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Comparative growing space efficiency of four tree species in mixed conifer–hardwood forests
Forest Ecology and Management 177 (2003) 361±377
Comparative growing space ef®ciency of four tree species in mixed conifer±hardwood forests Christopher R. Webster*, Craig G. Lorimer Department of Forest Ecology and Management, University of Wisconsin±Madison, 1630 Linden Drive, Madison, WI 53706, USA Received 26 February 2002; accepted 18 July 2002
Abstract The in¯uence of shade tolerance, canopy position, and tree size on growing space ef®ciency (GSE) in mixed stands of co-occurring conifer and hardwood species was investigated in hemlock±northern hardwood forests. Three alternative measures of twodimensional growing spaceÐtotal crown area (TCA), exposed crown area (ECA), and a projection of the total available growing space (AGS)Ðwere investigated to clarify the comparative importance of shaded and illuminated crown regions and unoccupied space in the forest canopy. GSE was expressed as ratios of stem volume increment and biomass increment per unit of growing space. Late-successional, shade-tolerant species have often been portrayed as slow growing, inef®cient users of their growing space; however, hemlock (Tsuga canadensis), which is one of the most shade-tolerant conifers in North America, was the most ef®cient canopy tree in our sample across all measures of GSE. Likewise, the mid-tolerant yellow birch (Betula alleghaniensis) tended to be less ef®cient than the more shade-tolerant maples (Acer rubrum and Acer saccharum). For all species, volume increment per unit of growing space increased with increasing tree height and canopy position, but within a given stratum decreased with increasing crown size. The relative ef®ciency of each species did not appear to be in¯uenced by the measure of growing space employed. In most cases, volume and biomass increments per unit of ECA and AGS were signi®cantly greater p < 0:05 for intermediate than dominant crown class trees. However, for a given level of ECA or AGS, ef®ciency did increase with increasing relative height, which suggests that ef®ciency is in¯uenced by the relative vertical position of growing space in the forest canopy. In general, the shaded area of a crown (i.e., TCA ECA) was not a signi®cant predictor of volume increment once height and ECA were known, suggesting that once 100% canopy closure is reached, packing trees more tightly may not increase stand-level production. However, mean volume increment per unit of TCA scaled more accurately to the stand-level than mean volume increment per unit of ECA. Potential scaling problems associated with mixed-species stands are discussed. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Hemlock±northern hardwoods; Growing space ef®ciency; Acer rubrum; Acer saccharum; Betula alleghaniensis; Tsuga canadensis
1. Introduction Growing space ef®ciency (GSE), traditionally de®ned as stem volume growth per unit of crown * Corresponding author. Present address: School of Forestry and Wood Products, Michigan Technological University, 1400 Townsend Dr. Houghton, MI 49931, USA. Tel.: 1-906-487-2454. E-mail address: [email protected] (C.R. Webster).
projection area (Hamilton, 1969; Assmann, 1970; Sterba and Amateis, 1998), provides a conceptual link between physiological aspects of tree growth and tangible measures of wood production. The underlying rationale behind this ratio is that trees allocate carbon to stem wood production only after other needs have been met (Kozlowski and Pallardy, 1997). Therefore, the amount of wood produced per unit of growing
0378-1127/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 2 ) 0 0 3 9 4 - 8
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C.R. Webster, C.G. Lorimer / Forest Ecology and Management 177 (2003) 361±377
space provides an index of the comparative vigor of individual trees and forest stands (Waring et al., 1980). Individual-tree ef®ciency is strongly in¯uenced by tree size and competitive status and tends to increase with increasing canopy position (Hamilton, 1969; Assmann, 1970; Roberts and Long, 1992; O'Hara, 1996). However, within a given height stratum, trees with large crowns, while often fast growing, tend to be less ef®cient per unit area than trees with smaller crowns (Assmann, 1970; O'Hara, 1988; Sterba and Amateis, 1998). Numerous thinning studies suggest a similar trend at the stand level since the greatest standlevel productivity typically is achieved when smallcrowned trees are tightly packed together (Zeide, 2001). As a result, large dominant and codominant trees have often been viewed as inherently inef®cient (Assmann, 1970). Most studies of GSE have focused on the ef®ciency of individual trees in monocultures (Hamilton, 1969; O'Hara, 1988; Long and Smith, 1990; Larocque and Marshall, 1994a; Stoneman and Whitford, 1995; Gilmore and Seymour, 1996; Sterba and Amateis, 1998; Kollenberg and O'Hara, 1999) with only limited research in mixed-species conifer stands (Kaufmann and Ryan, 1986; Smith and Long, 1992). Natural mixed-species stands are often vertically complex because shade-intolerant tree species with rapid juvenile height growth rates overtop slower-growing, shade-tolerant species following stand initiation (Hibbs, 1982; Kelty, 1986; Fajvan and Seymour, 1993; Oliver and Larson, 1996). Colonization of small canopy gaps by shade-tolerant species during later stages of stand development is another important source of structural heterogeneity (Fajvan and Seymour, 1993; Oliver and Larson, 1996). Assmann (1970) suggested that the spreading crowns typical of shade-grown trees in uneven-aged stands make them especially inef®cient once they reach the canopy. However, large canopy gaps in uneven-aged forest stands often have a dense sapling cohort that prevents individual trees from developing plagiotropic crowns. Also, recent work by Smith and Long (1992) suggests that GSE may actually increase with increasing shade tolerance in conifers. Nevertheless, relatively little is known about the comparative GSE of co-occurring conifer and hardwood species in vertically complex, uneven-aged stands, and it remains unclear whether shade-tolerant species are more or less ef®cient users
of their growing space than less shade-tolerant species. Likewise, even though shade-tolerant tree species are often perceived as slow growing because of comparatively slow initial heightgrowth rates, it is unclear whether these species are really less productive than more shadeintolerant species once they reach the canopy. Detailed analyses of GSE may help clarify different competitive strategies of species in natural mixed stands and also provide a means of scaling from tree-level to stand-level production. Leaf area allocation models that scale GSE from cohorts or strata to the stand level have provided valuable insights into the comparative ef®ciency of alternative stand structures (O'Hara, 1996, 1988; O'Hara et al., 2001). However, results from a loblolly pine (Pinus taeda) spacing experiment suggest that scaling mean crown ef®ciency to the stand level can be problematic because crowns of a given size may be more ef®cient in an open than a closed stand (Sterba and Amateis, 1998). Consequently, extrapolating the mean crown ef®ciency observed at a given stand density to stand structures with disparate densities may lead to erroneous estimates of stand-level ef®ciency. More detailed analyses of changes in tree growth in response to crown overlap and ef®ciency may help clarify this point and improve the accuracy associated with scaling from individual trees to stands. Uneven-aged, hemlock±northern hardwood forests represent an appropriate system in which to examine the in¯uence of shade tolerance and canopy position on GSE because of their vertical complexity and species composition. This forest type contains two of the most shade-tolerant tree species in eastern North America: eastern hemlock (Tsuga canadensis) and sugar maple (Acer saccharum) (Burns and Honkala, 1990). Other common species include the mid-tolerant yellow birch (Betula alleghaniensis) and the moderately shade-tolerant red maple (Acer rubrum). The objectives of this study are to: (1) examine the comparative GSE of eastern hemlock, sugar maple, red maple, and yellow birch in structurally complex forests, (2) examine the in¯uence of the shaded and illuminated portions of the crown on productivity and GSE, and (3) investigate the accuracy of scaling from individual strata to the stand-level using alternative measures of growing space in mixed-species forests. Since leaf area is highly correlated with stem volume increment (Waring, 1983; Long and Smith,
C.R. Webster, C.G. Lorimer / Forest Ecology and Management 177 (2003) 361±377
1984; O'Hara, 1988), it has been used commonly in recent years as a surrogate measure of occupied growing space (e.g. Waring et al., 1980). However, while leaf area offers some advantages over twodimensional crown projections, limitations include the inability to assess the unoccupied growing space available to individual trees and to differentiate between shaded and illuminated portions of the crown. When crown dimensions are measured directly in the ®eld, it is not dif®cult to differentiate unoccupied growing space around a tree crown and to distinguish shaded versus illuminated portions of the crown. We examined three alternative two-dimensional projections of growing space. 2. Methods 2.1. Study area This study was conducted on the Menominee Indian Reservation in northeastern Wisconsin, USA. Study locations were restricted to the Acer-Tsuga-Maianthemum (ATM) habitat type (Kotar and Burger, 1989) to reduce the effects of natural variation in site conditions on tree growth. This forest type on the reservation has traditionally been managed with single-tree selection on a 15-year cutting cycle. Soils are moderately fertile sandy loams and loams developed in outwash of the brown drift region (Kotar and Burger, 1989). Common soil series on this habitat type include the Onamia, Padus, Pence, and Chetek (Kotar and Burger, 1989). The topography is gently rolling and the climate is continental, with moderately heavy snowfall and cold winters (Albert, 1995). The mean monthly temperature in July is 21 8C (maximum 28 8C) and 8 8C (maximum 4 8C) in January. Mean annual precipitation is 80 cm with a mean annual snowfall of 122 cm (Shawano, WI Weather Station; National Oceanic and Atmospheric Administration, 2000). The stands we examined were all relatively well stocked and on average contained 502:8 40:1 trees ha 1 (12.7 cm DBH), 30:0 1:0 m2 ha 1 of basal area, and 287:0 18:0 m3 ha 1 of total volume. The mean height of sample trees on our plots was 19:6 0:4 m. Maximum tree height of sample trees on the plots ranged from 23.4 to 30.2 m, averaging 27:5 0:5 m. These stands contain somewhat larger
363
and older trees than typical selection stands in the Lake States because of their long management history and the marking guidelines used by Menominee Tribal Enterprises (Table 1). 2.2. Field methods We sampled 201 hemlocks, 91 red maples, 115 sugar maples, and 109 yellow birches. Sample trees were obtained from 20 randomly selected Menominee Continuous Forest Inventory (CFI) plots on the ATM habitat type. To obtain a wide range of tree sizes and canopy positions, we sampled all trees that were at least 12.7 cm diameter at breast height (DBH, 1.37 m) at the time of the last inventory. Data on broken and damaged trees are not reported here. Each tree was assigned to a crown class, as de®ned by Smith et al. (1997), based on its position in the canopy. We make a distinction between overtopped trees (with crowns immediately beneath crowns of the overstory trees) and suppressed trees (shorter saplings in the understory). On each tree, we measured DBH (cm) with a diameter tape and total height (m) with a ®berglass reel tape and clinometer. Two-dimensional projections of crown size and growing space available to each tree were measured as eight-sided polygons, using three alternative de®nitions (Fig. 1). Crown radii were measured by extending a ®berglass reel tape from the bole horizontally along each of eight compass bearings (458 intervals) to the crown edge. The position of the crown edge was determined using a clinometer. We de®ned and measured exposed crown radius to be that portion of the total crown radius not overtopped by branches of taller trees. To assess available growing space (AGS), we measured eight radii from each tree bole to the crown edge of the nearest competing tree. The maximum search distance for a competing crown was 16.67 m, two-thirds the height of a typical canopy tree. Crown radii measurements were used to calculate total crown area (TCA, m2), exposed crown area (ECA, m2), and AGS (m2) as the areas of eight-sided polygons by means of triangle geometry. 2.3. Data preparation and analysis Species-speci®c total stem volume estimates for eastern hemlock, sugar maple, and yellow birch were
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Table 1 Sample tree attributes by species and crown class (mean (1 standard deviation)) Total stem volume (m3)
Total stem biomass (kg)
n
(0.10) (0.10) (0.13) (0.11) (0.09)
0.13 0.23 0.35 0.77 2.68
(0.05) (0.13) (0.22) (0.51) (1.13)
49.97 89.03 136.50 295.61 1032.36
(17.87) (50.04) (85.94) (196.36) (436.65)
15 68 54 37 27
± 0.75 0.80 0.95 1.08
(0.06) (0.10) (0.09) (0.08)
± 0.21 0.28 0.58 1.11
(0.07) (0.12) (0.39) (0.52)
± 105.18 138.73 286.92 549.81
(34.84) (58.98) (195.60) (256.30)
0 10 25 36 20
(3.11) (2.27) (2.55) (2.59)
± 0.66 0.81 0.94 1.05
(0.14) (0.08) (0.10) (0.07)
± 0.19 0.27 0.51 1.26
(0.08) (0.12) (0.23) (0.54)
± 108.36 152.60 285.98 708.34
(43.57) (66.73) (130.57) (305.18)
0 20 25 47 23
(0) (2.55) (2.09) (2.54) (1.86)
0.26 0.66 0.75 0.92 1.02
(0) (0.11) (0.09) (0.08) (0.08)
0.06 0.18 0.27 0.66 1.42
(0) (0.08) (0.23) (0.41) (0.60)
30.08 96.91 144.75 358.10 772.60
(0) (43.14) (125.76) (222.94) (325.94)
1 11 35 42 20
Crown class
DBH (cm)
Height (m)
RH
Hemlock Suppressed Overtopped Intermediate Codominant Dominant
18.33 21.42 24.68 33.12 57.59
(2.58) (4.56) (6.28) (9.54) (12.00)
10.37 13.61 15.69 19.47 26.04
(2.49) (2.30) (2.87) (3.48) (2.61)
0.45 0.61 0.70 0.90 1.12
Red maple Suppressed Overtopped Intermediate Codominant Dominant
± 17.15 19.06 25.54 35.57
(3.12) (3.82) (7.25) (8.15)
± 18.09 19.46 22.54 25.23
(1.69) (2.23) (2.91) (2.83)
Sugar maple Suppressed Overtopped Intermediate Codominant Dominant
± 18.21 20.00 26.75 42.17
(3.52) (4.52) (6.14) (9.59)
± 15.55 19.00 21.72 24.96
Yellow birch Suppressed Overtopped Intermediate Codominant Dominant
13.50 17.14 19.43 28.17 41.55
(0) (3.23) (6.64) (7.64) (8.47)
6.75 15.38 17.11 21.26 23.25
Fig. 1. (a) Pro®le view of total and ECA (m2) and AGS (m2) of an intermediate crown class tree (white crown). Competitors are de®ned as trees whose height is greater than or equal to the height of the widest point of the subject tree's crown. (b) Overhead view of the same tree with additional competitors shown with dark stippling. The total projected crown area includes the shaded area indicated with the dotted line plus the ECA.
