Variation in shade tolerance of Douglas fir, western hemlock, and western red cedar in coastal British Columbia

Forest Ecology and Management, 55 ( 1992 ) 87-105

87

Elsevier Science Publishers B.V., Amsterdam

Variation in shade tolerance of Douglas fir, western hemlock, and western red cedar in coastal British Columbia R.E.

C a r t e r a a n d K. K l i n k a b

"Fletcher Challenge Canada, PO Box 10058, 700 West Georgia Street, Vancouver, B.C. V7Y 1J7, Canada bDepartment of Forest Sciences, University of British Columbia, Vancouver, B.C. V6T 1Z4, Canada (Accepted 28 January 1992)

ABSTRACT Carter, R.E. and Klinka, K., 1992. Variation in shade tolerance of Douglas fir, western hemlock, and western red cedar in coastal British Columbia. For. Ecol. Manage., 55: 87-105. This exploratory study examined the variation in shade tolerance of Douglas fir, western hemlock, and western red cedar in coastal British Columbia. To analyze the light environment-growth performance relationship, canopy transmittance, height increment, diameter at the base of the leader, vigor, vegetation competition, and ground surface materials data were obtained for naturally established or planted seedlings from 50 study sites which were located across a wide range of climatic and soil moisture conditions. Relationships between the percent of above-canopy light (in the photosynthetically active wavelengths (400-700 nm) associated with each seedling and its relative height increment and leader diameter were examined for individual study sites and groups of sites according to tree species and soil moisture regimes. Results indicated occurrence of both inter- and intraspecific variation in shade tolerance. Western red cedar performed relatively better at lower light levels than western hemlock, and western hemlock better than Douglas fir. The trend of increasing light required to achieve a given level of relative growth performance with increasing available soil moisture was consistent across all three species. It is concluded that the method developed has good utility but more rigorous studies are needed to define light environment-growth performance relationships and to explain ecological and physiological mechanisms involved.

INTRODUCTION

The presence and success of a plant species depends on deficiency or excess with respect to any one of several factors, such as heat, light, water, or nutrients, which may approach the limits of tolerance for that species. Thus, plant species have an ecological minimum and maximum, with the range beCorrespondence to: R.E. Carter, Fletcher Challenge Canada, PO Box 10058, 700 West Georgia Street, Vancouver, B.C. V7Y 1J7, Canada.

© 1992 Elsevier Science Publishers B.V. All rights reserved 0378-1127/92/$05.00

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R.E. CARTER AND K. KLINKA

tween these limits defining tolerance or ecological amplitude. In forest tree species, the term shade tolerance is primarily concerned with light-growth relationships, specifically, with tolerance to shade, i.e. the relative capacity of a species to grow under low light conditions and high root competition (Daniels et al., 1979; Kimmins, 1987 ). Thus, shade tolerance is an attribute determined by several interacting factors that has been learned through inference from observable morphological and growth characteristics of individual tree species and their stands. It is a useful concept but one which evades precise definition and characterization as a tree growing in forest understory is affected not only by reduced light but altered climatic, edaphic, and biotic conditions. A knowledge of shade tolerance and its implication for growth is essential for explaining and predicting vegetation succession and in supporting nearly every stand-level silvicultural decision. Surprisingly, there is very little sitespecific information relating establishment, survival, and initial growth of tree species to varying levels of light from full exposure to full shade. Characterization of inter- and intraspecific variation in shade tolerance is poor and physiological mechanisms explaining these variations remain inadequately understood. Contemporary understanding of shade tolerance has led to the classification or ranking of tree species based on their capacity to withstand low light conditions (e.g. Baker, 1950; Krajina, 1965; Minore, 1979; Burns and Honkala, 1991 ). As no direct measure of tolerance is available, much of this work has been based on field observations using indirect measures where light intensity was assumed to be the determinant of the occurrence and condition of advance regeneration of a species under varying canopy covers and several selected stand attributes (Daniels et al., 1979). Traditionally, tree species have been classified into two or more groups (e.g. tolerant and intolerant; or very tolerant, tolerant, intermediate, intolerant, and very intolerant) or ranked in order of comparative tolerance from the more to less tolerant species. These traditional approaches imply that the shade tolerance of a species is more or less constant across the range of sites in which the species may grow and through all stages of its growth. This implication is difficult to reconcile with the fact that both regional and local intraspecific variation in shade tolerance along environmental gradients and stages of growth have been reported in North American and European literature. Klinka et al. ( 1990 ) report that there are two opposing hypotheses explaining the basis for intraspecific variation in shade tolerance in relation to soil moisture availability. The first suggests that light requirements increase with increasing available soil moisture (Krajina, 1965 ). The second suggests that the light requirements of Douglas fir (Pseudotsuga menziesii Mirb. Franco) increase with decreasing available soil moisture (Atzet and Waring, 1970; Marshall, 1986). This study tested these hypotheses examining both intra-

