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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 presenting t h i s thes i s in p a r t i a l f u l f i l l m e n t of the requirements f o r an advanced degree at the Un iver s i t y of B r i t i s h Columbia, I agree that the L ib ra ry sha l l make i t f r e e l y ava i l ab le f o r reference and study. I f u r ther agree.that permission f o r extensive copying of th i s thes i s f o r s cho la r l y purposes may be granted by the Head of my Department or by his representat ives . It i s understood that copying or pub l i ca t ion of th i s thes i s f o r f i n a n c i a l gain sha l l not be allowed without my wr i t ten permiss ion. Chin-Sheng Tan Department of So i l Science Un ivers i ty of B r i t i s h Columbia Vancouver, B.C., Canada V6T 1W5 Date J 7 ~ ^ - 77 i i ABSTRACT This thes i s reports the re su l t s of a study of stomatal d i f f u s i o n res i s tance in a Douglas f i r f o re s t near Courtenay, in the dry eas t -coastal be l t . o f Vancouver I s land. Stomatal d i f f u s i o n res i s tance was measured by a ven t i l a ted d i f f u s i o n porometer designed by Turner e_t al_. The theory of the operation of th i s porometer as proposed by Turner and Parlange is reviewed.. The problems of using the theory to i n te rp re t ven t i l a ted porometer data are discussed and the porometer evaluated fo r use in the measurement of the stomatal d i f f u s i o n res i s tance of Douglas f i r needles. As part of a two-year study of the e f f e c t of th inning on evapotrans-p i r a t i on in Douglas f i r , the re l a t i on sh ip between stomatal d i f f u s i o n res i s tance and environmental parameters were s tud ied . Research was con-ducted in an unthinned stand (1840 stems ha~^) with n e g l i g i b l e undergrowth and a thinned stand (840 stems ha~^) with substant ia l s a l a l undergrowth. During the daytime, stomatal res i s tance was mainly re la ted to the s o i l water potent ia l and the vapour.pressure d e f i c i t of the canopy a i r . Daytime values of stomatal res i s tance f o r Douglas f i r ranged from 2 to 60 s cm - ' ' f o r values of vapour pressure d e f i c i t between 4 and 24 mb and values of s o i l water potent ia l between 0 and -12.5 bars. Although increas ing stomatal res i s tance was usua l ly associated with decreasing twig water p o t e n t i a l , increas ing stomatal res i s tance appeared to be associated with increas ing twig water potent ia l when the vapour pressure d e f i c i t was high. Stress history.was found to cause a s h i f t in the re l a t i on sh ip of stomatal resistance to twig water potential, 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 soil water potential. 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 ratio measure-ments of evapotranspiration. The typical steady increase in both canopy and stomatal resistance during daytime hours, even at high soil 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 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. Values of transpiration rate in the thinned stand calculated from a simple vapour diffusion model that uses the vapour pressure def i c i t of the canopy ai r , and measurements of the stomatal resistance and leaf area index of the canopy agreed well with those obtained from energy balance/ Bowen ratio measurements. Stomatal resistance characteristics were also used in the diffusion model instead of actual stomatal resistance measure-ments 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 deficit 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 for evapotranspiration: Rp - G - M (W m-*1) or porometer aperture area or needle area (cm ) Specific heat of moist air (J g"^ C~^) D.B.H. Diameter at breast height (cm) d Diameter of hole in calibration plate (cm) -2 -1 E Evapotranspiration or transpiration rate (g cm s ) e Diffusion porometer chamber vapour pressure (mb) e a Water vapour pressure of the air outside the boundary layer of the leaf (mb) e*(Ta) Saturation water vapour pressure at air 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 calibration plate thickness (cm) LAI Leaf area index or needle area (projected area basis)/unit area of ground surface (dimensionless) LAI. LAI of ith 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 calibration plate (cm ) vi RH Relative humidity _o Rn Net radiation flux density (Win ) -3 -1 -1 Rv Gas constant for water vapour (mb cm g k ) r^ Boundary layer resistance to water vapour diffusion (s cm - 1) r £ Canopy or surface resistance (s cm ^ r . Canopy resistance at the jth hour of the daytime (s cm - 1) c J r c Mean daytime canopy resistance (s cm - 1); defined by equation (6) in Chapter 3 r^ Aerodynamic resistance to heat exchange between the forest and the height z (s cm - 1) rp Porometer diffusion resistance (s cm 1) r g Stomatal diffusion resistance on a projected area basis or diffusion resistance of the calibration plate (s cm - 1) r s i Stomatal diffusion resistance of the ith canopy layer (s cm"1) r y Aerodynamic resistance to water vapour exchange between the forest and the height z (s cm - 1) s Slope of the saturation water vapour pressure curve (mb C 1) T Porometer temperature (K) T Air temperature (C) a T-| Leaf temperature (C) t Transit time (s) V Effective volume of the porometer (cm ); V = V c + V g 3 V c Actual porometer chamber volume (32 cm ) 3 V s Capacity of the sensor to absorb water vapour (cm ) 3 V s Average sensor capacity for the porometer (cm ) v.p.d. Vapour pressure d e f i c i t ; e*(Ta)-e g or e* z - e z (mb) v i i v.p. d. . Ar i thmet ic mean of values of v .p .d . during daytime (mb) v.p. d ' i v .p .d . at the i t h canopy layer (mb) v. p. d - j v .p .d . at the j t h hour of the daytime (mb) a 2 -1 D i f f u s i v i t y of water vapour in a i r (cm s ) B Bowen r a t i o ; H/LE (dimensionless) Y Psychrometric constant (mb C"^) (=R v T a pCp /L ) e 3 3 Volumetric water content of s o i l (cm of water/cm of s o i l ) p Density of moist a i r (g cm ) *s So i l water p o t e n t i a l ; includes only matr ic and osmotic terms (bar) * t Twig water p o t e n t i a l ; as measured by Scholander pressure chamber (bar) vi i i TABLE OF CONTENTS Page ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i i LIST OF SYMBOLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . v LIST OF TABLES . . . . . . . ., . . . . . . . . . . . . . . . . . . . . . xi LIST OF FIGURES . . . . . . . . . . . . . . . . .... • " x i i i ACKNOWLEDGEMENTS . . . . . . . . . . xix INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 CHAPTER 1 - EVALUATION OF A VENTILATED DIFFUSION POROMETER FOR THE MEASUREMENT OF STOMATAL DIFFUSION RESISTANCE OF DOUGLAS FIRNEEDLES . . . . . . . . . . . . . . . . . . . . . . . . 4 Abstract . . . . . . . . . . . . . . . . . . . 5 1. Introduction . . . . . 5 2. Porometer Sensing Head ... . . . . . . . . . . . . . . . . . 6 3. Porometer Circuitry . . . . . . . . . . . . . . . 8 4. Porometer Theory . . . . . . . . . . . . . . . . . . . . . . 9 5. Humidity Sensor Calibration and Porometer Resistance Determination 12 5.1 Humidity sensor calibration 14 5.2 Porometer resistance determination . . . . . . 14 6. Measurement Procedure . . . . . . . . ...... . . . . . •. . 19 7. • Performance Tests 23 7.1 Test plates of known diffusion 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 . . . . . . . . . 40 Abstract . . . . . . . . . 41 1. Introduction . . 42 ix Paoe 2. Experimental Procedure . . . . . . . . . 43 2.1 Experimental s i t e s . . . . . . . . . . . . . . . . 43 2.2 Measurements . . . . . . . . . . . . . . . •. . . . 44 3. Results and Discussion . . . . . . . . . . . 47 3.-1 Var ia t ion in stomatal res i s tance and twig water potent ia l during the summer . . . . . 47 3.2 Daytime va r i a t i on in stomatal res i s tance of Dougl as f i r and sa la l . . . . . . . . . . . . . . 50 3.3 Relat ionship of stomatal res i s tance to i r rad iance . . . . . . . . . . . . . . . . . . . 52 3.4 Relat ionship of stomatal res i s tance to twig water potent ia l and vapour pressure d e f i c i t . . . 54 3.5 Relat ionship of stomatal res i s tance to vapour pressure d e f i c i t and s o i l water potent ia l . . . . 59 4. Conclusions 62 References 63 CHAPTER 3 - FACTORS AFFECTING THE CANOPY RESISTANCE OF A DOUGLAS FIR FOREST . . . . . . . . . . . . . . . . . . . . . . . 66 Abstract . . . . . . . . . . . . . . . . . . . . . . . . 67 1. Introduction . . . . . . . . . . . . . . . . . . . . . . 68 2. Theory . . . . . . . . . . . . . . . . . . . . . . . . . . 69 3. Experimental Procedure . . . . . . . . . . . -. 70 3.1 Experiment s i t e . . . . . . . . . . . . •. . . . • 70 3.2 Measurements . . . . . . . . 71 4. Results and Discussion . . 74 4.1 Course 'of s o i l water potent ia l during the measurement period . . . . . . . . . . . . . . . . . 74 4.2 Daytime course of net rad ia t i on and l a t e n t heat f l ux f o r two se lected days . . . . . . . . . . . . . 76 4.3 Comparison of daytime courses of stomatal and canopy res i s tance . . . . . . . . . . . . . . . . . 79 4. -4 Relat ionship of stomatal res i s tance to p lant and environmental parameters . . . . . . . . . . . 81 4.5 Relat ionship of canopy res i s tance to s o i l water potent ia l and vapour pressure d e f i c i t . . . 86 5. Conclusions . . . . . . . . . . . . . . . . . . , . . •. 89 References . . . . . . . . . . . •. . . 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 Procedure . 98 3.1 Experimental s i t e 98 3.2 Measurements 99 4. Results and Discussion 102 4.1 Daytime v a r i a t i o n of t r an sp i r a t i on rate within the Douglas f i r canopy 102 4.2 Daytime va r i a t i on of t r an sp i r a t i on rate in Douglas f i r and sa la l 102 4.3 Comparison of t r an sp i r a t i on rates obtained from stomatal res i s tance and energy balance measurements 104 4.4 Dependence of stand t r an sp i r a t i on rate on vapour pressure d e f i c i t and so i l water potent ia l 112 5. Conclusions 119 References 121 SUMMARY 122 APPENDIX A - Comparison of l e a f and a i r temperature measurements . . .124 APPENDIX B - Needle surface area d i s t r i b u t i o n s in Douglas f i r . . . . 127 APPENDIX C - Daytime courses of t r an sp i r a t i on rate with in Douglas f i r canopy 134 APPENDIX D - Daytime courses of t r an sp i r a t i on rate of Douglas f i r and sa la l 137 APPENDIX E - Ve r t i c a l v a r i a t i on of v .p .d . within Douglas f i r canopy 140 APPENDIX F - Needle surface area versus D.B.H 143 APPENDIX G - Dirunal energy balance diagrams 146 x i LIST OF TABLES Table Page CHAPTER 1 1. E f f e c t of temperature on the average sensor capac i ty (V ) as.found by Turner and Parlange (1970) and in th i s study . . . . 13 2. The re l a t i on sh ip between the d i f f u s i v i t y of water vapour in a i r ( a ) and temperature ( T a ) . i . . . . . . . . . . . 25 3a. D i f fu s i on res i s tance ( r g ) data at 24.5 C f o r d r i l l e d brass p lates used with ven t i l a ted d i f f u s i o n porometer. For symbols, see text 26 3b. D i f fu s ion res i s tance (r ) data at 24.5 C fo r perforated p lates with and without a 0.3-cm space between p late and b l o t t i n g paper.- The p lates were purchased from Perforated Products, Ind., 68 Harvard S t . , Brook! ine, Mass. 02164 27 4. D i f fu s ion res i s tance (r ) data at 24.5 C f o r d r i l l e d brass s p lates used with unvent i la ted d i f f u s i o n porometer. For symbols, see text . . . . . . . . . . . . . 28 5. Comparison of stomatal d i f f u s i o n res i s tance (r ) of two bean plants measured by vent i la ted, and unvent i lated d i f f u s i o n por.ometers 35 6. Comparison of stomatal res i s tance (r ) of f ou r .1 - yea r -o l d Douglas f i r seedl ings measured with a vent i l a ted d i f f u s i o n porometer and ca l cu la ted from the weight loss of the potted seedl ings 37 CHAPTER 2 1. The re l a t i on sh ip of twig water potent ia l (\pt) to vapour pressure d e f i c i t (v.p.d.) and s o i l water potent ia l ) fo r Douglas f i r in the unthinned stand, during the two dry periods in 1974, and in the thinned stand in 1975, and Table salal undergrowth in the thinned stand. The 90% con-fidence interval on each mean is also shown . . . . . . . . . . . CHAPTER 4 1. Summary of leaf area (projected area basis) data for 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) for the thinned stand calculated using the simple diffusion model (with stomatal resistance measurements) with those measured using the energy balance/Bowen ratio method. Also shown are the average daytime transpiration rate of the salal and the ratio of these values to the corresponding values for the entire stand also calculated from stomatal resistance measurements x i i i 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 - aper-ture for insertion of needles; I - aperture for broad leaves; J - electrical connector 7 2. Metering circuit for ventilated and unventilated diffusion porometer . