UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Tolerance in Western hemlock Keller, Rodney Alan 1973

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Item Metadata


831-UBC_1973_A1 K44_5.pdf [ 9.29MB ]
JSON: 831-1.0101047.json
JSON-LD: 831-1.0101047-ld.json
RDF/XML (Pretty): 831-1.0101047-rdf.xml
RDF/JSON: 831-1.0101047-rdf.json
Turtle: 831-1.0101047-turtle.txt
N-Triples: 831-1.0101047-rdf-ntriples.txt
Original Record: 831-1.0101047-source.json
Full Text

Full Text

( TOLERANCE IN WESTERN HEMLOCK *y RODNEY ALAN KELLER B.A., University of British Columbia, 1954 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in the Department of BOTANY We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA In presenting t h i s thesis i n p a r t i a l f ulfilment of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t freely available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department The University of B r i t i s h Columbia Vancouver 8, Canada Date S l j f / ® / 7 3 i ABSTRACT Needle anatomy, water response and gas exchange characteristics of juvenile ecophenes or habitat forms of Western hemlock (Tsuga hetero-phylla) from sites of varying degrees of exposure were compared and con-trasted. This was done to gain information about the seemingly broad adaptative spectra displayed by ecophenes of this species within its natural habitat. Some differences were found in needle anatomy and chlorophyll content; the ratio of needle area to needle dry weight increased with the amount of shelter and chlorophyll content was less in the exposed ecophene. A scanning electron microscope was used to observe needle surfaces and cross sections were examined through a light microscope. The exposed ecophene had a thicker cuticle, thicker cell walls, more surface wax and a more circular cross section than sheltered eco-phene . A series of experiments, both in the field and in the labora-tory was run to test the postulate that response to changes in environ-mental water demand was different between the ecophenes. Results supported the postulate and suggested that the sheltered ecophene was unable to control transpirational loss under high evaporative demand. A relationship was established between carbon uptake and rela-tive turgidity. Laboratory experiments were run with juvenile whole trees to test the postulates that diffusion resistances, carbon uptake, light compensation, and carbon uptake under different degrees of evaporative demand were different for the two ecophenes. i i Carbon uptake by the sheltered ecophene did not reach light saturation as rapidly as uptake by the exposed ecophene. Increase of carbon uptake by the sheltered ecophene vas probably limited by diffusional processes. Increase in uptake by the exposed ecophene at high light was limited by non-diffusional processes. Although the shade ecophene displayed a higher carbon uptake and light saturation intensity than the sun ecophene on a unit area basis, the two ecophenes were statistically inseparable when uptake was expressed as a percentage of maximum uptake per tree. Lowered relative humidity lowered carbon uptake and raised transpiration rates of both sun and shade ecophenes. i i i ACKNOWLEDGEMEINTS The author gratefully acknowledges the support and direction provided by his supervisor, Dr. E. B. Tregunna. Special thanks are given to Dr. V. J . Krajina for his continued interest in the research project and further for his assistance with the loan of critical material. Dr. D. J . Wort provided scientific guidance and gave considerable editorial assistance with the preparation of the manuscript. Dr. G. H. N. Towers provided useful advice and direction while Dr. Jack Walters donated much of his time in the i n i t i a l phase of the research and made available the f u l l facilities of the University Research Forest. In addition I thank a l l the many persons in the Departments of Botany and Zoology who donated time and materials. iv Table of Contents INTRODUCTION 1 CHAPTER 1. STUDY SITES 4 1.1 Introduction. 4 1.2 Site description . . . . . 5 CHAPTER 2. GROWTH HABIT AND NEEDLE CHARACTERISTICS 8 2.0 Introduction. 8 2.1 Growth habit and morphology 8 2.2 Needle area, needle area dry weight ratio and chlorophyll content 9 2.3 Leaf anatomy. 11 2.4 Conclusion l8 CHAPTER 3. THE COMPARISON OF MOISTURE STRESS IN SUN AND SHADE ECOPHENES 23 3.1 Introduction 23 3.2 Measurement of 24 3.3 The comparison of relative turgidity with xylem 26 3.4 Carbon uptake and relative turgidity. 29 3.5 The effect of f u l l sunlight on shade ecophene 30 3.6 The effect of increasing exposure on shade plants 38 3.7 Light and air movement as combined factors 39 3.8 Summary and conclusions 5^ CHAPTER 4. GAS EXCHANGE CHARACTERISTICS 47 4.1 Carbon uptake and light adaptation 47 V 4.2 Diffusion resistance and light adaptation 50 4.3 The division of leaf resistances 53 4.4 Diffusion resistance and environment... 56 4.5 Relative humidity and photosynthetic rate 57 4.6 Materials and methods 58 4.7 Results 68 4.8 Conclusions 79 CHAPTER 5. SUMMARY 82 APPENDIX 83 LITERATURE CITED 85 vi List of Figures 0.1 Tip blight in western hemlock 2.1 Typical juvenile shade trees 2.2 Typical juvenile sun tree 2.5 Electron micrograph of abaxial surface of exposed ecophene needle 2.6 Electron micrograph of abaxial surface of sheltered ecophene needle 2.7 Electron micrograph of an adaxial surface of an exposed needle. 2.8 Electron micrograph of adaxial surface of sheltered ecophene needle 2.9 Electron micrograph of cut through adaxial surface of sheltered ecophene needle 2.10 Electron micrograph of stomata of exposed ecophene needle 2.11 Electron micrograph of inner surface of stomata 2.12 Micrograph of cross section of abaxial surface of exposed ecophene needle 17 2.13 High power view of stomata in f i g . 2.12 19 2.lk High power view of stomata from sheltered ecophene 19 2.15 Cross section of exposed ecophene needle. 20 2.16 Cross section of sheltered ecophene needle 20 2.17 Cross section of abaxial epidermis of exposed ecophene needle 21 2.18 Cross section through abaxial epidermis of needle from sheltered ecophene 21 3.3 Relative turgidity and drought 31 3.4 Relative turgidity and relative net C02 uptake 32 .3 10 10 Ik Ik 15 15 16 16 v i i 3.5 Effect of evaporative stress on shade ecophene 35 3.6 Infrared photo of freshly exposed shade tree 36 3.7 Infrared photo of a shade tree after 4 days of exposure 36 3.8 Infrared photo of a sun tree ..37 3.9 Evaporation and relative turgidities from Sites 5 and 7 3.10 The arrangement of air flow and lighting on a plot in site 7 - 4-1 3.12 Evaporation from 6 areas in site 7 43 3.13 Relative turgidity of trees in 6 areas of site 7 44 4.1 Diagram of a closed system for measurement of gas exchange 60 4.2 Spectral distribution of G.E. "cool beam" 62 4.3 Circuit for a "homemade" thermocouple welder 63 4.4 Performance of a thermocouple used in measurement of leaf temperature 65 4.5 Photograph of paper model of tree 65 4.8 The effects of air flow and light intensity on leaf temperature in shade ecophene 72 4.9 The effects of air flow and light intensity on leaf temperature in sun ecophene 73 4.10 The effects of air flow and light intensity on net CO2 uptake in shade ecophene 74 4.11 The effects of air flow and light intensity on net C02 uptake in sun ecophene 75 4.12 The effect of light intensity, relative humidity and ecotype on rate of CO2 uptake 76 4.13 The effect of light intensity, relative humidity and ecophene on transpiration and transpiration resistance 77 v i i i List of Tables 2.3 Needle area and needle area to weight ratios 12 2.4 Chlorophyll content of 2 ecophenes 12 3.1 Diurnal flux of relative turgidity 27 3-2 A comparison of R.T. and xylem 28 3.11 A comparison of total evaporative potential with average relative turgidity 42 4.6 The effects of air speed, relative humidity, light intensity and ecotype on leaf temperature 68 4.7 Rates of transpiration and percent of total radiation expended in transpiration 70 4.14 Resistances to C02 flux at 432 ki'lolux 79 INTRODUCTION 'Tolerance1 may be described as the ability of a plant to survive under a forest canopy or in the shade of other trees and does not refer specifically to 'shade tolerance' (Baker, 195°* Decker, 1950). On the other hand, shade tolerance refers only to a plant's ability to live under a particular light regime. The term tolerance suggests the possibility of more than one variable imposing limits on plant survival. Such limiting variables would be light, water, heat, soil pH etc. It should also be noted that tolerance within a species may be relative to the climate of the area; Douglas-fir (Pseudotsuga menziesii var menziesii) in moist climates is intolerant, whereas in a subhumid climate, the same species is tolerant and i t becomesshade requiring in xeric and subxeric hygrotopes (Krajina, 1969). This thesis reports on the importance of two variables, light intensity, and atmospheric evaporative potential to establishment of juvenile Western hemlock (Tsuga heterophylla). In addition, i t reports on differences in anatomy, cytology, physiology and pigmentation of exposed and sheltered habitat forms or fecophenes' (sensu Daubenmire 19&5) of this species and relates variations in these differences to adaptive mechanisms to the above variables. The major postulate of the thesis suggests that tolerance in Western hemlock trees is inseparably related to both light interception and water conservation, and that survival in deep shade necessitates sacrifices of the ability to retain moisture under stress conditions, i.e. a narrowing or readjustment of adaptive spectra to f i l l a highly specialized niche. 2 Several of my observations tend to support this postulate: 1. Western hemlock, when growing in the Coastal Western Hemlock Zone, displays a wide range of habitats, from deep within a dense Red alder (Alnus rubra) thicket where light intensity seldom exceeds 5^ 0 lux, to the top of tree stumps on exposed southern slopes where f u l l unshaded sunlight may be accompanied by a desic-cating 10 to 20 m.p.h. breeze. 2. When a stand of trees is logged off, the Hemlock trees and seedlings that were under the canopy 'scorch' and die. 3. If nursery grown Hemlock seedlings are transplanted in exposed sites they develop 'Red tip1 or 'Tip blight' (Baker, 195l)> a condition of the needle where the tip desiccates and dies (see f i g . O.l). This condition is caused by water stress (Kozlowski, 1968). Clearly, there is no implication that other environmental gradients associated with the transition from exposed to sheltered habitat are not critical to the trees' environment. The total picture of tolerance in the species will not be complete until adaptation to these gradients, particularly edaphic gradients, is investigated. Krajina (1969) classes Western hemlock as a submesotrophic stenotrophophyte growing well on soils poor in nutrients and mesic to submesic in water content. These soils are usually podzols to brunisols, lacking calcium and rich in ammonium ion rather than nitrate. When seedlings are found in richer sites they are frequently established on decayed wood rather than in the rich s o i l . Figure 0.1. Parch blight in Western hemlock. CHAPTER 1 STUDY SITES 1.1 Introduction A l l study sites were in the University of British Columbia Research Forest at Haney, B.C. (see Appendix). Site selection was also confined to altitudes under hOO meters; a l l were in the Coastal Western Hemlock Biogeoclimatic Zone of the Pacific Coast Mesothermal forest (Krajina, 1965). The primary criterion for differentiating exposed from shel-tered site was the degree of shading from direct daylight. A l l exposed sites were under open sky, while sheltered sites were under closed forest canopies and exposures, except for fleck light, were between twenty and one percent of f u l l sunlight. In view of the emphasis on atmospheric desiccation, a further index of degree of exposure was given by modified piche evaporimeters (Waring and Hermann, 1966). As there appears to be no procedure that satisfactorily relates evaporimetric data to any standard meteorological units (Piatt and Griffiths, 196!