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Light-growth response relationships in Pacific silver fir (Abies amabilis) and subalpine fir (Abies lasiocarpa)
K. KLINKA, Q. WANG, AND G. J. KAYAHARA Forest Sciences Department, The Ut~iversio of British Columbia, Vancouver, B. C., Canada V6T 124
R. E . CARTER Forest Sciences Department, The Universiry of Britidl Columbia, Vancouver, B. C . , Canada V6T 124
and Fletcher Challenge Canada, P. 0. Box 331, 700 West Georgia St., Vancouver, B. C., Canada V7Y 157
AND
B. A. BLACKWELL B. A. Blackwell and Associates, 3087 Hoskins Rd., North Vancouver, B. C., Canada V7J 3B5
Received June 23, 1992
KLINKA, K. , WANG, Q., KAYAHARA, G. J., CARTER, R. E., and BLACKWELL, B. A. 1992. Light-growth response relationships in Pacific silver fir (Abies arnabilis) and subalpine fir (Abies lasiocarpa). Can. J. Bot. 70: 1919- 1930.
Pacific silver fir and subalpine fir, both typically inhabiting high-elevation forests in northwestern North America, were considered shade-tolerant species, the former more tolerant than the latter. To determine their relative shade tolerance, estab- lished advance regeneration was sampled along a light gradient ranging from open areas to inside a forest stand, and analysis of irradiance, growth, and leaf measures was obtained. Relationships between the percentage of above-canopy light (in the photosynthetically active wavelengths) associated with each study tree and its 1991 height increment, 1991 lateral increment, caliper at the base of the 1991 leader, and specific leaf area were examined for individual sites, and sites were grouped accord- ing to soil moisture. There were strong, consistent, and similar relationships between irradiance, growth performance, and specific leaf area for both species. As irradiance decreased, growth performance decreased and specific leaf area increased. In spite of ecological differences between the study sites for each species, it was concluded that on fresh sites, Pacific silver fir and subalpine fir are very tolerant of shade. Both are equally well adapted to survive under high shade and snowpack by allocating more resources to caliper and lateral growth than to height growth and by increasing specific leaf area.
Key words: shade tolerance, advance regeneration, irradiance, growth performance, specific leaf area, Pacific silver fir, subalpine fir.
KLINKA, K., WANG, Q., KAYAHARA, G. J., CARTER, R. E., et BLACKWELL, B. A. 1992. Light-growth response relationships in Pacific silver fir (Abies arnabilis) and subalpine fir (Abies lasiocarpa). Can. J. Bot. 70 : 1919-1930.
Le sapin argent6 du Pacific et le sapin subalpin, tous deux se retrouvant typiquement dans des for& de haute altitude de nord-ouest de 1'AmCrique du Nord, sont considCrCs comme de espbces tolerant l'ombre, la prernibre Ctant plus tolCrante que la dernibre. Afin de determiner leur tolerance relative B l'ombre, les auteurs ont echantillonnC de la regeneration Ctablie en pourtour, selon un gradient de luminositC allant de la pleine ouverture jusqu'8 l'intkrieur du peuplernent. 11s ont mesure l'irra- diation, divers parambtres de croissance et la surface foliaire spicifique. 11s ont examine les relations entre le pourcentage de lurnibre au dessus de la canopee (longeurs d'ondes impliquCes dans la photosynthbse) en association avec chaque arbre CtudiC; ces observations ont CtC conduites pour la saison de 1991 et cornportent l'accroissernent en hauteur, l'accroissement en largeur, le diambtre de la base de la flbche terminale et la surface foliaire spCcifique. Ces observations on Cte conduites sur des sites individuels ainsi que sur des sites regroupks en fonction de l'humiditk du sol. On retrouve des relations fortes constantes et semblables entre l'irradiance, les performances de croissance et les surfaces foliaires specifiques chez les deux espbces. A mesure que diminue l'irradiance, les performance de croissance diminuent et les surfaces foliaires specifiques augmentent. En dCpit des differences Ccologiques entre les sites d'Ctude pour chacune des espkces, les auteurs concluent que sur des sites neufs, le sapin argente du Pacific et le sapin subalpin sont tous deux trbs tolirants a l'ombre. Les deux sont bien adaptCs pour survivre ii l'ombre et sous le couvert de neige en allouant plus de ressources h la croissance en diambtre et en largeur, qu ' i la croissance en hauteur, et an augmentant la surface foliaire spkcifique.
Mots clis : tolCrance l'ombre, regCnCration en pourtour, irradiance, performance de croissance, surface foliaire speci- fique, sapin argent6 du Pacific, sapin subalpin.
[Traduit par la rtdaction]
Introduction nutrient-rich sites (Krajina 1969).
Pacific silver fir (Abies amabilis (Dougl. ex Loud.) Forbes) On the basis of age and structure analyses, these two species and subalpine fir (Abies lasiocalpa (Hook.) Nutt.) are each a of fir are considered climatic climax species (e.g., Krajina major, often dominant, component of coastal and interior 1969; Thornburgh 1969; Brooke et al. 1970; Franklin and forests, respectively, typically in high-elevation in north- Dyrness 1973). In British Columbia, Pacific silver fir often western North America. In British Columbia, Pacific silver fir forms mixtures with western hemlock (Tsuga heterophylla is confined to wet cool mesothermal and maritime subalpine (Raf.) Sarg.) in the wetter, montane subzones of the Coastal boreal climates; the range of subalpine fir is wider, from wet Western Hemlock zone and with mountain hemlock (Tsuga cool temperate to wet montane boreal to wetter continental mertensiana (Bong.) Carr.) in the Mountain Hemlock zone. subalpine boreal climates (Krajina 1969). Relative to climate, Subalpine fir often forms mixtures with white spruce (Picea both species grow most vigorously on fresh to moist and glauca (Moench) Voss) in the Sublbareal Spruce and Boreal Printed in Canada i ImprimC au Canada
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TABLE 1. General location and the characteristics of study sites and the current year's foliage of the study seedlings (based on subsamples of 300 needles)
Pacific silver fir Subalpine fir
Characteristic Mean Min. Max. Mean Min. Max.
