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Root water uptake in a Douglas fir forest Nnyamah, Joseph U. 1977

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ROOT WATER UPTAKE IN A DOUGLAS FIR FOREST by JOSEPH U. NNYAMAH B.Sc. (Hons.), Un iver s i t y of N i ge r i 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 So i l Science (Soi l Physics/Biometeorology) • We accept th i s thes i s as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1977 (c) Joseph U. Nnyamah, 1977 In presenting th 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 ibrary sha l l make i t f r e e l y ava i l ab le for reference and study. I f u r ther agree that permission f o r extensive copying of t h 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 i on of th i s thes i s fo r f i n a n c i a l gain sha l l not be allowed without my wr i t ten permission. Department of So i l Science  The Un iver s i t y o f B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 JOSEPH U. NNYAMAH i i i ABSTRACT The fo res t water balance and root water re l a t i ons were studied in a thinned (840 stems/ha) and an unthinned (1840 stems/ha) stand of a Douglas f i r f o res t during two consecutive summers. So i l water content and poten-t i a l data were used to compute water ext rac t ion rates and patterns f o r the root zone in each stand over a four-week drying per iod. The re su l t s showed a gradual downward s h i f t of the zone of maximum root water uptake as the s o i l d r i ed . There was good co r re l a t i on between water uptake rate and root dens i ty . Water f l ux into the bottom of the root zone, estimated by the use of Darcy's Law, increased from 8 to 15% of the evapotranspirat ion at the thinned s i t e and from 2 to 8% at the unthinned s i t e . So i l p r o f i l e water dep le t i on , corrected fo r f l ux out of or into the bottom of the root zone, agreed well with evapotranspirat ion computed from micrometeorological energy balance data. Water withdrawal from trunk storage accounted fo r only 2% of the to ta l evapotranspirat ion over the four-week drying per iod. In the f i r s t two weeks, evapotranspirat ion from the thinned stand was 11% less than that from the unthinned stand, but was 18% more in the l a s t two weeks. At a p a r t i c u l a r s o i l water p o t e n t i a l , i nd iv idua l trees at the thinned s i t e t rans -p ired an average of 25% more than those at the unthinned s i t e on f i n e sunny days. When water uptake was compared over the four-week per iod , i t was found that the ind iv idua l trees at the thinned s i t e were t r an sp i r i ng 35% more than those at the Unthinned s i t e . Measurements of s o i l and root xylem water potent ia l s were made using a Wescor HR-33T dew point microvoltmeter and PT 51-10 hygrometers. Tens io -meters were used to measure s o i l water potent ia l s at values greater than -1 bar. Twig water potent ia l was measured by the pressure chamber i v technique. Root water potent ia l measurement required tangential i n se r t i on of the hygrometer into the root xylem and sensor protect ion from plant res ins using gypsum powder. So i l water potentia ls measured with hygrometers were compared with po tent i a l s computed using grav imetr ic s o i l water content measurements and laboratory so i l water re tent ion data, while root water potent ia l measurements were compared with those made on roots with the pressure chamber. The comparisons showed good agreement to within 0.3 bar over an 8-bar range. So i l water matric potent ia l on both dew point and psychrometric modes showed good agreement. So i l and root res i s tances to water uptake were studied in both stands. Resistances were obtained from water potent ia l d i f fe rences and evapotranspirat ion f l uxes . Root xylem water p o t e n t i a l , l i k e twig water p o t e n t i a l , showed a d e f i n i t e diurnal trend. So i l water potent ia l approached the root water potent ia l as the s o i l d r i e d . So i l res i s tance remained very small in comparison to root res i s tance even at a s o i l water potent ia l of -11 bars; however, i t was found that "contact re s i s tance " could account f o r as much as ha l f of the to ta l s o i l to root xylem res i s t ance . Root res i s tance var ied during the daytime becoming increas ing ly . important toward n i g h t f a l l . The p lot of rate of water uptake versus s o i l to root xylem potent ia l d i f f e rence showed a l i n e a r re l a t i on sh ip extending through zero. Root res i s tance remained r e l a t i v e l y constant as the s o i l d r i e d . , V TABLE OF CONTENTS ABSTRACT • i i i LIST OF TABLES • v i i i LIST OF FIGURES ix LIST OF SYMBOLS xix ACKNOWLEDGEMENTS x v i i INTRODUCTION . . . . . . . . . . 1 CHAPTER I - RATES AND PATTERNS OF WATER UPTAKE IN A DOUGLAS FIR FOREST . 3 Abstract . . . . . . . . 4 Introduction 5 Methods and Mater ia ls . . . . . . . 6 Experimental s i t e s 6 F i e l d measurement of hydrologic and meteorological var iab les . . . . . . . . 7 Measurement of t ree parameters . . . . . . . . 11 Laboratory measurement of s o i l hydrologic. parameters ,. . 12 Est imation of root water uptake and f luxes with in the root zone 16 Results and Discussion 17 So i l hydrologic propert ies . . 17 Rates and patterns of water uptake . . 18 Conclusions 41 L i t e ra tu re C i ted . . 43 CHAPTER II -EFFECT OF THINNING ON THE WATER BALANCE OF A DOUGLAS FIR FOREST 47 Abstract . . . . . . . . . . . . . . . 48 Introduction 48 Methods and Mater ia ls . . . . . . . . . . 50 Experimental s i t e s 50 Measurement of hydro log ic , meteorological and plant parameters . . 50 Results and Discuss ion . . . 52 Conclusions , 58 L i t e ra tu re C i ted . . . . . . . . 61 vi CHAPTER III - RESISTANCE TO WATER UPTAKE IN A DOUGLAS FIR FOREST 63 Abstract . . . . . . . ^ . . . . . . . . . . . . 64 Introduction . . . . . . 65 Methods and Mater ia l s 66 Experimental s i t e s . . . . . . . . 66 Measurement of hydro log ic , meteorological and plant parameters . . . . . . . 67 Estimation of res i s tances in the water transport pathway 71 Results and Discuss ion . . 74 Water potent ia l patterns as soi l , d r ied . . . 74 S o i l , root and xylem res i s tance estimates . 82 Relat ing R s and R r to water uptake model parameters . . 86 Conclusions 93 L i t e ra tu re C i ted . . . . 95 CHAPTER IV - FIELD PERFORMANCE OF THE DEW POINT HYGROMETER IN STUDIES OF SOIL-ROOT WATER RELATIONS . . . . . . . . . . . . . . . . . 99 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 Introduction . 1 0 0 Methods and Mater ia l s . . 101 Dew point hygrometer . . . . . . . . . . . . 101 S i te and s o i l c h a r a c t e r i s t i c s . . 102 So i l water matric potent ia l and s o i l water content measurement 102 Root xylem water potent ia l measurement . . . . . . . . . 104 Hygrometer c a l i b r a t i o n . • • • 104 Results and Discuss ion • . . . 1 0 5 So i l water matric potent ia l measurement comparison . . . 1 0 5 Root water potent ia l measurement comparison 108 Diurnal v a r i a t i on in ^ r and ipm I l l Seasonal v a r i a t i on in i j ; r and i j j m . . . • 113 Hygrometer c a l i b r a t i o n 115 Concl us ions 115 L i t e r a t u r e C i ted . 118 APPENDIX I - Neutron moisture meter c a l i b r a t i o n curve f o r s i t e s 1 and 2 120 APPENDIX II - Volumetric water content (0) as a funct ion of depth (z) at s i t e 1 during 1974 122 APPENDIX III - Volumetric water content ( e ) as a funct ion of depth (z) at s i t e 2 during 1975 124 vi i Page APPENDIX IV - Total s o i l water potent ia l {ty) at four depths fo r the period June 17 to August 19, 1974 at s i t e 1 1 2 6 APPENDIX V - Total s o i l water potential s{ty) at f i v e depths f o r the period June 18 to August 12, 1975 at s i t e 2 129 APPENDIX VI - Unsaturated hydraul ic conduct iv i ty as a funct ion of matric water potent ia l f o r Dashwood g rave l l y sandy loam 132 APPENDIX VII - Da i ly micrometeorological energy balance-Bowen r a t i o evapotranspirat ion data at the unthinned s i t e ( s i t e 1) at various s o i l water potent ia l s {ty) during 1974 . . . . 134 APPENDIX V I I I - D a i l y micrometeorological energy balance-Bowen r a t i o evapotranspirat ion data at the thinned s i t e ( s i t e 2) at various s o i l water potent ia l s {ty) during 1975 137 APPENDIX IX - Comparison of change in trunk water storage with average micrometeorological energy balance evapo-t r an sp i r a t i on 140 APPENDIX X - Root dens i ty data at s i t e 1 . . . . 142 APPENDIX XI - Root density data in one of the sample locat ions at s i t e 2 145 APPENDIX XII - Mid-day values of s o i l water potent ia l {tys), root xylem water potent ia l {tyr), twig water potent ia l {tyt), and Douglas f i r t r an sp i r a t i on (E) at s i t e 2 during 1975 147 APPENDIX XIII- Ca l i b r a t i on data fo r a batch of 30 hygrometers using 0.30 molal NaCl so lu t ion (-13.68 bars at 25C and -13.21 bars at 15C ) at two temperatures (25C and 15 C) on both the dew point and psychrometric modes 149 APPENDIX XIV - B r i t i s h Columbia showing the study area 151 APPENDIX XV - Mean annual values of temperature and p r e c i p i t a t i o n based on the period 1941-1970 fo r the Comox-Courtenay area 153 v i i i LIST OF TABLES Table Page CHAPTER I I Properties of the s o i l in the unthinned stand ( s i t e 1) , 8 II Properties of the s o i l in the thinned stand (s i t e 2) 9 III Percentage of water extracted from four s o i l layers during f i v e time periods as a function of percentage of roots and volumetric water content ( e ) in the unthinned stand ( s i t e 1 ), 1974 . .. . • 33 IV Percentage of water extracted from six s o i l layers during f i v e time periods as a function of percentage of roots and volumetric water content ( e ) in the thinned stand (si t e 2), 1975 3 4 V Average values of net radiation and evapotranspiration obtained by the water balance and energy balance methods for site s 1 and 2 4 0 CHAPTER III I Root density (L , cm root cm" s o i l ) data at s i t e 2 83 ix LIST OF FIGURES Figure Page CHAPTER I 1. Water retent ion curves fo r various layers of Dashwood g rave l l y sandy loam in the unthinned stand ( s i t e 1) . . . . . 19 2. Water re tent ion curves f o r various layers of Dashwood g rave l l y sandy loam in the thinned stand ( s i t e 2) 20 3. Unsaturated hydraul ic conduct i v i t y as a funct ion of matr ic potent ia l fo r a sample of Dashwood g rave l l y sandy loam with large stones removed 21 4. Relat ion between unsaturated hydraul ic conduct i v i t y and volumetric water content corrected fo r stone content fo r Dashwood g rave l l y sandy loam 22 5. Total s o i l water potent ia l (i/*) at four depths fo r the period June 17 to August 19, 1974 23 6. Total s o i l water potent ia l (ii) at f i v e depths f o r the period June 18 to August 12, 1975 24 7. Volumetric water content ( e ) as a funct ion of depth (z) at s i t e 1 26 8. Volumetric water content ( e ) as a funct ion of depth (z) at s i t e 2 27 9. So i l water f lux (v 2 ) in the root zone during the drying cyc le Ju l y 18 to August 17, 1974. Pos i t i ve values ind i ca te downward f lux and v ice versa,,. 29 10. So i l water f l ux (v z ) in the root zone during the drying cyc le June 30 to August 1, 1975. Pos i t i ve values ind ica te down-ward f l ux and v ice versa 30 11; Depth d i s t r i b u t i o n of water uptake by roots at s i t e 1 and s i t e 2 over f i v e time periods. .The values of R z giv.en were. . obtained by in tegrat ing r z over the en t i r e root 'zone (0- to 65-cm at s i t e 1 and 0- to 80-cm at s i t e 2) 3 2 X Figure % Page 12. Root biomass (dry weight) per uni t volume of s o i l as a funct ion of depth at s i t e s 1 and 2. Root biomass was computed using the predominant diameter c lass (< 2-mm diameter) which cons t i tu ted 95 to 98% of the to ta l root length . . . 36 13. Root zone dep let ion of water, f l u x at the bottom of the root zone, and evapotranspirat ion ca l cu la ted from these data f o r s i t e 1 38 14. Root zone dep let ion of water, f lux at the bottom of the root zone, and evapotranspirat ion ca l cu la ted from these data fo r s i t e 2; 3 9 CHAPTER II 1. Comparison of weekly evapotranspirat ion estimates obtained by the water balance and energy balance methods f o r s i t e s 1 and 2 53 2. Stand t ransp i ra t i on normalized to the net r ad i a t i on to the stand as a funct ion of to ta l s o i l water potent ia l (ip) on f i ne sunny days at s i t e s 1 and 2 . 54 3. Individual t ree t r ansp i r a t i on normalized to the net rad ia t i on to the stand as a funct ion of to ta l s o i l water potent ia l (^) on f i n e sunny days at s i t e s 1 and 2 56 4. Seasonal comparison of average weekly net rad ia t i on f l u x , tota l stand t r an sp i r a t i on normalized to the stand net r a d i a t i o n , and ind iv idua l t ree t r ansp i r a t i on normalized to the stand net rad ia t i on at s i t e s 1 and 2. T ransp i ra t ion values were ca l cu la ted from s o i l water deplet ion and drainage data' 57 5. Comparison of the average diameter at breast height (D.B.H.) of three thinned ( s i te 2) and three unthinned ( s i te 1) t ree s . Diameters were normalized to the values on May 22 . . 59 xi Figure Page CHAPTER III 1. Polygon of occupancy showing sample points at 1/8, 1/4 and 1/2 pos i t ions 70 2. Course of s o i l water potent ia l (^ s) at f i v e depths and root xylem water potent ia l (ty^) f o r the period June 18 to August 12, 1975 75 3. Diurnal trends of water potent ia l of root and twig xylem of Douglas f i r , components of the energy balance of the stand, Douglas f i r t r an sp i r a t i on (stand area bas is ) and root and xylem res i s tances of Douglas f i r at a s o i l water potent ia l of -1.0 bar. See text f o r d e f i n i t i o n of symbols. To convert Douglas f i r t r an sp i r a t i on (E) on a stand area basis to t r ansp i ra t i on on an ind iv idua l tree bas i s , d i v ide stand t ransp i ra t i on by 840 trees/ha 76 4. Diurnal trends of water potent ia l of root and twig xylem of Douglas f i r , components of the energy balance of the stand, Douglas f i r t r an sp i r a t i on (stand area basis) and root and xylem res i s tances of Douglas f i r at a s o i l water potent ia l of -2.3 bars. See text f o r d e f i n i t i o n of symbols. To convert Douglas f i r t r an sp i r a t i on (E) on a stand area basis to t r ansp i r a t i on on an ind iv idua l tree bas i s , d i v ide stand t r ansp i r a t i on by 840 t rees/ha 77 5. Diurnal trends of water potent ia l of root and twig xylem of Douglas f i r , components of the energy balance of the stand, Douglas f i r t r an sp i r a t i on (stand area bas is ) and root and xylem res i s tances of Douglas f i r at a s o i l water potent ia l of -7.4 bars. See text fo r d e f i n i t i o n of symbols. To convert Douglas f i r t r an sp i r a t i on (E) on a stand area basis to t r ansp i r a t i on on an ind iv idua l tree bas i s , d i v ide stand t r ansp i r a t i on by 840 trees/ha 78 x i i Figure Page 6. Diurnal trends of water potent ia l of root and twig xylem of Douglas f i r , components of the energy balance of the stand, Douglas f i r t r an sp i r a t i on (stand area bas is ) and root and xylem res i s tances of Douglas f i r at a s o i l water potent ia l of -10.1 bars. See text f o r d e f i n i t i o n of symbols. To convert Douglas f i r t r an sp i r a t i on (E) on a stand area basis to t r an sp i r a t i on on an ind iv idua l t ree bas i s , d i v ide stand t r an sp i r a t i on by 840 trees/ha . . . . . . . 79 7. Unsaturated hydraul ic conduct i v i t y as a funct ion of matric potent ia l f o r a sample of Dashwood g rave l l y sandy loam with large stones removed. (Same as F i g . 3 of Chapter I) . . . 85 8. The rate of water uptake of Douglas f i r on a stand area basis versus s o i l to root xylem potent ia l d i f f e rence (Aip ) at s i t e s 1 and 2. On cloudy days ( t r i ang le s ) the c l ea r day p a r t i t i o n i n g of Douglas f i r and sa la l t r a n s p i r -a t ion may underestimate water uptake of Douglas f i r . To convert water uptake on a stand area basis to uptake on an ind iv idua l tree bas i s , d i v ide water uptake value by the j appropriate stand dens i ty . . . . . . . . 87 9. The rate of water uptake of Douglas f i r on a stand area basis versus s o i l to twig potent ia l d i f f e rence (Aip f ) S u at s i t e 2. Cloudy day^is ind icated by t r i a n g l e . To convert water uptake on a stand area basis to uptake on an ind iv idua l t ree bas i s , d i v ide water uptake by 840 trees/ha . . 89 10. Mid-day values of combined so i l and root res i s tance ( R s r ) » s o i l res i s tance ( R s ) » root res i s tance(Py) and root r e s i s -tance with the e f f e c t of "contact re s i s tance " (R ) sub-i c t racted (R-H ) at various s o i l water potent ia l s at s i t e 2. The t r i ang le s and open squares are derived from the cloudy day. data of F i g . 8 . . . . -. . . . . i . . . . . 91 xi i i Figure 11. Mid-day values of s o i l re s i s tance (R s ) versus s o i l water potent ia l . . . 92 CHAPTER IV 1. Water re tent ion curves f o r various layers of Dashwood g rave l l y sandy loam in the unthinned stand ( s i te 1). (Same as F i g . 1 of Chapter I) 106 2. Comparison of s o i l water matric potent ia l measured with the dew point hygrometer at s i t e 1 with that determined from measurements of s o i l water content and the s o i l water retent ion curve 107 3. Comparison of s o i l water matric potent ia l measured on the dew point and psychrometric modes at s i t e 2. The data were from 8 hygrometers over a period of 13 days . . . . 109 4. Comparison of Douglas f i r root xylem water potent ia l measured with a dew point hygrometer and a pressure chamber at 5. Daytime course of s o i l water matric potent ia l (^  ), root and twig water po tent i a l s (^r and y^) on two days at s i t e 1 . . . 112 6. Course of t o ta l s o i l water potent ia l at four depths and root water potent ia l (^ r) fo r the per iod June 17 to August 19, 1974 114 7. • Typica l c a l i b r a t i o n curves fo r a hygrometer on both dew point and psychrometric modes at two temperatures (25C and 15C) . . . . . . . . . . . . 116 s i t e 1 110 xiv LIST OF SYMBOLS E Evapotranspirat ion or t r an sp i r a t i on (mm day " 1 ) G So i l heat f l ux (W m" 2 or MJ m" 2 day " 1 ) I Q 1 s Standardized gamma count r a t i o at water content (dimensionless) I Q 1 s Standardized gamma count r a t i o at water content 6 ^ (dimensionless) ' "»i:T K or K 2 So i l unsaturated hydrau l ic conduct i v i ty (cm day?' ) L Latent heat of vapor izat ion of water (J g" 1 ) _ r> L y Root dens i ty (cm of root cm" of s o i l ) M Rate of storage of sens ib le and l a ten t heat within the canopy on -2 - 2 - 1 aniarea basis (W m or MJ m" day" ) R c "Contact re s i s tance " between the s o i l and root surface (days or bar day cm" 1 ) -2 -2 -1 Rn Net r ad i a t i on f l u x (W m~ or MJ m" day" ) R^  Root res i s tance (root surface to root xylem) (days or bar day cm" 1 ) Rl|r^ Root res i s tance with "contact res i s tance" removed (days or bar day cm" 1 ) R s S o i l res i s tance or rhizosphere res i s tance (bulk s o i l to root surface) (days or bar day cm" 1 ) R^r Combined s o i l and root res i s tance (bulk s o i l to root xylem) (days or bar day c m - 1 ) R Xylem res i s tance (root xylem to twig xylem) (days or bar day cm" 1 ) f 2 -1 R Accumulated root ext rac t ion r a t e , I r dz (cm day ) z 0' z S Thickness of s o i l used in unsaturated hydraul ic conduct i v i t y determination (cm) XV T Transp i ra t ion rate or rate of water uptake (mm day " 1 ) 3 3 V^ Volume f r a c t i o n occupied by the < 2-mm so i l f r a c t i o n (cm cm" ) W.p Water content, dry mass bas i s , of the < 2-mm so i l f r a c t i o n (g g~^) c Half the distance between neighbouring roots (cm) f Fract iona l saturat ion o f : t h e s o i l , e / e s a t (dimensionless) q r Rate of water withdrawal at steady state by roots of r ad iu s , r, 3 -1 -1 (cm cm root " day" ) 3 - 3 - 1 1 r z Water uptake by roots at depth z (cm cm day" or day" ) v Flux of water at a given depth' in the s o i l (cm day"^) z Depth in the s o i l (cm) 3 Bowen. r a t i o : the r a t i o of the. f l ux dens i t i e s of sens ib le and l a tent heat (dimensionless) 00 Volumetric water content of the whole s o i l (coarse fragments and 3 -3 the < 2-mm f r a c t i o n ) (cm cm ) 3 3 6 s a t Volumetric water content of the whole s o i l at saturat ion (cm c m ) 2 1 v Attenuation c o e f f i c i e n t f o r water (cm q~ ) w Bulk dens i ty of the < 2-mm s o i l f r a c t i o n (g c m ) P w Density of water (g cm ) ty Total s o i l water potent ia l (bars) <Jj-j Leaf water potent ia l (bars) ^ m So i l water matric potent ia l (bars) ^ r Root xylem water potent ia l (bars) ^ r s Water potent ia l at the r o o t - s o i l i n te r f ace (bars) ^ s Total s o i l water potent ia l in Chapter 3 (bars) ty^ Twig water potent ia l (bars) A i ^ i So i l to l ea f water potent ia l d i f f e rence (bars) X V I A i p s r So i l to root xylem water potent ia l d i f f e rence (bars) AiJ> S£ So i l to twig water potent ia l d i f f e rence (bars) xvi i ACKNOWLEDGEMENTS I wish to express my s ince grat i tude and deep apprec iat ion to Dr. T.A. Black, my superv i sor , f o r h i s guidance, ingenui ty , whole-hearted p a r t i c i p a t i o n and support that has enabled me to complete th i s study. My spec ia l thanks go to Dr. J . de Vr ies fo r providing the gamma attenuation equipment and fo r the s t imulat ing d i scus s ions , comments and ideas that he has contr ibuted during the course of my study. I am very gratefu l to Drs. L.M. Lavku l i ch , C.A. Rowles and R.A. Freeze f o r serving as members of my doctoral committee and f o r the personal i n te re s t that they took in both my work and my personal wel l -be ing a l l through my course. I am very much indebted to Dr. N.K. Nagpal for his suggestions and help, e s p e c i a l l y during the data ana l y s i s ; to Dr. T.M. Ba l l a rd fo r pro-v id ing the d i s sec t ing microscope and helpfu l advice f o r the root density ana l y s i s ; to Mr. B. von Spindler f o r his e x c e l l e n t . d r a f t i n g and photography; to Ms. D.J. Green fo r typing th i s t h e s i s ; to my fe l low graduate students, Mr. C.S. Tan and Mr. P. Tang for t h e i r cooperation and help; to Dr. H. Br ix and Mr. M. Crown of the Canadian Forestry Service f o r t h e i r advice and cooperat ion; to the fo res te r s of Crown Ze l lerbach Company fo r t h e i r help and cooperat ion;.and to the Canadian Forestry Serv i ce , the National Research Council ofa-Canada and the B r i t i s h Columbia Department of A g r i - . cu l tu re f o r funding th i s p ro jec t . F i n a l l y , my deep apprec ia t ion goes to my dear f r i e n d Ms. P a t r i c i a E. Chute f o r her encouragement, i n s p i r a t i o n and understanding. 