1.18 except for J u l y 5, 1985.  - 67 -  The fact that the evapotranspiration rate did not increase i n response to an increase i n  on July 5, suggests that the  evapotranspiration rate was either s o i l water supply limited or that i t was poorly coupled  to A^,. Table 1.6 shows that on a 24 hr basis the  evapotranspiration from the lysimeter was between 37 and 54% of the equilibrium evapotranspiration rate (E q) below the tree canopy, which e  i s given by  E  eq  = [s/(s + ) ] (Rn - G)/L  The 2 mm d  (21)  Y  - 1  average evapotranspiration from the whole stand, f o r the  period June 26 to July 9, was approximately 50% of the 24 hr E q for e  the stand.  The understory  evapotranspiration presented  i n Table 1.5 was  between 30 and 42% of the daily t o t a l stand water use.  4.3  Forest Floor Evaporation Rates Following S a l a l Removal On July 8 the s a l a l growing i n the lysimeter was clipped and E  was measured f o r three days (Table 1.7).  s  The weather during the three  days was similar to that of the previous measurement days; however, the d a i l y average D on July 9 was greater than had previously been observed.  The mean 24 hr E  was 0.37 mn d " . This rate was 1  s  approximately 50% of the mean understory salal  evapotranspiration rate with  present. While the s a l a l canopy i n the lysimeter was not dense, i t i s  reasonable  to assume that removing i t would result i n a s i g n i f i c a n t  increase i n the net radiation flux density to the forest f l o o r and an  - 68 -  Table 1.6  Daily (24 hour) average temperature (T) measured below the tree canopy, daily understory equilibrium evapotranspiration rate (E q), and the r a t i o of measured daily understory evapotranspiration to the daily understory equilibrium evapotranspiration rate ( E ^ E ^ ) e  T  E  t  Date  (°C)  eq (mm)  June 21  15.3  1.89  0.37  22  11.8  1.51  0.47  25  14.5  1.41  0.48  26  14.5  1.36  0.54  July 04  17.4  1.62  0.52  05  14.3  1.20  0.53  06  15.7  1.35  0.44  07  18. 9  1.64  0.43  eq  - 69 -  Table 1.7  Daily (24 hour)forest s o i l evaporation ( E ) following s a l a l removal, daily t o t a l available energy flux density measured above (Ag) and below (A^,) the tree canopy, and daily average vapour pressure d e f i c i t (D) measured below the tree canopy s  E  Date  s (mm)  A (MJ  b (MJ  D  A  a m ) -2  m ) -2  (kPa)  July 09  0.41  17.1  5.6  1.76  10  0.36  14.9  4.7  1.12  11  0.34  20.4  6.0  1.18  - 70 -  increase i n s o i l surface temperature.  The top few centimetres of the  organic forest f l o o r would also increase i n temperature resulting i n a s i g n i f i c a n t increase i n the saturation vapour pressure of the s o i l a i r and the vapour pressure gradient between the vapour source and the atmosphere.  Removing the s a l a l would also reduce the aerodynamic  resistance to vapour transport due to increased windspeed and more e f f e c t i v e penetration by gusts down to the forest f l o o r .  The vapour  pressure would probably decrease, further increasing the vapour pressure gradient between the vapour source within the s o i l and the atmosphere. The combined effect of a l t e r i n g the meteorological regime at and close to the forest f l o o r , brought about by removing s a l a l , would appear to be a s i g n i f i c a n t enhancement of the forest s o i l evaporation rate. The average r  c  o  f o r the three days was 3530 s m  discussed l a t e r i n the context of E  4.4  s  • This w i l l be  under a s a l a l canopy.  Dependence of the Bulk Aerodynamic Resistance to Vapour Transport from the Salal Canopy on Windspeed and Leaf Area Index The bulk aerodynamic resistance to vapour transport (r^) was  calculated using data averaged over the f i r s t 3 minutes after wetting both surfaces of the s a l a l leaves with a plant mister, and the surrounding s a l a l with a manual f i r e pump.  The water sprayed on the  s a l a l leaves was observed to bead up, probably due to the waxy c u t i c l e . Using soap as a surfactant was ruled out because of possible deleterious effects on the leaves.  At least 80% of the leaf area appeared to be  covered by water, however exact estimates were not possible.  The depth  - 71 -  of water on the of s a l a l leaves, computed from the measured increase i n lysimeter mass after wetting, was  found to be 0.10  ± 0.02  compares well with K e l l i h e r ' s (1985) estimate of 0.12  mm  mm,  which  for s a l a l  canopy storage capacity, and with Spittlehouse and Black's (1982) experimental results of 0.1± 0.03  mm  of water.  Ten runs were conducted  with LAI=1, and four runs were conducted with the leaf area doubled by placing clipped s a l a l stems into the lysimeter. Figure 1.21  shows the values for r^, calculated using (12),  plotted against the mean windspeed (u) measured 0.3 m above the s a l a l . The boundary-layer  resistances ( r ) , for s a l a l canopies with v a  leaf area indices of 1 and 2, also shown i n F i g . 1.21,  were calculated  using the relationship  r  -,0.5 = [F 184(d/u) ] /2LAI va s  where F  (22)  L  s  i s a shelter factor (Kelliher 1985;  Spittlehouse and Black  1982) assumed to be 1 for IAI=1 and 1.5 for LAI=2 (Jarvis  a l . 1976),  d i s the average diameter of the leaves (0.035 m), and u i s the average windspeed. The natural logarithm of r ^ i s plotted against the natural logarithm of u i n Figure 1.22.  Using linear least squares regression  analysis the windspeed dependence of r ^ for the s a l a l canopy with LAI=1 was  found to be  r. = 25.2 u A  0.554  (23)  - 72 -  Figure 1.21  Bulk aerodynamic resistance to vapour transport (R^) from a wet s a l a l canopy to a height of 1.0 m calculated from equation (12) plotted against the mean windspeed (u) measured 0.3 m above the s a l a l canopy. Values of r ^ calculated from data collected with the lysimeter 1AI=1 (•) and LAI=2 (A). The curve through the data represents the function r . (s m ) = 25.2 u ~ (G i n m s ) , and the two lower curves represent the function _1  0 , 5 5 4  _ 1  >  va s (°- /u)°* ]/ » )» where the shelter factor equals 1.0 for LAI=1 ( r  =  [ F  1 8 4  0 3 5  5  1.5 for LAI=2 (  ).  