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Study of stomatal diffusion resistance in a Douglas Fir Forest Tan, Chin-Sheng 1977

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JI. (  A STUDY OF STOMATAL DIFFUSION RESISTANCE IN A DOUGLAS FIR FOREST  by  CHIN-SHENG TAN B.Sc, National  Chung-Hsing University, Taiwan, 1969  M.Sc, University of New Hampshire, U.S.A.,- 1972  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES Department of Soil Science  We accept this thesis as conforming to the required  standard  THE UNIVERSITY OF BRITISH COLUMBIA March, 1977 @  Chin-Sheng Tan, .1977  In p r e s e n t i n g t h i s  thesis  in p a r t i a l  f u l f i l l m e n t o f the requirements  f o r an advanced degree a t the U n i v e r s i t y o f B r i t i s h Columbia, I t h a t the L i b r a r y s h a l l I  agree  make i t f r e e l y 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 .  f u r t h e r a g r e e . t h a t p e r m i s s i o n f o r e x t e n s i v e copying o f t h i s  thesis  f o r s c h o l a r l y purposes may be granted by the Head o f my Department or by h i s r e p r e s e n t a t i v e s . of t h i s  thesis  It  for financial  is  understood t h a t c o p y i n g o r p u b l i c a t i o n  gain s h a l l  not be allowed w i t h o u t my w r i t t e n  permission.  Chin-Sheng Tan  Department o f S o i l S c i e n c e U n i v e r s i t y of B r i t i s h Columbia Vancouver, B.C., Canada V6T 1W5  Date  J  7  ~  ^  -  77  ii  ABSTRACT  This thesis resistance coastal  r e p o r t s the r e s u l t s  of a study of  stomatal  diffusion  i n a Douglas f i r f o r e s t near Courtenay, i n the dry  belt.of  Stomatal  Vancouver  east-  Island.  d i f f u s i o n r e s i s t a n c e was measured by a v e n t i l a t e d  porometer designed by Turner e_t al_.  The theory of the o p e r a t i o n o f  porometer as proposed by T u r n e r and Parlange  is  and the porometer e v a l u a t e d f o r use i n the measurement o f the r e s i s t a n c e o f Douglas f i r  p i r a t i o n i n Douglas f i r , the r e l a t i o n s h i p between stomatal r e s i s t a n c e and environmental parameters were s t u d i e d .  and a t h i n n e d stand  water p o t e n t i a l  soil  diffusion  Research was  salal  con-  undergrowth  undergrowth.  r e s i s t a n c e was mainly r e l a t e d to the s o i l Daytime  r e s i s t a n c e f o r Douglas f i r ranged from 2 to 60 s cm '' -  o f vapour p r e s s u r e d e f i c i t between 4 and 24 mb and values  water p o t e n t i a l  stomatal  evapotrans-  and the v a p o u r . p r e s s u r e d e f i c i t o f the canopy a i r .  v a l u e s o f stomatal f o r values  stomatal  (1840 stems ha~^) with n e g l i g i b l e  (840 stems ha~^) with s u b s t a n t i a l  During the daytime, stomatal  discussed  needles.  As p a r t of a two-year study o f the e f f e c t o f t h i n n i n g on  ducted in an unthinned stand  this  r e v i e w e d . . The problems  o f using the theory to i n t e r p r e t v e n t i l a t e d porometer data are  diffusion  diffusion  between 0 and -12.5 b a r s .  Although  increasing  r e s i s t a n c e was u s u a l l y a s s o c i a t e d with d e c r e a s i n g twig  p o t e n t i a l , i n c r e a s i n g stomatal  water  r e s i s t a n c e appeared to be a s s o c i a t e d  i n c r e a s i n g twig water p o t e n t i a l when the vapour p r e s s u r e d e f i c i t was Stress history.was  found to cause a s h i f t  of  i n the r e l a t i o n s h i p o f  with high.  stomatal  resistance to twig water p o t e n t i a l , but had l i t t l e e f f e c t on the relationship of stomatal resistance to vapour pressure d e f i c i t and soil water p o t e n t i a l .  Daytime values of stomatal resistance for salal ranged  from 2 to 45 s cm"^. The physiological nature of the forest canopy resistance was studied by comparing the stomatal and canopy resistance of the unthinned stand. Canopy resistance was calculated using energy balance/Bowen r a t i o measurements of evapotranspiration.  The typical steady increase in both canopy  and stomatal resistance during daytime hours, even at high s o i l water potentials^ indicated that the increasing canopy resistance was caused by gradually closing stomata.  During a dry period, the mean daytime  value of canopy.resistance increased value of the stomatal resistance.  in proportion  to the mean daytime  Values of canopy resistance calculated  from stomatal resistance and leaf area index.measurements agreed well with those calculated from energy balance measurements. Values of transpiration rate in the thinned  stand calculated from  a simple vapour d i f f u s i o n model that uses the vapour pressure d e f i c i t of the canopy a i r , and measurements of the stomatal resistance and l e a f area index of the canopy agreed well with those obtained from energy balance/ Bowen r a t i o measurements.  Stomatal resistance c h a r a c t e r i s t i c s were also  used in the d i f f u s i o n model instead of actual stomatal resistance measurements to calculate transpiration rate.. There was reasonable agreement between these transpiration values and energy balance measurements.  Both  the model and energy balance measurements showed that transpiration rate increased with increasing vapour pressure d e f i c i t until a certain vapour pressure d e f i c i t was reached after which the rate declined.  Calculations  iv  from the model indicated that the fraction of transpiration from the thinned stand transpired by the salal during daytime hours increased from approximately 45 to 70% during a four-week drying period.  V  LIST OF SYMBOLS  A  Available energy f o r evapotranspiration: R  - G - M (W m * ) -  p  1  or porometer aperture area or needle area (cm ) S p e c i f i c heat of moist a i r (J g"^ C~^) D.B.H.  Diameter at breast height (cm)  d  Diameter of hole in c a l i b r a t i o n plate (cm)  -2 E  Evapotranspiration or transpiration rate (g cm  e  Diffusion porometer chamber vapour pressure (mb)  e  -1 s  )  Water vapour pressure of the a i r outside the boundary layer of  a  the leaf (mb) e*(Ta)  Saturation water vapour pressure at a i r temperature T (mb) a  e  g  Saturation water vapour pressure within the stomatal cavity (mb)  e  z  Water vapour pressure at height z in the canopy (mb)  e*  z  Saturation water vapour pressure at height z in the canopy (mb)  G  Soil heat flux density (W m  )  H  Sensible heat flux density (W nf )  L  Latent heat of vapourization of water (J g~^), or c a l i b r a t i o n plate thickness (cm)  LAI  Leaf area index or needle area (projected area basis)/unit area of ground surface (dimensionless)  LAI.  LAI of i t h canopy layer  (dimensionless)  LE  Latent heat flux density (W m  )  _2 M  Rate of heat storage in canopy volume on an area basis (W m ")  n  Number of holes per unit area of c a l i b r a t i o n plate (cm  )  vi  RH  Relative humidity  R  Net radiation flux density (Win  n  _o  ) -3  R  Gas constant f o r water vapour (mb cm  v  -1 g  -1 k  )  r^  Boundary layer resistance to water vapour d i f f u s i o n (s  r r . cJ  Canopy or surface resistance (s cm ^ Canopy resistance at the j t h hour of the daytime (s  r  Mean daytime canopy resistance (s c m ) ; defined by equation  £  cm ) -1  cm ) -1  -1  c  (6) in Chapter 3 r^  Aerodynamic resistance to heat exchange between the forest and the height z (s  cm ) -1  rp  Porometer d i f f u s i o n resistance (s cm  r  Stomatal d i f f u s i o n resistance on a projected area basis or  g  1  )  d i f f u s i o n resistance of the c a l i b r a t i o n plate (s  cm ) -1  r  s i  Stomatal d i f f u s i o n resistance of the i t h canopy layer (s  r  y  Aerodynamic resistance to water vapour exchange between the forest and the height z (s  cm" ) 1  cm ) -1  s  Slope of the saturation water vapour pressure curve (mb C )  T  Porometer temperature (K)  T  1  a  A i r temperature (C)  T-|  Leaf temperature (C)  t  Transit time (s)  V  E f f e c t i v e volume of the porometer (cm ); V = V 3 Actual porometer chamber volume (32 cm )  V  c  V  s  V  s  v.p.d.  c  + V  3 Capacity of the sensor to absorb water vapour (cm ) 3 Average sensor capacity f o r the porometer (cm ) Vapour pressure d e f i c i t ; e*(Ta)-e  g  or e*  z  - e  z  (mb)  g  vii  v . p . d. v.p. v. p.  d  'i  d  -j  . A r i t h m e t i c mean o f v a l u e s  of v . p . d . during  v.p.d.  at the i t h canopy l a y e r  v.p.d.  at the j t h hour o f the daytime  (mb)  Diffusivity  B  Bowen r a t i o ; H/LE  Y  Psychrometric constant  e  V o l u m e t r i c water c o n t e n t o f s o i l  p  D e n s i t y o f moist a i r  *t  (mb) -1  2  a  *s  daytime (mb)  o f water vapour i n a i r  (cm  s  )  (dimensionless) (mb C"^)  (=R T pCp/L) v  a  3  Soil  (g cm  (cm  3 o f water/cm  of  soil)  )  water p o t e n t i a l ; i n c l u d e s o n l y m a t r i c and osmotic terms  (bar)  Twig water p o t e n t i a l ; as measured by Scholander p r e s s u r e chamber (bar)  vi i i  TABLE OF CONTENTS  Page  ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . LIST OF SYMBOLS  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  LIST OF TABLES . . . . . . . LIST OF FIGURES  ., . . . . . . . . . . . . . . . . . . . . .  . . . . . .....  . . . . . . .... • "  ACKNOWLEDGEMENTS  ii v xi xiii  . . . . . . . . . .  xix  INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  1  CHAPTER 1 - EVALUATION OF A VENTILATED DIFFUSION POROMETER FOR THE MEASUREMENT OF STOMATAL DIFFUSION RESISTANCE OF DOUGLAS FIRNEEDLES  . . . . . . . . . . . . . . . . . . . . . . . .  4  . . . . . . . . . . . . . . . . . . .  5  Abstract 1.  Introduction  . . . . .  5  2.  Porometer Sensing Head ... . . . . . . . . . . . . . . . . .  6  3.  Porometer C i r c u i t r y  8  4.  Porometer Theory . . . . . . . . . . . . . .  5.  Humidity Sensor Calibration Determination  . . . . . . . . . . . . . . . . . . . . . . .  and Porometer Resistance 12  5.1 Humidity sensor c a l i b r a t i o n  14  5.2 Porometer resistance determination 6.  Measurement Procedure  9  ......  14  . . . . . . . . ...... . . . . . •. .  19  7. • Performance Tests  23  7.1 Test plates of known d i f f u s i o n resistance  . . . . . .  23  7.2 Effect of fan speed' . . . . . . . . . . . . . . . . . . . .  24  7.3 Test of porometer theory . . . . . . . . . . . . . . .  32  CHAPTER 2 - CHARACTERISTICS OF STOMATAL DIFFUSION RESISTANCE IN A FOREST EXPOSED TO LARGE SOIL WATER DEFICITS Abstract 1.  Introduction  . . . . . . . . . . . . . . . . . .  . .  40 41 42  ix  Paoe  2.  Experimental Procedure 2.1  Experimental s i t e s  2.2 Measurements 3.  Results  . . . . . . . . .  43  . . . . . . . . . . . . . . . .  43  . . . . . . . . .  and D i s c u s s i o n  . . . . . . •. . . . . . . . . . . . . . .  47  3.-1 V a r i a t i o n i n stomatal r e s i s t a n c e and twig water p o t e n t i a l d u r i n g the summer . . . . .  47  3.2 Daytime v a r i a t i o n i n stomatal r e s i s t a n c e of Dougl as f i r and s a l a l . . . . . . . . . . . . . .  50  3.3 R e l a t i o n s h i p irradiance  52  of stomatal r e s i s t a n c e to . . . . . . . . . . . . . . . . . . .  3.4 R e l a t i o n s h i p of stomatal r e s i s t a n c e to twig water p o t e n t i a l and vapour p r e s s u r e d e f i c i t  4.  44  . . .  54  3.5 R e l a t i o n s h i p o f stomatal r e s i s t a n c e to vapour p r e s s u r e d e f i c i t and s o i l water p o t e n t i a l . . . .  59  Conclusions  62  References  63  CHAPTER 3 - FACTORS AFFECTING THE CANOPY RESISTANCE OF A DOUGLAS FIR  FOREST . . . . . . . . . . . . . . . . . . . . . . .  Abstract  . . . . . . . . . . . . . . . . . . . . . . . .  1.  Introduction  2.  Theory . . . . . .  3.  Experimental Procedure . . . . . . . . . . 3.1  . . . . . .  . . .  . . . .  Experiment s i t e  . . . . . . . . .  . . . . . . . . . . . . .  . . . . .  Results 4.1  67  . .  68  . . .  69 70  . •. . . . •  . . . . . . . .  and D i s c u s s i o n  70 71  . .  74  C o u r s e ' o f s o i l water p o t e n t i a l d u r i n g the measurement p e r i o d . . . . . . . . . . . . . . . . .  74  4.2 Daytime course o f net r a d i a t i o n and l a t e n t heat f l u x f o r two s e l e c t e d days . . . . . . . . . . . . .  76  4.3 Comparison o f daytime courses of stomatal and canopy r e s i s t a n c e . . . . . . . . . . . . . . . . .  79  4.-4 R e l a t i o n s h i p of stomatal r e s i s t a n c e to p l a n t and environmental parameters . . . . . . . . . . .  81  4.5 R e l a t i o n s h i p o f canopy r e s i s t a n c e to water p o t e n t i a l 5.  .  . -.  . . . . . .  3.2 Measurements 4.  .  66  Conclusions References  and vapour p r e s s u r e d e f i c i t  . . . . . . . . . . . . . . . .  .  soil  .  . . .  86  . . . . . . , . . •.  89  . . . . . •. . .  gg  X  Page CHAPTER 4 - A SIMPLE DIFFUSION MODEL OF TRANSPIRATION APPLIED TO A THINNED DOUGLAS FIR  STAND  93  Abstract  94  1.  Introduction  95  2.  Theory  96  3.  Experimental  4.  .  98  3.1  Experimental s i t e  98  3.2  Measurements  99  Results 4.1 4.2 4.3  4.4 5.  Procedure  and D i s c u s s i o n  102  Daytime v a r i a t i o n o f t r a n s p i r a t i o n r a t e w i t h i n the Douglas f i r canopy  102  Daytime v a r i a t i o n o f t r a n s p i r a t i o n r a t e Douglas f i r and s a l a l  102  in  Comparison o f t r a n s p i r a t i o n r a t e s o b t a i n e d from stomatal r e s i s t a n c e and energy balance measurements  104  Dependence o f stand t r a n s p i r a t i o n r a t e on vapour p r e s s u r e d e f i c i t and s o i l water p o t e n t i a l  112  Conclusions  119  References  121  SUMMARY  122  APPENDIX A - Comparison o f l e a f and a i r temperature measurements APPENDIX B - Needle s u r f a c e area d i s t r i b u t i o n s  i n Douglas f i r  . .  . . . .  .124 127  APPENDIX C - Daytime courses o f t r a n s p i r a t i o n r a t e w i t h i n Douglas f i r canopy  134  APPENDIX D - Daytime courses o f t r a n s p i r a t i o n r a t e o f Douglas and s a l a l  137  APPENDIX E - V e r t i c a l  v a r i a t i o n o f v . p . d . w i t h i n Douglas  canopy  fir 140  APPENDIX F - Needle s u r f a c e area versus APPENDIX G - D i r u n a l  fir  D.B.H  energy balance diagrams  143 146  xi  LIST OF TABLES  Table  Page  CHAPTER 1 1.  E f f e c t of temperature on the average sensor c a p a c i t y as.found  2.  The r e l a t i o n s h i p in a i r  3a.  by Turner and Parlange  a  resistance  (r ) g  . . . .  . . . . . . . . . .  25  data a t 24.5 C f o r d r i l l e d brass For  see t e x t  Diffusion  13  vapour  used with v e n t i l a t e d d i f f u s i o n porometer.  symbols, 3b.  study  between the d i f f u s i v i t y o f water  ( a ) and temperature ( T ) . i .  Diffusion plates  (1970) and i n t h i s  (V )  26  resistance  (r  ) data a t 24.5 C f o r p e r f o r a t e d  p l a t e s with and w i t h o u t a 0.3-cm space between p l a t e and b l o t t i n g paper.-  4.  Products,  Ind.,  Diffusion  resistance  plates  68 Harvard S t . ,  B r o o k ! i n e , Mass. 02164  27  ) d a t a a t 24.5 C f o r d r i l l e d brass s used with u n v e n t i l a t e d d i f f u s i o n porometer. For  symbols, 5.  The p l a t e s were purchased from P e r f o r a t e d  (r  see t e x t  . . . . . . . . . . . . .  Comparison o f stomatal  diffusion resistance  (r  28  ) of two  bean p l a n t s measured by v e n t i l a t e d , and u n v e n t i l a t e d diffusion 6.  por.ometers  Comparison o f stomatal Douglas f i r  seedlings  35 resistance  (r  ) of  four.1-year-old  measured with a v e n t i l a t e d  porometer and c a l c u l a t e d from the weight l o s s o f  diffusion the  potted seedlings  37  CHAPTER 2 1.  The r e l a t i o n s h i p of twig water p o t e n t i a l pressure d e f i c i t f o r Douglas f i r periods  ( v . p . d . ) and s o i l  (\p ) to vapour t  water p o t e n t i a l  )  i n the unthinned s t a n d , during the two dry  i n 1974, and i n the t h i n n e d stand  i n 1975, and  Table  salal undergrowth in the thinned stand.  The 90% con-  fidence interval on each mean i s also shown  . . . . . . . . . . .  CHAPTER 4 1.  Summary of leaf area (projected area basis) data f o r the Douglas f i r canopy of the thinned stand at Courtenay, B.C. The leaf area index values were obtained by multiplying by the stand density (840 stems ha~^).  Also included are the  leaf area index values of the undergrowth at three locations. . . 2.  Comparison of average daytime transpiration rates (E) f o r the thinned stand calculated using the simple d i f f u s i o n model (with stomatal resistance measurements) with those measured using the energy balance/Bowen r a t i o method.  Also  shown are the average daytime transpiration rate of the salal and the r a t i o of these values to the corresponding values f o r the entire stand also calculated from stomatal resistance measurements  xiii  LIST OF FIGURES  Figure  Page  CHAPTER 1 1.  Side view of ventilated diffusion porometer head (modified) from Turner et^ al_., 1969).  The parts labelled are: A -  humidity sensor and bead thermistor; B - fan; C - plunger containing desiccant; D - motor; E - rubber coupling; F sealing bearing; G - closed cell foam rubber pads; H - aperture for insertion of needles; I - aperture for broad leaves; J - electrical connector 2.  Metering circuit for ventilated and unventilated diffusion porometer  3.  7  . ...  . . . . . . . . . . . . . . . . . . . ...  10  . . . . . . . . . . . . . . . .  15  Calibration curves for lithium chloride humidity sensor suppl-ted by Hygrodynamics  4.  .  ...  The relative humidity of vapour versus density of aqueous solutions of sulphuric acid for equilibrium conditions at 20 C (from Handbook of Chemistry and Physics, 40th ed. 1958-1959)  5.  16  The relationship between relative humidity and resistance of lithium chloride humidity sensor  6.  ... . . . . .-. . . . . . .  The relationship between porometer current and sensor resis;  tance for three different humidity ranges 7.  . . . . . . . . . . .  20  Calibration curve for unventilated diffusion porometer at 24.5 C  9.  18  Porometer transit time versus time since the sampling of Douglas f i r needles  8.  17  29  Transit time versus fan speed for different values of diffusion resistance ( r ) g  30  xiv  Figure  Page  10.  V o l t a g e versus f a n speed f o r a 6-V Marx-Luder motor . . . . . .  11.  T r a n s i t time versus d i f f u s i o n r e s i s t a n c e of d r i l l e d and perforated plates. as  the s t r a i g h t  .31  The t h e o r e t i c a l r e l a t i o n s h i p , shown  line,  is  equation  (6) with RH-| = 25.3%,  RH = 29.5%, V = 88.4 c m , v = 3 2 c m , A = 2.85 cm and 2 s c r = 2.3 s c r r H . The poor agreement o f the t r i a n g l e data 3  3  2  0  point is  thought to be due to an e r r o r i n p o s i t i o n i n g  beneath porometer a p e r t u r e 12.  .  plate  . . . . . . . . .  ..33  T r a n s i t time versus d i f f u s i o n r e s i s t a n c e o f d r i l l e d and p e r f o r a t e d p l a t e s a t 5 temperatures . . . . . . . . .  34  CHAPTER 2 1.  Seasonal potential stomatal  course of s o i l (^ )> t  water p o t e n t i a l  vapour p r e s s u r e d e f i c i t  resistance  (r  U )J s  (v.pid.)  Seasonal  course of s o i l  potential stomatal 1  water p o t e n t i a l  48 twig water  s  vapour p r e s s u r e d e f i c i t ( v . p . d . ) resistance  (r  and  ) measured each day between 1300 and s  3. - Daytime course o f stomatal at 3 heights  and s a l a l  . . . . . . . . . . . . . 49  resistance . ( r )  o f Douglas  i n the t h i n n e d stand showing  the e f f e c t of d e c r e a s i n g s o i l  4.  .  w )»  1430 PST i n the thinned Douglas f i r stand  shown i s  and  ) measured each day between 1300 and  1430 PST i n the unthinned Douglas f i r stand 2"1  twig water  water p o t e n t i a l  Relationship  between the quanta f l u x d e n s i t y  0 . 4 - 0 . 7 ym band width and stomatal Douglas f i r  resistance  mainly Also  U ). S  the course o f the vapour p r e s s u r e d e f i c i t  (v.p.d.)  (r ) g  of salal  shown at the top of  the graph . Relationship potential  51  The approximate energy f l u x  d e n s i t y i n the 0 . 4 - 0 . 7 ym band width i s  5.  . . .  (Q) f o r the  i n the t h i n n e d and unthinned s t a n d s , . a n d  undergrowth i n the t h i n n e d s t a n d .  fir  53 between stomatal  (IK) for  (a)  resistance  Douglas f i r  (,r^)  and twig water  i n the unthinned s t a n d ,  Figure during the two drying periods in 1974, and in the thinned stand in 1975, and (b) salal undergrowth in the thinned stand 6.  . . . . . .  Relationship between.stomatal resistance ( r ) and twig g  water potential  f o r the 3-, 5- and 7-m heights in  the thinned Douglas f i r canopy.  