1 INTRODUCTION An understanding of the water balance and transport in the s o i l -plant-atmosphere system requires r e l i a b l e measurements in i t s various part s . A knowledge of the behavior of th i s system i s necessary in managing crops and forest s to achieve high water use e f f i c i e n c y . This thes i s research was ca r r i ed out as part of a large two-year p ro jec t aimed at studying the e f f e c t of th inn ing , o m water consumption and tree growth in a Douglas f i r f o res t on the droughty east coast of Vancouver Is land. The thes i s cons i s t s of four chapters. The research was f inanced under a contract with the Canadian Forestry Service and with grants from the National Research Council of Canada and the B r i t i s h Columbia Department of A g r i c u l t u r e . In Chapter 1, the rates and patterns of water uptake in the fo res t root zone were studied by use of s o i l physics (water balance) and micro-meteorological techniques to estimate water consumption. Comparison of these two independent methods of est imating water uptake has been c a r r i e d out in a g r i cu l tu re but not in f o r e s t s , e s p e c i a l l y at a remote l o ca t i on . In contrast to a g r i c u l t u r a l crops which have v e r t i c a l l y expanding root systems, the Douglas f i r trees had well es tab l i shed root systems. Due to the stoniness of the s o i l , large samples had to be taken in order to obtain a representat ive amount of roots. In Chapter 1, the root d i s t r i b u t i o n was re la ted to water uptake and la ter , in Chapter 3, the root dens i ty was appl ied to s o i l res i s tance c a l c u l a t i o n s . In Chapter 2, the bene f i c i a l e f f e c t of thinning in provid ing more 2 water f o r each ind iv idua l tree was inves t i ga ted. The tota l stand t ransp i ra t i on by the Douglas f i r trees dropped with th inning but the f l ux of water through each ind iv idua l tree increased r e s u l t i n g in f a s te r rate of growth in thinned t r e e s . . The r e l a t i v e magnitudes of s o i l and plant res i s tances have been a subject of much controversy. The present consensus appears to be that the s o i l res i s tance remains r e l a t i v e l y small in comparison to the p lant res i s tance even at a s o i l water potent ia l approaching -15 bars. The present research provided an opportunity to make these res i s tance measure-ments in the f o re s t and e s p e c i a l l y to p a r t i t i o n the plant pathway into two - root surface to root xylem, and root xylem to twig xylem. A recent ly advanced hypothesis that takes into account development of an a i r gap due to root shrinkage in a drying s o i l was tested in add i t ion to the c l a s s i c a l s i n g l e - roo t model. These res i s tances are discussed in Chapter 3. Chapter 4 deals with the in tens ive f i e l d tes t s performed on a new i s o p i e s t i c water potent ia l measuring device - the hygrometer. It deter -mines water potent ia l s by measuring the dew point temperature depression of water vapour in equ i l i b r ium with a sample. Its operation i s r e l a t i v e l y independent of the wetting c h a r a c t e r i s t i c s of the junct ion and the s ize and shape of the water drop let formed on the junc t i on . This makes the hygrometer quite adaptable f o r f i e l d use. 3 CHAPTER 1 RATES AND PATTERNS OF WATER UPTAKE IN A DOUGLAS FIR FOREST 4 RATES AND PATTERNS OF WATER UPTAKE IN A .DOUGLAS FIR FOREST ABSTRACT The f o re s t water balance was studied in a thinned (840 stems/ha) and an unthinned (1840 stems/ha) Douglas f i r f o res t during two consecutive summers. So i l water content was measured with the neutron moisture meter. So i l water potent ia l was measured with tensiometers and dew point hygro-meters over a range of 0 to -15 bars. These data were used to compute water ext rac t ion rates and patterns for the root zone over a four-week drying per iod . . The re su l t s showed a gradual downward s h i f t of the zone of maximum root water uptake as the s o i l d r i e d . The f u l l y developed root system of Douglas f i r showed les s hydrotropic response than the developing root systems of annuals reported in . the l i t e r a t u r e . There was good cor -r e l a t i o n between water uptake rate and root dens i ty . During the drying per iod , water f l ux into the bottom of the root zone, estimated by the use o f .Darcy ' s Law, increased from-8 to 15% of the evapotranspirat ion at the thinned s i t e and from 2 to 8% at the unthinned s i t e . So i l p r o f i l e water dep let ion corrected fo r f l ux out of or into the bottom of the root zone agreed well with evapotranspirat ion computed from micrometeorological energy balance data. Water withdrawal from trunk storage accounted fo r only 2% of the to ta l evapotranspirat ion over the four-week drying per iod . 5 INTRODUCTION The water transport process in the so i l -p lant-atmosphere system is a dynamic phenomenon. An understanding of root water uptake and the root zone water balance requires a knowledge of the so i l water re tent ion and conducting propert ies as well as the plant root dens i ty . Both so i l water content and so i l water potent ia l p r o f i l e s are required fo r the c a l c u l a t i o n of root zone water uptake patterns. Rose, and Stern (1967) measured so i l water content in the f i e l d and in fe r red s o i l water potent ia l from retent ion curves. Stone ef al_. (1973b) and Arya et_ al_. (1975a) used tensiometers to monitor s o i l water potent ia l and i n fe r red s o i l water content p r o f i l e s from retent ion curves, van Bavel et^ a l . (1968a), Rice (1975), and Allmaras et al_. (1975) made jjVjnj^,.measure-ments o f , s o i l water content with the neutron moisture meter and of s o i l water potent ia l with tensiometers. To compute accurate evapotranspirat ion estimates from s o i l water data, i t i s essent ia l that s o i l p r o f i l e water dep let ion be corrected f o r f lux into or out of the bottom of the root zone (Rose and Stern, 1967; van Bavel et al_., 1968a, 1968b; Black et a l . , 1970; Stone et al_., 1973b; R ice, . 1975; Allmaras et al_., 1975; and S c h o l l , 1976). Except fo r Schol l (1976), the tes t crops in a l l the cases c i t e d were .annuals with developing root systems and the range of s o i l water potent ia l invest igated was mostly between 0 and -1 bar. Ca lcu lated root water uptake has been shown to be in good agreement with independent . l y s i -metric measurements (van Bavel et al_., 1968a; Black et al_., 1970; and R ice , 1975) and with independent micrometeorological energy balance 6 measurements (Feddes, 1971). This study was designed ( i ) to descr ibe the rates and patterns of water uptake i n two stand dens i t i e s o f a Douglas f i r (Pseudotsuga  menzies i i (Mirb.) Franco) f o re s t over a s o i l water potent ia l range of 0 to -15 bars, and ( i i ) to r e l a t e root water uptake to root dens i ty . METHODS AND MATERIALS  Experimental S i tes The study was ca r r i ed o u t , i n a thinned (840 stems/ha) and an un-thinned (1840 stems/ha) stand of a 20-year-o ld Douglas f i r fo res t during two consecutive summers. The s i t e s were located approximately 27 km northwest of Courtenay, B.C., on the east coast of Vancouver I s land. The thinned s i t e was approximately 1 1/2 km southeast of the unthinned s i t e . The warm, droughty summers of th is region provided good drying cyc les f o r so i l - p l an t -water re l a t i on s s tud ies . The topography was gener-a l l y f l a t , with a number of r idges of approximately 20- to 30-m r e l i e f . E levat ion of the s i t e s was about 150 m. (See Appendix XIV f o r s i t e map). Trees at the unthinned s i t e ( s i t e 1) were 8- to 10-m t a l l and averaged 10.6 cm in diameter at breast height while at the thinned s i t e ( s i t e 2) the trees were 7- to 9-m t a l l and 10.9 cm in diameter. There was very scanty undergrowth at s i t e 1 but a t s i t e 2 there was a luxur ian t growth of sa l a l (Gaultheria sha l l on , Pursh) 1.\ 7 The s o i l i s a Duric Humo-Ferric Podzol (L.M. Lavku l i ch , personal com-municat ion). The s o i l type at both s i t e s i s Dashwood g rave l l y sandy loam. The s o i l i s underlain by compacted basal t i l l at a maximum depth of 70 cm at s i t e 1 and 85 cm at s i t e 2. Pa r t i c l e s la rger than 2 mm occupy 20% (by volume) of the s o i l at s i t e 1 and 7.5% (by volume) at s i t e 2. Some hydrologic char-a c t e r i s t i c s of the s o i l at both s i t e s are reported in Tables I and II. F i e l d Measurement of Hydrologic and Meteorological Var iab les The unthinned s i t e ( s i t e V) was instrumented in the summer of 1974 and the thinned s i t e ( s i t e 2) in the summer of 1975. So i l water matric p o t e n t i a l , ip , was measured with a tensiometer-pressure transducer system in the 0 to -1 bar range and with a Wescor HR-33T dew point microvoltmeter and PT 51-10 hygrometers f o r values less than -1 bar. The tensiometer-pressure transducer system i s descr ibed in de ta i l by Well ington (1971). At both s i t e s , one tensiometer each was located at the 15-, : 30-,' 45 - , and 60-cm depths. A l so , one tensiometer was located at the 75-cm .depth at s i t e 2. Six hygrometers were i n s t a l l e d at s i t e 1, two each at the 15- , and 30-cm depths and one each at the 45- , and 60-cm depths. At s i t e 2, three hygrometers each were i n s t a l l e d at the 15-, 30- , 45- , 60- , and 75-cm depths. The hor izonta l d istance between hygrometers at the same depth was 1 m. The i n s t a l l a t i o n procedure and performance tests of the hygrometers are discussed in Chapter 4. Twenty-four hours were allowed a f t e r i n s t a l l a t i o n to ensure e q u i l i b r a t i o n . So i l water poten-t i a l s measured with the tensiometer-transducer system were recorded at 1-hourly i n te rva l s on a Hewlett-Packard 2707A data logger, whi le s o i l water potent ia l s measured with the dew point hygrometers were manually 8 Table I - Propert ies of the s o i l in the unthinned stand ( s i t e 1). -Depth, (cm) P a r t i c l e S i z e + {%) Bulk density (g cm-3) Saturated K ! (cm day. ) Sand S i l t Clay pH 0-13 75.0 15.0 10.0 5.25 1.50 750.0 13-26 76.5 12.0 11.5 5.20 1.57 400.0 26-39 75.0 13.5 11.5 5.10 1.64 400.0 39-52 74.0 13.0 13.0 4.85 1.75 480,; 0 52-65 74.0 13.0 13.0 4.85 1.75 450.0 So i l texture at a l l depths i s g rave l l y sandy loam. Average coarse fragment content (0- to 65-cm) is 20% by volume. 9 Table II - Propert ies of the s o i l in the thinned stand ( s i t e 2) . Depth (cm) P a r t i c l e S i z e + ( X ) Bulk Saturated Sand S i l t Clay pH density (g cm"3) (cm day ) 0-13 74.5 14.0 11.5 5.25 1.05 2,700.0 13-26 72.0 14.0 14.0 5.20 1.12 2,520.0 26-39 76.0 11.0 13.0 5.10 1.10 3,058.0 39-52 76.5 12.0 11.5 5.20 1.10 2,637.0 52-65 76.0 12.5 11.5 5.35 1.10 1,623.0 65-78 74.5 15.5 10.0 5,45 1.15 1,080.0 So i l texture at a l l depths is g rave l l y sandy loam. Average coarse fragment content (0- to 78-cm) is 7.5% by volume. 10 recorded three times d a i l y . The g rav i ta t iona l potent ia l was added to the matric potent ia l to obtain the tota l s o i l water potent ia l (osmotic potent ia l was assumed n e g l i g i b l e ) . So i l water content was measured g rav imet r i ca l l y in the 0- to 10-cm layer and by the use of a neutron moisture meter (Troxler model 105A) at deeper depths. At s i t e 1, four aluminum access tubes with a wall t h i c k -ness of 3.6 mm were i n s t a l l e d down to the basal t i l l whi le at s i t e 2 s ix access tubes were i n s t a l l e d . The tubes were arranged in a c i r c l e around the meteorological tower at the center of each s i t e . Weekly neutron meter moisture measurements were made in these access tubes at depths of 20- , 35-, and 50-cm at s i t e 1 and at depths of 20-, 30-, 45- , 60- , and 75-cm at s i t e 2. One access tube at each s i t e was used fo r a f i e l d c a l i b r a t i o n of the neutron moisture probe. Continuous ha l f - hour l y measurements of evapotransp i rat ion were made using the Bowen rat io/energy balance method as part of the large p ro jec t . The Bowen ratio,B,wa: measured at the 10.5-m height at s i t e 1 and at the 11-m height at s i t e 2 using a psychrometric apparatus described by Black and McNaughton (1971). The separation between the pa i r of wet and dry bulb sensors was 1 m at s i t e 1 and 3 m at s i t e 2. The increased separation was used to increase the s e n s i t i v i t y of the instrument at the thinned s i t e where v e r t i c a l temperature and vapor pressure gradients above the canopy were smal ler than at the unthinned s i t e . The net rad ia t ion f l u x , R n,was measured at the top of the canopy with a Swissteco S- l net radiometer. The s o i l heat f l u x , G,was measured at the 5-cm depth with two heat f l u x p lates and corrected f o r storage in the upper 5 cm using an integrated temperature measured with two diode in tegrat ing thermometers (Tang et al_., 1974). Storage of sens ib le and l a tent heat wi th in the 11 canopy, M,was estimated from wet and dry bulb temperatures taken every 15 minutes at the 3-, 5-, and 7-m leve l s and from estimates of the heat capacity of the canopy based on the work of Stewart and Thorn (1973). The evapotranspirat ion r a te , E,was then ca l cu la ted using the equation: E =(Rn - G - M)/ [ L O + $)] (1) where L i s the l a tent heat of vapor iza t ion . Wind speed was measured by a sens i t i ve Case l la anemometer at a height o f 10.5-m at s i t e 1 and 11-m at s i t e 2. Wind d i r e c t i o n above the canopy was measured using a Cl imet wind vane. P r e c i p i t a t i o n was recorded d a i l y with a 10.2-cm diameter r a in located on top o f the instrument t r a i l e r , gauge/ Data s ignals were ca r r i ed back to the instrument t r a i l e r by 75-m long shie lded cables where they were recorded on a Hewlett-Packard 2707A data logger. Net r ad i a t i on and Bowen r a t i o data s i gna l s were integrated using voltage in tegrators (Tang et al_., 1976). Bowen r a t i o data were a l so monitored on a s t r i p - c h a r t recorder. Measurement of Tree Parameters Root dens i ty samples as a funct ion of depth were taken along 5 transects between a representat ive tree and the surrounding t r e e s . T r i p -l i c a t e core samples (7.3-cm diameter and 7.6-cm long) were taken at 1/8, 1/4 and 1/2 the d is tance along the transect from a representa t i ve t ree at 13-cm depth i n t e r v a l s . The roots were washed f ree of s o i l on a screen, dr ied at 70C and the root biomass as a funct ion of depth determined. 12 Two representat ive trees at s i t e 2 were cut down.in the summer of 1975; one, when the s o i l water potent ia l was -0.5 bar and the other, at a s o i l water potent ia l of -10.0 bars. Discs 3-cm th ick were sampled along the trunk and oven dr ied at 70C. The change in water content observed was used to evaluate the importance of trunk water storage as. a source of water f o r t r a n s p i r a t i o n . Laboratory Measurement of So i l Hydrologic Parameters Undisturbed s o i l cores (7.3-cm diameter and 7.6-cm long)v;were taken in three rep l i c a te s in 13-cm depth increments from the so i l surface to the basal t i l l at both s i t e s . . Saturated hydraul ic conduct i v i t y was det-ermined on these samples by the constant-head method of Klute (1965)-.-These core samples were then oven dr ied to determine the bulk dens i ty of the whole s o i l ( i . e . both coarse fragments and the < 2-mm f r a c t i o n ) as a funct ion of depth. The bulk dens i ty o f . the whole s o i l was also determined in the f i e l d by the excavation method. The<exe:a;yaied materia l was used to determine the coarse fragment content. The evaluat ion of the bulk dens i ty of the < 2-mm f r a c t i o n i s i l l u s t r a t e d with data from s i t e 1 (see Table I). The average bulk dens i ty fo r the whole s o i l was 1.61 g cm . An average of 20% of the s o i l volume was occupied by coarse fragments. Using these data and assuming that the coarse fragments had an average p a r t i c l e dens i ty of 2.65 _3 g cm , the bulk dens i ty of the < 2-mm f r a c t i o n was. ca l cu l a ted to be 1.35 -3 g cm . The grav imetr ic s o i l water content determined on the < 2-mm f r a c t i o n 13 was corrected f o r coarse fragments by the Reinhart (1961) approach: 9 = W f P fV f (2) where e i s the volumetric water content of the whole s o i l , ( c o a r s e f r a g -3 -3 ments and the < 2-mm f r a c t i o n ) , (cm cm )-, W^  i s the water content, dry mass bas i s , of the < 2-mm f r a c t i o n (g g - 1 ) " ; p f is the bulk dens i ty o f the < 2-mm f r a c t i o n j and i s the volume f r a c t i o n occupied by the < 2-mm 3 -3 f r a c t i on (cm cm ). P a r t i c l e s i ze ana lys i s as a funct ion of depth was done by the hydro-fo r the -0.1 bar to -15 bar range meter method. Water re tent ion curves/as a funct ion of depth were deter -mined on the < 2-mm so i l f r a c t i o n by the pressure p la te ext ract ion tech -nique, f "am 0 v-^r L . ' a . s . The water re tent ion data and grav imetr ic water content were used to obtain s o i l water potent ia l at a s o i l depth of 5 c m . The unsaturated hydraul ic conduct i v i ty over a so i l water matric potent ia l range of -0.002 bar to -15 bars was measured in the laboratory by the t rans ient outflow method using an experimental setup s im i l a r to that of Chow and de Vr ies (1972). A bulk s o i l sample was taken (0- to 40-cm depth) at s i t e 1. Large undisturbed s o i l samples were impossible to take due to the stoniness of the s o i l . This unsieved so i l sample was a i r dr ied and large stones removed. The a i r dr ied s o i l was packed into an a c r y l i c p l a s t i c cy l i nder which was 20-cm x 15-cm in cross sect ion and 30-cm high, with 0.6-cm wall th ickness . In order to ensure a bulk dens i ty s im i l a r to the one in the f i e l d , a preweighed amount of s o i l was packed in the cy l i nder in 2-cm increments. Each s o i l add i t ion was tapped leve l 14 with markings on the cy l i nder wa l l . This procedure ensured uniform pack-ing and very l i t t l e migration of the f i ne materia l towards the bottom of each sec t ion . The s o i l c y l i nder was supported by an unconsolidated glass bead (29-p diameter) porous p late which formed the bottom of the c y l i n d e r . A hanging water column attached to th i s porous p late provided the means fo r wetting ' the s o i l sample to saturat ion and fo r applying suct ion to the s o i l column during drainage. Two tensiometers each were i n s t a l l e d at the 5-, 10-, and 15-cm depths. The sample was then saturated and a l l the tensiometers allowed to e q u i l i b r a t e . The experiment was c a r r i e d out in two phases; ( i ) drainage - during which the top of the s o i l column was covered to prevent evaporation and suct ion was appl ied in 30-cm of water increments through the hanging water column attached to the porous p l a te , and ( i i ) evaporation - before which the porous p late was removed and during which two small o s c i l l a t i n g fans operated continuously 5'cm above the exposed s o i l sur face. So i l water content as a funct ion of time was measured at 2-cm depth i n te rva l s along the column with a co l l imated beam of 0.661 Mev gamma photons. This scanning was made poss ib le by the up and down motion of the power dr iven platform on which the s o i l column was located. I n i t i a l l y the s o i l water content scans were done every 5 hours, but as the change in water content with time dec l i ned , the scanning in te rva l was gradual ly increased to 100 hours by the end of the experiment. To minimize errors due to d r i f t in the counting equipment, an a c r y l i c p l a s t i c standard on the platform was scanned during each measurement sequence. The average water content over a 2-cm depth was ca l cu la ted from the standardized count r a t i o using the gamma rad i a t i on attenuat ion equation (Gardner, 1965; de V r i e s , 1969): 15 e l " e 2 = <1/(VwS)) l n ( W ^ I S * <3> 3 -3 where and are the volumetric water contents (cm cm ) at matric po tent ia l s ij> -j and ^ ^ (bars ) , I 25 and IQ-j^ are the standardized count r a t i o s at water contents e 9 and e , , y i s the attenuat ion c o e f f i c i e n t of L. I W 2 -1 water (= 0.0835 cm g fo r the p a r t i c u l a r co l l imat ion-count ing system used), p i s the dens i ty of water (g cm ), and S = 20 cm, the thickness of the w s o i l . So i l water potent ia l between 0 and -1 bar was measured with a t en s i o -meter-pressure transducer system. As each tensiometer went o f f s c a l e , i t was replaced by a Wescor dew point s o i l hygrometer which measured s o i l water potent ia l in the -1 to -15 bar range, These measurements were made at the same time in te rva l s as the s o i l water content. From plots of water content, e,and water p o t e n t i a l , ip,as a funct ion of t ime, water content p r o f i l e s , e .,and water potent ia l p r o f i l e s , ^ , «were i n f e r r e d . The matric potent ia l was ca l cu la ted by subtract ing the g r a v i -t a t i ona l component from the to ta l water p o t e n t i a l . The unsaturated hydraul ic c o n d u c t i v i t i e s , K z,(cm d a y - 1 ) were ca l cu la ted from Darcy 's Law: K z =-v z /(3i|»/3z) (4) where the f l u x , v z , (cm d a y - 1 ) can be expressed as: v z = / ( 3 8 / 3 t ) d z (5) where zQ i s a plane where v z i s equal to zero and z is the plane where the ca l cu la ted f l ux occurs. 