2  L A I  l n  m  s _ 1  ) and  - 73 -  T —  » -2.5  Figure  1.22  1  I  '  1  -2  -15  ln(0)  I  i  -1  I  1 -0.5  I  5  3  0  The natural logarithm of the bulk aerodynamic resistance to vapour transport (r^) calculated from equation (12) plotted against the natural logarithm of the mean windspeed ( u ) .  - 74 -  where r ^ i s i n s m  and u i s i n m s  . Equation (23) i s plotted as a  heavy s o l i d line through the data i n Figure 1.21. The fact that the power i n (23) i s similar to 0.5 suggests that the bulk of the resistance is leaf boundary-layer resistance. Calder et a l . (1984) found that the windspeed dependence of r ^ for an upland heather canopy was well described by the function r  A  = (20 ± 2) u ~  ( 0 , 6±  °-  0 6  \ h i c h i s very similar to (23). W  Black et^ a l . (1982) found that the windspeed within a s a l a l canopy with a LAI=3 was 50% of that measured above the canopy.  If the  windspeed within a sparser canopy (LAI=1) i s assumed to be 80% of that measured above the canopy then 70-75% of r ^ i s a t t r i b u t a b l e to r  v a  .  Increasing the LAI to 2 resulted i n a reduction i n the mean leaf boundary-layer resistance and an apparently at windspeeds below 0.5 m s  4.5  - 1  more rapid increase i n r ^  .  Applying the Penman-Montelth Equation to the Salal Canopy Hourly estimates of s a l a l transpiration (E') were calculated using  the Penman-Monteith equation (6) to test how well i t would estimate s a l a l transpiration under the stand canopy.  The canopy resistance  values used i n the calculations were based on hourly stomatal resistance measurements made on s a l a l leaves surrounding the lysimeter.  The bulk  aerodynamic resistance was computed using (23). To use (6) the latent heat flux from the forest s o i l below the s a l a l canopy i s required.  E  s  was computed using (10), and reasonable  - 75 -  estimates of the differences i n the values of meteorological parameters above and below the s a l a l canopy, as I l l u s t r a t e d i n the following example for 1200 - 1300 PST on June 25. The average r e a r l i e r , was  c o  for July 9-11,  3530 s m~*.  after s a l a l removal mentioned  This value was used for the forest f l o o r  surface resistance i n (10).  The net radiation flux density below the  s a l a l canopy was estimated to be 140 W m~ , i . e . , 60% of that measured above the s a l a l .  This estimate was  based on a leaf area based net  radiation flux density extinction model presented by Tanner and Jury (1976).  The a i r temperature  pressure was  was reduced from 23 to 20°C, the vapour  increased from 1.2 to 1.4 kPa, the aerodynamic resistance  to sensible and latent heat fluxes from the s o i l surface to the s a l a l canopy was assumed to be approximately 40 s m . -1  of meteorological parameters E was  s  was  For this  estimated to be 0.01  combination  mm h , which - 1  s l i g h t l y greater than 10% of the measured understory  evapotranspiration for this period. The resulting calculations of s a l a l understory transpiration rates for 0900 - 1600 PST and 1000 - 1700 PST on June 22 and 25 respectively, are presented i n Table 1.8. June 22 was  The calculated s a l a l transpiration (E') for  found to be 8% less than the t o t a l measured  evapotranspiration minus the 10% assumed to be attributable to E , s  on June 25, E' was  overestimated by  and  10%.  Table 1.9 shows the effect of a l t e r i n g the assumptions about the r a t i o of E  s  to E  underestimated  t  on E'.  Assuming Eg = 0 resulted i n E' being  by 20% on June 22 and overestimated by 3% on June 25.  At  - 76 -  Table 1.8  Hourly measured understory evapotranspiration ( E ) and calculated s a l a l transpiration (E') for June 22 and 25, 1985 t  June 22 Time  June 25 E'  E  E t (mm h" )  E'  (PST)  t (mm h )  0900 - 1000  0.05  0.05  -  -  1000 - 1100  0.04  0.04  0.02  0.04  1100 - 1200  0.07  0.06  0.05  0.05  1200 - 1300  0.14  0.11  0.06  0.07  1300 - 1400  0.09  0.07  0.08  0.07  1400 - 1500  0.08  0.06  0.07  0.07  1500 - 1600  0.09  0.07  0.09  0.08  1600 - 1700  -  -  0.06  0.06  - 1  (mm  h ) - 1  1  (mm  h ) _ 1  - 77 -  Table 1.9  The effect of a l t e r i n g the assumption about the amount of forest s o i l evaporation that occurred, while the s a l a l canopy was present , on the calculation of s a l a l transpiration  E'/(E E /E s t  June 22  t  -v June 25  0.0  0.8  1.03  0.1  0.92  1.10  0.2  1.02  1.17  0.5  1.18  1.30  - 78 -  the other extreme, assuming E s a l a l was  present,  evapotranspiration,  s  was  equivalent  to that measured when no  i . e . , 50% of the measured understory resulted i n E' being overestimated by 18-30%.  Tan et_ a l . (1978) computed s a l a l transpiration using the following approximation  E-P^ L Y  r  (2«) '  v  s  They noted that the s a l a l leaves were generally 1-2°C  warmer than a i r  temperature and that E computed using D would be underestimated, however, they f e l t that this would be offset by not including the boundary-layer resistance to latent heat f l u x .  Calculating s a l a l  transpiration using (24) for June 22 and 25 resulted i n 25% and underestimates of the measured rates for the two  5%  days respectively,  assuming that 10% of the measured understory evapotranspiration  was  due  to E . s  The  effect of windspeed on transpiration i s complicated  by the fact  that 1/r^ appears i n both the numerator and the denominator of (6). Monteith (1965) has shown that the transpiration rate w i l l be independent of windspeed when the canopy resistance to vapour f l u x equals a c r i t i c a l resistance given by  r* = [pc (1 + y/s) D ]/Y(Rn - G)  (25)  - 79 -  Grace (1977) presented experimental evidence demonstrating that at high rates of energy absorption  an increase i n windspeed causes a  decrease i n the surface temperature and a reduction i n the transpiration rate.  