Latitude Longitude Elevation (m) Age at the germination point (years) Percentage of above-canopy light 1991 height increment (cm) 1991 caliper (mm) Specific leaf area (cm2 g-')
NOTE: For Pacific silver fir there were 21 sites, 2072 study seedlings, and 198 foliar study seedlings; for subalpine fir there were 6 sites, 652 study seedlings, and 168 foliar study seedlings.
Black and White Spruce zones and with Engelmann spruce (Picea engelmannii Parry ex Engelm.) in the Engelmann Spruce - Subalpine Fir zone of British Columbia (Krajina 1969; Meidinger and Pojar 1991).
Thus, Pacific silver fir and subalpine fir, barring disturb- ance, replace themselves in advanced stages and can partially or fully replace less shade-tolerant associates during earlier stages of secondary succession. This has been attributed to their ability to establish and survive in shade and to persist as suppressed trees in the understory (e.g., Franklin and Dyrness 1973; Henderson 1982; Packee et al. 1982; Oliver and Larson 1990; Crawford and Oliver 1990; Alexander et al. 1990). With adequate seed source, suitable seedbeds, and suitable site conditions they regenerate or their advance regeneration releases on sites disturbed by cutting or windthrow.
This study used the approach and methodology of Carter and Klinka (1992) to search for a quantitative means to char- acterize shade tolerance of tree species. This objective was accomplished by (i) examining the variation in growth and specific leaf area along a light gradient in established advance regeneration of Pacific silver fir and subalpine fir and (ii) com- paring light-growth and light-specific leaf area relationships for the two species with their common associates, western hemlock and white spruce.
Materials and methods
Data from 21 sites for Pacific silver fir and 6 sites for subalpine fir were analyzed (Table 1). Each study site, approximately 0.5 ha, was relatively uniform in topography and soils and supported the growth of well-established advance regeneration (seedlings and sap- lings) exposed to a wide range of light conditions. In all study sites regeneration was within a relatively narrow range in height (approxi- mately 0.5 to 1.5 m) and had a similar history of disturbance. The most recent disturbance to forest canopy had occurred more than 5 years previously for a relatively uniform growth response to release in gaps and felled area (Herring and Etheridge 1976; Herring 1977).
On each study site, approximately 100 seedlings with little or no light competition from herbs and shrubs were sampled along the existing light gradient. Typically, the gradient ranged from open area in clearcuts to shaded area along north-facing stand-edges and right- of-ways, or less shaded areas along south-facing stand-edges to deep within adjacent mature stands. Canopy gaps were used to provide a relatively even sample distribution of various light conditions (Table 2).
Pacific silver fir sites were in the Very Wet Maritime Coastal Western Hemlock (CWHvm) subzone (Klinka et al. 1991a) on Vancouver Island and southern coastal mainland; subalpine fir sites were in the Moist Cold Sub-boreal Spruce (SBSmc) subzone (Meidinger and Pojar
1991) north and south of Smithers in central British Columbia (Table 1). Data recorded for each study site included altitude, aspect, soil
moisture regime (Table 2), ground surface materials, humus form, and vegetation. Soil moisture regimes were identified in the field using a combination of topography, soil morphological properties, indicator plants, and methods described by Klinka et a/. (1984, 1989). Ground surface materials and humus form were described and identified according to Klinka et a/ . (1989). These data were used to explain the variation of regeneration performance in relation to sites and to stratify the study sites into environmentally uniform groupings.
Four measures were made on individual trees (seedling or sapling): height increment (HI), caliper (C), lateral increment (LI), and canopy transmittance (Table 1). Height increment, the 1991 current annual height, was measured to the nearest millimetre using a steel ruler. Caliper, the diameter at the base of the 1991 leader, was measured using a digital micrometer to the nearest 0.1 mm. Lateral increment was measured on only two Pacific silver fir sites (CED2 and CED3), and for each study seedling it was calculated as the mean of no less than six measurements of 1991 lateral branch growth from the first and second whorls, which were measured to the nearest millimetre using a steel ruler.