16 The f l u x , vz,was determined by graphical integration between two successive water content p r o f i l e s at t-| and tg from Z q to z. For drainage, the interval of integration was from the s o i l surface down to a depth of 12.5 cm. During the evaporation phase, the integration interval was from the bottom of the s o i l up to a depth of 7.5 cm. The flux planes were chosen such that there were tensiometers or hygrometers on either side. These fl u x planes ensured that the fluxes under consideration were kept as large as possible in order to maintain a good degree of accuracy in the computed K values, Corresponding water potential gradients, (9ijj/9z) z,were obtained by graphical d i f f e r e n t i a t i o n of water potential p r o f i l e s , Estimation of Root Water Uptake and Fluxes Within the Root Zone The rates and patterns of water uptake by tree roots at any given depth were computed from the rate of change of s o i l water content and the water flux divergence at that depth as follows: r = 36/9t + 9v /9z (6) z z v ' where v z i s positive downward, z i s measured p o s i t i v e l y downward from zero 3 -3 -1 -1 at the s o i l surface and r z (cm cm day or day ) i s the water uptake by roots at depth, z. r z i s a negative quantity which on integration over a given depth interval in the root zone yie l d s Rz, the accumulated root extraction rate in cm day"^. Another approach would be to obtain Rz f i r s t by using the depth integrated form of Eq. (6) and then graphically d i f f e r e n t i a t i n g this to obtain r . van Bavel ejt al_. (1968a) discussed both methods and concluded 17 that the d i f f e r e n t i a l approach, as used in th i s paper, was more s a t i s -f ac to ry . To apply Eq. (6) to the present study, a four-week drying period was chosen at each s i t e ; Ju ly 18 to August 17, 1974 at s i t e 1 and Ju ly 1 to August T, 1975 at s i t e 2. From s o i l water content p r o f i l e data, ae/at was determined numerical ly at s pec i f i ed depths (5 - , 20-, 35- , and 50-cm at s i t e 1 and 5-, 20-, 30- , 45- , 60-, and 75-cm at s i t e 2) f o r f i v e time i n t e r v a l s , each in terva l ranging from 5 to 7 days. The f lux divergence term of Eq. (6) was determined as fo l lows. From ty p r o f i l e s , dty/dz were obtained by numerical d i f f e r e n t i a t i o n at s pec i f i ed depths (10-, 25- , 40 - , 50-, and 65-cm at s i t e 1 and 10-, 25-, 37.5- , 52.5- , 67.5, and 80-cm at s i t e 2. Nega-t i v e 9^/az values ind icate downward f luxes and v ice versa. The ty value at each s p e c i f i e d depth and the Htym) curve (see F i g . 3) were used to obtain the re levant K values. The f l u x , v z , a t each depth was ca lcu la ted using Eq. (4) and values from success ive p r o f i l e s averaged to obtain average v z values. These were d i f f e r e n t i a t e d with respect to depth to obtain 3v z/az values and r z values were computed. An estimate of the accumulated root ext rac t ion rate f o r the ent i re root zone was obtained from the same set of data by numerical ly in tegrat ing success ive water content p r o f i l e s over the root zone (0- to 65-cm at s i t e 1 and 0- to 80-cm at s i t e 2) and cor rec t ing f o r the f lux out of or into the bottom of the root zone. RESULTS AND DISCUSSION  Soi l Hydrologic Propert ies The water retent ion curves f o r the various layers sampled are shown 18 in F i g . 1 f o r the unthinned s i t e ( s i t e 1) and in F i g . 2 f o r the thinned 3 -3 s i t e ( s i t e 2). The water content, e,drops from 0.40 cm cm" at sa tur -3 -3 at ion to a range of 0.19-0.26 cm cm at a f value of -0.3 bar at s i t e 3 - 3 3 - 3 1 and from 0.50 cm cm to 0.19-0.24 cm cm for the same change in potent ia l at s i t e 2. This rap id change in water content is as would be expected f o r a g rave l l y sandy loam. The pore s i ze d i s t r i b u t i o n (slope of the curves) remains f a i r l y constant with depth. Values of K measured fo r the unsieved stoneless sample over the matric potent ia l range of -0.002 to -15 bars are shown in F i g . 3. The values are comparable to those obtained by Mehuys et al_. (1975) on a g rave l l y sandy loam s o i l . Mehuys et_ a_h found that when K is expressed as a funct ion of \pm, the unsaturated hydraul ic conduct i v i ty values obtained on s tone- f ree s o i l columns show very good agreement with values that would be obtained i f stones were present. Furthermore, they found that K versus 6 curves f o r stony and non-stony samples do not show good agreement. This appears to be mainly due to the f a c t that at the same volumetric water content, the sample conta in ing stones has a higher water content in the <2-mm f r a c t i o n than the sample from which stones have been removed, Since stones hold . no water, the water content of the s o i l f r a c t i o n in a stony sample i s under-estimated. So in F i g . 4, which shows K as a funct ion of 0 , the s o i l water content values were corrected f o r stones using the Reinhart (1961) approach. Rates and Patterns of Water Uptake Total s o i l water p o t e n t i a l , ,measurements at various depths fo r the en t i re measurement periods at s i t e s 1 and 2 are shown in F igs . 5 and 6 9 (cm3cm~3) 1 - Water retent ion curves f o r various layers of Dashwood g rave l l y sandy loam in the unthinned stand ( s i t e 1). 20 O. I 0 . 2 9 ( c m 3 cm" 3 ) 0 . 3 0.4 T 0 . 5 -4 Tm (bars) -81 - 1 2 - 1 6 " D A S H W 0 0 D G R A V E L L Y S A N D Y L O A M S I T E 2 Z ( c m ) •— — • 0 - 1 3 A A 1 3 - 2 6 O O 2 6 - 3 9 A A 3 9 - 5 2 • • 5 2 - 6 5 X— —X 6 5 - 7 8 Fig. 2 - Water retention curves for various layers of Dashwood gravelly sandy loam in the thinned stand (s i t e 2). Fig . 3 - Unsaturated hydraul ic conduct iv i ty as a funct ion of matric potent ia l fo a sample of Dashwood grave l ly sandy loam with large stones removed. Fig. 4 - Relation "between unsaturated hydraulic conductivity and volumetric water content corrected for stone content for Dashwood gravelly sandy loam. 5 - Total s o i l water potent ia l (ip) at four depths f o r the period June 17 to August 19, 1974. 24 F i g . 6 - Total s o i l water potent ia l (ty) at f i v e depths fo r the period June 18 to August 12, 1975. 25 r e s p e c t i v e l y , along with the p r e c i p i t a t i o n data. At s i t e 1, two drying cyc les separated by an in terva l of wet weather were observed f o r the period June 17 to August 19, 1974. At s i t e 2 only one major drying cyc le was observed f o r the period June 18 to August 12, 1975. The rap id rate of dec l ine of ip made i t poss ib le to study water uptake over a wide range of s o i l water po ten t i a l s . During wett ing, the upper s o i l layers showed a f a s t e r increase in ip as would be expected. At the beginning of each drying c y c l e , the deeper s o i l layers showed a lower water potent ia l than the upper s o i l layers i nd i ca t i ng downward drainage at th i s time. For the res t of each drying c y c l e , the deeper layers were at higher ip than the upper layers thus re su l t i n g in upward f luxes into the zone of maximum root dens i ty . The small amount of r a i n f a l l during the drying cyc les (4 mm on July 21, 1974 at s i t e 1 and 2.7-mm on Ju ly 14, 1975 at s i t e 2 ) , was not enough to wet the s o i l . The second drying cyc le at s i t e 1 and the drying cyc le at s i t e 2, each l a s t i n g approximately four weeks, were analyzed in d e t a i l to show root water uptake rates and patterns. The s o i l water content p r o f i l e s fo r the time in te rva l s considered are shown in F igs . 7 and 8 fo r s i t e s 1 and 2 re spec t i ve l y . Time zero f o r each drying cyc le corresponded to the end of a period of continuous r a i n f a l l . Water content decreased with time at a l l depths. As the season progressed, the s o i l p r o f i l e became d r i e r at the top than at the bottom due to intense water removal in the upper s o i l l ayer s . A l so , the time rate of change in water content dec l ined as the season pro-gressed. Over the en t i r e drying c y c l e s , more water was l o s t from the upper 3 - 3 3 - 3 ha l f of the root zone (0.18 cm cm at s i t e 1 and 0,15 cm cm at s i t e 2) 3 - 3 3 - 3 than from the lower ha l f (0.05 cm cm at s i t e 1 and 0.04 cm cm at s i t e 2). 9 (cm3cm"3) Fig. 8 - Volumetric water content (e) as a function of depth (z) at site 2. . • 28 I r respect ive of stand dens i ty d i f f e r e n c e , the s o i l p r o f i l e water dep let ion patterns f o r both s i t e s are very s i m i l a r . So i l water f l ux fo r four depth i n te rva l s at s i t e 1 and f o r f i v e depth i n te rva l s at s i t e 2 are shown in F i g s . 9 and 10 re spec t i ve l y . These f luxes were computed by the use of Eq. (4); the K values were obtained from F i g . 3 and the to ta l potent ia l gradients were obtained from to ta l water potent ia l p r o f i l e s deduced from F igs . 5 and 6. In the f i r s t week of the drying cyc le at each s i t e , flow d i r e c t i o n was downward in a l l layers except in the 37.5- to 52,5-cm depth in te rva l at s i t e 2. The predominant downward f l ux r e f l e c t p r o f i l e drainage from ra in before Ju ly 17, 1974 at s i t e 1 and June 30, 1975 at s i t e 2. The upward f l ux in the 37.5- to 52.5-cm depth in terva l at s i t e 2 may be due to flow toward the zone of water ex t r ac t i on . Subsequently, most of the s o i l layers showed upward f luxes of water. At s i t e 1, maximum upward f l ux occurred in the 0- to 10-cm layer ea r l y in the drying cyc le and then progressed with time from shallower to deeper depths.. At s i t e 2, upward f luxes in the surface layers were very smal l . The maximum upward f l ux occurred fu r ther down in the s o i l p r o f i l e , progressing again with time from shallower to deeper depths. The predom-inant ly upward f luxes in deeper layers ind icate that the tree roots were removing water in the top layers f i r s t . Maximum upward f l ux of 0.2 to 0.3 mm d a y - 1 occurred in deeper s o i l layers at s i t e 1 about ha l f way through the drying c y c l e . S imi lar values were observed at s i t e 2. Upward f lux values in the root zones of various annuals have been reported in the l i t -erature, van Bavel e_t al_. (1968a) observed a 4.0, mm day" 1 upward f l ux into a sorghum root zone in a c lay loam s o i l . Stone ejt al_. (1973a) obtained upward f luxes as large as 2.0 mm day" 1 at the bottom of sorghum root zone 29 F i g . 9 - So i l water f lux (v ) in the root zone during the drying cyc le Ju ly 18 to August 17, 1974. Pos i t i ve values ind icate downward f lux and v ice versa. F i g . 10 - So i l water f l ux (v z ) in the root zone during the drying cyc le June 30 to August 1, 1975. Pos i t i ve values ind ica te downward f lux and v ice versa. 31 in a s i l t loam s o i l . Allmaras et_ al_. (1975) showed upward f luxes of up to 8.0 mm d a y - 1 into corn and soybean root zones in a sandy c lay loam. The upward f luxes in the annual crops c i t e d are at l eas t one order of mag-nitude higher than that obtained in the present study. The root zones of the annual crops c i t e d average 150 cm in depth which i s about twice that of the Douglas f i r in t h i s study. Furthermore, in the case of the a g r i -c u l t u r a l crops, the s o i l extended well below the root zone, whereas in the Douglas f i r stand, a compacted basal t i l l def ined approximately the lower l i m i t of the root zone. A l l s o i l s on which the a g r i c u l t u r a l crops were grown had higher water retent ion a b i l i t y and higher unsaturated hy-d r a u l i c conduc t i v i t i e s than the g rave l l y sandy loam on which the Douglas f i r grows. The annual crops had developing root systems which d id not rami-fy in the s o i l as in t imate ly as the well developed root system of the 20-year -o ld Douglas f i r stand, For these reasons, the upward f l ux rates in the case of the Douglas f i r stand would be expected to be cons iderably less than in the case of the a g r i c u l t u r a l crops. The rates and patterns of root water uptake during each of the f ou r -week drying periods at s i t e s 1 and 2 are shown in F i g . 11. The water uptake by root s , r z , f o r spec i f i ed time i n te rva l s was ca l cu la ted using Eq. (6). Since each r z value was an average over a s pec i f i ed depth i n t e r v a l , a bar-type representat ion was used. In the f i r s t week of the drying per iods , 82% and 78% of the water uptake at s i t e s 1 and 2 re spec t i ve l y occurred in the upper ha l f of the root zone (Tables III and IV). As the water in the surface horizons was dep leted, an increas ing f r a c t i o n of the to ta l water uptake was extracted from deeper depths in the root zone. As such, in the l a s t week of the dry ing per iods , 46% and 48% of the to ta l water uptake at DOUGLAS FIR  C O U R T E N A Y . 1974 S I T E I JULY 1 8 - 2 2 Rz=2.9 mm doy"1 J i i i I r2 (doy"1) - 0 . 8 - 0 . 4 D O U G L A S FIR  C O U R T E N A Y . 1975  S ITE 2 J U L Y 1-6 R z s 3 . 7 m m day"1 -V ••'.•••.'.•••:v-;. .1 L-.-'.::*;sl-:.J. JULY 2 2 - 2 8 R z= 1.8mm doy"' -0 .8 - 0 . 4 JULY 6-11 R z "3.3 mm day - 1 I i i • ' • JULY 2 8 - A U G . 4 R z=2.l mm day - ' - 0 . 4 JULY 11-18 R j^Ommday" 1 ! - j i I.. AUG.4-II R z=l.6 I I I L. 0 -0.4 JULY 18-25 R Z ' I 6 1 L. 2 0 Z (cm) 4 0 AUG. 11-171: R z = 0.9 I L. n 6 0 0 -0.4 r i 11.1.... 2 0 JULY 2 5 -AUG. R z » 1.2 • • • Z (cm) 4 0 6 0 1 8 0 Fig . 11 - Depth d i s t r i b u t i o n of water uptake by roots at s i t e 1 and s i t e 2 over f i v e time periods. The values of R£ given were obtained by in tegra t ing r z over the ent i re root zone (0- to 65.cm at s i t e 1 and 0- to 80.cm at ro s i t e 2). Table III - Percentage of water extracted from 4 so i l layers during 5 time periods as a funct ion of percentage of roots and volumetric water content, 9 in the unthinned stand ( s i t e 1), 1974. Ju ly 18-22 July 22-28 Juil.yy28-Auq.4 Aug. 4-11 Aug.11-17 Root + Water Water Water Water Water biomass ext ract ion 4-4- extract ion ext ract ion ext ract ion extract ion Depth (% of (% of T r e (%-of e {% of 6 (% of 6 {% of e (cm) t o t a l ) t o t a l ) (%) t o ta l ) («) t o t a l ) (*) t o t a l ) (%) t o t a l ) («) 0-10 42 35 22.5 55 17.0 33 11.6 13 8.6 22 7.4 10-25 46 41 16.3 13 14.5 13 12.3 19 9.8 21 8.4 25-40 7 12 16.3 25 14.9 42 13.1 46 10.9 35 9.4 40-65 5 12 15.9 7 15.3 12 14.0 22 12.4 22 11.2 + See also F i g . 12 + + Average f o r time in terva l i nd i ca ted . CO 00 Table IV - Percentage of water extracted from 6 s o i l layers during 5 time periods as a funct ion of percentage of roots and volumetric water content, 8 in the thinned stand ( s i t e 2 ) , 1975. Ju l y 1-6 July 6-11 Ju ly 11-18 Ju l y 18-25 Ju ly 25-Aug.l Root + Water Water Water Water Water biomass ext ract ion j i _ extract ion ext ract ion ex t rac t ion ext ract ion Depth {% of (% of t t e (% of e • {% of e (% of 9 (% of e (cm) t o t a l ) t o t a l ) (*) t o t a l ) (*) t o t a l ) ( * ) • t o t a l ) (%) t o t a l ) (•X) 0-10 29 22 22.1 19 18.4 26 15.0 11 12.5 12 11.3 10-25 20 23 22.4 29 18.1 16 15.9 24 14.3 24 12.6 25-37.5 16 33 18.0 15 15.7 8 14.1 14 12.9 11 11.9 37.5-52.5 17 21 16.8 16 15.6 19 14.4 17 13.3 15 12.5 52.5-67.5 11 0.6 17.3 13 16.0 20 14.9 22 13.9 25 13.0 67.5-80 7 0.4 17.6 8 17.0 11 15.8 12 14.6 13 13.8 + See also F i g . 12 + + Average for time interva l ind icated 35 s i t e s 1 and 2 re spec t i ve l y occurred in the lower ha l f of the root zone (Tables III and IV). In t h i s study, there was a more gradual s h i f t of the zone of maximum root uptake than in the annual crops (sorghum, soybean and corn) reported in the l i t e r a t u r e by van Bavel et al_. (1968a), Stone et_ a l . (1973b), Rice (1975), and Allmaras ejt al_. (1975). The annual crops c i t e d had v e r t i c a l l y expanding root systems which showed more pronounced hydrotropic response than the f u l l y developed 20-year-o ld Douglas f i r root system. This would account f o r the more dramatic downward s h i f t of the root water uptake sink under a g r i c u l t u r a l crops. Klepper e_t a K (1973) suggested that s o i l d r i e r than -1 bar could have such low conduct i v i t y that water uptake by the developing roots of cotton would be s u f f i c i e n t l y reduced to cause p re fe ren t i a l extension growth into wetter s o i l l a ye r s . The zone of i n i t i a l maximum water uptake corresponded to the zone of highest root dens i ty (F ig . 12). (More root dens i ty data in terms of root length per un i t volume of s o i l fo r both s i t e s i s reported in Chapter 3). This cor re la ted well with the f a s te r dec l ine of $ in the surface layers shown in F igs . 5 and 6. However, the amount of water extracted from any layer depended on both root dens i ty and s o i l water content (Tables III and IV). The dependence on water status i s well i l l u s t r a t e d by the data during the l a s t week of the drying cyc les at both s i t e s . During th i s time per iod , the deepest s o i l l ayer was cont r ibut ing as much water as the top l ayer even though i t had a much smaller root dens i ty (Tables III and IV). A s im i l a r pattern was observed by Taylor and Klepper (1973) on corn , Arya et a l . (1975b) on soybeans and Taylor and Klepper (1975) on cotton. Evapotranspirat ion ca l cu la ted from s o i l p r o f i l e water dep let ion (F igs , 7 and 8) and f l ux at the bottom of the root zone (Figs. 9 and 10) are reported 36 20 Z ( c m ) 40 60 80 0.5 ROOT BIOMASS / VOL. OF SOIL (mgcm"3) 1.0 1.5 2.0 r • i SITE I SITE 2 "J—r- 2.5 DOUGLAS FIR COURTENAY F ig . 12 - Root biomass (dry weight) per un i t volume of s o i l as a funct ion of depth at s i t e s 1 and 2. Root biomass was computed using the predominant diameter c la s s (<2-mm diameter) which const i tuted 95 to 98% of the to ta l root length. 37 in F igs . 13 and 14. Evapotranspirat ion was a lso ca l cu la ted by in tegra t ing r z over the en t i r e root zone, thus obtaining the accumu-la ted root ext rac t ion rate fo r the root zone. Results from both c a l c u -l a t i o n procedures agreed w e l l , d i f f e r i n g by 5 to 15% at s i t e 1 and by 1 to 3% at s i t e 2. Such d i f fe rences have been reported in the l i t e r a t u r e ; 10 to 19% (Stone et al_., 1973b) and 20 to 25% (Allmaras et al_., 1975). If equated with s o i l p r o f i l e water dep le t i on , evapotranspirat ion would be over e s t i -mated in the f i r s t time period and under estimated in the other time periods during the drying cyc les at both s i t e s . The water f l ux into the bottom of the root zone became an increas ing ly more important f r a c t i o n of the water taken up as the s o i l dr ied out, increas ing from 2 to 8% at s i t e 1 and from 8 to 15% at s i t e 2. As an independent check, the weekly evapotranspirat ion estimates from water balance data were compared with those from micrometeorological energy balance data. The comparison (Table V) showed good agreement between both methods (see Chapter 2 fo r fur ther d i scuss ion of th i s comparison). Water withdrawal from trunk storage accounted f o r only 2% of the to ta l evapotranspirat ion during the four-week drying period at s i t e 2. It meant a 13% change in the volumetric water content of the t rees . This amount of water would support 12 hours of a t r e e ' s t r an sp i r a t i on at the average t r an sp i r a t i on rate (27 l i t r e s d a y - 1 s t e m - 1 or 2.27 mm d a y - 1 with a stand density of 840 trees/ha) over the drying c y c l e . It does appear that the importance of th i s source of water could l i e in i n i t i a t i n g predawn t r an sp i r a t i on as suggested by Ja rv i s (1975). He ind icated that the amount of exchangeable water stored in the sapwood could support the t r an sp i r a t i on requirement of a large tree f o r a few days. But i t i s not ce r t a in whether a l l of th i s water i s ava i l ab le f o r t r an sp i r a t i on or how r e a d i l y i t moves into the t r an sp i r a t i on stream. Waring (1975) has suggested that the amount 4 2 mm doy"1 0 -2 1 I 1 1 _ ^ DEPLETION IN 0-65cm ROOT ZONE p EVAPOTRANSPIRATION i ~"> (SOIL DATA) — — • - DOUGLAS FIR -COURTENAY 1 SITE 1 1 I I 18 22 28 4 if 17 JULY 1974 AUGUST F i g . 