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3. Calibration curves for lithium chloride humidity sensor suppl-ted by Hygrodynamics . . . . . . . . . . . . . . . . . . . 15 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) 16 5. The relationship between relative humidity and resistance of lithium chloride humidity sensor ... . . . . .-. . . . . . . 17 6. The relationship between porometer current and;sensor resis-tance for three different humidity ranges . . . . . . . . . . . 18 7. Porometer transit time versus time since the sampling of Douglas f i r needles 20 8. Calibration curve for unventilated diffusion porometer at 24.5 C 29 9. Transit time versus fan speed for different values of diffusion resistance (r g) 30 x iv Figure Page 10. Voltage versus fan speed fo r a 6-V Marx-Luder motor . . . . . . . 3 1 11. T rans i t time versus d i f f u s i o n res i s tance of d r i l l e d and perforated p la tes . The theore t i ca l r e l a t i o n s h i p , shown as the s t ra i gh t l i n e , i s equation (6) with RH-| = 25.3%, RH 0 = 29.5%, V = 88.4 cm 3 , v =32 cm 3 , A = 2.85 cm 2 and 2 s c r = 2.3 s c r rH . The poor agreement of the t r i ang l e data point i s thought to be due to an e r ro r in pos i t i on ing p la te beneath porometer aperture . . . . . . . . . . . . 3 3 12. T r an s i t time versus d i f f u s i o n res i s tance of d r i l l e d and perforated plates at 5 temperatures . . . . . . . . . 34 CHAPTER 2 1. Seasonal course of s o i l water potent ia l U s ) J twig water potent ia l (^t)> vapour pressure d e f i c i t (v .p id. ) and stomatal res i s tance (r ) measured each day between 1300 and 1430 PST in the unthinned Douglas f i r stand . 48 2"1 Seasonal course of s o i l water potent ia l ws)» twig water potent ia l vapour pressure d e f i c i t (v.p.d.) and stomatal res i s tance (r ) measured each day between 1300 and 1 s 1430 PST in the thinned Douglas f i r stand . . . . . . . . . . . . . 49 3. - Daytime course of stomatal res i s tance . ( r ) o f Douglas f i r at 3 heights and sa la l in the thinned stand showing mainly the e f f e c t of decreasing s o i l water potent ia l U S ) . Also shown i s the course of the vapour pressure d e f i c i t (v.p.d.) . . . 51 4. Re lat ionship between the quanta f l ux dens i ty (Q) f o r the 0.4-0.7 ym band width and stomatal res i s tance ( r g ) of Douglas f i r in the thinned and unthinned stands,.and sa la l undergrowth in the thinned stand. The approximate energy f l ux dens i ty in the 0.4-0.7 ym band width is shown at the top of the graph . 53 5. Re lat ionship between stomatal res i s tance (,r^) and twig water potent ia l ( I K ) fo r (a) Douglas f i r in the unthinned stand, 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 g) and twig water potential 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 d e f i c i t (v.p.d.) classes and 2 soil water potential ( i j ^ ) classes 7. Relationship between stomatal. resistance (r g) and vapour pressure d e f i c i t (v.p.d.) for Douglas f i r in the unthinned stand, during the two dry periods in 1974, and in the thinned stand in 1975 for 4 soil water potential (^g) ranges . . 8. Relationship between stomatal resistance (r g) and vapour pressure def i c i t (v.p.d.) for the salal undergrowth in the thinned stand for 4 soil water potential (^s) ranges . CHAPTER 3 1. Soil water potential ) in the unthinned stand during the period June 17 - August 19, 1974. Each datumpoint is the average value of the water potentials at the 15, 30 and 45-cm depths 2. The relationship between the 24-hour value of the ratio, LE/R and the soil water potential U ) for 18 fine (R Il r\ -I s n 14.0 MJ m day )•days in 1974 in the unthinned stand. The number adjacent to each data point is the date. Values of the average root.zone soil water content (e) corresponding to 4) are also shown rs 3. Daytime course of net radiation (Rn) and latent heat flux (LE)i for June 18 and July 25, 1974 in the unthinned stand, showing the effects of decreased soil water potential (^s) . 4. Daytime course of canopy resistance (r ) and stomatal resistance (r ) (projected leaf area basis) at the 8-m height xvi Figure Page in the unthinned stand f o r the days in F i g . 3 . . . . . . . . . . . 80 5. The re l a t i on sh ip between canopy res i s tance (r ) and stomatal res i s tance (r ) at the 8-m height in the unthinned stand for 10 days 82 6. Daytime courses of stomatal res i s tance (r ) at the 8-m height and vapour pressure d e f i c i t (v.p.d.) on two days with s im i l a r s o i l water potent ia l s in the unthinned stand . . . . 84 7. The re l a t i on sh ip between stomatal res i s tance (r ) at the 8-m height in the unthinned stand and s o i l water potent ia l (^ s). The data have been separated into s ix ranges of vapour pressure d e f i c i t (v.p.d.) and were daytime measure-ments made on 12 days 85 8. The re l a t i on sh ip between the mean daytime canopy res i s tance (F7) of the unthinned stand and s o i l water potent ia l U s ) . The data have been separated into three ranges of mean day-time vapour pressure d e f i c i t (v.p.d.) and were obtained over a period of 18 days . . . . . . . . . . . . . . 88 CHAPTER 4 1. Daytime courses of t r an sp i r a t i on rate (E) fo r 3 layers in the thinned Douglas f i r stand a t high and low values o f s o i l water potent ia l on 2 se lected f i n e days when stomatal res i s tance was i n ten s i ve l y measured. The values of net r ad ia t i on (R n) shown are fo r the 24-hour period 103 2. Daytime courses of t r an sp i r a t i on rate (E) fo r Douglas f i r and s a l a l 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 f o r each of the 3 layers shown in the previous f i g u r e . A lso shown are the courses o f the net rad ia t i on (R ) and vapour pressure d e f i c i t (v.p.d.) at 10.5-m height 105 3. Comparison of the daytime courses o f t r an sp i r a t i on rate (E) obtained from stomatal res i s tance and energy balance xvi i Figure Page measurements on 3 se lected f i n e days at the thinned stand when the s o i l water potent ia l (^ s) was r e l a t i v e l y high. The values of net r ad ia t i on (R ) shown are fo r n the 24-hour period . . . . . . . . . . 106 4. Comparison of the daytime courses of t r an sp i r a t i on rate (E) obtained from stomatal res i s tance and energy balance measurements on 3 se lected f i n e days at the thinned ; . stand when the s o i l water potent ia l ) was low. The values of net r ad ia t i on ( R ) shown are f o r the 24-hour n period . . 107 5. Values of t r ansp i ra t i on rate obtained from stomatal r e s i s -tance measurements [E(S.R.M.)] p lo t ted against c o r r e s -ponding values obtained from energy balance measurements [E(E.B.M.)]. The data were obtained from 7 se lected f i n e days when stomatal res i s tance was i n ten s i ve l y measured in the thinned stand. 110 6. Re lat ionsh ip between stomatal res i s tance ( r g ) and vapour pressure d e f i c i t (v.p.d.) f o r the 3-, 5- and 7-m heights in the thinned Douglas f i r stand. The data have been separated into 3 s o i l water potent ia l (^ s) c lasses and curves through the respect ive c lasses are described by.equations (7a, b and c) in the text . . . . . . . . . . . . . . . . . . . . 113 7. Re lat ionsh ip between stomatal res i s tance ( r $ ) and vapour pressure d e f i c i t (v.p.d.) f o r the sa l a l undergrowth in the thinned stand. The data have been separated into 3 s o i l water potent ia l (\p ) c la s ses and curves through the respect ive c lasses are descr ibed by equations (8a, b and c) in the text . . . 115 8. Transp i ra t ion rate (E) of Douglas f i r and sa l a l in the thinned stand as a funct ion of vapour pressure d e f i c i t (v.p.d.) fo r 3 d i f f e r e n t s o i l water potent ia l (^ s) c l a s se s . The curves were ca l cu la ted using the simple vapour d i f f u s i o n model (equation 6) together with stomatal res i s tance xviii Figure Page characteristics (equations 7 and 8) n 6 9. 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 ( s^) classes. Each datum point is the average of 6 energy balance values obtained over a 3-hour. period . . .... . . . . . n s XIX ACKNOWLEDGEMENTS I am great ly indebted to my.supervisor, Dr. T.A. B l ack , . f o r his d i r e c t i o n , ass i s tance and f r i endsh ip throughout the course of my doctoral s tud ies . Without his i n s i gh t and encouragement the success o f t h i s study would not have been pos s ib le . I extend s incere thanks to Drs. T.M. B a l l a r d , P.G. Haddock, T.R. Oke, and C A . Rowles, who served on my doctora l committee, and who have given advice and encouragement at a l l stages of t h i s study. I appreciate very much the cooperation of Mr. Joe Nnyamah, in generously supplying s o i l water potent ia l data necessary in my ana l y s i s . I wish to thank Mr. Paul Tang and Mr. Ron Toth f o r t h e i r va luable help in the instrumentat ion. The author acknowledges the valuable ass i s tance of Mr. Bernie von Sp ind ler in developing the photographic technique i used in th i s study, to determine projected needle area. Thanks are a lso given to Miss Ruth Hardy, Mr. John C u r t i s , Mr. Sandy Mcfar lane, f o r t h e i r help and f r i endsh ip in the f i e l d phase of th i s study, and to Miss Diane Green fo r typing the the s i s . S incere thanks go to Dr. Holger Brix and Mr. Mike Crown of the Canadian Forestry Service f o r t h e i r advice and cooperation during the f i e l d phase of t h i s s tudy. . A l so , I wish to acknowledge Crown-Zellerbach Company fo r al lowing us to conduct the research on i t s property. Thanks are expressed to the National Research Counci l of Canada f o r granting me a scholarsh ip during my doctoral study. The required funds f o r th i s research were made a v a i l a b l e through grants from the National Research Council of Canada and B.C. Department of Agriculture, and a contract with the Canadian Forestry Service. Last, but not least, the patience and encouragement of my wife, Chun-Chih, throughout my academic years, are acknowledged with my sincere affection. 1 INTRODUCTION The stomatal diffusion 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 diffusion resistance of leaves or needles plays a direct role in determining not only the rate at which water vapour is lost from the leaf but also the rate at which CO2 enters the leaf. An understanding of stomatal behaviour under f i e l d conditions is extremely important in developing accurate methods of estimating plant water use and in studying aspects of the problem of plant water use efficiency. The relationship between stomatal aperture and environmental factors for various species has important implications in watershed and stand management practices. Physiological characteristics such as stomatal resistance and leaf area index are required in any re a l i s t i c model used to predict the effect of thinning on water consumption. The relationships between the stomatal diffusion resistance and environmental parameters were investigated as part of a two-year study of the effect 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. Research was conducted 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 characteristics. 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 characteristics of a thinned and an unthinned stand. Chapter 3 describes how stomatal resistance . relates to an existing, well-known model of transpiration. Finally, Chapter 4 proposes a simple model of stand transpiration which uses stomatal resistance as an input. The unventilated diffusion porometer has been widely accepted in the measurement of the stomatal resistance of broad-leaved plants (Kanemasu et 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 significant portion of my research in 1973 was aimed at evaluating and improving the ventilated diffusion 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, is included in Chapter 1. The objectives of the research discussed in Chapter 2 were (1) to show the effect of thinning on the characteristics of stomatal diffusion resistance of Douglas f i r , (2) to determine the effect of stress history on the characteristics of stomal diffusion resistance of Douglas f i r , (3) and to assess the hydrologic importance of the stomatal diffusion resistance characteristics of undergrowth vegetation in the thinned stand. In this, as well as the following chapters, the importance of vapour pressure d e f i c i t in affecting the stomatal diffusion resistance is clearly evident. The two parameters in this study, other than light, that were most important in causing changes in stomatal diffusion resistance, were the vapour pressure d e f i c i t and soil water potential. 