+), evaporimetric readings were relative only to each other and are expressed as percentages of the maximum. A l l evaporimeters were at seedling height, 15 cm above the forest sur-face. Two evaporimeters were used in each site. Evaporimeters were not enclosed in meteorological screens but were kept in the open where they were subjected to unrestricted air currents and turbulence (Piatt and Griffiths, 19&-). In general, an exposed and a sheltered site were selected 5 within 100 meters of each other to facilitate simultaneous testing and minimize topographic and geological differences. 1.2 Site Description 1.2.1 Sites 1 and 2 (See map, Appendix l) Site 1 was situated under a dense 5 to 1 year old Red alder (Alnus rubra) thicket on a southerly exposed logging slash. This site was very shaded and light was seldom more than 1$ of daylight. Evapor-imetric data indicated a very low evaporative demand with an average weekly evaporation of 9.1 cubic centimeters for the month of July, 1968. Site 2 was approximately 50 meters from site 1 and was situ-ated on the south side of a low rise surrounding a tree stump. Only loose mineral soil was present and this had been strongly disturbed during the logging operation. Site 2 was exposed to f u l l daylight and considerable evaporative stress, and the evaporative index was the highest recorded for July, 1968 (100$ of maximum) with an average weekly evaporation of 19-1 cc. 1.2.2 Sites 3 and 4 (See map, Appendix l) Sites 3 and k were situated in the southern half of the research forest at an altitude of approximately 360 meters. Site 3 "was located on an old logging road and trees were growing in the well drained road gravel, sand and subsoil particles of unused roadbed. This site was exposed to wind and had some late afternoon shading from a nearby mixed immature stand of Western hemlock 6 and Douglas f i r . Evaporative potential was high. The average weekly-evaporation for July 1968 was 18.7 cc or 98-5$ of the maximum for the sites. Site h was 80 meters north of site 3 and was under the canopy of the above stand. Trees were growing on decayed wood in moist, dis-turbed s o i l . Light intensity reached 20$ of f u l l sunlight on a clear day and considerable fleck light of nearly 80$ f u l l daylight intensity reached the stand in the early morning. Evaporative potential was moderate, the weekly average for July, 1968, being 13.0 cc or 68$ of maximum. 1.2.3 Sites 5 and 6 (See map, Appendix l) Sites 5 and 6 were in the southern half of the forest at an altitude of approximately kOO meters. Site 5 was situated on top of a one meter diameter stump. The stump was on a southerly exposed logging slash. This site was continu-ously exposed to f u l l sunlight. The evaporative potential was high with an average weekly evaporation for the period extending from June 17 to July 17, 1970, of 35.0 cc or 100$ of the maximum 1970 readings. Site 6 was approximately 180 meters west of site 5 and was well within a closed canopy stand of Western hemlock and Douglas f i r . Trees in this site were generally rooted on fallen decayed tree hunks or on well rotted stumps. Average weekly evaporation for the same period as site 5 was 11.7 cc, or 32.2$ of the evaporation of site 5. 7 1.2.h Site 7 (See map, Appendix l) Site 7 was some 100 meters from the Blaney Lake road at an altitude of approximately 3^0 meters. The site was in a closed stand of Western hemlock and Red. cedar (Thuja piicata). The slope was northerly, and trees were rooted both in the forest soil and in the well rotted split logs used in the construction of an old abandoned wagon road. This site was very sheltered with a weekly evaporation of 7.1 cc or 20$ of site 5. CHAPTER 2 GROWTH HABIT AND NEEDLE CHARACTERISTICS OF SUN AND SHADE ECOPHENES OF WESTERN HEMLOCK 1.0 Introduction Moisture status, light intensity and temperature can modify the morphology and leaf anatomy of a plant (Wilson and Cooper, 1969a; Wilson and Cooper, 1969b; Kramer, 1962; Wassink et a l , I956; and Mitchell, 1953). In addition, Bjorkman and Holmgren (1963) and Loach (1967) have found that substantial differences in total chlorophyll content are associated with differences in light intensity, while Todd and Barker (I965) found a negative correlation between moisture stress and chloro-phyll content. This chapter describes experiments designed to determine the relative magnitude of morphological and anatomical differences between exposed and sheltered ecophenes of Western hemlock. 1*1 Growth Habit and Morphology Gross differences between sun and shade ecophenes are readily observable. Many of the features of the sun ecophene are characteristic of the plant grown under water stress (Kramer, 1962) : reduced growth, smaller needles, thicker needles and reduced stem elongation."'" In addition to this, exposed ecophenes are a light yellowish green, generally upright and much less flexible than the deep green, It should be noted that some of these characteristics are symptomatic of inorganic nutrient deficiency as well as chronic water deficiency. inclined, lax, sheltered trees. Illustrations of sheltered and exposed types may be seen in the photographs of figures 2.1 and 2.2. Rooting habit was cruite varied. Exposed plants grew in tree stumps, drained gravel road beds, soil pockets and rocky outcrops. Sheltered trees, however, were nearly always rooted on, or in close association with, decayed wood. Roots of the sun ecophene were thick, relatively deep, sinewy and difficult to remove from the rooting media while roots of the shade ecophene were thin, shallow, widely branched and very susceptible to breakage. This agrees with Shirley (194-5) w n 0 found that leaf development takes place at the expense of root development with plants growing in deep shade. 2.2 Needle Area, Needle Area-Dry Weight Ratio and Chlorophyll Content Leaf growth and development are affected by high light. Blackman and Wilson (1954) found the ratio of leaf area to leaf weight of Helianthus annuus to be inversely and logarithmically proportional to light level, while Brix (1967) reports that an increase in light intensity decreased the ratio of leaf area to leaf weight in seedling Douglas f i r . Chlorophyll content also varies with environment. Bjorkman and Holmgren (1963) investigated chlorophyll content in various ecophenes of Solidago virgaurea and found that plants grown in the shade had a higher chlorophyll content (on a fresh weight basis) than those grown in the sun, while Loach (1967) shown this tendency for several tolerant and intolerant species. Figure 2.1. Typical juvenile shade trees of Western hemlock. 11 2.2.1 Methods Six juvenile Hemlock trees between two and seven years of age were taken from each of sites 5, 4, and 7> representing a high, medium and low exposure site respectively. A l l needles were removed from a particular tree and placed into one of approximately 11, equal size groups. A dissecting microscope was used to measure length and width of 8 randomly selected needles from each group. Where there were less than 8 needles in a group, the entire group was counted. Needle area was established using the constant given by Barker (1968). Total leaf surface area was computed and the ratio of area to total oven dried (85 C) needle weight was established. Average leaf area per tree was also computed. Chlorophyll determinations were made on 8 trees from each of exposed site 3> exposed site 5, and sheltered site 7• Pigment extrac-tion was with acetone-water and determination of total chlorophyll followed the method of Strain and Svec (1966). 2.2.2 Results The mean area and area-to-weight ratios are shown in Table 2.3. Clearly there is a progression from needles of low area with small area to weight ratio, i.e. a small thick needle to a large, low density needle, with the change from exposed to sheltered habitat. Total chlorophyll data are shown in Table 2.U-. The data agree with the current literature, the sheltered site ecophene having Table 2.3 Average needle area (cm2) and needle area to weight ratio for 3 sites. Site 3 - exposed, site 5 = moderate exposure, site 7 = sheltered. Site 3 Site 5 Site 7 Av Av Av Tree Area cm2/gm Area cm2/gm Area cm2/gm 1 0.77 96.7 1.50 117.5 I.87 176.4 2 O.69 84.8 1.33 120.0 I.92 201.9 3 0.82 90.6 1.28 135-8 1.84 220.3 4 0.64 82.1 O.89 127.7 2.04 196.5 5 0.77 89.6 1.09 133.9 1.66 187.3 6 O.89 85.4 1.47 140.6 2.12 177.8 MEAN 0.74 88.2 1.26 122.3 1.92 191.7 considerably more chlorophyll per gram leaf weight than the exposed ecophene. Table 2.4 Total chlorophyll of ecophenes from sites 3 and 7. Total Chlorophyll in rag/gm Fresh Weight Tree Site 3 (Exposed) Site 7 (Sheltered) 1 O.76 I.83 2 0.73 1.64 3 O.76 1.93 4 0.81 2.12 5 0.79 1.80 6 0.90 1.76 7 O.72 2.09 8 0.69 2.11 MEAN 0.77 1.91 2.3 Leaf Anatomy 2.3.1 Leaf Surface and Stomata The boundary of any hydrated structure with a high to volume ratio is of particular importance in maintaining a surface area favorable water balance against a vapor pressure gradient. Conifer needles have an epidermis and cuticle that present a barrier to uncontrolled diffusion. Gaseous exchange between leaf and air is primarily controlled by the stomata. Scanning electron micrographs of the abaxial needle surface of both ecophenes show a continuous layer of cuticle contoured over the long, somewhat tubular epidermal cells (figures 2.5 and 2.6). The needle of the shade ecophene has a smoother surface with considerably less wax than the rougher sun ecophene. The adaxial surface is shown in figures 2.7 and 2.8. Stomata are confined to the adaxial surface. The adaxial cuticle extends over the stomatal pore. Free passage to the pore is afforded through cracks in the cuticle, usually around the sides of the stomatal ridge so that the cuticle becomes a raised plug suspended over the stomatal pores. This acts as a diffusion baffle to the stoma. This may be seen in figures 2.9 and 2.10. Stomata in both ecophenes are sunken, the accessory cells forming the distal part of the cavity. The inner surface of the stomata is shown in figure 2.11. Note the prominent guard cell ridges that are used to close the pore. The short rod like objects clustered along the ridges and on the guard cell walls are probably bacteria. In cross section, the stomatal geometry can be clearly seen; this is shown in figures 2.12, 2.13, and 2.1k. Guard cells of both ecophenes contain chloroplasts, and the guard cell ridges display a characteristic beak-like appearance. Wax plugs can be clearly seen in stomata of both ecophenes. The clear space between the accessory cell wall and the plug may well be the pathway of gaseous exchange. Figure 2 .5 . Electron micrograph of abaxial surface of exposed ecophene needle (X 24-750). Figure 2.6. Electron micrograph of abaxial surface of sheltered ecophene needle (X 24-500). Figure 2.7. Electron micrograph of adaxial surface of exposed ecophene needle (X 290). Figure 2.8. Electron micrograph of adaxial surface of sheltered ecophene needle (X 2^0). 16 Figure 2.10. Electron micrograph of stomata of exposed ecophene needle (X 5 0 0 0 ) . 17 Figure 2 . 1 2 . Micrograph of cross section of adaxial surface of exposed ecophene needle showing overall arrangement of epidermal cells (X 5 0 0 ) . 18 2.3.2 Leaf Geometry and Cell Wall Thickness Leaf cross sections of the two ecophenes show a marked dif-ference in leaf geometry and cell wall thickness. This may he seen in figures 2.13 to 2.18. The sun ecophene has a rounder cross section along with thicker cell walls. 2.3.3 Wax and Cuticle The cross sections shown in figures 2.17 and 2.18 show the needle of the shade ecophene to have a thinner, more contoured cuticle along with less wax than the sun ecophene. No explanation is given of the cystolith-like crystals in the upper layer of the cuticle. 2.3.4 Chloroplast Content Fresh sections show considerably more chloroplasts in palisade and mesophyll tissue of the shade ecophene. The sun ecophene has chloro-plasts in the same tissues as the shade ecophene; however, the chloro-plast frequency per cell is lower. 