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and interspecific variation in shade tolerance in Douglas fir, western hemlock ( Tsuga heterophylla Raf. Sarg. ), and western red cedar ( Thuja plicata Donn. ex D. Don in Lamb. ) in coastal British Columbia by examining the pattern of light-growth relationships. MATERIALS AND METHODS

Field analysis Selected study sites were relatively uniform in climate, topography, and soils, and supported growth of well-established natural regeneration or planted seedlings of the same origin, and exposed to a wide range of light conditions. The study trees were generally 1.0-2.0 m in height ( _+0.5 m ) . Usually seedlings were sampled from growing in open-area clear-cuts to slightly shaded areas along north-facing stand edges and right-of-ways into adjacent stands with increasing canopy cover (Fig. 1 ). Canopy gaps provided relatively even sample distribution of various light conditions. Seedling ages were quite variable; relatively open-grown seedlings were generally less than 10 years old, particularly for Douglas fir. However, western hemlock and western red cedar seedlings growing under low light conditions reached ages of up to 44 years. Since this approach utilizes established seedlings, seed bed and climatic conditions at the time of germination and establishment, and survival history could not be examined. The study sites were located across a wide range of climatic and soil mois-

N

j

~J

Fig. 1. Typical sampling situation where seedlingswere sampled along a light gradient from an open-area climate under full sunlight in a clear-cut to partial shade along a north-facing stand edge to full shade under continuous cover of a forest canopy.

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R.E. C A R T E R A N D K. K L I N K A

ture conditions. A total of 50 study sites were located across seven biogeoclimatic subzones. These 50 sites included 18 Douglas fir, 18 western hemlock and 14 western red cedar stands. An attempt was made to gain a representative sample of each species across a range of soil moisture (represented by actual soil moisture regimes or their groupings ) and climate (represented by biogeoclimatic subzones). The biogeoclimatic subzone of each study site was identified from the map of Nuszdorfer et al. ( 1985 ) according to its location. Soil moisture regimes (SMRs) were identified in the field using a combination of topographic and soil morphological properties and the methods described by Klinka et al. ( 1984, 1989 ). Canopy transmittance and 1990 height and caliper increment were measured for approximately 200 seedlings on each study site. All measured seedlings were experiencing little or no aboveground vegetative competition. Canopy transmittance of light in the photosynthetically active range (400-700 n m ) was used as the measure of seedling light environment. For the purpose of the study, this measure was assumed to integrate other, unmeasured, factors that determine shade tolerance (e.g. humidity, soil and air temperatures, air movement, etc. ). Using an approach similar to that of E m m i n g h a m and Waring ( 1973, 1977 ) the light environment of each seedling was then related to the seedling's annual height increment and caliper at the base of the current year's leader in order to quantify light environment-growth performance relationships for each species. Annual height increment was measured to the nearest millimeter using a steel ruler. Caliper at the base of the 1990 leader was measured using a digital micrometer to the nearest 0.1 mm. Where western hemlock was found to have elliptical caliper at the base of the leader, caliper measurements were made across the smaller diameter. Photosynthetically active radiation (PAR) was measured using a 'Sunfleck Ceptometer' (Model SF-80, Decagon Devices, Inc.), and canopy transmittance was determined using a technique similar to that described by Pierce and Running ( 1988 ). The instrument is a hand-held device designed to measure the instantaneous photon flux of PAR (400-700 nm; m m o l m -2 s-2). The ceptometer has 80 light sensors placed at 1-cm intervals along a linear 80 cm wand attached to a battery-powered digital datalogger. The microprocessor scans the 80 light sensors on d e m a n d and calculates the arithmetic average (Decagon Devices, 1987 ). The amount of PAR associated with each seedling (Qi) was sampled using two measurements taken at right angles directly above each measured seedling at two times during the day - - during the pre- and post-noon period when solar angles were between 50 ° and 65 ° from vertical. Open-sky PAR (Qo) was measured continuously using a Li-Cor LI-190SA quantum sensor and LI-1000 datalogger (LI-COR Inc., 1986). Measurements were made under either clear skies or continuous cloud cover avoiding days of changing cloud cover to minimize variability in Qo. All measurements