C.R. Webster, C.G. Lorimer / Forest Ecology and Management 177 (2003) 361±377
derived from multiple regressions (total volume f (tree height and DBH)) ®t to volume tables presented by Cloutier (1948). For red maple, the equation of Crow and Erdmann (1983) was used. The volume increment of each tree (dm3 per year) was calculated as the difference in total volume between the last two CFI measurement periods (winter 1989 and 1999, respectively) divided by the length of the interval. Because the four sample species vary considerably in speci®c gravity (hemlock 0:38, red maple 0:49, sugar maple 0:56, and yellow birch 0:55; Smith, 1985), a total stem biomass increment (kg per year) was also calculated to clarify potential differences in dry-matter production between species (Assmann, 1970). Total stem biomass was calculated from total volume estimates using species-speci®c relationships between dry weight and volume (Smith, 1985). Three basic measures of GSE (expressed as both dm3 m 2 per year and kg m 2 per year) were calculated for each tree. These included volume and biomass increments per unit of the TCA of the tree (GSEtca), ECA (GSEeca), and AGS (GSEags). Analyses of ECA and total AGS were limited to intermediate, codominant, and dominant crown class trees because suppressed and overtopped trees by de®nition have no ECA or AGS in the canopy. Likewise, trees with only
Table 2 Species-speci®c regressions of GSE (dm3 m
2
365
trace amounts of ECA (<5% of TCA exposed) were also excluded. The relative height (RH) of each tree was calculated by dividing the total height of the tree by the mean height of codominant and dominant trees on the sample plot. This measure provides a quantitative analog to the more subjective crown class assignments. Standard linear and nonlinear regression techniques were used to examine trends in GSE and volume increment as functions of tree stature and position (Bates and Watts, 1988; Chatterjee and Price, 1991). Two-way ANOVAs with blocking (by plot to reduce variance) were used to compare the GSE of different species and crown classes (Neter et al., 1996). Square root transformations on the response variable were used to homogenize variance. The Tukey±Kramer Studentized range test for unbalanced data was used for pair-wise comparisons (Kramer, 1956). The feasibility of scaling GSE from individual strata to stands was examined by selecting ®ve plots from the data set that contained only trees of the four species examined in this report. The TCAs and ECAs were summed for each species by crown class and multiplied by their respective mean ef®ciency. The predicted stand-level volume increments were then compared with the observed volume increment of the
per year) as a function of total height (HT, m), RH, TCA (m2), ECA (m2), and AGS (m2)a
Species
Equation
Adjusted R2
n
Hemlock
ln GSEtca 5:86 2:60 ln HT 0:696 ln TCA ln GSEtca 1:17 2:16 ln RH 0:408 ln TCA ln GSEeca 0:310 1:89 RH 0:648 ln ECA ln GSEags 0:197 2:18 RH 0:763 ln AGS p GSEtca 0:058 0:0657 HT 0:216 ln TCA p GSEtca 0:063 1:23 RH 0:152 ln TCA ln GSEeca 1:01 2:84 RH 0:704 ln ECA ln GSEags 1:12 3:03 RH 0:787 ln AGS p GSEtca 0:433 0:0494 HT 0:254 ln TCA p GSEtca 0:406 1:13 RH 0:238 ln TCA p GSEeca 1:82 1:17 ln RH 0:307 ln ECA ln GSEags 2:09 3:26 ln RH 0:835 ln AGS p GSEtca 0:287 0:055 HT 0:00915 TCA p GSEtca 0:298 1:24 RH 0:00831 TCA ln GSEeca 1:60 2:51 ln RH 0:734 ECA ln GSEags 1:71 2:84 ln RH 0:801 AGS
0.71 0.57 0.58 0.83
201 201 107 107
0.53 0.35 0.52 0.68
91 91 81 81
0.59 0.53 0.63 0.76
115 115 91 91
0.37 0.38 0.38 0.59
109 109 90 90
Red maple
Sugar maple
Yellow birch
a
The overall F-test for each regression was highly signi®cant p < 0:001.
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C.R. Webster, C.G. Lorimer / Forest Ecology and Management 177 (2003) 361±377
Fig. 2. Trends in individual-tree GSEtca (dm3 m 2 per year) as a function of tree height (HT, m) and TCA (m2) for hemlock, red maple, sugar maple, and yellow birch. See Table 2 for equations.
plot. This method is similar to the method proposed by Kollenberg and O'Hara (1999) for even- and multi-aged lodgepole pine (Pinus contorta) stands in Montana. This is not a formal statistical validation of a particular scaling method, but rather is presented for discussion because it illustrates some of the potential scaling problems encountered in mixed-species stands. 3. Results 3.1. GSE in relation to species, stature, and canopy position For all species, tall trees with relatively small crowns tend to be the most ef®cient producers of
volume per unit of TCA (Figs. 2 and 3 and Table 2). At a given height, hemlock was the most ef®cient species at using its TCA followed in decreasing order by red maple, sugar maple, and yellow birch (Figs. 2 and 3). However, while GSE increased continuously with increasing tree height (Figs. 2 and 3), it peaked at between 20 and 30 cm DBH in the hardwoods and at approximately 45 cm DBH for hemlock (Fig. 4a and Table 3). The trend between GSE and DBH was best described by a modi®ed periodic annual increment function (Y exp a b X 1 b X 2 ; Avery and Burkhart, 1994). Increasing tree height or DBH in multi-layered forests generally improves the canopy position and competitive status of a tree, which are usually positively correlated with volume increment (dm3 per year). The volume increment of hemlock, red maple,
Fig. 3. (a) Volume increment per unit of TCA (GSEtca, dm3 m 2 per year) of upper canopy trees (RH 1:0) as a function of TCA (m2) for hemlock, red maple, sugar maple, and yellow birch. (b) Volume increment per unit of ECA (GSEeca, dm3 m 2 per year) of upper canopy trees as a function of ECA (m2). (c) Volume increment per unit of AGS (GSEags, dm3 m 2 per year) of upper canopy trees as a function of AGS (m2). See Table 2 for equations.
Fig. 4. Trends between stem DBH (cm) and: (a) stem volume increment per unit of TCA (GSEtca, dm3 m 2 per year; see Table 3 for 2 equations); (b) annual stem volume increment (DV, dm3 per pyear); p (c) TCA (m ). Equations2 for part (b) are: hemlock 2 (p DV 0:268 0:116 ln DBH, R 0:72, n 201), red maple ( DV 0:279 p 0:123 ln DBH, R 0:65, n 91), sugar maple ( DV 0:156 0:0805 ln DBH, R2 0:49, n 115), and yellow birch ( DV exp 2:611 68:184=DBH(68:184=DBH2 ), raw R2 0:85, mean corrected R2 0:33, n 109). Equations for part (c) are: hemlock (TCA 73:348 30:501 ln DBH, R2 0:58, n 201), red maple (TCA 6:287 0:0709 DBH 0:0291 DBH2 , R2 0:74, n 91), sugar maple (TCA 10:001 1:519 DBH, R2 0:61, n 115), and yellow birch (ln TCA 1:588 1:489 ln DBH, R2 0:60, n 109). The overall F-test for each regression was highly signi®cant p < 0:001.