The data have been  separated into 4 vapour pressure d e f i c i t and 2 s o i l water potential 7.  (v.p.d.) classes  ( i j ^ ) classes  Relationship between stomatal. resistance ( r ) and vapour g  pressure d e f i c i t  (v.p.d.) f o r Douglas f i r in the unthinned  stand, during the two dry periods in 1974, and in the thinned stand in 1975 f o r 4 s o i l water potential 8.  (^ ) ranges g  . .  Relationship between stomatal resistance ( r ) and vapour g  pressure d e f i c i t  (v.p.d.) f o r the salal undergrowth in the  thinned stand f o r 4 s o i l water potential  (^ ) ranges  .  s  CHAPTER 3 1.  Soil  water potential  ) i n the unthinned  period June 17 - August 19, 1974.  stand during the  Each datumpoint i s the  average value of the water potentials at the 15, 30 and 45cm depths 2.  The relationship between the 24-hour value of the r a t i o , LE/R  U  and the s o i l water potential Il  14.0 MJ m  r\  s  -I  day  ) f o r 18 fine (R n  )•days in 1974 in the unthinned  number adjacent to each data point i s the date.  stand.  The  Values of  the average root.zone s o i l water content (e) corresponding to 4) are also shown s r  3.  Daytime course of net radiation  (R ) and latent heat flux n  (LE)i f o r June 18 and July 25, 1974 in the unthinned showing the effects of decreased 4.  stand,  s o i l water potential  (^ ) s  Daytime course of canopy resistance (r ) and stomatal resistance (r ) (projected leaf area basis) at the 8-m height  .  xvi  Figure  Page i n the unthinned stand f o r the days i n F i g . 3 . . . . . . . . . .  5.  The r e l a t i o n s h i p between canopy r e s i s t a n c e resistance for  6.  7.  (r  ) a t the 8-m h e i g h t  (r  ) and  i n the unthinned  stand 82  Daytime courses  of stomatal  resistance  (r  ) at the 8-m  h e i g h t and vapour p r e s s u r e d e f i c i t  ( v . p . d . ) on two days  with s i m i l a r  i n the unthinned stand  soil  The r e l a t i o n s h i p  (^ ).  water p o t e n t i a l s between stomatal  resistance  i n the unthinned stand and s o i l  vapour p r e s s u r e d e f i c i t  (r  . . . .  ) a t the  ranges  of  ( v . p . d . ) and were daytime measure-  ments made on 12 days 8.  The r e l a t i o n s h i p  84  water p o t e n t i a l  The data have been separated i n t o s i x  s  80  stomatal  10 days  8-m h e i g h t  .  85  between the mean daytime canopy  (F7) o f the unthinned stand and s o i l  resistance  water p o t e n t i a l  U ). s  The data have been s e p a r a t e d i n t o t h r e e ranges o f mean daytime vapour p r e s s u r e d e f i c i t  ( v . p . d . ) and were o b t a i n e d over  a p e r i o d o f 18 days  . . . . . . . . . . . . . .  88  CHAPTER 4 1.  Daytime courses  of t r a n s p i r a t i o n  rate  (E)  for 3 layers  in  the t h i n n e d Douglas f i r stand a t high and low v a l u e s o f water p o t e n t i a l  on 2 s e l e c t e d f i n e days when  r e s i s t a n c e was i n t e n s i v e l y measured. radiation 2.  (R ) n  Fig.  1.  The Douglas f i r  net  rate  (E)  103  f o r Douglas  fir  f o r each of the 3 l a y e r s  Also  shown i n the  previous  shown a r e the c o u r s e s o f the net r a d i a t i o n  (R )  ( v . p . d . ) a t 10.5-m h e i g h t  Comparison of the daytime courses o f t r a n s p i r a t i o n o b t a i n e d from stomatal  in  v a l u e s were o b t a i n e d from the sum o f  and vapour p r e s s u r e d e f i c i t 3.  The v a l u e s o f  i n the t h i n n e d stand f o r the same 2 days shown  the v a l u e s figure.  stomatal  shown are f o r the 24-hour p e r i o d  Daytime courses o f t r a n s p i r a t i o n and s a l a l  soil  r e s i s t a n c e and energy  rate  balance  105 (E)  xvi i  Figure  Page  measurements on 3 s e l e c t e d f i n e days at the t h i n n e d stand when the s o i l high.  water p o t e n t i a l  was r e l a t i v e l y  s  The v a l u e s o f net r a d i a t i o n  (R ) shown are f o r n . . . . . . . . . .  the 24-hour p e r i o d 4.  (^ )  Comparison o f the daytime courses o f t r a n s p i r a t i o n (E)  o b t a i n e d from stomatal  rate  r e s i s t a n c e and energy balance  measurements on 3 s e l e c t e d f i n e days a t the t h i n n e d ; stand when the s o i l  water p o t e n t i a l  v a l u e s of net r a d i a t i o n period 5.  106  (R) n  ) was low.  .  The  shown are f o r the 24-hour . .  107  Values o f t r a n s p i r a t i o n r a t e o b t a i n e d from stomatal  resis-  tance measurements [ E ( S . R . M . ) ] p l o t t e d a g a i n s t c o r r e s ponding values [E(E.B.M.)].  o b t a i n e d from energy balance measurements The d a t a were o b t a i n e d from 7 s e l e c t e d f i n e  days when stomatal  r e s i s t a n c e was i n t e n s i v e l y measured i n  the t h i n n e d s t a n d . 6.  Relationship  110  between stomatal  resistance  p r e s s u r e d e f i c i t ( v . p . d . ) f o r the 3 - , i n the t h i n n e d Douglas f i r separated i n t o 3 s o i l  stand.  (r ) g  and vapour  5- and 7-m  heights  The data have been  water p o t e n t i a l  (^ ) s  c l a s s e s and curves  through the r e s p e c t i v e c l a s s e s are d e s c r i b e d b y . e q u a t i o n s (7a, b and c) 7.  i n the t e x t  . . . . . . . . . . . . . . . . . . . .  R e l a t i o n s h i p between stomatal pressure d e f i c i t  resistance  ( v . p . d . ) f o r the s a l a l  $  and vapour  undergrowth i n the  thinned stand.  The data have been separated i n t o 3 s o i l  water p o t e n t i a l  (\p ) c l a s s e s and c u r v e s through the r e s p e c t i v e  c l a s s e s are d e s c r i b e d by equations 8.  (r )  Transpiration rate  (8a, b and c )  (E) o f Douglas f i r and s a l a l  i n the t e x t i n the  t h i n n e d stand as a f u n c t i o n o f vapour p r e s s u r e d e f i c i t (v.p.d.) for 3 d i f f e r e n t soil  water p o t e n t i a l  The curves were c a l c u l a t e d using model  113  (^ ) s  the simple vapour  (equation 6) t o g e t h e r with stomatal  classes. diffusion  resistance  . . .  115  xviii  Figure  Page  characteristics (equations 7 and 8) 9.  n6  Comparison of thinned stand transpiration rates (E) measured by.the energy balance method and calculated using the simple diffusion model (equation 6) together with stomatal resistance characteristics (equations 7 and 8) for 3 different soil water potential (^ ) classes. s  Each datum point is the average of 6 energy balance values obtained over a 3-hour. period . . .... . . .  . .  ns  XIX  ACKNOWLEDGEMENTS  I am g r e a t l y direction, studies.  indebted to m y . s u p e r v i s o r , Dr. T . A . B l a c k , . f o r  assistance Without h i s  his  and f r i e n d s h i p throughout the c o u r s e o f my d o c t o r a l i n s i g h t and encouragement the success o f  this  study would not have been p o s s i b l e . I T.R.  extend s i n c e r e thanks to D r s . T.M. B a l l a r d , P.G. Oke, and C A .  Haddock,  Rowles, who served on my d o c t o r a l committee, and who  have g i v e n a d v i c e and encouragement a t a l l  stages o f t h i s  study.  I a p p r e c i a t e very much the c o o p e r a t i o n of Mr. Joe Nnyamah, generously supplying  soil  in  water p o t e n t i a l data necessary i n my a n a l y s i s .  I wish to thank Mr. Paul Tang and Mr. Ron Toth f o r t h e i r v a l u a b l e i n the i n s t r u m e n t a t i o n .  The a u t h o r acknowledges  the v a l u a b l e  help  assistance  o f Mr. B e r n i e von S p i n d l e r i n d e v e l o p i n g the p h o t o g r a p h i c t e c h n i q u e i  used i n t h i s also  s t u d y , to determine p r o j e c t e d needle a r e a .  Thanks  given to Miss Ruth Hardy, Mr. John C u r t i s , Mr. Sandy  f o r t h e i r help and f r i e n d s h i p i n the f i e l d phase o f t h i s Miss Diane Green f o r t y p i n g the  are  Mcfarlane, s t u d y , and to  thesis.  S i n c e r e thanks go to Dr. Holger B r i x and Mr. Mike Crown o f the Canadian F o r e s t r y S e r v i c e f o r t h e i r a d v i c e and c o o p e r a t i o n d u r i n g the field  phase o f t h i s  Company f o r a l l o w i n g  study.. Also,  I wish t o acknowledge C r o w n - Z e l l e r b a c h  us to conduct the r e s e a r c h on i t s  Thanks are expressed to the N a t i o n a l  Research C o u n c i l o f Canada  g r a n t i n g me a s c h o l a r s h i p d u r i n g my d o c t o r a l for this  property.  study.  for  The r e q u i r e d funds  r e s e a r c h were made a v a i l a b l e through g r a n t s from the N a t i o n a l  Research Council of Canada and B.C. Department of Agriculture, and a contract with the Canadian Forestry Service. Last, but not l e a s t , the patience and encouragement of my wife, Chun-Chih, throughout my academic years, are acknowledged with my sincere a f f e c t i o n .  1  INTRODUCTION The stomatal d i f f u s i o n resistance of leaves has been recognized as an important physiological parameter in plant-water relations as well as in plant growth processes (Kramer, 1974).  The stomatal d i f f u s i o n  resistance of leaves or needles plays a direct role in determining not only the rate at which water vapour i s l o s t from the leaf but also the rate at which CO2 enters the l e a f .  An understanding of stomatal  behaviour under f i e l d conditions i s extremely important in developing accurate methods of estimating plant water use and in studying aspects of the problem of plant water use e f f i c i e n c y . stomatal aperture and environmental  The relationship between  factors f o r various species has  important implications in watershed and stand management practices. Physiological c h a r a c t e r i s t i c s such as stomatal resistance and l e a f area index are required in any r e a l i s t i c  model used to predict the e f f e c t of  thinning on water consumption. The relationships between the stomatal d i f f u s i o n resistance and environmental  parameters were investigated as part of a two-year study  of the e f f e c t of thinning on growth and use of water in a 21-year-old Douglas f i r forest.  This study was supported by grants from the National  Research Council of Canada and the B.C. Department of Agriculture, and a research contract with the Canadian Forestry Service. conducted  Research was  in an unthinned stand in 1974 and, in the following year, in  an adjacent thinned stand on the droughty eastern coast of Vancouver Island. Since, in the f i r s t year of the study, there were two dry periods separated by a short period when the soil was completely rewetted, I was provided with a unique opportunity to study the importance of stress history in changing stomatal resistance c h a r a c t e r i s t i c s .  2  The thesis has been organized as follows.  Chapter 1 deals with  the d i f f i c u l t problem of stomatal resistance measurement in conifers. Chapter 2 summarizes the stomatal resistance c h a r a c t e r i s t i c s of a thinned and an unthinned stand.  Chapter 3 describes how stomatal resistance .  relates to an e x i s t i n g , well-known model of transpiration.  Finally,  Chapter 4 proposes a simple model of stand transpiration which uses stomatal resistance as an input. The unventilated d i f f u s i o n porometer has been widely accepted in the measurement of the stomatal resistance of broad-leaved plants (Kanemasu e t al_., 1969).  The technique of using the ventilated porometer to measure  the stomatal resistance of needled species, in the conventional units of s cm , -1  has yet to be firmly established. For this reason, a s i g n i f i c a n t  portion of my research in 1973 was aimed at evaluating and improving the ventilated d i f f u s i o n porometer obtained from the Connecticut Agricultural Experimental Station, New  Haven, Conn. U.S.A. (Turner e_t al_., 1969), for  use on Douglas f i r needles.  This, as well as a discussion of the technique  of measuring stomatal resistance of needles, i s included in Chapter 1. The objectives of the research discussed in Chapter 2 were (1) to show the e f f e c t of thinning on the c h a r a c t e r i s t i c s of stomatal d i f f u s i o n resistance of Douglas f i r , (2) to determine  the effect of stress history  on the c h a r a c t e r i s t i c s of stomal d i f f u s i o n resistance of Douglas f i r , (3) and to assess the hydrologic importance of the stomatal d i f f u s i o n resistance c h a r a c t e r i s t i c s of undergrowth vegetation in the thinned stand. In t h i s , as well as the following chapters, the importance of vapour pressure d e f i c i t in affecting the stomatal d i f f u s i o n resistance i s c l e a r l y evident.  The two parameters in this study, other than l i g h t , that were  most important in causing changes in stomatal d i f f u s i o n resistance, were the vapour pressure d e f i c i t and s o i l water potential.  3  Little  i s known as to the physiological nature of the forest canopy  resistance and whether i t i s possible to model i t from an understanding of the behaviour of the stomata of the trees.  The objectives of the research  described in Chapter 3 were (1) to show the relationship between canopy and stomatal resistance in the unthinned stand, (2) to describe the relationship between canopy resistance and environmental  parameters,  and  (3) to assess whether canopy resistance behaviour can be inferred from knowledge of stomatal response to environmental  parameters.  Since research in the unthinned stand was completed was written before Chapter 2.  f i r s t , Chapter 3  However, since Chapter 2 i s a general  discussion of stomatal resistance c h a r a c t e r i s t i c s at both s i t e s , i t was f e l t that i t should precede Chapter 3 in the thesis. In the fourth chapter, I consider a simple physical model of stand transpiration that requires as inputs, the vapour pressure d e f i c i t of the canopy a i r , and the stomatal resistance and leaf area index d i s t r i b u t i o n within the canopy.  Estimates of stand transpiration using the model with  measured daytime values of stomatal resistance are compared with energy balance/Bowen ratio measurements of evapotranspiration. The model i s then used together with the stomatal d i f f u s i o n resistance c h a r a c t e r i s t i c s of both the Douglas f i r and the undergrowth to determine the relationship between transpiration rate and vapour pressure d e f i c i t at various degrees of s o i l  dryness.  References Kanemasu, E.T., G.W.  T h u r t e l l , and C B .  Tanner.  1969.  and f i e l d use of a stomatal d i f f u s i o n porometer. 44: 881-885.  Design c a l i b r a t i o n Plant Physiol.  3a  Kramer, P.J.  1974.  F i f t y years of progress i n water relations research.  Plant Physiol. 54: 463-471. Turner, N.C., F.C.C. Peterson, and W.H. d i f f u s i o n porometer f o r f i e l d use. B u l l . No. XXIX: 1-7.  Wright.  1969.  An aspirated  Conn. Agr. Exp. Sta. S o i l s  4  CHAPTER 1  EVALUATION OF A VENTILATED DIFFUSION POROMETER FOR THE MEASUREMENT OF STOMATAL DIFFUSION RESISTANCE OF DOUGLAS FIR NEEDLES  5  EVALUATION OF A VENTILATED DIFFUSION POROMETER FOR THE MEASUREMENT OF STOMATAL DIFFUSION RESISTANCE OF DOUGLAS FIR NEEDLES  Abstract  The theory of the operation of the ventilated d i f f u s i o n porometer as proposed by Turner and Parlange  i s reviewed.  The problems of using  the theory to interpret ventilated porometer data are discussed. ventilated porometer designed  The  by Turner ejt al_. i s evaluated f o r use  in the measurement o f the stomatal  d i f f u s i o n resistance o f Douglas f i r  needles.  1. Introduction  The f i r s t d i f f u s i o n porometer was described by Wallihan  (1964).  Since that time, many modifications of this type of porometer have been reported.  V e n t i l a t i o n of the porometer chamber was introduced to decrease  chamber d i f f u s i o n resistance and to reduce inaccuracies as a r e s u l t of buouyancy effects within the chamber (Turner e_t al_., 1969; Byrne et a l . , 1970).  The reduction of chamber resistance made i t possible to measure  the r e l a t i v e stomatal d i f f u s i o n resistance of small and irregular shaped  6  leaves placed within the chamber.  Turner and Parlange (1970) gave an  analysis of the operation and c a l i b r a t i o n of the ventilated d i f f u s i o n porometer of Turner e_t al_. (1969).  The analysis resulted in an equation  from which the stomatal d i f f u s i o n resistance, in conventional units of s/cmirj,could be calculated from measured changes in chamber humidity. Waggoner and Turner (1971) used this equation to calculate stomatal d i f f u s i o n resistance from ventilated porometer data in a study of transpiration and i t s control by stomata in a pine forest. This chapter evaluates the operation and c a l i b r a t i o n theory proposed by Turner and Parlange for the ventilated porometer when used to measure the stomatal d i f f u s i o n resistance of needle-shaped  leaves.  In addition,  i t describes a procedure f o r the use of the ventilated d i f f u s i o n  porometer  for the measurement of the stomatal d i f f u s i o n resistance of Douglas f i r needles.  2. Porometer Sensing Head  The only difference between the sensing head used in this study and that of Turner et al_. (1969) i s in the replacement of the original fan motor, D in F i g . 1.  Their Ideal Toy Corporation fan motor was found i n -  adequate for continuous use.  Instead, a 6 VDC  instrument motor (R. Marx-  Luder, 1721 Gemmrigheim/neckar, Germany, available from Frew and Gordon Co., Vancouver) was used.  Tests, to be discussed l a t e r , indicated that  7  Fig.  1.  Side view o f v e n t i l a t e d d i f f u s i o n porometer head from T u r n e r ejt al_., 1969).  The p a r t s  (modified  labelled are: A -  humidity sensor and bead t h e r m i s t o r ; B - f a n ; C -  plunger  c o n t a i n i n g d e s i c c a n t ; D - motor; E - rubber c o u p l i n g ; F sealing  bearing; G - closed c e l l  foam rubber pads; H - a p e r t u r e  f o r i n s e r t i o n of n e e d l e s ; I - a p e r t u r e f o r broad l e a v e s ; J electrical  connector.  8  this motor should be operated at 6 V resulting in a fan speed of 2550 rpm.  When stomatal d i f f u s i o n resistance measurements were made, the  a c r y l i c plug, H in Fig. 1, was removed and replaced by a plug formed with modelling clay which supported the needles (see Measurement Procedure).  The chamber drying system, comprised of the fan and the plunger  which contained desiccant (B and C respectively in F i g . 1), was found to be too slow.. Instead,.dry a i r was pumped through a column of s i l i c a gel into the chamber using a hand-pumped rubber bulb.  This system  was  successfully used by Kanemasu et_ aJL (1969) to quickly dry the chamber of their unventilated d i f f u s i o n  porometer.  3. Porometer C i r c u i t r y  The lithium chloride humidity sensor (Hygrosensor 4-4832, Hygrodynamics Inc., S i l v e r Springs, Md., stable AC supply.  U.S.A.), A in F i g . 1, requires a  The AC current through the sensor depends on sensor  resistance, which in turn depends on r e l a t i v e humidity.- Turner et a l . (1969) used a c i r c u i t v i r t u a l l y the same as that of Kanemasu et al_. (1969), in which a stable 24 V peak-to-peak square wave i s provided by a 4t r a n s i s t q r multivibrator c i r c u i t .  The current i s measured by being rec-  t i f i e d f i r s t , then passed through a DC microammeter. in a commercially available unventilated d i f f u s i o n  The c i r c u i t i s used  porometer.  The main problem with this c i r c u i t i s that i t i s c o s t l y , since i t  9  requires many e l e c t r i c a l it.  components and considerable  Consequently, a much simpler c i r c u i t was  of an RCA  time to assemble  designed that makes use  COS/MOS 4030AE integrated c i r c u i t chip (Fig. 2).  This chip  consists of 4 exclusive-OR gates, a l l of which are used to produce a square wave identical to that of Kanemasu et al_. (1969). seen in Fig. 2, the considerably  As can  be  simplified multivibrator c i r c u i t con-  s i s t s of only 4 r e s i s t o r s , 2 capacitors, 2 diodes, 2 6.75-V mercury c e l l s , and a 4030AE IC chip. The improved porometer c i r c u i t requires 1.5 mA, the 3.4 mA  as compared with  required in the c i r c u i t of Kanemasu et_al_. (1969).  the l i f e of the two a considerable  This extends  6.75-V mercury c e l l s to over 600 hours, resulting in  saving in battery expense.  The cost of a l l components  including the printed c i r c u i t , chassis box,,and microammeter i s approximately $60.  A l l components are available in most electronics stores..  The time required for assembling the porometer c i r c u i t r y  i s less than  4 hours.  