13 - Root zone deplet ion of water, f l ux at the bottom of the root zone, and evapotranspirat ion ca l cu la ted from these data f o r s i t e 1. T~ ^DEPLETION IN 0-80 cm ROOT ZONE 1 2 mm day"1 0 L, • EVAPOTRANSPIRATION (SOIL DATA) L I FLUX AT 80cm DEPTH • • *••••< -2h DOUGLAS FIR COURTENAY SITE 2 H 18 JULY 1975 25 AUG. 14 - Root zone deplet ion of water, f l ux at the bottom of the root zone, and evapotranspirat ion ca l cu la ted from these data fo r s i t e 2. 40 Table V - Average values of net r ad ia t i on (mm of water equivalent day" ) and evapotranspirat ion obtained by the water balance and  energy balance methods (mm day " 1 ) at s i t e s 1 and 2 fo r the  5 success ive time periods ind icated in F i g . 11. S i te 2 E EB R n EWB E EB 1 4.9 2.8 2.8 7.4 3.6 3.7 2 5.4 2.8 3.1 6.9 3.4 3.5 3 6.3 2.5 2.9 4.5 2.0 2.0 4 6.2 1.9 2.2 5.2 1.9 2.0 5 5.1 - - 5.2 1.2 1.3 S i te 1 Time R E. I D Period n W B 41 of water that can be withdrawn from the sapwood on a given day is r e l a -t i v e l y smal l . The change in volumetric t ree water content in th i s study i s s i m i l a r to that observed by Rothwell (1974) on lodgepole p ine. The e f f e c t of th inning on the water economy of the ind iv idua l t ree in th i s f o re s t i s discussed in Chapter 2. CONCLUSIONS The patterns of water uptake in the Douglas f i r f o re s t root zone c l e a r l y i l l u s t r a t e d the phenomenon of a root sink moving progress ive ly from shallower to deeper depths as the s o i l d r i e d . The amount of water taken up from any s o i l l ayer depended on both the water content and the root dens i ty . Thus when the s o i l p r o f i l e was uniformly wet from top to bottom, most of the water uptake occurred in the surface layers which a lso had the highest root dens i ty . But as the s o i l d r i e d , the deeper s o i l l a ye r s , which were then wetter than the surface l a y e r s , contr ibuted an increas ing f r a c t i o n of the t o t a l water uptake despite t h e i r lower root dens i ty . Evapotranspirat ion estimates using s o i l water dep let ion data were corrected f o r f lux out of or into the bottom of the root zone. Water f luxes in the s o i l were predominantly downward immediately a f t e r the cessat ion of r a i n f a l l . Subsequently, upward f luxes predominated becoming an inc reas ing ly higher f r a c t i o n of the to ta l water uptake as the s o i l d r i e d . However, they never exceeded 20% of the evapotransp i rat ion. Evapotranspirat ion estimates obtained by the water balance technique and by the micrometeorological energy balance technique showed good agreement over a s o i l water poten t i al range of 0 to -12 bars. 43 LITERATURE CITED 1. A l lmaras, R.R., W.W. Nelson, and W.B. Voorhees. 1975. Soybean and corn root ing in southwestern Minnesota: I. Water uptake s ink. So i l S c i . Soc, Amer. Proc. 39: 764-771. 2. Arya, L.M., G.R. Blake, and D.A. F a r r e l l . 1975a. A f i e l d study of s o i l water dep let ion patterns in presence of growing soybean roots : II. E f f e c t of plant growth on s o i l water pressure and water loss patterns. So i l S c i . Soc. Amer. Proc. 39: 430-436. 3. Arya, L.M., G.R. Blake, and D.A, F a r r e l l , 1975b. A f i e l d study of s o i l water dep let ion patterns in presence of growing soybean roots : III. Rooting c h a r a c t e r i s t i c s and root ext rac t ion of s o i l water. So i l S c i . Soc. Amer. Proc. 39: 437-444. 4. Black, T.A. and K.G. McNaughton. 1971. Psychrometric apparatus f o r Bowen-ratio determination over f o re s t s . Boundary Layer Meteorol. 2: 246-254. 5. Black, T .A . , C.B. Tanner, and W.R, Gardner, 1970. Evapotranspirat ion from a snap bean crop. Agron. J . 62: 66-69. 6. Chow, T . L . and J . de V r i e s . 1972. Dynamic measurement of hydrologic prop-e r t i e s of a layered s o i l during drainage and evaporat ion, fol lowed by wett ing. Proc. Second Symp. Fund. Transport Phen. in Porous Media. 2: 443-460. 7. de V r i e s , J . 1969. In s i t u determination of phys ica l propert ies of the surface layer of f i e l d s o i l s . So i l S c i . Soc, Amer. Proc. 33: 349-353. 8. Feddes, R.A. 1971. Water, heat and crop growth. Ph.D. Thes i s , A g r i c u l -tura l Un i ve r s i t y , Wagenningen, The Netherlands, No. 71-12. 44 9. Gardner, W.H. 1965. Water content, p. 82-127. In_: C A . Black et a]_., (ed. ) , Methods of s o i l ana l y s i s : Part 1. Physical and mineralog ica l p roper t i e s , inc lud ing s t a t i s t i c s of measurement and sampling. Amer. Soc. Agron., Madison, Wis. 10. J a r v i s , P. 1975. Water t rans fer in p lants , p. 369-394. In_: D.A. de Vr ies and N.H. Afgan (ed. ) , Heat and mass t rans fe r in the biosphere: I. Transfer processes in p lant environment. Sc r ip ta Book Co. , Wash., D.C. 11. Klepper, B., H.M. Tay lo r , M.G. Huck, and E.L. F i scus . 1973. Water r e l a t i on s and growth of cotton in drying s o i l . Agron. J . 65: 307-310. 12. K lute, A. 1965. Laboratory measurement of hydraul ic conduct i v i ty of saturated s o i l . p. 210-221. IrK C.A. Black et aJL , (ed . ) , Methods of s o i l ana l y s i s : Part 1. Physical and minera log ica l p roper t i e s , inc lud ing s t a t i s t i c s of measurement and sampling. Amer. Soc. Agron., Madison, Wis. 13. Mehuys, G.R., L.H. S to l zy , J . Letey, and L.V. Weeks. 1975. E f f e c t of stones on the hydraul ic conduct i v i ty of r e l a t i v e l y dry desert s o i l s . So i l S c i . Soc. Amer. Proc. 39: 37-42. 14. Reinhart, K.G. 1961. The problem of stones in so i l -mo i s ture measurement. So i l S c i . Soc. Amer. Proc. 25: 268-270. 15. R ice, R.C. 1975. Diurnal and seasonal s o i l water uptake and f l ux with in a bermudagrass root zone. So i l S c i , Soc. Amer. Proc. 39: 394-398. 16. Rose, C.W. and W.R. Stern. 1967. Determination of withdrawal of water from s o i l by crop roots as a funct ion of depth and time. Aust. J . So i l Res. 5: 11-19. 17. Rothwell, R.L, 1974. Sapwood water content of lodgepole p ine. Ph.D. Thes i s , Univ. of B r i t i s h Columbia. (Nat. L i b . Canada No. 25256). 45 18. S c h o l l , D.G. 1976, So i l moisture f l ux and evapotranspirat ion determined from s o i l hydraul ic propert ies in a chaparra l stand. So i l S c i . Soc. Amer. J . 40: 14-18. 19. Stewart, J .B . and A.S. Thorn. 1973. Energy budgets in a pine f o re s t . Quart. J . Roy. Meteorol. Soc. 99: 154-170. 20. Stone, L.R., M.L. Horton, and T.C. Olson. 1973a. Water loss from an i r r i g a t e d sorghum f i e l d : I. Water f l ux within and below the root zone. Agron. J . 65: 492-495. 21. Stone, L.R., M.L. Horton, and T,C. Olson. 1973b. Water loss from an i r r i g a t e d sorghum f i e l d : II. Evapotranspirat ion and root ex t r ac t i on . Agron. J . 65: 495-497. 22. 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. 23. Tang, P.A., K.G. McNaughton, and T.A. Black, 1976. Prec i s ion e l e c t r o n i c in tegra tor f o r environmental measurement. Transact ions of the ASAE 19: 550-552. 24. Tay l o r , H.M. and B. Klepper. 1973. Rooting dens i ty and water ext rac t ion patterns f o r corn (Zea mays L , ) . Agron. J . 65: 965-968. 25. Tay l o r , H.M. and B. Klepper, 1975. Water uptake by cotton root systems: An examination of assumptions in the s ing le root model. So i l S c i . 120: 57-67. 26. van Bavel , C.H.M., G.B. S t i r k , and K.J. Brust. 1968a. Hydraul ic proper-t i e s of a c lay loam s o i l and the f i e l d measurement of water uptake by roots : I. In terpretat ion of water content and pressure p r o f i l e s . So i l S c i . Soc. Amer. Proc. 32: 310-317. 46 27. van Bavel , C.H.M., K.J. Brust, and G.B. S t i r k , 1968b. Hydraul ic prop-e r t i e s of a c l ay loam s o i l and the f i e l d measurement of water uptake by roots : II. The water balance of the root zone. So i l S c i . Soc. Amer. Proc. 32: 317-321. 28. Waring, R.H. 1975. Water re l a t i on s and hydrologic cyc le s . (Unpub. data) . 29. W i l l i n g ton , R.P. 1971. Development and app l i ca t i on of a technqiue fo r eva luat ing root zone drainage. Ph.D. Thes i s , Univ. of B r i t i s h Columbia. (Nat, L i b . Canada No. 8343). CHAPTER 2 EFFECT OF THINNING ON THE WATER BALANCE OF A DOUGLAS FIR FOREST 48 EFFECT OF THINNING ON THE WATER BALANCE OF A DOUGLAS FIR FOREST ABSTRACT The e f f e c t of th inning on water consumption by Douglas f i r was studied during a four-week drying period during each of two consecutive summers. An unthinned stand was studied during the f i r s t year , while an adjacent, recent ly thinned stand was studied the fo l lowing year. Tree water uptake was measured by two independent methods, the water balance and the energy balance, in both stands. Both methods showed good agreement. In the f i r s t ,two weeks, evapotransp i rat ion, normalized to 24-hour net rad ia t ion t o t a l s , from the thinned stand was 11% less than that from the unthinned stand, but was 18% more in the l a s t two weeks. At a p a r t i c u l a r s o i l water p o t e n t i a l , ind iv idua l trees at the thinned s i t e t ransp i red an average of 25% more than those at the unthinned s i t e on f i n e sunny days. When water uptake was com-pared over the four-week per iod , i t was found that the ind iv idua l trees at the thinned s i t e were t r an sp i r i ng 35% more than those at the unthinned s i t e INTRODUCTION In order f o r the f o re s t industry to produce high qua l i t y timber p r o f i t -ab ly , i t i s e s sent ia l that the trees are managed to achieve an optimum 49 growth ra te . A major f ac to r f o r optimum growth i s water. Estimates of water uptake by t ree roots are required to va l i da te hypotheses and emp i r i -cal judgements as to the a v a i l a b i l i t y of water in fo res t stands of d i f f e r e n t d e n s i t i e s . Weekly estimates of tree water uptake can be obtained from so i l p r o f i l e water dep let ion data corrected fo r f luxes at the bottom of the root zone (Rose and Stern, 1967; van Bavel £ t al_., 1968a, 1968b; Black et al_., 1970; and S c h o l l , 1976. See a l so chapter 1). Da i ly e s t i -mates of t ree water uptake can be obtained by the energy balance/Bowen r a t i o technique (Black and McNaughton, 1971). So i l p r o f i l e water dep let ion in f o re s t stands have been studied by a number of researchers (Patr ic et a l . , 1965; S c h o l l , 1976). So i l water d i s t r i b u t i o n between trees was studied by Douglas (1960) in a il obi o i l y pine fo res t and by Brown and Bourn (1973) in a mixed oak stand. Hoover et al_. (1953) studied the e f fec t i veness of young l o b l o l l y pines as a p ro tec t i ve cover on watershed lands by monitoring so i l water cond i t ions . Influence of s o i l water a v a i l a b i l i t y on tree water s tress has been invest igated by .C l ine and Campbell (1976). Barrett and Youngberg (1965) studied the e f f e c t of t ree spacing and understory vege-t a t i on on to ta l water use in p'onderosa pine f o r e s t . It i s genera l ly accepted that th inning would promote growth of the remaining t rees . If ha l f the trees in a f o re s t at c e i l i n g l ea f area index are removed, the to ta l stand t r ansp i r a t i on often drops but the f l ux of water in each remaining t ree increases ( Ja rv i s , 1975). This re su l t s in a rapid increase of the sap-wood c ros s - sec t iona l area which i s a funct ion of l ea f biomass (Grier and Waring, 1974). There i s a pressing need to e s t a b l i s h , at l ea s t in semi-quant i ta t i ve terms, the magnitude of th i s improved growth. So th i s study was ca r r i ed out to inves t i ga te the e f f e c t of th inning on the water balance 50 of a Douglas f i r (Pseudotsuga menzies i i (Mirb.) Franco) fo res t both on a stand area basis and on an ind iv idua l t ree bas i s , on ind iv idua l days and over a sfour-week'drying period during ;mid-summer. METHODS AND MATERIALS  Experimental S i tes The research was ca r r i ed out in two stands of a Douglas f i r (Pseudotsuga menzies i i (Mirb.) Franco) fo res t of dens i t i e s 1840 trees/ha ( s i te 1; unthinned) and 840 trees/ha ( s i te 2; thinned).* Both s i t e s (about 1 1/2 km apart) were located on genera l ly f l a t t e r r a i n on the east coast of Vancouver Island about 27 km northwest o f Courtenay, B.C. At both s i t e s , t ree diameter was 10.6 to 10.9 cm and the trees ranged in height from 7 to.10 m.• The s o i l at both s i t e s i s Dashwood g rave l l y sandy loam underla in by compacted basal t i l l at a maximum depth of 70 cm at s i t e 1 ahd°85 G m v a t ' S i t e - 2 . - See Chapter 1 f o r de ta i l ed s i t e de sc r i p t i on . Measurement of Hydrologic, Meteorological and Plant Parameters The unthinned s i t e ( s i te 1) was instrumented in the summer of 1974 and the thinned s i t e ( s i t e 2) in the summer of 1975. So i l water matric p o t e n t i a l , tym,was measured with a tensiometer-pressure transducer system in the 0 to -1 bar range and with a Wescor HR-33T dew *The th inning at s i t e 2 was begun in 1974 (chemical ly) and completed in the spr ing of 1975 (mechanical ly) . 51 point microvoltmeter and PT 51-10 hygrometers for values les s than -1 bar. Tensiometers/hygrometers were i n s t a l l e d at depths of 15-, 30-, 45-, and 60-cm at s i t e 1 and at depths o f 15-, 30-, 45-, 60-, and 75-cm at s i t e 2. So i l water potent ia l measured with the tensiometer-pressure transducer system was recorded at 1-hourly i n te rva l s on a Hewlett-Packard 2707A data logger, whi le potent ia l s measured with the dew point hygrometer were manually recorded three times d a i l y . ' Weekly so i l .water content measurements were made g r av imet r i c a l l y in the 0- to 10-cm layer and by the use of a neutron moisture meter (Troxler model 105A) at depths o f 20-, 35- , and 50-cm at s i t e 1 and at depths of 20-, 30- , 45-, 60-, and 75-cm at s i t e 2. The neutron moisture probe was c a l i b r a t e d at each s i t e . Continuous ha l f -hour l y evapotranspirat ion rates were measured by the as part of the large p ro jec t . Bowen rat io/energy balance method/. The Bowen r a t i o , g,was measured at the 10.5-m height at s i t e 1 and at the 11-m height at s i t e 2 using a psychro-metric apparatus descr ibed by. Black and McNaughton (1971). The evapotrans-p i r a t i on r a t e , E,was ca l cu l a ted from the equation: E = ( R n - G - M)/[L(1 + g)] (1) where Rn i s the net r ad i a t i on f l u x , G is the s o i l heat f l u x , M i s the rate of storage of sens ib le and l a tent heat within the canopy, L i s the l a tent heat o f vapor i za t i on , and g is the r a t i o of the f lux dens i t i e s o f sens ib le and l a ten t heat. Deta i l s o f the so i l water and evaporation meas-urements are given in Chapter 1. Dendrometer bands were mounted on three comparable t rees a t each s i t e , during the summerof 1975, to monitor the increase in the diameter at 52 breast height (D.B.H.) as the s o i l d r i e d . The dendrometers were read once a week. RESULTS AND DISCUSSION Weekly evapotranspirat ion rates obtained by the water balance and the energy balance methods are compared in F i g . 1. At s i t e 1 (unthinned stand) , the energy balance estimates averaged 10% larger than the water balance est imates, while at s i t e 2 (thinned stand), t h i s f i gu re was only 3%. These d i f fe rences are with in the experimental e r ror involved in each method and suggest- that both methods can provide r e l i a b l e estimates of t ree water uptake fo r purposes of comparison of water consumption by stands of d i f f e r e n t d e n s i t y ^ . F i g . 2 shows a comparison of d a i l y micrometeorological energy balance evapotranspirat ion values (stand area bas is ) normalized to net rad ia t i on on -2 -1 f i n e sunny days (18.0 >,Rn > 14.0 MJ m day ) fo r both s i t e s . This com-parison indicateis that at a given s o i l 'Water potent ia l on f i n e sunny days, the two stands ( sa la l t r ansp i ra t i on at s i t e 2 included) were consuming water at approximately the same ra te . A l l the t r an sp i r a t i on at s i t e 1 was by Douglas f i r t r e e s , whereas at s i t e 2, the sa la l undergrowth was competing a c t i v e l y with the Douglas f i r . . Pa r t i t i on ing of the to ta l stand t r an sp i r a t i on at s i t e 2 between the Douglas f i r and the s a l a l showed that t r ansp i ra t i on by Douglas f i r accounted fo r approximately 60% of the t o t a l in the f i r s t 3 weeks and approximately 50% of the to ta l in the l a s t week of Fig. 1 - Comparison of weekly evapotranspiration estimates obtained by the water balance and energy balance methods for sites 1 and 2. V 0.6 3 E E H I _> CT 0) O £ 0.4 E E • 0.2 0 0 -o • STAND BASIS 0 0 o ° . ° SITE I • SITE 2 R„ >I4.0 MJm- 2 day- ' or 5.7mm H2O equiv. day DOUGLAS FIR COURTENAY o o 00 e • -2 - 6 V (bars) -8 -10 -12 Fig. 2 - Stand transpiration normalized to the net radiation to the stand ., as a function of total soil water potential (ip) on fine sunny days at sites 1 and 2. 'The values are averages for the root zone and were computed taking into account the zone of maximum root density and the rate of water depletion in the various soil layers. en 55 the dry ing cyc le (C.S. Tan, 1977. 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 . Ph.D. Thes i s . Un iver s i t y of B r i t i s h Columbia, Vancouver). Tan pa r t i t i oned stand evapotranspirat ion by making c a l c u -l a t i ons of t r an sp i r a t i on for three leve l s in the Douglas f i r canopy and f o r the sa l a l undergrowth using l ea f area index data and measurements of vapour pressure d e f i c i t and stomatal res i s tance every three hours on several days during the four-week per iod. F i g . 3 shows the data in F i g . 2 a f t e r con-vers ion to a s ing le t ree basis with the sa l a l cont r ibut ion to t r ansp i r a t i on at s i t e 2 removed. This comparison ind icated that at a p a r t i c u l a r s o i l water p o t e n t i a l , each tree at s i t e 2 was using an average of 25% more water than each tree at s i t e 1. Over the en t i re drying c y c l e s , average weekly evapotranspirat ion values at both s i t e s obtained from s o i l water data and normalized to net r ad i a t i on were compared (F ig . 4). On a stand area bas i s , when the normal-ized evapotranspirat ion values from both s i t e s ( sa la l t r an sp i r a t i on at s i t e 2 included) were compared, i t was observed that in the f i r s t ha l f of the dry ing period the thinned stand ( s i te 2) t ransp i red 11% less than the unthinned stand ( s i t e 1) but 18% more in the second ha l f of the drying per iod. This suggests that the unthinned stand had a rather high t ransp i ra t i on rate ear ly in the drying period and so did not have as much l e f t as the thinned stand l a t e r on. When only Douglas f i r trees were considered on a stand area bas i s , s i t e 1 was using an average of 38% more water than s i t e 2 over the four-week per iod. This i s l a r ge ly due to the d i f f e rence in stand dens i ty between the two s i t e s . When th i s comparison was c a r r i e d out on an ind iv idua l t ree bas i s , i t was found that each tree at s i t e 2 was t r an sp i r i ng an average of 35% more than each tree at s i t e 1. Thus both during f ine sunny days and over the en t i r e four-week per iod , 1 1 1 INDIVIDUAL TREE BASIS 1 I . ' — I — DOUGLAS FIR COURTENAY • • • • • • - *„ o SITE 1 & 0 • SITE 2 - o e R„>I4.0 MJm*2day_l • or 5.7 mm H20equiv. day"1 0 ° o 0 o ° ° 0 o e -• oo o oo * 1 1 1 1 1 1 -4 -6 V (bars) -10 -12 Individual tree transpiration normalized to the net radiation to the stand as a function of total soil water potential on fine sunny days at sites 1 and.2. The f values are the same as in Fig. 2. E E 2 0 16 12 8 4 0 0.6 0.4 3-I 0.2 -I 3 : 2 • I • 0 S I T E I 1 S I T E 2 DOUGLAS F IR . C O U R T E N A Y S I T E 2 ( T R E E S AND S A L A L ) i : S I T E I S I T E 2 ( T R E E S O N L Y ) S T A N D BAS IS S I T E 2 !.»-•-•-*-•- •- •- *~ •- •-•-»-; S I T E I • INDIVIDUAL T R E E BAS I S - 5 5-tt » _1_ 0 5 10 15 2 0 2 5 DAYS A F T E R J U L Y , 1974 (SITE I) A N D J U N E 3 0 , 1975 ( S I T E 2 ) 3 0 E E Fig..4 - Seasonal comparison of average weekly net rad iat ion, f l u x , t o ta l stand-trans-. p i r a t i on normalized to the stand net r a d i a t i o n , and ind iv idua l t ree t r ansp i r a t i on normalized to the stand net rad ia t ion at s i t e s 1 and 2. T ransp i ra t ion values were ca lcu la ted from so i l water deplet ion and drainage data. 58 the ind iv idua l trees at s i t e 2 were using more water than those at s i t e 1. Th i s de s i r ab le goal in f o re s t th inning co r re l a ted well with the f a s te r rate of growth of the thinned trees (F ig . 