C.R. Webster, C.G. Lorimer / Forest Ecology and Management 177 (2003) 361±377 Table 3 Nonlinear regression parameter estimates and asymptotic standard errors (ASE) for individual species regressionsa of volume growth per unit of TCA (GSEtca) as a function of tree DBH (1.37 m)b Species
Parameter
Estimate
ASE
Hemlock (0.70, 0.13)
a b
4.99 90.60
0.094 4.151
Red maple (0.74, 0.14)
a b
4.42 61.53
0.146 5.474
Sugar maple (0.66, 0.02)
a b
4.08 60.75
0.117 4.651
Yellow birch (0.62, 0.07)
a b
3.53 48.66
0.143 5.399
GSEtca exp a b DBH 1 b DBH 2 . These regressions correspond to the ®tted curves in Fig. 4a. The raw (1 residual SS/total SS) and mean corrected (1 residual SS/corrected SS) R2 values for each regression are provided under the species name (raw, mean corrected). a
b
and sugar maple all increased signi®cantly with increasing tree DBH (p < 0:001, Fig. 4b). However, the volume increment of yellow birch increased until approximately 30 cm DBH and then slowly declined
(Fig. 4b). TCA, on the other hand, increased rapidly with increasing DBH for red maple, sugar maple, and yellow birch, but increased at a decreasing rate with DBH in hemlock (Fig. 4c). This suggests that increases in tree diameter that improve the canopy position of a tree, without leading to signi®cant increases in crown area, increase the ef®ciency of volume production per unit of TCA (Fig. 4). In general, hemlock appeared to be the most ef®cient and productive large canopy tree in these forests, due at least in part to the high volume growth rate and modest crown areas of larger trees (Fig. 4). Results of a two-factor ANOVA suggested that dominant and codominant trees of each species were generally the most ef®cient users of their TCA (Fig. 5a and Table 4). While GSE tended to be highest in dominant hemlocks, GSE in red maple, sugar maple, and yellow birch appeared to peak in the codominant crown class (Fig. 5a and Table 4). The overall ANOVA was highly signi®cant (root MSE 0:196, F36;515 12:49, p < 0:0001), and both species and crown class were signi®cant determinants p < 0:0001 of volume increment per unit of TCA. A species by crown class interaction term was signi®cant p 0:0008,
Table 4 Species and crown class comparisons of volume increment per unit of TCA (GSEtca, dm3 m and total AGS (GSEags, dm3 m 2 per year)a Hemlock
369
Red maple
Sugar maple
N/Ab n 0 0.19 (0.03) efg n 10 0.49 (0.07) bcde n 25 0.75 (0.07) abc n 36 0.68 (0.08) abc n 20
N/A n 0 0.38 (0.12) 0.31 (0.04) 0.58 (0.04) 0.47 (0.05)
2
per year), ECA (GSEeca, dm3 m
2
per year),
Yellow birch
GSEtca Suppressed Overtopped Intermediate Codominant Dominant
0.17 0.33 0.57 0.85 0.99
GSEeca Intermediate Codominant Dominant
2.53 (0.34) a n 43 1.71 (0.25) abc n 37 1.12 (0.14) bcd n 27
2.30 (0.71) ab n 24 0.89 (0.10) cde n 36 0.69 (0.08) def n 20
1.12 (0.24) bcde n 22 0.83 (0.07) cde n 47 0.52 (0.06) ef n 23
0.81 (0.14) def n 35 0.50 (0.04) ef n 42 0.27 (0.05) f n 20
GSEags Intermediate Codominant Dominant
1.97 (0.35) a n 43 1.55 (0.41) ab n 37 0.53 (0.06) bcde n 27
1.51 (0.59) acb n 24 0.51 (0.09) cde n 36 0.29 (0.05) de n 20
0.93 (0.25) bcd n 22 0.53 (0.07) bcde n 47 0.27 (0.05) de n 23
0.55 (0.12) de n 35 0.33 (0.04) de n 42 0.13 (0.03) e n 20
a
(0.03) (0.02) (0.04) (0.08) (0.08)
efg n 15 cdef n 68 abcd n 54 ab n 37 a n 27
cdef n 20 cdef n 25 abcd n 47 bcde n 23
0.02 0.11 0.32 0.43 0.25
(0.00)c n 1 (0.04) fg n 11 (0.05) def n 35 (0.04) bcde n 42 (0.04) def n 20
For each measure of GSE, means (1 standard error) followed by the same letter are not signi®cantly different (Tukey±Kramer Studentized range test for unbalanced data, a 0:05). Only intermediate (ECA 5% of TCA), codominant, and dominant crown classes were included in analyses with GSEeca and GSEags. b No individuals in this class were represented in our sample. c No statistical inferences regarding this class are possible.
370
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Fig. 5. Means and standard errors for volume increment per unit of: (a) TCA (GSEtca, dm3 m 2 per year); (b) ECA (GSEeca, dm3 m 2 per year); (c) total AGS (GSEags, dm3 m 2 per year). Crown classes are abbreviated as follows: S, suppressed; OT, overtopped; I, intermediate; CD, codominant; and D, dominant.
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suggesting that the relationship between crown class and GSE was not constant across all species. For example, the mean GSE of overtopped sugar maples was unexpectedly greater than that of intermediates, deviating from the pattern observed for the other species (Fig. 5a). However, most of the overtopped sugar maples in our sample were less ef®cient than higher crown classes with a few notable exceptions among individual trees. These in¯uential points increased the overall mean ef®ciency of overtopped trees. To some extent, hemlock's comparatively light wood may contribute to its apparent superiority in growth ef®ciency compared to associated hardwood species. Incorporating wood density reduced the number of signi®cant differences among species; however, hemlock remained the most ef®cient species (Fig. 6a and Table 5). Again, the overall ANOVA was highly signi®cant (root MSE 0:019, F36;515 11:03, p < 0:0001) with species and crown class explaining signi®cant shares of the variation p < 0:0001. The species by crown class interaction also remained signi®cant p 0:0035.
3.2. Ef®ciency per unit of ECA and total AGS While dominants and codominants may use their TCA more ef®ciently than other crown classes, they appear to be relatively inef®cient users of their ECA and total AGS (Fig. 5b and c). Overall ANOVAs of growing space per unit of ECA (root MSE 0:408, F30;375 7:19, p < 0:0001) and AGS (root MSE 0:436, F30;375 5:67, p < 0:0001) were highly signi®cant (i.e., the full model; Table 4). Species and crown class were highly signi®cant in both ANOVAs p < 0:0001, but the interaction between these two variables was not (p 0:1659 and 0.3352, respectively). For all species, intermediates used their ECA and AGS more ef®ciently than higher crown classes. Dominant yellow birches were the least ef®cient trees based on ECA and total AGS. Similar trends were observed for total stem biomass increment per unit ECA (Fig. 6b and Table 5; root MSE 0:276, F30;375 5:32, p < 0:0001) and AGS (Fig. 6c and Table 5; root MSE 0:293, F30;375 4:55, p < 0:0001) with no signi®cant interactions (p 0:1337 and 0.3161, respectively).
Table 5 Species and crown class comparisons of total stem biomass increment per unit of TCA (GSEtca, kg m year), and total AGS (GSEags, kg m 2 per year)a Hemlock
371
2
per year), ECA (GSEeca, kg m
Red maple
Sugar maple
Yellow birch
N/Ab n 0 0.09 (0.02) efg n 10 0.24 (0.04) abcd n 25 0.37 (0.03) a n 36 0.34 (0.04) ab n 20
N/A n 0 0.21 (0.07) 0.18 (0.02) 0.32 (0.02) 0.26 (0.03)
0.01 0.06 0.17 0.23 0.14
2
per
GSEtca Suppressed Overtopped Intermediate Codominant Dominant
0.07 0.13 0.22 0.33 0.38
GSEeca Intermediate Codominant Dominant
0.98 (0.13) a n 43 0.67 (0.10) abc n 37 0.46 (0.05) bcd n 27
2.30 (0.35) ab n 24 0.89 (0.05) cd n 36 0.69 (0.04) cde n 20
0.62 (0.14) abcd n 22 0.47 (0.04) bcd n 47 0.29 (0.03) de n 23
0.44 (0.08) cde n 35 0.27 (0.02) de n 42 0.14 (0.03) e n 20
GSEags Intermediate Codominant Dominant
0.76 (0.13) a n 43 0.60 (0.16) abc n 37 0.20 (0.02) cde n 27
0.75 (0.29) ab n 24 0.25 (0.05) cde n 36 0.14 (0.03) de n 20
0.52 (0.14) abc n 22 0.30 (0.04) bcd n 47 0.15 (0.03) de n 23
0.30 (0.07) cde n 35 0.18 (0.02) cde n 42 0.07 (0.02) e n 20
(0.01) (0.01) (0.02) (0.03) (0.03)
efg n 15 cdef n 68 abcde n 54 ab n 37 a n 27
abcdef n 20 abcdef n 25 abc n 47 abcd n 23
(0.00)c n 1 (0.02) fg n 11 (0.03) bcdef n 35 (0.02) abcd n 42 (0.02) def n 20
a For each measure of GSE, means (1 standard error). Actual means and standard errors are presented, but data were square root transformed for ANOVA. Followed by the same letter are not signi®cantly different (Tukey±Kramer Studentized range test for unbalanced data, a 0:05). Only intermediate (ECA 5% of TCA), codominant, and dominant crown classes were included in analyses with GSEeca and GSEags. b No individuals in this class were represented in our sample. c No statistical inferences regarding this class are possible.
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Fig. 6. Means and standard errors for stem biomass increment per unit of: (a) TCA (GSEtca, kg m 2 per year); (b) ECA (GSEeca, kg m 2 per year); (c) total AGS (GSEags, kg m 2 per year). Crown classes are abbreviated as follows: S, suppressed; OT, overtopped; I, intermediate; CD, codominant, and D, dominant.
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In spite of the comparatively low ef®ciencies of trees in upper canopy layers, coef®cients for RH were positive and highly signi®cant p < 0:001 in speciesspeci®c regressions of volume increment per unit of ECA and AGS versus RH and the respective measure of growing space (Table 2). As with volume increment per unit of TCA, ef®ciency declined with increasing projection area at a given RH (Fig. 3b and c). Therefore, the relative vertical position of a tree's growing space may be an important determinant of ef®ciency, regardless of the measure of growing space employed. 3.3. Differential contributions of exposed and shaded crown regions to productivity ECA appears to be the best single crown-based predictor of volume increment, but differences among crown measures were not great (Table 6). For all species, volume increment increased at a decreasing rate with increasing AGS (Table 6). To assess the contribution of the shaded portion of a tree's crown, the projection area of the shaded portion of the crown (i.e., TCA ECA) was entered into a regression of volume increment in which ECA and total tree height
373
were already present as independent variables. RH was used as an independent variable for yellow birch because neither ECA p 0:236 nor any other crown variable was signi®cant after total height was added. The shaded portions of red maple, sugar maple, and yellow birch crowns were not signi®cant predictors of volume increment once ECA and total or RH were known (p 0:209, 0.390, and 0.463, respectively). The shaded area of hemlock crowns p 0:073 was marginally insigni®cant. This suggests that the moderate predictive power of TCA may result more from the ECA being encapsulated in it than the contribution of the shaded portion of the crown to volume production (Table 6). 3.4. Scaling from strata to stands The mean annual volume increment on the subset of ®ve plots (containing no species aside from hemlock, red maple, sugar maple, and yellow birch) was 4.2 m3 ha 1 per year. By multiplying the sum of TCA per crown class for each species by its respective mean ef®ciency (Table 4), we predicted a total annual volume increment of 4.1 m3 ha 1 per year. Using the same approach with ECA, we predicted a volume
Table 6 Individual species regression equations for mean volume increment (DV, dm3 per year) as a function of TCA (m2), ECA (m2), and AGS (m2)a Species
Equation
d.f.
F-statistic
R2
Hemlock
ln DV p 0:0925 0:9064 ln TCA DV 1:3639 1:2586 ln ECA p DV p 1:8259 0:8583 ln AGS DV 1:7591 0:2837 HT 0:2748 ln ECA
1, 1, 1, 2,
106 106 106 106
114.29 140.49 110.73 301.91
0.52 0.57 0.51 0.85
Red maple
ln DV p 0:1156 0:7653 ln TCA DV 2:4555 0:0555 ECA p DV 1:277 0:7191 ln AGS p DV 3:5617 0:3029 HT 0:0216 ECA p DV p 0:1137 1:0383 ln TCA DV 1:212 0:8171 ln ECA p DV 1:6168 0:5587 ln AGS p DV 2:5882 0:2464 HT 0:2696 ln ECA p DV 0:0837 0:8912 ln TCA p DV 1:0226 0:6814 ln ECA p DV 1:1907 0:5132 ln AGS p DV 1:1993 3:562 RH 0:3523 ln ECA
1, 1, 1, 2,
80 80 80 80
58.87 77.91 54.44 123.66
0.43 0.50 0.41 0.75
1, 1, 1, 2,
90 90 90 90
31.11 54.28 40.29 80.11
0.26 0.38 0.31 0.64
1, 1, 1, 2,
90 90 90 90
31.28 38.47 30.37 25.29
0.26 0.30 0.25 0.35
Sugar maple
Yellow birch
a
Regressions of DV as a function of ECA and total tree height (HT, m) or RH are also presented (adjusted R2 values are presented for multiple regressions). All F-tests were highly signi®cant p < 0:001. Only intermediate (ECA 5% of TCA), codominant, and dominant crown class trees were included in this analysis.