Canopy transmittance of light in the photosynthetically active range (400-700 nm) was used as the measure of seedling light conditions. Photosynthetically active radiation (PAR) was measured using a Sunfleck ceptometer (model SF-80; Decagon Devices Inc., 1987), and canopy transmittance was determined using a technique similar to that described by Pierce and Running (1988) and used by Carter and Klinka (1992). Measurements were taken between August 5 and September 15, 1991. The amount of PAR associated with each seed- ling (Qi) was sampled using the average of two measurements taken at right angles above each measured seedling twice during the day, i.e., during the pre- and post-noon period that solar angles were between 50" and 65" from vertical. Open-sky PAR (Qo) was mea- sured continuously using a Li-Cor LI-190SA quantum sensor and LI-1000 datalogger (Li-Cor Inc. 1988). Measurements were made under either clear skies or continuous cloud cover, avoiding days of changing cloud cover to minimize variability in Qo. The study sites for each species were within one degree of latitude of each other, minimizing latitudinal variation in the absolute amount of incoming solar radiation. The percentage of above-canopy light (PACL) asso- ciated with each study seedling was calculated as the mean of the morning and afternoon measurements of PACL that were calculated as
Qi [I] PACL = - x 100 Qo
where Qi to Qo is canopy transmittance. Three seedlings, randomly selected from each of seven PACL
classes ( < 5 % , 5-9.9%, 10-14.9%, 15-19.9%, 20-29.9%, 30-49.9%, and r 5 0 % ) , for a total of 21 sample seedlings per study site, in 10 Pacific silver sites and @subalpine fir sites, were used to
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determine specific leaf area (Chiariello er al. 1991). Only the current year's foliage from the laterals of the uppermost whorl was sampled. Collected samples were dried at 105°C for 8 h within 2 to 4 days of sampling. Needles were then stripped off the twigs and counted to prepare subsamples of 300 needles. The subsamples were redried at 105°C for 8 h, weighed to the nearest 0.01 g (W,), and measured twice for the one-sided projected leaf area (A (cm2)) of 300 needles using LI-3000 surface area meter. Specific leaf area (SLA) (cm2 g-I) was calculated as
Percentage of relative height increment (RHI), relative caliper (RC), relative lateral increment (RLI), and relative specific leaf area (RSLA) were used for interspecific comparisons of growth and leaf characteristics (Carter and Klinka 1992). For each seedling, these percentages were calculated as
where X, is the HI, LI, C, or SLA for the ith seedling, (i = 1, 2, . . . , 11); Xo is the reference HI, LI, C, or SLA defined, respectively, as the mean current height increment, lateral increment, caliper, or SLA of the upper half portion of all seedlings growing in greater than 60% of full sunlight in each study site. The X, values were used to represent the potential HI, LI, C, and SLA attainable on each study site in full light conditions (Table 2).
Height increment and caliper of the reference seedlings were found to be the best measure for the denominators of these ratios for three reasons: (i) the largest seedlings were not always growing under full sunlight; (ii) nonlinear regression examining growth-light relation- ships showed that microsite had a greater contribution to height and caliper growth than light at levels greater than 60% of full sunlight; and (iii) this resulted in an adequate number of reference seedlings to give a reliable approximation of potential growth rates when light was not limiting.
The analysis of light-growth relationships was carried out in several steps following Carter and Klinka (1992). Box plots and scat- tergrams were used to examine the distribution and growth pattern of seedlings in relation to PACL. Outliers and the study sites showing excessive variability were checked for possible measurement errors and stability of both morning and afternoon sets of datalogger values. When both sets were stable, the mean of morning and afternoon values was used; when only one set was stable, that set was used for the study seedlings involved. If neither set was stable for a portion of the data set, the study seedlings associated with the instability were deleted; if neither set was stable over the entire data set then that set was deleted from the analysis. The last condition occurred for five Pacific silver fir sites that showed an extreme variability in Qo and a poor (severely truncated) distribution of study seedlings. This was attributed to a very clumpy regeneration pattern, a great variation in contrasting ground surface materials (decaying wood versus forest floor), and a nonuniform growth response of old residual advance regeneration to release.
Visual interpretation of the box plots and scattergrams implied con- sistency in the light-growth relationships between fresh and moist Pacific silver fir sites, but two subalpine fir sites (one moderately dry and another very moist) showed an entirely different pattern than the fresh study sites. Further analysis was conducted on combined data from 21 Pacific silver fir sites and 6 subalpine fir sites, since there were no replications for moderately dry and very moist subalpine fir sites.
All dependent variables were graphically examined for normality using probability plots and for homogeneity of variances by examin- ing scattergrams. For subalpine fir, natural logarithmic transforma- tions were done for RHI, RC, and RHI to RC, since there were increasing variances with increasing PACL. For Pacific silver fir, natural logarithmic transformations were done for RLI, RHI to RC,
TABLE 2. Mean actual height increment (HIREF) and caliper (C,,,) of the reference seedlings used to represent potential growth at full
light conditions for each study site
Study HI,,, CREF Altitude Soil site (cm) (mm) (m) Aspect moisture
CED3 ARR2 BAL2 CAP2 ARRl BAL 1 CAI2 CAP5 CAP4 JAM 1 GOLl GOL3 GOL4 GOL2 CLI 1 JAM3 CED4 ORRl GOL5 CED5 CED2
CHA7 CHA5 HOU6 RIC 13 SKI 1 CHA3
Pacific silver fir
20.1 5.1 480 Gaps 21.4 6.7 10 10" Gaps 26.1 6.3 610" West edge 30.1 7.6 - 470 North edge 31.0 8.0 1010" Gaps 32.0 7.0 630" North edge 32.0 7.6 725" South edge 32.6 7.2 575" North edge 34.3 7.3 545 North edge 34.5 6.9 580" North edge 35.2 7.1 430 Gaps 37.0 7.3 435 Gaps 37.6 7.8 430 North edge 38.8 8.3 435 Gaps 41.1 10.1 760" South edge 41.2 8.6 505 South edge 42.0 8.6 450 South edge 45.0 8.7 575 South edge 48.8 8.9 440 South edge 51.3 9.5 410 South edge 65.2 12.3 535 South edge
Subalpine fir
14.2 6.4 880 North edge 15.7 6.1 880 North edge 19.5 8.1 970 Gaps 20.8 9.0 910 East edge 21.6 11.9 910 North edge 22.0 6.8 880 North edge
Fresh Fresh Fresh Moist Fresh Fresh Fresh Moist Moist Fresh Fresh Fresh Fresh Fresh Moist Moist Moist Fresh Fresh Moist Moist
Fresh Fresh Fresh Fresh Fresh Fresh
"Elevations for montane sites
and SLA, and a square root transformation for RC, since the distribu- tions were nonnormal.