4. Porometer Theory  -2 The rate of transpiration,(E,(g cm under isothermal  conditions i s given i  E  =  V  -  e s  r  s  +  -1 s  ) into the porometer chamber  by:  e  r  p  (1)  10  Fig.  2.  Metering  c i r c u i t f o r v e n t i l a t e d and u n v e n t i l a t e d  porometer.  diffusion  11  where r  i s the stomatal d i f f u s i o n resistance (s c m ) , r -1  s  p  i s the  porometer resistance, which i s determined mainly by the l e a f boundary layer resistance (s c m ) , T i s the porometer temperature (K), R -1  3 the gas constant for water vapour (4620 mb cm vapour pressure (mb) and e  y  is  1 1 g  -  K  ), e i s the chamber  i s the saturation vapour pressure (mb) at  g  the porometer temperature. If the porometer has an e f f e c t i v e volume (V, cm ) and an aperture 2 -1 area (A, cm ), then the rate of increase of the vapour pressure (mb s ) in the chamber i s given by: A  de dt Substituting  A  E  T  R  v V  =  ( ) 2 K C )  (1) into (2) gives:  de dt  A s " V r + r s p e  =  e  ^  Turner and Parlange (1970) found that the e f f e c t i v e volume i s made up of the actual chamber volume (V )(which i s 32 cm c  f o r t h e i r porometer sensing  head) and the capacity of the sensor to absorb water vapour ( V ) . g  The time (t,s) required for the vapour pressure in the chamber to increase from e-j to e^ i s found by integration of (3). Rearrangement of the resulting expression gives:  r  s  =  A  t r  .  (V. + V J l n  e  6  where ?  s  . . , s l - e  -  r  p  P  s " 2. e  i s the average sensor capacity which i s defined by:  (4)  12  V de _s  V In s  Turner and Parlange found that V  g  (5)  i s independent of chamber vapour  pressure (e) but varied with temperature as shown in Table 1. 9  Since  the lithium chloride sensor responds to relative humidity, i t i s convenient to rewrite (4) in terms of relative humidity (RH, %) as: _  At (V + V )].n s  c  (6)  100 - RH 100 - RH,  The values of RH^ and RH£ are determined by arbitrary end-points on the microammeter (in this study, 3.0 and 6.0 y A ) , and the porometer temperature.  This is because the humidity sensor resistance i s a function  of both relative humidity and temperature.  The time (t) i s referred to  as the transit time, since i t i s the time for the microammeter needle to make a complete transit from 3.0 to 6.0 uA.  5. Humidity Sensor Calibration and Porometer Resistance Determination  In addition to the value of V  c  and the relationship between ¥  $  and  temperature, the two other pieces of information which are required before (6) can be used are ( i ) the dependence of the microammeter current on RH and temperature and ( i i ) the dependence of r on temperature.  13  Table 1.  Effect of temperature on the average sensor capacity (V ) as found by Turner and Parlange (1970) and in this study.  Temperature (C) This study  15  20  25  30  139  112  85  58  137  110  83  56  35  V (cm ) 3  s  Turner and Parlange (1970)  29  14  5.1  Humidity sensor  calibration  Rather than s u p p l y i n g  an RH versus  temperatures with each humidity s e n s o r , o f RH versus  the d i a l  temperatures, as  i n F i g . 3.  the r e l a t i o n s h i p  r e s i s t a n c e graph,  is  the sensor  for  several  the sensor was  i n a c l o s e d vacuum d e s i c -  The r e l a t i o n s h i p between RH and s u l p h u r i c a c i d  solution  5.  dial  calibrated  solution  shown i n F i g . 4.  solutions  The data from a 20 C c a l i b r a t i o n run a r e  Because of the time necessary f o r e q u i l i b r i u m to  o c c u r , one complete run took about 4 days.  Using  the 20 C c u r v e s  5, the r e l a t i o n s h i p between the Hygrodynamics  r e s i s t a n c e was d e t e r m i n e d . in F i g .  instruments  a curve  over a range o f  p l o t t e d in F i g .  3 and F i g .  supplies  between RH and r e s i s t a n c e was determined) a t 20 C.  c a t o r above s u l p h u r i c a c i d - w a t e r  density  test  several  In o r d e r to c o n v e r t from the RH versus  T h i s was accomplished by suspending  densities.  Hygrodynamics  r e a d i n g o f one of i t s  r e a d i n g graph to an RH versus (i.e.  r e s i s t a n c e curve f o r  Using  this  relationship,  dial  in  reading  the remaining  Fig. and  curves  5 were o b t a i n e d .  The r e l a t i o n s h i p s  between microammeter c u r r e n t and sensor  tance f o r the t h r e e c i r c u i t humidity ranges are shown i n F i g .  resis6.  Conse-  q u e n t l y , knowing the v a l u e o f both c u r r e n t and t e m p e r a t u r e , the c o r r e s ponding v a l u e o f RH can be o b t a i n e d u s i n g had s u p p l i e d F i g .  Figs.  5 and 6.  If  Hygrodynamics  5 r a t h e r than F i g . 3, c o n s i d e r a b l e time would  have  been saved.  5.2 Porometer r e s i s t a n c e  determination  The porometer r e s i s t a n c e  (r  )can be e a s i l y c a l c u l a t e d from  (6)  15  ig.  3.  C a l i b r a t i o n curves f o r l i t h i u m c h l o r i d e humidity sensor s u p p l i e d by  Hygrodynamics.  16  Fig. 4.  The r e l a t i v e humidity of vapour versus density of aqueous solutions of sulphuric acid for equilibrium conditions at 20 C (from Handbook of Chemistry and Physics, 40th ed. 1958-1959).  5.  The r e l a t i o n s h i p between r e l a t i v e humidity and r e s i s t a n c e o f l i t h i u m c h l o r i d e humidity  sensor.  18  RESISTANCE  Fig. 6.  (KAJ  The relationship between porometer current and sensor resistance for three different humidity ranges.  5000  19  when the stomatal diffusion resistance(r ) i s zero. v s  In the case of  7  f l a t leaves, this i s accomplished by placing a sheet of wet blotting paper between the foam pads, G i n F i g . 1, then noting the t r a n s i t time, t.  In the case of needles, rp i s obtained by inserting dead needles,  that have been dipped in water containing wetting agent, into the chamber (see Measurement Procedure).  The number of needles used to obtain rp  should be the same as that used i n stomatal d i f f u s i o n resistance measurements.  The value o f . r  mately 0.8 s cm . -1  0.5%/C.  f o r four Douglas f i r needles at 20 C i s approxi-  I t decreases with increasing temperature by roughly  This variation need not be taken into account when stomatal  resistances are greater than 10 s cm" . 1  6. Measurement Procedure  Unlike broad leaves, needles must be removed from the tree in order to make a measurement with the ventilated d i f f u s i o n porometer.  The  measurement should be made as soon as possible and as close as possible to the o r i g i n a l location of the needles on the tree.  A change in l i g h t  i n t e n s i t y can cause a change in stomatal d i f f u s i o n resistance.  Fig. 7  shows how t r a n s i t time, and therefore stomatal d i f f u s i o n resistance, increases with time since a sample of Douglas f i r needles were removed from the tree.  This experiment was done during August, 1972, at the  U.B.C. Research Forest at Haney, B.C. in a 13-year old plantation.  The  20  TIME  Fig.  7.  Porometer t r a n s i t o f Douglas f i r  (min )  time versus  needles.  time s i n c e the  sampling  21  needles e x p e r i e n c e d a d e c r e a s e i n l i g h t  i n t e n s i t y s i n c e they were  removed from r o u g h l y the middle o f the t r e e and taken to near level.  ground  S i n c e the measurements were made on the same sample, the  d r y i n g probably hastened c l o s u r e .  C o n s e q u e n t l y , the graph p r o v i d e s an  e s t i m a t e o f the minimum time b e f o r e r e s i s t a n c e  increase is  The graph suggests t h a t measurement should be made w i t h i n needle removal  from the  excessive  significant. 5 minutes  of  tree.  Exposure o f the needles to porometer chamber a i r , which has an RH near 25%, should be.kept to a minimum.  The c h o i c e o f the a p p r o p r i a t e  humidity range switch p o s i t i o n should keep t r a n s i t A c c u r a t e stomatal  d i f f u s i o n r e s i s t a n c e measurement depends on the  number and placement o f needles Turner small  times to l e s s than 60 s.  i n the porometer chamber.  (1971) i n s e r t e d 5 f a s c i c l e s  (10 n e e d l e s ) o f red pine through  a p e r t u r e , c o n t a i n i n g the a c r y l i c p l u g , H i n F i g . 1.  the best way o f using measurement, t e s t s  Waggoner and  the instrument f o r Douglas f i r  In  stomatal  the  studying resistance  were f i r s t made i n which a 2-cm s e c t i o n o f a t w i g  b e a r i n g a p p r o x i m a t e l y 8 to 10 needles was i n s e r t e d through the a p e r t u r e , I  i n F i g . 1,  since this  the s m a l l e r a p e r t u r e . that inserting  amount o f m a t e r i a l  c o u l d not be i n s e r t e d  T h i s proved u n s a t i s f a c t o r y .  4 individual  turn was i n s e r t e d t i g h t l y  through  I n s t e a d , i t was  found  needles i n t o a m o d e l l i n g c l a y p l u g , which  i n t o the s m a l l e r a p e r t u r e gave good  in  results.  The broken ends o f the needles must be c a r e f u l l y i n s e r t e d i n t o the c l a y to p r e v e n t them from being vapour s o u r c e s .  The needle area .(A, p r o j e c t e d 2  area b a s i s ) , If sensor  of f o u r Douglas f i r needles i s  a p p r o x i m a t e l y 1.5  to 2 cm .  the porometer has not been used f o r s e v e r a l days or more, the should be hydrated and dehydrated s e v e r a l  times b e f o r e use.  The  22  sensor  should be s t o r e d with d e s i c c a n t when not being used f o r an  extended p e r i o d of t i m e . t a n c e , , the f o l l o w i n g . <;  To make a measurement o f the stomatal  steps  should be f o l l o w e d :  1. A d j u s t microammeter needle to zero when porometer switch 2. Switch t o p o s i t i o n FS ( f u l l with f u l l  scale adjust  scale)  and a d j u s t  s  In  off.  knob. (H^ or Hg f o r  < 20 s cm"^, H„ f o r r > 20 s cm~^). 2 s  4. Pump dry a i r 5.  is  needle t o 10 yA  3. S e l e c t and switch t o the a p p r o p r i a t e humidity range r  resis-  i n t o the chamber u n t i l  the microammeter reads 1.0  the case o f n e e d l e s : Shape a plug from c l e a n m o d e l l i n g c l a y  r e p l a c e the a c r y l i c p l u g , H i n F i g . 1. ( i n the case o f Douglas f i r )  yA. to  G e n t l y i n s e r t f o u r needles  i n t o the end o f the p l u g .  Make sure  the broken end o f the needles are s e a l e d i n t o the c l a y . In the case o f broad l e a v e s : . P l a c e the l e a f between the foam pads, G i n F i g .  1.  6. Turn on the f a n , and r e c o r d . t h e time  (transit  time) f o r the  needle to move from 3.0 yA to 6.0 yA. 7. Switch to p o s i t i o n T and r e c o r d c u r r e n t through the t h e r m i s t o r . The temperature can be o b t a i n e d from the t h e r m i s t o r  calibration  curve. 8. Switch to OFF to conserve b a t t e r y power. 9. In  the case o f n e e d l e s : Remove the f o u r needles and p l a c e them i n  a sample bag f o r l a t e r d e t e r m i n a t i o n of needle a r e a , A. In  the case o f broad l e a v e s :  aperture I  in Fig.  The area i s  simply the area  of  1.  To determine r p , the above procedure i s  f o l l o w e d except t h a t 4 dead  needles dipped i n water c o n t a i n i n g w e t t i n g agent are used.  The d e t e r m i n a t i o n  23  of r  serves  to check the porometer e l e c t r o n i c s , so i t . i s  doing at l e a s t once a day. mination of r  The use of wet b l o t t i n g  well  worth  paper i n the d e t e r -  can be used i n the f i e l d to check the e l e c t r o n i c s , which P  i s more c o n v e n i e n t than using  the wet n e e d l e s .  7. Performance T e s t s  7.1  Test plates  of.known d i f f u s i o n  resistance  In o r d e r to t e s t the a c c u r a c y o f were r e q u i r e d .  These d i f f u s i o n  p e r f o r a t e d p l a t e s w i t h holes  (6),  resistances  known d i f f u s i o n  resistances  were o b t a i n e d by  l e s s than 130 microns  using  i n diameter and d r i l l e d  brass p l a t e s with holes o f 0.5 and 1.0 mm i n d i a m e t e r .  Drilled  brass  p l a t e s were used by Kanemasu e t al_. (1969) i n the c a l i b r a t i o n o f an unventilated d i f f u s i o n i t was o r i g i n a l l y  porometer.  thought  p e n e t r a t e the l a r g e  The p e r f o r a t e d p l a t e s were used  t h a t e d d i e s from the porometer f a n would  holes and c o n s e q u e n t l y reduce the d i f f u s i o n  of the brass p l a t e s .  because  L a t e r i t was found t h a t t h i s  e f f e c t was  resistance  not  observable. The d i f f u s i o n r e s i s t a n c e ( r from the Brown and Escpmbe  (L +  r S  where n i s  ncnrd^  )of  the brass p l a t e s was  calculated  (1900) f o r m u l a :  —  )  (7)  4  the number o f holes per .unit Jarea? o f ' . p l a t e  ! ?  L is  the. p l a t e  24  thickness  (cm), d i s  the hole diameter 2  o f water vapour i n a i r  (cm  using  (7)  the  diffusivity  -1 s~  tance data f o r . t h e brass p l a t e s The d i f f u s i o n  (cm), and a i s  resistances  ) (see T a b l e 2 ) . is  The d i f f u s i o n  resis-  shown i n T a b l e 3a.  o f the p e r f o r a t e d p l a t e s were not c a l c u l a t e d  because o f t h e i r i r r e g u l a r  measured by an u n v e n t i l a t e d d i f f u s i o n  hole geometry.  I n s t e a d , they were  porometer c a l i b r a t e d using  a separate  s e t of d r i l l e d brass p l a t e s with c a l c u l a t e d d i f f u s i o n r e s i s t a n c e s i n T a b l e 4. 24.5 C i s  The c a l i b r a t i o n curve f o r t h i s  shown i n F i g . 8.  perforated plates  shown  u n v e n t i l a t e d porometer  The measured d i f f u s i o n  resistances  of  at the  are shown i n T a b l e 3b.  Each brass p l a t e was p l a c e d between the foam pads s u r r o u n d i n g I  (Fig.  aperture,  1) o f the v e n t i l a t e d porometer head.. Wet b l o t t i n g paper was  between the brass p l a t e and the o u t e r pad. perforated plate a c r y l i c frame.  and the b l o t t i n g  placed  A 0.3-cm space between the  paper was maintained by means o f an  T h i s prevented l i q u i d water from e n t e r i n g the holes  of  the p e r f o r a t e d p l a t e .  7.2 E f f e c t o f fan  speed  The r e l a t i o n s h i p between t r a n s i t for plates  of d i f f e r e n t d i f f u s i o n  time  (3 to 6 yA) and f a n speed  resistance  is  shown i n F i g . 9.  transit  time approaches  This  a r e s u l t o f the porometer r e s i s t a n c e approaching a steady  is  a steady v a l u e f o r f a n speeds  as shown by the curve o f zero d i f f u s i o n r e s i s t a n c e . voltage/fan  The  exceeding 2500 rpm.  These r e s u l t s  value, and  spee.d r e l a t i o n s h i p shown i n F i g . 10 suggest t h a t the fan motor  be operated at no l e s s than the maximum motor v o l t a g e  o f 6 V.  T a b l e 2.  The r e l a t i o n s h i p between the d i f f u s i v i t y o f water in a i r  (a)and temperature •••(T ).  T (0 a  -  10  2 a  (cm  -1 s  0.211  0  0.226  10  0.241  20  0.257  30  0.273  40  0.289  )  vapour  Table 3a. Diffusion resistance (r ) data at 24.5 C for drilled brass plates used with ventilated diffusion porometer. For symbols, see text.  Plate No.  L (cm)  n  d (cm)  a r (cm)  1  0.155  31  0.1016  2.73  10.3  2  0.160  19  0.1016  4.56  17.3  3  0.315  19  0.1016  7.52  28.5  4  0.140  31  0.0508  8.39  31.8  g  r  g  (s cm" )  27  T a b l e 3b.  Diffusion  resistance  (r ) g  data at 24.5 C f o r p e r f o r a t e d p l a t e s  with and without a 0.3-cm space between p l a t e and b l o t t i n g paper.  The p l a t e s were purchased from P e r f o r a t e d P r o d u c t s ,  68 Harvard S t . ,  B r o o k l i n e , Mass. 02146.  with P l a t e No.  ar  s  (cm)  without  space r  s  (s cm" )  r  s  (s  space cm" )  20 W ( C o n i c a l )  2.88  10.9  9.8  25 W ( C o n i c a l )  2.14  8.1  7.0  50 W (Conical  1.16  4.4  3.3  60 W ( C o n i c a l )  0.92  3.5  2.4  125 W ( C o n i c a l )  0.66  2.5  1.4  40 T  (Conical)  0.90  3.4  2.3  60 T  (Conical)  0.63  2.4  1.3  100 T  (Conical)  0.61  2.3  1.2  1.0  3.8  2.7  1.0  3.8  2.7  60QT (WentUKi) 100 T  (Venturi)  1  Inc.,  28  Table 4.  Diffusion resistance•(r ) data at 24.5 C for drilled brass plates used with unventilated diffusion porometer.  For symbols,  see text.  Plate No.  L (cm)  n  d (cm)  ar  g  (cm)  r  g  (s cm" )  1  0.310  —  —  0.31  1.2  2  0.160  30  0.10  2.45  9.3  3  0.305  30  0.10  3.94  14.9  4  0.160  35  0.05  7.02  26.6  5  0.310  35  0.05  12.31  46.6  U N V E N T I L A T E D DIFFUSION ( M E T E R NO. 2 )  POROMETER  24 5 C  Fig. 8.  Calibration curve for unventilated diffusion porometer at 24.5 C.  9.  Transit time versus fan speed for different values of diffusion resistance  (r ).  Fig. 10.  Voltage versus fan speed f o r a 6-V Marx-Luder motor.  32  7.3  Test o f porometer t h e o r y Ten p e r f o r a t e d p l a t e s  diffusion  t e s t are shown i n F i g . 11.  against plate diffusion  (6).  is  known  r e s i s t a n c e were used to t e s t v e n t i l a t e d porometer t h e o r y .  Results of t h i s  figure  and f o u r d r i l l e d brass p l a t e s o f  resistance.,  Transit  The s t r a i g h t  time was  plotted  l i n e shown i n the  the t h e o r e t i c a l r e l a t i o n s h i p expected and was c a l c u l a t e d  F i g . 11 shows e x c e l l e n t agreement between the experimental  and porometer t h e o r y .  T h i s agreement f o r brass p l a t e s  observable reduction in t h e i r d i f f u s i o n p e n e t r a t i n g the d r i l l e d Fig.  results  indicates  no  r e s i s t a n c e as a r e s u l t o f  a t 5 t e m p e r a t u r e s . . The l i n e s drawn r e p r e s e n t the i n d i v i d u a l Using  eddies  holes.  12 shows c a l i b r a t i o n data f o r the v e n t i l a t e d d i f f u s i o n  equations..  using  these s l o p e s  and a V  c a l c u l a t e d f o r each temperature. same temperatures f o r which V and are shown i n T a b l e 1.  g  c  v a l u e of 32 cm  in  porometer  regression  (6),  ^  s  was  These r e s u l t s were e x t r a p o l a t e d to the  was r e p o r t e d by Turner and Parlange  As e x p e c t e d , the agreement i s  (1970)  good.  Two bean p l a n t s were a l s o used to t e s t v e n t i l a t e d porometer t h e o r y . In  this  c a s e , the c a l i b r a t e d u n v e n t i l a t e d porometer was used to measure  the stomatal  d i f f u s i o n r e s i s t a n c e o f the a b a x i a l  Measurement,  using  (lower)  leaf  the v e n t i l a t e d porometer, was made w i t h i n 2 minutes  of each u n v e n t i l a t e d porometer measurement.  The r e s u l t s  t h a t the v e n t i l a t e d porometer g i v e s s l i g h t l y  h i g h e r stomatal  values.  T h i s probably r e s u l t s  a f t e r using still  w i t h i n experimental  i n T a b l e 5 show resistance  from the tendency o f the stomata  the u n v e n t i l a t e d porometer.  As a f i n a l  surfaces.  to  close  N e v e r t h e l e s s , the agreement  error.  t e s t , the stomatal  diffusion  r e s i s t a n c e o f needles on  is  33  Fig.  11.  T r a n s i t time versus  d i f f u s i o n r e s i s t a n c e of d r i l l e d and  perforated plates.  The t h e o r e t i c a l r e l a t i o n s h i p , shown as  the s t r a i g h t  l i n e , is  equation  • 29.5%, V = 88.4 c m , \/ =-32 s ' c 3  s cm" .  (6) w i t h RH-j = 25.3%, Rh^ = c m , A = 2.85 cm 3  The poor agreement of the t r i a n g l e  to be due to an e r r o r i n p o s i t i o n i n g aperture.  2  and r  p  =  data p o i n t i s  2.3 thought  p l a t e beneath porometer  34  VENTILATED POROMETER LiCI SENSOR No.598037  TRANSIT TIME (sec)  Fig.  12.  T r a n s i t time versus d i f f u s i o n perforated plates  r e s i s t a n c e o f d r i l l e d and  a t 5 temperatures.  35  Table 5.  Comparison of stomatal diffusion resistance (r ) of two bean plants measured by ventilated and unventilated diffusion porpmeters.  r  (s cm" ) s  Plant No.  Unventilated Porometer  Average  Average  Venti1ated Porometer  3.5  4.0  2.8  2.9  3.0  3.0  3.0  Average  3.3  5.0  5.3  5.5  5.7  4.8  5.5  5.0  Average  5.5  36  f o u r 1-year o l d Douglas f i r  seedlings  was measured with the v e n t i l a t e d  porometer and c a l c u l a t e d from the weight l o s s of the same p o t t e d s e e d lings.  The average stomatal  r e s i s t a n c e of the needles o f a  s e e d l i n g was c a l c u l a t e d using  (1).  Aluminum f o i l  prevented e v a p o r a t i o n from the s o i l .  