3 L i t t l e is known as to the physiological nature of the forest canopy resistance and whether i t is 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 f i r s t , Chapter 3 was written before Chapter 2. However, since Chapter 2 is a general discussion of stomatal resistance characteristics at both sites, 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 distribution 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 is then used together with the stomatal diffusion resistance characteristics 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 soil dryness. References Kanemasu, E.T., G.W. Thurtell, and CB. Tanner. 1969. Design calibration and f i e l d use of a stomatal diffusion porometer. Plant Physiol. 44: 881-885. 3a Kramer, P.J. 1974. Fifty years of progress in water relations research. Plant Physiol. 54: 463-471. Turner, N.C., F.C.C. Peterson, and W.H. Wright. 1969. An aspirated diffusion porometer for f i e l d use. Conn. Agr. Exp. Sta. Soils Bull. No. XXIX: 1-7. 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 diffusion porometer as proposed by Turner and Parlange is reviewed. The problems of using the theory to interpret ventilated porometer data are discussed. The ventilated porometer designed by Turner ejt al_. is evaluated for use in the measurement of the stomatal diffusion resistance of Douglas f i r needles. 1. Introduction The f i r s t diffusion porometer was described by Wallihan (1964). Since that time, many modifications of this type of porometer have been reported. Ventilation of the porometer chamber was introduced to decrease chamber diffusion resistance and to reduce inaccuracies as a result 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 relative stomatal diffusion resistance of small and irregular shaped 6 leaves placed within the chamber. Turner and Parlange (1970) gave an analysis of the operation and calibration of the ventilated diffusion porometer of Turner e_t al_. (1969). The analysis resulted in an equation from which the stomatal diffusion 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 diffusion resistance from ventilated porometer data in a study of trans-piration and its control by stomata in a pine forest. This chapter evaluates the operation and calibration theory proposed by Turner and Parlange for the ventilated porometer when used to measure the stomatal diffusion resistance of needle-shaped leaves. In addition, i t describes a procedure for the use of the ventilated diffusion porometer for the measurement of the stomatal diffusion 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) is in the replacement of the original fan motor, D in Fig. 1. Their Ideal Toy Corporation fan motor was found in-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 later, indicated that 7 F i g . 1. Side view of ven t i l a ted d i f f u s i o n porometer head (modified from Turner ejt al_., 1969). The parts l a b e l l e d are: A -humidity sensor and bead thermis tor ; B - f an ; C - plunger conta in ing des i ccant ; D - motor; E - rubber coup l ing ; F -sea l ing bear ing; G - c losed c e l l foam rubber pads; H - aperture f o r i n se r t i on of needles; I - aperture fo r broad leaves ; J -e l e c t r i c a l connector. 8 this motor should be operated at 6 V resulting in a fan speed of 2550 rpm. When stomatal diffusion resistance measurements were made, the acrylic plug, H in Fig. 1, was removed and replaced by a plug formed with modelling clay which supported the needles (see Measurement Pro-cedure). The chamber drying system, comprised of the fan and the plunger which contained desiccant (B and C respectively in Fig. 1), was found to be too slow.. Instead,.dry air 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 diffusion porometer. 3. Porometer Circuitry The lithium chloride humidity sensor (Hygrosensor 4-4832, Hygro-dynamics Inc., Silver Springs, Md., U.S.A.), A in Fig. 1, requires a stable AC supply. The AC current through the sensor depends on sensor resistance, which in turn depends on relative humidity.- Turner et a l . (1969) used a ci r c u i t virtually the same as that of Kanemasu et al_. (1969), in which a stable 24 V peak-to-peak square wave is provided by a 4-transistqr multivibrator c i r c u i t . The current is measured by being rec-t i f i e d f i r s t , then passed through a DC microammeter. The circuit is used in a commercially available unventilated diffusion porometer. The main problem with this c i r c u i t is that i t is costly, since i t 9 requires many electrical components and considerable time to assemble i t . Consequently, a much simpler c i r c u i t was designed that makes use of an RCA COS/MOS 4030AE integrated c i r c u i t chip (Fig. 2). This chip consists of 4 exclusive-OR gates, all of which are used to produce a square wave identical to that of Kanemasu et al_. (1969). As can be seen in Fig. 2, the considerably simplified multivibrator c i r c u i t con-sists of only 4 resistors, 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, as compared with the 3.4 mA required in the circuit of Kanemasu et_al_. (1969). This extends the l i f e of the two 6.75-V mercury cells to over 600 hours, resulting in a considerable 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 is approxi-mately $60. All components are available in most electronics stores.. The time required for assembling the porometer circuitry is less than 4 hours. 4. Porometer Theory -2 -1 The rate of transpiration,(E,(g cm s ) into the porometer chamber under isothermal conditions is given by: i e s - e  E = V r s + r p (1) 10 F i g . 2. Metering c i r c u i t f o r ven t i l a ted and unvent i la ted d i f f u s i o n porometer. 11 where r is the stomatal diffusion resistance (s cm - 1), r is the s p porometer resistance, which is determined mainly by the leaf boundary layer resistance (s cm - 1), T is the porometer temperature (K), Ry is 3 1 1 the gas constant for water vapour (4620 mb cm g - K ), e is the chamber vapour pressure (mb) and e g is the saturation vapour pressure (mb) at the porometer temperature. If the porometer has an effective 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 is given by: A A E R T de = v ( 2) dt V K C ) Substituting (1) into (2) gives: de = A e s " e dt V r + r ^ s p Turner and Parlange (1970) found that the effective volume is made up of the actual chamber volume (Vc)(which is 32 cm for their 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^ is found by integration of (3). Rearrangement of the resulting expression gives: rs = A t r . . . , - rp (4) P (V. + V J l n es - e l 6 s " e2. where ? s is the average sensor capacity which is defined by: 12 V de _s V In s (5) Turner and Parlange found that Vg is independent of chamber vapour pressure (e) 9but varied with temperature as shown in Table 1. Since the lithium chloride sensor responds to relative humidity, i t is con-venient to rewrite (4) in terms of relative humidity (RH, %) as: _ At (Vs + Vc)].n 100 - RH 100 - RH, (6) 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 tem-perature. This is because the humidity sensor resistance is a function of both relative humidity and temperature. The time (t) is referred to as the transit time, since i t is 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 Vc 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) 15 20 25 30 35 V s(cm 3) This study 139 112 85 58 Turner and Parlange 137 110 83 56 29 (1970) 14 5.1 Humidity sensor c a l i b r a t i o n Rather than supplying an RH versus res i s tance curve f o r several temperatures with each humidity sensor, Hygrodynamics suppl ies a curve of RH versus the d i a l reading of one of i t s tes t instruments f o r several temperatures, as in F i g . 3. In order to convert from the RH versus d i a l reading graph to an RH versus res i s tance graph, the sensor was c a l i b r a t e d ( i . e . the re l a t i on sh ip between RH and res i s tance was determined) at 20 C. This was accomplished by suspending the sensor in a c losed vacuum d e s i c -cator above su lphur ic acid-water so lut ions over a range of so lu t ion d e n s i t i e s . The re l a t i on sh ip between RH and su lphur ic ac id so lu t ion dens i ty is shown in F i g . 4. The data from a 20 C c a l i b r a t i o n run are p lot ted in F i g . 5. Because of the time necessary f o r equ i l i b r ium to occur, one complete run took about 4 days. Using the 20 C curves in F i g . 3 and F i g . 5, the r e l a t i o n s h i p between the Hygrodynamics d ia l reading and res i s tance was determined. Using th i s r e l a t i o n s h i p , the remaining curves in F i g . 5 were obta ined. The re l a t i on sh ip s between microammeter current and sensor r e s i s -tance fo r the three c i r c u i t humidity ranges are shown in F i g . 6. Conse-quent ly, knowing the value of both current and temperature, the c o r r e s -ponding value of RH can be obtained using F ig s . 5 and 6. If Hygrodynamics had suppl ied F i g . 5 ra ther than F i g . 3, cons iderable time would have been saved. 5.2 Porometer res i s tance determination The porometer res i s tance (r )can be e a s i l y ca l cu l a ted from (6) 15 i g . 3. Ca l i b r a t i on curves fo r l i th ium ch lo r ide humidity sensor suppl ied by Hygrodynamics. 16 Fig. 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. The re l a t i on sh ip between r e l a t i v e humidity and res i s tance of l i th ium ch lo r ide humidity sensor. 18 5000 RESISTANCE ( K A J Fig. 6. The relationship between porometer current and sensor resistance for three different humidity ranges. 19 when the stomatal diffusion resistance(r )is zero. In the case of v s 7 f l a t leaves, this is accomplished by placing a sheet of wet blotting paper between the foam pads, G in Fig. 1, then noting the transit time, t. In the case of needles, rp is 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 in stomatal diffusion resistance measure-ments. The value of.r for four Douglas f i r needles at 20 C is approxi-mately 0.8 s cm - 1. It decreases with increasing temperature by roughly 0.5%/C. 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 diffusion porometer. The measurement should be made as soon as possible and as close as possible to the original location of the needles on the tree. A change in light intensity can cause a change in stomatal diffusion resistance. Fig. 7 shows how transit time, and therefore stomatal diffusion 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 (min ) F i g . 7. Porometer t r a n s i t time versus time s ince the sampling of Douglas f i r needles. 21 needles experienced a decrease in l i g h t i n ten s i t y s ince they were removed from roughly the middle of the tree and taken to near ground l e v e l . Since the measurements were made on the same sample, the excessive drying probably hastened c lo sure . Consequently, the graph provides an estimate of the minimum time before res i s tance increase i s s i g n i f i c a n t . The graph suggests that measurement should be made with in 5 minutes of needle removal from the t ree . Exposure of the needles to porometer chamber a i r , which has an RH near 25%, should be.kept to a minimum. The choice of the appropr iate humidity range switch pos i t i on should keep t r a n s i t times to less than 60 s. Accurate stomatal d i f f u s i o n res i s tance measurement depends on the number and placement of needles in the porometer chamber. Waggoner and Turner (1971) inserted 5 f a s c i c l e s (10 needles) of red pine through the small aperture, contain ing the a c r y l i c p lug, H in F i g . 1. In studying the best way of using the instrument f o r Douglas f i r stomatal res i s tance measurement, te s t s were f i r s t made in which a 2-cm sect ion of a twig bearing approximately 8 to 10 needles was inserted through the aperture, I in F i g . 1, s ince th i s amount of materia l could not be inser ted through the smaller aperture. This proved unsa t i s f ac to ry . Instead, i t was found that i n se r t ing 4 ind iv idua l needles into a modell ing c lay p lug, which in turn was inser ted t i g h t l y into the smaller aperture gave good r e s u l t s . The broken ends of the needles must be c a r e f u l l y inserted into the c l ay to prevent them from being vapour sources. The needle area .(A, projected 2 area b a s i s ) , of four Douglas f i r needles i s approximately 1.5 to 2 cm . I f the porometer has not been used f o r several days or more, the sensor should be hydrated and dehydrated several times before use. The 22 sensor should be stored with des iccant when not being used f o r an extended period of t ime. To make a measurement of the stomatal r e s i s -tance,, the fo l lowing steps should be fo l lowed: . <; 1. Adjust microammeter needle to zero when porometer switch is o f f . 2. Switch to pos i t i on FS ( f u l l sca le ) and adjust needle to 10 yA with f u l l sca le adjust knob. 3. Se lect and switch to the appropriate humidity range (H^ or Hg for r < 20 s cm"^, H„ f o r r > 20 s cm~^). s 2 s 4. Pump dry a i r into the chamber un t i l the microammeter reads 1.0 yA. 5. In the case of needles: Shape a plug from clean modell ing c l ay to rep lace the a c r y l i c p lug, H in F i g . 