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increment of 4.7 m3 ha 1 per year. Using TCA to scale from individual trees in different strata to the standlevel therefore resulted in an underestimation of standlevel volume increment by approximately 2.7%, while using ECA resulted in an overestimation of 14.8%. Most of the overestimation associated with ECA resulted from overpredicting the potential of hemlock. Consequently, removing crown overlap from estimates of mean ef®ciency did not improve predictions of stand-level production over more traditional approaches using TCA. 4. Discussion Rapid juvenile height growth and relatively inef®cient light capture have often been associated with shade-intolerant, early successional species (Daniel et al., 1979). Nevertheless, relatively few studies have examined the relationship between GSE and shade tolerance. Work by Smith and Long (1992) in conifer forests of the Rocky Mountains suggests that subalpine ®r (Abies lasiocarpa) is more ef®cient than the intolerant lodgepole pine (P. contorta). However, in eastern deciduous forests, Chapman and Gower (1991) found that the mid-tolerant northern red oak (Quercus rubra) was signi®cantly more ef®cient than the tolerant sugar maple. In spite of marked differences in shade tolerance among the four species we examined, no clear trends based on shade tolerance rank were observed in these mixed-species stands. Hemlock was typically more ef®cient than associated hardwood species, but results for the three hardwood species were mixed. In general, red maple was more ef®cient than sugar maple, which was more ef®cient than yellow birch, but few differences were signi®cant once speci®c gravity was taken into account. All four species tended to respond in concert to changes in canopy position and stature regardless of the measure of growing space used. Tall trees with relatively small amounts of growing space were most ef®cient. These results are consistent with work in simpler forests using both leaf area and projected crown area approaches. For example, early work by Hamilton (1969) and Assmann (1970) as well as more recent work by O'Hara (1988) has suggested that tall, narrow-crowned trees tend to be the most ef®cient producers of volume increment on an area basis.
Assmann (1970) synthesized much of the early work on GSE in Europe in his classic ``The Principles Of Forest Yield Study''. Based on observations from numerous research projects, Assmann (1970) concluded that while the smallest crowned trees in a given canopy layer were most ef®cient, medium crowned trees actually represented optimal production. Primary reasons included the relatively low vigor and slow growth rate of small-crowned trees. For example, deep, full crowns and rapid individual-tree growth are generally considered a sign of high stand vigor in lodgepole pine; however, GSE is greatest in stands with short, compact crowns and modest individual-tree growth (Smith and Long, 1989). When measures of illumination are used as surrogates for growing space, this discontinuity becomes more pronounced. Using potential radiation absorption (irradiance projected leaf area) as a surrogate for growing space, Kaufmann and Ryan (1986) found that suppressed and overtopped lodgepole pine, Engelmann spruce (Picea engelmannii), and subalpine ®r were more ef®cient producers of volume increment than higher crown classes. Likewise, intermediate crown class trees of all species examined in our study used their ECA more ef®ciently than higher crown classes. These results illustrate that some measures of GSE may not be suitable as direct indicators of tree and stand vigor and that caution must be used in their application. Measures of ECA do, however, provide a rough approximation of potential radiation, which is a strong determinant of photosynthetic production (Russell et al., 1989). Our results suggest that the contributions of the shaded regions of the crown to stem volume production may be negligible compared to those of exposed regions. The shaded area of the crown was not a signi®cant predictor of volume production once the exposed area of the crown and the height of the tree were known for any species examined. Total AGS, on the other hand, was somewhat less informative than TCA for most species, and once a crown had a small amount of open space around it, further increases in growing space did not lead to further increases in volume increment. These results suggest that heavy release treatments in hemlock±hardwood forests may not be necessary to maximize the volume increment of individual trees. However, for hardwood species, maximizing volume production, even on merchantable
C.R. Webster, C.G. Lorimer / Forest Ecology and Management 177 (2003) 361±377
trees, is usually secondary to maintaining stem quality due to the premium placed on veneer and large sawlogs in these forests. Shade-tolerant, late-successional species, such as hemlock and sugar maple, are commonly considered slow growing, perhaps because they are often overtopped by less shade-tolerant species in even-aged stands as a consequence of differences in initial height growth rate. However, in this study, both hemlock and sugar maple canopy trees had higher rates of volume growth than yellow birch. This is consistent with work in hemlock±hardwood canopy gaps where dominant yellow birch saplings had higher height growth rates in large openings than hemlock and sugar maple saplings, but lower basal area growth rates (Webster and Lorimer, 2002). Similarly, Chapman and Gower (1991) found that at a given diameter, overstory sugar maples were signi®cantly more productive than the less shade-tolerant red oak. These results suggest that comparisons of initial height growth rates may not provide an accurate depiction of comparative productivity once trees reach the canopy, and that there might be cause to reconsider the common assumption that shade-tolerant, late-successional species are inherently slow growing. Our results build on a growing body of evidence that large diameter conifers may be at least as ef®cient as smaller trees. For example, in Scotland, Hamilton (1969) found that volume production per unit of crown projection area in Sitka spruce (Picea sitchensis) increased with increasing tree diameter. Some recent research in the western United States suggests that older cohorts of ponderosa (Pinus ponderosa) and lodgepole pine may be more ef®cient users of their projected leaf area than smaller, younger cohorts (O'Hara, 1996; Kollenberg and O'Hara, 1999). Similarly, Maguire et al. (1998) found that growth ef®ciency in red spruce (Picea rubens) was highest among large and tall trees. Nevertheless, declines in ef®ciency with increasing tree size and age have been observed in lodgepole pine (Long and Smith, 1990), subalpine ®r (Roberts and Long, 1992), and red pine (Pinus resinosa; Larocque and Marshall, 1994a). Our results for hemlock suggest that GSE peaks between 40 and 50 cm DBH and then slowly declines. This range is similar to the range reported by Assmann (1970) for Norway spruce (Picea abies) and silver ®r (Abies alba) in Europe. Regardless of the measure of
375
growing space used, if the occupied growing space increases more rapidly than volume production, ef®ciency declines. Thus, controls on crown and leaf area such as phenotypic differences between species, competition (Assmann, 1970; Larocque and Marshall, 1994b), crown architecture (Smith and Long, 1989), stand structure (Roberts et al., 1993), and stand density (Long and Smith, 1990; Jack and Long, 1991) may ultimately control GSE in large trees. Given that the shaded portion of a tree's crown had no signi®cant effect on volume increment, one might infer that once 100% crown closure is reached, no further increase in production would be gained by packing trees more tightly with overlapping crowns. However, there is evidence that shaded portions of crowns may have at least a minor positive impact on stand-level production. For example, Sterba and Amateis (1998) found that basal area increment per hectare of loblolly pine (P. taeda) increased until crown closure approached 130%. Additionally, if shade-tolerant species are grown as an understory beneath a canopy of shade-intolerant species, they can contribute an ``additive increment'' (Assmann, 1970), and in some cases, stands strati®ed in this fashion may be more productive than single-species stands of either species in the overstory (Kelty, 1989; Menalled et al., 1989; Man and Lieffers, 1999). This may help to explain why TCA in our study provided a better estimate of stand-level production than ECA (2.7% underestimate versus a 14.8% overprediction, respectively). However, it does not explain why use of ECA to extrapolate from tree-level to stand-level production resulted in an overestimate. Most of the overprediction was due to overestimating the volume production of hemlock. One possible explanation may be seasonal variation in ECA. For example, the ECA of a hemlock with mostly hardwood competitors may increase greatly following leaf fall, providing the tree with a few months of off-season photosynthesis over a larger than expected area. A mean ef®ciency value based on hemlocks with hardwood competitors could potentially lead to an overestimate of production. However, if a hemlock has mostly evergreen competitors, competition rather than seasonal factors would in¯uence the amount of crown area exposed to illumination. Therefore, to scale accurately from individual strata to the stand-level in mixed conifer± hardwood forests, it may be necessary to incorporate
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the species composition of neighboring trees into estimates of mean ef®ciency. Acknowledgements Financial support for this study was provided by the McIntire-Stennis Cooperative Forestry Research Program, project WISO4167. We gratefully acknowledge the following groups and individuals: the Menominee Nation for allowing us to conduct this research on their tribal lands; the staff of the Menominee Tribal Enterprises Forestry Center, especially Marshall Pecore, Dan Pubanz, and Matt Duvall, for logistical support; Erik Nordheim for advice on statistical analyses; Eric Kruger for helpful discussions and a review of an earlier draft of the manuscript; and Theran Stautz for assistance with data collection and entry. References Albert, D.A., 1995. Regional landscape ecosystems of Michigan, Minnesota, and Wisconsin: a working map and classi®cation. USDA Forestry Service General Technical Report NC-178. Assmann, E., 1970. The Principles of Forest Yield Study. Pergamon Press, Oxford. Avery, T.E., Burkhart, H.E., 1994. Forest Measurements, fourth ed. McGraw-Hill, Boston. Bates, D.M., Watts, D.G., 1988. Nonlinear Regression Analysis and its Applications. Wiley, New York. Burns, R.M., Honkala, B.H. (Tech. Coords.), 1990. Silvics of North America: (1) Conifers; (2) Hardwoods. USDA Forest Service Agricultural Handbook No. 654. Chapman, J.W., Gower, S.T., 1991. Aboveground production and canopy dynamics in sugar maple and red oak trees in southwestern Wisconsin. Can. J. For. Res. 21, 1533±1543. Chatterjee, S., Price, B., 1991. Regression Analysis by Example, second ed. Wiley, New York. Cloutier, E., 1948. Form-class Volume Tables. Canada Department of Mines and Resources, Ottawa. Crow, T.R., Erdmann, G.G., 1983. Weight and volume equations and tables for red maple in the Lake States. USDA Forestry Service Research Paper No. NC-242. Daniel, T.W., Helms, J.A., Baker, F.S., 1979. Principles of Silviculture, second ed. McGraw-Hill, New York. Fajvan, M.A., Seymour, R.S., 1993. Canopy strati®cation, age structure, and development of multicohort stands of eastern white pine, eastern hemlock, and red spruce. Can. J. For. Res. 23, 1799±1809. Gilmore, D.W., Seymour, R.S., 1996. Alternative measures of stem growth ef®ciency applied to Abies balsamea from four canopy positions in central Maine, USA. For. Ecol. Manage. 84, 209±218.