c o v e r i n g the  rate.  l a y e r r e s i s t a n c e o f the needles was made n e g l i g i b l y  four seedlings  sample.  small  The boundary compared to  4 - n e e d l e samples were removed from the  r e s p e c t i v e l y , at roughly  V e n t i l a t e d porometer t r a n s i t  The needle area o f each sample  e q u a l l y spaced s e e d l i n g  ;  height  times were measured f o r  This c a l c u l a t i o n  to w i t h i n 6% o f a photographic method of needle area measurement i n Chapter 4.  The v e n t i l a t e d porometer v a l u e o f the stomatal  r e s i s t a n c e o f each sample w a s . c a l c u l a t e d The r e s u l t s  of this  diffusion  was  agreed discussed  diffusion  using (6).  comparison a r e shown.in T a b l e 6.  average values o f stomatal  each  ( p r o j e c t e d l e a f area b a s i s )  c a l c u l a t e d from length anduWnidth measurements.  measurements  pressure  r e s i s t a n c e , using a f a n .  T h r e e , f o u r , f i v e and s i x  intervals.  The temperature  thermocouples, w h i l e the vapour  o f the a i r was measured with an a s p i r a t e d psychrometer.  the stomatal  soil  The weight l o s s over a p e r i o d o f  one hour was used as an e s t i m a t e of the t r a n s p i r a t i o n of the needles was measured u s i n g  single  Since  the  r e s i s t a n c e o b t a i n e d from porometer  agreed to w i t h i n 10% o f the v a l u e s o b t a i n e d from the  weight  l o s s . t e c h n i q u e , the porometer was c o n s i d e r e d to be a c c u r a t e enough to make f i e l d measurements o f stomatal  r e s i s t a n c e o f Douglas f i r n e e d l e s .  r e c e n t s t u d y , Gandar and Tanner (1976) suggest t h a t m a t e r i a l s  such  stainless  steel  diffusion  porometers because o f t h e i r r e l a t i v e l y low water vapour  In a as  or waxed s u r f a c e s a r e p r e f e r a b l e f o r the c o n s t r u c t i o n  of  37  T a b l e 6.  Comparison o f stomatal Douglas f i r s e e d l i n g s  resistance  (r  ) of four  1-year-old  measured with a v e n t i l a t e d d i f f u s i o n  porometer and c a l c u l a t e d from the weight l o s s o f the p o t t e d seed!ings..  r  Seedling  No.  s  (s cm" )  1  2  3  4  4.2  7.2  9.5  12.4  4.1  8.4  10.7  10.1  3.4  7.9  11.6  11.1  4,8  7.1  10.1  11.8  5.2  9.8  11.5  10.0  11.8  C a l c u l a t e d from weight  loss  Measured with , ventilated porometer  12.1 AVERAGE  4.1  7.2  10.4  11.4  38  sorptivities.  It should be noted that the e f f e c t of water vapour  sorption on the a c r y l i c walls of the porometer  used in this study i s  taken into account by c a l i b r a t i n g over a reasonably broad d i f f u s i o n resistance range and by drying out of the chamber to the same humidity value before every measurement.  39  References  Brown, H. and F. Escombe. 1900.  S t a t i c d i f f u s i o n of gases and  liquids i n relation to the assimilation of carbon and translocation i n plants.  P h i l . Trans. Roy. Soc. London, Ser. B.,  B i o l . S c i . 193: 322-291. Byrne, G.F., C.W.  Rose and R.O. Slatyer.  1970.  An aspirated  d i f f u s i o n porometer. Agr. Meteorol. 7: 39-44. Gandar, P.W.  and C B , Tanner.  1976.  Water vapour sorption by the  walls and sensors of stomatal d i f f u s i o n porometers. Agron. J . 68: 245-249. Kanemasu, E.T., G.W.  Thurtell and C B . Tanner.  1969.  Design,  c a l i b r a t i o n and f i e l d use of a stomatal d i f f u s i o n porometer. Plant Physiol. 44: 881-885. Turner, N.C,  F.C.C. Pederson and W.H.  Wright.  d i f f u s i o n porometer for f i e l d use.  1969.  An aspirated  Conn. Agr. Sta. Special  Soils B u l l . 29, 12 p. Turner, N.C. and J.Y. Parlange.  1970.  Analysis of operation and  c a l i b r a t i o n of a ventilated d i f f u s i o n porometer.  Plant Physiol.  46: 175-177. Waggoner, P.E. and N.C. Turner. by stomata in a pine forest. Wallihan, E.F. 1964.  1971.  Transpiration and i t s control  Conn. Agr. Exp. Sta. B u l l . 726.  Modification and use of an e l e c t r i c hygrometer  for estimating r e l a t i v e stomatal aperture. 86-90.  Plant Physiol 39:  CHAPTER 2  CHARACTERISTICS OF STOMATAL DIFFUSION RESISTANCE IN A FOREST EXPOSED TO LARGE SOIL WATER DEFICITS  41  CHARACTERISTICS OF STOMATAL DIFFUSION RESISTANCE IN A FOREST EXPOSED TO LARGE SOIL WATER DEFICITS  Abstract  As part of a two-year study of the e f f e c t of thinning on evapotranspiration in Douglas f i r , the relationship between stomatal d i f f u s i o n resistance (r ) and environmental parameters were studied.  Research was  conducted in an unthinned stand (1840 stems h a ) with negligible under- 1  growth and a thinned stand (840 stems h a ) with substantial salal under- 1  growth. the  A ventilated d i f f u s i o n porometer was used to measure r . In  thinned stand, during the daytime, r  g  was mainly related to the  s o i l water potential (\\> ) and the vapour pressure d e f i c i t (v.p.d.) of the  canopy a i r .  The same relationship was observed in the upper portion  of the unthinned stand. from 2 to 60 s cm  -1  Daytime values of r  g  f o r Douglas f i r ranged  f o r values of v.p.d. between 4 and 24 mb and values  of ip between 0 and -12.5 bars.  Although increasing r  ated with decreasing twig water potential to be associated with increasing ^  g  was usually associ-  (^ )» increasing r t  when the v.p.d. was high.  history was found to cause a s h i f t in the relationship of r had l i t t l e effect on the relationship of r values of r  s  s  f o r salal ranged from 2 to 45 s cm .  appeared Stress t o b u t  s  to v.p.d. and \\>. -1  g  g  Daytime  These stomatal  Presented by the author at 68th Annual American Society of Agronomy Meeting, Nov. 29 to Dec. 2, 1976.  42  c h a r a c t e r i s t i c s indicate that as the soil dried out, salal  transpiration  accounted f o r an increasing f r a c t i o n of the total water loss by the thinned stand.  1.  Introduction  Both tree growth and transpiration are d i r e c t l y related to stomatal resistance and water potential of leaves or needles.  Consequently there  has been a good deal of research in recent years aimed at determining how stomatal resistance and l e a f water potential are related to parameters describing 1974,  the physical  environment (Slavik, 1970; Schulze et a l . ,  1975; Jordan et al_., 1975; Hinckley et al_., 1975; Running, 1976;  Fetcher, 1976; Cline and Campbell, 1976; Tan and Black, 1976). authors (Jordan and Ritchie, 1971; McCree,, 1974;  Several  Hinckl ey et al_., 1975;  Brown et_ al_., 1976; Thomas et al_., 1976) have also indicated  that  stress history could change the relationship between stomatal d i f f u s i o n resistance and l e a f water p o t e n t i a l .  Physically based models of forest  growth and water use require these relationships. tion of post-thinning  In addition,.the  descrip-  stress r e l i e s on a knowledge of the relationship  between these physiological  and environmental parameters.  While the  importance of the role of the stomata has been recognized f o r some time, only recently has the behaviour of the stomata of conifers been studied (Waggoner and Turner, 1971; Running, 1976).  This i s because of the  43  development of ventilated and null-balance types of d i f f u s i o n porometers and methods of c a l i b r a t i n g them (Turner and Pariange, 1970; Beardsell et/ al_., 1972). As part.of a two-year study of the e f f e c t of thinning on tree water stress in a 21-year-old Douglas f i r forest, the relationships between, the stomatal d i f f u s i o n resistance and environmental studied.  Research was conducted  parameters were  in an unthinned stand with n e g l i g i b l e  undergrowth and in an adjacent thinned stand with substantial undergrowth. Since there.were two dry periods separated by a short period when the s o i l was completely rewetted in the f i r s t year of the study, " This provided a unique opportunity to study the importance  of stress  history i n changing stomatal resistance c h a r a c t e r i s t i c s . The objectives of this Chapter are (1) to show the e f f e c t of thinning on the c h a r a c t e r i s t i c s of stomatal d i f f u s i o n resistance of Douglas f i r , (2) to describe the e f f e c t of stress history.on the c h a r a c t e r i s t i c s of stomatal d i f f u s i o n resistance of Douglas f i r ,  (3) and to assess the hydro-  logic importance of the stomatal d i f f u s i o n resistance c h a r a c t e r i s t i c s of undergrowth vegetation in the thinned stand.  2. Experimental  2.1 Experimental  Procedure  sites  The research was conducted on Crown-Zellerbach  land approximately 27  km northwest of Courtenay,.B.C. on the eastern coast of Vancouver Island.  44  The topography i s generally f l a t , although there are several ridges of approximately 20-30 m r e l i e f , and some depressional areas of a swampy nature which dry out during the summer.  The sites were located at an  elevation of 150 m and were approximately  1 1/2 km apart.  The measure-  ments were made i n an unthinned and a thinned stand of Douglas f i r (Pseudotsuga unthinned  menziesii (Mirb.) Franco) trees planted in 1953. In the  stand which was studied in 1974, there were approximately 1840  stems ha"^ and the trees ranged in height from 7 to 10 m, and averaged 10.6 cm in diameter at breast height.  In the thinned stand which was  studied in 1975, there were approximately 840 stems ha~^ and the trees were very similar in height and diameter to those i n the unthinned  stand.  There was v i r t u a l l y no undergrowth i n the unthinned stand, while in the thinned stand, there was considerable undergrowth, which was mainly salal (Gaultheria shall on Pursh).  Over half of the thinning was done in 1974  and i t was completed in May, 1975. The s o i l to the same soil sandy  i n the thinned stand  series as that of the unthinned  belonged  stand (Dashwood gravelly  loam,, Duric Humo-Ferric Podzol).  2.2 Measurements A l l the measurements in the unthinned stand were made over the period June 12 to August 18, 1974 and in the thinned stand, over the period June 18 to August 11, 1975. In 1974, there were two extensive periods of dry weather; the f i r s t ended on July 8, the second began July 17.  Heavy rains f e l l at the s i t e between these two dates.  In 1975, there  was only one extensive period of dry weather which began June 29 following  45  prolonged r a i n . The same measurement techniques were used f o r both the unthinned and the  thinned stands.  Soil water potential  ) between 0 and -1 bar was  measured by a tensiometer^-pressure transducer system.  Four tensiometers  were used and were located at depths of 15, 30, 45 and 60 cm.  Soil water  potential less than -1 bar was measured using a Wescor HR-33T dew point microvoltmeter and PT 51-10 hygrometers.  Six hygrometers, two each at the  15- and 30-cm depth, and one each at the ,45- and 60-m depth, were used. Besides the above-mentioned measurements used in both stands, an additional tensiometer and hygrometer measurements were made at the 75-cm depth in the thinned stand.  Soil water potential measured by the tensiometer-transducer  system was recorded at 15-minute i n t e r v a l s , while s o i l water potentials measured by the hygrometers were recorded 3 times each day. only d a i l y average values of-^ For 1977.  In this study,  were used in the analysis.  further details of the s o i l water measurements, see J.U. Nnyamah,  Root water uptake in a Douglas f i r forest. Ph.D. Thesis, Department  of Soil Science, University of B r i t i s h Columbia;,Twig water potentials Gj^) were measured with a Scholander pressure chamber (Scholander e_tal_., 1965).  In the unthinned stand, ^  t  at the 8-m  height was measured on a routine basis 3 times a day (0700 to 0830, 1300 to 1430 and 1600 to 1730 PST) on the majority of days during the period June 12 to August 18.  Measurements of ^  t  at the same height were made  every 2 hours during the daytime on 12 days.  In the thinned stand, ^  was  measured over the period June.18 to. August 4, 1975 at the 0.5- ( s a l a l ) , 3-, 5- and 7-m heights.  Over this same period, routine measurements (3  times during the daytime) were made on 18 days, while intensive measure^ ments (every 2 to 3 hours during the daytime) were made on 4 days. least 2 twig samples were used to obtain each value of ^  f  At  and 2 readings  46  were taken f o r each twig.  The xylem sap osmotic potential was v e r i f i e d  to be n e g l i g i b l y small using a Wescor HR-33T dew point microvoltmeter and hygrometer chamber. Stomatal resistance ( r ) measurements were made using a ventilated g  porometer designed by Turner e_t al_. (1969). was discussed by Turner and Parlange (1970).  The theory.of this  porometer  Modification of the porometer  electronics and the sensing head, and the c a l i b r a t i o n procedure used are described in 'Chapter 1.  ?  !  In the unthinned stand, the stomatal resistance at the 8-m height was measured on a routine basis 3 times a day on the majority of days during the measurement period., Intensive measurements (every 2 hours during the daytime) at this same height were made on 12 days. thinned stand, the measurements of r 5- and 7-m heights.  g  In the  were made at the 0.5- ( s a l a l ) , 3-,  Over the experiment period,.routine measurements  (3 times during the daytime) were made on 22 days, while intensive measurements (every 2 to 3 hours during the daytime) were made on 7 days.  A  sample of four needles was brought down from the tree, attached to the clay plug for insertion as in the case of the seedlings.  The samples  were stored in p l a s t i c bags for the determination of needle.area l a t e r in the day.  The time between the removal  of the needles from the branch and  the completion of a measurement was kept to less than 4 minutes.  It was  found that stomatal closure does not occur in Douglas f i r needles u n t i l after about,5 minutes  (see Cjhapter 1).  At least 2 samples of 4 needles  each were used to obtain a single value of r .• On August 6, 1974, in the unthinned stand, r  g  and l i g h t quanta flux  (0.4 to 0.7 ym) were measured every 3 hours at the 4-, 6- and 8-m heights,  47  while on August 1 and 2 these measurements were made at the 6- and 8-m heights.  Light quanta flux was measured with a Lambda model LI-l85  l i g h t meter with the quantum sensor held by hand, near the point at which the needles were removed from the tree.  Wet and dry bulb  were measured with 3 silicon-diode psychrometers  temperatures  located at 4-, 6- and  8-m heights and recorded every 15 minutes by a data logger, which provided canopy vapour pressure d e f i c i t (v.p.d.) p r o f i l e s over the measurement period.In the thinned stand, the v.p.d. and l i g h t quanta flux measurements were made at the 0.5- ( s a l a l ) , 3-, 5- and 7-m heights.- The l i g h t p r o f i l e measurements were made on 6 days, while the v.p.d. p r o f i l e measurements were made on 36 days.  Wet and dry bulb temperatures  were measured either  with an Assmann psychrometer or 3 silicon-diode psychrometers the 3-, 5- and 7-m heights. temperature  Since needle temperature  located at  was close to a i r  (Appendix A), the v.p.d. can be regarded as a good approxi-  mation of the difference between the vapour pressure within the stomatal c a v i t i e s and in the canopy a i r .  3. Results and Discussion  3.1 Variation in stomatal resistance and twig Water potential during the summer In Figs. 1 and 2, the values of if; , IJJ. , r D  L  and v.p.d. at the 8-m  o  height in the unthinned stand and the 7-m height in the thinned stand  48  J  10  I  I  20  I  30  I  10  JUNE  I  20 JULY  •  I  30  1  I  L  10  20  AUGUST  Fig. 1. Seasonal course of soil water potential (^ )> twig water s  potential  (<J> )» t  vapour pressure deficit (v.p.d.), and  stomatal resistance ( r ) measured each day between 1300 and g  1430 PST in the unthinned Douglas f i r stand.  49  r—'  0  1  10  • — 1  1  20  •  30  JUNE  Fig.  2.  Seasonal  1—:—,  10  i  i  20  and stomatal  C^ ), t  i  r  . 10  l 20  AUGUST  course of s o i l  water p o t e n t i a l  . 30  JULY  •  1  water p o t e n t i a l  ),  vapour p r e s s u r e d e f i c i t  resistance  (r  twig (v.p.d.),  ) measured each day between  1300 and 1430 PST i n the t h i n n e d Douglas f i r  stand.  50  measured between 1300 and 1430 PST are shown f o r the r e s p e c t i v e measurement p e r i o d s .  The r e s u l t s  f o r the unthinned stand show t h a t the minimum  v a l u e o f ijj^ d u r i n g the f i r s t d r y p e r i o d was a p p r o x i m a t e l y - 2 1 . 4 w h i l e the minimum v a l u e of ^  bars,  d u r i n g the second dry p e r i o d was a p p r o x i -  mately - 2 4 . 7 b a r s , when t/j was between - 7 to -10 bars s  o t h e r hand, the minimum v a l u e of ^  (Fig.  1).  On the  i n the thinned stand near the end  o f the d r y p e r i o d was a p p r o x i m a t e l y - 2 1 . 9 bars  (Fig. 2).  These r e s u l t s  suggest t h a t t r e e s , which had been s u b j e c t e d to a p e r i o d o f low  soil  m o i s t u r e c o n d i t i o n s e a r l i e r i n the same summer, had a lower twig water potential  than t r e e s t h a t had not had p r i o r exposure to low s o i l  conditions.  The e f f e c t o f t h i n n i n g on ijj  f i r s t d r y p e r i o d o f the unthinned stand of the t h i n n e d stand  (Fig.  a l s o c l e a r from F i g s . r  s  2).  t  can be seen by comparing the  (Fig.  1) with the o n l y dry p e r i o d  The e f f e c t appears t o be s m a l l .  1 and 2 t h a t t h e r e i s  i n high v a l u e s o f - r  s  and high v . p . d . shown i n F i g s .  The daytime v a r i a t i o n i n r  3 d i f f e r e n t values o f ^  salal  In  several  1 and  cases,  2,.resulted  s  r e s i s t a n c e of Douglas f i r and a t the 3 - ,  Douglas f i r canopy and i n the s a l a l  heights  is  between 1300 and 1430 PST.  3.2 Daytime v a r i a t i o n i n stomatal  a t the 7-m h e i g h t .  It  a marked c o r r e l a t i o n between  and v . p . d . i n both the thinned and unthinned s t a n d s .  the combination of low ^  moisture  It  s  5- and 7-m h e i g h t s  i n the  undergrowth o f the t h i n n e d stand  are shown i n F i g . 3.  was found t h a t when ^  i n the thinned stand was small  undergrowth was s l i g h t l y  salal  Also  for  shown i s the v . p . d .  was h i g h , . r  a t the 3  and very s i m i l a r , w h i l e r  $  lower than t h a t o f the Douglas f i r  i n the during  51  C O U R T E N A Y , DOUGLAS FIR JUNE 30 T  4  8  1  1  12  16  , THINNED STAND, 1975  JULY II n  20  r i  4  JULY 30  1  1  1  8  12  16  1 i  20  4  1  8  r  1  12  16  20  HOUR , PST  Fig. 3. Daytime course of stomatal resistance•(r ) of Douglas f i r at 3 heights and salal in the thinned stand showing mainly the effect of decreasing soil water potential Ol^).  Also shown is  the course of the vapour pressure deficit (v.p.d.)  52  most of the daytime (Figs. 3a and 3b). When $  was very low, f o r  example, near the end of the dry period when i t reached a value of -11.3 bars, r  g  i n the canopy displayed more variation than at high  (Fig. 3c). Furthermore,  at low $  t  r  g  ^  of the salal undergrowth was  considerably lower than that of the Douglas f i r (Fig. 3c). In the unthinned stand, r canopy.  s  At low ij;  generally decreased with increasing height in the i t decreased more rapidly than at high \ j j . These, s  results are to be expected since the l i g h t at the ground level of the unthinned stand was often less than 10% of that above the canopy, while the l i g h t at the ground level of the thinned stand often equalled that above the canopy. 3.  The e f f e c t of v.p.d. on r  In p a r t i c u l a r , when iji  was very low, r  g  g  can also be seen from F i g .  increased rapidly during the  morning, as shown in F i g . 3c, r e f l e c t i n g the rapid r i s e in the v.p.d. during this time.  As was observed i n 1974 in the unthinned stand, r  g  sometimes rose to a daytime maximum between 1300 and 1600 PST, then declined temporarily before f i n a l l y increasing at'sunset (Fig. 3c). It should also be noted that high resistances early in the morning and late in the afternoon are the result of low l i g h t l e v e l s .  3.3 Relationship of stomatal resistance to irradiance Fig. 4 shows the stomatal resistance of Douglas f i r and salal undergrowth plotted against the measured incident quanta flux density and the corresponding energy flux density of the 0.4-0.7 ym band width.  These  results suggest that stomatal closure of Douglas f i r and salal began at -2 a quanta flux density of between 250 and 350 yE m  -1 s  .  