1. Gently inser t four needles ( in the case of Douglas f i r ) into the end of the p lug. Make sure the broken end of the needles are sealed into the c l a y . In the case of broad leaves : .P lace the l e a f between the foam pads, G in F i g . 1. 6. Turn on the f an , and record.the time ( t r an s i t time) f o r the needle to move from 3.0 yA to 6.0 yA. 7. Switch to pos i t i on T and record current through the thermis tor . The temperature can be obtained from the thermistor c a l i b r a t i o n curve. 8. Switch to OFF to conserve battery power. 9. In the case of needles: Remove the four needles and place them in a sample bag f o r l a t e r determination of needle area, A. In the case of broad leaves: The area i s simply the area of aperture I in F i g . 1. To determine rp , the above procedure is fol lowed except that 4 dead needles dipped in water conta in ing wetting agent are used. The determination 23 of r serves to check the porometer e l e c t r o n i c s , so i t . i s well worth doing at l ea s t once a day. The use of wet b l o t t i n g paper in the deter -mination of r can be used in 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 convenient than using the wet needles. 7. Performance Tests 7.1 Test p lates of.known d i f f u s i o n res i s tance In order to te s t the accuracy of (6), known d i f f u s i o n res i s tances were requ i red . These d i f f u s i o n res i s tances were obtained by using perforated p lates with holes less than 130 microns in diameter and d r i l l e d brass p lates with holes of 0.5 and 1.0 mm in diameter. D r i l l e d brass plates were used by Kanemasu et al_. (1969) in the c a l i b r a t i o n of an unvent i lated d i f f u s i o n porometer. The perforated plates were used because i t was o r i g i n a l l y thought that eddies from the porometer fan would penetrate the large holes and consequently reduce the d i f f u s i o n res i s tance of the brass p l a tes . Later i t was found that t h i s e f f e c t was not observable. The d i f f u s i o n res i s tance ( r ) o f the brass p lates was ca l cu la ted from the Brown and Escpmbe (1900) formula: r (L + — ) (7) S ncnrd^ 4 where n i s the number of holes per .unit Jarea? o f ' . p l a te ! ? L i s the. p late 24 thickness (cm), d i s the hole diameter (cm), and a i s the d i f f u s i v i t y 2 -1 of water vapour in a i r (cm s~ ) (see Table 2). The d i f f u s i o n r e s i s -tance data f o r . the brass p lates is shown in Table 3a. The d i f f u s i o n res i s tances of the perforated p lates were not ca l cu l a ted using (7) because of t h e i r i r r e g u l a r hole geometry. Instead, they were measured by an unvent i lated d i f f u s i o n porometer c a l i b r a t e d using a separate set of d r i l l e d brass p lates with ca l cu la ted d i f f u s i o n res i s tances shown in Table 4. The c a l i b r a t i o n curve fo r th i s unvent i lated porometer at 24.5 C i s shown in F i g . 8. The measured d i f f u s i o n res i s tances of the perforated p lates are shown in Table 3b. Each brass p late was placed between the foam pads surrounding aperture, I (F ig . 1) o f the ven t i l a ted porometer head.. Wet b l o t t i n g paper was placed between the brass p late and the outer pad. A 0.3-cm space between the perforated p late and the b l o t t i n g paper was maintained by means of an a c r y l i c frame. This prevented l i q u i d water from enter ing the holes o f the perforated p l a te . 7.2 E f f e c t o f fan speed The re l a t i on sh ip between t r a n s i t time (3 to 6 yA) and fan speed for plates of d i f f e r e n t d i f f u s i o n res i s tance is shown in F i g . 9. The t r a n s i t time approaches a steady value fo r fan speeds exceeding 2500 rpm. This i s a r e s u l t of the porometer res i s tance approaching a steady va lue, as shown by the curve of zero d i f f u s i o n re s i s t ance . These re su l t s and vo l tage/fan spee.d r e l a t i on sh ip shown in F i g . 10 suggest that the fan motor be operated at no less than the maximum motor voltage of 6 V. Table 2. The re l a t i on sh ip between the d i f f u s i v i t y of water vapour in a i r (a)and temperature •••(T ). Ta(0 2 -1 a (cm s ) - 10 0.211 0 0.226 10 0.241 20 0.257 30 0.273 40 0.289 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) ar g (cm) r g (s 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 27 Table 3b. D i f fus ion res i s tance ( r g ) data at 24.5 C f o r perforated p lates with and without a 0.3-cm space between p late and b l o t t i n g paper. The plates were purchased from Perforated Products, Inc., 68 Harvard S t . , Brook l ine, Mass. 02146. P late No. a r s (cm) with space r s (s cm" ) without space r (s cm" 1 ) s 20 W (Conical) 2.88 10.9 9.8 25 W (Conical) 2.14 8.1 7.0 50 W (Conical 1.16 4.4 3.3 60 W (Conical ) 0.92 3.5 2.4 125 W (Conical) 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 60QT (WentUKi) 1.0 3.8 2.7 100 T (Venturi) 1.0 3.8 2.7 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 UNVENTILATED DIFFUSION POROMETER ( METER NO. 2 ) 2 4 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 for a 6-V Marx-Luder motor. 32 7.3 Test of porometer theory Ten perforated plates and four d r i l l e d brass p lates o f known d i f f u s i o n res i s tance were used to test vent i l a ted porometer theory. Results of t h i s te s t are shown in F i g . 11. T rans i t time was p lot ted against p late d i f f u s i o n res i s tance. , The s t ra i gh t l i n e shown in the f i gure is the theore t i ca l r e l a t i o n s h i p expected and was ca l cu l a ted using (6). F i g . 11 shows exce l len t agreement between the experimental r e su l t s and porometer theory. This agreement fo r brass p lates ind ica tes no observable reduct ion in t h e i r d i f f u s i o n res i s tance as a r e s u l t of eddies penetrat ing the d r i l l e d holes. F i g . 12 shows c a l i b r a t i o n data for the vent i l a ted d i f f u s i o n porometer at 5 temperatures.. The l i n e s drawn represent the ind iv idua l regress ion equations.. Using these slopes and a V c value of 32 cm in (6), ^ s was ca l cu la ted fo r each temperature. These re su l t s were extrapolated to the same temperatures fo r which V g was reported by Turner and Parlange (1970) and are shown in Table 1. As expected, the agreement i s good. Two bean plants were a lso used to tes t ven t i l a ted porometer theory. In th i s case, the ca l i b r a ted unvent i la ted porometer was used to measure the stomatal d i f f u s i o n res i s tance of the abaxial (lower) l ea f sur faces. Measurement, using the ven t i l a ted porometer, was made with in 2 minutes of each unvent i lated porometer measurement. The re su l t s in Table 5 show that the ven t i l a ted porometer gives s l i g h t l y higher stomatal res i s tance values. This probably re su l t s from the tendency of the stomata to c lose a f t e r using the unvent i lated porometer. Nevertheless, the agreement i s s t i l l with in experimental e r r o r . As a f i n a l t e s t , the stomatal d i f f u s i o n res i s tance of needles on 33 F i g . 11. T r an s i t time versus d i f f u s i o n res i s tance of d r i l l e d and perforated p la tes . The theo re 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 gh t l i n e , i s equation (6) with RH-j = 25.3%, Rh^ = • 29.5%, V = 88.4 cm 3 , \/ =-32 cm 3 , A = 2.85 cm 2 and r = 2.3 s ' c p s cm" . The poor agreement of the t r i ang l e data point i s thought to be due to an error in pos i t i on ing p la te beneath porometer aperture. 34 VENTILATED POROMETER LiCI SENSOR No.598037 TRANSIT TIME (sec) F i g . 12. T rans i t time versus d i f f u s i o n res i s tance of d r i l l e d and perforated p lates at 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 Venti1ated Porometer 3.5 2.8 3.0 Average 3.0 4.0 2.9 3.0 Average 3.3 5.0 5.5 4.8 5.3 5.7 5.5 Average 5.0 Average 5.5 36 four 1-year o ld Douglas f i r seedl ings was measured with the ven t i l a ted porometer and ca l cu la ted from the weight loss of the same potted seed-l i n g s . The average stomatal res i s tance of the needles of a s ing le seedl ing was ca l cu la ted using (1). Aluminum f o i l covering the s o i l prevented evaporation from the s o i l . The weight loss over a per iod of one hour was used as an estimate of the t ransp i ra t i on r a te . The temperature of the needles was measured using thermocouples, whi le the vapour pressure of the a i r was measured with an asp i rated psychrometer. The boundary layer res i s tance of the needles was made neg l i g i b l y small compared to the stomatal re s i s t ance , using a fan. Three, f ou r , f i v e and s ix 4-needle samples were removed from the four seedlings r e s p e c t i v e l y , at roughly equal ly spaced seedl ing height i n t e r v a l s . Vent i l a ted porometer t r a n s i t times were measured f o r each sample. The needle area of each sample (projected l e a f area bas is ) was ca l cu l a ted from length ;anduWnidth measurements. This c a l cu l a t i on agreed to with in 6% of a photographic method of needle area measurement discussed in Chapter 4. The ven t i l a ted porometer value of the stomatal d i f f u s i o n res i s tance of each sample was.ca lculated using (6). The re su l t s o f th i s comparison are shown.in Table 6. Since the average values of stomatal d i f f u s i o n res i s tance obtained from porometer measurements agreed to with in 10% of the values obtained from the weight lo s s . techn ique, the porometer was considered to be accurate enough to make f i e l d measurements of stomatal res i s tance of Douglas f i r needles. In a recent study, Gandar and Tanner (1976) suggest that mater ia l s such as s ta in le s s s teel or waxed surfaces are pre ferab le f o r the construct ion of d i f f u s i o n porometers because of t h e i r r e l a t i v e l y low water vapour 37 Table 6. Comparison of stomatal res i s tance (r ) of four 1-year-old Douglas f i r seedl ings measured with a ven t i l a ted d i f f u s i o n porometer and ca l cu la ted from the weight loss of the potted seed! ings.. r (s cm" ) s Seedling No. 1 2 3 4 Ca lcu lated from weight loss 4.2 7.2 9.5 12.4 4.1 8.4 10.7 10.1 Measured with , 3.4 7.9 11.6 11.1 ven t i l a ted 4,8 7.1 10.1 11.8 porometer 5.2 9.8 11.5 10.0 11.8 12.1 AVERAGE 4.1 7.2 10.4 11.4 38 sorptivities. It should be noted that the effect of water vapour sorption on the acrylic walls of the porometer used in this study is taken into account by calibrating over a reasonably broad diffusion 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. Static diffusion of gases and liquids in relation to the assimilation of carbon and trans-location in plants. Phil. Trans. Roy. Soc. London, Ser. B., Biol. Sci. 193: 322-291. Byrne, G.F., C.W. Rose and R.O. Slatyer. 1970. An aspirated diffusion porometer. Agr. Meteorol. 7: 39-44. Gandar, P.W. and CB, Tanner. 1976. Water vapour sorption by the walls and sensors of stomatal diffusion porometers. Agron. J. 68: 245-249. Kanemasu, E.T., G.W. Thurtell and CB. Tanner. 1969. Design, calibration and f i e l d use of a stomatal diffusion porometer. Plant Physiol. 44: 881-885. Turner, N.C, F.C.C. Pederson and W.H. Wright. 1969. An aspirated diffusion porometer for f i e l d use. Conn. Agr. Sta. Special Soils Bull. 29, 12 p. Turner, N.C. and J.Y. Parlange. 1970. Analysis of operation and calibration of a ventilated diffusion porometer. Plant Physiol. 46: 175-177. Waggoner, P.E. and N.C. Turner. 1971. Transpiration and its control by stomata in a pine forest. Conn. Agr. Exp. Sta. Bull. 726. Wallihan, E.F. 1964. Modification and use of an electric hygrometer for estimating relative stomatal aperture. Plant Physiol 39: 86-90. 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 effect of thinning on evapo-transpiration in Douglas f i r , the relationship between stomatal diffusion resistance (r ) and environmental parameters were studied. Research was conducted in an unthinned stand (1840 stems ha - 1) with negligible under-growth and a thinned stand (840 stems ha - 1) with substantial salal under-growth. A ventilated diffusion porometer was used to measure r . In the thinned stand, during the daytime, r g was mainly related to the soil water potential (\> ) and the vapour pressure def i c i t (v.p.d.) of the canopy air. The same relationship was observed in the upper portion of the unthinned stand. Daytime values of r g for Douglas f i r ranged from 2 to 60 s cm-1 for values of v.p.d. between 4 and 24 mb and values of ip between 0 and -12.5 bars. Although increasing r g was usually associ-ated with decreasing twig water potential ( t^)» increasing r g appeared to be associated with increasing ^ when the v.p.d. was high. Stress history was found to cause a shift in the relationship of r s t o b u t had l i t t l e effect on the relationship of r s to v.p.d. and \\>s. Daytime values of r g for salal ranged from 2 to 45 s cm - 1. These stomatal Presented by the author at 68th Annual American Society of Agronomy Meeting, Nov. 29 to Dec. 2, 1976. 