If the canopy resistance i s less than the c r i t i c a l resistance the  transpiration rate w i l l increase i n response to increased windspeed. The hourly average canopy resistance and computed  critical  resistances for June 22 and 25 are presented i n Table 1.10.  The canopy  resistance was generally much greater than r*; however, during the two hours from 1000 - 1200 PST on June 25, A, was very low and the computed D  r* was similar to the measured canopy resistances. c  During the day, when  At, i s large, increased windspeed w i l l tend to reduce the transpiration rate of s a l a l . Use of the Penman-Monteith equation to estimate understory transpiration requires estimates of u below the tree canopy.  Generally  u below the tree canopy was found to be within 6-15% of u above the trees; however, there i s a good deal of scatter i n the relationship which i s not unexpected i n view of the complexity of turbulent  transport  i n the tree canopy. Monteith (1965) demonstrated that the r e l a t i v e transpiration rate, written as E, d _ E * w  or  8  s + y + y d + r /r ) c  A  (26)  - 80 -  Table 1.10  Hourly measured canopy resistances and computed c r i t i c a l canopy resistances for June 22 and 25, 1985  June 22 Time (PST)  June 25  (s  r  r* c  r^ m- ) 1  (s  m" ) 1  (s  m" ) 1  r* c (s  m" ) 1  0900 - 1000  546  266  -  -  1000 - 1100  594  287  865  766  1100 - 1200  505  251  787  1,008  1200 - 1300  554  61  787  176  1300 - 1400  510  126  767  183  1400 - 1500  405  188  767  202  1500 - 1600  535  90  753  115  7 59  186  1600 - 1700  - 81 -  _ ! - [ ! + s( + TY )r Is.]"1  E  1  w  K  J  '  A  A  where Ey i s the evaporation rate from wet foliage and E,j i s the transpiration rate from dry foliage under the same meteorological conditions, depends only on the a i r temperature and the r a t i o of r to c  r^.  The average r e l a t i v e transpiration rate for s a l a l during June 22  and 2 5 was found to be 0.19.  The average r e l a t i v e t r a n s p i r a t i o n rate  calculated from the lysimeter measurements made immediately before, during, and after the wetting experiments was 0.21.  This compares with  an average r e l a t i v e rate of 0.19, calculated using (26), for the meteorological conditions prevalent during the canopy wetting experiments and assuming that the stomatal resistance was similar to that which was measured on s a l a l surrounding  4.6  the lysimeter.  The Relative Importance of Net Radiation and Vapour Pressure D e f i c i t i n Determining the Latent Heat Flux Density from the S a l a l Canopy McNaughton and J a r v i s (1983) rearranged  the Penman-Monteith  equation, i n order to show e x p l i c i t l y the equilibrium or radiative term and the advective term, as follows:  LE =  s (Rn - G) + s + y  (27)  - 82 -  where D q (the equilibrium vapour pressure d e f i c i t ) i s given by e  Yr D = —~—req (s + y) p c  (Rn - G)  (28)  p  The equilibrium vapour pressure d e f i c i t can be interpreted as being the vapour pressure d e f i c i t required to obtain a latent heat flux, across an existing canopy resistance, which i s equal to the equilibrium evaporation rate for a given available energy flux density.  As r  c  increases there i s an increase i n the r a t i o of sensible to latent heat flux density which results i n warming of the a i r above the exchange surface.  As the a i r warms D increases and given s u f f i c i e n t time i t w i l l  increase u n t i l i t equals D  eq  equilibrium evaporation rate.  and the evaporation Finnegan  rate equals the  and Raupach (1985) demonstrate  that there may not be time for this to happen because the quiescent periods between penetrating gusts of a i r from above the forest canopy are at best 3 minutes long. The measured canopy resistance of the s a l a l around the lysimeter was quite large, and consequently, D q exceeded the measured D e  whenever the available energy flux density was much greater than 50 W m . -2  This resulted i n the advective term i n (27) becoming negative,  which, when combined with the observation that D above the s a l a l was similar to D above the trees, suggests  that there was a high rate of  exchange between the atmosphere above the s a l a l canopy and the atmosphere above the tree crowns.  Furthermore, the atmosphere above the  trees i s well coupled to the atmosphere i n the well mixed portion of the planetary boundary-layer,  and therefore the vapour pressure d e f i c i t  above the trees i s determined  by regional advective processes.  Since D  -  above the s a l a l was l e v e l below D q e  advective  83  -  maintained, through e f f e c t i v e turbulent mixing, at a  for the s a l a l canopy i t appears that there  depression  of the evaporation  was  rate.  McNaughton and Jarvis (1983) also presented the Penman-Monteith equation in a form which included a factor (ft) that gives the r e l a t i v e importance of the equilibrium term and a term that represents transpiration rate that would occur i f the vapour pressure  the  deficit in  the outer portion of the planetary boundary-layer were imposed at the surface with no l o c a l adjustment, as follows  LE = Q(  I s +  )(R y n  pc - G) + (1 - fl)(  P  D ) vr ' c  (29)  m  where  (30)  D  m  i s the vapour pressure  d e f i c i t at a reference height i n the well  mixed portion of the planetary boundary-layer, and r  is the  a s  aerodynamic resistance to vapour f l u x from the exchange surface to the reference height within the planetary boundary-layer. be seen that ft i s determined by the r a t i o of r Equations (29) and  c  to r  a s  From (30) i t can .  (30) can be used to describe the degree of  coupling of s a l a l transpiration to the available energy flux density vapour pressure that r  a s  d e f i c i t below the Douglas-fir canopy.  