Thinning  to have l i t t l e or no e f f e c t on the l i g h t response of the stomata of  appeared  53  COURTENAY, DOUGLAS FIR R, (A. = 0.4 -0.7pm) (Wm-2) 100 200  300  50  40  (s cm")  o • *  UNTHINNED STAND, 1974 THINNED STAND, 1975 SALAL, 1975  1  30  20  10 V*.  o• *. 500  Fig.  4. R e l a t i o n s h i p  1000 Q (pE r r T V )  resistance  t h i n n e d and unthinned s t a n d s ,  width i s  2000  between the quanta f l u x d e n s i t y  ym band width and stomatal  stand.  1500  (r  and s a l a l  (Q) f o r the  ) o f Douglas f i r  0.4-0.7 i n the  undergrowth i n the thinned  The approximate energy f l u x d e n s i t y i n the 0 . 4 - 0 . 7 ym band shown a t the top of the graph.  54  Douglas f i r .  3.4 R e l a t i o n s h i p o f stomatal  r e s i s t a n c e to twig water p o t e n t i a l  and  vapour p r e s s u r e d e f i c i t Fig.  5 shows a p l o t o f r -2  g  versus ^  t  when the l i g h t  quanta  flux  -1  exceeded 350 yE m~  s~  (y solar  radiation  (0.4-4 pm) f l u x d e n s i t y o f  _o  a p p r o x i m a t e l y 150 wm  ).  Stomata began to c l o s e a t a ^  value  of.-19.5  bars d u r i n g the f i r s t dry p e r i o d i n the unthinned stand and i n the t h i n n e d s t a n d , w h i l e d u r i n g the second dry p e r i o d i n the unthinned s t a n d , began to c l o s e a t a ^  v a l u e of - 2 3 . 5 bars  (Fig.  5a).  McCree  H i n c k l e y et a1_. (1975),  Brown et a 1.  showed s i m i l a r r e s u l t s  i n d i c a t i n g t h a t a f t e r drought h i s t o r y  M i l l a r e t al_. (1971),  (1971)  reported that cotton plants  indicating  stomatal  than  plants.  closure.  i n the leaf  Jordan and R i t c h i e  grown i n the greenhouse had v e r y  r e s i s t a n c e when l e a f water p o t e n t i a l was -16  stomatal  before.  r e s i s t a n c e a t a given v a l u e o f  than growth-chamber-grown  values of stomatal  (1976)  i n a study o f seed o n i o n s , found t h a t p l a n t s  f i e l d had a lower v a l u e o f stomatal water p o t e n t i a l  (1974),  (1976), and Thomas et a l .  c l o s u r e o c c u r r e d a t a lower v a l u e of l e a f water p o t e n t i a l  stomata  However, the r e s i s t a n c e o f the  high  bars,  field-grown  plants  remained low even a t l e a f water p o t e n t i a l v a l u e s o f -27  bars.  Millar  e t al_. (1971), Turner and Begg (1973)  suggested  t h a t the d i f f e r e n c e  i n the stomatal  and Turner  (1974)  response o f growth-chamber  versus  f i e l d - g r o w n p l a n t s was due t o a r e d u c t i o n i n the osmotic p o t e n t i a l leaves o f - t h e f i e l d - g r o w n p l a n t s . Thomas e t al_. (1976)  of  More r e c e n t l y Brown e t al_. (1976)  have shown t h a t the osmotic p o t e n t i a l  l e a v e s decreases as a r e s u l t o f drought h i s t o r y .  It  and  of the p l a n t  would thus  appear  55  COURTENAY 50  n  (a)  DOUGLAS  FIR  (b)  SALAL, 1975  40 o  I*  •  2"* DRY PERIOD, 1974  DRY PERIOD, 1974  »  1975  0  0  30  (s cm") 1  20  10  -10  Fig.  5.  -10  R e l a t i o n s h i p between stomatal potential  for  (a)  -20  resistance  Douglas f i r  (r  ) and twig water  i n the unthinned s t a n d ,  d u r i n g the two d r y i n g p e r i o d s i n 1974, and i n the t h i n n e d stand i n 1975, and (b) stand.  salal  undergrowth i n the t h i n n e d  56  that the osmotic potential of the Douglas f i r needles decreases as a r e s u l t of drought history within the same growing season.  This has yet  to be confirmed.. In the salal undergrowth, stomatal closure was found to occur at a ^  t  value of approximately -16 bars (Fig. 5b). Since salal was not  present in the unthinned stand i n 1974 the e f f e c t of drought history was not observed. In order to determine how much of the scatter in F i g . 5a was caused by stomatal response to the vapour pressure d e f i c i t , the data from the thinned stand at the 3-, 5- and 7-m heights were plotted as shown in F i g 6.  ;  The data in F i g . 6 are separated into four v.p.d. classes as shown.  As can be seen, the v.p.d. does account f o r much of the v a r i a t i o n .  It  should be noted that, when the v.p.d. i s high,.the stomata begin to close at much higher values of  i|>  t>  thus preventing further desiccation in very  dry atmospheric conditions. At very low values of-ty at a given value of higher ^  s  < -9 bars), r  g  and v.p.d. was observed to be higher than at  values (Fig. 6).  The relationship of ^  to v.p.d. and ^  unthinned stands can be seen in Table 1. age value of  ij>  t  s  f o r both thinned and  The table shows that the aver-  i n most cases, including the s a l a l , decreases with  increasing v.p.d. in the range from 8 to 16 mb. however, the value of ^  For higher v.p.d.,  appears to r i s e s l i g h t l y .  Camacho-B et a l .  (1974), Hall and Yermanos (1975), Schulze et al_. (1975), Johnson et al_. (1976) and Hall and Hoffman (1976) have reported that, when stomata p a r t i a l l y close as a r e s u l t of a high vapour pressure gradient between leaf and a i r , leaf water status may be maintained at i t s present level or improved.  57  COURTENAY, THINNED  DOUGLAS  STAND,  FIR  1975  1  l  v.p.d. (mb)  - 9 bars  •  60  r - 9 bars  < 10  o  < 10  •  10-15  •  10-15  •  15-20  X  > 20  ( s cm*') 40  20  -10  -15  -20 Y,  -25  (bars)  Fig. 6. Relationship between stomatal resistance (r ) and twig water potential ( i j ^ ) for the 3-, 5- and 7-m heights in the thinned Douglas f i r canopy.  The data have been separated into 4 vapour  pressure deficit (v.p.d.) classes and 2 soil water potential (iji ) classes.  Table 1: The relationship of twig water potential Ut,) to vapour pressure d e f i c i t (v.p.d.) and s o i l water potential U ) f o r Douglas f i r in the unthinned stand, during the two dry periods in 1974, and in the thinned stand in 1975, and salal undergrowth in the thinned stand. The 90% confidence interval on each mean i s also shown. s  Twig Water Potential 0 bar > $ > -6.5 bars s Salal Douglas f i r  (-bars) -6.5 bars > \p > -12.5 bars s Salal Douglas f i r  v.p.d. (mb)  1st 1974  2nd 1974  1975  1975  1st 1974  2nd 1974  0 -8  17.0  17.0 ±2.2  16.5 ±1.8  8.9 ±1.1  17.3 ±1.8  8 - 16  18.4 ±1.5**  19.3 ±1.0  19.7 ±0.8  15.3 ±2.4  16.7  21.9 ±0.7  17.0 ±1.9  11.8 ±3.7  16 - 24 24-32  *  *  -  1975  1975  20.2  *  18.7 ±3.1  13.0  20.0 +0.4  22.3 ±0.9  20.0 ±1.4  14.2 ±1.4  19.3 ±2.9  22.7 ±3.2  19.2 ±2.4  13.9  *  *  20.5 ±1.9  * < 2 data values ** 90% confidence interval on the mean  OO  59  3.5 Relationship of stomatal resistance to vapour pressure d e f i c i t  and  s o i l water potential As we have seen, the relationship between r The value of r  g  could vary from 2 to 60 s cm  and \p. i s poorly  defined.  with only a small change  -1  in \p^. While only turgor potential controls stomatal opening and c l o s i n g , ij>  t  measurements include both turgo$ and osmotic components.  As indicated  e a r l i e r , the e f f e c t of stress history can change the osmotic potential resulting in a s h i f t in the r e l a t i o n s h i p of r  g  to \p^.  It was  f e l t that  rather than using the leaf water p o t e n t i a l , a parameter that was of water supply to the tree, such as \p  might be more successful in  t  describing stomatal resistance c h a r a c t e r i s t i c s . Cline and Campbell  a measure  In two other studies,  (1976), Tan and Black (1976) have also shown that r  g  during the daytime appeared to be controlled by a combination of v.p.d. and ip^.  Figs. 7 and 8 show r  g  of Douglas f i r and salal plotted against  v.p.d. for d i f f e r e n t classes of  In general, with high \p , r s  increases s l i g h t l y with increasing v.p.d. (Figs. 7a and 8a). becomes d r i e r ( i . e . as ip decreases), s  r  g  g  As the s o i l  of Douglas f i r r i s e s more quickly  in response to increasing v.p.d. (Figs. 7b, 7c and 7d).  Stress history  and thinning had l i t t l e impact on these r e l a t i o n s h i p s .  At values of iji  greater than -3.5  bars, salal has stomatal resistance c h a r a c t e r i s t i c s  similar to those of Douglas f i r (Fig. 7a vs. Fig. 8a). between -3.5  and -12.5  bars!)* r  g  At values of ^  s  of salal has a smaller response to  increasing v.p.d. than that of Douglas f i r (Figs. 7b, 7c and 7d vs. Figs. 8b, 8c and 8d).  This suggests that salal takes an increasing proportion^  of the soil water as the s o i l becomes d r i e r . The p o s s i b i l i t y that temperature might account for the variation observed within each of the s o i l water potential classes in F i g . 7 was  60  COURTENAY, DOUGLAS FIR  0  1  0  •  '  •  10  1  .—I I  20  0  .  I  v.p.d. (mb)  Fig.  7.  Relationship  stand, during  i_ 20  v. p.d.(mb)  between stomatal  pressure d e f i c i t  ,  10  resistance  ( v . p . d . ) f o r Douglas f i r  the two dry p e r i o d s  stand i n 1975 f o r 4 s o i l  (r  ) and vapour  i n the unthinned  i n 1974, and i n the t h i n n e d  water p o t e n t i a l  (ty )  ranges:.  61  8.  Relationship between stomatal resistance (r ) and vapour pressure deficit (v.p.d.) for the salal undergrowth in the thinned stand for 4 soil water potential (^ ) ranges. $  62  examined.  Federer and Gee  (1976) have shown that for broad-leaved  tree species, the stomatal resistance increased as v.p.d. increased, and decreased,to some extent, as the temperature potentials were high.  increased when s o i l water  In the present study, temperature  appeared  to  account for very l i t t l e additional v a r i a t i o n .  4. Conclusions  During the daytime, stomatal resistance in a Douglas f i r stand was found to be mainly related to the s o i l water potential and vapour pressure deficit.  Stomatal  resistance of Douglas f i r and salal undergrowth  increases as the vapour pressure d e f i c i t increases and s o i l water potential decreases.  The stomatal resistance of salal undergrowth has a smaller  response to vapour pressure d e f i c i t change than that of Douglas f i r , especia l l y when the s o i l becomes d r i e r .  Evidence was found to suggest that  stomatal closure caused by high vapour pressure d e f i c i t can result in maintaining or s l i g h t l y improving leaf water status.  Stress history appeared to  cause a s h i f t in the relationship of stomatal resistance to twig water p o t e n t i a l , but had l i t t l e effect on the relationship of stomatal resistance to vapour pressure d e f i c i t and s o i l water potential.  There was  little  effect of thinning on the stomatal resistance c h a r a c t e r i s t i c s of Douglas f i r observed within a year of the thinning operation.  This study shows  that s o i l water potential i s extremely useful in describing stomatal  resis-  tance c h a r a c t e r i s t i c s in forests where there i s a seasonal change in s o i l moisture.  63  References Beardsell, M.F.,  P.G.  J a r v i s , and B. Davidson.  1972.  A' null-balance  d i f f u s i o n porometer suitable for use with leaves of many shapes. J. Appl. Ecol. 9: 677-690. Brown, K.W.,  W.R.  Jordan and J.C. Thomas.  1976.  Water stress induced  a l t e r a t i o n in the stomatal response to leaf water p o t e n t i a l . Physiol. Plant. 37: Camacho-B., S.E.,  A.E.  1-5.  Hall and M.R.  Kaufman.  1974.  E f f i c i e n c y and  regulation of water transport in some woody and herbaceous species. Plant Physiol. 54: 169-172. Cline, R.G.  and G.S.  Campbell.  1976.  of selected forest species. Federer, CA.  and G.W.  Gee.  in stress and unstressed Fetcher, N.  1976.  Ecol. 57: 367-373.  1976.  Diffusion resistance and xylem potential  northern  hardwood trees.  Yermanos.  H a l l , A.E. and G.J. Hoffman.  1975.  1976.  and water transport in plants. M.O.  pine  Ecol. 57: 339-345. Leaf conductance and l e a f water status  of Sesame strains in hot, dry climates.  Hinckley, T.M.,  Ecol. 57: 975-984.  Patterns of l e a f resistance to lodgepole  transpiration in Wyoming. H a l l , A.E. and D.M.  Seasonal and Diurnal water r e l a t i o n s .  Crop S c i . 15: 789-793.  Leaf conductance response to humidity Agron. J . 68: 876-881.  Schroeder, J.E. Roberts and D.N.  Bruckerhoff.  1975.  Effect of several environmental variables and xylem pressure potential on l e a f surface resistance in white oak. Johnson, D.A.  and M.M.  Caldwell.  1976.  Forest S c i . 21: 201-211.  Water potential components,  stomatal function, and l i q u i d phase water transport resistance of four a r c t i c and alpine species in relation to moisture stress. Physiol. Plant 36: 271-278.  64  Jordon, W.R.  and J.T. Ritchie.  1971.  Influence of s o i l water stress  on evaporation, root absorption and internal water status of cotton.  Plant Physiol. 48: 783-788.  Jordon, W.R.,  K.W.  Brown and J.C. Thomas.  1975.  Leaf age as a  determination in stomatal control of water loss from cotton during water stress. McCree, K.J.  Plant Physiol. 56: 595-599.  1974.  Changes in the stomatal response c h a r a c t e r i s t i c s of  grain sorghum produced by water stress during growth.  Crop. S c i .  14: 273-278. M i l l a r , A.A., W.R.  Gardner and S.M.  Goltz.  1971.  and water transport in seed onion plants. Running, S.W. conifers.  1976.  Internal water status  Agron. J . 63: 779-784.  Environmental control of leaf water conductance in  Can. J. For. Res. 6: 104-112.  Scholander, P.F., H.T. Hammel, E.D. Bradstreet and E.A. Hemmingsen. 1965.  Sap pressure in vascular plants.  S c i . 148: 339-346.  Schulze, E.-D., O.L. Lange, M. Evenari, L. Kappen and U. Buschbom.  1974.  The role of a i r humidity and l e a f temperature in c o n t r o l l i n g stomatal resistance of Prunus armeniaca L. under desert conditions. simulation of the daily course of stomatal resistance.  I.  A  Oecologia  (Berl.) 17: 159-170. Schulze, E.-D., O.L. Lange, L. Kappen, M. Evenair and U. Buschbom.  1975.  The role of a i r humidity and leaf temperature in c o n t r o l l i n g stomatal resistance of Prunus armeniaca L. under desert conditions.  I I . The  significance of leaf water status and internal carbon dioxide  65  concentration. Oecologia Slavik, B. 1970.  (Berl.) 18: 219-233.  Transpiration resistance in leaves of maize grown in  humid and dry a i r . p. 269-270. In: R.O.  Slatyer (ed.). Plant response  to c l i m a t i c factors. Unesco, Paris. (Proc. of the Uppsala Symposium, 1970).  v Tan, C S .  and T.A.  Black. 1976.  Factors a f f e c t i n g the canopy resistance of  a Douglas f i r f o r e s t . Boundary Layer Meteorol. 10: Thomas, J . C ,  K.W.  Brown, and W.R.  Jordan. 1976.  (in press).  Stomatal response to leaf  water potential as affected by preconditioning water stress in the f i e l d . Agron. J. 68: 706-708. Turner, N.C,  F.C.C. Peterson, and W.H.  Wright. 1969.  porometer for f i e l d use. Conn. Agr. 1-7.  An aspirated d i f f u s i o n  Exp. Sta. S o i l s B u l l . No. XXIX:  "  Turner, N.C.  and J.Y.  Parlange, 1970.  Analysis of operation and c a l i b r a t i o n  of a ventilated d i f f u s i o n porometer. Plant Physiol. 46: 175-177. Turner, N.C.  and J.E. Begg. 1973.  Stomatal behaviour and water status of  maize, sorghum and tobacco under f i e l d conditions. I. At high s o i l water potential. Plant Physiol. 51: 31-36. Turner, N.C.  1974.  Stomatal behaviour and water status of maize, sorghum  and tobacco under f i e l d conditions.::il. At low s o i l water p o t e n t i a l . Plant Physiol. 53: 360-365. Waggoner, P.E.  and N.C.  Turner. 1971.  Transpiration and i t s control by  stomata in a pine forest. B u l l . Conn. Agr. Exp. Sta, No.  726.  CHAPTER 3  FACTORS AFFECTING THE CANOPY RESISTANCE OF A DOUGLAS FIR FOREST  67  FACTORS AFFECTING THE CANOPY RESISTANCE OF A DOUGLAS FIR FOREST  Abstract  The physiological nature of the canopy resistance was studied by comparing the stomatal and canopy resistance of a 10-m high Douglas f i r forest.  Stomatal resistance of the needles was measured using porometry,  while the canopy resistance was calculated using energy balance/Bowen r a t i o measurements of evapotranspiration.  The typical steady increase in the  forest canopy resistance during daytime hours, even at high s o i l water potentials, was observed.  A s i m i l a r trend in the stomatal resistance  indicated that increasing canopy resistance during the daytime was caused by gradually closing stomata.  During a dry period when s o i l water potentials  declined from 0 to -10.5 bars, the mean daytime value of canopy resistance increased in proportion to the mean daytime value of the stomatal resistance. Values of canopy resistance calculated from stomatal resistance and leaf area index measurements agreed well with those calculated from energy balance measurements.  The dependence of stomatal resistance on l i g h t ,  vapour pressure d e f i c i t , twig and s o i l water potentials are summarized.  Paper accepted for publication in Boundary-Layer Meteorology (1976).  68  1. Introduction  Improved procedures of estimating evapotranspiration are required in hydrological studies of vegetated areas.  Hydrologists are well aware  that potential evaporation estimates can often greatly exceed actual values of evapotranspiration. Any r e a l i s t i c evapotranspiration model must include plant or s o i l parameters  to  provide an accurate description of  the process when i t i s limited by the supply of water.  One such parameter  i s the canopy or surface resistance which i s contained in the Penman Monteith model of transpiration (Monteith, 1965).  In view of the complexity  of many plant canopies this i s a greatly simplified model; however, i t has been remarkably successful in i t s application to agricultural  crops  (Monteith et al_., 1965; Black et_ al_., 1970; Szeicz et_ al_., 1973).  These  workers have shown that for well ventilated agricultural crop canopies, the canopy resistance i s approximately equal to the stomatal resistance of the leaves acting in p a r a l l e l .  Recently several forest micrometeorologists  have calculated, using the Penman - Monteith model, hourly, daily and monthly values of canopy resistance of forests in various parts of the world (e.g. Szeicz and Long, 1969; Stewart and Thorn, 1973; McNaughton and Black, 1973; Gash and Stewart, 1975).  L i t t l e i s known as to the physiol-  ogical nature of the forest canopy resistance and whether i t i s possible to model i t from an understanding of the behaviour of the stomata of the trees (Federer, 1975).  The objectives of this Chapter are (1) to show the  relationship between canopy and stomatal resistance in a 21-year-old Douglas f i r f o r e s t , (2) to describe the relationship between canopy r e s i s tance and environmental  parameters and (3) to assess whether canopy  69  resistance behaviour can be inferred from knowledge of stomatal response to environmental parameters.  2. Theory  Total evapotranspiration can be considered as mainly transpiration with small error f o r forests, with no intercepted water present, since evaporation from the s o i l i s sjriall.  The Penman -Monteith model of trans-  piration assumes that the canopy i s isothermal and that the canopy r e s i s tance (r ) i s the resistance of a l l parallel.  stomata of the leaves acting in  The equation expressing the model can be written as sA  +  P  c (e* p  - e )/r  z  z  H  ( ] )  E = [s + ( l Y  +  r  / r c  H  ) ] L  which assumes that the aerodynamic resistances, r  v  and r ^ are equal.  (For the meaning of remaining symbols, see the L i s t of Symbols f o r thesis), Thorn (1972) gives the form of (1) with r  y  f r ^ and Tanner and Fuchs (1968)  givenan alternative equation to (1) f o r the case of a non-isothermal canopy. On re-arrangement of (1), the canopy.resistance can be written as pc (e* r  c=  p  YLE  - e .) Z  fl  ^ H ^ ' E E -  1  ' - ^  (  2  )  Both Stewart and Thorn (1973) and McNaughton and Black 0 973) have shown that when r  u  n  (and r ) <<r , as in a forest, the second term of (2) i s V C  70  much smaller than the f i r s t so that to a good approximation  E =  pc — M e * - e )/r yL z z c  (3)  This r e l a t i o n indicates the importance of the vapour pressure d e f i c i t (e*  z  - e ) in the transpiration process in the forest. The canopy resistance i s a function of stomatal resistance (r)\  the  t  leaf area index (LAI)and the d i f f u s i o n resistance to water vapour through the canopy a i r volume. much smaller than r  The assumption that the l a t t e r resistance i s  in forest canopies would imply that r  can be  calculated from n 1/r  =  E i - 1  (LAI./r .)  (4)  where r ^ and LAI^ are respectively the r  g  and LAI of the i t h canopy  layer and n i s the number of layers.  