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were taken between 22 July and 15 October 1990. Canopy transmittance, or percent above-canopy light (PACE) was calculated as P A C E = (Qi/ao) x 100

( 1)

Three additional measures were collected on an individual seedling basis: a subjective assessment of vigor, vegetative competition, and soil substrate. Vigor was assessed on a 1-5 scale defined as follows: ( 1 ) seedling not expected to survive more than 2 years and is of very poor vigor with typical symptoms including poor foliage retention, dieback of leading shoots, arrested growth or loss of apical tendency; (2) seedling is of poor vigor usually showing many of the symptoms described above but continues to produce new foliage and annual increment; (3) seedling is of medium vigor showing poor height and diameter growth but little or none of the symptoms described above; (4) seedling is of good vigor showing adequate height and diameter growth and none of the symptoms described above; ( 5 ) seedling is of very good vigor showing height and diameter growth commensurate with site quality and none of the symptoms described above. Ground surface materials were cOded as follows: ( 1 ) compact forest floors (Mors), (2) friable forest floors (Moders and Mulls), (3) exposed mineral soil, and (4) decaying wood (Klinka et al., 1989).

Data analysis Light environment The percent above-canopy light associated with each seedling was calculated as the mean of the morning and afternoon measurements of PACE. This gave an imperfect measure of light available to the study seedlings due to inherent limitations in PAR measurements and constraints of the study. Firstly, PACE does not properly account for incoming radiation in the form of sunflecks due to their periodicity. Secondly, no compensation was made for differences in PACL due to measurements being made under diffuse versus direct solar beam radiation. Thirdly, an appropriate weighting of temporal and seasonal variation in the light environment of each seedling was not used due to the exploratory nature of the study. The use of both morning and afternoon canopy transmittance measurements above each seedling attempted to minimize some of the error associated with points ( 1 ) and (3), above. Efforts were also made to avoid locating sample seedlings in close proximity to deciduous cover to minimize seasonal changes in light environment. All study sites were within one degree of latitude of each other mini-

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mizing latitudinal variation in the absolute amount of incoming solar radiation.

Growth performance Relative height increment (RHI) and relative caliper (RC) were used as measures of growth performance. This was necessary as meaningful comparison of light environment-growth performance relationships among study sites and species can only be achieved by converting actual values into percentage values. For each seedling, RHI and RC were calculated as current annual height or caliper divided by the mean annual height or caliper of the tallest half of all seedlings growing in greater than 60% of full sunlight (the reference seedlings). Actual height and caliper measurements used to represent RHI and RC values of 100% are given by SMRs in Table 1. In consequence, both variables in the light-growth relationship analyzed were expressed on a relative basis - - light environment as a proportion of above-canopy light and growth as a proportion of the growth of the reference seedlings. Height increment and caliper of the reference seedlings were found to be the best measures for the denominators of these ratios for three reasons: ( 1 ) the largest seedlings were not always growing under conditions of full sunlight (particularly with western red cedar); (2) non-linear regression examining growth-light relationships generally showed that microsite had a greater contribution to height and caliper growth than light at levels greater than 60% PACL; (3) the number of reference seedlings was adequate to give a reliable approximation of potential growth rates when light was not limiting. TABLEI Mean values of actual height increment and caliper for the tallest half of all trees growing at PACL values above 60%. These average values were used to represent a relative height increment (RHI) and a relative caliper (RC) of 100% for each soil moisture regime (SMR) Tree species