42 characteristics indicate that as the soil dried out, salal transpiration accounted for an increasing fraction of the total water loss by the thinned stand. 1. Introduction Both tree growth and transpiration are directly 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 leaf water potential are related to para-meters describing the physical environment (Slavik, 1970; Schulze et a l . , 1974, 1975; Jordan et al_., 1975; Hinckley et al_., 1975; Running, 1976; Fetcher, 1976; Cline and Campbell, 1976; Tan and Black, 1976). Several authors (Jordan and Ritchie, 1971; McCree,, 1974; 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 diffusion resistance and leaf water potential. Physically based models of forest growth and water use require these relationships. In addition,.the descrip-tion of post-thinning stress relies on a knowledge of the relationship between these physiological and environmental parameters. While the importance of the role of the stomata has been recognized for some time, only recently has the behaviour of the stomata of conifers been studied (Waggoner and Turner, 1971; Running, 1976). This is because of the 43 development of ventilated and null-balance types of diffusion porometers and methods of calibrating them (Turner and Pariange, 1970; Beardsell et/ al_., 1972). As part.of a two-year study of the effect of thinning on tree water stress in a 21-year-old Douglas f i r forest, the relationships between, the stomatal diffusion resistance and environmental parameters were studied. Research was conducted in an unthinned stand with negligible undergrowth and in an adjacent thinned stand with substantial undergrowth. Since there.were two dry periods separated by a short period when the soil 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 in changing stomatal resistance characteristics. The objectives of this Chapter are (1) to show the effect of thinning on the characteristics of stomatal diffusion resistance of Douglas f i r , (2) to describe the effect of stress history.on the characteristics of stomatal diffusion resistance of Douglas f i r , (3) and to assess the hydro-logic importance of the stomatal diffusion resistance characteristics of undergrowth vegetation in the thinned stand. 2. Experimental Procedure 2.1 Experimental 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 is 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 in an unthinned and a thinned stand of Douglas f i r (Pseudotsuga menziesii (Mirb.) Franco) trees planted in 1953. In the unthinned 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 in the unthinned stand. There was virtually no undergrowth in 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 soil in the thinned stand belonged to the same soil series as that of the unthinned stand (Dashwood gravelly sandy loam,, Duric Humo-Ferric Podzol). 2.2 Measurements All 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 site between these two dates. In 1975, there was only one extensive period of dry weather which began June 29 following 45 prolonged rain. The same measurement techniques were used for 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 intervals, while soil water potentials measured by the hygrometers were recorded 3 times each day. In this study, only daily average values of-^ were used in the analysis. For further details of the soil water measurements, see J.U. Nnyamah, 1977. Root water uptake in a Douglas f i r forest. Ph.D. Thesis, Department of Soil Science, University of British 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- (salal), 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. At least 2 twig samples were used to obtain each value of ^ f and 2 readings 46 were taken for each twig. The xylem sap osmotic potential was verified to be negligibly small using a Wescor HR-33T dew point microvoltmeter and hygrometer chamber. Stomatal resistance ( r g) measurements were made using a ventilated porometer designed by Turner e_t al_. (1969). The theory.of this porometer was discussed by Turner and Parlange (1970). Modification of the porometer electronics and the sensing head, and the calibration 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. In the thinned stand, the measurements of r g were made at the 0.5- (salal), 3-, 5- and 7-m heights. Over the experiment period,.routine measurements (3 times during the daytime) were made on 22 days, while intensive measure-ments (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 plastic bags for the determination of needle.area later 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 until 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 light 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 light meter with the quantum sensor held by hand, near the point at which the needles were removed from the tree. Wet and dry bulb temperatures were measured with 3 silicon-diode psychrometers located at 4-, 6- and 8-m heights and recorded every 15 minutes by a data logger, which provided canopy vapour pressure def i c i t (v.p.d.) profiles over the measurement period.-In the thinned stand, the v.p.d. and light quanta flux measurements were made at the 0.5- (salal), 3-, 5- and 7-m heights.- The light profile measurements were made on 6 days, while the v.p.d. profile measurements were made on 36 days. Wet and dry bulb temperatures were measured either with an Assmann psychrometer or 3 silicon-diode psychrometers located at the 3-, 5- and 7-m heights. Since needle temperature was close to air temperature (Appendix A), the v.p.d. can be regarded as a good approxi-mation of the difference between the vapour pressure within the stomatal cavities and in the canopy air. 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 and v.p.d. at the 8-m D L o height in the unthinned stand and the 7-m height in the thinned stand 48 J I I I I I • I 1 I L 10 20 30 10 20 30 10 20 JUNE JULY AUGUST Fig. 1. Seasonal course of soil water potential ( s^)> twig water potential (<J>t)» vapour pressure deficit (v.p.d.), and stomatal resistance (r g) measured each day between 1300 and 1430 PST in the unthinned Douglas f i r stand. 49 r — ' 1 • r 0 1 • 1— 1 • 1—:—, i i . i . l 10 20 30 10 20 30 10 20 JUNE JULY AUGUST F i g . 2. Seasonal course of s o i l water potent ia l ), twig water potent ia l C ^ t ) , vapour pressure d e f i c i t ( v .p .d . ) , and stomatal res i s tance (r ) measured each day between 1300 and 1430 PST in the thinned Douglas f i r stand. 50 measured between 1300 and 1430 PST are shown fo r the respect ive measure-ment per iods. The re su l t s fo r the unthinned stand show that the minimum value of ijj^ during the f i r s t dry period was approximately -21.4 bars, while the minimum value of ^ during the second dry per iod was approx i -mately -24.7 bars, when t/js was between -7 to -10 bars (F ig . 1). On the other hand, the minimum value of ^ in the thinned stand near the end of the dry period was approximately -21.9 bars (F i g . 2) . These r e s u l t s suggest that t r ee s , which had been subjected to a per iod of low s o i l moisture condit ions e a r l i e r in the same summer, had a lower twig water potent ia l than trees that had not had p r i o r exposure to low s o i l moisture cond i t i ons . The e f f e c t of thinning on ijj t can be seen by comparing the f i r s t dry period of the unthinned stand (F ig . 1) with the only dry period of the thinned stand (F ig . 2). The e f f e c t appears to be smal l . It i s a lso c l ea r from F igs . 1 and 2 that there i s a marked c o r r e l a t i o n between r and v .p .d . in both the thinned and unthinned stands. In several cases, s the combination of low ^ s and high v .p .d . shown in F ig s . 1 and 2 , . resu l ted in high values o f - r between 1300 and 1430 PST. 3.2 Daytime va r i a t i on in stomatal res i s tance of Douglas f i r and sa la l The daytime va r i a t i on in r s at the 3-, 5- and 7-m heights in the Douglas f i r canopy and in the sa l a l undergrowth of the thinned stand fo r 3 d i f f e r e n t values of ^ s are shown in F i g . 3. Also shown i s the v .p .d . at the 7-m height. It was found that when ^ was h i gh , . r at the 3 heights in the thinned stand was small and very s i m i l a r , whi le r $ in the sa l a l undergrowth was s l i g h t l y lower than that of the Douglas f i r during 51 COURTENAY, DOUGLAS FIR , THINNED STAND, 1975 JUNE 30 JULY II JULY 30 T 1 1 n r i 1 1 1 1 i 1 1 r 4 8 12 16 20 4 8 12 16 20 4 8 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, for example, near the end of the dry period when i t reached a value of -11.3 bars, r g in 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 un-thinned stand, r s generally decreased with increasing height in the canopy. At low ij; i t decreased more rapidly than at high \ j j s . These, results are to be expected since the light at the ground level of the unthinned stand was often less than 10% of that above the canopy, while the light at the ground level of the thinned stand often equalled that above the canopy. The effect of v.p.d. on r g can also be seen from Fig. 3. In particular, when iji was very low, r g increased rapidly during the morning, as shown in Fig. 3c, reflecting the rapid rise in the v.p.d. during this time. As was observed in 1974 in the unthinned stand, r g sometimes rose to a daytime maximum between 1300 and 1600 PST, then dec-lined 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 light levels. 3.3 Relationship of stomatal resistance to irradiance Fig. 4 shows the stomatal resistance of Douglas f i r and salal under-growth 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 -1 a quanta flux density of between 250 and 350 yE m s . Thinning appeared to have l i t t l e or no effect on the light response of the stomata of 53 COURTENAY, DOUGLAS FIR 50 40 (s cm"1) 30 20 10 R, (A. = 0.4 -0.7pm) (Wm-2) 100 200 300 V*. o UNTHINNED STAND, 1974 • THINNED STAND, 1975 * SALAL, 1975 o • * . 500 1000 Q (pE rrTV) 1500 2000 F i g . 4. Re lat ionship between the quanta f l ux dens i ty (Q) fo r the 0.4-0.7 ym band width and stomatal res i s tance (r ) of Douglas f i r in the thinned and unthinned stands, and sa la l undergrowth in the thinned stand. The approximate energy f l ux dens i ty in the 0.4-0.7 ym band width is shown at the top of the graph. 54 Douglas f i r . 3.4 Relat ionship of stomatal res i s tance to twig water potent ia l and  vapour pressure d e f i c i t F i g . 5 shows a p lo t of r g versus ^ t when the l i g h t quanta f l ux -2 -1 exceeded 350 yE m~ s~ (y so lar r ad ia t i on (0.4-4 pm) f l ux dens i ty of _o approximately 150 wm ). Stomata began to c lose at a ^ value o f . -19.5 bars during the f i r s t dry period in the unthinned stand and in the thinned stand, while during the second dry period in the unthinned stand, stomata began to c lose at a ^ value of -23.5 bars (F ig . 5a). McCree (1974), Hinckley et a1_. (1975), Brown et a 1. (1976), and Thomas et a l . (1976) showed s im i l a r re su l t s i nd i ca t ing that a f t e r drought h i s to ry stomatal c losure occurred at a lower value of l e a f water potent ia l than before. M i l l a r et al_. (1971), in a study of seed onions, found that p lants in the f i e l d had a lower value of stomatal res i s tance at a given value of l e a f water potent ia l than growth-chamber-grown p lants . Jordan and R i t ch ie (1971) reported that cotton plants grown in the greenhouse had very high values of stomatal res i s tance when l e a f water potent ia l was -16 bars, i nd i ca t i ng stomatal c lo sure . However, the res i s tance of the f ie ld-grown plants remained low even at l ea f water potent ia l values of -27 bars. M i l l a r et al_. (1971), Turner and Begg (1973) and Turner (1974) suggested that the d i f f e rence in the stomatal response of growth-chamber versus f ie ld-grown plants was due to a reduction in the osmotic potent ia l of leaves o f - the f ie ld-grown p lants . More recent ly Brown et al_. (1976) and Thomas et al_. (1976) have shown that the osmotic potent ia l of the plant leaves decreases as a r e s u l t of drought h i s t o ry . It would thus appear 55 50 COURTENAY n (a) DOUGLAS FIR 40 o I* DRY PERIOD, 1974 • 2"* DRY PERIOD, 1974 0 » 1975 0 30 (s cm"1) 20 10 -10 (b) SALAL, 1975 -10 -20 F i g . 5. Re lat ionsh ip between stomatal res i s tance (r ) and twig water potent ia l fo r (a) Douglas f i r in the unthinned stand, during the two drying periods in 1974, and in the thinned stand in 1975, and (b) sa l a l undergrowth in the thinned stand. 56 that the osmotic potential of the Douglas f i r needles decreases as a result 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 in 1974 the effect of drought history was not observed. In order to determine how much of the scatter in Fig. 