5) . E f f ec t of th inning on the ra te of t ree growth had been reported by Bar re t t and Youngberg (1965). They observed that in a 100-year-old ponderosa pine f o r e s t , the trees in a 312 stem/ha stand increased in diameter at breast height at the rate of 10.4 cm per decade while t rees in a 2500 stem/ha stand increased by only 4.3 cm for the same time per iod . As in th i s study, Barrett { (1965) Youngberg/ a t t r i bu ted the f a s te r rate of growth at the thinned s i t e to a higher water consumption per t ree . Normalization of the evapotransp ir -at ion to the stand net rad ia t i on considerably reduces any confounding e f fec t s that the d i f f e rence in rad ia t ion regime at the two s i t e s might have had. This i s c l e a r l y i l l u s t r a t e d by the f a c t that in the l a s t time per iod studied at each s i t e (F ig . 4) , the rad ia t ion regimes were almost exact ly the same and the ind iv idua l trees at the thinned s i t e were using s i g n i f i -cant ly more water than those at the unthinned s i t e . Furthermore, the d i f f e rence in root zone depth between the two s i te s was accounted fo r by computing the s o i l water storage on a unit s o i l volume bas i s . CONCLUSIONS Weekly energy and water balance estimates of evapotransp i rat ion showed good agreement, thus suggesting that both approaches are r e l i a b l e in making comparison o f water uptake by stands of d i f f e r e n t dens i ty . Com-parison of the water uptake at s i t e s 1 and 2 on a stand area bas is suggested that the trees at the unthinned s i t e were somewhat wasteful of water ea r l y .010 NORM. D.B.H. 1.005 1.000 DOUGLAS FIR COURTENAY ^^SITF.,2 SITE I J3 O X 10 20 JULY X 30 10 AUG. 1975 Fig . 5 - Comparison of the. average diameter at breast height (D.B.H.) of three-~ thinned ( s i te 2) and three unthinned ( s i t e 1) t rees . Diameters;were normalized to the values on May 22. to 60 in the drying per iod. Thinning was found to be bene f i c i a l in terms of provid ing more water fo r each ind iv idua l t r ee . Comparison of ind iv idua l t ree water uptake on f i ne sunny days showed that at a p a r t i c u l a r s o i l water p o t e n t i a l , each t ree at the thinned s i t e ( s i t e 2) was using 25% more water than each tree at the unthinned s i t e ( s i t e 1). In comparing water uptake over the en t i re four-week per iod , each tree at the thinned s i t e was using 35% more water than each tree at the unthinned s i t e . This co r re l a ted well with the f a s te r rate of growth of the trees at the thinned s i te . 61 LITERATURE CITED 1. Ba r re t t , J.W. and C.T. Youngberg. 1965. E f fec t of t ree spacing and understory vegetation on water use in a pumice s o i l . So i l S c i . Soc. Amer. Proc. 29: 472-475. 2. Black, T.A. and K.G. McNaughton. 1971. Psychrometric apparatus fo r Bowen-ratio determination over f o r e s t s . Boundary Layer Meteorol. 2: 246-254. 3. Black, T .A . , C.B. Tanner, and W.R. Gardner. 1970. Evapotranspirat ion from a snap bean crop. Agron. J . 62: 66-69. 4. Brown, J . H . and T.G. Bourn..1973. Patterns of s o i l moisture deplet ion in a mixed oak stand. For. S c i . 19: 23-30. 5. C l i n e , R.G. and G.S. Campbell. 1976. Seasonal and d iurnal water r e l a -t ions of se lected fo res t spec ies . Ecology 57: 367-373. 6. Douglas, J . E . 1960. So i l moisture d i s t r i b u t i o n between trees in a thinned l o b l o l l y pine p l an ta t i on . J . Forestry 58: 221-222. 7. G r i e r , C C . and R.H. Waring. 1974. Coni fer f o l i a g e mass re l a ted to sapwood area. For. S c i . 20: 205-206. 8. Hoover, M.D.,- D.F. Olson, and G.E. Greene. 1953. So i l moisture under a young l o b l o l l y pine p l an ta t i on . So i l S c i . Soc. Amer. Proc. 17: 147-150. 9. J a r v i s , P. 1975. Water t rans fe r in p lant s , p. 369-394. In: D.A. de Vr ies and N.H. Afgan (ed.) Heat and mass t rans fe r in the biosphere: I. Transfer processes in plant environment. Sc r ip ta Book Co., Wash., D.C. 10. P a t r i c , J . H . , J . E . Douglass, and J.D. Hewlett. 1965. So i l water 62 absorpt ion by mountain and piedmont f o r e s t s . So i l S c i . Soc. Amer. Proc. 29: 472-475. 11. Rose, C.W. and W.R. Stern. 1967. Determination of withdrawal of water from so i l by crop roots as a funct ion of depth and time. Aust. J . So i l Res. 5: 11-19. 12. S c h o l l , D.G. .1976. So i l moisture f l ux and evapotranspirat ion determined from s o i l hydraul ic propert ies in a Chapparal stand. So i l S c i . Soc. Amer. J . 40: 14-18. 13. van Bavel, C.H.M., G.B. S t i r k , and K.J. Brust. 1968a. Hydraul ic prop-e r t i e s o f a c l ay loam.soi l and the f i e l d measurement of water uptake by roots : I. In terpretat ion of water content and pressure p r o f i l e s . So i l S c i . Soc. Amer. Proc. 32: 310-317. 14. van Bavel, C.H.M., G.B. S t i r k , and K.J. Brust. 1968b. Hydraul ic prop-e r t i e s of a c l ay loam s o i l and the f i e l d measurement of water uptake by roots : II. The water balance of the root zone. So i l S c i . Soc. Amer; Proc. 32: 317-321. CHAPTER 3 RESISTANCE TO WATER UPTAKE IN A DOUGLAS FIR FOREST 64 RESISTANCE TO WATER UPTAKE IN A DOUGLAS FIR FOREST ABSTRACT So i l and root res i s tances were studied in two stands of a Douglas f i r f o re s t . So i l water potent ia l was measured with a tensiometer-pressure transducer system in the 0 to -1 bar range and with Wescor HR-33T dew point microvoltmeter and hygrometers at values less than -1 bar. Root xylem water potent ia l was measured with hygrometers inserted a x i a l l y into the t ree roots and protected from plant res ins by the use of gypsum powder. Twig water potent ia l was measured by the pressure chamber technique. Trans-p i r a t i o n rates were ca lcu la ted from energy balance and stomatal d i f f u s i o n res i s tance measurements. So i l and root res i s tances were obtained from the water potent ia l differences and t r an sp i r a t i on f l uxe s . Root dens i ty was de ter -mined from intens ive sampling. Root xylem water p o t e n t i a l , l i k e twig water p o t e n t i a l , showed a d e f i n i t e d iurnal t rend. So i l water potent ia l approached the root water potent ia l as the s o i l d r i e d . So i l res i s tance remained very small in comparison to root res i s tance even at a s o i l water potent ia l of -11 bars, whether or not "contact re s i s tance " was taken into account. Root res i s tance var ied d i u r n a l l y becoming increas ing ly important at n ight. Ana lys i s of mid-day data during the drying period showed that root res i s tance remained r e l a t i v e l y constant with decreasing s o i l water potent ia l and that rate of water uptake was l i n e a r l y re l a ted to s o i l to root xylem potent ia l d i f f e rence . 65 INTRODUCTION L iqu id phase water movement in the s o i l and plant occurs along a gradient of decreasing water potent ia l from the s o i l to the root and to the leaves (van den Honert, 1948). Root water uptake has been mathemat-i c a l l y descr ibed by s i ng le - roo t models which regard the root as an i n f i n i t e l y long cy l i nder of uniform radius and water absorbing propert ies and s o i l water is assumed to move only r a d i a l l y ( P h i l i p , 1957; Gardner, 1960; Cowan, 1965). For the p rac t i c a l app l i c a t i on of these models, Gardner (1960) suggested that the t rans ient drying of the s o i l by roots can be approximated as a ser ies of steady s ta tes . Many researchers have used these models in an attempt to determine the r e l a t i v e magnitudes of the s o i l and plant res i s tances and furthermore, to locate the la rges t res i s tance within the p lant . It has been shown that f o r normal root ing dens i t i e s the s o i l res i s tance is qu i te small in comparison to the plant res i s tance even in s o i l s c lose to a matric potent ia l of -15 bars (Newman, 1969a, 1969b; Lawlor, 1972; Hansen, 1974a, 1974b; Taylor and Klepper, 1975; Herke l rath, 1975; Reicosky and R i t c h i e , 1976). From several experiments c i t e d by Ja rv i s (1975) there is evidence that root res i s tance is large in r e l a t i o n to stem and l ea f res i s tances in most p lants . Inasmuch as the models c i t e d are approximations of a much more complex system, research re su l t s have shown that they are useful in gaining an important ins i ght into water transport in the so i l and p lant. A f u l l u t i l i z -a t ion of these models requires d i r e c t measurement of a l l the parameters to be used in the ana ly s i s . So f a r , due to measurement technique l i m i t a t i o n s , i t has not been poss ib le to make in s i t u measurement of the potent ia l at 66 the root surface. Furthermore, root xylem water potent ia l has not been measured d i r e c t l y in any res i s tance to water uptake study. At best i t has been i n fe r red from equ i l i b r a ted s o i l water potent ia l in a d i v i d e d -root experiment by Herkelrath (1975). The formulat ion of r e a l i s t i c water uptake models requires more prec i se knowledge of the root sink strength. In th i s regard, Tay lor and Klepper (1975) have suggested that th i s could be achieved i f in the app l i ca t i on of the Gardner (1960) model, the outer edge of the root xylem rather than the root surface i s used as a boundary. T h i s , of course, necess i tates accurate in s i t u measurement of root xylem p o t e n t i a l . In such a system i t would then be poss ib le to re l a te root res i s tance to such water uptake parameters as t r an sp i r a t i on and s o i l water p o t e n t i a l . The purpose of th i s study was ( i ) to develop a technique to measure i_n s i t u root xylem potent ia l cont inuously over a period of several weeks, ( i i ) to study the water potent ia l patterns in a fo res t root zone, ( i i i ) to evaluate the r e l a t i v e magnitudes of s o i l , roo t , and xylem re s i s t ances , and ( iv ) to r e l a te root res i s tance to various root water uptake model para-meters such as t r a n s p i r a t i o n , s o i l water p o t e n t i a l , and the water potent ia l difference from the s o i l to the root xylem. METHODS AND MATERIALS  Experimental S i tes The experiment was ca r r i ed out in two stands of a Douglas f i r 67 (Pseudotsuga menzies i i (Mirb.) Franco) f o re s t of dens i t ie s 1840 trees/ha ( s i te 1; unthinned) and 840 trees/ha ( s i te 2; th inned). Both s i t e s were located on genera l ly f l a t t e r r a i n on the east coast of Vancouver Island about 27 km northwest of Courtenay, B.C. Average tree diameter was 10.6 to 10.9 cm and the trees ranged in height from 7 to 10 m. The s o i l at both s i t e s i s Dashwood g rave l l y sandy loam underla in by compacted basal t i l l at a maximum depth of 70 cm at s i t e 1 and 85 cm at s i t e 2. S i te 1 was instrumented in the summer of 1974 and s i t e 2 in the summer of 1975. (See Chapter 1 f o r d e t a i l s of s i t e d e s c r i p t i o n ) . Measurement of Hydrolog ic, Meteorological- and Plant Parameters So i l water p o t e n t i a l , as a funct ion of depth was measured by the use of a tensiometer-pressure transducer system in the 0 to -1 bar range and by the use of Wescor HR-33T dew point microvoltmeter and PT 51-10 hygrometers at values less than -1 bar. Tensiometer data were recorded automat ica l ly on a Hewlett-Packard 2707A data logger at 1-hourly i n te rva l s while hygrometer data were recorded manually three times d a i l y . Root xylem water p o t e n t i a l , tp r,was a l so measured with hygrometers. This involved ax ia l i n se r t i on of hygrometers into the tree roots with a layer of gypsum powder to protect the sensors from plant re s i n s . Deta i l s of th i s technique are reported in Chapter 4. At s i t e 1, ^ r was measured on three roots of diameter 3 to 4.5 cm located at a depth of 15-to 20-cm on three d i f f e r e n t t rees . S imi la r roots were instrumented at s i t e 2. Two hygrometers were i n s t a l l e d 1 m apart along one root . This was done on two roots on two d i f f e r e n t t rees . One more hygrometer was i n s t a l l e d in another 68 root of one of these two t rees . The root hygrometers were read rou t ine l y three times each day. On two days at s i t e 1 and on f i v e days at s i t e 2, the root xylem water p o t e n t i a l , ip r,was measured every two hours from predawn t i l l sunset. Twig water p o t e n t i a l , ty^.at the 7-m height was measured at the same 2-hourly i n te rva l s by the pressure chamber technique (Scholander et_ al_., 1965). Ha l f -hour ly evapotranspirat ion rates at both s i t e s were measured by as part of the large p ro jec t , the Bowen rat io/energy balance method/ Deta i l s of th i s methodology are reported in Chapter 1. The evapotranspirat ion r a t e , E,was ca l cu la ted from the equation: E = (R n - G - M)/[L(1 + 3)] [1] where Rn i s the net r ad i a t i on f l u x , G is the s o i l heat f l u x , M is the storage rate of sens ib le and l a tent heat within the canopy, L is the l a tent heat of vapor i za t i on , and B, Bowen r a t i o , i s the r a t i o of the f lux dens i t ie s of sens ib le and l a tent heat. A s im i l a r method fo r est imating f lux rates was used by H e l l k v i s t et_ al_. (1974). At s i t e 1, s ince there was v i r t u a l l y no undergrowth, Douglas f i r t r an sp i r a t i on rate was equated with the energy balance measurement of evapotransp i rat ion. At s i t e 2, the to ta l stand t r an sp i r a t i on on s ix f i ne sunny days during a four-week drying period was pa r t i t i oned at 3-hourly i n te rva l s between the Douglas f i r and the sa la l (Gaulther ia sha l l on , Pursh ) undergrowth. This was extrapolated to the other days during the drying per iod. The p a r t i t i o n i n g was done by use of l ea f area index data and vapour pressure and stomatal res i s tance measurements at three l eve l s in the Douglas f i r canopy and on the sa la l undergrowth (Tan, 1977). 69 The design that was used f o r root density sampling i s c a l l e d the "po l y -gon of occupancy". It i s s im i l a r to that of Santantonio (1974). The polygon of occupancy i s formed by the i n te r sec t i on of the perpendicular l i ne s which pass through the midpoints of the l i ne s connecting the centre of the sample tree to the centres of the nearest neighbouring trees (F ig . 1). This design i s advantageous f o r mapping root d i s t r i b u t i o n over large areas s ince the po ly -gons def ine a unique area fo r each t ree . Furthermore, because no a r b i t r a r y distances are used, none of the polygons overlap and no area in the stand is l e f t undefined, no matter how the stocking density of the stand va r i e s . At each s i t e , f i v e t ransects between the sample t ree and the neighbouring trees were de l ineated. Root samples were taken along the transects at locat ions 1/2, 1/4, and 1/8 the distance from the centre of the sample t ree to the centre of the neighbouring trees (depicted by x ' s in F i g . 1). The sampling at each l oca t i on was in two s o i l l a ye r s : 0 to 30-cm and 30-cm to the basal t i l l . Due to the stoniness of the s o i l , rather large s o i l samples had to be dug up in order to get a representat ive amount of roots . The volume of s o i l 3 3 dug up var ied from 4,000 cm to 13,000 cm . The roots were separated from the s o i l by washing on a screen with 0.2-cm diameter holes. The washed roots were stored in j a r s contain ing 10% formaldehyde so lut ion at 2C un t i l measurements of root length, diameter, and weight could be made. Root diameter and root length measurements were ca r r i ed out as fo l lows: The roots were c l a s s i f i e d into three diameter c lasses s im i l a r to those of Meyer and Gottsche (1971), f i n e s t roots ( inc luding the mycorrhizae), <0.5 mm; f i ne root s , 0 . 5 - 2 mm; small roo t s , 2 - 5 mm. In each sample, 50 roots of each diameter c lass were randomly se lected fo r diameter measurements. The diameter was measured at the middle of each root segment with a 70 71 b inocular d i s sec t ing microscope equipped with an ocular micrometer. From these measurements, an average root diameter fo r each diameter c lass was obtained. The res t of the roots were sorted into t h e i r appropriate diameter c la s ses . Samples taken along two transects at each s i t e were se lected fo r de ta i l ed length measurements. Total root length of each diameter c lass in each sample was estimated by a modi f i cat ion of the l i n e i n te r sec t i on tech -nique of Newman (1966) and Marsh (1971). The method was ca l i b r a ted using pieces of sewing thread of known lengths. Three independent estimates of root length f o r each sample were made. The average value was d iv ided by the s o i l volume to obtain root ing dens i ty , L v , (cm root length cm s o i l ) fo r the sample. The roots in each diameter c lass were then dr ied at 70C. From the measured length and oven dry weight, a length-weight f ac to r f o r each diameter c lass was obtained. Root lengths of the samples taken along the other three transects at each s i t e were obtained by use of the oven dry weights and the length-weight f a c to r s . The root lengths were then expressed as L y . The l_v values in a l l the samples were averaged to obtain an average L v value fo r the f o re s t . The unsaturated hydraul ic conduc t i v i t y , K,of the s o i l was determined by the t rans ien t outflow method over a s o i l matr ic p o t e n t i a l , g r a n g e of -0.002 to -15 bars. Deta i l s of th i s method are reported in Chapter 1. Estimation of Resistances in the Water Transport Pathway At steady s t a te , the pathway of water movement in the s o i l and plant 72 can be considered as being comprised of res i s tances in ser ies (van den Honert, 1948) using the equation: j _ ^s - Vs *Vs - ^r _ ^r - ^t r 9 - . where T i s the t r an sp i r a t i on rate in cm d a y - 1 ; ip s , > <Pt are the water potent ia l s in the s o i l matr ix, at the root sur face , in the root xylem, and in the tree twigs re spec t i ve l y (cm of water or bars ) , and R s , R ,^ and R are res i s tances of the s o i l (bulk s o i l to root su r face ) , root (root A surface to root xylem), and xylem (root xylem to twig xylem) pathways re spec t i ve l y (days or bar day cm" 1 ) . Transp i ra t ion in most plants i s con-t r o l l e d by the stomatal aperture and the gradient in water vapour pressure from the l ea f stomatal cav i ty to the a i r . Increases in s o i l and root r e s i s -tances operate i n d i r e c t l y to reduce t r ansp i r a t i on by increas ing stomatal re s i s t ance . Since i p r s cannot yet be measured f o r roots growing in s o i l due to measurement technique l i m i t a t i o n s , the present approach i s to compute a combined s o i l and root res i s tance and then estimate s o i l res i s tance from the theory of Gardner (1960). Root res i s tance i s then obtained by d i f f e r e n c e . So Eq. [2] can be rewri t ten as: T = = -J1-=—1~ [3] K s r K x where R g r i s the combined s o i l and root res i s tance (bulk s o i l to root xylem). Gardner (1960) proposed a model f o r computing the d i f f e rence in s o i l matric potent ia l between the root surface and the bulk s o i l . The root was considered as a uniform cy l i nde r of r ad iu s , r (cm), withdrawing water at steady s t a te , q r (cm /cm root/day) . The water was assumed to be taken up only from a d i s tance,c (cm), which i s ha l f the d is tance between 73 -112 neighbouring roots , assuming uniform root ing or c = (TTL )~ , where L Y i s the root density (cm root/cm s o i l ) . Assuming n e g l i g i b l e s o i l osmotic p o t e n t i a l , Gardner's formula i s : *rs " *s = ^ l n ^/r2) [4] where K is the unsaturated s o i l hydraul ic conduct iv i ty df the bulk s o i l . To express T in Eq. [2] as f l ux per uni t ground area, we assume hor izonta l and v e r t i c a l uni formity of roots growing to depth,d (cm), and that a l l roots are equal ly e f f e c t i v e in water absorpt ion, then q r = T / L v d [5] -1/2 Subst i tu t ing Eq. [5] into Eq. [4] and assuming c = UL- v ) ' , R g from Eq. [2] becomes 2 R = In (1/r TTU) R F I L  K s 47rKLvd L b J and so R r = ~ T - R s C 7 ] When "contact re s i s tance " due to root shrinkage in a drying s o i l i s taken into account, a co r rec t i on f a c t o r , f , has to be inserted into Eq. [3] (Herkelrath, 1975). Herkelrath assumed that the rate of water uptake was proport ional to the wetted area of the root surface. Hence, as a f i r s t approximation, f was assumed to be equal to the f r a c t i o n a l s a tu ra t i on , 9/9 . sat (9„ . i s the s o i l water content at saturat ion) of the bulk s o i l . Then the sat f i r s t part of Eq. [3] can be rewr i t ten as 74 T = if—) [8] w s a t R;i sr where R- „ i s the sum of s o i l and root resistances with contact resistance sr i removed. This estimate of root resistance, R • , i s calculated by sub-tracting R„ from R as follows 3 s sr i 9 \ *s - * r r y s a t 1 s RESULTS AND DISCUSSION  Water Potential Patterns as Soil Dried The relationship of Douglas f i r root xylem water po t e n t i a l , i p r , at mid-day to s o i l water po t e n t i a l , i p s , a t various depths over a growing season at the thinned s i t e are reported in Fig. 2. As the s o i l dried, mid-day $ values were lower than ip values as would be expected with transpiring trees. This response r e f l e c t s the dependence of i|y on y>s and atmospheric demand. A si m i l a r trend was obtained at the unthinned s i t e (see Chapter 4). During a wetting period at the unthinned s i t e , ip showed values higher than i p s in some of the s o i l layers. This indicates that the roots were responding quickly to the wetting up of the surface s o i l layers. Figs. 3a, 4a, 5a, and 6a show the diurnal trends in root and twig water potentials at various s o i l water potentials during a drying period at the thinned s i t e . A consistent feature of these data i s that the root water 75 F i g . 