2.4 Conclusion A comparison of sun and shade ecophenes shows the sun ecophene to have many of the qualities of the chronically moisture stressed plant: reduced stem elongation, short thick needles, and thicker cell walls. The lowered chlorophyll content could also be the result of moisture stress (Todd and Barker, 1965; Hemming, 1965; and Sestak and Vaclavik, 1963). However, Bjorkman and Holmgren (1963) found lowered 19 Figure 2.1k. High power view of stomata from sheltered ecophene (X ll8o). 20 Figure 2 . l 6 . Cross section of sheltered ecophene needle. Figure 2.17. Cross section through abaxial epidermis of exposed ecophene needle (X l l 8 o ) . Figure 2.18. Cross section through abaxial epidermis of needle from sheltered ecophene (X l l 8 o ) . 22 chlorophyll in high light grown plants that were raised without evident moisture stress. The leaf area-weight data agree with the previous findings of Brix (1967) and Blackman and Wilson (195*0. In addition the dispersion of the scores around the means suggests that the means can "be used to compute area. 23 CHAPTER 3 THE COMPARISON OF MOISTURE STRESS IN SUN AND SHADE ECOPHENES OF WESTERN HEMLOCK 3.1 Introduction Studies of shade tolerance in plants have largely neglected comparisons of moisture status despite the obvious differences in heat balance or moisture status between sun and shade environments. Because the damage noted in Chapter 1 suggested "parch blight" or "red belt" (Boyce, 196l), a condition which results from high water stress, i t was decided to provide an array of experiments to test the null postulate that ecophenes from sun and shade habitats exhibit similar moisture stresses in response to similar climatic variation. Plant water stress or water deficit implies less than turgid cells or tissues (Kramer, 1969) a n <^ W be described in terms of water content {-&-) or energy status of the contained water . Reliance on -0~ as a parameter has been criticized by Kramer and Brix (19^5) and Weatherly (1965) in favor of the energy status para-meter Iff . However, Slatyer (i960) found different species may have widely different relative water content (50$ and 90$) but the same (-20 bars) and states that at least in the case of high water deficits -0-is more appropriate than . In this work, measurements were generally of as the tech-nique used was less destructive. However, and were compared to assess the feasibility of measuring each parameter in the fie l d . 3 -2 The Measurement of -Q» The parameter-0- is described by the relative turgidity (R.T.) (Weatherly, 1950) of the needles. R.T. expresses the water content of the fresh needle as a percentage of the water content of the saturated leaf, i.e., R.T. = fresh weight-dry weight (l) saturated weight-dry weight The value is easily converted to water saturation deficit.''" 100-R.T. = water saturation deficit (2) Harms and McGregor (1962) and Clausen and Kozlowski (1965) have measured R.T. in conifers. Both of these papers suggest a minimum leaf weight of .5 grams which would require 25 to 35 hemlock needles and thus severely limit the number of samples that could be drawn from a single juvenile tree. The first experiment was designed to show the feasibility of determining R.T. with 8 needles with total weights considerably less than .5 grams. Method Four dormant field grown Western hemlock seedlings from Green Timbers Provincial Nursery, Surrey, B.C., were used. The two-year-old seedlings were transplanted into gallon crocks of s o i l , transported to "Heater deficit is that deficit defined by the two basic parameters -©-and "fr ; water saturation deficit is confined to that deficit described by-©-only (after Barrs, I968). 2 5 a greenhouse at the University of B.C., and acclimatized there for 3 months. At the time of use dormancy had broken and the seedlings had flushed. On the day of sampling, the soil was saturated with water at 9 A.M. At 11:30 A.M., 2:30 P.M., and 6:30 P.M., 8 needles were picked from each of three branches of each of four trees. Each group of needles was immediately weighed and placed tip up in water-saturated polyurethane sponges in petri dishes. After kQ hours the needles were removed, blotted once on toilet tissue, weighed, and placed in an oven at 105 C. After 2k hours the needles were weighed and R.T. was calculated for each sample (see chapter introduction). An analysis of variance was used to test the following null postulates: ( = .05) 1. R.T. values within each tree were the same. 2. R.T. values among the trees were the same. 3. R.T. values through the day were the same. Results and Discussion Results are shown in Table 3«1« Means within trees and among each tree appear quite similar and were.not significantly different. The means through the day were significantly different, the 2:30 means being considerably lower than the other means. The method uses less than half the needle weight required for the previously mentioned methods and yet furnishes reasonably consistent within and among tree means and is sensitive to a midday rise in water deficit as would be expected to occur (Barrs, I968). 26 3.3 The Comparison of Relative Turgidity with Xylem Slatyer and McElroy (196l) and Brix (1970) suggest that measurements of water deficit based on relative water content lack sensitivity at high values. This experiment was designed to estab-lish a relationship between R.T. and xylem Sunder field conditions and further to test the postulate that this relationship was the same for both sun and shade ecophenes. Method Trees in this experiment were naturally seeded 2-to k-year-old hemlock from site 3 (exposed) and h (sheltered). Two trees from each habitat were used in each sample. R.T. was measured as described in the previous experiment and xylem potential was determined with a pressure bomb as described by Scholander et a l (1965). The procedure consisted of picking ca. 10 needles just below the leader flush. While these needles were being weighed and placed in polyurethane, the stem was cut off an inch from the ground and the severed end pushed through the sealing ring of the removable cap. In this way, when the cap was screwed down on the bomb, the shoot was inside the bomb with the fresh-cut stem sticking through the cap. Air pressure inside the bomb was gradually raised (10 psi/sec) until xylem fluid became visible at the cut surface of the stem. A record was kept of the relative humidity and light intensity throughout the day. Table 3.1 Diurnal flux of relative turgidity in juvenile Western hemlock. T = tree, S = sample, frwt = fresh weight, satw = saturated weight and dryw = dry weight. A l l weights are milligrams and R.T. is expressed as a percentage. 11:30 A.M. 2:30 P.M. 6:30 P.M. T S frwt satw dryw RT frwt satw dryw RT frwt satw dryw RT 1 1 15.6 18.4 5.2 73 22.7 26.2 16.6 78 23.3 25.2 9.4 88 2 16.2 17.8 6.5 86 21.5 24.8 15.6 79 17.8 20.3 7.1 81 3 13.8 15.7 5.6 82 18.4 20.8 7.9 81 17.8 19.5 6.7 86 TREE AV. 80 79 85 2 1 22.2 23.2 8.7 88 25.0 29.8 10.8 75 23.9 25.4 9-3 91 2 22.8 23.6 8.3 89 29.6 32.5 12.6 85 16.9 19.3 6.7 88 3 26.2 28.1 9.6 90 28.8 32.2 12.8 77 20.8 22.1 8.0 91 TREE AV. 89 79 90 3 1 22.3 24.8 8.7 85 20.6 24.8 16.0 74 17.0 19.7 6.8 89 2 29.3 32.8 11.3 84 28.8 32.2 12.8 81 20.4 22.0 8.0 89 3 22.9 24.7 8.9 88 24.7 30.5 10.6 71 20.8 22.8 8.0 91 TREE AV. 86 75 90 4 1 26.2 28.5 10.0 88 23.0 26.4 9.0 80 23.0 24.6 8.7 90 2 25.8 28.7 10.5 85 21.2 23.7 8.4 84 31.4 33.4 12.7 90 3 30.0 32.5 11.4 88 24.0 27.4 10.8 81 22.4 25.4 9.1 85 TREE AV. 87 82 88 TOT. AV. 87 79 88 Results and Discussion The results are shown in Table 3-2. Relative turgidity in the exposed site rose to 90.5$ at 2:30 P.M. possibly in response to either low light or elevated relative humidity. This trend was not evident with the shade ecophene which displayed very l i t t l e variation through-out the day. There was no significant correlation ( = 0.05) between R.T. and xylem ^ . Xylem changes rapidly with changes in light intensity (Fraser, 1970). This suggests that the wide variation within the shade samples may be the result of the plant being or not being exposed to fleck light prior to sampling. This, however, does not explain similar variation in exposed trees. It is concluded that R.T. is a much more suitable parameter for use in the following studies because: 1. R.T. measurements may be made on the same trees several times. 2. R.T. is less sensitive to sudden stresses at high values and serves to integrate or "average out" transient stresses. Table 3.2 The Comparison of R.T. with Xylem K . RT = Relative Turgidity, PR = Pressure in Bars, TEMP = Temperature in Degrees Fahrenheit, RH = Relative Humidity and INT = Light Intensity in Klx. Exposed Shelt ered TIME RT PR TEMP RH INT RT PR TEMP RH 8:00 57-5 80 57.0 82 8:30 95 10.0 94 6.0 8:30 85 10.0 91 8.7 9:00 60.5 73 .6 59.0 72 10:00 61.5 73 32.4 60.0 73 10:30 84 10.7 88 6.7 10:30 82 12.7 93 9-3 11:00 68.5 58 60.5 62.0 64 12:00 68.0 58 67.O 64.0 65 12:30 86 10.7 94 8.0 12:30 84 20.0 94 14.0 1:30 68.0 67 67.O 67.O 62 2:00 68.0 66 27.0 67.0 71 2:30 91 12.7 96 16.7 2:30 90 9-3 98 19.4 3:00 69.5 63 19.4 65.5 70 4:00 66.0 6l 19.4 65.5 66 8:00 92 6.0 93 2.0 8:00 92 1.4 94 2.7 8:30 62.0 74 61.0 73 3.4 Carbon Uptake and Relative Turgidity The purpose of this experiment was to correlate R.T. with net COg uptake. Farmer (1969) and Pallas et a l (1967) a n d many others have demonstrated that water stress in vascular plants results in decreased growth. Pallas et a l (1967) also correlated low leaf water potential with lowered carbon uptake, while Pharis (I966) established a lethal R.T. of about 43$ for Ponderosa pine (Pinus ponderosa). Method The eight trees used for this study were two-year-old plants from Green Timbers Provincial Nursery, Surrey, B.C. They were trans-planted while dormant into gallon crocks of soil and transported to a greenhouse at the University of B.C. and acclimatized there for 6 months. They were then placed in a growth room for a further two months of acclimatization. Throughout the growth room acclimatization and the testing period, the pots were randomly rearranged weekly. The testing sequence began by removing ca. 10 needles from a tree. These needles were immediately weighed and placed in saturated polyurethane sponges for determination of needle R.T. as described in section 3.2. Testing consisted of sampling for R.T. and then enclosing the plant in a plexiglass cuvette which was part of a "closed" system consisting of an air pump, a Beckman infrared CO2 analyzer, and the cuvette. The net CO2 uptake at 10.8 klx. was measured between concen-trations of 35° and 250 parts per million (ppm). Tests were started at 1:30 P.M. Relative humidity was kept at 86$ by the use of saturated NaCl solution. The gas analyzer was zeroed and calibrated with gases at 86$ relative humidity. The "upscale" calibration gas had a C 0 2 concen tratIon of 300 ppm. For the f i r s t 12 days a l l plants were watered every day. After this period, plants 5, 6, 7 and 8 were not watered while plants 1 , 2 , 3 and k were watered every day. Results and Discussion Relative turgidity results are graphed i n figure 3-3, while R.T. against relative percentage net CO2 uptake assimilation i s shown i n figure 3-k. The maximum rate of CO^ assimilation by each plant was set at 1 0 0 $ . Clearly by the twenty-eighth day of the experiment there had been a marked decrease in R.T. i n the droughted group (Fig. 3.3). In general, R.T.'s below 80$ are associated with restricted CO2 uptakej however, there i s considerable variation i n correlation of uptake and R.T. between R.T. = 70$ and R.T. = 80$. R.T.'s below 7 0 $ have strongly limited uptake, while an R.T. below 55$ indicates a com-p lete lack of net C 0 g assimilation. 3.5. The Effect of F u l l Sunlight on Shade Ecophenes. When the canopy over juvenile Hemlocks i s cleared away as in a logging operation, most of the very sheltered trees die. It i s not certain whether this i s because of the shade seedlings' susceptibility to pigment photooxidation or the seedlings' i n a b i l i t y to maintain a satisfactory water balance. Death could also be caused by a combination of the two factors, with pigment destruction occurring at high water Figure 3-3- Relative turgidity and drought. y= droughted trees fr= vatered trees . 