3. Experimental  3.1  Procedure  Experimental s i t e The s i t e was approximately 27 km northwest of Courtenay, B.C.,  the eastern coast of Vancouver Island.  on  The measurements were made in an  unthinned stand of Douglas f i r planted in 1953, located on the r e l a t i v e l y extensive coastal plain between Courtenay and Campbell  River.  The  wet,  71  mild winters and dry, warm summers experienced range of s o i l moisture conditions.  by the area provide a wide  The topography i s generally f l a t ,  although there are several ridges of approximately  20-30 m r e l i e f , and  some depressional areas of a swampy nature which dry out during the summer. The s i t e was located at an elevation of 150 m. At the time of the research, the trees at the s i t e ranged in height from 7 to 10 m, and averaged 10.6 cm in diameter at breast height. were approximately  There  -1 2 -1 1840 stems ha and the basal area was 27.5 m ha .  The s o i l , belonging to the Dashwood series, was a well-drained gravelly sandy loam of 45-60 cm depth overlying a deep layer of compacted basal till.  Measurements of root density indicated that the majority of the  roots were in the upper 45 cm of the s o i l . The area i s considered an excellent one for micrometeorological research.  Effects of local advection were f e l t to be absent with 8 km of  forested land separating the s i t e from the Straight o f Georgia, and forest extending  upwards of 32 km in either direction along the coast.  Although the west-to-south sector included h i l l y t e r r a i n and deforested areas on the lower slopes of mountains within 3 to 4 km o f the s i t e , winds were v i r t u a l l y never from that d i r e c t i o n .  Prevailing winds were  from the east to northeast.  3.2 Measurements Continuous half-hourly measurements of evapotranspiration were made from June 14 until August 15, 1974 using the Bowen ratio/energy technique.  balance  The Bowen ratio*- (g)was measured at the 10.5-m level using the  72  psychrometric apparatus described in Black and McNaughton (1971). The net radiation flux,;.(R ) was measured at the top of the canopy with a Swissteco S-l net radiometer, continuously ventilated with dried a i r from an aquarium-type pump.  The s o i l heat flux .(G.). was measured at  the 5-cm depth with two heat flux plates, and corrected f o r storage in the upper 5 cm using an integrated temperature measured with a diode integrating thermometer (Tang ejt al_., 1974).  Storage of sensible and  latent heat within the canopy;(M.) was estimated from wet and dry bulb temperatures  taken every 15 minutes at the 3-, 5- and 7-m l e v e l s , and from  estimates of the heat capacity of the canopy based on.the work of Stewart and Thorn (1973).  The evapotranspiration rate, (E ) was then calculated r  using the equation:  E = (R - G - M)/[L(1 + (3)] n  (5)  where L i s the latent heat of vapourization. Wind speed was measured by a sensitive Casella anemometer located at a height of 10.5 m.  The aerodynamic resistance was estimated using the  logarithmic wind p r o f i l e equation and assumed values of surface roughness and displacement height. Supporting climatological measurements were made continuously of incident solar radiation above the canopy, and at.various levels through the canopy at selected times using a Ki pp and Zonen solarimeter. Wind direction above the canopy was measured using a Climet wind vane.  Precipi-  tation was recorded each day with a 10.2 cm diameter rain gauge at the data logging t r a i l e r .  73  Data signals were carried back to the t r a i l e r by 75 m shielded cables where they were recorded with a Hewlett-Packard 2707 A data logger.  Net r a d i a t i o n , solar radiation and Bowen r a t i o data signals  were integrated using voltage integrators (Tang e_t al_., 1976).  Bowen  ratio data was further monitored on a strip-chart.recorder. Soil water potential between 0 and -1 bar was measured by a tensiometer-transducer system.  Four tensiometers were used and were located  at depths of 15, 30, 45 and 60 cm.  Soil water potential  less than -1  bar was measured using a Wescor HR-33T dew point microvoltmeter and PT51-10 hygrometers.  Six hygrometers, two each at the 15- and 30-cm depth, and  one each at the 45- and 60-cm depth, were used.  Soil water potential  measured by the tensiometer-transducer system was recorded at 15 minute i n t e r v a l s , while s o i l water potentials measured by the hygrometers were recorded three times each day.  In this study, only d a i l y values of s o i l  water potential are used in the analysis. Soil water content was measured both gravimetrically and by use of the neutron moisture meter.  Neutron moisture measurements were made every  seven days in six access tubes.  Gravimetric sampling of the root zone  was taken every one to two weeks, depending on the stage of the drying period.  For d e t a i l s of s o i l water measurements see Nnyamah (1977).  The stomatal resistance j.(r ); and twig water potential" s  (ij^),  at the  8-m height were measured on a routine basis three times a day (0700 to 0830, 1300 to 1430 and 1600 to 1730 PST) on the majority of days during the period June 12 to August 18.  Measurements of r  g  and ^  at the same  height were made every two hours during the daytime on twelve days. eight of the twelve days that  was measured, r  On  was also measured at  74  approximately the same time.  A pressure chamber located in the data  logging t r a i l e r was used to make the ^ 1965).  measurements (Scholander et a l . ,  At least two twig samples were used to obtain each value of  and two readings were taken f o r each twig.  ^  Stomatal resistance measure-  ments were made using a ventilated porometer designed by Turner et a l . (1969).  Modifications of the porometer electronics and the sensing head,  and the c a l i b r a t i o n procedure used are described in {Chapter 1.  A sample  of four needles was brought down from the tree, attached to a modelling clay plug and inserted into the porometer.  At least two samples of four  needles each were used to obtain a value of r . s On August 6, r  g  and l i g h t quanta flux (0.4 to 0.7 ym) were measured  every three hours at the 8-m,  6-m  and 4-m  2 these measurements were made at the 8-m  heights, while on August 1 and and 6-m  heights.  Light quanta  flux was measured with a Lambda model LI-185 l i g h t meter with the quantum sensor held by hand near the point at which the needles were removed from the tree,(see also Chapter 2).  4. Results and Discussion  4.1 Course of s o i l water potential during the measurement period Figure 1 shows the average soil water potential-;(^ ) for the period s  June 17 to August 19, 1974 for the root zone (0 to 45 cm depth). values of p r e c i p i t a t i o n are shown for reference.  The average s o i l  Daily water  75  Fig. -1.  Soil water potential  ) in the unthinned stand during  the period June 17 - August 19, 1974.  Each data point i s  the average value of the water potentials at the 15, 30 and 45-cm depths.  :  76  content for the root zone over the same period ranged approximately from 0.23 to 0.08 on a volumetric basis which included stones.  The  d a i l y (24-hour) value of the r a t i o of latent heat flux to the net radiation flux (LE/R ) ranged from 0.58 n  to 0.27 over the period of measurement. ?  The dependence of LE/R  n  on ^  days i s shown in Figure 2. are  s  for eighteen fine (R  > 14.0 MJ m  1  day  )  (Note that Figs. 2, 5, 7 and 10 of Curtis (1975)  preliminary results of iithbse. reported in Figs. 1 to 4 in this Chapter).  4.2 Daytime course of net radiation and latent heat flux f o r two selected days The days selected for the comparison tances were July 25 and June 18.  of stomatal and canopy r e s i s -  The former day had an average  soil  water potential of -0.6 bar, while the l a t t e r had a potential of -6.5 bars.. Consequently there was a considerable difference in the a v a i l a b i l i t y of s o i l water to the trees on these days. LE for both days i s shown in Figure 3.  The daytime course of R  n  and  The latent heat flux on the July  25 did not display the mid-afternoon peak observed by McNaughton and.Black (1973) in a Douglas f i r forest where ^  s  was always greater than -0.5 bar.  This suggests some r e s t r i c t i o n to transpiration during the afternoon when the vapour pressure d e f i c i t (v.p.d.) was at i t s daily maximum.  The  reason may well be that in the coarse s o i l , the hydraulic conr d u c t i v i t y i s low near the roots.  The 24-hour value of.LE/R  n  of 0.58 on  July 25 (Figure 2) was the highest observed during the study. On June 18, R  n  was much higher than on July 25.  A reduction in trans-  piration was apparent at 1030 PST by which time LE had reached i t s d a i l y maximum.  The latent heat flux decreased during the late morning and early  77  9  Fig.  2.  ( c m H 0 / c m 3 soil) 3  2  The relationship between the 24-hour value of the ratio-j LE/R  n  and the s o i l water potential  > 14.0 MJ m"  r\  -  I  (ip ) f o r 18 f i n e (R s n  day" ) days in 1974 in the unthinned  The number adjacent to each data point i s the date.  stand. Values  of the average root zone s o i l water content (e) corresponding to ip are also shown, s  78  Fig. 3.  Daytime course of net radiation  (R ) and latent heat n  flux (LE) f o r June 18 and July 25, 1974 in the unthinned stand, showing the effects of decreased s o i l water potential  (^ ). s  79  afternoon to a minimum of 80 W rn  at 1430 PST when the v.p.d. and  stomatal resistance both reached maximum daily values (see Figure 6). Resistance to transpiration i s quite apparent in this case with the 24 hour value of LE/R  n  being only  0.36.  4.3 Comparison of daytime courses of stomatal and canopy resistance From the measurements o f - r 8-m  heights, i t was found that r  height in the canopy.  made on August 6 at the 4-m, g  6-m  and  increased s l i g h t l y with decreasing  The value of r  g  at the 4-m  height was between 10%  and 50% higher than that at the 8-m  height.. Since the stomatal resistance  measured at the 8-m  to be representative of a large  height appeared  proportion of the canopy, i t was  used in subsequent comparisons with  canopy resistance,- f^V Figure 4 compares r 18.  c  with r  g  at the 8-m  height for July 25 and June  Canopy resistance was calculated from (2) but d i f f e r e d very l i t t l e  from that calculated from (3) and the difference became n e g l i g i b l e as r increased.  £  The low values of canopy resistance in the morning and i t s  steady increase during the daytime on July 25 were also observed by McNaughton and Black (1973).  Similar daytime courses of canopy resistance  have been reported by Stewart and Thorn (1973) and Gash and Stewart for a pine forest with high values of s o i l water p o t e n t i a l .  (1975)  The pattern  of the daytime course of stomatal resistance on July 25 i s very similar to that of canopy resistance.  On June 18, when soil water potential was  the stomatal resistance rises to a maximum of approximately 60 s cm  -1  low, at  about 1500 PST, then declines markedly before stomatal closure i s caused  80  Fig. 4.  Daytime course of canopy resistance (r ) and stomatal tance ( r ) (projected leaf area basis) at the 8-m g  the unthinned stand f o r the days in Fig. 3.  resis-  height in  81  by low irradiance.  The canopy resistance shows a s i m i l a r pattern  but  displays some variation which i s a consequence of scatter in the energy balance data.  Figure 5 shows a good c o r r e l a t i o n between canopy resistance  and stomatal resistance at the 8-m which stomatal resistance was  height for ten of the twelve days on  intensively measured.  Stomatal resistance  measurements on the other two days were not used because of energy balance instrumentation  maintenance.  Leaf area index>(LAl). measurements were made on four trees in a stand recently thinned within the same f o r e s t , but Ih s i t e (see Chapter 4 for measurement d e t a i l s ) .  km from the study  Trees at this location  were the same age, height and diameter at breast height as those at the study s i t e .  Results of these measurements suggested that the average  value of LAI at the study s i t e was basis).  between 7.5  and 8.0  (projected area  Assuming the stomatal resistance shown in Figures 4 and 5 applied  to the whole canopy, then from ( 4 ) , v T  c  one eighth of the stomatal resistance. ship between r  £  and r  g  should be between one seventh and This i s the approximate r e l a t i o n -  in Figures 4 and 5.  The close s i m i l a r i t y of  patterns and the reasonable relationship between values of the two tances indicates that the forest canopy resistance i s l a r g e l y of  resisphysio-  logical o r i g i n , e s p e c i a l l y in the case of the water-short f o r e s t .  4.4  Relationship of stomatal resistance to plant and environmental parameters Modelling of the canopy resistance requires knowledge of how  plant  and environmental parameters a f f e c t the stomatal resistance of.each tree species.  The response of stomata to l i g h t , internal water d e f i c i t s of the  82  Fig. 5.  The relationship between canopy resistance (r ) and stomatal resistance (r ) at the 8-m height in the unthinned stand f o r 10 days.  83  tree, v.p.d. and temperature 1974;  are p a r t i c u l a r l y important  Schulze et_ al_., 1974).  Stomatal  (e.g. J a r v i s ,  resistance began to increase  rapidly when solar radiation decreased to approximately 150 W m -2 yE m  (350  -1 s  f o r the 0.4 - 0.7 ym wave band).  The relationship between  stomatal resistance and twig water potential was poorly defined.  Although  stomatal closure was observed at a twig water potential of -18 bars»the stomatal resistance varied from 3 s cm  -1  to 50 s cm"  at -20 barsduring  1  the daytimei;(see Chapter 2). . As can be seen from (3) the v.p.d. can be considered a water demand parameter in forests.  The e f f e c t of the v.p.d. was p a r t i c u l a r l y evident  as can be seen in Figure 6 f o r June 18 and 19, both sunny days.  The s o i l  water potential on June 18 was -6.5 bars and decreased s l i g h t l y to -7.2 bars on June 19.  On June 18, the decrease in stomatal resistance between  1500 and 1900 PST appears to be related to the decrease in v.p.d. starting at 1400 PST.  On the following day, the high and almost constant value  of the v.p.d. throughout the late afternoon seems to maintain the stomatal resistance at a high value during this time. Rather than to consider only plant and atmospheric stomatal resistance, the use of s o i l and atmospheric some practical advantages.  parameters affecting  parameters may have  Since both the s o i l water content and conduc-  t i v i t y decrease with decreasing s o i l water potential, the l a t t e r parameter will  be used as a water supply parameter.  Figure 7 shows stomatal  resis-  tance plotted against s o i l water potential and indicates that the r e l a t i o n ship depends strongly on atmospheric demand for water.  The data have been  separated into s i x v.p.d. ranges and a l i n e has been drawn by eye through each range.  At a given p o t e n t i a l , the stomatal resistance increases with  84  Hour  Fig.  6.  Daytime courses o f stomatal  PST  resistance  (r  s  ) a t the 8-m  h e i g h t and vapour p r e s s u r e d e f i c i t  ( v . p . d . ) on two days  with s i m i l a r  i n the unthinned s t a n d .  s o i l water p o t e n t i a l s  85  50 r—  0' 0  1  •  1  r.  I  J  -2  1  —i  l  •  7.  The r e l a t i o n s h i p height  1  I  .  -6  -4 Y  Fig.  1  s  between stomatal  resistance  The data have been s e p a r a t e d i n t o s i x  made on 12 d a y s .  L  -8  (bar)  i n the unthinned stand and s o i l  pressure d e f i c i t  r  (r  ) at the 8-m  water p o t e n t i a l ranges o f  ( v . p . d . ) and were daytime  vapour  measurements  86  increasing v.p.d., as suggested in the previous paragraph.  Szeicz et  a l . (1973) analyzed the relationship between stomatal conductance and s o i l water potential for sorghum.  Brady e_t al_. (1975), working on soy-  beans, plotted stomatal resistance against s o i l water p o t e n t i a l .  Much  of the scatter in the data of both of these groups of workers may  be  due to variation in the v.p.d.  4.5  Relationship of canopy resistance to s o i l water potential and vapour pressure d e f i c i t Since the canopy resistance i s c l o s e l y related to the stomatal  resistance, i t should be possible to relate the canopy resistance to the same environmental parameters that influence the stomatal resistance. In view of the relationship between stomatal resistance and canopy r e s i s tance shown in Figures 4 and 5, canopy resistance would be related to soil water potential and v.p^d. in a manner similar to that for stomatal resistance in Figure 7.  The  value of the canopy resistance of the forest  in this study for given values of soil water potential and v.p.d.,.can be obtained by dividing the value of the stomatal resistance on ordinate of Figure 7 by about 7.5 at the 8-m  (the r a t i o of the stomatal  the  resistance  height to the canopy resistance taken from Figure 4).  For the hydrologist interested in estimates of  evapotranspiration  over periods of days or weeks, a practical question  is whether daytime  mean values of canopy resistance can be related to daytime mean values of environmental parameters.  Szeicz et al_. (1973) related daytime canopy  resistance of sorghum to the s o i l water potential of the root zone.  They  87  found that the resistance began to increase with decreasing potential when the potential was less than about -7 bars. Szeicz and Long (1969) found that when plotting mean monthly canopy resistance of a grass-clover crop against s o i l water potential the resistance began to increase at a potential of -3.5 bars. In an a l f a l f a  crop, van Bavel (1967) found no  s i g n i f i c a n t change i n canopy resistance until the soil water potential decreased to -4 b a r  Si  In each of these cases there was considerable  scatter in the plots of resistance against p o t e n t i a l . In Figure 8, values of the daytime mean canopy resistance f o r the forest in the present study are plotted against the daily mean s o i l water potential.  The daytime mean canopy resistance,.r , was obtained by  weighting the hourly values by the v.p.d. as follows:  (6) j =1 where v.p.d.  i s the arithmetic average of the daytime v.p.d., v.p.d.. <J  and r . are the hourly values of the v.p.d. and r  respectively.  Much of  the scatter in Figure 8 i s removed by separating the data into v.p.d. ranges as suggested by the stomatal response shown in Figure 7. averaging o f . r  The weighted  results in a set of straight lines rather than the curved  lines in Figure 7... Canopy resistance appears to increase s i g n i f i c a n t l y at a s o i l water potential ;f less, soil  and the shallow root zone.  than -2.5 bars, r e f l e c t i n g the coarse The separation of data into v.p.d.  ranges indicates that i f (1) or (3) are to be used to estimate daytime forest evapotranspiration using mean daytime values of the required parameters, then i t i s necessary to consider the dependence of canopy resistance  88  Fig. 8. The relationship between the mean daytime canopy resistance (?P  of the unthinned stand, and soil water potential  ).  The data have been separated into three ranges of mean daytime vapour pressure deficit (v.p.d.) and were obtained over a period of 18 days.  89  on both soil water potential and v.p.d.  It points out the influence  of both water supply and demand on the canopy resistance.  As we have  seen earlier, this is mainly due to the action of the stomata in regulating water loss.  The results of this study suggest that a good deal can be  inferred about the canopy resistance from a knowledge of stomatal resistance characteristics of vegetation.  5. Conclusions  It has been shown that the canopy resistance of a 21-year-old Douglas f i r forest is largely of physiological origin.  Both hourly and daytime  values of canopy resistance depend on vapour pressure deficit as well as soil water potential of the root zone.  Daytime mean values of  canopy resistance should be calculated by weighting hourly values by the vapour pressure deficit.  The results of this study suggest that from a  knowledge of the response of stomata to environmental parameters, useful relationships of canopy resistance to environmental parameters can be obtained.  90 References Black, T.A. and K.G. McNaughton.  1971.  Psychrometric apparatus for  Bowen-ratio determination over forests. 2:  Boundary Layer Meteorol.  246-254.  Black, T.A., C B .  