2 - Course of s o i l water potent ia l -(ip ) at f i v e depths and root xylem water potent ia l (ip ) f o r the period June 18 to August 12, 1975. The bars on ip ind icate the range of values averaged. / 76 (jl "8 (bars) -12 -16 -20 -24 8 ENERGY FLUX DENSITY (100 W m"2) — • — i 1 1 r 1 SOIL -\ \ ROOT^X V TWIG/""< I (a) 1 • —1 1 1 ' -2 2 LED.FIR (100 W m"2) I 0 8 R (I04days) 4 L DOUGLAS FIR T | , - COURTENAY SITE 2 • JULY 3.1975 / H/. / / , W '•i -* • > _ — ' - - ^ - ^ V-. *^.. / t . i i (0 1.2 0.9 0.6 0.3 0.3 0.3 E (mm hr~') 0.15 E (mm hr"1) 1 i • , - T — , >XYLEM ---"(d) i ROOT -l l I l 8 12 16 20 24 HOURS P.S.T. Diurnal trends of water potent ia l of root and twig xylem of Douglas f i r , components of the energy balance of the stand, Douglas f i r t r ans -p i r a t i o n (stand area basis) and root and xylem res i s tances of Douglas f i r at a s o i l water potent ia l of - 1 .0 .bar . See text fo r d e f i n i t i o n of symbols, To convert Douglas f i r t r an sp i r a t i on (E) on a stand area basis to t r ansp i r a t i on on an ind iv idua l t ree bas i s , d iv ide stand t r an sp i r a t i on by 840 t rees /ha. 77 • i " " F 1 - i r -SOIL -4 --8 (bars) -12 / -16 /TWIG -20 \ -24 V L _ J . ENERGY FLUX DENSITY (100 W m"2) DOUGLAS FIR  COURTENAY  SITE 2 JULY 11,1975 / -i r ...Rn _ LE * [ 3 ^ ^ - ^ ^ 1.2 0.9 0 6 (mmhr-l) 0.3 LED.FIR (100 W m"2) 4 8 12 16 20 24 HOURS P.S.T. F i g . 4- - D iurnaVt rends of water potent ia l of root and twig xylem of Douglas f i r , components of the energy balance of the stand, Douglas f i r t r an sp i r a t i on (stand area basis) and root and xylem res i s tances of .Douglas f i r at a soi l, water potent ia l of -2.3 bars. See text fo r d e f i n i t i o n of symbols. To convert Douglas f i r t r an sp i r a t i on (E) on a stand area basis to t r an sp i r a t i on on an ind iv idua l tree bas i s , d iv ide stand t r an sp i r a t i on by 840 t rees /ha . 78 -4 l|l -8 (bars) -12 r -20 -24 — 1 1 1 1 1 SOIL (a) i i i 1 1 — • 8 1 1 r DOUGLAS FIR 1 T 1.2 6 • COURTENAY • 0.9 ENERGY FLUX DENSITY 4 SITE 2 • JULY 24,1975 •,H : , V V 0.6 E (mm hr (IOOW m" 2) 2 , . _ 0.3 0 G*M •— 0 -2 (b) 1 1 1 L 1 • -0.3 L ED.FIR (100 W rrf2) I 0 R 8 (c) 0.3 0.15 0 E ( m m hr"1) (I04days) i —i - i — -/*--•» XYLEM 4 •ROOT (d) 8 12 16 20 24 HOURS P.S.T. F ig . 5 - Diurnal trends of water potent ia l of root and twig xylem of Douglas f i r , components of the energy balance of the stand, Douglas f i r t r an sp i r a t i on (stand area basis) and root and xylem res i s tances of Douglas f i r at a.so.il water potent ia l of -7.4 bars. :Se.e text f o r d e f i n i t i o n of symbols. To convert Douglas f i r t r an sp i r a t i on (E) on a stand area basis to t r an sp i r a t i on on an ind iv idua l t ree bas i s , d iv ide stand t r an sp i r a t i on by 840 t rees /ha, 79 (bars) -8 -12 -16 -20 -24 8 1 i r i T ^ SOIL -^ — - -(a) , i . • ENERGY FLUX DENSITY (100 W m - 2) 2 0 -2 2 1 1 — DOUGLAS FIR — — i 1 1 % -COURTENAY SITE 2 - JULY 50,1975 . ' V. • •, .' i": :V. :'i • i LE •J V\ -" ^ • M ^ "(b) i i i 1_ 1 _ . i L E D. FIR (100 W m - 2 ) (c) 1.2 0.9 E 0 6 (mmhr-1) 0.3 -0.3 0.3 MO. 15 (mm hr"') R (I04days) •~ir - * * "*XYLEM. — i i, ^ 8 — « — • ROOT " 8 12 16 20 24 HOURS P.S.T. 6 - Diurnal trends of water potent ia l o f root and twig xylem of Douglas f i r , components of the energy balance of the stand, Douglas f i r t r an sp i r a t i on (stand area basis) and root and xylem res i s tances of Douglas f i r at a - s o i l water potent ia l of -10.1 bars. See text f o r d e f i n i t i o n of symbols. To convert Douglas f i r t r ansp i r a t i on (E) on a stand area basis to t r an sp i r a t i on on an ind iv idua l t ree bas i s , d i v ide stand t r an sp i r a t i on by 840 t rees /ha . 80 potent ia l was always between the twig and s o i l water p o t e n t i a l s , thus i nd i ca t i n g root water uptake at a l l of the measurement times. As the drying cyc le progressed, ^ s approached 41 thus d iminishing the d i f f e rence between s o i l and-root- 'potent ia ls . This was a l so observed at the unthinned s i t e . S imi la r f ind ings were reported by Kaufmann (1968) and Fiscus (1972). Over the drying per iod , the amplitude of the d iurna l swing of the root potent ia l dec l ined much more than that in the twig. This shows that atmospheric demand has much greater in f luence on twig than on root water p o t e n t i a l . A s im i l a r observation was made by H e l l k v i s t et_ al_. (1974). As the s o i l became progress ive ly d r i e r , the at predawn assumed a progress ive ly lower value. Also the ip r l a te at night (20.00 to 22.00 hours P.S.T.) showed a s im i l a r response. The technique f o r measuring used in th i s study has cons iderable advantage over prev ious ly reported methods. For ins tance, even though Kaufmann (1968) could show a d iurna l trend in ip^ determined on new roots growing out of a s o i l mass into a humid chamber, his procedure had the weak-ness that the water potent ia l was measured on roots that were not d i r e c t l y a part of the pathway f o r water movement from the s o i l to the shoot. De Roo (1969) used the pressure chamber on well -developed and wel1-branched advent i t ious roots of f ie ld-grown tobacco. This technique involved des t ruc -t i v e sampling and the use of d i f f e r e n t plants to fo l low a t rend. This could in part account f o r the lack of a regular d iurnal trend in his ip^. In the "d iv ided - root experiment of Herkelrath (1975) where the tp^  was i n fe r red from the s o i l water potent ia l of a s o i l compartment from which no fur ther water uptake was occur r ing , i t was not poss ib le to fo l low rap id changes in <Pr because the approach to equ i l i b r ium was too slow. Furthermore, only one 81 4> r value could be obtained during a given drying c y c l e . In cont ras t , the root water potent ia l technique used in th i s study made measurements d i r e c t l y in the pathway of water flow from s o i l to shoot. It d id not involve des t ruc t i ve sampling and measurements were made continuously at one loca t ion which allowed \b to be monitored both on a d iurna l and on a r seasonal bas i s . The average ip s in the root zone was computed taking into account the zone of maximum root dens i ty and the rate of water deplet ion in the various s o i l l ayers . The iji value was obtained by averaging \> measured c lose to the base of the stem in three t rees . This was considered appropriate s ince according to Sl.atyer (1960) and Kramer (1969), the root system tends to integrate the range of s o i l water potent ia l s encountered and the ip measured c lose to the base of the stem gives a reasonable estimate of ip in the sect ion of the root zone where most of the water i s being absorbed. From parts a, b and c of F igs . 3, 4, 5, and 6, i t can be seen that l a rger d i f f e r -ences between the s o i l and the root xylem potent ia l were associated with l a rger Douglas f i r t r an sp i r a t i on rates . Furthermore, the larger s o i l to root d i f fe rences occurred in wetter s o i l (greater than -1.0 bar) . By comparing the evapotransp irat ion trend in part b and the Douglas f i r t r an sp i r a t i on rate in part c , i t i s apparent that the Douglas f i r competed better with the s a l a l in the morning and evening than around mid-day. Further d i s -cuss ion on th i s i s contained in Tan (1977). 82 S o i l , Root and Xylem Resistance Estimates Soil resistance calculations using Eq. [6] required both measure-ments of Douglas f i r root density, L y,and the s o i l unsaturated hydraulic conductivity, K. Root density values along 5 transects at given distances away from a representative tree at the thinned s i t e are reported in Table 1. A consistent feature of these data i s that the root density was highest close to the trunk and declined away from the trunk. Similar results were obtained at the unthinned s i t e . An average root density value of 0.66 3 cm of root/cm of s o i l which included roots of a l l ages was used in a l l Rg calculations for s i t e 2. The corresponding value for s i t e 1 was 0.64 cm of 3 root/cm of s o i l . These values are s i m i l a r to those of Roberts (1948) and Kramer and Bullock (1966) working on pine. The average L y value was con-sidered appropriate since i t has been shown that suberized roots of conifers play a s i g n i f i c a n t role in water uptake. For instance, Kramer and Bullock (1966) have pointed out that suberized roots of a 34-year-old pine stand contributed about 93% of the total absorbing area and although th e i r conduc-t i v i t y was about one tenth that of unsuberized roots, they absorbed about three-fourths of the tot a l water taken up. A similar study on 1-year-old pine seedlings (Chung and Kramer, 1975) showed that about 60% of the root absorbing surface was suberized and about 50% of the total water uptake occurred through suberized roots'. The smallest root diameter measured in this study was 25u. . The mycorrhizae were c l a s s i f i e d together with the f i n e s t roots. The mean root diameter used for resistance calculations was 0.08 cm. Root hairs were ignored in this analysis. If the trees in the drying s o i l had an abundance of functional root hairs, then the average Table 1. Root dens i ty , L y (cm root cm" s o i l ) data at s i t e 2. Fract ion of -3 s o i l ) /distance to L v (cm root cm adjacent t ree TRANSECT NO. 1 2 3 4 5 1/8 0.64 0.74 1.22 0.72 0.81 1/4 0.46 ' 0.65 0.66 0.52 0.77 1/2 0.30 0.60 0.62 0.46 0.75 84 distance which water had to move to reach a root or a root ha i r would have been less than that assumed in these c a l c u l a t i o n s , so s o i l r e s i s -tance would be even smal ler. Values of K as a funct ion of matric s o i l water potent ia l are shown in F i g . 7. The K values are comparable to those reported by Mehuys et a l . (1975) on a s o i l of s im i l a r texture. A f te r due cons iderat ion of the d i f f i c u l t i e s involved in measuring K, i t was f e l t that the K values in th i s study could be s l i g h t underestimates.. Furthermore, the K of the bulk s o i l would be higher than that in the immediate v i c i n i t y of the roots . The d iurna l trends of the root re s i s t ance , Py, (Eq.[ 7]) and xylem re s i s t ance , R , (Eq. [3 ]) are shown in part d of F igs . 3, 4, 5, and 6. X R and R tended to increase over the ac t i ve t r an sp i r a t i on period on any I A given day. This tendency f o r root res i s tance to increase between mid-day and sunset i s s i m i l a r to the resu l t s of Skidmore and Stone (1964) and Barrs and Klepper (1968) who appl ied suct ion to decapitated root systems in nutr ient so lut ions in growth chambers under simulated day and night cond i t ions . R was cons i s ten t l y higher than R . The d i f f e rence increased A t as the drying cyc le progressed. This i s a r e f l e c t i o n of the f a s te r dec l ine of the s o i l to root potent ia l d i f f e rence in comparison to the root to twig potent ia l d i f fe rence as the s o i l d r i ed . Data from Boyer (1971) and several papers c i t e d by Ja rv i s (1975) show the same type of re l a t i on sh ip between R f and R x. 10 1 1 1 1 DASHW00D 1 GRAVELLY SANDY LOAM -SITE 1 (Z=0-40cm) 10"' •') -lO"2 -io-' e 10-4 in-8 1 1 1 1 -0.001 -0.01 -0.1 -1 -10 Vm (bars) F i g . 7 - Unsaturated hydraul ic conduct iv i ty as a funct ion of matric potent ia l f o r a sample of Dashwood grave l l y sandy loam with large stones removed. (Same as F i g . 3 of Chapter I). CO cn 86 Relat ing R and R r to Water Uptake Model Parameters A knowledge of root res i s tance and i t s response to water uptake model parameters such as t r a n s p i r a t i o n , s o i l water p o t e n t i a l , and root xylem potent ia l are required in the formulat ion of r e a l i s t i c models of water uptake (Allmaras et_ al_., 1975). In th i s s e c t i on , an attempt i s made to provide some of t h i s information using mainly mid-day data obtained at the thinned s i t e . Assuming that there was neg l i g i b l e root shrinkage and hence a n e g l i g i b l e a i r gap between the s o i l and the root surface as the s o i l d r i e d , R r was computed from Eq. [7] . When i t was assumed that suf -f i c i e n t root shrinkage could occur in the s o i l to create an a i r gap between the s o i l and the root , R r was ca l cu la ted from Eq. [9] . The values of R r and R* r used in th i s sect ion were those ca l cu la ted from i p s , <jy, and T data f o r a one-hour in terva l around mid-day. These were used because the f l ux divergence with in the tree around mid-day was considered to be at i t s minimum fo r the day-time per iod. The ca l cu la ted f l ux divergence within the tree over a one-day period was very small in comparison to the d a i l y evapotransp irat ion ra te . Deta i l s of th i s c a l c u l a t i o n are reported in Chapter 1. S imi la r re su l t s have been reported by Cowan and Mi lthorpe (1968) and Rothwell (1974). The re l a t i on sh ip between the rate of water uptake (Douglas f i r t r ans -p i r a t i on ) and the s o i l to root xylem potent ia l difference, ^ s r , i s shown in F i g . 8. Higher uptake rates and larger A ^ s r were associated with wetter s o i l while lower uptake rates and smaller h\\>sr were associated with d r i e r s o i l . For s i t e 2, the re l a t i on sh ip between Douglas f i r t r an sp i r a t i on and A ^ s r w a s ^ i n e a r a n c * ' w n e n extrapolated to a zero A ^ s r va lue, ind icated to a 87 A Y8r (bars) F i g . 8 - The rate of water uptake of Douglas f i r on a stand area basis versus s o i l to root xylem potent ia l d i f f e rence (Aip s r ) at s i t e s 1 and 2. On cloudy days ( t r i ang les ) the c l ea r day p a r t i t i o n i n g of Douglas f i r and sa l a l t r an sp i r a t i on may underestimate water uptake of Douglas f i r . Tc convert water uptake on a stand area basis to uptake on an ind iv idua l t ree bas i s , d i v ide water uptake value by the appropriate stand dens i ty . 88 good approximation to a zero flux. The l i n e shown was drawn through the s i t e 1 data assuming that a constant resistance occurred as suggested by s i t e 2 data. A similar conclusion was arrived at by Taylor and Klepper (1975). This conclusion d i f f e r s from that of Herkelrath (1975) who suggested that the relationship between the uptake rate and Aip s r was non-linear. There are two important differences between the present study and that of Herkelrath (1975). Whereas Herkelrath (1975) studied the effect of s o i l dryness on root extraction without stressing the plants, this research dealt with effect of s o i l dryness on root extraction as the trees got progressively stressed. Secondly, whereas Herkelrath (1975) assumed a constant root xylem potential over his entire drying period, in t h i s study root xylem potential was measured at the same time as s o i l water potential and evapotranspiration. The present measurements showed (Fig. 2) that root xylem potential was not constant over a drying period. Prior to the two papers c i t e d , water uptake had been compared with s o i l to leaf potential difference, Ai|> i . Since in both cases the van den Honert (1948) model i s being used, there i s a basis for comparison between the two sets of data. Hailey et a]_. (1973) did a detailed analysis of water uptake versus Aip ^ trends. The results in Fig. 8 would correspond with th e i r second type of response where the transpiration rate varies l i n e a r l y or near l i n e a r l y with A ^ - J . The rate of water uptake versus s o i l to twig potential difference i s shown in Fig. 9. It shows a similar trend to Fig. 8 except that the curve i s displaced further to the right because the minimum s o i l to twig potential difference was always larger than the minimum s o i l to root potential difference. 89 >, o •o E E Id < »-0. 3 U . o LU CC -1 i I l 1 DOUGLAS FIR " COURTENAY e • SITE 2 • • - • • • - • • • •A • I i • i i 12 16 A Y S } (bars) 20 24 F i g . 9 - The rate of water uptake of Douglas f i r on a stand area basis versus s o i l to twig potent ia l d i f f e rence (Aip t ) at s i t e 2. Cloudy day is ind icated by t r i a n g l e . To convert water Uptake on a stand area basis to uptake on an ind iv idua l t ree bas i s , d i v ide water uptake by 840 t rees /ha. 90 R „ and R ca l cu la ted from the s i t e 2 data in F i g . 8 remained sr r r e l a t i v e l y constant with decreasing s o i l water potent ia l (F ig . 10). Reicosky and R i t ch ie (1976) working on sorghum, reported a plant r e s i s -tance ( so i l to l ea f ) of 1.15 x 10^ days at a s o i l water potent ia l of -0.1 4 bar and 1.06 x 10 days at a s o i l water potent ia l of -1 bar, suggesting a constant res i s tance in a drying s o i l . On the other hand, Rg increased qui te r ap id l y as the s o i l dr ied (F ig . 11). I r respect ive of th i s rap id rate of increase, the absolute magnitude of Rg remained small in comparison to R p such that at a s o i l water potent ia l , of -10 bars, R^  was s t i l l about two orders of magnitude greater than R g. Furthermore, as the s o i l d r ied from -0.3 to -11.3 bars, R g /R r increased from 0.002% to 1.5%, while R s /R' ' r increased from 0.004% to 4.5%. S imi la r re su l t s were obtained at the unthinned s i t e . The poss ib le s l i g h t underestimate of the K values mentioned e a r l i e r would mean that these r a t i o s are s l i g h t overest imates. Thus, whether or not there was a "contact re s i s tance " present, the s o i l r e s i s -tance (rhizosphere res i s tance) was much smaller than the root re s i s tance . Assuming there i s no "contact r e s i s t a n c e " , the major res i s tance to water uptake into the root xylem appears to be ins ide the root. However, from F i g . 10 i t appears that the "contact r e s i s t a n c e " , Rc, could cons t i tu te a large f r a c t i o n of the to ta l s o i l to root xylem res i s tance . A cons iderat ion of a poss ib le 20% overestimate of the tree root dens i ty due to the roots of the sa l a l undergrowth increased the R g very s l i g h t l y such that the R s /R p r a t i o s remained v i r t u a l l y unchanged. If the s o i l to twig potent ia l d i f fe rence i s used in th i s res i s tance c a l c u l a t i o n , the r a t i o of s o i l to plant res i s tance would be much smal ler. This has been the approach adopted by many researchers in evaluat ing the importance of s o i l res i s tance (Andrews and Newman, 1969; 91 R (I04 days) DOUGLAS FIR, COURTENAY  SITE 2 e 8 8 8 s .8 8 88* . a s • °° •IB. • \ I •::::RS ••• t R'r * • RSr * ° Rr • * R'r -2 - 4 -6 f s (bars) -8 -10 -12 F i g . 10 - Mid-day values of combined s o i l and root res i s tance (R s r )> s o i l res i s tance (R s)> root res i s tance (R r ) and root res i s tance with the e f f e c t o f . " con tac t re s i s tance " (R c) subtracted ( R ' r ) at various s o i l water potent ia l s at s i t e 2. The t r i ang le s and open squares are derived from the cloudy! day data of F i g . 8. 93 Lawlor, 1972; Hansen, 1974a, 1974b; Tay lor and Klepper, 1975; Arya et al_., 1975; Reicosky and R i t c h i e , 1976). The present approach separated the plant pathway into two d i s t i n c t parts with d i f f e r e n t water conducting c h a r a c t e r i s t i c s : ( i ) The root t i s sues external to the vascular system in which water would have to move through c e l l protoplasm at one or more points and ( i i ) The xylem tracheids con-s i s t i n g of open c e l l s in which water does not move through protoplasm. One disadvantage of p red i c t ing trends in plant res i s tance from only s o i l and l ea f water potent ia l measurements i s that there is an inherent assumption that the root water potent ia l would increase and decrease propor t ionate ly , to the d i f f e rence between s o i l and lea f water potent ia l s at d i f f e r e n t s o i l water po ten t i a l s . But as shown in part a of F igs. 3, 4, 5, and 6, whereas the s o i l to root xylem potent ia l d i f f e rence dec l ined r ap id l y as the so i l d r i e d , the s o i l to twig potent ia l d i f f e rence did not. CONCLUSIONS The present technique of measuring 4 r had some advantages over tech -niques reported in the l i t e r a t u r e . For example, measurement was made d i r e c t l y and continuously in the path of water movement from the s o i l to the shoot. During the drying per iod , $ values were between ips and ^ values as would be expected fo r t r an sp i r i ng t rees , approached 4 r as the s o i l d r i ed . ips showed a d e f i n i t e d iurnal response l i k e but the amplitude of the d iurna l swing of $ dec l ined f a s te r than that of 4>. in the drying s o i l due to the 94 f a c t that tyr was more responsive to tys while ^ t was more responsive to atmospheric demand. Root res i s tance var ied during the daytime, becoming increas ing ly important toward n i g h t f a l l . R g remained very small in comparison to R r over the range of s o i l water potent ia l s studied ( 0 to -11 bars) whether or not "contact re s i s tance " was taken into account. Ana lys i s of mid-day data during the drying cyc le ind icated that R r remained r e l a t i v e l y constant with decreasing s o i l water potent ia l and that the rate of water uptake was l i n e a r l y re la ted to A b -using Herke l rath ' s "contact re s i s t ance " model, i t was found that "contact re s i s tance " between the s o i l and root could account fo r up to one ha l f of the to ta l s o i l to root xylem res i s tance . However, R* s t i l l exhib i ted re l a t i onsh ip s to T, A ^ s r > and ^ s that were s im i l a r to those of R r-95 LITERATURE CITED 1. Al lmaras, R.R., W.W, Nelson, and W.B. Voorhees. 