Considering the exponential nature of increase in growth over time (Blackman 1919), the applicability of several nonlinear growth func- tions to the data was examined. The equation most appropriate for height growth (as well as lateral growth and the ratio of height to lateral growth) data was
where ln(RH1) is transformed RHI by natural logarithm, a, b, and c are parameters to be estimated, e is the base of the natural logarithm, and E is the error term of the equation.
Equation 4 is one of the forms of Chapman-Richards growth func- tion (Pienaar and Turnbull 1973), also known as the monomolecu- lar or Mitcherlich function (Richards 1969). The parameter c was included because when measured PACL values were equal to 0, the actual light condition was not necessarily zero, although very low (i.e., the light conditions were so low as not to be detected by the cep- tometer). Consequently, RHI was not necessarily zero at measured PACL values of 0.
Since there was no apparent nonlinear relationship, the following equation was used for Pacific silver fir to describe the relationship between PACL and a square root transformation of RC:
where a and b are parameters to be estimated. For subalpine fir, the most appropriate equation to describe the
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TABLE 3. Models for the regression of relative height increment (RHI), relative caliper (RC), RHI to RC ratio, relative lateral increment (RLI), RHI to RLI ratio, and specific
leaf area (SLA) on percentage of above-canopy light (PACL)
R2 SEE
Pacific silver fir
[4a] ln(RH1) = 4.446 - 2.529 exp[-0.039(PACL)] [5a] (RC)"' = 6.135 + 0.041(PACL)
RHI [8] - = 1.06 - 0.819 exp[-0.025(PACL)]
RC [4b] ln(RL1) = 4.711 - 1.213 exp[-0.013(PACL)]
Subalpine fir
[4d] ln(RH1) = 4.472 - 2.634 exp[-0.03(PACL)] [6] ln(RC) = 3.237 + 0.129(PACL)'/'
RH1 - 0.355 + 0.007(PACL) [91 - - RC
"IZ2 and SEE are in logarithmic scale. "corrected R' for nonlinear regression. Calculated by SYSTAT as for normal linear regression R' (personal com-
munication, P. J. Fleury, Director of Tecnical Support, SYSTAT Inc., Evanston, Ill.).
relationship between PACL and RC transformed by the natural or without scattergrams are given by species and dependent logarithm was variables (Figs. 1-3). While there were many Pacific silver
fir seedlings growing at PACL levels less than l o % , there [6] ln(RC) = a + b(PACL)I1' + E were only a few subalpine fir seedlings growing at these low where ln(RC) is RC transformed by the natural logarithm and a and light levels. This reflected higher light levels in the understory b are parameters to be estimated. of the interior, lodgepole pine-dominated study stands (Figs.
For both species of fir the equation used to relate PACL to specific IB and ID). The models in Table 3 were used to calculate leaf area (SLA) transformed by the natural logarithm was RHI. RC. RHI to RC. and RSLA values for both fir s~ecies.
where ln(SLA) is SLA transformed by the natural logarithm and a and b are parameters to be estimated.
For Pacific silver fir the most appropriate equation to describe the relationship between PACL and the ratio of relative height increment to relative caliper (RHIIRC) was
RHI [8] - = a [ l - c . e-*(PACL)] + E
RC
where a , b, and c are parameters to be estimated, e is the base of the natural logarithm, and E is the error term of the equation.
Since there was no apparent nonlinear relationship between PACL and RHI over RC for subalpine fir, the following equation was used:
RHI [9] - = a + b(PACL) + E
RC
where a and b are parameters to be estimated and E is the error term of the equation.
For each regression model, the distribution of residuals was graphi- cally examined for normality (Chambers et al. 1983). When deemed appropriate, a logarithmic scale of the x-axis was chosen for present- ing scattergrams and regression lines since it offered the appropriate expression of the light -growth relationships over a wide segment of a light gradient (Carter and Klinka 1992).
Results Ten regression models described the strength of relation-
ships between PACL and growth and leaf variables (Table 3). All models were significant at p < 0.01. Regression lines with
and RLI ind RHI over RLI values for two Pacific siiver fi; sites at nine selected levels of PACL (Table 4).
All dependent variables showed consistent trends, coher- ence, and moderate to strong relationships with PACL; how- ever, there was great variability for any given variable at any given level of PACL (Figs. 1 and 3). In general, variability increased with increasing PACL, indicating that with increas- ing irradiance factors other than light increasingly affect these relationships (Chazdon 1988; Carter and Klinka 1992). The variability in the portion of study seedlings located under high shade was attributed to PACL measurements not properly accounting for irradiance in the form of sunflecks (Carter and Klinka 1992). Chazdon (1988) reported that light usually limits growth at less than 20% PACL and that variations in sunfleck activities among understory microsites are correlated with differences in seedling growth rates. The variability in the portion of study trees located in clearcuts was attributed to the contrasting pattern of microsites and differential responses of seedlings to release from low to high irradiance (Herring and Etheridge 1976; Herring 1977; Oliver and Larson 1990).
Relationships between light and measures of growth Both species showed strong and similar relationships between
PACL and growth, with PACL accounting for 45% or more of the variation in RHI, RC, and RHI to RC (Table 3; Fig. 1). Interspecific differences in any of these variables could not be shown to be statistically significant.
HI and C increased with increasing PACL, with the greatest change occurring between PACLs of -10 and 30% (Fig. 1).