31a. o •tw o * ft o tffl o o tffl o o a o + 9 o 4 o o o + 4 o 4- ai o a o 8 8 8 o o 8 3 o o Figure 3.4. Relative t u r g i d i t y and r e l a t i v e net CO, uptake. ICO.. 90.1 B0.1 7 0 -BD>± 5 0 • 4 0 . | M ID 30-4-LJ £ 2 0 - * EC 10.1 X X X - f - -+- -t-X X xxxx X X X ' X X X X -+- •4- -+ 0' 10« E0» 3 0 - 40« 50« BO. 7 0 • BO- 9 0 - 100« RELATIVE CARBON UPTAKE ro 0> v.-33 stress. The object of this experiment was to measure the moisture regime and the photosynthetic capability of the sheltered trees when they were exposed in situ to direct sunlight. The trees of an adjacent exposed site were used as an additional basis for comparison. Method Shaded site number 2, under dense Red alder, and exposed site number 1, as previously described (Chapter l) were used. Light was measured with a Gossen foot candle meter and hourly evaporative potential of the air at each site was found with a modified Piche evaporimeter (Waring and Hermann, 1$66). Carbon uptake was measured by placing a plexiglas cuvette over a seedling. The cuvette was part of a closed system previously described in section 3-3- Photo-synthetic rates were determined at 20 C. The cuvette was removed when-ever the temperature rose about 22 degrees C. Light intensity was lowered to 10.8 klx by covering the cuvette with layers of cheesecloth or, i f the ambient light was less than 10.8 klx (as was the case with the deeply shaded plants), the intensity was raised to 10.8 klx with a portable tungsten light source. A Spectronic 20 spectrophotometer was used for chlorophyll determinations. Pigment extraction followed the method of Arnon (19^9). Water status was determined by R.T. sampling as described in section 3-2. On the first day, R.T., light intensity, relative evaporative potential and relative chlorophyll content of the needles were measured in the two sites. Trees were cleared away from the shaded site by 9 A.M. 3+ of the third day and the same measurements were taken. Kodak infrared false colour pictures were taken of the plants on days 1, 2 and k in an attempt to detect damage. Results and Discussion Results are shown in figure 3»5' There was a cessation of photosynthesis in the shade trees in the morning of the third day and "by the afternoon the trees were distinctly wilted and some of the needles had developed areas of whitish overlay that may have heen the beginning of "bleaching". However, neither the infrared photographs (figure 3«6) nor the pigment assays gave any evidence of massive pigment destruction. By day h there was obvious bleaching and infrared photo-graphs (figures 3«7 and 3-8) clearly delineate the shoot tips as areas of tissue destruction. Visual inspection of these plants showed bleached needle tips. Clearly, relative turgidity is correlated to some extent with the evaporative potential of the a i r . Moreover, shade seedlings are unable to control excessive water loss when they are exposed. The cessation of net COg uptake is very probably a result of low R.T. (see section 3-4), while the needle bleaching and necrosis results primarily from a large water saturation deficit. The latter conclusion is based on two suppositions: 1. Water loss in conifers is fir s t apparent in the needle and shoot tips (Kozlowski, 1968). 2. Pigment destruction through photooxidation or any photo-chemical process would not be confined to a particular Figure 3.5. Effect of evaporative stress on shade ecophene ii = shaded site - exposed site jf = shaded tree 1 j> = shaded tree 2 <} = exposed tree 1 *7 = exposed tree 2 Figure 3.6. Infrared photo of shade trees at time of clearing of site. Figure 3.7. Infrared photo of shade trees after k days of exposure. Figure 3.8. Infrared photo of sun trees. region of a leaf. 3.6 The Effect of Increasing Exposure of Shade Plants without Changing Light Intensity The results of experiment 3'^ do not separate effects of light from an increase in overall exposure. The purpose of this experiment was to demonstrate the importance of increased exposure rather than increased light intensity to the development of necrotic tips. This was achieved by raising the evapor-ative potential of a shaded site without changing the light regime. An exposed site was included for comparison. Method Two adjacent sites, numbers 5 and 7 (see Chapter l) were used in this experiment. Site number 5, a tree stump site, was approximately 180 meters from the heavily shaded site 6. Two sections of the shaded site were used. These sections were approximately 10 meters apart. One of these sections was subjected to continuous air movement supplied by a large ventilation fan, while the other was subjected to only the natural air movements. The evaporative potential of the air at each area was deter-mined by periodic readings of pairs of Piche evaporimeters located 5-6 inches above the ground. On day 1, the fan was started at 9 A.M. and evaporimetric information and R.T.'s for the 3 sites were gathered throughout the day and into day 2. 39 Results and Discussion Results are shown in figure 3.9. On day 1 the evaporative demand of the treatment area has been clearly raised above the shaded site potential, but not to the level of the exposed site. R.T.'s of trees in the treatment area are clearly below R.T.'s of both exposed and shaded sites. By early morning of day 2 and up to the last R.T. measurements, a light rain precluded the possibility of high evaporative potentials. Obviously, high light intensity is not required to generate high water saturation deficit in shade plants; moreover, shade plants do not appear to have the ability to control water loss in response to high evaporative potential. Exposed plants, however, tolerate very high degrees of exposure without excessive water loss. 3.7 Light and Air Movement as Combined Factors This experiment was done to see how light and air movement interacted to affect the R.T. of shaded seedlings in the fiel d . It had already been demonstrated (section 3>5) that an increase in evapor-ative potential was associated with a rise in plant water stress and this experiment was designed to show to what extent light-stimulated stomatal opening is required to allow water loss with consequent drop in R.T. The null postulate in this experiment is that light and evaporative potential do not interact to produce lowered R.T. in shade trees. Figure 3.9. Evaporation and Relative Turgidities from Sites 5 and 7• X = shade site A= treatment site t> - exposed site k l Method The very sheltered site 7 was used for this experiment. Within the site a 2 m x 3 m plot was established along one of the long sides. A 2 m 3 tube fluorescent unit was placed at a height of approximately 0.7 m above the forest floor. Light intensity at plant level was 3-2 klx. A large ventilating fan was placed about 1.5 m in front of the plot and the airflow directed over the site as is illustrated in figure 3.10. Five evaporimeters were placed beside 5 trees within the plot Figure 3'10' The arrangement of air flow and lighting on a plot in site 7-(see figure 3-10)j one evaporimeter-tree combination, number 6, was 7 m in front of the fan where it would be subjected to the environment of the site without the influence of the plot environment. Evaporative potential was computed on days 0, 1, 3 and 6. R.T. was measured at 2 P.M. on the same days. On day 0 the fan and the lights were turned on at 9 A.M. and were left on until the end of the experiment. To facilitate comparison of relative evaporative po-tential with relative turgidity, evaporimeter readings from each area were expressed as a percentage of the maximum. Average relative turgidity was also expressed as a percent of maximum. Results Results are shown in Table 3.11 and in the graphs of figures 3.12 and 3.13- Although day 0 was a warm and dry day, i t rained on days 1 and 2; day 3 was cloudy and cool. The environmental manipulation Table 3.11 A comparison of total evaporative potential with average relative turgidity among 6 areas of site 7. Note that a negative order of magnitude is used with relative $> R.T. to facilitate comparison. Rel. $, E.P. = relative percentage evaporation potential. Neg. Rel. # Order Rel j> Order Area E.P. of Mag. R.T. of Mai 1 87.I -2nd 81.5 1st 2 100.0 1st 81.9 3rd 3 66.7 3rd 85.7 5th 4 52.7 4th 81.5 1st 5 39.8 5th 84.5 4th 6 29.0 6th 100.0 6th strongly affected the evaporative potential of the plot (see graph, figure 3.12). The 'without plot' area (area 6) displayed less than l/3 the evaporative potential of the highest potential area (area 2) of the 'within plot' areas. R.T.'s show an obvious response to evaporative demand, and are lowered by high evaporative demand (see graph, figure 3.13). fa Figure 3.12. Evaporation from 6 areas in site 7. DAYS CF EXPERIMENT Figure 3.13. Relative turgidity of trees in 6 areas of site 7. x= area 1 += area 2 •*.= area 3 v= area h t>= area 5 A= area 6 D A Y S O F E X P E R I M E N T 45 Grouping of R.T.'s within the plot, however, was so close that corre-lation of evaporative potential with R.T. was not possible. Evidence to reject the n u l l postulate i s lacking and i t should be noted that the "under light", area number 1, has an identical average R.T. to that of the unlit area number 4. It should also be noted that the unlit area 4 has the lowest R.T. on the wet days, raising the possi-b i l i t y that the lighting had very l i t t l e to do with evaporative loss i n this shaded si t e . 3.8 Summary and Conclusion The experiments i n t h i s chapter have: 1. Established the v a l i d i t y of a method for sampling small quantities of Western hemlock needles for relative turgidity determinations. 2. Simultaneously compared relative turgidity with xylem potential and did not establish a correlation between the two factors. 3. Determined the inhibitory effect of decreased relative turgidity on carbon uptake. 4. Shown that depression of relative turgidity i n shade eco-phenes can be brought about by simply increasing evaporative 46 potential. An increase in light intensity or duration is not necessary to lower relative turgidity. On the basis of these conclusions i t is inferred that the primary cause of tip necrosis in freshly exposed, unadapted shade trees is needle water deficit. Photooxidation or any other form of pigment destruction appears to be secondary to the water deficit and may occur because of water deficit. CHAPTER k GAS EXCHANGE CHARACTERISTICS h.1 Carbon Uptake and Light Response The literature suggests definite gas exchange differences between tolerant and intolerant^" species, sun and shade ecophenes of the p same species, and sun and shade leaves of the same plant. Kramer and Kozlowski ( i960) define shade tolerant plants as those that reach a maximum rate of photosynthesis at a relatively low light intensity while shade intolerant plants are those that require relatively high light intensities to achieve maximum photosynthetic rate. This definition is based partly on the results of Bohning and Burnside (1956) who, working with single attached leaves, made extensive comparisons of tolerant and intolerant plants that substantiate this definition. In addition, Monsi and Saeki (1953) indicate that photosyn-thetic efficiency is of major importance in determing which species will succeed in any particular plant community. •^ee definition of tolerance in Chapter 1. 