Hamilton, G.J., 1969. The dependence of volume increment of individual trees on dominance, crown dimensions, and competition. Forestry 42, 133±144. Hibbs, D.E., 1982. Gap dynamics in a hemlock±hardwood forest. Can. J. For. Res. 12, 522±527. Jack, S.B., Long, J.N., 1991. Response of leaf area index to density for two contrasting species. Can. J. For. Res. 21, 1760±1764. Kaufmann, M.R., Ryan, M.G., 1986. Physiographic, stand, and environmental effects on individual tree growth ef®ciency in subalpine forests. Tree Physiol. 2, 47±59. Kelty, M.J., 1986. Development patterns in two hemlock±hardwood stands in southern New England. Can. J. For. Res. 16, 885±891. Kelty, M.J., 1989. Productivity of New England hemlock/hardwood stands as affected by species composition and canopy structure. For. Ecol. Manage. 28, 237±257. Kollenberg, C.L., O'Hara, K.L., 1999. Leaf area and tree increment dynamics of even-aged and multiaged lodgepole pine stands in Montana. Can. J. For. Res. 29, 687±695. Kotar, J., Burger, T.L., 1989. Habitat type classi®cation system for the Menominee Indian Reservation. Department of Forestry, University of Wisconsin±Madison. Kozlowski, T.T., Pallardy, S.G., 1997. Physiology of Woody Plants, second ed. Academic Press, New York. Kramer, C.Y., 1956. Extension of multiple range tests to group means with unequal numbers of replications. Biometrics 12, 307±310. Larocque, G.R., Marshall, P.L., 1994a. Crown development in red pine stands. I. Absolute and relative growth measures. Can. J. For. Res. 24, 762±774. Larocque, G.R., Marshall, P.L., 1994b. Crown development in red pine stands. II. Relationship with stem growth. Can. J. For. Res. 24, 775±784. Long, J.N., Smith, F.W., 1984. Relationship between size and density in developing stands: a description and possible mechanisms. For. Ecol. Manage. 7, 191±206. Long, J.N., Smith, F.W., 1990. Determinants of stemwood production in Pinus contorta var. latifolia forests: the in¯uence of site quality and stand structure. J. Appl. Ecol. 27, 847± 856. Maguire, D.A., Brissette, J.C., Gu, L., 1998. Crown structure and growth ef®ciency of red spruce in uneven-aged, mixed-species stands in Maine. Can. J. For. Res. 28, 1233±1240. Man, R., Lieffers, V.J., 1999. Are mixtures of aspen and white spruce more productive than single species stands? For. Chron. 75, 505±513. Menalled, F.D., Kelty, M.J., Ewel, J.J., 1989. Canopy development in tropical tree plantations: a comparison of species mixtures and monocultures. For. Ecol. Manage. 104, 249±263. Neter, J., Kutner, M.H., Nachtsheim, C.J., Wasserman, W., 1996. Applied Linear Statistical Models. Irwin, Chicago. O'Hara, K.L., 1988. Stand structure and growing space ef®ciency following thinning in an even-aged Douglas-®r stand. Can. J. For. Res. 18, 859±866. O'Hara, K.L., 1996. Dynamics and stocking-level relationships of multi-aged ponderosa pine stands. Monograph No. 33. For. Sci. 42.
C.R. Webster, C.G. Lorimer / Forest Ecology and Management 177 (2003) 361±377 O'Hara, K.L., Lahde, E., Laiho, O., Norokorpi, Y., Saksa, Y., 2001. Leaf area allocation as a guide to stocking control in multi-aged, mixed-conifer forests in southern Finland. Forestry 2, 171±185. Oliver, C.D., Larson, B.C., 1996. Forest Stand Dynamics. Wiley, New York. Roberts, S.D., Long, J.N., 1992. Production ef®ciency of Abies lasiocarpa: in¯uence of vertical distribution of leaf area. Can. J. For. Res. 22, 1230±1234. Roberts, S.D., Long, J.N., Smith, F.W., 1993. Canopy strati®cation and leaf area ef®ciency: a conceptualization. For. Ecol. Manage. 60, 143±156. Russell, G., Jarvis, P.G., Monteith, J.L., 1989. Absorption of radiation by canopies and stand growth. In: Russell, G., Marshall, B., Jarvis, P.G. (Eds.), Plant Canopies: Their Growth, Form and Function. Cambridge University Press, Cambridge, pp. 21±39. Smith, W.B., 1985. Factors and equations to estimate forest biomass in the North Central Region. USDA Forest Service Research Paper No. NC-268. Smith, F.W., Long, J.N., 1989. The in¯uence of canopy architecture on stemwood production and growth ef®ciency of Pinus contorta var. latifolia. J. Appl. Ecol. 26, 681±691. Smith, F.W., Long, J.N., 1992. A comparison of stemwood production in monocultures and mixtures of Pinus contorta
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var. latifolia and Abies lasiocarpa. In: Cannell, M.G.R., Malcolm, D.C., Robertson, P.A. (Eds.), Ecology of Mixed Species Stands of Trees. Blackwell Scienti®c Publications, Oxford, pp. 87±98. Smith, D.M., Larson, B.C., Kelty, M.J., Ashton, P.M.S., 1997. The Practice of Silviculture: Applied Forest Ecology, ninth ed. Wiley, New York. Sterba, H., Amateis, R.L., 1998. Crown ef®ciency in a loblolly pine (Pinus taeda) spacing experiment. Can. J. For. Res. 28, 1344±1351. Stoneman, G.L., Whitford, K., 1995. Analysis of the concept of growing space ef®ciency in Eucalyptus marginata (jarrah) in relation to thinning, fertilising and tree characteristics. For. Ecol. Manage. 76, 47±53. Waring, R.H., 1983. Estimating forest growth and ef®ciency in relation to canopy leaf area. Adv. Ecol. Res. 13, 327±354. Waring, R.H., Thies, W.G., Muscato, D., 1980. Stem growth per unit leaf area: a measure of tree vigor. For. Sci. 26, 112± 117. Webster, C.R., Lorimer, C.G., 2002. Single-tree versus group selection in hemlock±hardwood forests: are smaller openings less productive? Can. J. For. Res. 32, 591±604. Zeide, B., 2001. Thinning and growth: a full turnaround. J. For. 99, 20±25.
Comparative growing space ef®ciency of four tree species in mixed conifer±hardwood forests Christopher R. Webster*, Craig G. Lorimer Department of Forest Ecology and Management, University of Wisconsin±Madison, 1630 Linden Drive, Madison, WI 53706, USA Received 26 February 2002; accepted 18 July 2002
Abstract The in¯uence of shade tolerance, canopy position, and tree size on growing space ef®ciency (GSE) in mixed stands of co-occurring conifer and hardwood species was investigated in hemlock±northern hardwood forests. Three alternative measures of twodimensional growing spaceÐtotal crown area (TCA), exposed crown area (ECA), and a projection of the total available growing space (AGS)Ðwere investigated to clarify the comparative importance of shaded and illuminated crown regions and unoccupied space in the forest canopy. GSE was expressed as ratios of stem volume increment and biomass increment per unit of growing space. Late-successional, shade-tolerant species have often been portrayed as slow growing, inef®cient users of their growing space; however, hemlock (Tsuga canadensis), which is one of the most shade-tolerant conifers in North America, was the most ef®cient canopy tree in our sample across all measures of GSE. Likewise, the mid-tolerant yellow birch (Betula alleghaniensis) tended to be less ef®cient than the more shade-tolerant maples (Acer rubrum and Acer saccharum). For all species, volume increment per unit of growing space increased with increasing tree height and canopy position, but within a given stratum decreased with increasing crown size. The relative ef®ciency of each species did not appear to be in¯uenced by the measure of growing space employed. In most cases, volume and biomass increments per unit of ECA and AGS were signi®cantly greater p < 0:05 for intermediate than dominant crown class trees. However, for a given level of ECA or AGS, ef®ciency did increase with increasing relative height, which suggests that ef®ciency is in¯uenced by the relative vertical position of growing space in the forest canopy. In general, the shaded area of a crown (i.e., TCA ECA) was not a signi®cant predictor of volume increment once height and ECA were known, suggesting that once 100% canopy closure is reached, packing trees more tightly may not increase stand-level production. However, mean volume increment per unit of TCA scaled more accurately to the stand-level than mean volume increment per unit of ECA. Potential scaling problems associated with mixed-species stands are discussed. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Hemlock±northern hardwoods; Growing space ef®ciency; Acer rubrum; Acer saccharum; Betula alleghaniensis; Tsuga canadensis
1. Introduction Growing space ef®ciency (GSE), traditionally de®ned as stem volume growth per unit of crown * Corresponding author. Present address: School of Forestry and Wood Products, Michigan Technological University, 1400 Townsend Dr. Houghton, MI 49931, USA. Tel.: 1-906-487-2454. E-mail address: [email protected] (C.R. Webster).