i n (30) be replaced by r , and % A  This  in (2 9) by D.  requires  With  and  - 84 -  this change, i t can be seen by comparing (30) and (26) that Q for the s a l a l understory i s equal to i t s r e l a t i v e transpiration rate (E j/E ), which was found to be approximately 0.2. This indicates (  w  that the advective  term i n (29) i s the dominant term.  Since D above the  s a l a l was very similar to that above the tree canopy, this analysis shows that the evapotranspiration of the regional vapour pressure  rate Is responding to the imposition  d e f i c i t at the s a l a l canopy with  e s s e n t i a l l y no l o c a l adjustment. As part of the derivation of (29) and (30), McNaughton and J a r v i s (1983) showed that the vapour pressure  d e f i c i t at the surface i s given  by D = Q D + (1 - Q) D o eq m  (31)  In this case Q establishes the r e l a t i v e importance of the available energy flux density at the surface and turbulent mixing above the surface i n determining the vapour pressure exchange surface.  d e f i c i t immediately above the  When Q i s large, the surface i s isolated from the  planetary boundary-layer by a large aerodynamic resistance and D approaches D q. e  D  When Q i s small, due to e f f e c t i v e turbulent mixing,  then the atmosphere at the exchange surface i s coupled to the atmosphere within the well mixed portion of the planetary boundary-layer and D approaches D . m  surface, D advective  Q  Q  Since 1^ i s only slowly altered by the underlying  becomes a regionally set value controlled by regional processes.  The small Q values computed for the s a l a l canopy indicate that D  0  - 85 -  at the s a l a l leaves was controlled by D above the s a l a l canopy.  As  indicated e a r l i e r , the l a t t e r was similar to D above the trees.  The  values of D were substantially less than the vapour pressure d e f i c i t which would have existed had the a i r above the s a l a l canopy been e f f e c t i v e l y decoupled from the atmosphere above the stand.  Stewart  (1984) reported a similar result for a bracken fern understory i n Thetford Forest, U.K.; however, i n that case the humidity d e f i c i t above the forest was 8.2 g k g  - 1  whereas, f o r the 48 W m~  and 1.24 m above the ground i t was 8.6 g k g  - 1  ,  of available energy and the r e l a t i v e l y low  canopy resistance the equilibrium humidity d e f i c i t would have been 1.0 g kg . - 1  In this case the understory evapotranspiration was being  advectively enhanced by the imposition of a regional D which was much greater than D . eq  K e l l i h e r (1985) showed that s a l a l understory  evapotranspiration was much higher than E q -Q, thus demonstrating e  advective enhancement.  Again D above the s a l a l was similar to that  above the tree canopy; however, low r greater than D . eq  U  c  resulted i n D being much  - 86 -  5.  CONCLUSION  Two 400 kg weighing lysimeters, with a resolution of 0.006 mm of water, were constructed to measure hourly rates of forest f l o o r evaporation and understory transpiration i n a 23-year-old Douglas-fir plantation.  The  weighing mechanism was an above-ground counterbalanced beam which pivoted on two, 5 cm long, knife edges and was held i n suspension by two opposing springs. A linear variable d i f f e r e n t i a l transformer resolved changes of 0.01 mm i n the v e r t i c a l displacement of the suspension beam approximately 1.25 m from the fulcrum.  Positioning the weighing  mechanism above ground simplified construction and minimized the impact on the vegetation surrounding the lysimeter well.  The high degree of  s p a t i a l v a r i a b i l i t y i n solar irradiance below the tree canopy caused d i f f e r e n t i a l heating of the suspension beam on either side of the fulcrum.  This problem was corrected by completely shading the beam from  direct solar irradiance. A technique was developed for i s o l a t i n g undisturbed lysimeter s o i l columns (0.6 m diameter x 0. 75 m deep) containing understory vegetation, without causing s i g n i f i c a n t damage to the plants' root systems. Porometry measurements of the stomatal resistance of s a l a l growing i n the lysimeter were found to be similar to those of plants growing outside the lysimeter. Lysimeter measurements of daily forest s o i l evaporation rates, following s a l a l removal i n a 30 m x 40 m plot, on 8 consecutive days i n August 1984, when the s o i l moisture content was low, ranged from 0.08 -  - 87 -  0.34 mm d  .  These rates were 15-18% of the daily t o t a l stand  evapotranspiration.  In early July 1985, after removing s a l a l from a  lysimeter positioned under a less dense tree canopy, the forest s o i l evaporation days.  rate was found to vary from 0.34 - 0.41 mm d  These forest s o i l evaporation  average daily understory the previous  - 1  , over three  rates were approximately 50% of the  evapotranspiration measured on 8 days, out of  17 days, for which daily totals were a v a i l a b l e .  The daily  forest s o i l evaporation was 17-21% of the t o t a l stand evapotranspiration. Forest floor d i f f u s i v e resistances, computed by rearranging the Penman-Monteith equation, were found to range between 900 and 3500 s m" . 1  These values are i n close agreement with the range of  forest floor d i f f u s i v e resistances reported by K e l l i h e r (1985) for a similar organic forest f l o o r . The measured daily evapotranspiration rate from a plot with understory  salal  present was 0.60 - 0.84 mm d * , which was equivalent to -  30-42% of the daily t o t a l stand evapotranspiration.  Diurnal  measurements of hourly evapotranspiration rates were conducted on 8 days when s a l a l was growing i n the lysimeter.  The maximum hourly  evapotranspiration rate observed was 0.13 mm h d a i l y t o t a l understory  - 1  .  