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Percentage of above-canopy l ight
1 10 100
Percentage of above-canopy l ight
Percentage of above-canopy l ight Percentage of above-canopy l ight
FIG. 1. Scattergrams and fitted regression lines of relative height increment (RHI) on percentage of above-canopy light (PACL) for (A) Pacific silver fir and (B) subalpine fir using [4a] and [4d] (Table 3), respectively, and relative caliper (RC) on PACL for (C) Pacific silver fir and (D) subalpine fir using [5a] and [6] (Table 3), respectively.
Increments exceeding 75 % of the potential were approached at PACLs of approximately greater than or equal to 75 %, but the largest and most vigorous study seedlings always occurred under conditions of full irradiance (cf. Murray et al. 1991). In both fir species, there was little change in HI and C between 0.1 and 10% of PACL, and in this PACL region HI was minute (Table 4; Figs. 1A and 1B). RHI was more sensitive than RC to decreasing irradiance. At less than 1 % PACL, RC was approximately 38 % for Pacific silver fir and 27 % for subalpine fir, compared with 7 % and 6% for RHI, respec-
tively, indicating lesser loss in caliper than height growth at low irradiance (Table 4; Figs. 1C and ID).
Using the reference height increment (HIREF) as a measure of growth performance for Pacific silver fir (Table 2), several relationships were inferred but not quantified. First, as one would expect, HIREF appeared to be correlated with the refer- ence caliper (CREF). Second, HIREF appeared to be correlated with elevation, local climate, and soil moisture. Trees on mon- tane sites (approximately 2 575 ..m in elevation) showed a lower height increment than those ori submontane sites. Local
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TABLE 4. The regression models from Table 3 used to estimate the values of relative height incre- ment (RHI), relative caliper (RC), RHI to RC ratio, relative lateral increment (RLI), RHI to RLI ratio, and specific leaf area (SLA) at nine levels of percentage of above-canopy light (PACL)
Characteristic 0.1 1 5 10 20 35 50 75 90
RHI 6.9 7.5 RC 37.7 38.1 RHI to RC 0.24 0.26 RLI 33.1 33.6 RHI to RLI 0.21 0.22 SLA 55.7 53.9
RHI 6.3 6.8 RC 26.5 29.0 RHI to RC 0.36 0.36 SLA 51.2 49.1
Pacific silver fir
Subalpine fir
9.1 12.4 20.6 34.0 38.3 45.3 0.39 0.43 0.50
45.4 42.8 39.4
NOTE: The actual values of HI and C for a given PACL value can be calculated by multiplying the relative value in this table by the reference value in Table 2 and dividing by I00 for the appropriate species and characteristic.
climate, expressed by the aspect of the stand edge adjacent to height growth under high shade, then there must be a PACL the clearcut portion of study sites, appears to influence value at which both HI and LI are equal. Using the equations response to release considerably. In both submontane and for HI and LI (Fig. 2), this level was predicted to occur at montane strata, CREF increased from north-facing to south- 33.4% of PACL. facing clearcut edges: This trend was confounded by increases from fresh to moist (seepage), presumably nitrogen-richer sites. For subalpine fir, however, such comparisons were not evident because of the similarity in elevation and local climate.
The ratio of RHI to RC was examined to identify how these morphological characteristics vary under differing light condi- tions. RHI to RC ratios decreased with decreasing PACL from approximately 1.0 under full irradiance to 0.24 for Pacific sil- ver fir and to 0.36 for subalpine fir at less than l % PACL, with the greatest chance occurring between PACLs of 10 and 30% (Tables 3 and 4). Decreases in RHI to RC ratios and less than 1.0 values manifest a faster rate of decrease in HI than C, the opposite trend to etiolation. This suggests that the response of both species to low irradiance is to suppress height growth in favour of caliper growth.
Our observations and several studies (Herring and Etheridge 1976; Herring 1977; Packee et al. 1982; Henderson 1982; Kimmins 1987; Oliver and Larson 1990) describe the growth form of suppressed advance regeneration for both species of fir as being flat-topped, with little or no leader dominance and lateral branch growth greatly exceeding stem height growth. By resampling two Pacific silver fir sites, this was confirmed by cpantifying relationships between PACL and HI or RLI, LI or RLI, and HI to LI or RHI to RLI ratios (Tables 3 and 4; Fig. 2). LI increased with increasing PACL, with the greatest change occurring between PACLs of 10 and 30%, but virtu- ally no change occurring between PACLs less than 1 and 10%. At PACLs of less than 1 %, LI was nearly fivefold higher (33%) than HI (7%). Similar relationships were noted for subalpine fir but not quantified.
Both RHI to RC ratios and RHI to RLI ratios for Pacific sil- ver fir decreased with decreasing PACL from approximately 0.80 under full irradiance to 0.21 at PACLs of less than 1 %, with the greatest change occurring between PACLs of 10 and 30% (Tables 3 and 4). The decrease in the RHI to RLI ratio and less than 1.0 values reflect a faster rate of decrease in HI than LI. Knowing that the lateral branch growth exceeds the
Relationships between light and specific leaf area Both leaf area and weight increased with increasing irradi-
ance, but the rate of increase in weight was greater than in leaf area. Although not measured, the main effects of light on leaf dimensions were attributed to a decrease in leaf length and an increase in leaf thickness as irradiance increased; the minor confounded effect was attributed to a decrease in leaf width with increase in temperature along the light gradient from high shade to open areas (Brix 1967).
Both species of fir had similar SLA values, albeit Pacific sil- ver fir was slightly higher than subalpine fir, and showed simi- lar SLA - and RSLA -PACL relationships (Tables 3 and 4; Figs. 3 and 5). Equations 7a and 7b accounted for 48% of the variation in SLA, without interspecific differences shown to be statistically significant. SLA increased gradually from approximately 30 to 50 c m Q g - I, i.e., approximately just under a twofold increase from high to low irradiance. The effect of light availability on SLA was reported for many other plants, especially those that were shade tolerant (e.g., Black- man and Wilson 1954; Brix 1967; Jackson 1967; Daubenmire 1974; Fitter and Hay 1989; Kozlowski et al. 1991).