Tanner and W.R.  from a snap bean crop. Brady, R.A.,  S.M.  Gardner.  1970.  Evapotranspiration  Agron. J . 62: 66-69.  Goltz, W.L.  Powers and E.T. Kanemasu.  1975.  Relation  of soil water potential to stomatal resistance of soybean.  Agron.  J. 67: 97-99. Curtis, J.R.  1975.  Evapotranspiration from a dry Douglas f i r f o r s t .  M.Sc. Thesis, Department of Soil Science, University of B r i t i s h Federer, CA.  1975.  Evapotranspiration.  Columbia.  Reviews of Geophysics and  Space Physics 13(3): 442-445, 487-494. Gash, J.H.C. and J.B. Stewart.  1975.  The average surface resistance of a  pine forest derived from Bowen-ratio measurements.  Boundary Layer  Meteorol. 8: 453-464. J a r v i s , P.  1975.  Water transfer in plants.  I_n_ D.A. de Vries and  Afgan (eds.): Heat and mass transfer in the biosphere.  I.  N.H.  Transfer  processes in plant environment, Scripta Book Co., Wash., D.C. McNaughton, K.G. and T.A. Black.  1973.  A study of evapotranspiration from  a Douglas f i r forest using the energy balance approach.  Water Resour.  Res. 9: 1579-1590. Monteith, J.L.  1965.  Evaporation and environment.  Symp. Soc. Exp. B i o l .  19: 205-234. Monteith, J.L., G. Szeicz and P.E. Waggoner.  1965.  The measurement and  control of stomatal resistance in the f i e l d , J . Appl. Ecol. 2: 345-355. Nnyamah, J.U.  1977.  Root water uptake in a Douglas f i r forest.  Thesis, Department of Soil Science, University of B r i t i s h  Ph.D.  Columbia.  91  Scholander, P.F.,  H.T.  Hammel, E.D.  Bradstreet and E.A. Hemmingsen.  1965. Sap pressure in vascular plants. S c i . 148: 339-346. Schulze, E.-D., O.L.  Lang, M. Evanari, L. Kappen, and U..Buschbom. 1974.  The role of a i r humidity and leaf temperature  in c o n t r o l l i n g stomatal  resistance of Prunus armeniaca L. under desert conditions. I. A simul a t i o n of the d a i l y course of stomatal resistance. Oecologia (Berl.) 17: 159-170. Stewart, J.B. and A.S. Thorn. 1973.  Energy budgets in a pine forest. Quart.  J. Roy. Meteorol. Soc. 99: 154-170. Szeicz, G. and I.F. Long. 1969. Surface resistance of crop canopies. Water Resour. Res. 5: 622-633. Szeicz, G., CH.M.  van Bavel and S. Takami. 1973. Stomatal  factor, in the  water use and dry matter production by sorghum. Agr. Meteorol. 12: 361-389. Tang, P.A.,  K.G. McNaughton and T.A.  Black. 1974.  Inexpensive diode  thermometry using integrated c i r c u i t components. Can. J . Forest. Res. 4: 250-254. Tang, P.A.,  K.G. McNaughton and T.A.. Black. 1976. Precision electronics  integrator f o r environmental measurement. Transactions of ASAE 19: 550-552. Tanner, C.B. and M. Fuchs. 1968.  Evaporation from unsaturated surfaces:  a generalized combination method. J . Geophys. Res. 73: 1299-1303-.Thorn, A.S. 1972. Momentum, mass and heat exchange of vegetation.,Quart. J . Roy. Meteorol. Soc. 98: 124-134. Turner, N.C,  F.C.C. Peterson and W.H.  Wright. 1969. An aspirated d i f f u s i o n  porometer for f i e l d use. Conn. Agr. Exp. Sta. Soils B u l l . No. XXIX 1-7.  92  van Bavel, C.H.M. 1967. Changes i n canopy resistance to water loss from a l f a l f a induced by soil water depletion. Agr. Meteorol. 4: 165-176.  CHAPTER 4  A SIMPLE DIFFUSION MODEL OF TRANSPIRATION APPLIED TO A THINNED DOUGLAS FIR STAND  94  A SIMPLE DIFFUSION MODEL OF TRANSPIRATION APPLIED TO A THINNED DOUGLAS FIR STAND  Abstract  Values of forest transpiration rate calculated from a vapour d i f f u s i o n model that uses the vapour pressure d e f i c i t  (v.p.d.) of the canopy a i r ,  and measurements of the stomatal resistance (every 2 to 3 hours) and leaf area index of the canopy agreed well with those obtained from energy balance,/Bowen r a t i o measurements.  Stomatal resistance c h a r a c t e r i s t i c s  were also used in the d i f f u s i o n model instead of actual stomatal resistance measurements to calculate transpiration rate.  There was reasonable agree-  ment between these transpiration values and energy balance measurements averaged over 3-hour periods.  Both the model using stomatal resistance  characteristics,.and energy balance measurements showed that transpiration rate increased with increasing v.p.d. u n t i l a certain v.p.d. was reached after which the rate declined. transpiration rate occurred  The values of v.p.d. at which maximum  were 15, 13 and 11 mb for s o i l water potential  ranges of 0 to -3.5 bars, -3.5 to -9.5 bars and -9.5 to -12.5 bars respectively.  Calculations from the model, using stomatal resistance measure-  ments (every 2 to 3 hours) on 7 days, indicated that the f r a c t i o n of transpiration from the thinned hours increased period.  stand transpired by the salal during daytime  from approximately 45 to 70% during a four-week drying  95  1. Introduction  Practical methods of estimating evapotranspiration are required in water use studies of forest stands.  In recent years, several researchers  have examined an empirical approach to evapotranspiration estimation ,in which the r a t i o of the latent heat flux to the net radiation i s correlated with the s o i l water potential 1973).  (e.g. Ritchie ejtal_., 1972; Davies and A l l e n ,  While t h i s approach i s useful in many situations, i t i s to a large  degree empirical.  Recently Black et al_. (1976) found that in a forest  stand the value of the above ratio depends not only on s o i l water potential but also on net radiation.  Another common approach i s the Penman-Monteith  isothermal extensive leaf model (Monteith, 1965).  As indicated in Chapter  3, this i s a "single source" model that requires an estimate of canopy resistance to vapour d i f f u s i o n , and has worked well in well-ventilated canopiesy (e.g. Black et al_., 1970).  The canopy resistance required in the  Penman-Monteith model of transpiration was shown in Chapter 3 to be related to stomatal resistance and leaf area index in an unthinned stand. this chapter, the Penman-Monteith model i s not used.  In  As was indicated  in the previous chapter, since the aerodynamic resistance of the PenmanMonteith model w.is small compared to stomatal resistance,. their:model should be approximated  by a model of transpiration from a,leaf at the same temper-,  ature as the a i r . In this chapter, this very simple d i f f u s i o n model i s used to estimate fluxes from three tree canopy layers and the salal undergrowth in a thinned Douglas f i r stand.  These are summed to obtain an estimate o f '  stand transpiration rate which i s compared with energy balance measurements  96  of evapotranspiration rate.  In addition, the model, along with stomatal  resistance c h a r a c t e r i s t i c s , is used to predict the relationship between transpiration rate and v.p.d. f o r d i f f e r e n t s o i l water potential classes in the thinned stand.  2. Theory  -2 The rate of d i f f u s i o n of water vapour (E, " . g cm"  -1 s  ) from a  transpiring leaf or needle i s given by: be E =  ZjL L  Y  e - e -§ §r + r s  (1)  b  -3 where p v i s the density of moist a i r (1.2 x 10" heat of moist a i r (1.01 J g~^  -3 g cm"  ), c^ i s the s p e c i f i c  C~^), L i s the latent heat of vapourization  of water (2452 J g"^), y i s the psychrometric constant (0.66 mb is the vapour pressure within the stomatal c a v i t i e s (mb), e  =  C~^), e  g  i s the vapour  a pressure of the a i r outside the boundary layer of the leaf (mb), r the stomatal resistance to water vapour d i f f u s i o n (s cm"^)  g  is  and r ^ i s the  boundary layer resistance to water vapour d i f f u s i o n (s cm~^).  Since the  r e l a t i v e humidity of stomatal c a v i t i e s are approximately 100%,  e  assumed to be equal to the saturation vapour pressure at leaf  temperature.  This can be expressed as: e ~ e*(Ti) s  g  is  (2)  97  where  i s the leaf temperature  ( C). In well-ventilated coniferous  canopies, leaf temperature i s not too d i f f e r e n t from a i r temperature (see Appendix A).  For example, i n the thinned stand of this study,  infrared thermometer measurements indicated that Douglas f i r twig temperatures were usually within 1 C of a i r temperature.  Even the salal  leaf  temperatures were within 2 C of a i r temperature,partly the r e s u l t of shading by the trees.  As a r e s u l t , the following approximation can be  used: e  s  ^  e*(T )  (3)  a  where T i s the a i r temperature. a be approximated e*(T )-e . a  a  This suggests that e  s p  - e, in (1) can a  by the vapour pressure d e f i c i t (v.p.d.) of the a i r ,  Values of r ^ for the Douglas f i r needles were estimated from  a mass transfer formula determined by Landsberg and Ludlow (1969) using detached Sitka spruce shoots.  Since wind speeds in the mid-portion of the  Douglas f i r canopy in this study were usually much greater than 90 cm s  - 1  during the daytime hours, values of r ^ were usually less than 0.2 s cm . -1  (  Even at 0.5 m above the s o i l surface, windspeeds were usually much greater than 50 cm s" owing to the openness of the canopy. 1  From the work of  Parlange e_t al_. (1971) on broad leaves, these values suggest that r ^ of salal was usually much less than 0.4 s cm . -1  Since in this study r  g  was  usually greater than 4 s cm , r ^ could be considered much smaller than r -1  g  and (1) could be approximated by:  E  =  v.p.d.  (4)  It should be noted that the underestimate of transpiration rate obtained by  98  using v.p.d. as an estimate of the vapour pressure difference between leaf and a i r i s compensated somewhat by using r  g  alone in equation (4).  In order to use (4) to calculate the rate of transpiration from a forest canopy, the canopy must be s p l i t into layers and the leaf area index of each layer determined.  The transpiration rate per unit ground  surface from a p a r t i c u l a r layer i with leaf area index LAI^. and  average  stomatal resistance r . i s given by: pc  LAI. — !  E. =  v.p.d.. 1  (5)  The transpiration rate per unit ground surface from the entire canopy is obtained by summing (5) over a l l n layers and can be written as:  E  =  pc —2L Y  n LAI. v.p.d.. z 3 1 1 =1  (6)  This equation was suggested but not used in chapter 3.  3. Experimental  3.1  Experimental  Procedure  site  The experimental s i t e was  located approximately 27 km northwest  Courtenay, B.C. on the eastern coast of Vancouver Island.  of  The stand,planted  in 1953, contained trees ranging in height from 7 to 10 m and averaging 10.6 cm in diameter at breast height.  The stand density was 840 stems  ha~\  99  and thinning (from 1470 stems ha" ) was completed in May, 1975. Salal undergrowth  in the stand completely covered the s o i l .  A detailed s i t e  description : i s given in Chapter 2.  3.2 Measurements Continuous half-hourly measurements of evapotranspiration were made from June 29 to August 11, 1975 using the energy balance/Bowen ratio method.  This measurement;procedure  was described in 'Chapter 3.  Stomatal  resistance was measured using a ventilated d i f f u s i o n porometer (Turner et al_., 1969) described in Chapter 1.  Over the experiment period, routine  measurements (3 times during the daytime) were made on 22 days, while intensive measurements (every 2 to 3 hours during the daytime) were made on 7 days.  At least 2 samples of 4 needles each were used to obtain a  single value of r^.  Stomatal resistance and v.p.d. were measured at the  following heights: 0.5 m ( s a l a l ) , 3, 5 and 7 m.  Further d e t a i l s of these  stomatal resistance measurements <-war.e'contained in .Chapter 2. Leaf area index measurements were made on 4 trees in the thinned stand during the f i r s t 2 weeks of August, 1975. On 3 of the trees, the number of branches on each whorl were counted and an average branch on each whorl was removed.  The total dry weight of needles on this branch was  measured as well as the needle area to dry weight r a t i o of 100 needles from this branch.. On the fourth tree a l l needles were removed and the total dry  weight r a t i o was also determined.  This procedure was used on the fourth  tree as a check on the v a l i d i t y of measuring the needle weight of an average branch and multiplying by the number of branches per whorl in the case of  100  the other trees. The plan area of the 100 needles was determined by f i r s t placing them on photographic paper in darkness and then exposing the paper to light.  This operation was done in the field the evening after the needles  were removed from the tree.  Later in the laboratory, the photosensitive  paper was used to make negative slides.  The final step was to expose the  slide to a light source and measure the intensity of the transmitted light by means of a photo cell under diffusing.plastic.  This technique proved  to be accurate to within 2% and to be very convenient. The leaf area index of the salal undergrowth (including a small quantity of other species) was determined by removing a l l undergrowth within a square meter of ground surface at 3 locations in the thinned stand. The area was measured with an automatic photo-electric leaf area meter. A summary of the leaf area (projected area) data for the Douglas f i r canopy of the thinned stand is given in Table 1 (see also Appendix B). The data from Table 1 may be converted into leaf area index values by 2 dividing by 11.9 m /tree, which is the reciprocal of the stand density. Also included are the 3 salal undergrowth leaf area index values. Since the stomatal resistance and v.p.d. measurements were made at 3 heights in the Douglas f i r canopy and at the 0.5-m height in the salal undergrowth, the stand was considered as being composed of 4 layers. The LAI value of each layer was as follows: (Douglas f i r canopy) 0 to 3.5-m height, 2.0; 3.5 to 6.0-m height, 1.2; 6-m height to tree top, 0.4; (salal undergrowth) 0 to 0.5-m height, 3.0. The Douglas f i r canopy values were obtained by weighting the LAI distribution for each tree in Table 1 by the percentage of trees in the stand that were in the following D.B.H.  101  Table 1: Summary of leaf area (projected area basis) data f o r the Douglas f i r canopy of the thinned stand at Courtenay,.B.C. The leaf area index values were obtained by multiplying by the stand density ( 8 4 0 stems ha~^).'  Also included are the leaf area  index values of the undergrowth at three locations.  2  Douglas f i r needle area (m )  Height Interval  Tree No.  (m)  1  2  3  4  0 -  1  0.82  0.41  1.53  1.94  1 -  2  8.05  7.14  5.58  4.54  2 -  3  11.14  8.98  6.69  5.76  9.81  13.15  8.01  2.69  8.97  7.96  3.13  0.93  6.05  4.77  2.20  0.12  3 - 4 4 -  5  5 - 6 6 -  7  6.33  2.38  0.38  0.02  7 -  8  2.21  0.34  0.29-  0  8 -  9  0.83  0.07  0.01  0  9 - 1 0  0.37  0  0  0  >10  0.10  0  0  0  54.7  45.2  27.8  16.0  D.B.H. (cm)  12.3  11.0  9.6  6.0  Height  10.5  8.9  8.1  6.4  v •- : 2 , 3 . J :  .1.4.  Total  (m ) 2  (m)  LAI  4.6  1  3; 8  Salal undergrowth (LAI) Location No. LAI  1  2  3  4.0  2.9  1.7  102  ranges and then summing: 6.0 tb 7,5, 7.5 to 10, 10 to 1.2 and 12 to 15 cm-. The percentages in these ranges were-'.6,.<!|9j'3j£/a;nd..35 v  respectively.  4. Results and Discussion  4.1 Daytime variation of transpiration rate within the Douglas f i r canopy Fig. 1 shows the daytime variation of transpiration rate in the 0 to 3.5-m, 3.5 to 6.0-m, and >6.0-m layers calculated using (5) for the Douglas f i r canopy on 2 selected days when r^ was intensively measured. The soil water potentials (^ ) on the 2 days shown in Fig. 1 were -0.5 s  and -10.5 bars respectively.  Regardless of the value of ^ , the rate of  transpiration from each layer decreased with increasing layer height. This was largely attributed to the higher LAI values of the lower canopy layers.  While stomatal resistance variation with height increased as soil  water potential decreased, in general, variation with height was relatively small (see Fig. 3, "Chapter 2). Fig. 1 also shows that as the soil dries out, the rate of transpiration from each layer substantially decreases as expected (see also Appendix C).  i  4.2 Daytime variation of transpiration rate in Douglas f i r and salal The daytime variation of transpiration rate in Douglas f i r and salal  103  Fig. 1.  Daytime courses of transpiration rate (E) for 3 layers in the thinned Douglas f i r stand at high and low values of soil water potentials (ip ) on 2 selected fine days when stomatal resistance was intensively measured.  The values  of net radiation (R ) shown are for the 24-hour period. n  104  is compared in Fig. 2. Also shown are the net radiation (R ) and v.p.d. p  at the top of the canopy.  The Douglas f i r values were obtained by using  (6) with n = 3 or by summing the 0 to 3.5-m, 3.5 to 6.0-m and >6.0-m layer values shown in Fig. 1. The results obtained show that on June 30, when the value of ^ was high (Fig. 2a), the Douglas f i r had a higher s  transpiration rate than that of the salal during the early morning and late afternoon.  During the hours between 1000 and 1700 PST, salal trans-  piration rate was higher.  During this time, Douglas f i r transpiration rate  showed l i t t l e relationship to the net radiation.  This is largely a result  of stomatal closure in response to increasing v.p.d. the value of ij>  On July 29, when  was low (Fig. 2b), the rate of transpiration from both  species was much less than on June 30; however, the decrease in the rate of transpiration from the salal was much less than that of the Douglas fir.  Consequently, when the value of ^ was low,,the salal was consuming s  more water than the Douglas f i r .  This is because the stomatal resistance  of the salal increases much more slowly in response to increasing v.p.d. and decreasing i|» than does the stomatal resistance of the Douglas f i r (see Fig. 5 and 6).  Additional results similar to those shown in Fig. 2 are  shown in Appendix D.  4.3 Comparison of transpiration rates obtained from stomatal resistance and energy balance measurements Figs. 3 and 4 show values of the transpiration rate obtained from both stomatal resistance and energy balance measurements during the daytime on 6 of the 7 fine days on which intensive stomatal resistance measurements were made during the drying period. Those obtained from stomatal  105  1  1  1  HOUR ,  1  :  t  r  P.S.T.  Fig. 2.- Daytime courses of-transpiration rate (E) f o r Douglas f i r and salal in the thinned stand f o r the same 2 days shown in F i g . 1. The Douglas f i r values were obtained from the sum of the values for each of the 3 layers shown in the previous figure.  Also shown  are the courses of the net radiation (R^) and vapour pressure d e f i c i t (v.p.d.) at 10.5-m height.  106  6  9  12  15  18  21  HOUR, P.S.T.  Fig. 3. Comparison of the daytime courses of transpiration rate (E) obtained from stomatal resistance and energy balance measurements on 3 selected fine days at the thinned stand when the soil water potential (ty^) was relatively high. (R ) n  The values of net radiation  shown are for the 24-hour period.  107  (a)  0.2  JULY 28, 1975  O.I h  E  (b)  0.2  JULY 29, 1975  f =-l0.5 bars  (mm hr"')  s  R = l5.6 MJm^day"  0.1  0 0.2  1  n  °  o STOMATAL RESISTANCE MEASUREMENT  •  • ENERGY BALANCE MEASUREMENT  L  (0 JULY 30,1975 V-H.3  bars  R = l4.6 MJrrf^y, n  0.1  12  15  18  21  HOUR , P.S.T.  Fig. 4.  Comparison of the daytime courses of transpiration rate (E) obtained from stomatal resistance and energy balance measurements on 3 selected fine days at the thinned stand when the s o i l water potential  ( i p ) was low. . The values of net radiation ( R ) shown s  are f o r the 24-hour period.  n  108  resistance measurements involved using equation (6) with r , LAI and v.p.d. data for the 3 Douglas f i r canopy layers and the salal layer. . Consequently, the results in Figs. 3a and 4b are simply the sum of the Douglas f i r and salal data shown in Fig. 2.  June 30, July 5 and 11 were  days at the beginning of the drying period when values of ^  s  were rela-  tively high (Fig. 3), There is excellent agreement between stomatal resistance and energy balance measurement values from 0830 to 1730 PST.  This  agreement lends support to the assumption that there is l i t t l e evaporation -from;the: soil in the thinned stand because of the complete coverage of undergrowth. In Fig. 4, both methods of obtaining stand transpiration rate are compared for July 28, 29 and 30, near the end of the drying period, when the value of ^  was approximately -10 bars.  transpiration rates.  