SMR

RHI = 100% (cm)

RC = 100% (mm)

Douglas fir

Moderately dry Slightly dry Fresh Moist

20.4 23.1 52.7 65.2

3.8 4.6 7.7 8.9

Western hemlock

Slightly dry Fresh

15.6 37.5

1.7 3.3

Western red cedar

Slightly dry Fresh

14.9 17.6

2.7 2.6

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IN COASTAL B.C.

Light environment-growth performance relationships Due to the imperfect measure of available light to the study seedlings, there was great variability in PACL for any given growth performance level within each study site. Two approaches were used to visually examine the nature of this variability. The first approach used the mean growth performance measure (height or caliper) for each of 50 PACL intervals (each interval spanning a range of 2% PACL). The second approach used the median relative growth performance measure for each interval. The median method was included to reduce the influence of possible outliers on the mean. An example of the RHI-PACL relationship using the mean and median approaches is given in Fig. 2. Both the mean and median methods gave very similar results; therefore, the more easily calculated mean value was used in further analysis. Non-linear regression was then used to relate RHI and RC to PACL values. Several non-linear biological growth models were examined with the Chapman-Richard's generalization of the Von Bertalanffy model (Draper and Smith, 1966; Pienaar and Turnbull, 1973) used in the final analysis. This model has the following form: G = [a( 1 -exp -btQi/Q°)) ]c

(2)

where G is relative growth, Qi and Qo are simultaneous measurements of PAR taken over each study tree and under open sky, respectively, and a (control150

100

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I ......

r

O

I

0

13

[

26

I

I

39

52

mean values

I

65

I

78

91

PACL

Fig. 2. Relationship between relative height increment (RHI) and percent of above-canopy light (PACL) for a western hemlock study site using 2% intervals and the median and mean for each PACL interval.

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ling asymptote ), b (controlling the rate of change ), and c (controlling shape ) are parameters to be estimated. Significant regressions of PACL on RHI or RC using eqn. (2) were found for all but four of the study sites. The results for both RHI and RC were very comparable. Comparison of R 2 values and visual residual analysis of the nonlinear regression analyses suggested there were no important differences between the use of height increment or caliper as the measure of seedling growth performance. RC did have a somewhat higher R z than RHI for Douglas fir. This is likely to be due to the greater tendency of Douglas fir to etiolate under low PACL values. Because the results found for both growth performance measures were so similar all further analysis and discussion is based on relationships to RHI alone. RESULTS

PACL generally had a maximum value of 80-100% and a minimum of less than 1.0% of full sunlight, although several study sites represented truncated intervals of this gradient. Actual height increment and caliper values generally increased with improving ecological site quality and increasing PACL values. Relationships between RHI and PACL were examined on each study site. The distribution of sampled seedlings was examined in relation to PACL and the general shape of the height-light relationship was compared using box plots (Fig. 3a) and non-linear regression (eqn. (2)) across the full range of PACL (Fig. 3b) and across a truncated range from 0 to 30% (Fig. 3c). The truncated range of 0-30% PACL was examined as this was considered to be the range of PACL having the greatest influence on growth performance, with growth performance at light levels greater than 30% PACL becoming increasingly influenced by other environmental factors (Atzet and Waring, 1970; Strothman, 1972 ). Non-linear regression using this truncated range of light conditions improved the overall fit of the model at PACL levels less than 1015% for some species on some sites. However, residual analysis suggested that this analysis tended to overestimate growth response to increasing light at these low PACL levels and greatly underestimated growth response to PACL levels greater than 20-25% (Fig. 3c). Plotting of the results of the regression model on a logarithmic scale over the complete range of PACL levels offered the greatest opportunities for visual interpretation and comparison among sites and species (Fig. 3d). Variance explained by the non-linear regression of PACL on RHI was very high with a mean R2= 0.84 across all 50 sites with only four sites having nonsignificant regressions ( P < 0.05 ). However, despite these high R 2 values, visual residual analysis found the fit to the regression to be quite variable with the 'best fit' often at PACL levels less than approximately 20-30%. This find-