projection area (Hamilton, 1969; Assmann, 1970; Sterba and Amateis, 1998), provides a conceptual link between physiological aspects of tree growth and tangible measures of wood production. The underlying rationale behind this ratio is that trees allocate carbon to stem wood production only after other needs have been met (Kozlowski and Pallardy, 1997). Therefore, the amount of wood produced per unit of growing
0378-1127/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 2 ) 0 0 3 9 4 - 8
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space provides an index of the comparative vigor of individual trees and forest stands (Waring et al., 1980). Individual-tree ef®ciency is strongly in¯uenced by tree size and competitive status and tends to increase with increasing canopy position (Hamilton, 1969; Assmann, 1970; Roberts and Long, 1992; O'Hara, 1996). However, within a given height stratum, trees with large crowns, while often fast growing, tend to be less ef®cient per unit area than trees with smaller crowns (Assmann, 1970; O'Hara, 1988; Sterba and Amateis, 1998). Numerous thinning studies suggest a similar trend at the stand level since the greatest standlevel productivity typically is achieved when smallcrowned trees are tightly packed together (Zeide, 2001). As a result, large dominant and codominant trees have often been viewed as inherently inef®cient (Assmann, 1970). Most studies of GSE have focused on the ef®ciency of individual trees in monocultures (Hamilton, 1969; O'Hara, 1988; Long and Smith, 1990; Larocque and Marshall, 1994a; Stoneman and Whitford, 1995; Gilmore and Seymour, 1996; Sterba and Amateis, 1998; Kollenberg and O'Hara, 1999) with only limited research in mixed-species conifer stands (Kaufmann and Ryan, 1986; Smith and Long, 1992). Natural mixed-species stands are often vertically complex because shade-intolerant tree species with rapid juvenile height growth rates overtop slower-growing, shade-tolerant species following stand initiation (Hibbs, 1982; Kelty, 1986; Fajvan and Seymour, 1993; Oliver and Larson, 1996). Colonization of small canopy gaps by shade-tolerant species during later stages of stand development is another important source of structural heterogeneity (Fajvan and Seymour, 1993; Oliver and Larson, 1996). Assmann (1970) suggested that the spreading crowns typical of shade-grown trees in uneven-aged stands make them especially inef®cient once they reach the canopy. However, large canopy gaps in uneven-aged forest stands often have a dense sapling cohort that prevents individual trees from developing plagiotropic crowns. Also, recent work by Smith and Long (1992) suggests that GSE may actually increase with increasing shade tolerance in conifers. Nevertheless, relatively little is known about the comparative GSE of co-occurring conifer and hardwood species in vertically complex, uneven-aged stands, and it remains unclear whether shade-tolerant species are more or less ef®cient users
of their growing space than less shade-tolerant species. Likewise, even though shade-tolerant tree species are often perceived as slow growing because of comparatively slow initial heightgrowth rates, it is unclear whether these species are really less productive than more shadeintolerant species once they reach the canopy. Detailed analyses of GSE may help clarify different competitive strategies of species in natural mixed stands and also provide a means of scaling from tree-level to stand-level production. Leaf area allocation models that scale GSE from cohorts or strata to the stand level have provided valuable insights into the comparative ef®ciency of alternative stand structures (O'Hara, 1996, 1988; O'Hara et al., 2001). However, results from a loblolly pine (Pinus taeda) spacing experiment suggest that scaling mean crown ef®ciency to the stand level can be problematic because crowns of a given size may be more ef®cient in an open than a closed stand (Sterba and Amateis, 1998). Consequently, extrapolating the mean crown ef®ciency observed at a given stand density to stand structures with disparate densities may lead to erroneous estimates of stand-level ef®ciency. More detailed analyses of changes in tree growth in response to crown overlap and ef®ciency may help clarify this point and improve the accuracy associated with scaling from individual trees to stands. Uneven-aged, hemlock±northern hardwood forests represent an appropriate system in which to examine the in¯uence of shade tolerance and canopy position on GSE because of their vertical complexity and species composition. This forest type contains two of the most shade-tolerant tree species in eastern North America: eastern hemlock (Tsuga canadensis) and sugar maple (Acer saccharum) (Burns and Honkala, 1990). Other common species include the mid-tolerant yellow birch (Betula alleghaniensis) and the moderately shade-tolerant red maple (Acer rubrum). The objectives of this study are to: (1) examine the comparative GSE of eastern hemlock, sugar maple, red maple, and yellow birch in structurally complex forests, (2) examine the in¯uence of the shaded and illuminated portions of the crown on productivity and GSE, and (3) investigate the accuracy of scaling from individual strata to the stand-level using alternative measures of growing space in mixed-species forests. Since leaf area is highly correlated with stem volume increment (Waring, 1983; Long and Smith,
C.R. Webster, C.G. Lorimer / Forest Ecology and Management 177 (2003) 361±377
1984; O'Hara, 1988), it has been used commonly in recent years as a surrogate measure of occupied growing space (e.g. Waring et al., 1980). However, while leaf area offers some advantages over twodimensional crown projections, limitations include the inability to assess the unoccupied growing space available to individual trees and to differentiate between shaded and illuminated portions of the crown. When crown dimensions are measured directly in the ®eld, it is not dif®cult to differentiate unoccupied growing space around a tree crown and to distinguish shaded versus illuminated portions of the crown. We examined three alternative two-dimensional projections of growing space. 2. Methods 2.1. Study area This study was conducted on the Menominee Indian Reservation in northeastern Wisconsin, USA. Study locations were restricted to the Acer-Tsuga-Maianthemum (ATM) habitat type (Kotar and Burger, 1989) to reduce the effects of natural variation in site conditions on tree growth. This forest type on the reservation has traditionally been managed with single-tree selection on a 15-year cutting cycle. Soils are moderately fertile sandy loams and loams developed in outwash of the brown drift region (Kotar and Burger, 1989). Common soil series on this habitat type include the Onamia, Padus, Pence, and Chetek (Kotar and Burger, 1989). The topography is gently rolling and the climate is continental, with moderately heavy snowfall and cold winters (Albert, 1995). The mean monthly temperature in July is 21 8C (maximum 28 8C) and 8 8C (maximum 4 8C) in January. Mean annual precipitation is 80 cm with a mean annual snowfall of 122 cm (Shawano, WI Weather Station; National Oceanic and Atmospheric Administration, 2000). The stands we examined were all relatively well stocked and on average contained 502:8 40:1 trees ha 1 (12.7 cm DBH), 30:0 1:0 m2 ha 1 of basal area, and 287:0 18:0 m3 ha 1 of total volume. The mean height of sample trees on our plots was 19:6 0:4 m. Maximum tree height of sample trees on the plots ranged from 23.4 to 30.2 m, averaging 27:5 0:5 m. These stands contain somewhat larger
363
and older trees than typical selection stands in the Lake States because of their long management history and the marking guidelines used by Menominee Tribal Enterprises (Table 1). 2.2. Field methods We sampled 201 hemlocks, 91 red maples, 115 sugar maples, and 109 yellow birches. Sample trees were obtained from 20 randomly selected Menominee Continuous Forest Inventory (CFI) plots on the ATM habitat type. To obtain a wide range of tree sizes and canopy positions, we sampled all trees that were at least 12.7 cm diameter at breast height (DBH, 1.37 m) at the time of the last inventory. Data on broken and damaged trees are not reported here. Each tree was assigned to a crown class, as de®ned by Smith et al. (1997), based on its position in the canopy. We make a distinction between overtopped trees (with crowns immediately beneath crowns of the overstory trees) and suppressed trees (shorter saplings in the understory). On each tree, we measured DBH (cm) with a diameter tape and total height (m) with a ®berglass reel tape and clinometer. Two-dimensional projections of crown size and growing space available to each tree were measured as eight-sided polygons, using three alternative de®nitions (Fig. 1). Crown radii were measured by extending a ®berglass reel tape from the bole horizontally along each of eight compass bearings (458 intervals) to the crown edge. The position of the crown edge was determined using a clinometer. We de®ned and measured exposed crown radius to be that portion of the total crown radius not overtopped by branches of taller trees. To assess available growing space (AGS), we measured eight radii from each tree bole to the crown edge of the nearest competing tree. The maximum search distance for a competing crown was 16.67 m, two-thirds the height of a typical canopy tree. Crown radii measurements were used to calculate total crown area (TCA, m2), exposed crown area (ECA, m2), and AGS (m2) as the areas of eight-sided polygons by means of triangle geometry. 2.3. Data preparation and analysis Species-speci®c total stem volume estimates for eastern hemlock, sugar maple, and yellow birch were
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Table 1 Sample tree attributes by species and crown class (mean (1 standard deviation)) Total stem volume (m3)
Total stem biomass (kg)
n
(0.10) (0.10) (0.13) (0.11) (0.09)
0.13 0.23 0.35 0.77 2.68
(0.05) (0.13) (0.22) (0.51) (1.13)
49.97 89.03 136.50 295.61 1032.36
(17.87) (50.04) (85.94) (196.36) (436.65)
15 68 54 37 27
± 0.75 0.80 0.95 1.08
(0.06) (0.10) (0.09) (0.08)
± 0.21 0.28 0.58 1.11
(0.07) (0.12) (0.39) (0.52)
± 105.18 138.73 286.92 549.81
(34.84) (58.98) (195.60) (256.30)
0 10 25 36 20
(3.11) (2.27) (2.55) (2.59)
± 0.66 0.81 0.94 1.05
(0.14) (0.08) (0.10) (0.07)
± 0.19 0.27 0.51 1.26
(0.08) (0.12) (0.23) (0.54)
± 108.36 152.60 285.98 708.34
(43.57) (66.73) (130.57) (305.18)
0 20 25 47 23
(0) (2.55) (2.09) (2.54) (1.86)
0.26 0.66 0.75 0.92 1.02
(0) (0.11) (0.09) (0.08) (0.08)
0.06 0.18 0.27 0.66 1.42
(0) (0.08) (0.23) (0.41) (0.60)
30.08 96.91 144.75 358.10 772.60
(0) (43.14) (125.76) (222.94) (325.94)
1 11 35 42 20
Crown class
DBH (cm)
Height (m)
RH
Hemlock Suppressed Overtopped Intermediate Codominant Dominant
18.33 21.42 24.68 33.12 57.59
(2.58) (4.56) (6.28) (9.54) (12.00)
10.37 13.61 15.69 19.47 26.04
(2.49) (2.30) (2.87) (3.48) (2.61)
0.45 0.61 0.70 0.90 1.12
Red maple Suppressed Overtopped Intermediate Codominant Dominant
± 17.15 19.06 25.54 35.57
(3.12) (3.82) (7.25) (8.15)
± 18.09 19.46 22.54 25.23
(1.69) (2.23) (2.91) (2.83)
Sugar maple Suppressed Overtopped Intermediate Codominant Dominant
± 18.21 20.00 26.75 42.17
(3.52) (4.52) (6.14) (9.59)
± 15.55 19.00 21.72 24.96
Yellow birch Suppressed Overtopped Intermediate Codominant Dominant
13.50 17.14 19.43 28.17 41.55
(0) (3.23) (6.64) (7.64) (8.47)
6.75 15.38 17.11 21.26 23.25
Fig. 1. (a) Pro®le view of total and ECA (m2) and AGS (m2) of an intermediate crown class tree (white crown). Competitors are de®ned as trees whose height is greater than or equal to the height of the widest point of the subject tree's crown. (b) Overhead view of the same tree with additional competitors shown with dark stippling. The total projected crown area includes the shaded area indicated with the dotted line plus the ECA.
C.R. Webster, C.G. Lorimer / Forest Ecology and Management 177 (2003) 361±377
derived from multiple regressions (total volume f (tree height and DBH)) ®t to volume tables presented by Cloutier (1948). For red maple, the equation of Crow and Erdmann (1983) was used. The volume increment of each tree (dm3 per year) was calculated as the difference in total volume between the last two CFI measurement periods (winter 1989 and 1999, respectively) divided by the length of the interval. Because the four sample species vary considerably in speci®c gravity (hemlock 0:38, red maple 0:49, sugar maple 0:56, and yellow birch 0:55; Smith, 1985), a total stem biomass increment (kg per year) was also calculated to clarify potential differences in dry-matter production between species (Assmann, 1970). Total stem biomass was calculated from total volume estimates using species-speci®c relationships between dry weight and volume (Smith, 1985). Three basic measures of GSE (expressed as both dm3 m 2 per year and kg m 2 per year) were calculated for each tree. These included volume and biomass increments per unit of the TCA of the tree (GSEtca), ECA (GSEeca), and AGS (GSEags). Analyses of ECA and total AGS were limited to intermediate, codominant, and dominant crown class trees because suppressed and overtopped trees by de®nition have no ECA or AGS in the canopy. Likewise, trees with only
Table 2 Species-speci®c regressions of GSE (dm3 m
2
365
trace amounts of ECA (<5% of TCA exposed) were also excluded. The relative height (RH) of each tree was calculated by dividing the total height of the tree by the mean height of codominant and dominant trees on the sample plot. This measure provides a quantitative analog to the more subjective crown class assignments. Standard linear and nonlinear regression techniques were used to examine trends in GSE and volume increment as functions of tree stature and position (Bates and Watts, 1988; Chatterjee and Price, 1991). Two-way ANOVAs with blocking (by plot to reduce variance) were used to compare the GSE of different species and crown classes (Neter et al., 1996). Square root transformations on the response variable were used to homogenize variance. The Tukey±Kramer Studentized range test for unbalanced data was used for pair-wise comparisons (Kramer, 1956). The feasibility of scaling GSE from individual strata to stands was examined by selecting ®ve plots from the data set that contained only trees of the four species examined in this report. The TCAs and ECAs were summed for each species by crown class and multiplied by their respective mean ef®ciency. The predicted stand-level volume increments were then compared with the observed volume increment of the
per year) as a function of total height (HT, m), RH, TCA (m2), ECA (m2), and AGS (m2)a
Species
Equation
Adjusted R2
n
Hemlock
ln GSEtca 5:86 2:60 ln HT 0:696 ln TCA ln GSEtca 1:17 2:16 ln RH 0:408 ln TCA ln GSEeca 0:310 1:89 RH 0:648 ln ECA ln GSEags 0:197 2:18 RH 0:763 ln AGS p GSEtca 0:058 0:0657 HT 0:216 ln TCA p GSEtca 0:063 1:23 RH 0:152 ln TCA ln GSEeca 1:01 2:84 RH 0:704 ln ECA ln GSEags 1:12 3:03 RH 0:787 ln AGS p GSEtca 0:433 0:0494 HT 0:254 ln TCA p GSEtca 0:406 1:13 RH 0:238 ln TCA p GSEeca 1:82 1:17 ln RH 0:307 ln ECA ln GSEags 2:09 3:26 ln RH 0:835 ln AGS p GSEtca 0:287 0:055 HT 0:00915 TCA p GSEtca 0:298 1:24 RH 0:00831 TCA ln GSEeca 1:60 2:51 ln RH 0:734 ECA ln GSEags 1:71 2:84 ln RH 0:801 AGS
0.71 0.57 0.58 0.83
201 201 107 107
0.53 0.35 0.52 0.68
91 91 81 81
0.59 0.53 0.63 0.76
115 115 91 91
0.37 0.38 0.38 0.59
109 109 90 90
Red maple
Sugar maple
Yellow birch
a
The overall F-test for each regression was highly signi®cant p < 0:001.