Comparison of light-growth relationships of the$r species to their climax associates
In the absence of significant differences in growth and leaf measures along a light gradient for the two fir species, it might be expected that using the same approach, methods, and mea- sures, there should not be significant differences between these two species and their most common associates, western hemlock and white spruce. To examine this hypothesis, the data from this study was compared with those for western hemlock (Carter and Klinka 1992) and white spruce (G. J. Kayahara, R. E. Carter, K. Klinka, and D. Coates, unpub- lished data). The white spruce sites were in the same climatic area as subalpine fir sites, while the western hemlock sites were distributed across both drier and wetter subzones of the Coastal Western Hemlock zone-of southern British Columbia.
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FIG. 2. Fitted regression lines of the actual height increment (HI) (HI = 0.521 exp(4.637 - 2.66 exp[-0.02(PACL)] ) ), lateral incre- ment (LI) (LI = 0.271 exp(4.711 - 1.213 exp[-O.O13(PACL)])), and HI to LI ratio (HIILI = exp{-0.182 - 1.373 exp[-0.036- (PACL)])) on percentage of above-canopy light (PACL) for two Pacific silver fir sites.
Visual interpretation of the fitted regression curves for growth and leaf measures showed little interspecific variation (Figs. 4 and 5). RHI - , RC - , and RHI to RC -PACL rela- tionships were very similar for the fir species and white spruce but somewhat different for western hemlock. At PACL values of l o % , the RHI for western hemlock was greater than 30% but less than 15% for other species (Fig. 4A). From PACL values of approximately 5 % , western hemlock consistently attained a given height increment at lower irradiance than other species. In examining net photosynthesis, Thornburgh (1969) found western hemlock to be slightly superior to Pacific silver fir, especially under low temperature and irradi- ance. At PACL values of 50%, RHI for white spruce was approximately 50%, which is greater than the 25 % growth reduction (i.e., RHI = 75%) reported by Logan (1969).
At very low irradiance, RC for western hemlock was con- siderably lower than for other species (Fig. 4B). The poor caliper growth appeared to be manifested in the growth form of suppressed advance regeneration that had flat-topped crowns and a poorly developed (often lacking) recumbent and pendulous leader, offering little resistance to the snow. Again, from PACL levels of approximately 5 %, caliper increased with increasing PACL at a higher rate than in other species. This suggests a capacity of western hemlock to respond rapidly in caliper growth to increased irradiance.
There were no significant differences between the fitted regression curves describing the RSLA-PACL relationship for the fir species and western hemlock, but the curve for white spruce was notably different (Fig. 5A). While there was a nearly twofold increase for the fir species and western hem- lock, RSLA for white spruce remained virtually unchanged across the light gradient. Examination of the actual SLA- PACL relationship (Fig. 5B) was complementary in demon- strating that at any PACL level (i) the actual SLA decreased in order from western hemlock to Pacific silver fir to subalpine
Percentage of above-canopy light
Percentage of above-canopy light
FIG. 3. Scattergrams and fitted regression lines of specific leaf area (SLA) on percentage of above-canopy light (PACL) for (A) Pacific silver fir using [7a] (Table 3) and (B) subalpine fir using [7b] (Table 3).
fir to white spruce, with the SLA for western hemlock being considerably greater for all levels of PACL, (ii) the slope of the curve for the true fir species and western hemlock was similar, and (iii) the curve for white spruce was almost parallel to the x-axis.
Discussion
This study, examining variations in growth and leaf mea- sures of advance regeneration along a light gradient, found the approach and methodology developed by Carter and Klinka (1992) useful in characterizing and providing insight into growth-light relationships in Pacifi: silver fir and subalpine
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Percentage of above-canopy l igh t Percentage of above-canopy l igh t
Percentage of above-canopy l igh t
FIG. 4. Fitted regression lines of (A) relative height increment (RHI) on percentage of above-canopy light (PACL), (B) relative caliper (RC) on PACL, and (C) RHI to RC ratio on PACL for Pacific silver fir (Ba), subalpine fir (Bl), western hemlock (Hw), and white spruce (Sw) on fresh sites.
fir. As in the previous study, growth. measures were found to decrease exponentially with decreasing irradiance, with the best fit of the developed regressions having occurred at PACL levels of less than 30%. This implies that light is the limiting factor at low irradiance (Shirley 1929). Both species of fir have similar growth and specific leaf area patterns in relation to light and thus could be considered to have equal tolerance of shade.
Constraints of the study The inherent and imposed constraints of the approach and
methods were discussed by Carter and Klinka (1992). In addi- tion to PACL being an imperfect measure of a seedling's irradiance, this exploratory study used established advance regeneration and assumed a link between growth, specific leaf area, and photosynthetic capacity. There was no opportunity to examine changes through time i? mortality and the variation
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in growing space within sites. Under certain light conditions, seedlings might have established but died, or may not have occurred for reasons other than the light environment. This study did not use other growth measures, such as root mass and shoot to root ratio, and most importantly, physiological measures, which are needed to determine whether the assump- tion of the link between photosynthetic capacity and growth is valid.