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Fig. 3. An example of the analytical approaches used for each study site: height incrementPACL relationship using box plots across a range from 1 to 99% PACL ((a); centre line and outside edge of each box (hinges) indicate the median and range of inner quartile around the median, respectively; whiskers represent values falling within 1.5 times the absolute value of the difference between the value of the two hinges; asterisks represent outside values), nonlinear regression across the full range of PACL (b), non-linear regression across a range from 0 to 30% PACL (c), and non-linear regression across the full range of PACL using a log x-scale to allow closer examination of RHI at low PACL levels (d). (All examples given are for a western hemlock study site. )

ing is s u p p o r t e d by several other studies w h i c h f o u n d net p h o t o s y n t h e s i s in coastal western h e m l o c k , D o u g l a s fir, and western red cedar was close to saturation at light levels o f 3 0 - 4 0 % full sunlight ( L e v e r e n z , 1981; Major, 1990; G r o s s n i c k l e and Arnott, 1 9 9 2 ) and also supports our earlier assertion that

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environmental factors other than light play an increasing role above PACL levels in this range. Visual interpretation of the estimated regression curves between each species and within a species across climatic and soil moisture gradients suggested that both inter- and intraspecific variation occurred in the RHI-PACL relationship. At lower PACL, RHI values increased in order from Douglas fir to western hemlock to western red cedar. All three species showed uniform trends in intraspecific variation with the PACL required to achieve a given RHI increasing with increasing available soil moisture regardless of variation in regional climate (biogeoclimatic subzones). Intraspecific variation in the RHI-PACL relationship was then further examined by stratifying and combining all data for each species according to SMRs. The Douglas fir study sites were combined into four groups including a combined very dry and moderately dry group, and slightly dry, fresh and moist groups. Western hemlock and western red cedar were stratified into two groups with moderately dry study sites combined with slightly dry, and moist study sites combined with fresh study sites. These two groups reflected observed affinities in intraspecific variation and a poor representation of very dry, moderately dry, and moist SMRs for the respective species. This allowed development of several soil-moisture specific regressions for each species (Table 2) and, using these regressions, calculation of RHI at eight selected levels of PACL (Table 3 ). Soil-moisture specific regression curves are presented by species in Figs. 4, 5, and 6. These curves display a consistent TABLE 2 Coefficients a, b, and c from the non-linear regression model used to develop species- and soil-moisture specific relationships between RHI and PACL (eqn. (2): G= [a( 1 - e x p -b(Qi/Q°) ) ]c Tree species

Soil moisture regime

Coefficients a

b

c

Moderately dry Slightly dry Fresh Moist

10.05 9.67 15.85 10.39

0.066 0.048 0.006 0.022

9.58 9.67 15.93 11.53

Slightly dry Fresh

11.49 10.20

0.058 0.038

9.89 10.15

Slightly dry Fresh

9.85 10.59

0.266 0.062

9.85 9.31

Douglas fir

Western hemlock

Western red cedar

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TABLE 3 Relative height increment (RHI) of Douglas fir, western hemlock, and western red cedar according to soil moisture regime (SMR) at eight levels of above canopy light (PACL) calculated using eqn. (2) and the regression coefficients given in Table 21 SMR

Tree species

PACL 5%

10%

20%

30%

40%

50%

75%

90%

Moderately dry Slightly dry Fresh Moist

27 20 8 13

47 36 15 24

71 58 29 43

83 71 42 58

89 80 54 70

93 85 65 80

96 91 91 97

96 92 105 103

Slightly dry Fresh

30 18

50 33

78 55

94 70

102 81

107 88

112 98

113 100

Slightly dry Fresh

71 26

90 46

96 70

97 83

97 90

97 94

97 98

97 98

Douglas fir

Western hemlock

Western red cedar

~Actual height increment for a given PACL value can be calculated by multiplying the RHI values in this table by the R H I = 100 values in Table 2 for the appropriate species and SMR.