2 A sun leaf is considered to be a leaf that is on the surface of the canopy, unshaded by other leaves and exposed to direct solar radiation, while shade leaves are within the crown or below the forest canopy and shielded from direct solar radiation. ^Kramer and Kozlowski used the term "tolerance" in their definition, but state that tolerance in this case refers only to the plant's capacity to endure shade, rather than the capacity to endure the whole complex of environmental factors. In keeping with the definition of shade t o l -erance set out in chapter 1, I am referring to it as "shade tolerance". Precedence for this change was established by Wuenscher and Kozlowski (1970) who give the same definition as defining "shade tolerant" species. Decker (1955) questions a l l comparisons of net COg uptake "based on a per unit basis as i t assumes a constant that has universal significance and can be used for meaningful comparisons between species and clones. An uptake rate, according to Decker, may have l i t t l e sig-nificance in terms of dominance or survival. He suggests that compari-sons be made on a percentage maximum basis and warns that no single factor can be translated into a broad ecological index. Work done by Kramer and Decker (1944) and Bourdeau (1959) compares C02 uptake by tolerant climax species of the southeast United States with the intol-erant successional Loblolly pine on a percentage maximum basis and shows the tolerant species to be more efficient at low light intensities. Work with sun and shade grown plants of various tolerances indicates that in many cases light environment can condition CCv, uptake efficiency. Burnside and Bohning (1957) showed that intolerant plants, grown in shade, tended to achieve carbon uptake saturation at lower light intensities than when they were grown in direct sunlight. Bjorkman and Holmgren (1963, 1966) showed that shade ecotypes of Solidago virgaurea and several other species were more efficient than were their sun ecotypes. Bormann(1956) found that shade grown Loblolly pine seedlings at saturation had higher photosynthetic rates than sun grown seedlings. Kusumoto (1957) found that shade leaves of tolerant broad leaved evergreen trees were more efficient than their sun leaves while intolerant trees had sun leaves that were more efficient than were their shade leaves. Bourdeau and Laverick (1958) found the shade needles of Red pine (Pinus resinosa) and White pine (Pinus strobus) more efficient that the sun needles while the intolerant Ailanthus altissima had sun leaves that were more efficient than the shade leaves. In addition, 49 Bourdeau and Laverick (1958) correlated photosynthetic efficiency at low light with degree of tolerance. Bohning and Burnside suggest two adaptations to low light intensity: 1. Low light saturation intensity. 2. Lower light compensation point. Light compensation is that light intensity at which no net COg exchange occurs. Bohning and Burnside demonstrated these adapta-tions in several less tolerant species when grown in shade. Tomato, however, developed a higher saturation intensity when i t was grown at low intensity. Boysen Jensen (1929) and Tranquillini (1954) found lower light compensation points in shade leaves than sun leaves of several European hardwood species. Bourdeau and Laverick (1958) established the same relationship for sun and shade needles of Red pine, but found that shade leaves of the intolerant Ailanthus had higher compensation points than the sun leaves. Although the literature is sometimes contradictory, some general tendencies may be discerned: 1. Tolerant plants usually have a higher photosynthetic efficiency at low light intensities than intolerant plants. 2. Plants and leaves grown in the shade often have lower light compensation points and higher photosynthetic efficiency at low light intensity. 50 h.2 Diffusion Resistances and Light Adaptation Photosynthesis involves 3 major groups of reactions: 1. The trapping of light quanta with concomittant photo-chemical reactions. 2. Thermochemical enzyme-controlled reactions. 3. Physical diffusion processes preceeding C0^ assimilation. Any one of these reactions may limit photosynthesis. Clearly, when light is low, carbon uptake will vary directly with light intensity, or, i f temperature is too extreme, the speed of the enzymatic reaction limits the rate of carbon uptake. It is generally considered that CO^  diffusion limits photo-synthesis when light is saturating (Gaastra, 1959; Wuenscher and Kozlowski, 1969). A leaf has its own microenvironment, separated from the external environment by a series of diffusion barriers. Diffusion of gases through these barriers is generally considered to follow Ficks law of diffusion. Ficks law states that the rate of diffusion of a gas is directly proportional to the cross sectional area of the path and to the difference in density of the gas along the path and inversely proportional, to the length of the path (Meidner and Mansfield, 1968). Ficks law is frequently analogized to Ohms law (Milthorpe, 196l; Bange, 1953? Lake, 1967). Ficks law maybe stated: ra/t = D (c-c1) o/l (3) 51 Where: m/t is the mass of gas flowing in unit time t . D is the diffusion coefficient, c-cl is the concentration gradient. 0 is the cross sectional area of the path or conductor. 1 is the length of the path or conductor. Ohms law is stated: e/t = f (v-v1) o/l (4) Where: e/t is the quantity of electrical energy flowing in unit time, f is the conductivity constant. v-v1 is the electrical gradient or voltage. When I (amps) is substituted for e/t, equation 4 can be restated to demonstrate a l l the factors that offer resistance to current flow (R eiect): v-v1 1 = 1 . 1 (5) f o Clearly then: Relect « -l/o (6) Continuing the analogy, those factors resisting the diffusive flow of a gas (R<jiff) may be shown: Rd i f f = ^  ' ^ ° W 52 i f : R = r x + r 2 + r n (8) then, when several resistances are in series: m = c-c3- t r i s + r2s + r n s (9) of, i f in parallel: m = c-c1 rl p + r 2p + rnp Combining 9 and 10 we get a general equation for determining diffusion resistance: m = c-c1 - t rl p x r 2 p x - rnp + r l s + r 2 s rns rip + r2p + r n p (11) Provided the analogy holds true, only c and cx are needed. Intermediate concentrations are not required. The constant D is defined as the mass of gas diffusing across a cross sectional area of a path in response to a difference in gradient, or: D = m . 1 = gm cm = cnrVsec^ t o (c-cl) sec cm2 (gm/cm3) (12) 53 In general, D for various gases is a function of the molecular weight of the gas, the relative rates of diffusion being inversely pro-portional to the square root of its density or molecular weight (Grahams law). D for water and CCvj is practically constant over a wide range of densities of water and CCg (Meidner and Mansfield, 1968). In this work DJJ^O ^s considered to be 0 . 1 4 cm2/sec and D ^ Q ^ as 0 . 2 4 cm2/sec. These values are in general use throughout the literature (Gaastra, 1959; Holmgren et a l , 1965; Gale et a l , 1968; Wuenscher and Kozlowski, 1970)^ and are based on the supposition that D for the gases in question is the same through a leaf as i t is through the air . This supposition has shown to be a simplification as Milthorpe and Penman (1967) have shown that D is not independent of pore size. 4 . 3 The Division of Leaf Resistance Total leaf resistance to gas diffusion is divided, somewhat arbitrarily, into several composite resistances. Most of these resist-ances are in series, but the resistance offered by the stomata is con-sidered in parallel to the resistance of the epidermis and cuticle. 4.3.I Boundary layer resistance (r a) This resistance is associated with the diffusive shell sur-rounding the leaf. This shell is relatively thick in s t i l l air but can ^These values, however, are not without question; Fuller et a l (1967) and Lee and Wilke (195*0 suggest values of O.I65 and 0.258" cm2 sec - 1 respectively for B C O 2 a nd D'.jfoo. This information was utilized by Gale et a l (1968). 54 be greatly reduced by a i r motion (Gates, 1968). In addition, leaf shape, size and orientation with respect to a i r movement also affect r Q . Monteith (1965) summarizes the knowledge concerning r a H 2 Q : i a H 2 o = -65x5 x u 5 (13) and Hunt (I968) gives an equivalent expression f o r FQ^Q^: r aco 2 = , 9 5 x 5 x u 5 where x i s the le a f width and u the wind speed. 4.3.1 Cuticular Resistance ( r c ) Although a t h i r d to a quarter of t o t a l t ranspiration i n certain species may occur through the c u t i c l e (Freeland, 1948; Oppenheimer, i960) i t has been found that plants with t h i c k c u t i c l e and/or epidermis have a very high resistance to non-stomatal d i f f u s i o n (Freeland, 1948) while Pisek and T r a n q u i l l i n i (195l) and Pisek and Berger (1938) have found very low rates of cu t i c u l a r transpiration i n various species of conifers. Wo attempt was made to measure r c i n these experiments. 4.3.2 Stomatal Resistance ( r s ) Stomatal resistance describes that resistance offered by the stomatal pores and the substomatal mesophyll spaces and i s generally considered the p r i n c i p a l resistance to transpiration (Slatyer, i960; Shimshi, I963). Stomatal opening has been shown to respond to changes i n l i g h t i n t e n s i t y , C02 concentration, leaf temperature, water status, and r e l a t i v e 55 humidity (Meidner and Mansfield, 1968). 4.3.3 Wall Resistance (r w) The question of a resistance occurring at the wet mesophyll cell wall is a matter of dispute. Brown and Enscombe (1900) working with Helianthus, found the calculated stomatal opening too large to account for the small water loss and developed a theory of mutual inter-ference of pores to account for the deficiency. Others accounted for the deficiency by suggesting an additional resistance associated with the incipient drying of the cell walls. This was called wall resistance. Renner (1910) found that Brown and Enscombe did not include r a in their computations and most of the discrepancy could be accounted for when this was done. Bange (1953) also showed the importance of r a which he demonstrated was the prime controller of transpiration in s t i l l air, and demonstrated that in moving air stomatal control was of prime importance. Meidner (1955) corroborates Renner and Bange, but adds: there remains, however, a persistent suggestion that a marked in-crease in internal resistance to water loss develops owing to incipient drying of cell walls". Milthorpe and Spencer (1957), Rawlins (I963), Fry (1965), and Jarvis and Slatyer (1970) have measured discrepancies in the correlation of r_ with transpiration which they have attributed to r „ . 4.3.4 Mesophyll Resistances Net photosynthesis may be lowered before transpiration or r„ 56 responds to water stress (Monteith, 1963; Baker and Musgrave, 1964; Whiteman and Roller, 1964a) suggesting the existence of a variable limiting CCvj diffusion other than stomatal opening. This variable is referred to as mesophyll resistance (r m) and describes that diffusion resistance offered by the mesophyll cells from the outer membrane to the carboxyla-tion sites. It should not describe the chemical resistance of carboxyla-tion, ideally, and is measured only when C02 concentration at the reaction sites is close to zero (Gaastra, 1959)' Because of the impossibility of separating carboxylation resistance from mesophyll resistance, r m shall include carboxylation resistance (r x) in this thesis. RmH20 i s generally considered negligible and Shimshi (1963) found that while water stress can appreciably affect rmr»o2 ^n e e^ec^ o n transpirational levels was relatively less pronounced. 