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 Fig ; 6. The data in Fig. 6 are separated into four v.p.d. classes as shown. As can be seen, the v.p.d. does account for much of the variation. It should be noted that, when the v.p.d. is 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 < -9 bars), r g at a given value of and v.p.d. was observed to be higher than at higher ^ s values (Fig. 6). The relationship of ^  to v.p.d. and ^ s for both thinned and unthinned stands can be seen in Table 1. The table shows that the aver-age value of ij> t in most cases, including the salal, decreases with increasing v.p.d. in the range from 8 to 16 mb. For higher v.p.d., however, the value of ^  appears to rise slightly. 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 partially close as a result of a high vapour pressure gradient between leaf and air, leaf water status may be maintained at its present level or improved. 57 (s cm*') 6 0 4 0 20 -10 COURTENAY, DOUGLAS FIR  TH INNED S T A N D , 1975 1 l r v.p.d. (mb) - 9 bars - 9 bars • < 10 o < 10 • 10-15 • 10-15 • 1 5 - 2 0 X > 20 -15 - 2 0 Y, (bars) - 2 5 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 deficit (v.p.d.) and soil water potential Us) for 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 is also shown. Twig Water Potential (-bars) 0 bar > $ > -6.5 bars -6.5 bars > \p > -12.5 bars s s Douglas f i r Salal Douglas f i r Salal v.p.d. 1st 2nd 1st 2nd (mb) 1974 1974 1975 1975 1974 1974 1975 1975 0 - 8 17.0 17.0 16.5 8.9 17.3 20.2 18.7 13.0 * ±2.2 ±1.8 ±1.1 ±1.8 * ±3.1 * 8 - 16 18.4 19.3 19.7 15.3 20.0 22.3 20.0 14.2 ±1.5** ±1.0 ±0.8 ±2.4 +0.4 ±0.9 ±1.4 ±1.4 16 - 24 16.7 21.9 17.0 11.8 19.3 22.7 19.2 13.9 * ±0.7 ±1.9 ±3.7 ±2.9 ±3.2 ±2.4 * 24-32 - 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  soil water potential As we have seen, the relationship between r and \p. is poorly defined. The value of r g could vary from 2 to 60 s cm-1 with only a small change in \p^. While only turgor potential controls stomatal opening and closing, ij> t measurements include both turgo$ and osmotic components. As indicated earlier, the effect of stress history can change the osmotic potential resulting in a shift in the relationship of r g to \p^. It was f e l t that rather than using the leaf water potential, a parameter that was a measure of water supply to the tree, such as \p t might be more successful in describing stomatal resistance characteristics. In two other studies, Cline and Campbell (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 different classes of In general, with high \ps, r g increases slightly with increasing v.p.d. (Figs. 7a and 8a). As the soil becomes drier (i.e. as ips decreases), r g of Douglas f i r rises 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 relationships. At values of iji greater than -3.5 bars, salal has stomatal resistance characteristics similar to those of Douglas f i r (Fig. 7a vs. Fig. 8a). At values of ^ s between -3.5 and -12.5 bars!)* r g 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 soil becomes drier. The possibility that temperature might account for the variation observed within each of the soil water potential classes in Fig. 7 was 60 COURTENAY, DOUGLAS FIR 0 1 • ' • 1 .—I I . I , i_ 0 10 20 0 10 20 v.p.d. (mb) v. p.d.(mb) Relat ionship between stomatal res i s tance (r ) and vapour pressure d e f i c i t (v.p.d.) fo r Douglas f i r in the unthinned stand, during the two dry periods in 1974, and in the thinned stand in 1975 fo r 4 s o i l water potent ia l (ty ) ranges:. F i g . 7. 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 increased when soil water potentials were high. In the present study, temperature appeared to account for very l i t t l e additional variation. 4. Conclusions During the daytime, stomatal resistance in a Douglas f i r stand was found to be mainly related to the soil water potential and vapour pressure def i c i t . Stomatal resistance of Douglas f i r and salal undergrowth increases as the vapour pressure def i c i t increases and soil water potential decreases. The stomatal resistance of salal undergrowth has a smaller response to vapour pressure def i c i t change than that of Douglas f i r , especi-ally when the soil becomes drier. Evidence was found to suggest that stomatal closure caused by high vapour pressure def i c i t can result in main-taining or slightly improving leaf water status. Stress history appeared to cause a shift in the relationship of stomatal resistance to twig water potential, but had l i t t l e effect on the relationship of stomatal resistance to vapour pressure def i c i t and soil water potential. 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. This study shows that soil water potential is extremely useful in describing stomatal resis-tance characteristics in forests where there is a seasonal change in soil moisture. 63 References Beardsell, M.F., P.G. Jarvis, and B. Davidson. 1972. A' null-balance diffusion 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 alteration in the stomatal response to leaf water potential. Physiol. Plant. 37: 1-5. Camacho-B., S.E., A.E. Hall and M.R. Kaufman. 1974. Efficiency and regulation of water transport in some woody and herbaceous species. Plant Physiol. 54: 169-172. Cline, R.G. and G.S. Campbell. 1976. Seasonal and Diurnal water relations. of selected forest species. Ecol. 57: 367-373. Federer, CA. and G.W. Gee. 1976. Diffusion resistance and xylem potential in stress and unstressed northern hardwood trees. Ecol. 57: 975-984. Fetcher, N. 1976. Patterns of leaf resistance to lodgepole pine transpiration in Wyoming. Ecol. 57: 339-345. Hall, A.E. and D.M. Yermanos. 1975. Leaf conductance and leaf water status of Sesame strains in hot, dry climates. Crop Sci. 15: 789-793. Hall, A.E. and G.J. Hoffman. 1976. Leaf conductance response to humidity and water transport in plants. Agron. J. 68: 876-881. Hinckley, T.M., M.O. Schroeder, J.E. Roberts and D.N. Bruckerhoff. 1975. Effect of several environmental variables and xylem pressure potential on leaf surface resistance in white oak. Forest Sci. 21: 201-211. Johnson, D.A. and M.M. Caldwell. 1976. Water potential components, stomatal function, and liquid phase water transport resistance of four arctic and alpine species in relation to moisture stress. Physiol. Plant 36: 271-278. 64 Jordon, W.R. and J.T. Ritchie. 1971. Influence of soil 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. Plant Physiol. 56: 595-599. McCree, K.J. 1974. Changes in the stomatal response characteristics of grain sorghum produced by water stress during growth. Crop. Sci. 14: 273-278. Millar, A.A., W.R. Gardner and S.M. Goltz. 1971. Internal water status and water transport in seed onion plants. Agron. J. 63: 779-784. Running, S.W. 1976. Environmental control of leaf water conductance in conifers. 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. Sci. 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 leaf temperature in controlling stomatal resistance of Prunus armeniaca L. under desert conditions. I. A simulation of the daily course of stomatal resistance. Oecologia (Berl.) 17: 159-170. Schulze, E.-D., O.L. Lange, L. Kappen, M. Evenair and U. Buschbom. 1975. The role of air humidity and leaf temperature in controlling stomatal resistance of Prunus armeniaca L. under desert conditions. II. The significance of leaf water status and internal carbon dioxide 65 concentration. Oecologia (Berl.) 18: 219-233. Slavik, B. 1970. Transpiration resistance in leaves of maize grown in humid and dry air. p. 269-270. In: R.O. Slatyer (ed.). Plant response to climatic factors. Unesco, Paris. (Proc. of the Uppsala Symposium, 1970). v Tan, CS. and T.A. Black. 1976. Factors affecting the canopy resistance of a Douglas f i r forest. Boundary Layer Meteorol. 10: (in press). Thomas, J.C, K.W. Brown, and W.R. Jordan. 1976. 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. An aspirated diffusion porometer for f i e l d use. Conn. Agr. Exp. Sta. Soils Bull. No. XXIX: 1-7. " Turner, N.C. and J.Y. Parlange, 1970. Analysis of operation and calibration of a ventilated diffusion 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 soil 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 soil water potential. Plant Physiol. 53: 360-365. Waggoner, P.E. and N.C. Turner. 1971. Transpiration and its control by stomata in a pine forest. Bull. 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 ratio measurements of evapotranspiration. The typical steady increase in the forest canopy resistance during daytime hours, even at high soil water potentials, was observed. A similar trend in the stomatal resistance indicated that increasing canopy resistance during the daytime was caused by gradually closing stomata. During a dry period when soil 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 light, vapour pressure d e f i c i t , twig and soil 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 soil parameters to provide an accurate description of the process when i t is limited by the supply of water. One such parameter is the canopy or surface resistance which is contained in the Penman -Monteith model of transpiration (Monteith, 1965). In view of the complexity of many plant canopies this is a greatly simplified model; however, i t has been remarkably successful in its 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 is approximately equal to the stomatal resistance of the leaves acting in parallel. 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 is known as to the physiol-ogical nature of the forest canopy resistance and whether i t is 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 forest, (2) to describe the relationship between canopy resis-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 for forests, with no intercepted water present, since evaporation from the soil is sjriall. The Penman -Monteith model of trans-piration assumes that the canopy is isothermal and that the canopy resis-tance (r ) is the resistance of a l l stomata of the leaves acting in parallel. The equation expressing the model can be written as sA + P c p ( e * z - e z ) / r H ( ] ) E = [s + Y ( l + r c / r H ) ] L which assumes that the aerodynamic resistances, r v and r^ are equal. (For the meaning of remaining symbols, see the List of Symbols for thesis), Thorn (1972) gives the form of (1) with r y f r^ and Tanner and Fuchs (1968) givenan alternative equation to (1) for the case of a non-isothermal canopy. On re-arrangement of (1), the canopy.resistance can be written as pc (e* - e .) fl rc= p YLE Z ^ H ^ ' E E - 1 ' - ^ ( 2 ) Both Stewart and Thorn (1973) and McNaughton and Black 0 973) have shown that when r u (and r ) <<r , as in a forest, the second term of (2) is n V C 70 much smaller than the f i r s t so that to a good approximation pc E = — M e * - e )/r (3) yL z z c This relation 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 is a function of stomatal resistance (r)\t the leaf area index (LAI)and the diffusion resistance to water vapour through the canopy air volume. The assumption that the latter resistance is much smaller than r in forest canopies would imply that r can be calculated from n 1/r = E (LAI./r .) (4) i - 1 where r ^ and LAI^ are respectively the r g and LAI of the ith canopy layer and n is the number of layers. 3. Experimental Procedure 3.1 Experimental site The site was approximately 27 km northwest of Courtenay, B.C., on the eastern coast of Vancouver Island. The measurements were made in an unthinned stand of Douglas f i r planted in 1953, located on the relatively extensive coastal plain between Courtenay and Campbell River. The wet, 71 mild winters and dry, warm summers experienced by the area provide a wide range of soil moisture conditions. The topography is 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 site was located at an elevation of 150 m. At the time of the research, the trees at the site ranged in height from 7 to 10 m, and averaged 10.6 cm in diameter at breast height. There -1 2 -1 were approximately 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 t i l l . 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 is 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 site from the Straight of 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 terrain and deforested areas on the lower slopes of mountains within 3 to 4 km of the sit e , winds were virtually never from that direction. 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 balance technique. 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 air from an aquarium-type pump. The soil heat flux .