100

I

75

-In-

50

25

F

I

10

100

PACL

Fig. 4. Non-linear regression of percent of above-canopy light (PACL) on relative height increment ( R H I ) of Douglas fir on moderately dry ( M D ) , slightly dry (SD), fresh (F), and moist ( M ) study sites.

98

R.E.CARTERANDK. KLINKA 120

I

100

80

g

6o 4O 20 0

I 10

100

PACL

Fig.5.N•n••inearregressi•ns•fpercent•fab•ve•can•py•ight(PACL)•nre•ativeheightincre• ment (RHI) of western hemlock on slightly dry (SD) and flesh (F) study sites. 120

100

80

"~ 80 40

2O

0

I 10

_

IO0

PACL

Fig. 6. Non-linear regressions of percent of above-canopy light (PACL) on relative height increment (RHI) of western red cedar on slightly dry (SD) and fresh (F) study sites.

trend which implies a decrease in shade tolerance with an increase in available soil moisture for each species. The only exception to this trend was for Douglas fir growing on a fresh and moist study site where the trend showed an apparent reversal, although these two regressions were not found to be significantly different. The same soil-moisture specific regressions (Table 3 ) were used to examine interspecific variation in the R H I - P A C L relationship on slightly dry and

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fresh sites (Fig. 7 ). Similar to the intraspecific comparison above, there is an apparent trend which implies a decrease in shade tolerance in order from western red cedar to western hemlock to Douglas fir regardless of variation in SMR. These apparent interspecific differences could not be shown to be statistically significant. Vigor values were used to determine whether RHI and RC relationships with PACL corresponded with overall visual impressions of seedling performance. An examination of the vigor-PACL relationship showed very similar trends to R H I - P A C L across all species and sites. Douglas fir vigor scores were less than or equal to 3 until PACL levels were above 10%, were 2-4 for PACL levels between 10 and 40%, and 4 and 5 for PACL levels greater than 120

1 oo

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eo

2O

i 10

100

PACL

120

~

100

80

Cw -

60

4O

2O 0

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100

PACL

Fig. 7. Non-linear regressionof percent of above-canopylight (PACL) on relative height increment (RHI) of Douglas fir (Fd), western hemlock (Hw), and western red cedar (Cw) on slightly dry (a) and fresh (b) study sites.

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40-50%. There was no obvious effect on western hemlock and red cedar vigor until PACL levels were less than 5-8%. Ground surface material was found to have a significant effect on growth performance for all species, particularly above 20-30% PACL levels. In general, RHI was greatest on microsites featuring friable forest floors (Moder or Mull humus forms), followed by compact forest floors (Mor humus forms) and exposed mineral soil, with the lowest RHI on microsites featuring decaying wood (Lignomor humus forms). This trend is supported by the results of several previous studies (Klinka et al., 1981; Lowe and Klinka, 1981; Carter, 1982; Kabzems and Klinka, 1987 ), which indicate decreasing nutrient availability in order from Mulls to Moders to Mors to Lignomors. This trend did not continue for western hemlock and western red cedar growing under low light conditions (PACL less than 8% ). Western hemlock seedlings were found to occur exclusively on Mor humus forms (47%) and decaying wood (53%) when growing at PACL levels of less than 8%. Western red cedar seedlings were found to occur almost exclusively on Mor humus forms at PACL levels less than 8%, with less than 3% of all measured seedlings found to occur on decaying wood. Seedlings of both species rarely had a RHI greater than 20% when growing under these low light conditions. DISCUSSION