As much as 10% of the  evapotranspiration was observed to occur  during  the night (2000 - 0600 PST). U t i l i z i n g the lysimeter to measure the rate of evaporation completely  from  wetted foliage proved to be a r e l i a b l e method for estimating  the windspeed dependence of the bulk aerodynamic resistance to vapour  - 88 -  transport. was  A relationship between r ^ (s m  -  ) and windspeed (m s  )  found which has the form r. = b u , where b = 25.2 and m = -0.554 A m  for a s a l a l canopy with LAI=1.  Approximately  70-75% of r ^ was  estimated to be attributable to the leaf boundary-layer  resistance, with  the remaining 25-30% of the t o t a l resistance being due to the resistance of the atmosphere between the s a l a l canopy and the meteorological sensors. The evaporation rate from wet foliage was  found to be 5 times  greater than the transpiration rate from dry f o l i a g e .  This i s  s i g n i f i c a n t when modelling the evaporation of intercepted p r e c i p i t a t i o n , especially during the growing season when p r e c i p i t a t i o n events are short i n duration and often followed by fine weather. water was  Approximately  0.1 mm of  observed to accumulate on one side of a unit area of l e a f .  Using hourly average meteorological measurements, measured stomatal resistance of s a l a l leaves surrounding the lysimeter, and estimates of s o i l evaporation rate and r^, as inputs to the Penman-Monteith equation, hourly s a l a l transpiration rates were computed to within 10% of the measured rates ( E  t  by the leaf boundary-layer  - E ). s  Since r ^ was  largely  determined  resistance i t has been shown that the  Penman-Monteith equation can be used with a reasonable estimate of leaf boundary-layer  resistance.  This requires windspeed estimates within the  s a l a l ( i n this study windspeed near the s a l a l was the stand).  6-15%  of that above  The simple vapour d i f f u s i o n model (» LAI D/r ) was  to underestimate  s  the s a l a l transpiration rate by 5-25%.  found  - 89 -  For 2 days during which s a l a l leaf stomatal resistance was measured hourly the canopy resistance was generally much greater than the c r i t i c a l resistance at which the latent heat flux density i s unaffected by windspeed, and consequently,  increased windspeed resulted i n a  reduction of the latent heat f l u x . A range of Q values between 0.15 and 0.22 were computed for two days during which hourly measurements of r understory  c  were made.  The measured  evapotranspiration was s u b s t a n t i a l l y lower than E q and was e  found to be largely determined by D. large r  s  r e l a t i v e to r ^ .  These values of Q r e f l e c t the  The vapour pressure d e f i c i t above the  s a l a l canopy was very similar to that above the trees.  The D which  would have existed above the s a l a l canopy, i n the absence of e f f e c t i v e turbulent mixing of the atmosphere, i . e . D q, was found to be much e  larger than that above the stand.  The imposition of the regionally  established D on the s a l a l understory of the understory  resulted i n advective  evapotranspiration.  depression  The lack of l o c a l Influence on  the below tree canopy D suggests that extensive s a l a l removal would not result i n an increase i n the vapour pressure d e f i c i t below the tree canopy. Mechanically removing s a l a l from a 30 by 40 meter plot required approximately  20 person days of labour.  Substantial resprouting was  observed to occur during the growing season.  The e f f i c a c y of currently  available herbicides registered for extensive use i n forestry projects has not yet been demonstrated for s a l a l c o n t r o l . The operational removal of s a l a l , whether by mechanical or chemical methods, w i l l be an expensive procedure i f i t i s carried out i n juvenile stands.  This  - 90 -  expense may  not be j u s t i f i e d i n view of the fact that there may  only be  a 50% reduction i n the understory latent heat flux density i n response to  t o t a l l y eliminating s a l a l .  When the s o i l surface layers are moist  the reduction i n latent heat flux density may  be even l e s s .  Kelliher  (1985) reported that s a l a l removal reduced the understory evapotranspiration by 40-60%. This study tends to support the suggestion by Black and Spittlehouse (1981) that higher stand densities should be maintained  on  s i t e s which are droughty and have s a l a l as an understory component.  In  order to keep the understory evapotranspiration rate down to 0.02 requires that the below canopy available energy flux density not 20 W m  ; and that the tree canopy be dense enough to prevent  mm  h  - 1  exceed  efficient  mixing of the below canopy a i r with the a i r above the trees, so that the dryness of the a i r surrounding the understory r e f l e c t s the canopy c h a r a c t e r i s t i c s and the energy regime of the understory. With respect to regeneration and stand tending strategies f o r Douglas-fir stands on droughty s i t e s , the results of this study together with the findings of the other studies reviewed suggest that  initial  planting densities should be high i n order to obtain early crown closure and f u l l s i t e occupancy.  Salal growing i n shade has reduced leaf area  and i n d i v i d u a l leaves are larger which results i n an increased boundary-layer  resistance to vapour flux from the leaf (Kelliher 1985).  Thinning operations should be designed to avoid creating large gaps in the tree canopy, as these gaps act l i k e chimneys and increase the v e n t i l a t i o n below the tree canopy which results i n the maintenance of a  - 91  larger atmospheric vegetation.  -  vapour pressure d e f i c i t around the understory  In order to minimize the number and size of the gaps  created i n the tree canopy, targeted stand density objectives should be achieved through several light thinning operations rather than a single heavy thinning.  