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Fig. 2. Trends in individual-tree GSEtca (dm3 m 2 per year) as a function of tree height (HT, m) and TCA (m2) for hemlock, red maple, sugar maple, and yellow birch. See Table 2 for equations.
plot. This method is similar to the method proposed by Kollenberg and O'Hara (1999) for even- and multi-aged lodgepole pine (Pinus contorta) stands in Montana. This is not a formal statistical validation of a particular scaling method, but rather is presented for discussion because it illustrates some of the potential scaling problems encountered in mixed-species stands. 3. Results 3.1. GSE in relation to species, stature, and canopy position For all species, tall trees with relatively small crowns tend to be the most ef®cient producers of
volume per unit of TCA (Figs. 2 and 3 and Table 2). At a given height, hemlock was the most ef®cient species at using its TCA followed in decreasing order by red maple, sugar maple, and yellow birch (Figs. 2 and 3). However, while GSE increased continuously with increasing tree height (Figs. 2 and 3), it peaked at between 20 and 30 cm DBH in the hardwoods and at approximately 45 cm DBH for hemlock (Fig. 4a and Table 3). The trend between GSE and DBH was best described by a modi®ed periodic annual increment function (Y exp a b X 1 b X 2 ; Avery and Burkhart, 1994). Increasing tree height or DBH in multi-layered forests generally improves the canopy position and competitive status of a tree, which are usually positively correlated with volume increment (dm3 per year). The volume increment of hemlock, red maple,
Fig. 3. (a) Volume increment per unit of TCA (GSEtca, dm3 m 2 per year) of upper canopy trees (RH 1:0) as a function of TCA (m2) for hemlock, red maple, sugar maple, and yellow birch. (b) Volume increment per unit of ECA (GSEeca, dm3 m 2 per year) of upper canopy trees as a function of ECA (m2). (c) Volume increment per unit of AGS (GSEags, dm3 m 2 per year) of upper canopy trees as a function of AGS (m2). See Table 2 for equations.
Fig. 4. Trends between stem DBH (cm) and: (a) stem volume increment per unit of TCA (GSEtca, dm3 m 2 per year; see Table 3 for 2 equations); (b) annual stem volume increment (DV, dm3 per pyear); p (c) TCA (m ). Equations2 for part (b) are: hemlock 2 (p DV 0:268 0:116 ln DBH, R 0:72, n 201), red maple ( DV 0:279 p 0:123 ln DBH, R 0:65, n 91), sugar maple ( DV 0:156 0:0805 ln DBH, R2 0:49, n 115), and yellow birch ( DV exp 2:611 68:184=DBH(68:184=DBH2 ), raw R2 0:85, mean corrected R2 0:33, n 109). Equations for part (c) are: hemlock (TCA 73:348 30:501 ln DBH, R2 0:58, n 201), red maple (TCA 6:287 0:0709 DBH 0:0291 DBH2 , R2 0:74, n 91), sugar maple (TCA 10:001 1:519 DBH, R2 0:61, n 115), and yellow birch (ln TCA 1:588 1:489 ln DBH, R2 0:60, n 109). The overall F-test for each regression was highly signi®cant p < 0:001.
C.R. Webster, C.G. Lorimer / Forest Ecology and Management 177 (2003) 361±377 Table 3 Nonlinear regression parameter estimates and asymptotic standard errors (ASE) for individual species regressionsa of volume growth per unit of TCA (GSEtca) as a function of tree DBH (1.37 m)b Species
Parameter
Estimate
ASE
Hemlock (0.70, 0.13)
a b
4.99 90.60
0.094 4.151
Red maple (0.74, 0.14)
a b
4.42 61.53
0.146 5.474
Sugar maple (0.66, 0.02)
a b
4.08 60.75
0.117 4.651
Yellow birch (0.62, 0.07)
a b
3.53 48.66
0.143 5.399
GSEtca exp a b DBH 1 b DBH 2 . These regressions correspond to the ®tted curves in Fig. 4a. The raw (1 residual SS/total SS) and mean corrected (1 residual SS/corrected SS) R2 values for each regression are provided under the species name (raw, mean corrected). a
b
and sugar maple all increased signi®cantly with increasing tree DBH (p < 0:001, Fig. 4b). However, the volume increment of yellow birch increased until approximately 30 cm DBH and then slowly declined
(Fig. 4b). TCA, on the other hand, increased rapidly with increasing DBH for red maple, sugar maple, and yellow birch, but increased at a decreasing rate with DBH in hemlock (Fig. 4c). This suggests that increases in tree diameter that improve the canopy position of a tree, without leading to signi®cant increases in crown area, increase the ef®ciency of volume production per unit of TCA (Fig. 4). In general, hemlock appeared to be the most ef®cient and productive large canopy tree in these forests, due at least in part to the high volume growth rate and modest crown areas of larger trees (Fig. 4). Results of a two-factor ANOVA suggested that dominant and codominant trees of each species were generally the most ef®cient users of their TCA (Fig. 5a and Table 4). While GSE tended to be highest in dominant hemlocks, GSE in red maple, sugar maple, and yellow birch appeared to peak in the codominant crown class (Fig. 5a and Table 4). The overall ANOVA was highly signi®cant (root MSE 0:196, F36;515 12:49, p < 0:0001), and both species and crown class were signi®cant determinants p < 0:0001 of volume increment per unit of TCA. A species by crown class interaction term was signi®cant p 0:0008,
Table 4 Species and crown class comparisons of volume increment per unit of TCA (GSEtca, dm3 m and total AGS (GSEags, dm3 m 2 per year)a Hemlock
369
Red maple
Sugar maple
N/Ab n 0 0.19 (0.03) efg n 10 0.49 (0.07) bcde n 25 0.75 (0.07) abc n 36 0.68 (0.08) abc n 20
N/A n 0 0.38 (0.12) 0.31 (0.04) 0.58 (0.04) 0.47 (0.05)
2
per year), ECA (GSEeca, dm3 m
2
per year),
Yellow birch
GSEtca Suppressed Overtopped Intermediate Codominant Dominant
0.17 0.33 0.57 0.85 0.99
GSEeca Intermediate Codominant Dominant
2.53 (0.34) a n 43 1.71 (0.25) abc n 37 1.12 (0.14) bcd n 27
2.30 (0.71) ab n 24 0.89 (0.10) cde n 36 0.69 (0.08) def n 20
1.12 (0.24) bcde n 22 0.83 (0.07) cde n 47 0.52 (0.06) ef n 23
0.81 (0.14) def n 35 0.50 (0.04) ef n 42 0.27 (0.05) f n 20
GSEags Intermediate Codominant Dominant
1.97 (0.35) a n 43 1.55 (0.41) ab n 37 0.53 (0.06) bcde n 27
1.51 (0.59) acb n 24 0.51 (0.09) cde n 36 0.29 (0.05) de n 20
0.93 (0.25) bcd n 22 0.53 (0.07) bcde n 47 0.27 (0.05) de n 23
0.55 (0.12) de n 35 0.33 (0.04) de n 42 0.13 (0.03) e n 20
a
(0.03) (0.02) (0.04) (0.08) (0.08)
efg n 15 cdef n 68 abcd n 54 ab n 37 a n 27
cdef n 20 cdef n 25 abcd n 47 bcde n 23
0.02 0.11 0.32 0.43 0.25
(0.00)c n 1 (0.04) fg n 11 (0.05) def n 35 (0.04) bcde n 42 (0.04) def n 20
For each measure of GSE, means (1 standard error) followed by the same letter are not signi®cantly different (Tukey±Kramer Studentized range test for unbalanced data, a 0:05). Only intermediate (ECA 5% of TCA), codominant, and dominant crown classes were included in analyses with GSEeca and GSEags. b No individuals in this class were represented in our sample. c No statistical inferences regarding this class are possible.
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Fig. 5. Means and standard errors for volume increment per unit of: (a) TCA (GSEtca, dm3 m 2 per year); (b) ECA (GSEeca, dm3 m 2 per year); (c) total AGS (GSEags, dm3 m 2 per year). Crown classes are abbreviated as follows: S, suppressed; OT, overtopped; I, intermediate; CD, codominant; and D, dominant.
C.R. Webster, C.G. Lorimer / Forest Ecology and Management 177 (2003) 361±377
suggesting that the relationship between crown class and GSE was not constant across all species. For example, the mean GSE of overtopped sugar maples was unexpectedly greater than that of intermediates, deviating from the pattern observed for the other species (Fig. 5a). However, most of the overtopped sugar maples in our sample were less ef®cient than higher crown classes with a few notable exceptions among individual trees. These in¯uential points increased the overall mean ef®ciency of overtopped trees. To some extent, hemlock's comparatively light wood may contribute to its apparent superiority in growth ef®ciency compared to associated hardwood species. Incorporating wood density reduced the number of signi®cant differences among species; however, hemlock remained the most ef®cient species (Fig. 6a and Table 5). Again, the overall ANOVA was highly signi®cant (root MSE 0:019, F36;515 11:03, p < 0:0001) with species and crown class explaining signi®cant shares of the variation p < 0:0001. The species by crown class interaction also remained signi®cant p 0:0035.
3.2. Ef®ciency per unit of ECA and total AGS While dominants and codominants may use their TCA more ef®ciently than other crown classes, they appear to be relatively inef®cient users of their ECA and total AGS (Fig. 5b and c). Overall ANOVAs of growing space per unit of ECA (root MSE 0:408, F30;375 7:19, p < 0:0001) and AGS (root MSE 0:436, F30;375 5:67, p < 0:0001) were highly signi®cant (i.e., the full model; Table 4). Species and crown class were highly signi®cant in both ANOVAs p < 0:0001, but the interaction between these two variables was not (p 0:1659 and 0.3352, respectively). For all species, intermediates used their ECA and AGS more ef®ciently than higher crown classes. Dominant yellow birches were the least ef®cient trees based on ECA and total AGS. Similar trends were observed for total stem biomass increment per unit ECA (Fig. 6b and Table 5; root MSE 0:276, F30;375 5:32, p < 0:0001) and AGS (Fig. 6c and Table 5; root MSE 0:293, F30;375 4:55, p < 0:0001) with no signi®cant interactions (p 0:1337 and 0.3161, respectively).