The growth performance of advance regeneration of both species of fir was influenced by variation in (i) microsites, (ii) mutual competition, and (iii) physiological conditions (vigour) of individual seedlings (Herring and Etheridge 1976; Herring 1977). Nearly every study site featured an intricate pattern of microsites that were characterized, in order of improved nutrient availability (Klinka et al. 1991b) and growth, by the following humus forms: Resimors (compacted substrates that originated from coarse woody debris and were dominated by a dense thicket of ericaceous shrubs), Lignomors (substrates composed nearly entirely of soft decaying wood), Humimors (amorphous humus materials dominated by bryo- phytes), and commonly in clearcuts, Mormoders and Lepto- moders (friable forest floor materials supporting the growth of fireweed).
The stocking pattern of advance regeneration on the study sites was highly variable and clumpy, similar to that described for both species of fir by Herring and Etheridge (1976) and Herring (1977). They concluded that the high mutual competi- tion in overstocked clumps would decrease growth perfor- mance. They also concluded that the height growth response of suppressed advance regeneration to release may be delayed for several years and may continue to increase over a number of years. This delay and its variation is likely the result of time needed for foliar adaptation to full irradiance as well as root expansion, and the ability to respond is closely related to seed- ling vigour (age, size, and growth form) before release (Oliver and Larson 1990).
Shade tolerance Pacific silver fir is rated as one of the most. if not the most.
shade-tolerant coastal species; similarly, subalpine fir is rated as more shade-tolerant relative to its interior associates (Baker 1950; Krajina 1969; Daniel et al. 1979; Minore 1979; Hender- son 1982; Packee et al. 1982; Crawford and Oliver 1990; Packee 1990; Nienstaedt and Zasada 1990). As direct mea- surements of shade tolerance are not available, there are con- tinuing uncertainties regarding their true shade tolerance. For example, western hemlock is considered to have equal, greater, or lower shade tolerance than the fir species, and white spruce to have equal or lower shade tolerance than the fir species and western hemlock.
Shade-tolerant tree species survive in low irradiance, but their growth is reduced. Very shade-tolerant species survive at PACLs of less than 5 %, while very shade-intolerant species need a minimum of 30% PACL (Oliver and Larson 1990). The longevity of advance regeneration (Table 1) and consistent (greater than 5 % RHI) growth at PACLs of 1 % indicates that on this basis, all species examined in this study qualify to be ranked as tolerant of shade. However, when established, all of them perform best at full irradiance, suggesting that in the environments studied, they require neither shade nor full light for growth (Klinka et al. 1990).
Shade-tolerant plants usually respond to low irradiance (i) by reducing respiratory rate, which lowers the compensation
Percentage of above-canopy light
Percentage of above-canopy l ight
FIG. 5. Fitted regression lines of (A) relative specific leaf area (RSLA) on percentage of above-canopy light (PACL) and (B) specific leaf area (SLA) on PACL for Pacific silver fir (Ba), subalpine fir (Bl), western hemlock (Hw), and white spruce (Sw) on fresh sites.
point and growth rate, and (ii) by increasing SLA (by increas- ing leaf area and by decreasing leaf mass), which provides a greater surface for absorption of photosynthetically active radiation (Fitter and Hay 1989).
The first mechanism was implied as having been adopted by all four species in the present study, since under low irradiance there was no evidence of etiolation and suppression of lateral growth as has been observed in shade-intolerant species (Fitter and Hay 1989; Kozlowski et a1:- 1991). Concurrently with
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decreased HI under high shade, a physiological shift occurred that favoured growth on horizontal shoots instead of vertical ones, i.e., HI to LI ratio decreased with decreasing irradiance, as supported by Oliver and Larson (1990); however, increased lateral growth was conclusively proven only for Pacific silver fir. Terminals of the true fir species and white spruce were very short compared with the laterals but sturdy because of relative maintenance of caliper growth giving the trees a flat top but erect appearance (Henderson 1982; Packee et al. 1982; Alexander et al. 1990; Crawford and Oliver 1990). This growth form suggests a continuing strong epinastic control under high shade. However, the decurrent instead of recurrent growth form of western hemlock under high shade suggested a weakened epinastic control as its terminal grew sideways as lateral branches and frequently with no distinct central leader remaining (Oliver and Larson 1990; Packee 1990).
SLA is-considered an im~ortant index of leaf structure since it is largely a function of'leaf thickness and the amount of mechanical tissues in leaves. It varies considerably between species and is plastic within individuals (Chiariello et al. 1991). Therefore, variation in SLA within and between spe- cies indicates (i) the ability of leaves to utilize intercepted radi- ation better and (ii) responses in leaf morphology to varying irradiance. Twofold increases in SLA for both fir species and western hemlock from PACLs of less than 1 % to 100% sug- gest the presence of the second mechanism and a similar degree of shade-induced physiological adaptation to low irradiance. The fact that the same study seedlings, which now experience open-sky light conditions, had been growing several years ago at marginal irradiance emphasizes the rapid- ity and plasticity with which these species can adjust their SLA to full irradiance. This response, as well as some of the mor- phological and physiological mechanisms involved, has been reported for western hemlock following clear- and shelter- wood cuttings (Tucker and Emmingham 1977) and has been documented for Pacific silver fir over a 3-year period (Tucker et al. 1987).
In relation to the fir species, the SLA of western hemlock was approximately over 60% higher at any PACL level and that of white spruce approximately 60% lower at a PACL of 1 %. However, it is problematic to consider western hemlock more tolerant than the fir species on the basis of SLA alone, particularly without any available photosynthetic measures. The consistently low SLA of white spruce, which implies that there is little difference between its sun and shade leaves (e.g., Kimmins 1987; Fitter and Hay 1989; Kozlowski et al. 1991), justifies considering white spruce less tolerant than both spe- cies of fir and western hemlock.