4.4 Diffusion Resistance and Environment Over the past few years, several workers have investigated tolerance and shade adaptation from the standpoint of leaf diffusion resistance. Holmgren et a l (1965) found that light saturated photosynthesis in tolerant plants was limited by the rate of chemical processes, while light saturated photosynthesis in less tolerant plants was limited at normal concentrations by C02 diffusion. Holmgren (1968) working with sun and shade ecotypes of Solidago virgaurea found that plants grown at higher irradiances had appreciably lower r m than plants grown at low irradiances. This was particularly true for the exposed ecotypes. Holm-gren (1968) also found r to be higher than r in most plants. Wuenscher 57 and Kozlowski (1970) concluded that resistance to COg diffusion in cer-tain species of hardwoods strongly influences the net rate of photosyn-thesis over a wide range of environmental conditions. Tolerant plants had a relatively high TQQ^ with the greater part being r m . The least tolerant species had a much lower YQO2> the greater part of which was rs + ra (£ rH20^' W u e n s c n e r a n <* Kozlowski (1970) also found ^ r^o to be very high at low light intensities and because of its correlation with light intensity, considered i t the photosynthetic limit over a wide range of light intensity. The tolerant Acer sacharrum, however, reached minimal before reaching maximum photosynthetic rate, suggesting that maximum photosynthetic rate is limited in this case by a variable other than r s + r a . k.5 Relative Humidity and Photosynthetic Rate There is a strong correlation between leaf water potential and photosynthetic rate (Brix, 1962; Hodges, 1967). The moisture status of a seedling depends not only on the absorption of soil moisture but also on the release of moisture into the atmosphere through transpiration (Kramer, 1958). Factors controlling transpiration are: 1. Leaf Morphology, 2. Stomatal opening, 3. Steepness of vapour pressure gradient between leaf meso-phyll and ambient air. It is probable that lower relative humidities would increase plant moisture stress with concomittant lowering of photosynthetic rate. 58 Miller (1959) found with Abies alba that the rate of C02 uptake at 30$ relative humidity was less than half of the rate at 80$ relative humidity, and Tranquillini (I963) demonstrated a similar phenomenon for Picea excelsa, Pinus cembra and Larix decidua. Thompson et al (1965) investigated interaction between soil moisture and relative humidity and concluded that although the effects of soil "suction" (energy status of moisture in the soil or soil fr ) above 20 centibars had the greatest effect on physiological response. The evaporative potential of the air determined the degree of that response. Hodges (1967) correlated the midday rise in vapour pressure deficit with the midday drop in photo-synthetic rate of several coniferous species and Bosian (1968) states that along with light intensity, atmospheric humidity is a primary deter-miner of photosynthetic rate. Because variation in relative humidity affected the moisture dynamics of both ecophenes (see Chapter 3) the effect of variations in relative humidity on gas exchange were included in this study. h.6 Materials and Methods 4.6.1 Trees A l l trees were juvenile Western hemlock from a shaded or exposed site (sites 7 and 3 as described in Chapter l) in the University Research Forest, Haney, B.C. The trees were 2 to 6 years old and naturally seeded. In late September a l l seedlings were lifted, immediately potted in adjacent soil, and taken to the university greenhouse. In mid December they were transferred to a growth chamber with 16 hrs light per day and a temperature of 20 C. while illuminated. Dark temperature was 15 C. 59 Both in the greenhouse and in the growth chamber the shade plants were kept on shaded shelves. The maximum light intensity reaching the shade plants during this period was 1.3 klx. Maximum intensity for the exposed plants was ca 50 klx. 4.6.2 Measurement of Gas Exchange Intact seedlings were sealed at the stem, in a water jacketed plexiglass chamber. This was part of a closed system which included a Beckman model 215 infrared C02 gas analyzer and a Hygrodynamics narrow range lithium chloride humidity sensor (see f i g . 4.l). To measure transpiration, the system's relative humidity was adjusted by passing a fraction of the air over dessicant bottle number 1 until the desired relative humidity was attained. After 20 minutes of stabilization, the airflow was switched to the pre-weighed dessicant bottle number 2 and the same humidity maintained for 20 minutes. At the end of the measuring period the airflow was again switched to bottle number 1 and bottle 2 was disconnected and weighed. Net CO2 uptake was measured between 35° and 250 ppm unless otherwise noted. A l l plants were acclimatized to the chamber for 12 hours at 10.8 klx before the low light measurements were made. The measurements were made sequentially from low light to high light with at least 30 minutes of higher light acclimatization between steps. 4.6.3 Light Source Light was furnished by a G.E. "cool beam" 300 watt incandescent lamp and was filtered through 12 cm of water. Spectral distribution of this light was determined in situ with an "ISCO" spectral radiometer and Figure k.l. Diagram of a close system for measurement of gas exchange. 6 l is shown in figure 4.2. Total spectral energy was measured with a Y.S.I. "Kettering" radiometer and foot-candle power with a Gossen silicon foot-candle meter. 4.6.4 Leaf Temperature Determination of leaf temperature was done with thermocouples. The narrowness of the needles predicated the use of a very small sensor. This requirements was met with size 36 copper constantan thermocouple wire (Thermo Electric Co. #2-3170). A commercial thermocouple welder proved useless when used with this fine wire, and a homemade welder was developed using a "variac" and a worn out 9 volt battery (see figure 4.3). For use, the outer insulation of the thermocouple wire was stripped back one to one and a half inches and the inner insulation burnt back one half to three quarter inches. Ends of the wire were exposed, twisted together and connected to the variac. The voltage was adjusted to approximately 60 and the free end of thermocouple wire touched lightly to the battery anode for 1 to 2 seconds. The resulting bead was 0.2 to 0.3 mm in diameter. The response to temperature of a typical thermocouple is shown in figure 4.4. For leaf temperature measurements, the bead was pressed against a 1- or 2-year-old needle already at right angles to the light source (after Stoutdesjick, 1970). Enough pressure was applied to very slightly displace the leaf. yx Figure 4.2. Spectral distribution of G.E. "cool beam", x = with 10 centimeter water f i l t e r \>- without f i l t e r 6 2 4 RELATIVE ENERGY i i 63' V A R I A C 6 b 110 V O L T s B A T T E R Y V W W H T H M C P L . W I R F S Figure 4.3. Circuit of the "homemade" thermocouple velder described in the text. 4.6.5 Determination of Leaf Diffusion Resistances The r could not be determined with conventional means (see Gaastra, 1959) as paper models of the same dimension of the needles would not retain their shape when wet. An estimate of the upper limit of r a was attained by cutting 2 by 5 cm pieces of Whatman #1 f i l t e r paper into 2 groups of strips (see photograph in figure 4.5). These strips were fastened at the middle by a piece of waterproof tape. In use, the paper was fastened in the chamber in the same plane as the stem would have been in. The "leaves" were saturated with water, the chamber closed and the pump turned on; as soon as the surface temper-ature stabilized, measurement of water loss was begun. The equation used was: 2"H20 = wi " v a/E (after Gaastra, 1959) (15) 6 4 where: W. and W are water vapour concentrations at the needle x a surface and in the chamber respectively (in gm/cm^ ). E is the evaporation rate (gms/cm2/sec). In addition to the paper model, a qualitative estimate was made by measuring the decreasing effect of r on C0? uptake by increasing the air flow over both ecophenes until there was l i t t l e i f any change in COg uptake. Other resistances determined were ^ rH20 » rm > a n (^ ^  r i C02' ^*rg Q w a s determined with equation l 6 : ^rH 20 = Wi " Wa/E (after Gaastra, 1959) (16) While r m was determined only after determining the intercellular C02 concentration ( C0^ n-^  ). The equation used is from Moss and Rawlins (1963): C0.nt = C0 a t m - P ( £ r H 2 0 x DH20/DC ) (1 7 ) 65-Figure 4*5. Paper model attempting to simulate the boundary l a y e r conditions of a. c o n i f e r . 66 Where: coatm i s t n e concentration of C02 in the atmosphere in mg/cm^ . P is the photosynthetic rate in mg COg/cm^ . DJJ^Q and "DQQ^ are diffusion coefficients of water and carbon dioxide in a i r . D^oA'cC^ w a ^ considered 1.7 after Gaastra (1959) and Slatyer and Jarvis (1966). r m was calculated by the method of Whiteman and Koller (1964b). rm - C 0i n t /p (l8) 4.6.6 Experimental Sequences 1. The Effect of Air Flow, Light Intensity and Ecotype on Leaf  Temperature The purpose of this experiment was to observe the pattern of leaf temperature response to changes in light intensity, ecophene and air flow. In addition it attempted to qualitatively determine at what pump speed r a was effectively reduced and i f this reduction was the same for both ecotypes at various temperatures and light intensities. Each value is an average of four readings. Air temperature was 20±0.5 C and relative humidity was held at approximately 86$ through the use of saturated NaCl solution. At the end of each run, needles were stripped from the tree, placed in an oven at 80 C for 24 hours before weighing. 67 2. Leaf Temperature and Transpiration The purpose of this experiment vas to estimate the magnitude of the effect of transpiration on leaf temperature and to determine i f this effect is different vith different ecophenes at various air flow rates and light intensities. The temperatures of 2 needles were averaged for each value. Air temperature was kept at a constant 20-0.5 C.^ * 3. Boundary Layer Resistance and the Paper Model This experiment is described in section 4.6.5. The purpose was to quantitatively rather than qualitatively estimate r & . Measurement was made at an intensity of 43.2 klx and a pump speed of 4600 cc/min. 4. The Effect of Air Flow and Light Intensity by Ecophene on  Rate of COp Uptake The purpose of this experiment was to determine the pattern of carbon uptake response to the above variables and to ascertain that air flow within the cuvette was rapid enough to reduce r a to an acceptable minimum for both ecophenes at various light intensities. 5. The Effect of Light Intensity, Relative Humidity by Ecophene  on Transpiration, Carbon Uptake, and Diffusion Resistance The readings from 4 trees were averaged for each value. Air temperature was a constant 20-0.5 C. Photosynthesis was expressed as a percentage of the maximum for each ecophene. The trees in this experiment and in a l l experiments in Chapter 4 were used in the cuvette only once and then discarded. 4.7 Results 4.7.1 The Effects of Air Flow, Light Intensity and Ecophene on Leaf Temperature The effects of the air flow and light intensity on leaf temper-ature are graphed in figure 4.8 and figure 4.9. A l l variables had sig-nificant («^.