1975. Soybean and corn root ing in southwestern Minnesota: II. Root d i s t r i b u t i o n s and re l a ted water inf low. So i l S c i . Soc. Amer. Proc. 39: 771-777. 2. Andrews, R.E. and E.I. Newman. 1969. Resistance to water flow in s o i l and p lant : III. Evidence from experiments with wheat. New Phyto l . 68: 1051-1058. 3. Arya, L.M., G.R. Blake, and D.A. F a r r e l l . 1975. A f i e l d study of s o i l water dep let ion patterns in presence of growing soybean roots : III. Rooting c h a r a c t e r i s t i c s and root ext rac t ion of s o i l water. So i l S c i . Soc. Amer. Proc. 39: 437-444. 4. Barrs , H.D. and B. Klepper. 1968. C y c l i c va r i a t i ons in p lant propert ies under constant environmental cond i t ions . Phys io l . P lant . 21: 711-730. 5. Boyer, J .S . 1971. Resistances to water transport in soybean, bean, and sunflower. Crop.Sc i . 11: 403-407. 32 6. Chung, H. and P.J. Kramer. 1975. Absorption of water and P through suberized and unsuberized roots of L o b l o l l y pine. Can. J . For. Res. 5: 229-235. 7. Cowan, I.R. 1965. Transport of water in the so i l -p lant-atmosphere system. J . Appl . Eco l . 2: 221-239. 8. Cowan, I.R. and F.L. Mi l thorpe. 1968. Plant fac tor s in f luenc ing the water status of p lant t i s sues , p. 137-193. In: T .T . Kozlowski (ed.) Water d e f i c i t s and plant growth: Vo l . 1. Development, contro l and meas-urement. Academic Press, New York. 9. De Roo, H.C. 1969. Water s t ress gradients in plants and s o i l - r o o t systems. 96 Agron. J . 61: 511-515. 10. F i s cus , E.L. 1972. In s i t u measurement of root-water p o t e n t i a l . Plant Phys io l . 50: 191-193. 11. Gardner, W.R. 1960. Dynamic aspects of water a v a i l a b i l i t y to p lants . So i l S c i . 89: 63-73. 12. Ha i ley , J . L . , E.A. H i l e r , W.R. Jordan, and C.H.M. van Bavel. 1973. Resistance to water flow in Vigna s inens i s L. (Endl.) at high rates of t r a n s p i r a t i o n . Crop S c i . 13: 264-267. 13. Hansen, G.K. 1974a. Resistance to water transport in s o i l and young wheat p lants . Acta Ag r i c . Scand. 24: 37-48. 14. Hansen, G.K. 1974b. Resistance to water flow in s o i l and p lant s , p lant water s ta tus , stomatal res i s tance and t r an sp i r a t i on of I t a l i an ryegrass, as in f luenced by t r an sp i r a t i on demand and s o i l water dep le t ion . Acta Ag r i c . Scand. 24: 83-92. 15. H e l l k v i s t , J . , G.P. Richards and P.G. J a r v i s . 1974, V e r t i c a l gradients of water potent ia l and t i s sue water re l a t i on s in S i tka spruce trees measured with the pressure chamber. J . App l . E c o l . 11: 637-667. 16. Herke l ra th, W.N. 1975. Water uptake by plant roots . Ph.D. Thes i s . Univ. of Wisconsin. (Diss. Abstra. 36: 3710). 17. J a r v i s , P. 1975. Water t rans fe r in p lants , p. 369-394. In: D.A. de Vr ies and N.H. Afgan (ed.) Heat and mass t rans fer in the biosphere: I. Transfer processes in plant environment. Sc r ip ta Book Co., Wash., D.C. 18. Kaufmann, M.R. 1968. Water r e l a t i on s of pine seedl ings in r e l a t i o n to root and shoot growth. Plant Phys io l . 43: 281-288. 19. Kramer, P .J . 1969. Plant and s o i l water r e l a t i on sh i p s : A modern syn-the s i s . McGraw-Hill Book Co., Toronto. 97 20. Kramer, P.J. and H.C. Bul lock. 1966. Seasonal var i a t ions in the proportions of suberized and unsuberized roots of trees in r e l a t i o n to the absorption of water. Am. J . Bot. 53: 200-204. 21. Lawlor, D.W. 1972. Growth and water use of Loliurn perenne. I. Water t ransport . J . Appl. Eco l . 9: 79-98. 22. Marsh, B. 1971. Measurement of length in random arrangement of l i n e s . J . Appl . Eco l . 8: 265-267. 23. Mehuys, G.R., L.H. S to l zy , J . Letey, and L.V. Weeks. 1975. E f f ec t of stones on the hydraul ic conduct i v i ty of r e l a t i v e l y dry desert s o i l s . So i l S c i . Soc. Amer. Proc. 39: 37-42. 24. Meyer, F.H. and D..Gtittsche. 1971. D i s t r i bu t i on of root t i p s and tender roots of beech, p. 48-52. In: H. E l lenberg (ed.) Integrated experimental ecology: Methods and re su l t s of eco-system research in the German s o i l i n g p ro jec t . Eco l . Studies Vo l . 2. Spr inger -Ver lag , New York. 25. Newman, E.I. 1966. A method of est imating the to ta l length of root in a sample. J . Appl. Ecol. 3: 139-145. 26. Newman, E.I. 1969a. Resistance to water flow in s o i l and p lant : I. So i l res i s tance in r e l a t i o n to amount of root : Theoret ica l est imates. J . Appl . Eco l . 6: 1-12. 27. Newman, E.I. 1969b. Resistance to water flow in s o i l and p lant : II. A review of experimental evidence on the rhizosphere res i s tance . J . Appl . Eco l . 6: 261-272. 28. P h i l i p , J.R. 1957. The phys ica l p r i n c i p l e s of s o i l water movement during the i r r i g a t i o n c y c l e . Proc. Int. Congr. I r r i g . Drain. 8: 124-154. 29. Reicosky, D.C. and J . T . R i t ch i e . 1976. Re lat ive importance of s o i l res i s tance and plant res i s tance in root water absorpt ion. So i l S c i . 98 Soc. Amer. J . 40: 293-297. 30. Roberts, F.L. 1948. A study of the absorbing surfaces of the roots of L o b l o l l y pine. M.S. Thes i s . Duke Un iver s i t y . 31. Rothwell, R.L. 1974. Sapwood water content of Lodgepole pine. Ph.D. Thes i s . Univ. of B r i t i s h Columbia. (Nat. L i b . Canada No. 25256). 32. Santantonio, D. 1974. Root biomass studies of old-growth Douglas f i r . M.S. Thes i s . Oregon State Univ. 33. Scholander, P.F. , H.T. Hammel, E.D. Bradstreet , and E.A. Hemmingsen. 1965. Sap pressure in vascular p lants . Science 148: 339-346. 34. Skidmore, E.L. and J . F . Stone. 1964, Phys io log ica l r o l e in regu lat ing t r an sp i r a t i on rate of the cotton p lant . Agron. J . 56: 405-410. 35. S l a t ye r , R.O. 1960. Absorption of water by p lants . Bot. Rev. 26: 331-392. 36. Tan, C.S. 1977. 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 . Ph.D. Thes i s . Univ. of B r i t i s h Columbia. 37. Tay lo r , H.M. and B. Klepper. 1975. Water uptake by cotton root systems: An examination of assumptions in the s ing le root model. So i l S c i . 120: 57-67. 38. van den Honert. 1948. Water t ransport in plants as a catenary process. Discuss. Faraday Soc. 3: 146-153. CHAPTER 4 FIELD PERFORMANCE OF THE DEW POINT HYGROMETER IN STUDIES OF SOIL-ROOT WATER RELATIONS 100 FIELD PERFORMANCE OF THE DEW POINT HYGROMETER IN STUDIES OF SOIL-ROOT WATER RELATIONS ABSTRACT In s i t u measurements of so i l and root xylem water potent ia l s in a Douglas f i r f o re s t using a Wescor HR-33T dew point microvoltmeter and PT51-10 hygrometers are descr ibed. Root water potent ia l measurement required tangent ia l i n se r t i on of the hygrometer into the root xylem and sensor pro-t ec t i on from plant res ins using gypsum powder. Soi l water potent ia l meas-urements were compared with potent ia l s computed using gravimetr ic so i l water measurements and laboratory so i l water retent ion data, while root water potent ia l measurements were compared with those made with the pressure chamber. The comparisons showed agreement to with in ± 0.3 bar over an 8-bar range. So i l water matric potent ia l on both dew point and psychrometric modes showed good agreement. The root water potent ia l showed a d e f i n i t e d iurnal response to atmospheric demand. INTRODUCTION The study of water transport phenomena in the so i l -p lant -atmosphere system requires the in s i t u measurement of water potent ia l in the various 101 parts of the system. In s i t u measurements are cons iderably eas ier to make in the laboratory than in the f i e l d . F i e l d d i f f i c u l t i e s a r i s e p r i n c i p a l l y from instrument l i m i t a t i o n s in an uncontro l led environment. In t h i s regard, the dew point hygrometer developed by Neumann and Thur te l l (1972) and improved by Campbell et al_. (1973) shows cons iderable promise f o r water potent ia l measurement. As part of a study of water r e l a t i on s in a thinned and an unthinned stand of a Douglas f i r f o r e s t , the f i e l d performance of the dew point hygrometer was evaluated over two growing seasons. This paper reports the re su l t s o f the eva luat ion. MATERIALS AND METHODS Dew Point Hygrometer Water potent ia l measurements were made using a Wescor HR-33T dew point microvoltmeter and PT51-10 hygrometers. The P e l t i e r cooled thermo-couple dew point hygrometer determines water potent ia l s by measuring the dew point temperature depression of water vapor in equ i l ib r ium with a sample (Neumann and T h u r t e l l , 1972). This -is in contrast to the thermo-couple psychrometer which detects wet bulb temperature depress ion. Measure-, ments by the dew point technique are r e l a t i v e l y independent of the wetting c h a r a c t e r i s t i c s of the j u n c t i o n , and the s i ze and shape of the water drop-l e t formed on the junct ion (Neumann and T h u r t e l l , 1972; Campbell et a l . , 102 1973). These features make i t advantageous to use the dew point hygro-meter rather than the thermocouple psychrometer. S i t e and So i l Cha rac te r i s t i c s The f i e l d evaluat ion of the dew point hygrometer was ca r r i ed out in a 20-year-o ld Douglas f i r fo res t .near Courtenay on -ithe east coast of Vancouver I s land. The unthinned stand (1840 stems/ha; s i t e 1) was i n s t r u -mented in the summer of 1974 and the thinned stand (840 stems/ha; s i t e 2) , in the summer of 1975. The trees were 7 to 10 m t a l l . The s o i l at both s i t e s i s Dashwood g rave l l y sandy loam. It i s under la in by compacted t i l l at a maximum depth of 70 cm at s i t e 1 and 85 cm at s i t e 2. The s o i l water retent ion c h a r a c t e r i s t i c s were determined by the pressure p la te ext ract ion method. Soi l Water Matric Potent ia l and So i l Water Content Measurement Soi l water matric p o t e n t i a l , w a s measured by a tensiometer-pressure transducer system for potent ia l s greater than -1 bar and by dew point hygrometers fo r po tent ia l s less than -1 bar. At each s i t e a 2 m~ x 6 m p lo t on a very gentle slope was chosen fo r instrumentat ion. Due to the stoniness o f the s o i l , a specia l i n s t a l l a t i o n procedure was adopted f o r the hygrometers. A 30epx>30<.criv! trench was dug down to the basal t i l l . The s o i l and coarse fragments were removed in 10-cm layers and stored in 103 p l a s t i c bags to minimize moisture los ses . At s i t e 1, s ix hygrometers were i n s t a l l e d , two each at the 15- and 30-cm depths and one each at the 45- and 60-cm depths. At s i t e 2, three hygrometers each were i n s t a l l e d at the 15-, 30-, 45- , 60-, and 75-cm depths. To i n s t a l l a hygrometer at a p a r t i c u l a r depth, a 0.9-cm diameter s tee l rod was dr iven 30 cm into the s ide of the ho le, the hygrometer was then inserted into the hole and the s o i l replaced and packed around the wire leads. The hygrometers at any given depth were or iented in d i f f e r e n t d i rec t i on s to improve t h e i r averaging c a p a b i l i t y . This i n s t a l l a t i o n procedure a l so ensured that at leas t 30 cm of lead was at the same depth as the hygrometer in order to minimize heat conduction e r r o r s . The s o i l was f i l l e d back in the trench a f t e r i n s t a l l a t i o n at each depth. Digging a trench created some disturbance of the s o i l but the i n s t a l l a t i o n of the hygrometers 30 cm away from the trench wall and the prevention of moisture loss from the dug-up s o i l , minimized the e f fec t s of the d i s turbance. Twenty four hours were allowed a f t e r i n s t a l l a t i o n to ensure e q u i l i b r a t i o n . Hygrometer readings were taken three times d a i l y . At s i t e 2, s o i l water potent ia l was measured on both the dew point and psychrometric modes using 8 hygrometers located at the 15-, 30- , and 45-cm depths over a period of 13.days. So i l water content measurements were ca r r i ed out by means of g r a v i -metr ic sampling and by use of neutron moisture meter (Troxler model 105A). The volumetric s o i l water content was corrected fo r stones using the Reinhart (1961) approach. The water potent ia l s measured by the hygrometers were compared with those determined from s o i l water content and water re tent ion data. 104 Root Xylem Water Potent ia l Measurement Measurements o f root xylem water p o t e n t i a l , ^ywere made on three roots of diameter 3 to 4.5-cm located at a depth of 15 to 20-cm at both s i t e s . To i n s t a l l a root hygrometer, the root was exposed and a l l adhering s o i l cleaned o f f . A s l ant i n c i s i o n was made on the root and then extended ho r i zon ta l l y to form a l i p . The i n c i s i o n was made deep enough to reach the xylem. A f te r the cut surface was l i ned with dry gypsum powder, a hygrometer was then placed a x i a l l y against the xylem beneath the l i p . The cut was sealed by three layers of e l e c t r i c i a n ' s tape and a coating of Dow Corning 781 s i l i c o n e rubber sea lant . The s o i l was then f i l l e d back around the root and 48 hours were allowed fo r e q u i l i b r a t i o n . The root hygrometers were read three times each day at the same time as the so i l hygrometers. On two days at s i t e 1 and on f i v e days at s i t e 2, the root xylem water potent ia l was measured every two hours from predawn to sunset.. Twig water potent ia l s at the 7-m height were measured at the same two-hourly i n te r va l s by the pressure chamber technique (Scholander et al_., 1965). As a check on the performance of the root hygrometers, root water potent ia l was a l so measured by the pressure chamber using root s , 1.5 to 2.0-mm in diameter and 15-cm long dug up from the 15 to 20-cm depth. Hygrometer Ca l i b r a t i on A laboratory c a l i b r a t i o n at two temperatures, 25C and 15C, was done on each hygrometer using d i s t i l l e d water and sodium ch lo r ide so lut ions of 105 known m o l a l i t y . Solut ions used were of m o l a l i t i e s 0.10, 0.30, 0.50, and 0.90 corresponding to water potent ia l s of -4.62, -13.68, -22.81, and -41.58 bars re spec t i ve l y at 25C. The hygrometers were immersed in NaCl so lu t ion in p l a s t i c bot t les which were then immersed in water in a Haake FK-2 constant temperature c i r c u l a t o r . Ba/th temperature was monitored with a Hewlett-Packard 2802A d i g i t a l thermometer.. It var ied by about ± 0.2C from the set temperature. Four hours were a l lowed;for temperature and vapor e q u i l i b r a t i o n . Readings were then taken on the dew point and psychrometric modes. A f te r removal from each NaCl s o l u t i o n , the ceramic cup of each hygrometer was thoroughly washed in several changes of d i s t i l l e d water and then a i r d r i e d . RESULTS AND DISCUSSION  Soi l Water Matr ic Potent ia l Measurement Comparison The water re tent ion curves fo r the various layers sampled at s i t e 1 3 3 are shown in F i g . 1. The water content drops from 0.39 cm /cm at sa tur -3 3 at ion to a range of 0.19 to 0.26 cm /cm at a potent ia l of -0.3 bar as would be expected fo r a g rave l l y sandy loam. The retent ion curves at s i t e 2 are very s i m i l a r . F i g . 2 compares values of tym measured by hygrometer at the 30-cm depth with those obtained from so i l water content and retent ion data at s i t e 1. The so i l water content data used in the comparison was an average F i g . 1 - Water re tent ion curves f o r various layers of Dashwood g rave l l y sandy loam in the unthinned stand ( s i te 1). (Same as F i g . 1 of Chapter I). Tm ( R e t e n t i o n C u r v e ) (bars) -12 -8 -4 0 Fig. 2 - Comparison of soil water-matric potential measured with the dew point hygrometer at site 1 with that determined from measurements of soil water content and the soil water retention curve. 108 fo r the 20 to 30-cm and 30 to 40-cm layers while the re tent ion curve data was fo r the 26 to 39-cm l aye r . There was good agreement between the two methods being with in ± 0.3 bar o f each other (r = 0.99). Good agreement between values of' *j> measured by thermocouple psychrometer and values determined from so i l water content and retent ion data have "been reported e a r l i e r (Chow and de V r i e s , 1973). However, Chow and de V r i e s , who reported agreement to with in +0 .4 bar, made t h e i r measurements in the laboratory. Values of \pm measured on the dew point and psychrometric modes using 8 hygrometers at s i t e 2 over a period of 13 days are compared in F i g . 3. There was good agreement between both modes of measurement being with in ± 0.3 bar o f each other (r = 0.99). Root Water Potent ia l Measurement Comparison Comparison of measurements of ijy made with hygrometers and the pressure chamber i s shown in F i g . 4. The measurements agree to with in ± 0.3 bar (r = 0.99). The v a l i d i t y of $ measurements on excavated roots with the pressure chamber has been shownby De Roo (1969) working on tobacco and H e l l k v i s t et al_. (1974) working on S i t ka spruce. The pressure chamber readings were not corrected fo r xylem osmotic potent ia l s ince values of the l a t t e r were higher than -1 bar. This measurement was made using a Wescor C-52 chamber. Campbell and Campbell (1974) had reported good agreement (r = 0.93) between l ea f water potent ia l measured with a pressure chamber and a l e a f hygrometer f o r various t ree spec ies . F i g . 3 - Comparison of s o i l water matric potent ia l measured on the dew point and psychrometric modes at s i t e . 2. The data were from 8 hygrometers over a period of 13 days. g. 4 - Comparison of Douglas f i r root xylem water potential measured with a dew point hygrometer and a pressure chamber at site 1. I l l In th i s study, the ax ia l placement of the root hygrometer was found preferab le to the rad i a l i n se r t i on because i t permitted eas ier sea l ing and v i r t u a l l y ensured no leaks i f the tape and s i l i c o n e rubber sealant were proper ly app l i ed . The use of gypsum powder ensured that plant res ins secreted at the s i t e of the cut did not penetrate and seal the ceramic cup (P. J a r v i s , personal communication). The gypsum powder d id not a f f e c t the root xylem p o t e n t i a l . This i s because CaS0^.2H 20 is o n l y . s i i g h t l y ion ized and therefore only s l i g h t l y so lub le in water. Thus the osmotic potent ia l of a super saturated so lut ion measured with the dew point hygrometer was -found-to „be3 negTigibly. .d i f ferent from zero Diurnal Var i a t ion in i> and t r-r vm F i g . 5 shows the d iurna l trends in s o i l (30-cm depth), root , and twig (7-m height) water potent ia l s on two days during the growing season at s i t e 1. The so i l water matric potent ia l f o r the 30-cm depth showed the leas t v a r i a t i on over the day. The root water potent ia l showed a d e f i n i t e d iurna l response to evaporative demand. The root water potent ia l was lowest around midday or ear ly afternoon and then gradual ly increased. The twig potent ia l showed a t y p i c a l l y marked va r i a t i on during the daytime r e f l e c t i n g changes in evaporative demand. With t r an sp i r a t i on measurements, res i s tances to water movement through the so i l -p lant-atmosphere system were computed from these measurements of potent ia l during the day. Deta i l s of the res i s tance ca l cu l a t i on s are reported in Chapter 3. The d iurnal trend in the roots ind icates that the response of the hygrometer i s adequate to - 5 - 1 0 ( bars) -15 - 2 0 - 2 5 —1 r~—I 1——I 1 • — D O U G L A S - F I R . COURTENAY, SITE I J L J L 8 12 16 Hours (P.S.T.) 20 5 - Daytime course of soil..water matric potential U^) ' root and twig water potentials (ijy and ty^) on two days at site 1. 113 fo l low diurnal changes in the water potent ia l o f the t ree roots . Seasonal Va r i a t i on in ty and ty T) rm Water potent ia l measurements in the root and at four depths (15-, 30-, 45- , and 60-cm) in the s o i l during the growing season at s i t e 1 are shown in F i g . 6 along with the p r e c i p i t a t i o n data. From June 23 to Ju ly 8, the root water potent ia l was lower than the s o i l water matric potent ia l at a l l depths but was much more responsive than matric potent ia l to the r a i n f a l l between June 26 and Ju l y 4. From.July 9 to 16, the root water potent ia l was higher than the s o i l water potent ia l at the 15- and 30-cm depths. A to ta l of 8.6 cm of p r e c i p i t a t i o n f e l l during that week. During th i s period the twig water potent ia l remained lower than the root water p o t e n t i a l , i nd i ca t i ng that there was p re fe ren t i a l deplet ion of root zone water content. Fiscus (1972) had observed gradient reversa l s in a study of plant water r e l a t i on s in corn. A f t e r the ra iny per iod , the root water potent ia l was again lower than the s o i l water po ten t i a l s . Thus, during both drying per iods , the root water potent ia l remained lower than so i l water potent ia l s at a l l depths. In a d d i t i o n , the potent ia l at the 15-, and 30-cm depths dec l ined more r ap id l y than those at other depths. This was expected s ince the zone of maximum root .dens i ty was the 15 to 30-cm layer . The so i l water potent ia l s at the 45-, and 60-cm depths were higher than those at the 15-, and 30-cm depths throughout the measurement period except fo r the week in Ju ly when the so i l was being r ap id l y wetted. This 114 F i g . 