The trees removed, during each stand entry, should come  from the smaller diameter  classes and lower crown positions.  Since the economics of thinning operations are strongly related to the number of times a stand must be entered and the harvestable volume available at each entry, the strategy described above may economically unacceptable.  prove to be  However, a single heavy thinning i s also  expensive to conduct and i t has been demonstrated that the t o t a l productivity of the stand may  decline due to the f a i l u r e of the  remaining crop trees to f u l l y exploit the available s o i l moisture. The f i n a l management option therefore, i s to plant at i n i t i a l l y high densities, i n order to obtain early crown closure, and to allow the stand to self thin through natural mortality. requiring the least investment,  This option, while  provides the manager with no  opportunities for c o n t r o l l i n g stand development, and no harvestable volume midway through  the rotation.  a s i g n i f i c a n t percentage  For forest firms planning to obtain  of their timber supply from thinning operations  the loss of mid-rotation harvest opportunities w i l l be costly, especially i f replacement open log market.  timber supplies have to be purchased  from the  - 92 -  BIBLIOGRAPHY  Ainscough, G.L. 1981. The Designed Forest System of MacMillan Bloedel Limited: An example of i n d u s t r i a l forest management. The H.R. MacMillan Lectureship i n Forestry, U.B.C., Van., B.C. Anonymous. 1979. Biogeoclimatic Units: Nootka-Nanaimo. of Forests, V i c t o r i a , B.C. Anonymous. 1980. Forests and Range Resource Analysis. of Forests, V i c t o r i a , B.C.  B.C. B.C.  Ministry Ministry  Aston, A.R. 1984. Evaporation from Eucalypts growing i n a weighing lysimeter: a test of the combination equations. Agric. For. Meteorol. 31: 241-249. Black, T.A., C.B. Tanner and W.R. Gardner. 1970. from a snap bean crop. Agron. J. 62: 66-69.  Evapotranspiration  Black, T.A., C.S. Tan and J.U. Nnyamah. 1980. Transpiration rate of Douglas-fir trees i n thinned and unthinned stands. Can. J . S o i l S c i . 60: 625-631. Black, T.A. and D.L. Spittlehouse. 1981. Modeling the water balance for watershed management, pp. 117-129. In: D.M. Baumgartner (ed.) Proc. Symp. Interior West Watershed Mgt. Apr. 8-10, 1980, Spokane, WA, U.S.A. Black, T.A., D.L. Spittlehouse, F.M. K e l l i h e r and D.T. P r i c e . 1982. Water balance and management of young Douglas-fir stands. Contract Research Report, E.P. 855, B.C. Min. of For., V i c t o r i a , B.C. Black, T.A., D.T. Price, F.M. K e l l i h e r and P.M. Osberg. 1984. Effect of understory removal on seasonal growth of a young Douglas-fir stand. Contract Research Report, E.P. 855, B.C. Min. of For., V i c t o r i a , B.C. Black, T.A., D.T. Price, P.M. Osberg and D.G. G i l e s . 1985. Effect of reduction of s a l a l competition on evapotranspiration and growth of early stage Douglas-fir plantations. Contract Research Report to Research Branch, B r i t i s h Columbia Min. of For., V i c t o r i a , B.C. Bradley, E.F., O.T. Denmead and A.W. T h u r t e l l . 1983. Measurements of the turbulence and heat and moisture transport i n a forest canopy. Q.J.R. Meteorol. Soc. ( i n preparation). Calder, I.R., R.L. H a l l , R.J. Harding and I.R. Wright. 1984. The use of a wet-surface weighing lysimeter system i n r a i n f a l l interception studies of heather (Calluna v u l g a r i s ) . J. Climate and Appl. Meteorol. 23: 461-473.  - 93 -  Carlson, H. 1974. Springs: Troubleshooting and F a i l u r e Analysis. W.H. Middendorf (ed.) Engineering Troubleshooting. Vol. 1. Marcel Dekker, Inc., New York. Daniel, T.W., J . Helms and F.S. Baker. 1979. Principles of S i l v i c u l t u r e . Second E d i t i o n . McGraw-Hill Book Company. York.  In:  New  Denmead, O.T. 1984. Plant physiological methods for studying evapotranspiration: problems of t e l l i n g the forest from the trees. Agric. Wat. Mgt. 8: 167-189. Finnegan, J . J . and M.R. Raupach. 1985. Transfer processes i n plant canopies i n r e l a t i o n to stomatal c h a r a c t e r i s t i c s . In: E. Zieger, G. Farquar and I. Cowan. (EDS.) Stomatal Function. Stanford Univ. Press, Stanford, CA., U.S.A. Forest Act. 1979. Available from the Queens Printer for B r i t i s h Columbia, V i c t o r i a , B.C. F r i e s , J . and S. Hagner. 1970. Economic decision models for s i l v i c u l t u r a l operations. In: Proceedings s i l v i c u l t u r a l decison models. American Pulpwood Association, Atlanta, Georgia. Garratt, J.R. methods.  1984. The measurement of evaporation by meteorologial Agric. Wat. Mat. 8: 99-117.  G i f f o r d , H.H., D. Whitehead, R.S. Thomas and D.S. Jackson. 1982. Design of a new weighing lysimeter for measuring water use by individual trees. N.Z.J. For. S c i . 12: 448-456. Giles, D.G. 1983. S o i l water regimes on a forested watershed. Thesis, Univ. of B.C., Vancouver, B.C. Grace, J . 1977. pp. 60-69.  Plant Response to Wind.  M.Sc.  Academic Press, New York,  Jackson, D.S., E.A. Jackson and H.H. G i f f o r d . 1983. S o i l water i n deep Pinaki sands: some interactions with thinned and f e r t i l i z e d Pinus radiata. N.Z.J. For. S c i . 1392: 183-196. J a r v i s , P.G., G.B. James and J . J . Landsberg. 1976. Coniferous forest, pp. 171-240. In: J.L. Monteith (ed.) Vegetation and the Atmosphere. Vol. 2, Case Studies, Academic Press, New York. J a r v i s , P.G., W.R.N. Edwards and H. Talbot. 1981. Models of plant and crop water use. pp. 151-194. In: D.A. Rose and D.A. Charles-Edwards (eds.) Mathematics and Plant Physiology. Academic Press, London. K e l l i h e r , F.M. 1985. Salal understory removal effects on the s o i l water regime and tree transpiration rates i n a Douglas-fir forest. Ph.D. thesis, Univ. of B.C., Vancouver, B.C. Klinka, K., F.C. Nuszdorfer and L. Skoda. 