The method developed to examine possible inter- and intraspecific variation in shade tolerance in tree species showed good utility for characterization of light environment-growth performance relationships, which in turn, presents a possible means for classifying tree species according to their tolerance of shade. Despite shortcomings of the approach and methodology, the analysis of the RHI-PACL relationships in 50 study sites implies that inter- and intraspecific variation in shade tolerance of Douglas fir, western hemlock, and western red cedar does occur in the study area. Use of light environment as the sole measure of those conditions determining shade tolerance - - the ability of plant species to establish, survive, and grow under altered light conditions - - is recognized to be simplistic (Kimmins, 1987). However, examination of the light environment-growth performance relationship which considers environmental factors with direct influence on plants, such as soil moisture and nutrient availability, does have direct utility for improving present knowledge on shade tolerance. This study found light environment-growth performance relationships to be strongest at less than 30% PACL suggesting that light is the most limiting factor at PACLs less than 30%. There were several inherent and imposed constraints of the approach and methodology used in this study. Only established seedlings were studied with little or no opportunity to account for differences in their growth history (e.g.

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mortality, previous suppression by competing vegetation, recent release and undergoing adaptation to higher light intensities, etc. ). Seedlings might not occur under certain light conditions for reasons other than understory light environment such as an unsuitable seed bed, environment-specific predation on seed and/or germinants, etc. Growth performance of seedlings in any study site may have been influenced by microsite variability, and differences in seedling age and height. Finally, our measures of growth performance did not compensate for strategic adaptations the study trees may employ to compensate for changing light environments (e.g. changes in height/diameter ratio due to etiolation and changes in specific leaf area). Changes in specific leaf area in relation to different light regimes are being examined in a follow-up study. These four shortcomings, however, could be easily eliminated by establishing and monitoring a wide range of morphological and growth characteristics in field test plantations located across a variety of light, site, and microsite conditions as was done in this study, or by conducting controlled experimental studies. Both studies would achieve characterization of the limits of shade tolerance, with the former including and the latter excluding competition. Our measurement of PACL using both direct and diffuse radiation, and at only two times during one day of the growing season introduced unknown (assumed to be random) errors in seedling light environment (Reifsynder and Lull, 1965; Reifsynder et al., 1971 ). This imperfection imposed by the exploratory nature of our study could be eliminated in future studies by more frequent measurement of PACL throughout the growing season (mid-June through mid-September) with measures of direct as well as diffuse radiation. Existing knowledge of shade tolerance lacks an absolute scale and objectively defined classes - - thus is often contradictory. For example, Krajina (1965) listed amabilis fir (Abies amabilis Dougl. ex Loud. Forbes) as the most shade-tolerant tree species in the Pacific Northwest, and considered both western hemlock and western red cedar to be of equal tolerance but less tolerant than amabilis fir. Minore ( 1979 ) placed western hemlock into the same class as amabilis fir, and considered western red cedar to be less tolerant. Tolerance of Douglas fir is classed as intermediate (Krajina, 1965; Minore, 1979 ). Although it is accepted that shade tolerance of a tree species is not constant (Daniels et al., 1979 ), characterization of possible regional and local intraspecific variation in tree species population has remained poorly developed. It is proposed that the results of this study can be used to differentiate these classes by selecting appropriate measures of growth and light. Klinka et al. (1990) suggested that the existing tolerance classification or ranking (Baker, 1950; Krajina, 1969; Minore, 1979) recognize initially three classes along a shade tolerance gradient - - tolerant, moderately tolerant, and intolerant. In devising an interpretive tolerance classification, we chose an RHI equal to 25% or more as the minimum arbitrary level of the relative growth capacity.