Both methods indicate very low  During most of the daytime, agreement is good;  however, in some.cases, values obtained from stomatal resistance measurements appear to be larger than the corresponding energy balance values. This is likely because, at high Bowen ratio values (low transpiration rates), the energy balance/Bowen ratio approach using a psychrometric apparatus is prone to considerable error (Fuchs and Tanner, 1970). interesting to note that when values of  It is  were low, a marked decrease in  the transpiration rate as a result of stomatal closure occurred on the second of the 3 consecutive days shown in Fig. 4.  On the third day,.the  transpiration rate showed a marked increase over the previous day as a result of partial stomatal opening.  This stomatal behaviour appears to  be due to an overall improvement in tree water status made possible by the stomatal closure on the previous day.  109  Using the data from Figs. 3 and 4 and also data for July 6, values of transpiration rate obtained from stomatal resistance measurements are shown in Fig. 5 plotted against corresponding values obtained using energy balance measurements.  In general, the correlation is good;  although as previously observed, differences between corresponding values increased as the soil became drier. In Table 2, the average daytime values of stand transpiration rate calculated from stomatal resistance measurements are compared with energy balance values for the 7 days when intensive stomatal resistance measurements were made. Agreement is within 15% except on July 28 when the values obtained from stomatal resistance measurements are 30% higher than the energy balance measurements. Stomatal resistance measurement has another practical application besides providing a means of estimating stand transpiration rate.  As  shown earlier in Figs. 1 and 2, this measurement provides information as to the source distribution of transpired water in the stand.  Of particular  interest to foresters is how much water is consumed by undergrowth vegetation after stand thinning in water-short areas.. Table 2 shows average daytime rates of transpiration of the salal calculated from stomatal resistance measurements. These values were divided by the corresponding values for the entire stand, which also were calculated from stomatal resistance measurements, to provide an estimate of the proportion of stand transpiration originating from the salal.  Table 2 shows that this proportion  rose from roughly 45% at high soil water potential ( 0 » t ( j roughly 70% at low soil water potential (-9.5  s  > -3.5 bars) to  > -12.5 bars). This  indicates that the transpiration rate of the Douglas f i r decreased more  no  E (E.B.M.)  (mm hr' ) 1  Fig. 5. Values of transpiration rate obtained from stomatal resistance measurements [E(S.R.M.)] plotted against corresponding values obtained from energy balance measurements [E(E.B.M.)].  The  data were obtained from 7 selected fine days when stomatal resistance was intensively measured in the thinned stand.  Table 2: Comparison of average daytime transpiration rates (E) f o r the thinned stand calculated using the simple d i f f u s i o n model (with stomatal resistance measurements) with those measured using the energy Bowen r a t i o meithod.  Also shown are the average daytime transpiration rate of the salal and the r a t i o of  these values to the corresponding values f o r the entire stand also calculated from stomatal measurements.  Average Daytime Transpiration Rates (mm hr" ) Stomatal Period (PST)  Date, 1975  Energy balance measurements  resistance measurements  Douglas f i r and salal  Salal  E  s a l a l ', %  E  total  June 30  5 - 21  0.21c  0.240 i  o.io:  42  5  7 - 21  0.29,..  0.2?v  0.13<  50  6  8h-  0.27i  0.26;,  0.11  44  July  19  11  5-19  0.244-  0.193  o.n •  55  28  SH-  0.07,8  0.09)  0.06 V  62  29  6-19  0.06/  o.om  0.05!:  71  30  6V  0.11,  0.12"?'.?  0.09  72  19%  19  balance/  resistance  112  rapidly than that of the salal undergrowth as the soil became drier. These results suggest that one of the advantages of thinning (i.e. increased water supply per tree) can be partly offset, in some situations, by the water consumption of the undergrowth.  4.4 Dependence of stand transpiration rate on vapour pressure deficit and soil water potential In this section, the vapour diffusion model together with stomatal resistance characteristics are used to examine the dependence of Douglas f i r and salal transpiration rate on vapour pressure deficit and soil water potential. Fig. 6 shows stomatal resistance measurements at 3-, 5-, and 7-m heights made during the drying periodon the thinned Douglas f i r canopy plotted against v.p.d. for 3 classes of $ . These data were found to be best described by the following regression equations: 0 > xLi > -3.5 bars s " r  = exp[1 .4581 + O.OO27(v.p.d.));0 2  s  (7a)  -3.5 > \b > -9.5 bars s r  r  s  = exp[l.9901 + 0.0034(v.p.d.)j]  (7b)  -9.5 > ty > -12.5 bars s  r  s  = exp[2.6906 + 0.0057(v.p.d.)§]  Lines representing these equations are shown in Fig. 6.  (7c) This figure is  113  v.p.d.  Fig. 6.  (mb)  Relationship between stomatal resistance ' C r ) and vapour pressure deficit (v.p.d.) for the 3-, 5- and 7-m heights in the thinned Douglas f i r stand. potential U ) s  The data have been separated into 3 soil water  classes and curves through the respective classes  are described by equations (7a, b and c) in the text.  114  very similar to Fig. 7 in Chapter 2, except that in the latter case only measurements from the 7-m height were used and the lines were fitted by eye to the data. There has been no height separation in Fig. 6 because sampling variation at a particular height was often not much less than that between heights. Figv 7 shows a similar relationship between r  g  and v.p.d. in salal for the same classes of ip . Also shown are lines representing the following regression equations: 0 > ^  > -3.5 bars  s  r  = exp[1.4418 + 0.0019 (v.p.d.)?]  (8a)  -3.5 > ib > -9.5 bars s r = exp[1.7436 + 0.0031 ( v . p . d . ) ? ]  (8b)  s  r  s  -9.5 > ii > -12.5 bars s r  r  s  = exp[2.1768 + 0.0027(v.p.d.)j]  (8c)  The same figure is shown in Chapter 2 (Fig. 8) except that eye-fitted lines were used in Chapter 2. (-3.5 > ifi  In Figs. 6 and 7, the data for the middle class  > -9.5 bars) covered a wider soil water potential range than  the other 2 classes because of less available data. discussion of these characteristics  There is further  , in Chapter 2.  Fig. 8 shows the transpiration rate of Douglas f i r and salal as a function of v.p.d. for 3 different classes of ij; calculated using (6), (7) and (8). It should be noted that i t is meaningful  to compare the  transpiration rates of Douglas f i r and salal at the same v.p.d. value using Fig. 8, since in the thinned stand vertical variation in v.p.d. was  115  ,  .  :  1—  1  1—  1  o |-3.5 >V 2 -9.5 bars S  SALAL  F i g . 7.  Relationship between stomatal resistance (r ) and vapour pressure d e f i c i t (v.p.d.) for the salal undergrowth in the thinned stand. The data have been separated into 3 s o i l water potential (JJ ) S  classes and curves through the respective classes are described by equations (8a, b and c) in the text.  116  DOUGLAS FIR  v.p.d. (mb)  Fig. 8.  Transpiration rate (E) of Douglas f i r and salal in the thinned stand as a function of vapour pressure deficit (v.p.d.) for 3 different soil water potential  (ij; ) s  classes.  The curves were calculated using  the simple vapour diffusion model (equation 6) together with stomatal resistance characteristics (equations 7 and 8).  117  relatively small. For example, the regression relationship between v.p.d. measurement at 0.5-m  height and the 10.5-m height.was as follows,  (see also Appendix E): vip'.d.g g 0.65 and  x  = 2.2 mb..  m  = 0.217 + 0.981  In the high ^  s  v.p.d.-jg ^ with R =  range (0 > ^  > -3.5 bars), the  transpiration rate of the Douglas f i r exceeded that of the salal when the v.p.d. was less than 15 mb, while at higher v.p.d. values the reverse was true.  In the medium  IJJ  s  range (-3.5 >  tp  s  > -9.5 bars), the transpir-  ation rate of the salal remained slightly higher than that of the Douglas f i r for all v.p.d. ranges.  In the low  range (-9.5 > ^  s  > -12.5 bars),  the salal had a much higher transpiration rate than that of the Douglas f i r over the whole v.p.d. -range. ,j,Fig. 8 also shows that the transpiration rate of the Douglas f i r decreased more rapidly than that of the salal as the soil became drier (see also Fig. 2 and Table 2). Fig. 9 compares stand transpiration rates measured by the energy balance method and calculated using the vapour diffusion model together i  I with stomatal resistance characteristics.  Each datum; po-int is the average  of 6 energy balance values obtained over a 3-hour period. This value is plotted against the average value of the v.p.d. at the 10.5-m height. The curves in Fig. 9 are simply the sum of each pair of curves (Douglas f i r and salal) shown in Fig. 8.  As indicated earlier, there is small  error in this procedure because of small change in v.p.d. with height. The results show a general agreement between measured and calculated values; however, agreement is closer at higher than at lower values of ip . Measurement difficulties at low values of  were discussed earlier.  As suggested in Fig. 8, there appears to be an optimum value of v.p.d. for a particular ty range at which maximum stand transpiration rate occurs. c  118  0.4 r  v.p.d.  Fig. 9.  (mb)  Comparison of thinned stand transpiration rates (E) measured by the energy balance method and calculated using the simple d i f f u s i o n model (equation 6) together with stomatal resistance c h a r a c t e r i s t i c s (equations 7 and 8) f o r 3 d i f f e r e n t s o i l water potential  (^ ) s  classes.  Each datum point i s the average of 6  energy balance values obtained over a 3-hour period.  119  As can be seen in Fig 9, the model indicates that these v.p.d. values are roughly 15, 13 and 11 mb for the high, medium and low ranges of ^ respectively.  The energy balance data certainly confirms this at high  values of ip , and does not show substantial disagreement  in the other two  ranges of ip . The relationship shown in Figs. 8 and 9 probably have important implications to tree growth.  Since the stomata control, to a  large extent, CC^ entry into the needles, i t seems reasonable to speculate that the optimum v.p.d. range for maximum photosynthesis varies with the soil water potential.  5. Conclusions  Stomatal resistance measurements were used to study the daytime trends of transpiration rate in Douglas f i r and salal.  It was found that the  transpiration rate of the Douglas f i r decreased more rapidly than that of the salal as the soil became drier.  Calculations of stand transpiration  rate,based on a simple vapour.diffusion model,showed good agreement with energy balance/Bowen ratio measurements. This agreement suggests that the model can be used to determine the vertical distribution of transpiration rate within a thinned stand.  The fraction of total stand transpir-  ation that originated from the salal undergrowth was found to increase from roughly 45% at soil water potential values greater than -3.5 bars to roughly 70% at soil water potential values between -9.5 and -12.5 bars.  120  Both the model and energy balance measurements indicated that the v.p.d., at which maximum stand transpiration rate occurred, increased with increasing soil water potential.  121  References  Black, T.A.,  C.B. Tanner, and W.R.  Gardner. 1970. Evapotranspiration  from a snap bean crop. Agron. J . -62: 66-69. Black, T.A.,  C S . Tan, and J.U. Nnyamah. 1976. Effect of thinning on  evapotranspiration from a Douglas f i r forest. Final Report to the P a c i f i c Forest Research Center, Department of the Environment, Victoria,  B.C.  Davies, J.A. and CD.  A l l e n . 1973. Equilibrium, potential and actual  evaporation from cropped surfaces in southern Ontario. J . of Applied Meteorol. 12: 649-657. Fuchs, M. and C.B. Tanner. 1970. Error analysis of Bowen r a t i o measured by d i f f e r e n t i a l psychrometry. Agr. Meteorol. 7: 329-334. Landsberg, J . J . and M.M.  Ludlow. 1969. Technique f o r determining resistance  to mass transfer through the boundary layers of plants with complex structure. J . of Applied Ecology 7: 187-192. Monteith, J.L. 1965. Evaporation and Environment. Symp. Soc. Exp. B i o l . 19: 205-234. Parlange, J.Y., P.E. Waggoner, and G.H.  Heichel. 1971. Boundary layer  resistance and temperature d i s t r i b u t i o n on s t i l l  and flapping leaves.  Plant Physiol. 48:.437-442. Ritchie, J.T., E. Burnett, and R.C. Henderson. 1972. Dryland evaporative flux in a subhumid climate: I I I . Soil water influence. Agron. J . 64: 168-173. Turner, N.C,  F.C.C. Peterson, and W.H.  Wright. 1969. An aspirated d i f f u s i o n  porometer f o r f i e l d use. Con. Agr. Exp. Sta. S o i l s B u l l . No. XXIX: 1-7.  122 SUMMARY The ventilated diffusion porometer designed by Turner et al_. has been evaluated and a calibration procedure has been developed that can be satisfactorily used to measure stomatal diffusion resistance of Douglas f i r needles. In a Douglas f i r forest, during the daytime, stomatal diffusion resistance was found to be mainly related to soil water potential and vapour pressure deficit of the air.  The stomatal resistance of salal  undergrowth has a smaller response to vapour pressure deficit change than that of Douglas f i r , especially at low soil water potential.  This suggests  that salal takes an increasing proportion of the soil water as the soil becomes drier.  There was l i t t l e effect of thinning on the stomatal  resistance characteristics of Douglas f i r observed within a year of the thinning operation.  Stress history appeared to cause a shift in the  relationship of stomatal resistance to twig water potential, but has l i t t l e effect on the relationship of stomatal resistance to vapour pressure deficit and soil water potential. It has been shown that the canopy resistance of a Douglas f i r forest is largely of physiological origin.  Calculations of stand transpiration rate  in the thinned stand, based on a simple vapour diffusion model, showed good agreement with energy balance/Bowen ratio measurements.  This agreement  suggests that the model can be used to determine the vertical distribution of transpiration rate within a thinned stand.  The fraction of total stand  transpiration from the thinned stand that was transpired by the salal increased from roughly 45% at soil water potential values greater than -3.5 bars to roughly 70% at soil water potential values between -9.5 and -12.5 bars.  Both the model and energy balance measurements indicated that  123  the vapour pressure deficit corresponding to maximum stand transpiration rate increased with increasing soil water potential.  It seems reasonable  to speculate that the optimum vapour pressure deficit range for maximum photosynthesis varies with the soil water potential.  APPENDIX A  COMPARISON OF LEAF AND AIR TEMPERATURE MEASUREMENTS  COMPARISON OF LEAF AND AIR TEMPERATURE MEASUREMENTS  The figure shows the relationship between leaf temperature (T-|) and air temperature (T ) for Douglas f i r and salal. g  equations for both species are shown in the figure.  The regression  APPENDIX B  NEEDLE SURFACE AREA DISTRIBUTIONS IN DOUGLAS FIR  128  NEEDLE SURFACE AREA DISTRIBUTIONS IN DOUGLAS FIR  The figures show the distributions of the projected needle surface area (N.S.A.) for 4 trees in the thinned Douglas f i r stand at Courtenay, B.C. Also shown is a table summarizing the projected needle surface area and height (h) for each whorl of the same 4 Douglas f i r trees.  129  10  ~l  1  r  1  r  -i  T R E E No. I h=l0.5m D.B.H. = 12.3 cm  8h h (m)  0' 0  .1  I  4  8 N.S.A. ( m ) 2  I  L  12  TREE  No.2  h = 8.9 m D. B.H. =  8  0  N.S.A.  11.0 cm  12  TREE  No. 3  h =8.1 m D.B.H. = 9.6 cm  0  8 N.S.A. ( m ) 2  1:32  i  1  T R E E No. 4.h = 6.4 m D.B.H. = 6.0 cm  h (m)  0;  J  L  N.S.A. (m ) 2  :  r  h(m) N.S.A.(m ) 2  Tree No. 4  Tree No. 3  Tree No. 2  Tree No. 1  h(m) N.S.A.'m) 2  h(m) N.S.A.(m ) 2  h(m) fLS.A.Cm ) 2  0.7  0.82 •  0.7  0.41  0.3  0.06  0.4  0.11  1.1  3.54  1.2  1.32  0.5  0.01  0.6  0.26  1.5  4.51  1.5  2.30  0.7  0.76  0.8  1.06  2.0  7.03  1.9  6.64  0.9  0.70  1.0  0.53  2.5  4.11  2.5  5.88  1.3  2.54  1.3  1.19  3.3  4.72  3,1  7.25  1.6  3.04  1.6  1.69  3.9  5.09  3.5  5.90  2.2  3.34  1.9  1.66  4.5  4.45  4.1  4.63  2.6  3.35  2.2  1.81  5.0  5.45  4  -  3.33  3.2  4.75  2.5  1.51  5.6  5.12  5,3  2.76  3.7  3.26  2.8  1.71  6.3  4.41  5.5  2.01  4.3  2.40  3.0  0.73  6.9  2.70  6.3  1.64  4.9  0.73  3.4  1.56  7.9  1.77  7.0  0.74  5.3  0.46  §.8  1.13  8.6  0.62  7.7  0.34  5.4  0.59  4/3  0.63  9.4  0.30  8.4  0.06  5.9  1.15  4.9  0.30  10.1  0.03  8.9  0.01  6.5  0.38  5.6  0.10  10.5  0.01  7.1  0.24-  6.0  0.02  7.7  0.05  6.4  0  8.1  0.01  Total 45.2 (m ) 2  7  54.7  27.8  16.0  APPENDIX C  DAYTIME COURSES OF TRANSPIRATION RATE WITHIN DOUGLAS FIR CANOPY  DAYTIME COURSES OF TRANSPIRATION RATE WITHIN DOUGLAS FIR CANOPY  The figures show daytime courses of transpiration rate (E) for 3 layers in the Douglas f i r canopy at high and low values of soil water potential (ip ) on 4 selected fine days when stomatal resistance was intensively measured.  136  V-I^bors^  JULY 5, 1975  o.io  R = 18.7 M J m" day" 2  1  n  (a) 0.05  0 0.10  V =-2.4 bars  DOUGLAS FIR CANOPY LAYER  s  JULY II, 1975  R  n  ,  l  8  .  O  M  J  m  - 2  d  o  y  - ,  (b)  *  0.05  0 -3.5 m o 3.5 -6.0 m * >6.0m  o  E (mm hr ) -1  JULY 28, 1975 0.03  ^ -9.9 bars  R„ = 16.0 MJ m^day"  1  (0  0.02 0.01 0 0.03 (. JULY 30, 1975 0.02-  V.--II.3 bars  R = 14.6 M J m"day"' z  n  (d)  0.01 0 9  12 H O U R ,  15 P.  S T .  APPENDIX D  DAYTIME COURSES OF TRANSPIRATION RATE OF DOUGLAS FIR AND SALAL  DAYTIME COURSES OF TRANSPIRATION RATE OF DOUGLAS FIR AND SALAL  The figures show daytime courses of transpiration rate (E) as f i r and salal f o r the same 4 days shown in appendix C.  139  APPENDIX E  VERTICAL VARIATION OF V.P.D. WITHIN DOUGLAS FIR CANOPY  141  VERTICAL VARIATION OF V.P.D. WITHIN DOUGLAS FIR CANOPY  The figure shows the relationship between vapour pressure deficit (v.p.d.) at 0.5-m height and that at 10.5-m height. equation is shown in the figure.  The regression  142  143  APPENDIX F  NEEDLE SURFACE AREA VS. D.B.H.  144  NEEDLE SURFACE AREA VS. D.B.H.  The figure shows the relationship between total projected needle surface area (N.S.A.) and diameter at breast height (D.B.H.) for the thinned Douglas f i r stand.  APPENDIX G  DIURNAL ENERGY BALANCE DIAGRAMS  147  DIURNAL ENERGY BALANCE DIAGRAMS  The figures are diurnal energy balance diagrams f o r the thinned Douglas f i r stand at.Courtenay, B.C.  These are the days on which  stomatal resistance was intensively measured.  148  154  PUBLICATIONS: Tan, CS. and G.W. Gee. 1972. Influence of root pressure on transpiration and net C0 assimilation in the S.P.A.C. Paper presented at 64th Annual American Society of Agronomy Meeting, Oct. 29 to Nov. 2, 197.2. 2  Gee, G.W., B.E. Janes and CS. Tan. 1973. A chamber for applying pressure to root of intact plants. Plant Physiol. 52: 472-474. Black, T.A., P.A. Tang, CS. Tan, J.R. Curtis and K.G. McNaughton. 1974. Measurement techniques used in forest hydrometeorology. Final Report for the Director, Pacific Forest Research Center, Dpet. of the Environment, Victoria, B.C. Black, T.A., CS. Tan, J.R. Curtis, J.U. Nnyamah. 1975. Factors affecting evapotranspiration of an unthinned Douglas f i r forest. Final Report for the Director, Pacific Forest Research Center, Dept. of the Environment, Victoria, B.C. Black, T.A., CS. Tan and J.U. Nnyamah. 1976. Effect of thinning on evapotranspiration from a Douglas f i r forest. Final Report for the Director, Pacific Forest Research Center, Dept. of the Environment, Victoria, B.C. Tan, CS. and T.A. Black. 1976. Factors affecting the canopy resistance of Douglas f i r forest. Boundary Layer Meteor. 10: 475-488. Tan, C.S., T.A. Black and J.U. Nnyamah. 1977. Characteristics of stomatal diffusion resistance in a forest exposed to large soil water deficits. Paper presented at 68th Annual American Society of Agronomy Meeting, Nov. 29 to Dec. 2, 1976 and submitted to Can. J. of For. Res. 7. Tan, CS., T.A. Black and J.U. Nnyamah. 1977. A simple diffusion model of transpiration applied to a thinned Douglas f i r stand. Ecology: 58 (submitted). Nnyamah, J.U., T.A. Black and CS. Tan. 1977. Resistance of water uptake in a Douglas f i r forest. Soil Sci. (submitted). Nnyamah, J.U., T.A. Black and CS. Tan. 1977. Effect of thinning on the water balance of a Douglas f i r forest. Can. J. of For. Res. 7 (submitted). Nnyamah, J.U. and CS. Tan. 1977. The role of thinning in improving the forest soil moisture regime. Paper presented at 6th B.C. Soil Science Workshop. April 20-21, 1977.  

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