Based on the consideration of all relationships, both species of fir showed similar growth characteristics under different levels of shading. Kimmins (1987) suggested that increased caliper growth to support the lateral growth is one of the adap- tations of Pacific silver fir for surviving at low irradiance. By strongly suppressing height growth, both species apportion more resources to lateral growth to intercept more radiation available from sunflecks while maintaining strong epinastic control. By doing so, seedlings must also apportion more resources to caliper growth to support a large branch biomass, which provides additional structural support in the high snow- pack environment of the Mountain Hemlock and Engelmann Spruce - Subalpine Fir zones (Brooke et al. 1970; Klinka et al. 1991a; Meidinger and Pojar 1991). Thus the seedling growth form at low irradiance is vertically stunted but sturdy,
erect, and horizontally expanded (Crawford and Oliver 1990). It is quite common to find in the understory of closed-canopy stands saplings of Pacific silver fir that are less than 1 m tall but have branches over 1 m long (Kimmins 1987; Tucker et al. 1987).
Shade tolerance represents a useful but evasive plant attrib- ute because of the influence of factors other than light on the capacity of a tree to compete under low irradiance. Kozlowski et al. (1991) alluded to the near impossibility of making close comparisons of shade tolerance between species from different regions. There is a consensus that shade tolerance expresses the relative genetic and physiological capacity of the plant to develop in a given environment, with the capacity to withstand low irradiance being generally the most important characteris- tic (Daniel et al. 1979). Hence, tolerance is not a constant for a species under all circumstances and will likely vary with site (e.g., Krajina 1969; Daniel et al. 1979; Kozlowski et al. 1991 ; Carter and Klinka 1992).
Because the study sites for each of the fir species were cli- matically and edaphically similar, intraspecific variation in the studied growth and leaf measures could not be shown. How- ever, such variation was reported by Krajina (1969), Scott et al. (1976), Henderson (1982), Crawford and Oliver (1990), and Alexander et al. (1990). This study showed that Pacific silver fir and subalpine fir developed a special growth behav- iour to endure high shade and snowpack. This behaviour is absent in western hemlock, which prevails in low snowpack and high rainfall climates. In the environments studied for each species, western hemlock, Pacific silver fir, and sub- alpine fir have approximately equal shade tolerance that is greater than that of white spruce.
Applications to management Manipulation of the stand canopy can create the desired
environment for photosynthesis of trees below the canopy including advance regeneration. This manipulation presup- poses knowledge of how various tree species grow under a given set of light and ecological site quality conditions. Per- haps the most significant contribution of this study is the quan- tification of light-growth relationships that for application to silvicultural management still needs to be related to structural characteristics of forest stands.
On a biological basis, very shade-tolerant Pacific silver fir and subalpine fir could be regenerated by a variety of regener- ation cuttings (e.g., Franklin and DeBell 1973; Burns 1983; Smith 1986; Alexander 1987; Klinka and Carter 1991). Con- sidering the abundance of advance regeneration under high shade in many mature and old-growth stands (e.g., Herring and Etheridge 1976; Herring 1977), the most efficient way to regenerate these species may be by utilizing advance regenera- tion. Its longevity offers a bonus of increasing flexibility in scheduling regeneration cuttings.
In situations where advance regeneration exists as seedlings and saplings, release or selection cutting can be carried out depending on the management objective (Oliver and Larson 1990). When opting for a selection cutting, adequate irradi- ance (PACL of greater than 30%) should be created to provide for the height growth to equal or exceed lateral growth; how- ever, the growth potential is realized at PACLs of greater than 75%.
There is considerable amount of evidence relating seedling vigour to response to release (Oliver and Larson 1990). The advance regeneration of pacific-silver fir that most vigorously
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responsed to release was less than 30 years old (Long 1976; Wagner 1980). Therefore, we recommend (i) the present criteria for assessing the suitability of advance regeneration of Pacific silver fir and subalpine fir for release be expanded to include a vigour rating and (ii) recognition of their potential response to release be improved by the characterization of microsite patterns present in a stand.
In a situation where natural regeneration is a potentially viable option (i.e., fir trees are seed-producing and major spe- cies on a site, appropriate seedbeds are present in the stand, and potential competing vegetation will not impede natural seeding) and where advance regeneration has not yet devel- oped because of excessive high shade, action is needed to induce the understory reinitiation stage. A light thinning (a preparatory cut) in late-immature stands or a light seed cut in mature stands to reduce high shade may be all that is necessary to stimulate the establishment of a seedling bank in the under- story.
Conclusions There were strong, similar, and consistent relationships
between irradiance, growth measures, and specific leaf area for Pacific silver fir and subalpine fir. Growth performance decreased with decreasing irradiance, with height growth decreasing more rapidly than caliper and branch growth. Specific leaf area increased approximately twofold along the same gradient. These results suggest that (i) percentage of above-canopy light is a useful measure of plant light environ- ment, (ii) the measures of growth and specific leaf area are useful criteria for the characterization of shade tolerance, (iii) both species are equally well adapted to survive in high shade and snowpack, and (iv) in the environment studied, western hemlock, Pacific silver fir, and subalpine fir could be considered to have equal shade tolerance that is greater than that of white spruce.
Acknowledgments The authors thank Dr. H. Qian, Forest Sciences Depart-
ment, The University of British Columbia, for assistance in data analysis, Dr. D. P. Lavender, Forest Sciences Depart- ment, The University of British Columbia, and D. Coates, B.C. Ministry of ~ o r e s t s , for advice and comments on the manuscript. Financial support for the study was provided by the Prince Rupert and Vancouver Forest Regions to the B.C. Ministry of Forests, and the Natural Sciences and Engineering Research Council of Canada. This support is gratefully acknowledged.
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