6 - Course of to ta l s o i l water potent ia l (ty) at four depths and root water potent ia l (ty ) f o r the period June 17 to August 19, 1974. 115 resu l ted in upward water movement in the so i l p r o f i l e . Deta i l s o f t h i s are reported in chapter 1. This had been observed by several other workers (e.g. van Bavel e_t al_., 1968). It can be seen from the re su l t s in F i g . 6 that during the en t i re measurement per iod the hygrometers responded well to both the drying and wetting per iods . One hygrometer at the 30-cm depth f a i l e d to funct ion at the end of the ra iny per iod. This was l i k e l y due to contamination of the junct ion resu l t ingyfrom leakage during the heavy r a i n . Hygrometer Ca l i b r a t i on The c a l i b r a t i o n curves f o r one.of the hygrometers at 25C and 15C on both the dew point and the psychrometric modes are shown, i n - F i g . 7. The higher s e n s i t i v i t y of the dew point mode,is very advantageous at low yv outputs. Furthermore, the dew point mode showed a smaller change in s e n s i t i v i t y f o r a given temperature change than the psychrometric mode. A batch of 30 hygrometers used showed a standard dev iat ion of 3% from the mean dew point mode s e n s i t i v i t y o f 0.75 yv ba r " 1 at 25C. CONCLUSIONS Measurements, of s o i l water matric po tent ia l s by hygrometers agreed to wi th in ± 0.3 bar of those determined from s o i l water content and so i l .40 HYGROMETER CALIBRATION DEW POINT 25°C H30 OUTPUT (MV) -120 Fig. 7 - Typical c a l i b r a t i o n curves for a hygrometer on both dew point and psychrometric modes at two temperatures (25C and 15C). 117 water re tent ion data. Also measured on both the dew point and psychro-metric modes showed very good agreement (± 0.3 bar) . Good agreement (± 0.3 bar) between values of root water potent ia l measured by the hygro-meter and pressure chamber was observed. It was found that very care fu l attachment to the root was requ i red . The time response c h a r a c t e r i s t i c of the hygrometers was good enough to monitor d iurna l trends in root water p o t e n t i a l . They are durable, having l a s ted over a growing season. The hygrometers showed less change in s e n s i t i v i t y with the temperature on the dew point mode than on the psychrometric mode. The hygrometer has an important app l i c a t i on in studying t ree response to atmospheric demand and s o i l water a v a i l a b i l i t y . 118 LITERATURE CITED 1. Campbell, G.S. and M.D. Campbell. 1974. Evaluat ion of a thermocouple hygrometer f o r measuring l e a f water potent ia l in s i t u . Agron. J . 66: 24-27. 2. Campbell, E.G., G.S. Campbell, and W.K. Barlow. 1973. A dew point hygrometer for water potent ia l measurement. Agr. Meteorol. 12: 113-121. 3. Chow, T . L . and J . de V r i e s . 1973. Dynamic measurement of s o i l and l e a f water potent ia l with a double l o o p . p e l t i e r type thermocouple psychro-meter. So i l S c i . Soc. Amer. Proc. 37: 181-188. 4. De Roo, H.C. 1969. Water s t ress gradients in plants and s o i l - r o o t systems. Agron. J . 61: 511-515. 5. F i s cus , E.L. 1972. In s i t u measurement of root-water p o t e n t i a l . Plant Phys io l . 50: 191-193. 6. H e l l k v i s t , J . , G.P. Richards and P.G. J a r v i s . 1974. V e r t i c a l gradients of water potent ia l and t i s sue water r e l a t i on s in S i tka spruce trees measured with the pressure chamber. J . App l . E c o l . 11: 637-667. 7. Neumann, H.H. and G.W. T h u r t e l l . 1972. A p e l t i e r cooled thermocouple dew point hygrometer f o r in s i t u measurement of water po ten t i a l s , p. 103-112. In: R.W. Brown and B.P. van Haveren (ed.) Psychrometry in water re l a t i on s research. Utah Agr. Exp. S t a . , Logan. 8. Reinhart, K.G. 1961. The problem of stones in so i l -mo i s ture measurement. So i l S c i . Soc. Amer. Proc. 25: 268-270. 9. Scholander, P.F., H.T. Hammel, E.D. Bradstreet , and E.A. Hemmingsen.. 1965. Sap pressure in vascular p lant s . Science 148: 339-346. 10. van Bavel, ..C.H.M., K.J. Brust, and G.B. S t i r k . 1968. Hydraul ic propert ies of a c l ay loam s o i l and the f i e l d measurement of water uptake by roots : II. The water balance of the root zone. So i l Sci.Soc.Amer.Proc.32:317-321. 119 SUMMARY The water balance and water transport aspects o f t ree water con-sumption were studied in two Douglas f i r stands of d i f f e r e n t stocking dens i ty . In order to carry out th i s i n v e s t i g a t i o n , r e l i a b l e s o i l and root water potent ia l data were considered e s s e n t i a l . Consequently, the f i e l d performance of the dew point hygrometer in measuring s o i l and root xylem water potent ia l s was inves t i ga ted, tym values measured with the dew point hygrometer agreed well with those computed from retent ion curves and s o i l water content measurements. A l so , in s i t u tyr values showed good agreement with values obtained on sampled roots using the pressure chamber. Energy balance estimates of evapotranspirat ion were used as an independent check on water balance est imates. Both methods agreed w e l l . The zone of maximum root water uptake moved progress ive ly deeper into the s o i l p r o f i l e as the s o i l d r i e d . A l so , the amount of water taken up from any s o i l layer depended on both the s o i l water content and the root dens i ty . Thinning was bene f i c i a l in that i t provided about 30% more water fo r the thinned t rees . The rate of water uptake of the Douglas f i r var ied l i n e a r l y with the s o i l to root xylem water potent ia l d i f f e rence i nd i ca t i ng a constant s o i l to root xylem res i s tance . However, the so i l res i s tance increased rap id l y as the s o i l d r i e d . I r respect ive of th i s rapid increase , the s o i l res i s tance remained small in comparison to the root res i s tance.over the range of s o i l water potent ia l s studied (0 to -11 bars ) . This study was ca r r i ed out soon a f t e r th inning when the Douglas f i r canopy was s t i l l open. It would be h ighly des i rab le to carry out a s i m i l a r study in 3 or 4 years time when the Douglas f i r canopy would have c losed sub s t an t i a l l y . 120 APPENDIX I Neutron moisture meter c a l i b r a t i o n curve f o r s i t e s 1 and 2. 122 APPENDIX II Volumetric water content (e) as a funct ion of depth (z) at s i t e 1 during 1974. 123 Volumetric water content (e) as a funct ion of depth (z) at s i t e 1  during 1974 The fo l lowing values are the average f o r four neutron meter access tubes. 6 (cm cm" ) Depth (cm) Ju ly 18 Ju ly 22 Ju ly 28 Aug. 4 Aug. 11 Aug. 17 5 002250 0.200 0.140 0.093 0.080 0.068 20 0.173 0.153 0.138 0.109 0.087 0.080 35 0.170 0.155 0.142 0.119 0.098 0.090 50 0.161 0.157 0.148 0.132 0.116 0.108 124 APPENDIX III Volumetric water content (e) as a funct ion of depth (z) at s i t e 2 during 1975 r 125 Volumetric water content (o) as a funct ion of depth (z) at s i t e 2  during 1975. The fo l lowing values are the average f o r s ix neutron meter access tubes. e (cm cm" ) Depth _ _ _ _ _ _ _ ~ ~ (cm) Ju ly 1 Ju ly 6 Ju ly 11 Ju ly .18 Ju ly 25 Aug. 1 5 0.241 0.200 0.168 0.131 0.118 0.107 20 0.250 0.197 0.166 0.151 0.133 0.119 30 0.192 0.168 0.147 0.134 0.122 0.114 45 0.175 0.161 0.150 0.138 0.128 0.121 60 • 0.181 0.165 , 0.155 0.143 0.133 0.126 75 0.176 0.175 0.163 0.151 0.141 0.134 1 126 / APPENDIX IV Total s o i l water potent ia l {$) at four depths fo r the period June 17 to August 19, 1974 at s i t e 1. 127 Total s o i l water po ten t i a l (ty) at four depths fo r the period June 17  to August 19, 1974 at s i t e 1. The water potent ia l s were referenced to the s o i l sur face. •ty (bars) Date 15 Depth (cm) 30 45 60 June 17 5.8 6.5 5.0 4.2 18 6.9 7,4 5.2 5.2 19 7.8 8.1 6.1 5.2 20 8.0 8.2 6.1 5.2 21 8.2 8.4 * 6.3 5.6 22 8.8 9.0 6.6 5.8 23 9.1 9.2 6.6 6.1 24 9.3 9.3 6.6 ^ 6.4 25 9.6 9.3 7.1 6.4 26 10.9 9,8 7.4 6.8 27 10077. 9.9' 7.4 6.8 28 10] .7.7 10.2 7.4 6.9 29 10.20,2. 10.1 7.7 7.5 30 10.3 9.9 7.6 7.5 Ju ly 1 10.3 10.3 . 7.8 7,4 2 10.5 10.5 7.9 7.7 3 10.5 10.8 8.3 8.3 4 10.7 10.9 8*6 8.3 5 10.8 10.9 8.6 8.3 6 10.8 11.0 8.8 8.0 7 11.0 11.2 8.9 8.4 8 11.0 11.2 9.2 8.0 9 11.2 11.6 9.0 8.4 10 11.4 12.0 8.8 8.8 11 9.5 11.4 5,6- 8.4 12 7,3 10.8 4.9 8.3 13 4.1 9.5 4.55 7.6 14 1.6 8.3 4.8 7.4 15 0.8 6.2 4.8 7.3 16 0.5 5.7 4.5 6.9 17 0.3 0.3 1.5 1.7 18 0.6 0.6 0.7 0.8 Continued . . . . 128 Continued . . . . -i> (bars) Date 15 Depth (cm) ,30 45 60 Ju ly 19 0.8 0.6 0.7 0.8 20 0.9 0.7 0.7 0.8 21 1.0 0.7 0.7 0.8 22 1.0 0.9 0.8 0.8 23 1.1 0.9 0.9 0.8 24 1.2 1.1 0.9 0.8 25 1.2 1.1 0.9 0.8 26 1.3 1.2 1.0 0.9 27 1.3 1.3 1.0 0.9 28 1.4 1.5 1.1 1.0 29 1.4 2.3 1.2 1.0 30 1.5 3.0 1.4 1.1 31 1.6 3.5 1.8 1.1 Aug. 1 2.0 3.4 1.8 1.1 2 2.5 3.8 1.9 1.1 3 2.8 4.2 2.0 1.2 4 3.0 4.4 2.2 1.4 5 3.8 4.7 2.2 1.5 6 4.8 5.2 2.8 2.2 7 5.4 5.3 3,0 2.0 8 6.2 5.5 3.3 2.5 9 7.1 5.7 3.3 2.5 10 7.3 6.5 4.3 2.9 11 8.3 7.5 4.5 3.9 12 8.3 7.6 5.3 3.9 13 8.8 7.9 5.8 4.4 14 10.1 8.1 5.6 4.4 15 10.1 8.1 5.6 4.5 16 10.0 8.5 6.3 5.2 17 10.7 8.9 6.5 5.6 18 11.7 9.4 7.1 6.1 19 11.7 9.4 7.1 6.1 APPENDIX V Total s o i l water potent ia l (ty) at f i v e depths f o r the period June 18 to August 12, 1975 at s i t e 2. 130 Total s o i l water potent ia l ($) at f i v e depths f o r the period June 18  to August 12, 1975 at s i t e 2. The water potent ia l s were referenced to the so i l sur face. -ip (bars) Depth (cm) Date 15 30 45 60 75 June 18 0.8 0.9 0.7 19 1.4 1.3 0.9 0.8 _ 20^ 1.4 1.5 1.2 0.9 0.8 21 1.7 1.6 1.5 1.0 0.9 22 1.6 1.6 1.5 1.0 0.9 23 1.6 1.7 1.5 1.0 1.0 24 2.1 1.8 1.7 1.0 1.0 25 1.4 1.3 - 0.7 0.7 26 0.8 0.8 - 0.5 0,6 27 0.3 0.3 - 0.2 0.4 28 0.1 0.3 - 0.2 0.4 29 0.1 0.3 0.2 0.5 30 0.1 0.3 0.3 0.-5 Ju ly 1 0.2 0.5 0.6 0.5 0.8 2 0.4 1.0 0.8 0.6 0.8 3 0.5 1.6 0.9 0.7 0.8 4 0.7 1.7 1.1 0.8 0.8 5 1.4 2.0 2.1 1.5 1.1 6 1.7 2.0 2.2 1.6 1.2 77 2.1 2.1 2.3 1.6 1.2 8 2.0 2.1 2.4 1.7 1.2 9 2.2 2.1 2.5 2.0 1.2 10 2.2 2.1 2.6 2.3 1.2 11 2.2 2.1 2.7 2.3 1.2 12 2.4 2.3 2.8 2.3 1.2 13 2.5 - 2.4 2.8 2.3 1.2 14 2.6 2.5 2.8 2.3 1.2 15 2.7 2.6 2.9 2.3 1.2 16 3.1 3.2 3.0 2.3 1.4 17 3.7 3.3 3.0 2,3 1.7 18 4.6 4.0 3.6 2.6 1.8 19 5.5 4.7 4.2 3.0 1.9 20 6.0 5.1 4.7 3.5 2.0 21 6.6 5.5 5.3 3.9 2.0 J Continued . . . . 131 Continued -ty (bars) Date 15 30 Depth (cm) 45 60 75 Ju ly 22 7.1 6.2 5.8 4,5 2.4 23 7.3 7.4 6.6 4.9 2.4 24 8.3 7.9 7.5 6.0 2.7 25 8.7 8.5 7.9 6.7 3.2 26 9.1 9.0 8.3 7.5 3.6 27 9.6 9.4 8.7 8.2 4.1 28 10.3 9.7 9.5 8.6 4.9 29 11.0 10.0 10.5 9.2 5.6 30 11.8 11.0 11.1 10.1 6.4 31 12.5 12.0 11.9 11.2 7.5 Aug.. 1 12.4 12.0 . 12.4 11.5 8.0 2 12.2 12.1 13.0 11.8 8.4 3 11.4 . 11.8 12.8 11.8 8.5 4 10.5 11.6 12.5 11.7 8.7 5 10.8 11.9 12.8 12.1 9;;6 6 10.8 11.9 12.7 12.1 9.4 7 10.8 IT.9 12.4 12.2 9.2 8 19.8 12.0 12.7 12.5 9.8 9 IO.O ; 11.8 N 12.8 12.2 9.9 10 1.1.3 12.3 13.2 12.6 10.1 11 12.8 12.7 13.6 13.1 10.3 12 13.3 13.2 14.0 12.7 10.4 132 APPENDIX VI Unsaturated hydraul ic conduct i v i t y as a funct ion of matric water potent ia l fo r Dashwood g rave l l y sandy loam 133 Unsaturated hydraulic conductivity (K) as a function of matric water potential U m ) . for Dashwood gravelly sandy loam The unsaturated hydraulic conductivity was determined in the labor-atory using the transient outflow method. m (bars) K (cm day" 1) 0.002 1.7 0v005 1.7 0.009 1.4 0.011 1.2 0.015 8.7 x 0.017 9.2 x 0.018 7.6 x 0.021 7.2 : x 0.024 6.3 x 0.026 6.1 x 0.030 5.3 x 0.033 4.4i x 0.039 3.7 x 0.045 3.0 x 0.227 6.5 x 0.267 5.7 x 0.294 5.2 x 0.308 3.2 x 0.335 3.0 x 0.376 2.3 x 0.409 1.0 x 0.451 7.7 x 0.557 3.0 x 0.600 2.4 x 1.4 5.7 x 1.6 4.5 x 2.1 2.7 x 2.4 2.7 x 3.2 2.3 x 5.1 1.3 x 5.9 1.1 X 6.0 1.1 X 6.8 7.7 x 7.5 7.4 vx 7.8 7.1 x 8.1 5.6 x 8.8 5.5 x 9.2 5.4 x 10.0 5.3 x 15.0 5.3 x 2 2 2 -2 -2 -2 2 3 3 3 4 -4 4 -4 4 4 4 4 5 5 5 5 5 5 5 5 APPENDIX VII Da i ly micrometeorological energy balance-Bowen ra t i o evapo-t ransp i ra t i on data at the unthinned s i t e ( s i te 1) at various s o i l water potent ia l s (ty) during 1974. 135 Dai ly micrometeorological energy balance-Bowen r a t i o evapotranspirat ion  data at the unthinned s i t e ( s i te 1) during 1974 at various s o i l water  potent ia l s (if;). The as ter i sked (*) days are those:on which de ta i l ed ha l f -hour l y energy balance ca lcu lat ions(were done. The E on the res t of the days were obtained from a p l o t . o f LE/R v e r s u s ^ . Date R n (MJ m-2 day " 1 ) E (mm d a y - 1 ) LE/R n -ty (bars) * June 17 17.66 2.76 0.38 5.8 * 18 18.28 2.69 0.36 6.5 * 19 1 8; 30 3.06 0.41 7.3 * 20 18.60 2.61 0.34 7/4 21 11.78 1.50 0.32 7.6 22 16.49 2.04 0.31 8.1 23 13.67 1.69 0.31 8.3 24 - - - 8.4 * 25 16.29 1.83 0.27 8.7 r 26 2.43 0.27 0.28 9.4 27 11.71 1.33 0.28 9.3 28 7.13 0.80 0.28 9.4 29 4.83 0.55 i 0.28 9.3 • 30 8.62 0.98 0.28 9.3 JuJuly 1 9.36 1.05 0.28 9.5 2 14.01 1.56 0.28 9.6 " 3 10.76 1.16 0.27 9.9 4 11.88 1.28 0.27 10.1 * 5 18.60 2.12 0.28 10.1 * 6 18.60 2.15 0.28 10.2 * 7 16*59 2.03 0.30 10.4 * 8 16.86 2.00 0.29 10.5 9 3.09 0.32 0.26 10.6 10 14.41. 0.45 0.25 10.8 11 1 iii. 5 6 : 1.38 0.30 8.8 12 j8?55 1.09 0.32 7.6 * 13 16.60! 2.18 0.32 4.4 * 14 14.28 2.59 . 0.44 3.2 15 7.62 1.27 0.42 3.9 16 2.11 0.36 0.43 3.6 17 9.97 2.38 0.60 0.7 18 99333 2.21 0.58 0.6 * 19 1613737 3.7 0.56 0.7 * 20 16.44 3.86 0.58 0.8. Continued . . . . 136 Continued .... Rn E. Date, (MJ m-2 day" 1) (mm day - 1) LE/Rn (bars) July 21 8.33 1.93 0.57 0.8 22 9.26 2.15 0.57 0.9 23 12.54 2.87 0.56 1.0 24 14.19 3.18 0.55 1.0 25 14.21 3.39 0.58 1.1 26 14.28 3.10 0.54 1.2 27 14.48 3.19 0.54 1.2 28 14.23 3.00 0.53 1.3 29 14.96 3.16 0.53 1.7 30 13.52 2.80 0.51 2.0 31 14.63 2.73 0.46 2.3 Aug. 1 16.93 3.12 0.45 2.4 2 16.39 3.06 0.45 2.7 3 16.29 2.82 0.42 3.0 4 16.17 2.84 0.44 3.2 5 15.04 2.52 0.41 3.6 6 15.07 2.46 0.40 4.2 7 15.75 . 2.51 0.39 4.6 8 15.70 2.37 0.37 5.0 9 15.34 2.29 0.37 5.4 10 14.26 2.04 0.35 6.1 11 13.87 1.88" 0.34 6.7 12 15.14 1.99. 0.33 7.1 13 15.66 2.03 0.32 7.5 14 5.22 0.66 0.31 7.9 2.45 MJ m~ day" i s equivalent to 1 mm of water day" Latent heat of vaporization of water at 20C i s 2454 J g" 1. APPENDIX VIII Da i ly micrometeorological energy balance-Bowen r a t i o evapo-t r an sp i r a t i on data at the thinned s i t e ( s i t e 2) at various water potent ia l s ( i p ) during 1975. 138 Dai ly mi crometeorological energy ba1 ance-Bowen r a t i o .evapotranspi ra t ion  data at the thinned s i t e ( s i te 2) during 1975 at various s o i l water  potent ia l s (ty). -Date Rn (MJ m-2 day-1) E (mm d a y - 1 ) LE/R n -ty (bars *June 29 16.74 3.37 0.50 0.2 30 17.00 3.54 0.51 0.3 Ju ly 1 17.82 3.74 0.52 0.4 2 17.96 3.23 0.44 0.8 3 17.70 3.59 0.50 1.0 4 18.61 3.71 0.49 1.2 5 18.71 4.40 0.58 1.8 6 18.06 . 3.55 0.48 2.0 * 7 15.21 3.56 0.57 2.0 * 8 15.40 3.02 0.48 2.1 * 9 17.46 3.42 0.48 2.2 * 10 18.29 3.59 0.48 2.3 11 17.95 3.51 0.48 2.3 * 12 18.89 4.16 0.54 2.4 13 10.18 2.35 0.57 2.5 14 4.76 1.20 0.62 2.5 15 9.19 1.80 0.48 2.6 16 9.52 1.78 . 0.46 2.9 17 9.26 1.47 0.39 3.1 18 8.92 2.01 0.55 3.7 19 13.55 2.34 0.42 4.4 20 12.47 1.98 0.39 4.8 21 13.20 2.04 0.38 5.3 22 16.87 ' 2.53 0.37 5;9 23 14.44 2.37 0.40 6.6 24 8.47 1.59 0.46 7.4 25 13.84 • 1.93 0.34 8.0 26 16.57 1.59 0.24 8.4 27 5.91 0.93 0.39 ' 9.0 28 16.03 0.87 0.14 9.5 29 15.64 0.73 0.11 10.0 30 14.58 1.44 0.24 10.1 31 15.25 1.96 0.32 11.0 Continued . . . . / Continued . . . . Date R n (MJ m-2 day-1) E 1 (mm day " 1 ) LE/R n (bars) Aug. 1 5.21 1.42 0.67 11.3 2 5.50 1.05 0.47 11.5 3 5.86 1.42 0.60 11.3 4 11.31 ,1,97/ 0.43 11.0 5 8.97 2.00 0.55 11.5 6 8.67 T . 3 4 " 0.38 11.4 7 7.11 2.,29 0.79 11.3 8 8.31 1.49' 0.44 11.4 9 5.46 1.15 0.52 11.4 10 6.94 l . « : 0.40 •IT. 9 11 14.30 1.43 0.25 12.5 -2 -1 -1 2.45 MJ m day i s equivalent to 1 mm of water day Latent heat of vapor izat ion of water at 20C i s 2454 J g " 1 . The as ter i sked (*) days are those on which evapotranspirat ion were estimated from a p lo t of LE/R n versus ip; APPENDIX IX Comparison of change in trunk water storage with average micrometeorolggical-energy balance evapotransp i rat ion. 141 Tree Water Content 3 -3 I n i t i a l value (Tree No. 1) 1.23 cm cm at s o i l water potential of -0.5 bar. 3 -3 Final value (Tree No. 2) 1.10 cm cm at s o i l water potential of -10 bars. _3 Bulk Density of Wood •= 0.88 g cm Time Interval = 30 days Volume Height D.B.H. (cm3) (m) (cm) Tree No. 1 69,000 9.0 10.5 Tree No. 2 64,000 9.0 9.5 Volumetric change in water content = 0.13 cm cm" . This i s equivalent to 5.90 mg of water s" 1 stem" 1. Average micrometeorological energy balance evapotranspiration for this 2 - 1 - 1 time period = 3.13 X 10 mg of water s stem or 2.27 mm of water day" 1. 142 APPENDIX X . Root dens i ty data at s i t e 1. 143 A. Root dens i ty data in one of the sample locat ions at s i t e 1. The fo l lowing data are t yp i ca l of a l l the locat ions sampled. Root diameter c lass Average diameter Length (mm) (mm) (cm) F inest roots (<0.5) 0.30 2917 Fine roots (0.5-2) 1.00 3800 Small roots (2-5) 3.0 210 Volume of s o i l = 7500 cm d Depth of sample hole = 0 to 40 cm. 144 B. Root dens i ty (L , cm root cm" s o i l ) along f i v e transects at s i t e 1. L v (cm root cm~^ s o i l ) Fract ion of d istance to adjacent trees 1 2 Transect No. 3 4 5 1/8 0.92 1.15 0.79 1.01 0.84 1/4 0.50 0.41 0.56 0.52 0.59 1/2 0.48 0.37 0.47 0.47 0.47 145 APPENDIX XI Root density data in one of the sample locations at site 2. / Root density data in one of the sample locat ions at s i t e 2. The fo l lowing data are t yp i ca l of a l l the locat ions sampled. Root diameter c lass Average diameter Length (mm) (mm) (cm) F inest roots (<0.5) 0.29 2213 Fine roots (0.5-2) 0.95 5750 Small roots (2-5) 2.5 300 146 Volume of s o i l = 13,000 cm Depth of sample hole = 0 to 60 cm. APPENDIX XII Mid-day values of s o i l water potent ia l U $ h root xylem water potent ia l (ty ), twig water potent ia l (ty.), and Douglas f i r t r an sp i r a t i on (E) at s i t e 2 during 1975. 148. Mid-day values of s o i l water po tent i a l ( i | Q , root xylem water potent ia l  (^r)> twig water po ten t i a l ( ^ h and Douglas f i r t r an sp i r a t i on (E) at  s i t e 2 during 1975. Date ys (bars) (bars) (bars) E (mm d a y - 1 ) E D . F . E D.F.+Salal June 30 0.3 10.5 17.0 4i05 • 0.45* Ju ly 1 0.4 12.3 - 4.50 0.45 2 0.8 10.5 20.2 3.69 0.45 3 1.0 11.5 22.1 4.01 0.45 4 _ - - - 0.45 5 1.8 12.0 - 3.87 0.43* 6 1.9 12.2 - 4.43 0.50* 7 2.0 13.0 22.5 4.33 0.45 8 2.1 13.1 22.2 4.33 0.45 9 - - • - - 0.42 10 2.2 10.3 24.1 3.10 • 0.42 11 2.3 10.6 - . 2.96 0.38* 12 2.4 11.0 22.0 3.28 0.41 . 13 2.5 8.8 22.0 2.61 0.39 14 - - . - - • 0.38 15 2.6 8.8 2.20 0.3& 16 2.9 9,1 - 2.00 0.36' 17 3.1 8.0 20.0 1.56 0.36 18 3.7 9.2 - . 1.92 0.35 19 - - - • - 0.34-= 20 4.8 9.3 • -• 1.54 0,34 • 21 5,3 10.6 1.80 0.32 22 5.9 -10.9 ' 22.0 1.94 0.31 • 23 6.6 11.5 23.1 1.61 0.31 24 7.4 11.4 21.2 1.57 0.30 25 8.0 12.1 21.8 1.63 0.29 26 8.4 11.5 - 1.59 0.29/ 27 9.0 11.5 21.5 1.16 0.28 28 - - - • - 0.33* 29 -• - - - 0.26* 30 10.1 13.1 21.2 1.0 0.22* 31 11.0 13.3 - 1.0 0.24, : Aug. 1 11.3 13.3 - 1.0 0.24 -The as ter i sked <*> E D . F . ^ E D.F.+Sala l r a t i o s were taken from Tan (1977). u•I• u • 1 • 1 J U I W I The other values were then extrapolated from these. APPENDIX XIII Ca l i b r a t i on data for a batch of 30 hygrometers using 0.30 molal NaCl so lu t ion (-13.68 bars at 25C and -13.21 bars at 15C). at two temperatures (25C and 15C) on both the dew point and psychrometric modes. 150 Calibration data for a batch of 30 hygrometers using 0.30 moial NaCl  solution (-13.68 bars at.25C and -13.21 bars at 15C) at two temperatures  (25C and 15C) on both the dew point and psychrometric modes. Se n s i t i v i t y (pv bar" ) Dew Point Mode Psychrometric Mode No. Temperature Temperature 25C 15C 25C 15C 1 0.74 •, "0.72 0.41 ••: 0.28 2 0.77 0.71 0.41 ' 0.29 3 =0.74 0.68 0.39 0.29 4 0.74 0.67 0.41 0.29 5 0.77 0.73 0.41 0.30 6 0,73 0.68 0.38 0.25 7 0.74 0.69 0.38 0.25 8 0.73 0.70 0.38 0.26 9 0.75 0.70 0.39 0.28 10 0.73 0.68 0.39 0.27 11 0.72 0.70 0.37 0.28 12 0.73 0.72 0.38 0.25 13 0.73 0.70 0.38 0.28 14 0.75 0.73 0.38 0.28 15 0.73 0.69 0.39 0.29 16 0.74 0.70 0.38 0.26 17 0.73 0.70 0.39 0.27 18 0.74 0.68 0.38 0.26 19 0.77 0.73 0.39 0.27 20 0.75 0.70 0.37 0.25 21 0.77 0.75 0,37 0.28 22 0.75 0.72 0.40 0.30 23 0.77 0.72 0.40 0.30 24 0.77 0.72 0.38 0.29 25 0.77 0.73 > 0.40 0.30 26 0.77 0.73 0.39 0.30 27 0.77 0.72 0.37 0.26 28 0.76 0.70 0.40 0.28 29 0.77 0.73 0.39 0.28 30 0.75 0.73 0.39 0.28 \ 151 APPENDIX XIV B r i t i s h Columbia showing the study area 152 APPENDIX XV Mean annual values of temperature and p r e c i p i t -at ion based on the period 1941-1970 for the Comox-Courtenay area. 154 i Mean annual values of temperature and p r e c i p i t a t i o n based on the period  1941-1970 for the Comox-Courtenay area. Mean Mean Mean Mean Mean Mean Total Temperature . Maximum . Minimum Ra in fa l l Snowfall P r e c i p i t a t i o n (°C) Temperature Temperature (cm) (cm) ' (cm) (°C) (°C) 9.3 13.4 5.2 124.4 112.8 135.6 

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