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Each of the three classes was differentiated by assigning the limits of minimum PACL values to reach the required RHI as follows: tolerant less than or equal to 5% PACL, moderately tolerant less than or equal to 10% PACL, and intolerant less than or equal to 25% PACL. Obviously, different growth and light measures and their limits would result in different classifications, as would the choice of a different number of interpretive classes. The application of these differentiating characteristics resulted in ( 1 ) western red cedar being classified as tolerant on slightly dry and fresh sites (albeit showing a considerably higher tolerance on slightly dry sites); (2) western hemlock being classified as tolerant on slightly dry sites and moderately tolerant on fresh and moist sites; (3) Douglas fir being classified as tolerant on moderately dry sites, moderately tolerant on slightly dry sites, and, intolerant on fresh and moist sites (see Table 3 ). These results impart a considerable intraspecific variation in shade tolerance, thus questioning the usefulness of ranking or classification at the species level alone as any of the tree species studied may be more or less tolerant depending on the available soil moisture of the site in which it grows. A review of the literature suggests that the best growth performance of Douglas fir growing on water-deficient sites, independent of initial survival, is generally found under full or partial (i.e. 75% of full sunlight) light conditions (Strothman, 1972 ). The present study indicates that on moderately dry sites a RHI of 90% or more occurs at 50% PACL. Increases in light intensity are often associated with increases in temperature. The relationship between shade tolerance and temperature is difficult to evaluate as the temperature relations of photosynthesis and respiration, the two processes that determine rates of net photosynthesis, are quite different. All three species required greater PACL levels to reach a given RHI as soil moisture availability increased. This is in direct contrast to the 'increasing light requirement with decreasing soil moisture availability' hypothesis supported by the findings of Marshall (1986 ). This is not simply an artifact of the relative measures of growth performance. Actual height increment generally did not increase with increasing soil moisture availability at PACL levels less than 30% with the exception of western hemlock where actual height was somewhat greater on fresh sites than slightly dry sites (see Tables l and 3 ). Although the results of this study do not support the findings of Atzet and Waring (1970) and Marshall (1986) this may be due to differences in the range of soilwater deficits examined. Further, more intensive studies are required to identify the various eco-physiological processes involved. The issue of shade tolerance, to be resolved, should be separated into survival, and subsequent growth performance. On warm and/or droughty sites survival is greatest where germinants or seedlings are protected by partial shade. This reduces seed bed temperatures and provides a boundary layer reducing vapor-pressure deficits - - both reducing maintenance respiration

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requirements (Strothman, 1972; Marshall, 1986; Livingston and Black, 1987 ). Minore ( 1988 ) found that seedling survival under low light conditions was strongly affected by temperature, with improved survival under relatively lower (6-22 ° C ) temperatures than under higher ( 17-38 ° C) ones. To obtain successful seedling establishment, we must consider the need for its protection or exposure. This requires considering another categorical level in tolerance classification (Klinka et al., 1990) by recognizing (a) protection-requiring tree species (tolerant of shade but whose seedlings require protection from exposure in order to survive), (b) exposure-tolerant tree species (tolerant and moderately tolerant of shade but whose seedlings can survive and grow successfully with or without protection), and (c) exposure-requiring tree species (intolerant of shade but whose seedlings require high levels of light for survival and growth ). CONCLUSIONS

Non-linear regression was able to successfully account for much of the variation in the relationship between percent above-canopy light and relative measures of growth performance in seedlings. This result suggests that the method developed in this study has good utility in estimating general trends in light environment-growth performance relationships. The examination of these relationships indicated that both inter- and intraspecific variation in shade tolerance in the three tree species studied does exist in the study area, with shade tolerance decreasing in all three tree species with increasing soil moisture. An examination of interspecific variation in shade tolerance suggests that shade tolerance is greatest in western red cedar, followed by western hemlock and Douglas fir. More rigorous field and experimental studies are recommended to explain the various ecological and physiological mechanisms involved in shade tolerance of tree species. ACKNOWLEDGMENTS

The authors would like to thank to Dr. D.P. Lavender, Department of Forest Sciences, University of British Columbia, Dr. S.C. Grossnickle, Forest Biotechnology Centre, British Columbia Research Corp., and Dr. D. Minore, Forest Sciences Lab. Pacific Northwest Forest and Range Experimental Station, US Forest Service, for helpful advice and comments on the manuscript. Financial support for this study was provided by the Vancouver Forest Region of the B.C. Ministry of Forests, and the National Science and Engineering Council of Canada. This support is gratefully acknowledged.

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