= 0.05) effects. The sun ecophene displayed higher temperatures at almost a l l but the lowest light intensity and the effect of air speed was greatest at the highest light intensity. Leaf temperature increased linearly with light intensity and both ecophenes displayed l i t t l e change in leaf temper-ature between the air flows of 4600 cm3 min and 2000 cm^  min. 4.7.2 Leaf Temperature and Transpiration Table 4.6 shows leaf temperature for the two ecophenes at 5° and 80$ relative humidity at various intensities and at 4 different air flow rates. In a l l cases, air temperature was 20^ 0.5 C. Table 4.6 The effects of air speed, relative humidity, light intensity and ecophene on leaf temperature. Air temperature was 20 degrees C. Pump Speed in cc/min 250 1008 2000 4600 Relative Humidity 80$ 50$ 80$ 50$ 80$ 50$ 80$ 50$ 43.2 klx (0.400 cal cnrVmin) Sun Shade 27.3 25.0 27.1 24.8 27.2 24.2 27.1 24.2 24.0 22.0 24.2 22.0 23.6 22.0 23.5 21.8 10.8 klx (0.140 cal cm2/min Sun Shade 22.0 20.9 21.6 20.9 21.0 20.4 21.0 20.9 20.8 20.2 21.0 20.6 21.0 20.4 21.2 20.4 2.2 klx (0.040 cal cm2/min) Sun Shade 20.4 20.1 20.6 20.2 20.3 20.2 20.5 20.3 20.3 20.3 20.5 20.4 20.4 20.2 20.3 20.2 • 5 klx (0.004 cal cnr/min) Sun Shade 20.0 20.1 19.9 20.0 19.9 20.0 19.T 20.0 19.9 20.0 19.9 20.0 19.9 20.0 19-9 20.0 69 Air flow, light intensity and ecophene significantly affect leaf temperature; however, a significant difference in leaf temperature was not shown to he associated with changes in relative humidity. These data suggest that in terms of leaf energy budget, the loss of energy through evaporation is small enough that i t may be disregarded. The energy budget of the leaf can be expressed as: Q = Qr - C + LE (after Parkhurst and Gates, I 9 6 6 ) 1 (19) Where: Q is total incident radiation. Qr the radiation reflected. C the convection from leaf to air or air to leaf. L the latent heat of vaporization. E the rate of transpiration. For juvenile ecotypes of Western hemlock, the equation may be rewritten: Q = Qr t C (20) In an attempt to determine the actual energy consumed by trans-piration, data from section 4 .7 .1 were utilized Clearly, only a small portion of total irradiance is expended to vaporize the water transpired. This suggests that the bulk of energy dissipation rests with the other two terms in the energy budget, Qr and C. If Qr is ca. 30$ of Q as suggested by Gates (1968), then some 62 ^This equation disregards metabolic and storage terms in the interest of simplification. Both terms are relatively small in comparison with the 3 main terms (Parkhurst and Gates, I966). to Gtfo of Q should he dissipated by C. This information demonstrates why leaf temperature is not demonstrably influenced by relative humidity but is affected by air speed. Table 4.7 Rates of transpiration of sun and shade ecophenes and percentage of total radiant energy expended in the transpiration process. RH = Relative humidity. Air speed at pump = k600 cvaP/mln. Light i n -tensity = 43.2 klx. Ecotype RH E gm cmx L x E cal cm2 min L x E as $ of Q (=.40 cal cm2/min) Shade Shade QOfo 50$ .0416 .0445 .0241 • 0257 6.2 6.4 Sun Sun 80$ 50$ .0327 .0520 .0240 .0300 6.2 7.6 4.7.3 Boundary Layer Resistance and the Paper Model The paper model showed a resistance of 0.9 sec/cm. This is only an approximation and is included only to indicate the evaporative power of the air within the cuvette. On a single leaf basis, the model would have a considerably largi r than the smaller surfaced, differently shaped needles. However, i t is a entirely possible that bunched needles such as are found on the sun ecotype may "share" an r a which, because of the increased common area, is larger than a single needle r a . 71 k.J.k The Effect of Air Flow, Light Intensity and Ecophene on C02 Uptake The effects of air flow and light intensity on net C02 uptake are shown in figure 4.10 and figure 4.11. Leaf areas are for one side. A l l factors had significant effect 0.05). The shade ecophene not only had a more efficient uptake at low light hut also displayed a high uptake at what was expected to be the light saturation level. The shade ecophene was s t i l l showing net CO2 uptake at 324 lux while the sun ecophene showed no uptake at this intensity. The shade ecophene had not yet developed a saturation plateau at 43.2 klx while the sun ecophene was light saturated by 10.8 klx. As with leaf temperature air flow between 2000 and 4600 cm^ /min caused l i t t l e change in C02 uptake. In summary, the data from this section suggest that the r a for the ecophenes is less than 0.9 and may be minimized by using an air flow of at least 2000 cm^/min. In addition, the higher leaf temperature build up in sun ecophene points to the possibility of a larger boundary layer around this ecophene rather than around the shade ecophene. ,The data also show that the shade ecophene had a higher C02 uptake (on an area basis) at a l l light intensities and did not develop a light saturation plateau up to 43.2 klx. 4.7.5 The Effect of Light Intensity, Relative Humidity and Ecophene on Rate of Net CO2 Uptake and Transpiration Measured Simultaneously The effects of these factors on relative net C02 uptake are graphed in figure 4.12. Only light intensity and relative humidity were shown to affect this variable. Figure 4.8. The effects of a i r flow and light intensity on leaf temperature i n the shade ecophene. Figure k.9. The effects of air flow and light intensity on leaf temperature in the sun ecophene. 73a ANALYSTS OF VAJ'T ' MC5 SOURCK • DF SUM ?:<> F PP.03 1 Lisht 3 192.91 64.302 554.92 33.17 0.0 2 Flow 2 3.3462 4.4231 0.0 3 Eco v - i 7.9219 7.9219 63.37 0.0 4 1 X 2 6 16.114 2.6356 23.18 0.0 5 1 X 3 3 16.601 5.5335 47.75 0.0 6 2 X 3 2 ./>0376 .20^33 1.76 0.1370 7 Error 30 3.4763 .11533 S Total 47 246.27 Figure 4.10. The effect of air flow and light intensity on net CO2 uptake in the shade ecophene. Figure 4.11. The effects of a i r flow and light intensity on net C0 2 uptake i n the sun ecophene. 753 ANALYSIS OF VARI'vNCE SOURCE DF SUM SO M2AN SQ F PF.OB 1 Lifrlit 3 462.94 15'.. 31 646.51 0.0 • 2 Flow 3 15-SO? 5.2690 22.03 0,0 3 Eco- 1 136.33 5.7S82 136.33 571.18 0.0 h L X i' 9 0.64313 2.69 0.0074 5 i n 3 A 5 - i> 5 5 15.152 63.48 0.0 6 F X E 3 .77236 0.25945 1.09 0.3535 7 Error 105 25.062 0.23S69 Total 127 692.16 Figure 4.12. The effect of light intensity, relative humidity and ecophene on rate of COg uptake. 4 = shade ecophene 80$ R.H. V ; = shade ecophene 50$ R.H. >< = exposed ecophene 80$ R.H. >= exposed ecophene 50$ R.H. I N T E N S I T Y I N K I L O L U X Figure 4.13. The effect of light intensity, relative humidity and ecophene on transpiration and transpiration resistance. ^= shade ecophene 80$ R.H. X= shade ecophene 50$ R.H. V= exposed ecophene 80$ R.H. C>= exposed ecophene 50$ R.H. 78 The results on an area basis, at the high relative humidity, are comparable to the results i n the previous section. However, the lower relative humidity i s associated with reduced uptake rates i n both ecophenes. The low relative humidity treatment produced a definite saturation plateau in the shade ecophene starting around 5.k klx, while carbon uptake i n the exposed ecophene saturated at 10.8 klx. The effect of light intensity, relative humidity and ecophene on transpiration and ^  rjjgQ a r e shown in figure 4.13. This variable i s significantly affected by a l l 3 factors. Note that although the sun ecophene tended to transpire more at low light intensities, a plateau was established around 5-h klx. Transpiration i n the shade ecophene had not reached a plateau by 10.8 klx. This suggests a very definite mechanism operates i n the sun ecophene to increase r g(and possibly r w ) . This mechanism i s not evident i n ^\ rjj 2o of "the shade ecophene (see figure 4.9). The rapid drop In *y rH2o i n t h e s n a d e < * ecophene i s significant ( c v = 0.05) and correlates with the rise i n C02 uptake. In addition, the rise i n C02 uptake rate at high humidity between 10.8 and 43.2 klx i n this ecophene i s reflected i n a significant drop i n ^  rjj 2Q between the same intensities. The drop shown between the same intensities for shade ecophene at 50$ relative humidity i s not significant. Resistance i n the sun ecophene rose significantly between 10.8 and 43.2 klx. This rise was not reflected i n the percentage C02 uptake data, suggesting a rise in r m . At intensities of these magnitudes there is a po s s i b i l i t y of enzyme saturation or some other non-diffusive process limiting carbon uptake by this ecotype. tended to vary inversely with relative humidity and there ^ ^ 2° 79 was si g n i f i c a n t interaction between r e l a t i v e humidity and ecophene, sug-gesting that the ecophenes do not react i n the same way to changes i n humidity. rjj^O ^ o r shade plants was much higher at low l i g h t and i n d i -cates the p o s s i b i l i t y of d i f f u s i o n resistance l i m i t i n g net COg uptake at low l i g h t i n t e n s i t y as w e l l as high. This can be contrasted with the report of Wuenscher and Kozlowski (1970) who found the same pattern speci-f i c a l l y f o r two intolerant Quercus species. Values f o r r m are shown i n Table 4.14. Individual r m ' s of the shade ecophene were s i g n i f i c a n t l y (Off = 0.05) correlated with t h e i r cor-responding COg uptake rates. Correlation between r m and carbon uptake i n the sun ecophene was poor. Because r m values are the largest part of r ^ e a f , we may say that at least i n the case of the shade ecophene, r f f l governs most of the carbon exchange at high i n t e n s i t y . In the sun ecophene where v a r i a t i o n i n r s has l i t t l e effect on carbon uptake, some other non-diffusive variable i s l i m i t i n g carbon uptake. 4.8 Conclusions Table 4.14-Resistance to C0 2 f l u x at 43.2 k l x Net Psyn. Ecophene r f f i (sec/cm) r a + r s (sec/cm) r l e a f (sec/cm) mg/dm hr. Shade 80$ 21.6 10.0 31.6 6.36 Shade 50$ 33-8 15-3 ^9-6 4.54 Sun 80$ 25.6 22.2 48.4 5.04 Sun 50$ 26.0 15.8 41.9 4.58 1. In response to increased light intensity, the sun ecophene tends to develop higher needle temperatures than the shade eco-phene (figure 4.9 and figure 4.10). 2. An effect of change in relative humidity on leaf temperature was not demonstrated and i t was concluded that the greatest part of leaf energy dissipation was by convection. 3. At high relative humidities, the shade ecophene had a higher light saturation point than the sun ecophene which was saturated by 10.8 klx. 4. The shade ecophene had a higher net C0 2 uptake rate on a leaf area basis than the sun ecophene. In addition, the shade eco-phene had a lower light compensation point than the sun ecophene. 5. Lowered relative humidity significantly lowered relative net C02 uptake in both ecophenes and resulted in similar light saturation points for the two ecophenes. 6. Lowered relative humidity raised transpiration rates in both ecophenes. In the sun ecophene transpiration rates on a leaf area basis were higher, at the lower light levels. 7. In the sun ecophene, resistance to water diffusion rose as light intensity increased (above 5.4 klx). This rise was not accompanied by a drop in carbon uptake. 8. In the shade ecophene, resistance to water diffusion dropped with an increase in light intensity. This trend was reflected 81 in increased net COg uptake. This suggests that carbon uptake by the shade ecophene may be limited by diffusion resistances over a wide range of light intensities. 9. When rates of CO2 uptake were expressed relative to the maximum uptake by the tree, the two ecophenes were statistically inseparable in their overall response to changes in relative humidity and light intensity. 82 CHAPTER 5 SUMMARY Marked differences between sun and shade ecophenes of juvenile Western hemlock have heen demonstrated. The most important point is that the two ecophenes are adapted to survive in the microclimate of their particular habitat. The shade ecophene is adapted to opening its stomata as light intensity increases. In the shaded condition this response is appropriate as i t lowers ^N rC02* I n e x I ) 0 s e ^ condition i t i s inappropriate as i t raises water stress. Under high light intensity the exposed ecophene does not assimi-late as much COg as the shade ecophene. However, it is able to close its stomata to restrict water loss. The sun ecophene is adapted to control-ling j^rjjgQ, while the shade ecophene is adapted to controlling )^^QQ^> Anatomical features of the ecophenes support the above state-ments. The sun ecophene has cylindrical needles with thick cuticles and extensive surface wax. The shade ecophene has fl a t , high surface-to-volume needles with thin cuticles and less surface wax than the sun ecophene. The restriction of gaseous exchange appears to be of major importance in the design of the sun ecophene needle. One of the outstanding features of the shade ecophene is the high degree of stomatal control at lower light intensities. The 'ig Q is very high at 0.5 klx, and a change of 30$ in relative humidity results in far less transpiration change than in the sun ecophene. This degree of diffusion control at low light intensity is not shown by the sun ecophene. The sun ecophene exerts diffusion control only at high light intensities. APPENDIX LITERATURE CITED 1. Arnon, D.I. 194-9. Copper enzymes i n i s o l a t e d c h l o r o p l a s t s . Polyphenoloxidase i n Beta v u l g a r i s . Plant P h y s i o l . 24: 1-15. 2. Baker, D.S. 1950. P r i n c i p l e s of S i l v i c u l t u r e . McGraw-Hill, New York. 3. Baker, D.N. and R.B. Musgrave. 1964. The e f f e c t of low l e v e l moisture stress on the rate of apparent photosynthesis i n corn. Crop S c i . k: 249-253. 4. Bange, G.G.J. 1953. On the q u a n t i t a t i v e Explanation of stomatal t r a n s p i r a t i o n . Acta Bot. Neerl. 