1979. Biogeoclimatic units of central and southern Vancouver Island. B.C. Min. of For., V i c t o r i a , B.C.  - 94 -  Leverenz, J . , J.D. Deans, E.D. Ford, P.G. J a r v i s , R. Milne and D. Whitehead. 1982. Systematic s p a t i a l variation of stomatal conductance i n a Stika spruce plantation. J . Appl. Ecol. 19: 835-851. McNaughton, K.G. and P.G. J a r v i s . 1982. Predicting effects of vegetation changes on transpiration and evaporation, pp. 1-47. In: T.T. Kozlowski (ed.). Water D e f i c i t s and Plant Growth, Vol. VII. Academic Press, New York. Milne, R., J.D. Deans, E.D. Ford, P.G. J a r v i s , J . Leverenz and D. Whitehead. 1985. A comparison of two methods of estimating transpiration rates from a Sitka spruce plantation. Boundary-Layer Meteorol. 32: 155-175. Monteith, J.L. 1965. Evaporation and environment, B i o l . XIX: 205-234.  symp. Soc.  Exp.  Plamondon, A.P. 1972. Hydrologic properties and water balance of the forest f l o o r of a Canadian west coast watershed. Ph.D. thesis, Univ. of B.C., Vancouver, B.C. Price, D.T., T.A. Black and F.M. K e l l i h e r . 1986. Effects of s a l a l understory removal on photosynthetic rate and stomatal conductance of young Douglas-fir trees. Can. J . For. Res. 16: 90-97. Raupach, M.R. and B.J. Legg. 1984. The uses and limitations of flux-gradient relationships i n micrometeorology. Agric. Water Mgt. 8: 119-131. Roberts, J . , C F . Pymar, J.S. Wallace and R.M. Pitman. 1980. Seasonal changes i n leaf area, stomatal conductance and transpiration from bracken below a forest canopy. J . Appl. Ecol. 17: 409-422. Roberts, J., R.M. Pitman and J.S. Wallace. 1982. A comparison of evaporation from stands of Scots pine and corsican pine in Thetford Chase, East Anglia. J . Appl. Ecol. 19: 859-872. Roberts, J., J.S. Wallace and R.M. Pitman. 1984. Factors a f f e c t i n g stomatal conductance of bracken below a forest canopy. J. Appl E c o l . 21: 643-655. Shuttleworth, W.J. 1978. A simplified one-dimensional theoretical description of the vegetation-atmosphere i n t e r a c t i o n . Boundary-Layer Meteorol. 14: 3-27. Shuttleworth, W.J. 1979. Below-canopy fluxes i n a simplified one-dimensional theoretical description of the vegetation-atmosphere interaction. Boundary-layer Meteorol. 17: 315-331. Shuttleworth, W.J. and J.S. Wallace. 1985. Evaporation from sparse crops - an energy balance combination theory. Q.J.R. Meteorol. Soc. I l l : 839-855.  -  95 -  Smith, D.M. 1962. The Practice of S i l v i c u l t u r e . Wiley and Sons, Inc., New York.  7th E d i t i o n , John  Spittlehouse, D.L. and T.A. Black. 1982. A growing season water balance model used to p a r t i t i o n water use between trees and understory. pp. 195-214. In: P r o c Can. Hydrol. Symp. 82, Hydrol. processes i n f o r . areas. June 14-15, 1982, Fredericton, N.B. Stewart, J.B. 1983. A discussion of the relationships between the p r i n c i p a l forms of the combination equation f o r estimating crop evaporation. Agric. Meteorol. 30: 111-127. Stewart, J.B. 1984. Measurement and prediction of evaporation from forested and a g r i c u l t u r a l catchments. Agric. Water Mgt. 8: 1-28. Tan, C.S., T.A. Black and J.U. Nnyamah. 1977. Characteristics of stomatal d i f f u s i o n resistance i n a Douglas-fir forest exposed to s o i l water d e f i c i t s . Can. J . For. Res. 7: 595-604. Tan, C.S., T.A. Black and J.U. Nnyamah. 1978. A simple d i f f u s i o n model of transpiration applied to a thinned Douglas-fir stand. Ecol. 59: 1221-1229. Tanner, C.B. 1968. Evaporation of water from plants and s o i l . pp. 73-106. In: T.T. Kozlowski (ed.) Water D e f i c i t s and Plant Growth. Vol. 1. Academic Press, New York. Tanner, C.B. and W.A. Jury. 1976. Estimating evaporation and transpiration from a row crop during incomplete cover. Agron. J . 68: 239-243. Thorn, A.S. 1971. Momentum absorption by vegetation. Meteorol. Soc. 97: 414-428.  Q.J.R.  Thorn, A.S. 1975. Momentum, mass and heat exchange of plant communities, pp. 57-109. In: J.L. Monteith (ed.) Vegetation and Atmosphere. Vol. 1, P r i n c i p l e s , Academic Press, New York. Wahl, A.M. 1944. Mechanical Springs. F i r s t Edition. Publishing Company, Cleveland, Ohio.  Penton  Zahner, R. 1958. Hardwood understory depletes s o i l water i n pine stands. For. Sci. 4: 178-184.  - 96 -  APPENDIX I  S o i l P r o f i l e Description - Dunsmuir Creek S i t e  -  97  -  APPENDIX  I  S o i l P r o f i l e Description - Dunsmuir Creek S i t e  Horizon  Depth  Description  (cm) LF  2-0  Undecomposed and semi-decomposed  Douglas-fir  needles; abundant very fine and fine horizontal roots; abrupt, smooth boundary; 1 to 3 cm thick.  Ae  0-3  Grayish  brown (10 YR 5/2 d) gravelly sandy loam;  weak, medium granular;  s o f t ; p l e n t i f u l , fine and  medium oblique roots; abrupt, i r r e g u l a r boundary; 2 to 5 cm thick.  Bml  3-20  Brown (10 YR 4/3 m) gravelly sandy loam; weak, fine subangular blocky; f r i a b l e ; p l e n t i f u l , fine and medium oblique roots; d i f f u s e , wavy boundary; 15 to 22 cm thick.  Bm2  20-60  Dark yellowish brown (10 YR 4/4 m) gravelly sandy loam; weak, fine subangular blocky; f r i a b l e ; few, fine and medium oblique roots; gradual, wavy boundary; 30 to 50 cm thick.  - 98 -  Horizon  Depth  Description  (cm) BCgc  60-80  Grayish brown (2.5 Y 5/2  m) gravelly loamy sand;  common, medium, d i s t i n c t strong brown (7.5 YR 5/8  m)  mottles; massive; firm; very few, very fine oblique roots; discontinuous, weak cementation; clear, wavy boundary; 25 to 40 cm thick.  BCc  88 +  Dark grayish brown (2.5 Y 4/2 m) gravelly sandy loam; massive; extremely firm; no roots;  continuous  moderate cementation.  Classification:  Duric Dystric Brunisol.  Comments:  The upper part of the solum apears disturbed by tree-throw  and/or logging.  Occasional fragments of  duric horizon (BCc) were found throughout the Bm horizons.  - 99 -  APPENDIX II  Neutron Probe C a l i b r a t i o n Equation  - 100 -  2.5  I  i  i  1  0  I  i  i  1  0.00  0.10  6  y  ure A l l . I  —  i  i  r  i  020  i  0.30  i  i  0.40  (mV)  Neutron probe count r a t i o ( c / c s ) vs v o l . s o i l water content (0) at the Dunsmuir Creek s i t e . The equation of the repression l i n e i s c / c s = 0.348 + 4.619 9 V J r 2 = 0.88, Se = 0.11 or Gy = 0.217 c/cs - 0.075.  - 101 -  APPENDIX I I I  Theory of Counteracting Spring Balance  -  102 -  APPENDIX  III  Theory of Counteracting Spring Balance  When the l y s i m e t e r weighing mechanism i s balanced and the s u s p e n s i o n beam i s l e v e l , springs  F  the f o r c e s a c t i n g r o t a t i o n a l l y about the fulcrum  lys  lys  n  where F ^ y is F  c w  the  being h e l d i n suspension by two  S  of the suspension beam from the f u l c r u m  s p r i n g s , and d  fulcrum If  by the mass of the l y s i m e t e r ,  r e  the f o r c e s exerted  i s the l e n g t h  s  t o the p o i n t the l y s i m e t e r  F  TS  diy  S  c w  is  t o the by the bottom and  of the suspension beam from the  of s p r i n g attachment. l o s e s some mass through e v a p o r a t i o n  then the sum  of the s p r i n g f o r c e opposing r o t a t i o n i n the d i r e c t i o n of F increase,so  (AIII.l) '  to the l y s i m e t e r ,  by the mass of the counterweight, d  c o u n t e r w e i g h t s , Fgg and F^s a top  v  of the suspension beam from the f u l c r u m  i s the f o r c e exerted length  are given by  = F d + (F - F„) d cw cw BS TS s  i s the f o r c e e x e r t e d  the length  counteracting  c w  will  that  - Fgg = k [ ( X + AX ) - (X T  T  fi  - AX )]  (AIII.2)  B  however Xg = X j and AXg ( p o s i t i v e f o r c o n t r a c t i o n of bottom spring)  = AX^ ( p o s i t i v e f o r e l o n g a t i o n  FBS ~ T S = "2 k AX F  S  of top s p r i n g )  = AXg so ?  (AIII.3)  - 103 -  where k i s the spring constant  (N c m ) , X>p and Xg represents the -1  i n i t i a l elongation of the top and bottom springs, and AX  S  i s the  increase i n the elongation of the top spring as a result of a decrease in lysimeter mass. Substituting (AIII.3) into (AIII.l) gives F  d, = F d - 2k AX d lys cw cw s s  lys  (AIII.4)  and, therefore, F AX  s  =  d  C  W  C  - F,  W  y 2  ^VDT Since — AA s  k  d  d,  S  7  (AHI.5)  5  s  LVDT , , , , . " —: , where AX.,.-,- i s the increase i n the displacement d LVD I s d  A V  t U  k U  of the LVDT probe, and d^y^T i s the length of the suspension beam from the fulcrum to the LVDT, then to obtain AX yDT f °  ra  L  AF^yg requires that (AIII.5) be multiplied by dLVDT/^s  AY - cw cw " l y s l y s LVDT r~2 2k d (F  d  F  d  )  AX  t  0  given give  ^VDT  (AIII.6)  s If d  c w  = d  l y s  , then (F  =  L V U 1  L-  - F iy  ) d iy  )  2k <T s  d LVDT  (  A  I  I  I  .  7  }  - 104 -  *f s d  LVDT>  =  d  n  LVDT  e  (F  -  AY  AX  t  n  - F ) lys 2lTd s  cw  d,  lys  (  (AIII.8)  If l y s = d , then d  s  AX  LVDT  Zk' "  =  1  At balance,  5  ( A I I I  = ^gAL' * ' ' ^ V D T i  e  =  °' '  w h e r e  V  LVDT  a  n  d  V  BAL  a  r  '  9 )  e  the output voltages of the LVDT when the weighing mechanism i s i n operation and balanced.  As the lysimeter loses mass, F AX  greater than  V  V  +  b  becomes  LVDT  < D — ^ — , and  , AX^^, increases, i . e . ,  LVDT = BAL  c w  ^VDT  ( A I I I  where b i s the LVDT c a l i b r a t i o n factor (mv mm  1  '  1 0 )  ) . Substituting  (AIII.10) into (A1II.6) gives V  -V  LVDT  BAL  =  ( F d ^ cw cw  F d ) ( L l y s lys'' T.VDT  (AIII.ll)  2k d s  b  2  which can be rearranged to give F^yg as follows  F  =  ^  S  d  _£W_£W. d  lys  2k d s 2  _  LVDT l y s d  V.. - V LVDT mT  b  )  (AIII.12)  - 105 -  Differentiating F i  with respect to  y s  2k d  AF^y  VLVDT  yields  2  can also be written i n terms of the change i n the depth of water  S  i n the lysimeter as follows AF. = n r lys  2  AW  p dw  (AIII.14) '  K  where r i s the radius of the lysimeter, A W<j i s the depth of water evapotranspired, and  i s the density of l i q u i d water, therefore,  2 k d AW, = - ( LVDT l y s 2  d  b  d  A V ) ^ n r p u  n  T  (AIII.15)  2  w  K  When A W,j i s negative evaporation i s occurring, i . e . , A  VLVDT  ^  ^  and V y x i s increasing. L  D  The lysimeter balance c a l i b r a t i o n factor (mm mV ) -1  (reciprocal of  s e n s i t i v i t y ) i s obtained by rearranging (AIII.15) to give  AW  2 k d d  A V LVDT  2  8  2 b d d. it r z p LVDT lys w T T T T V P  (AIII.16)  K  The temperature s e n s i t i v i t y of the counteracting springs i s related to the difference i n elongation between the springs so that when the weighing mechanism i s i n a balanced  or neutral condition both springs  - 106 -  are elongated equally and the effect of temperature on the modulus of r i g i d i t y for each spring i s the same, and consequently, there are no changes i n the r e l a t i v e forces acting on the suspension beam (Carlson 1974; Wahl 1944). The depth of water i n the lysimeter i s given by:  W., = W - k' (1 - m(T - T )) (V , - V dl ref o LVDT BAL v  T¥Trin  v  W A T  )  (AIII.17) '  where W f i s an i n i t i a l reference depth of water i n the lysimeter, re  T  0  Is the temperature when the lysimeter was balanced, T i s the  temperature during the period when weight changes are being measured, m is the temperature s e n s i t i v i t y of the modulus of r i g i d i t y for the two springs and 2 k  2 k d  —  . b  d  LVDT l y s d  71  ^  w  P  The depth of water remaining i n the lysimeter after a period during which evaporation was taking place i s given by W,„ = W „ + A W, dZ dl d Differentiating  A W  D  (AIII.18)  A W<j with respect to temperature and voltage gives  aAW = (-^)  a AW AT + (-^-)  AV  (AIII.19)  therefore aAw - - k'm 3T  (V - V ) LVDT BAL' 1T  (AIII.20) '  - 107 -  and 3AW(  IT  - k' (1 - m(T - T )) Q  (AIII.21)  

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