(G.). was measured at the 5-cm depth with two heat flux plates, and corrected for 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 levels, and from estimates of the heat capacity of the canopy based on.the work of Stewart and Thorn (1973). The evapotranspiration rate, (Er) was then calculated using the equation: E = (Rn - G - M)/[L(1 + (3)] (5) where L is 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 profile 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 tr a i l e r by 75 m shielded cables where they were recorded with a Hewlett-Packard 2707 A data logger. Net radiation, solar radiation and Bowen ratio 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 tensio-meter-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 intervals, while soil water potentials measured by the hygrometers were recorded three times each day. In this study, only daily values of soil 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 details of soil water measurements see Nnyamah (1977). The stomatal resistance j.(r s); and twig water potential" ( i j ^ ) , 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. On eight of the twelve days that was measured, r 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 ^ measurements (Scholander et a l . , 1965). At least two twig samples were used to obtain each value of ^ and two readings were taken for 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 calibration 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 light quanta flux (0.4 to 0.7 ym) were measured every three hours at the 8-m, 6-m and 4-m heights, while on August 1 and 2 these measurements were made at the 8-m and 6-m heights. Light quanta flux was measured with a Lambda model LI-185 light 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 soil water potential during the measurement period Figure 1 shows the average soil water potential-;(^ s) for the period June 17 to August 19, 1974 for the root zone (0 to 45 cm depth). Daily values of precipitation are shown for reference. The average soil water 75 Fig. -1. Soil water potential ) in the unthinned stand during the period June 17 - August 19, 1974. Each data point is 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 daily (24-hour) value of the ratio of latent heat flux to the net radiation flux (LE/Rn) ranged from 0.58 to 0.27 over the period of measurement. ? 1 The dependence of LE/Rn on ^ s for eighteen fine (R > 14.0 MJ m day ) days is shown in Figure 2. (Note that Figs. 2, 5, 7 and 10 of Curtis (1975) are preliminary results of iithbse. reported in Figs. 1 to 4 in this Chapter). 4.2 Daytime course of net radiation and latent heat flux for two selected  days The days selected for the comparison of stomatal and canopy resis-tances were July 25 and June 18. The former day had an average soil water potential of -0.6 bar, while the latter had a potential of -6.5 bars.. Consequently there was a considerable difference in the availability of soil water to the trees on these days. The daytime course of Rn and LE for both days is shown in Figure 3. 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 restriction to transpiration during the afternoon when the vapour pressure def i c i t (v.p.d.) was at its daily maximum. The reason may well be that in the coarse s o i l , the hydraulic conr ductivity is low near the roots. The 24-hour value of.LE/Rn of 0.58 on July 25 (Figure 2) was the highest observed during the study. On June 18, Rn was much higher than on July 25. A reduction in trans-piration was apparent at 1030 PST by which time LE had reached its daily maximum. The latent heat flux decreased during the late morning and early 77 9 (cm 3 H 2 0/cm3 soil) Fig. 2. The relationship between the 24-hour value of the ratio-j LE/R and the soil water potential (ip ) for 18 fine (R n r\ - I s n > 14.0 MJ m" day" ) days in 1974 in the unthinned stand. The number adjacent to each data point is the date. Values of the average root zone soil water content (e) corresponding to ip are also shown, s 78 Fig. 3. Daytime course of net radiation (R n) and latent heat flux (LE) for June 18 and July 25, 1974 in the unthinned stand, showing the effects of decreased soil 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 is quite apparent in this case with the 24 hour value of LE/Rn being only 0.36. 4.3 Comparison of daytime courses of stomatal and canopy resistance From the measurements of-r made on August 6 at the 4-m, 6-m and 8-m heights, i t was found that r g increased slightly with decreasing height in the canopy. 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 height appeared to be representative of a large proportion of the canopy, i t was used in subsequent comparisons with canopy resistance,- f^V Figure 4 compares r c with r g at the 8-m height for July 25 and June 18. Canopy resistance was calculated from (2) but differed very l i t t l e from that calculated from (3) and the difference became negligible as r £ increased. The low values of canopy resistance in the morning and its 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 (1975) for a pine forest with high values of soil water potential. The pattern of the daytime course of stomatal resistance on July 25 is very similar to that of canopy resistance. On June 18, when soil water potential was low, the stomatal resistance rises to a maximum of approximately 60 s cm-1 at about 1500 PST, then declines markedly before stomatal closure is caused 80 Fig. 4. Daytime course of canopy resistance (r ) and stomatal resis-tance (r g) (projected leaf area basis) at the 8-m height in the unthinned stand for the days in Fig. 3. 81 by low irradiance. The canopy resistance shows a similar pattern but displays some variation which is a consequence of scatter in the energy balance data. Figure 5 shows a good correlation between canopy resistance and stomatal resistance at the 8-m height for ten of the twelve days on which stomatal resistance was 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 forest, but Ih km from the study site (see Chapter 4 for measurement details). Trees at this location were the same age, height and diameter at breast height as those at the study site. Results of these measurements suggested that the average value of LAI at the study site was between 7.5 and 8.0 (projected area basis). Assuming the stomatal resistance shown in Figures 4 and 5 applied to the whole canopy, then from ( 4 ) , v T c should be between one seventh and one eighth of the stomatal resistance. This is the approximate relation-ship between r £ and r g in Figures 4 and 5. The close similarity of patterns and the reasonable relationship between values of the two resis-tances indicates that the forest canopy resistance is largely of physio-logical origin, especially in the case of the water-short forest. 4.4 Relationship of stomatal resistance to plant and environmental parameters Modelling of the canopy resistance requires knowledge of how plant and environmental parameters affect the stomatal resistance of.each tree species. The response of stomata to light, internal water deficits of the 82 Fig. 5. The relationship between canopy resistance (r ) and stomatal resistance (r ) at the 8-m height in the unthinned stand for 10 days. 83 tree, v.p.d. and temperature are particularly important (e.g. Jarvis, 1974; Schulze et_ al_., 1974). Stomatal resistance began to increase rapidly when solar radiation decreased to approximately 150 W m (350 -2 -1 yE m s for 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"1 at -20 barsduring 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 effect of the v.p.d. was particularly evident as can be seen in Figure 6 for June 18 and 19, both sunny days. The soil water potential on June 18 was -6.5 bars and decreased slightly 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 parameters affecting stomatal resistance, the use of soil and atmospheric parameters may have some practical advantages. Since both the soil water content and conduc-t i v i t y decrease with decreasing soil water potential, the latter parameter will be used as a water supply parameter. Figure 7 shows stomatal resis-tance plotted against soil water potential and indicates that the relation-ship depends strongly on atmospheric demand for water. The data have been separated into six v.p.d. ranges and a line has been drawn by eye through each range. At a given potential, the stomatal resistance increases with 84 Hour PST Fig . 6. Daytime courses of stomatal res i s tance (r ) at the 8-m s height and vapour pressure d e f i c i t (v.p.d.) on two days with s im i l a r s o i l water potent ia l s in the unthinned stand. 85 5 0 r — 1 1 r. 1 — i 1 1 r 0' • I J l • I . L 0 -2 -4 -6 -8 Y s (bar) F ig . 7. The re l a t i on sh ip between stomatal res i s tance (r ) at the 8-m height in the unthinned stand and s o i l water potent ia l The data have been separated into s ix ranges of vapour pressure d e f i c i t (v.p.d.) and were daytime measurements made on 12 days. 86 increasing v.p.d., as suggested in the previous paragraph. Szeicz et a l . (1973) analyzed the relationship between stomatal conductance and soil water potential for sorghum. Brady e_t al_. (1975), working on soy-beans, plotted stomatal resistance against soil water potential. 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 soil water potential and vapour  pressure d e f i c i t Since the canopy resistance is closely 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 resis-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 the ordinate of Figure 7 by about 7.5 (the ratio of the stomatal resistance at the 8-m 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 soil water potential of the root zone. They 87 found that the resistance began to increase with decreasing potential found that when plotting mean monthly canopy resistance of a grass-clover crop against soil water potential the resistance began to increase at a potential of -3.5 bars. In an al f a l f a crop, van Bavel (1967) found no significant change in canopy resistance until the soil water potential decreased to -4 bar S i In each of these cases there was considerable scatter in the plots of resistance against potential. In Figure 8, values of the daytime mean canopy resistance for the forest in the present study are plotted against the daily mean soil water potential. The daytime mean canopy resistance,.r , was obtained by weighting the hourly values by the v.p.d. as follows: where v.p.d. is 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 is removed by separating the data into v.p.d. ranges as suggested by the stomatal response shown in Figure 7. The weighted averaging of.r results in a set of straight lines rather than the curved lines in Figure 7... Canopy resistance appears to increase significantly at a soil water potential ;f less, than -2.5 bars, reflecting the coarse soil and the shallow root zone. 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 para-meters, then i t is necessary to consider the dependence of canopy resistance when the potential was less than about -7 bars. Szeicz and Long (1969) j = 1 (6) 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 resis-tance 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. Boundary Layer Meteorol. 2: 246-254. Black, T.A., CB. Tanner and W.R. Gardner. 1970. Evapotranspiration from a snap bean crop. Agron. J. 62: 66-69. Brady, R.A., S.M. 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 forst. M.Sc. Thesis, Department of Soil Science, University of British Columbia. Federer, CA. 1975. Evapotranspiration. 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. Jarvis, P. 1975. Water transfer in plants. I_n_ D.A. de Vries and N.H. Afgan (eds.): Heat and mass transfer in the biosphere. I. 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. Biol. 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. Ph.D. Thesis, Department of Soil Science, University of British Columbia. 91 Scholander, P.F., H.T. Hammel, E.D. Bradstreet and E.A. Hemmingsen. 1965. Sap pressure in vascular plants. Sci. 148: 339-346. Schulze, E.-D., O.L. Lang, M. Evanari, L. Kappen, and U..Buschbom. 1974. The role of air humidity and leaf temperature in controlling stomatal resistance of Prunus armeniaca L. under desert conditions. I. A simu-lation of the daily 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 for 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 diffusion porometer for f i e l d use. Conn. Agr. Exp. Sta. Soils Bull. No. XXIX 1-7. 92 van Bavel, C.H.M. 1967. Changes in 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 diffusion model that uses the vapour pressure def i c i t (v.p.d.) of the canopy air, 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 ratio measurements. Stomatal resistance characteristics were also used in the diffusion 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. until a certain v.p.d. was reached after which the rate declined. The values of v.p.d. at which maximum transpiration rate occurred were 15, 13 and 11 mb for soil water potential ranges of 0 to -3.5 bars, -3.5 to -9.5 bars and -9.5 to -12.5 bars res-pectively. Calculations from the model, using stomatal resistance measure-ments (every 2 to 3 hours) on 7 days, indicated that the fraction of trans-piration from the thinned stand transpired by the salal during daytime hours increased from approximately 45 to 70% during a four-week drying period. 