2: 255-299. 5. Barker, H. 1968. Methods of measuring l e a f surface area of some co n i f e r s . Can. F o r e s t r y Branch Departmental P u b l i c a t i o n number 1229. 6. Barrs, H.D. 1968. E f f e c t of c y c l i c v a r i a t i o n s i n gas exchange under constant environment on r a t i o of t r a n s p i r a t i o n t o net photosyn-t h e s i s . P h y s i o l . P l a n t . 21: 918-929. 7. Bjorkman, 0. and P. Holmgren. 1963. A d a p t a b i l i t y of the photo-synthetic apparatus t o l i g h t i n t e n s i t y i n ecotypes from exposed and shaded h a b i t a t s . P h y s i o l . P l a n t . 16: 889-914. 8. Bjorkman, 0. and P. Holmgren. 1966. Photosynthetic adaptation t o l i g h t i n t e n s i t y i n plants native t o shaded and exposed h a b i t a t s . P h y s i o l . Plant. 19: 854-864. 86 9. Blackman, G.E. and G.L. Wilson. 1954. Physiological and ecological studies in the analysis of plant environment. VII. An analysis of the differential effects of light intensity on the net assimilation rate, leaf area ratio, and relative growth rate of different species. Ann. Bot. N.S. 15: 373-408. 10. Bohning, R.H. and CA. Burnside. 1956. The effect of light intensity on rate of apparent photosynthesis in leaves of sun and shade plants. Am. J . Bot. 43: 557-561. 11. Bormann, F.H. 1956. Percentage light readings, their intensity-duration aspects and their significance in estimating photo-synthesis. Ecol. 37: 473-476. 12. Bosian, G. 1968. Die Bedeutung der stomata, der Luftfeuchte und der Temperatur fur den COg und Wasserdampfgaswechsel der Pflanzen. Photosynthetica 2: 105-125. 13. Bourdeau, P.F. 1959' Seasonal variation in the photosynthetic efficiency of evergreen conifers. Ecology 40: 63-67. 14. Bourdeau, P.F. and P.L. Laverick. 1958• Tolerance and photosyn-thetic adaptability to light intensity in white pine (Pinus  strobus) red pine (Pinus resinosa) hemlock and Ailanthus seedlings. Forest Sci. k: 196-207. 15. Boyce, J.S. 1961. Forest Pathology. McGraw H i l l , New York. 16. Boysen-Jensen, P. 1932. Du Stoffproduktion der Pflanzen: G. Fischer, Jena. 17. Brix, H. 1962. The effect of water stress on the rates of photo-synthesis and respiration in tomato plants and loblolly pine seedlings. Physiol. Plant. 15: 10-20. 18. Brix, H. 1967. An analysis of dry matter production of Douglas Fir seedlings in relation to temperature and light. Can. J . Bot. k5: 2063-2072. 19. Brown, H. and F. Enscombe. 1900. Static diffusion of gases and liquids in relation to the assimilation of carbon and trans-location in plants. Phil. Trans. Roy. Soc. B. 193: 223-291. 20. Burnside, CA. and R.H. Bohning. 1957* The effect of prolonged shading on the light saturation curves of apparent photosynthesis in sun plants. Plant Physiol. 32: 61-63. 21. Daubenmire, R.F. 1965. Plants and Environment. John Wiley and Song, New York. 22. Clausen, T. and T.T. Kozlowski. 1965. Use of relative turgidity technique for measurement of water stress in gymnosperm leaves. Can. J . Botany 4-3: 305-316. 23. Decker, J.P. 1950' Tolerance is a good technical term. J . For. 50: 40-41. 24. Decker, J.P. 1955' The uncommon denominator in photosyntheses as related to tolerance. Forest Sci. 1: 88--89. 25. Farmer, R.E., Jr. 1969. Transpiration and leaf temperature in eastern cottonweed. Forest Sci. 15: 151-153. 88 26. Fraser, Bruce E. C. 1970. Vegetation development on recent alpine glacier forelands in Garibaldi Park, British Columbia. Ph. D. thesis, University of British Columbia. 27. Freeland, R.O. 19J+8. Photosynthesis in relation to stomatal frequency and distribution. Plant Physiol. 23: 595-600. 28. Fry, Kenneth Edward. 1965. A study of transpiration and photo-synthesis in relation to the stomatal resistance and internal water potential in Douglas F i r . Ph. D. thesis, University of Washington. 29. Fuller, E.N., P.D. Schettler, and J.C. Giddings. 1967. A new method for prediction of binary gas-phase diffusion coefficients. Ind. Eng. Chem. 58: 18-27. 30. Gaastra, P. 1959« Photosynthesis of crop plants as influenced by light, carbon dioxide, temperature, and stomatal diffusion resistance. Medd. Landbouwhogeschool (Wageningen) 59: 1-68. 31. Gale, J., Alexandra Poljakoff-Mayber. 1968. Resistances to the diffusion of gas and vapour in leaves. Physiol. Plant. 21: 1170-1176. 32. Gates, D.M. 1968. Transpiration and leaf temperature. Ann. Rev. Plant Physiol. 19: 211-238. 33. Harms, W.R. and W.H.D. McGregor. I962. A method for measuring the water balance of pine needles. Ecology 4-3: 531-532. 34. Hemming, I.V. 1965. Chlorophyll formation and water deficit. Physiol. Plant. 18: 994-1000. 89 35• Hodges, J.D. 1967. Patterns of photosynthesis under natural environ-mental conditions. Ecology 48: 234-242. 36. Holmgren, P. 1968. Leaf factors affecting light saturated photo-synthesis in ecotypes of Solidago virgaurea from exposed and shaded habitats. Physiol. Plant. 21: 676-698. 37. Holmgren, P., P.G. Jarvis and M. Jarvis. 1965. Resistance to carbon dioxide and water vapour transfer in leaves of different plant species. Physiol. Plant. 18: 557-573. 38. Hunt, L. A. 1968. Use of the Cionco model to obtain further infor-mation on the nature of leaf boundary layers. Can. J . Botany 46: 177. 39. Jarvis, P.G. and R.O. Slatyer. 1970. The role of the mesophyll cell wall in leaf transpiration. Planta (Berl) 90: 303-322. 40. Kozlowski, T.T. 1968. Introduction. In Water Deficits and Plant Growth. Academic Press, New York. Edited by T.T.Kozlowski. 1-19-41. Krajina, V.J. 1965. Biogeoclimatic zones and classification of British Columbia. Ecology of Western North America 1: 1-17. 42. Krajina, V.J. 1969. Ecology of Western North America. Department of Botany, University of British Columbia. 43. Kramer, P.J. 1958. Photosynthesis of trees as affected by their environment. In "Physiology of Forest Trees". Edited by K.V. Thimann. Academic Press, N.Y. 90 44. Kramer, P.J. 1962. The role of water in tree growth. In Tree Growth. Edited fry T.T. Kozlowski. Ronald Press, New York. 171-182. 45. Kramer, P.J. 1969. Plant and Soil Water Relations - A Modern Synthesis. McGraw H i l l , New York. 46. Kramer, P.J. and H. Brix. 1965. Measurement of Water Deficits in Plants. Arid Zone Res. 25: 343-352 UNESCO Switzerland. 4 7 . Krmaer, P.J. and J.P. Decker. 1944. Relation fretween light intensity and rate of photosynthesis of Loblolly pine and certain hardwoods. Plant Physiol. 19: 350-358. 48. Kramer, P.J. and T.T. Kozlowski. i960. Physiology of Trees. McGraw H i l l , New York. 49. Kusumoto, T. 1957• Physiological and ecological studies of the plant production in plant communities. Botan. Mag. Tokyo 70: 299-304. 50. Lake, J.V. 19^7• Respiration of leaves during photosynthesis. Aust. J . of Biol. Sci. 20: 487-493. 51. Larcher, W. 1969. ^n e effect of environmental and physiological variables on the carbon dioxide gas exchange of trees. Photo-synthetica 3: 167-198. 52. Lee, C.Y. and Wilke, C.R. 1954. Measurement of vapour diffusion coefficient. Ind. Eng. Chem. 46: 2381-2387. 91 53 • Loach, K. 1967. Shade tolerance in tree species. New Phytol. 66: 607-621. 54-. Meidner, H. 1955- Changes in the resistance of the mesophyll tissue with changes in the leaf water content. J . Exp. Botany 6: 94-99. 55. Meidner, H. and T.A. Mansfield. 1968. Physiology of Stomata. McGraw H i l l , London. 56. Miller, R. 1959' Assimilationsuntersuchungen an Tonnen und Fichten einer Naturverjungung im Bayerischen Wald. Forstwiss Cbl. 78: 297-313. 57. Milthorpe, F.L. 1961. Plant factors involved in transpiration. In "Plant Water Relationships in Arid and Semi-Arid Conditions". Proceedings of the Madrid symposium, 1959. UNESCO. 107-113. 58. Milthorpe, F.L. and H.L. Penman. 1967. The diffusive conductivity of the stomata of wheat leaves. J . Exptl. Botany 18: 422-457. 59. Milthorpe, F.L. and E.J. Spencer. 1957- Experimental studies of the factors controlling transpiration. J . Exptl. Botany 8: 413-437. 60. Mitchell, K.J. 1953* Influence of light and temperature on the growth of Ryegrass (Lolium spp.). 1. The patterns of vegetative development. Physiologia PI. 6: 21-27. 61. Monsl, M. and T. Saeki. 1953- Uber den Lichtfaktor in den Pflanzengesellschaften und seine Bedeutung fur die Stoffproduktion. Jap. J . Botany 14: 22-52. 92 62. Monteith, J.L. 1963. Gas exchange in plant communities. In Environmental Control of Plant Growth. Edited by L.T. Evans. Academic Press. New York and London. 63. Monteith, J.L. 1965. Evapotranspiration and environment. Proc. Symp. Soc. Expt. Biol. 19: 205-234. 64. Moss, D. A. and S.L. Rawlins. 1963. Concentration of C02 inside leaves. Nature Land, 197: 1320-1321. 65. Oppenheimer, H.R. i960. Adaptation to drought: Xerophytism. In plant water relationships in arid and semi-arid conditions. Arid Zone Research 25. UNESCO, Paris. 66. Pallas, J.E., B.E. Michel and D.G. Harris. 1967- Photosynthesis, transpiration, leaf temperature and stomatal activity of cotton plants under varying water potential. Plant Physiol. 42: 76-88. 67. Parkhurst, D.F. and D.M. Gates. 1966. Transpiration resistance and energy budget of Populus sargentii leaves. Nature 210: 172-174. 68. Pharis, R.P. 1966. Comparative drought resistance of five conifers and foliage moisture content as a viatility index. Geology 47: 211-217. 69. Pisek, A. and E. Berger. 1938. Kutikulare transpiration und trockenresistenz isolierter blatter und sprosse. Planta 28: 124-155. 93 70. Pisek, A. and W. Tranquillini. 1951. Transpiration und wasserhaus-halt der fichte (Picea excelsa) bei zunemender luft-undhodentrockenheit. Physiol. Plant, 4: 1-27. 71. Piatt, R.B. and J.F. Griffiths. 1964. Environmental measurement and interpretation. Reinhold Publishing Corporation. New York. 72. Rawlins, S.L. 1963. Resistance to water flow in the transpiration stream. In Stomata and Water Relations in Plants. Conn. Agr. Expt. Stn. Bull. 664: 69-85. As quoted by Fry 1965. 73. Renner, 0. 1910. Beitrage zur physik der transpiration. Flora 100: 451-548. 74. Scholander, P.F. I965. Sap pressure in vascular plants. Science 148: 339. 75• Shimshi, D. 1963. Effect of soil moisture and phenyl mercuric ace-tate upon stomatal aperature, transpiration and photosynthesis. Plant Physiol. 38: 713-721. 76. Shirley, H.L. 1945. Light as an ecological factor and its measure-ment. Bot. Rev. 11: 497-532. 77. Slatyer, R.O. i960. Aspects of the tissue relationships of an important arid zone species (Acacia aneura, F. Muell.) in comparison with two mesophytes. Bull. Res. Council Israel, sect. D 8: 159-168. 78. Slatyer, R.O. and I. C. Mcllroy. 1961. Practical Microclimatology UNESCO, Paris. 79« Slatyer, R.O. and P.G. J a r v i s . 1966. Gaseous d i f f u s i o n parameter for continuous measurement of d i f f u s i v e resistance of leaves. Science 151: 574-576. 80. Stoutjesdijk, P. 1970. Some measurements of l e a f temperature of t r o p i c a l and temperate plants and t h e i r interpretation. Acta. Botan. Neerl. 19: 373-384. 81. S t r a i n , H.H. and W.A. Svec. 1966. Extraction, separation, estima-t i o n , and i s o l a t i o n of the chlorophylls. In The Chlorophylls. Edited by Vernon, L.P. and G.R. Seeley. Academic Press, New York. 22-66. 82. Thompson, C.R., L.H. Stolzy and G.C. Taylor. 1965. Effect of s o i l suction, r e l a t i v e humidity and temperature on apparent photo-synthesis and t r a n s p i r a t i o n of rough lemon (Citrus .jambii). Proc. Amer. Soc. Hort. S c i . 87: 168-175-83. Todd, G.W. and E. Barker. 1965. Fate of various protoplasmic con-stituents i n droughted wheat plants. Phyton 22: 78-85. 84. T r a n q u i l l i n i , W. 1954. Die Lichtabhangikeit der Assimilation. In Sonnenund Schlattenblattern einer Buche unter Okologischen Bedingungen. Rapp. Congr. I n t . Bot. P a r i s , sect 13: 100-102. As quoted by Larcher 1963• 85. T r a n q u i l l i n i , W. 1963. Die Abhangikeit der Kohlensaureassimilation Junger Larcher, Fichten und Zirben von der Luft-und Bodenfeuch-t i g h e i t . Planta 60: 70-71. 


Citation Scheme:


Citations by CSL (citeproc-js)

Usage Statistics



Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            async >
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:


Related Items