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 ratio of the latent heat flux to the net radiation is correlated with the soil water potential (e.g. Ritchie ejtal_., 1972; Davies and Allen, 1973). While this approach is useful in many situations, i t is 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 soil water potential but also on net radiation. Another common approach is the Penman-Monteith isothermal extensive leaf model (Monteith, 1965). As indicated in Chapter 3, this is a "single source" model that requires an estimate of canopy resistance to vapour diffusion, and has worked well in well-ventilated can-opiesy (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. In this chapter, the Penman-Monteith model is not used. As was indicated in the previous chapter, since the aerodynamic resistance of the Penman-Monteith 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 ai r . In this chapter, this very simple diffusion model is 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 of' stand transpiration rate which is compared with energy balance measurements 96 of evapotranspiration rate. In addition, the model, along with stomatal resistance characteristics, is used to predict the relationship between transpiration rate and v.p.d. for different soil water potential classes in the thinned stand. 2. Theory -2 -1 The rate of diffusion of water vapour (E, " . g cm" s ) from a transpiring leaf or needle is given by: be e - e E = ZjL -§ §- (1) L Y r s + r b -3 -3 where p v i s the density of moist air (1.2 x 10" g cm" ), c^ is the specific heat of moist air (1.01 J g~^ C~^), L is the latent heat of vapourization of water (2452 J g"^), y is the psychrometric constant (0.66 mb C~^), e g is the vapour pressure within the stomatal cavities (mb), e = is the vapour a pressure of the air outside the boundary layer of the leaf (mb), r g is the stomatal resistance to water vapour diffusion (s cm"^) and r^ is the boundary layer resistance to water vapour diffusion (s cm~^). Since the relative humidity of stomatal cavities are approximately 100%, e g is assumed to be equal to the saturation vapour pressure at leaf temperature. This can be expressed as: e s ~ e*(Ti) (2) 97 where is the leaf temperature ( C). In well-ventilated coniferous canopies, leaf temperature is not too different from air temperature (see Appendix A). For example, in the thinned stand of this study, infrared thermometer measurements indicated that Douglas f i r twig temper-atures were usually within 1 C of air temperature. Even the salal leaf temperatures were within 2 C of air temperature,partly the result of shading by the trees. As a result, the following approximation can be used: e s ^ e*(T a) (3) where T is the air temperature. This suggests that e p - e, in (1) can a s a be approximated by the vapour pressure def i c i t (v.p.d.) of the air, e*(T a)-e a. 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 soil surface, windspeeds were usually much greater than 50 cm s"1 owing to the openness of the canopy. 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 - 1, r^ could be considered much smaller than r 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 air is 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 particular layer i with leaf area index LAI^ . and average stomatal resistance r . is given by: pc LAI. v.p.d.. E. = — ! 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: p c n LAI. v.p.d.. E = —2- z 3 1 (6) L Y 1 = 1 This equation was suggested but not used in chapter 3. 3. Experimental Procedure 3.1 Experimental site The experimental site was located approximately 27 km northwest of Courtenay, B.C. on the eastern coast of Vancouver Island. 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 site description : is 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 diffusion 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 (salal), 3, 5 and 7 m. Further details 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 ratio of 100 needles from this branch.. On the fourth tree a l l needles were removed and the total dry weight ratio was also determined. This procedure was used on the fourth tree as a check on the validity 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 first 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 all 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 for 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. Height Interval Douglas f i r needle 2 area (m ) 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 3 - 4 9.81 13.15 8.01 2 .69 4 - 5 8.97 7.96 3.13 0.93 5 - 6 6.05 4.77 2 .20 0.12 6 - 7 6.33 2.38 0.38 0.02 7 - 8 2.21 0.34 0 . 2 9 - 0 8 - 9 0 .83 0.07 0.01 0 9 - 1 0 0.37 0 0 0 >10 0.10 0 0 0 Total (m2) 54.7 45.2 27 .8 16 .0 D.B.H. (cm) 12.3 11 .0 9.6 6.0 Height (m) 10.5 8 .9 8.1 6.4 LAI 4.6 1 3; 8 v •- : 2 , 3 . J : .1 .4 . Salal undergrowth (LAI) Location No. 1 2 3 LAI 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,v.<!|9j'3j£/a;nd..35 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 ( s^) on the 2 days shown in Fig. 1 were -0.5 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 transpir-ation 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 (Rn) shown are for the 24-hour period. 104 is compared in Fig. 2. Also shown are the net radiation (Rp) and v.p.d. 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 ^ s was high (Fig. 2a), the Douglas f i r had a higher 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. On July 29, when the value of ij> 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 f i r . Consequently, when the value of ^ s was low,,the salal was consuming 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 day-time on 6 of the 7 fine days on which intensive stomatal resistance measure-ments were made during the drying period. Those obtained from stomatal 105 1 1 1 1 : t r HOUR , P.S.T. Fig. 2.- Daytime courses of-transpiration rate (E) for Douglas f i r and salal in the thinned stand for the same 2 days shown in Fig. 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 defi 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. The values of net radiation (Rn) shown are for the 24-hour period. 107 E (mm hr"') 0.2 O.I h (a) JULY 28, 1975 0.2 0.1 0 L (b) JULY 29, 1975 fs=-l0.5 bars R n = l5.6 MJm^day"1 ° o STOMATAL RESISTANCE MEASUREMENT • • ENERGY BALANCE MEASUREMENT 0.2 0.1 (0 JULY 30,1975 V - H . 3 bars Rn = l4.6 MJrrf^y, 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 soil water potential ( i p s ) was low. . The values of net radiation ( R n ) shown are for the 24-hour period. 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 resis-tance 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. Both methods indicate very low transpiration rates. During most of the daytime, agreement is good; however, in some.cases, values obtained from stomatal resistance measure-ments 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). It is interesting to note that when values of 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 measure-ments 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 trans-piration originating from the salal. Table 2 shows that this proportion rose from roughly 45% at high soil water potential (0 » t ( j s > -3.5 bars) to roughly 70% at low soil water potential (-9.5 > -12.5 bars). This indicates that the transpiration rate of the Douglas f i r decreased more n o 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) for the thinned stand calculated using the simple diffusion model (with stomatal resistance measurements) with those measured using the energy balance/ Bowen ratio meithod. Also shown are the average daytime transpiration rate of the salal and the ratio of these values to the corresponding values for the entire stand also calculated from stomatal resistance measurements. Average Daytime Transpiration Rates (mm hr" ) Date, 1975 Period (PST) Energy balance measurements Stomatal resistance measurements E s a l a l ', % Douglas f i r and salal Salal E t o t a l June 30 5 - 21 0.21c 0.240 i o . i o : 42 July 5 7 - 21 0.29,.. 0.2?v 0.13< 50 6 8h- 19 0.27i 0.26;, 0.11 44 11 5 - 1 9 0.244- 0.193 o . n • 55 28 SH- 19% 0.07,8 0.09) 0.06 V 62 29 6 - 1 9 0.06/ o.om 0.05!: 71 30 6 V 19 0.11, 0.12"?'.? 0.09 72 112 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 s = exp[1 .4581 + O.OO27(v.p.d.))2;0 (7a) -3.5 > \b > -9.5 bars rs -r s = exp[l.9901 + 0.0034(v.p.d.)j] (7b) -9.5 > tys > -12.5 bars r s = exp[2.6906 + 0.0057(v.p.d.)§] (7c) Lines representing these equations are shown in Fig. 6. This figure is 113 v.p.d. (mb) Fig. 6. 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. The data have been separated into 3 soil water potential Us) 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 i p . Also shown are lines representing the following regression equations: 0 > ^ s > -3.5 bars r s = exp[1.4418 + 0.0019 (v.p.d.)?] (8a) -3.5 > ib > -9.5 bars rs r s = exp[1.7436 + 0.0031 (v.p.d.)?] (8b) -9.5 > ii > -12.5 bars rs -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. In Figs. 6 and 7, the data for the middle class (-3.5 > i f i > -9.5 bars) covered a wider soil water potential range than the other 2 classes because of less available data. There is further discussion of these characteristics , 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 >VS 2 -9.5 bars SALAL Fig. 7. Relationship between stomatal resistance (r ) and vapour pressure def i c i t (v.p.d.) for the salal undergrowth in the thinned stand. The data have been separated into 3 soi l water potential (JJS) 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 ( i j ; 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 m = 0.217 + 0.981 v.p.d.-jg ^ with R = 0.65 and x = 2.2 mb.. In the high ^ s 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 IJJs 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 . Measure-ment 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 tyc range at which maximum stand transpiration rate occurs. 118 0.4 r v.p.d. (mb) Fig. 9. 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 (^s) classes. Each datum point is 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 transpir-ation 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., CS. Tan, and J.U. Nnyamah. 1976. Effect of thinning on evapotranspiration from a Douglas f i r forest. Final Report to the Pacific Forest Research Center, Department of the Environment, Victoria, B.C. Davies, J.A. and CD. Allen. 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 ratio measured by differential psychrometry. Agr. Meteorol. 7: 329-334. Landsberg, J.J. and M.M. Ludlow. 1969. Technique for 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. Biol. 19: 205-234. Parlange, J.Y., P.E. Waggoner, and G.H. Heichel. 1971. Boundary layer resistance and temperature distribution 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: III. Soil water influence. Agron. J. 64: 168-173. Turner, N.C, F.C.C. Peterson, and W.H. Wright. 1969. An aspirated diffusion porometer for f i e l d use. Con. Agr. Exp. Sta. Soils Bull. 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 (Tg) for Douglas f i r and salal. The regression equations for both species are shown in the figure. 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. 10 8h ~l 1 1 r - i r TREE No. I h=l0.5m D.B.H. = 12.3 cm 129 h (m) 0 ' .1 I I L 0 4 8 N.S.A. (m 2 ) 12 T R E E No.2 h = 8.9 m D. B.H. = 11.0 cm 0 N.S.A. 8 12 T R E E No. 3 h =8.1 m D.B.H. = 9.6 cm 0 N.S.A. (m 2) 8 1 h (m) i : r T R E E No. 4.-h = 6.4 m D.B.H. = 6.0 cm 1:32 0; J L N.S.A. (m 2) Tree No. 1 Tree No. 2 Tree No. 3 Tree No. 4 h(m) N.S.A.(m2) h(m) N.S.A.'m2) h(m) N.S.A.(m2) h(m) f LS.A.Cm2) 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 - 7 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 (m2) 45.2 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 o.io 0.05 0 0.10 0.05 E o (mm hr -1) 0.03 0.02 0.01 0 JULY 5, 1975 V - I ^ b o r s ^ Rn = 18.7 M J m"2day"1 (a) Vs =-2.4 bars JULY II, 1975 R n , l 8 . O M J m - 2 d o y - , (b) DOUGLAS FIR CANOPY LAYER 0 -3.5 m o 3.5 -6.0 m * * >6.0m JULY 28, 1975 ^ -9.9 bars R„ = 16.0 MJ m d^ay"1 (0 0.03 0.02-0.01 -0 (. JULY 30, 1975 V.--II.3 bars Rn = 14.6 M J m"zday"' (d) 9 12 15 H O U R , 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 for 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. The regression equation is shown in the figure. 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 for 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 C02 assimilation in the S.P.A.C. Paper presented at 64th Annual American Society of Agronomy Meeting, Oct. 29 to Nov. 2, 197.2. 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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