Table 5 Species and crown class comparisons of total stem biomass increment per unit of TCA (GSEtca, kg m year), and total AGS (GSEags, kg m 2 per year)a Hemlock
371
2
per year), ECA (GSEeca, kg m
Red maple
Sugar maple
Yellow birch
N/Ab n 0 0.09 (0.02) efg n 10 0.24 (0.04) abcd n 25 0.37 (0.03) a n 36 0.34 (0.04) ab n 20
N/A n 0 0.21 (0.07) 0.18 (0.02) 0.32 (0.02) 0.26 (0.03)
0.01 0.06 0.17 0.23 0.14
2
per
GSEtca Suppressed Overtopped Intermediate Codominant Dominant
0.07 0.13 0.22 0.33 0.38
GSEeca Intermediate Codominant Dominant
0.98 (0.13) a n 43 0.67 (0.10) abc n 37 0.46 (0.05) bcd n 27
2.30 (0.35) ab n 24 0.89 (0.05) cd n 36 0.69 (0.04) cde n 20
0.62 (0.14) abcd n 22 0.47 (0.04) bcd n 47 0.29 (0.03) de n 23
0.44 (0.08) cde n 35 0.27 (0.02) de n 42 0.14 (0.03) e n 20
GSEags Intermediate Codominant Dominant
0.76 (0.13) a n 43 0.60 (0.16) abc n 37 0.20 (0.02) cde n 27
0.75 (0.29) ab n 24 0.25 (0.05) cde n 36 0.14 (0.03) de n 20
0.52 (0.14) abc n 22 0.30 (0.04) bcd n 47 0.15 (0.03) de n 23
0.30 (0.07) cde n 35 0.18 (0.02) cde n 42 0.07 (0.02) e n 20
(0.01) (0.01) (0.02) (0.03) (0.03)
efg n 15 cdef n 68 abcde n 54 ab n 37 a n 27
abcdef n 20 abcdef n 25 abc n 47 abcd n 23
(0.00)c n 1 (0.02) fg n 11 (0.03) bcdef n 35 (0.02) abcd n 42 (0.02) def n 20
a For each measure of GSE, means (1 standard error). Actual means and standard errors are presented, but data were square root transformed for ANOVA. Followed by the same letter are not signi®cantly different (Tukey±Kramer Studentized range test for unbalanced data, a 0:05). Only intermediate (ECA 5% of TCA), codominant, and dominant crown classes were included in analyses with GSEeca and GSEags. b No individuals in this class were represented in our sample. c No statistical inferences regarding this class are possible.
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Fig. 6. Means and standard errors for stem biomass increment per unit of: (a) TCA (GSEtca, kg m 2 per year); (b) ECA (GSEeca, kg m 2 per year); (c) total AGS (GSEags, kg m 2 per year). Crown classes are abbreviated as follows: S, suppressed; OT, overtopped; I, intermediate; CD, codominant, and D, dominant.
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In spite of the comparatively low ef®ciencies of trees in upper canopy layers, coef®cients for RH were positive and highly signi®cant p < 0:001 in speciesspeci®c regressions of volume increment per unit of ECA and AGS versus RH and the respective measure of growing space (Table 2). As with volume increment per unit of TCA, ef®ciency declined with increasing projection area at a given RH (Fig. 3b and c). Therefore, the relative vertical position of a tree's growing space may be an important determinant of ef®ciency, regardless of the measure of growing space employed. 3.3. Differential contributions of exposed and shaded crown regions to productivity ECA appears to be the best single crown-based predictor of volume increment, but differences among crown measures were not great (Table 6). For all species, volume increment increased at a decreasing rate with increasing AGS (Table 6). To assess the contribution of the shaded portion of a tree's crown, the projection area of the shaded portion of the crown (i.e., TCA ECA) was entered into a regression of volume increment in which ECA and total tree height
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were already present as independent variables. RH was used as an independent variable for yellow birch because neither ECA p 0:236 nor any other crown variable was signi®cant after total height was added. The shaded portions of red maple, sugar maple, and yellow birch crowns were not signi®cant predictors of volume increment once ECA and total or RH were known (p 0:209, 0.390, and 0.463, respectively). The shaded area of hemlock crowns p 0:073 was marginally insigni®cant. This suggests that the moderate predictive power of TCA may result more from the ECA being encapsulated in it than the contribution of the shaded portion of the crown to volume production (Table 6). 3.4. Scaling from strata to stands The mean annual volume increment on the subset of ®ve plots (containing no species aside from hemlock, red maple, sugar maple, and yellow birch) was 4.2 m3 ha 1 per year. By multiplying the sum of TCA per crown class for each species by its respective mean ef®ciency (Table 4), we predicted a total annual volume increment of 4.1 m3 ha 1 per year. Using the same approach with ECA, we predicted a volume
Table 6 Individual species regression equations for mean volume increment (DV, dm3 per year) as a function of TCA (m2), ECA (m2), and AGS (m2)a Species
Equation
d.f.
F-statistic
R2
Hemlock
ln DV p 0:0925 0:9064 ln TCA DV 1:3639 1:2586 ln ECA p DV p 1:8259 0:8583 ln AGS DV 1:7591 0:2837 HT 0:2748 ln ECA
1, 1, 1, 2,
106 106 106 106
114.29 140.49 110.73 301.91
0.52 0.57 0.51 0.85
Red maple
ln DV p 0:1156 0:7653 ln TCA DV 2:4555 0:0555 ECA p DV 1:277 0:7191 ln AGS p DV 3:5617 0:3029 HT 0:0216 ECA p DV p 0:1137 1:0383 ln TCA DV 1:212 0:8171 ln ECA p DV 1:6168 0:5587 ln AGS p DV 2:5882 0:2464 HT 0:2696 ln ECA p DV 0:0837 0:8912 ln TCA p DV 1:0226 0:6814 ln ECA p DV 1:1907 0:5132 ln AGS p DV 1:1993 3:562 RH 0:3523 ln ECA
1, 1, 1, 2,
80 80 80 80
58.87 77.91 54.44 123.66
0.43 0.50 0.41 0.75
1, 1, 1, 2,
90 90 90 90
31.11 54.28 40.29 80.11
0.26 0.38 0.31 0.64
1, 1, 1, 2,
90 90 90 90
31.28 38.47 30.37 25.29
0.26 0.30 0.25 0.35
Sugar maple
Yellow birch
a
Regressions of DV as a function of ECA and total tree height (HT, m) or RH are also presented (adjusted R2 values are presented for multiple regressions). All F-tests were highly signi®cant p < 0:001. Only intermediate (ECA 5% of TCA), codominant, and dominant crown class trees were included in this analysis.
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increment of 4.7 m3 ha 1 per year. Using TCA to scale from individual trees in different strata to the standlevel therefore resulted in an underestimation of standlevel volume increment by approximately 2.7%, while using ECA resulted in an overestimation of 14.8%. Most of the overestimation associated with ECA resulted from overpredicting the potential of hemlock. Consequently, removing crown overlap from estimates of mean ef®ciency did not improve predictions of stand-level production over more traditional approaches using TCA. 4. Discussion Rapid juvenile height growth and relatively inef®cient light capture have often been associated with shade-intolerant, early successional species (Daniel et al., 1979). Nevertheless, relatively few studies have examined the relationship between GSE and shade tolerance. Work by Smith and Long (1992) in conifer forests of the Rocky Mountains suggests that subalpine ®r (Abies lasiocarpa) is more ef®cient than the intolerant lodgepole pine (P. contorta). However, in eastern deciduous forests, Chapman and Gower (1991) found that the mid-tolerant northern red oak (Quercus rubra) was signi®cantly more ef®cient than the tolerant sugar maple. In spite of marked differences in shade tolerance among the four species we examined, no clear trends based on shade tolerance rank were observed in these mixed-species stands. Hemlock was typically more ef®cient than associated hardwood species, but results for the three hardwood species were mixed. In general, red maple was more ef®cient than sugar maple, which was more ef®cient than yellow birch, but few differences were signi®cant once speci®c gravity was taken into account. All four species tended to respond in concert to changes in canopy position and stature regardless of the measure of growing space used. Tall trees with relatively small amounts of growing space were most ef®cient. These results are consistent with work in simpler forests using both leaf area and projected crown area approaches. For example, early work by Hamilton (1969) and Assmann (1970) as well as more recent work by O'Hara (1988) has suggested that tall, narrow-crowned trees tend to be the most ef®cient producers of volume increment on an area basis.
Assmann (1970) synthesized much of the early work on GSE in Europe in his classic ``The Principles Of Forest Yield Study''. Based on observations from numerous research projects, Assmann (1970) concluded that while the smallest crowned trees in a given canopy layer were most ef®cient, medium crowned trees actually represented optimal production. Primary reasons included the relatively low vigor and slow growth rate of small-crowned trees. For example, deep, full crowns and rapid individual-tree growth are generally considered a sign of high stand vigor in lodgepole pine; however, GSE is greatest in stands with short, compact crowns and modest individual-tree growth (Smith and Long, 1989). When measures of illumination are used as surrogates for growing space, this discontinuity becomes more pronounced. Using potential radiation absorption (irradiance projected leaf area) as a surrogate for growing space, Kaufmann and Ryan (1986) found that suppressed and overtopped lodgepole pine, Engelmann spruce (Picea engelmannii), and subalpine ®r were more ef®cient producers of volume increment than higher crown classes. Likewise, intermediate crown class trees of all species examined in our study used their ECA more ef®ciently than higher crown classes. These results illustrate that some measures of GSE may not be suitable as direct indicators of tree and stand vigor and that caution must be used in their application. Measures of ECA do, however, provide a rough approximation of potential radiation, which is a strong determinant of photosynthetic production (Russell et al., 1989). Our results suggest that the contributions of the shaded regions of the crown to stem volume production may be negligible compared to those of exposed regions. The shaded area of the crown was not a signi®cant predictor of volume production once the exposed area of the crown and the height of the tree were known for any species examined. Total AGS, on the other hand, was somewhat less informative than TCA for most species, and once a crown had a small amount of open space around it, further increases in growing space did not lead to further increases in volume increment. These results suggest that heavy release treatments in hemlock±hardwood forests may not be necessary to maximize the volume increment of individual trees. However, for hardwood species, maximizing volume production, even on merchantable
C.R. Webster, C.G. Lorimer / Forest Ecology and Management 177 (2003) 361±377
trees, is usually secondary to maintaining stem quality due to the premium placed on veneer and large sawlogs in these forests. Shade-tolerant, late-successional species, such as hemlock and sugar maple, are commonly considered slow growing, perhaps because they are often overtopped by less shade-tolerant species in even-aged stands as a consequence of differences in initial height growth rate. However, in this study, both hemlock and sugar maple canopy trees had higher rates of volume growth than yellow birch. This is consistent with work in hemlock±hardwood canopy gaps where dominant yellow birch saplings had higher height growth rates in large openings than hemlock and sugar maple saplings, but lower basal area growth rates (Webster and Lorimer, 2002). Similarly, Chapman and Gower (1991) found that at a given diameter, overstory sugar maples were signi®cantly more productive than the less shade-tolerant red oak. These results suggest that comparisons of initial height growth rates may not provide an accurate depiction of comparative productivity once trees reach the canopy, and that there might be cause to reconsider the common assumption that shade-tolerant, late-successional species are inherently slow growing. Our results build on a growing body of evidence that large diameter conifers may be at least as ef®cient as smaller trees. For example, in Scotland, Hamilton (1969) found that volume production per unit of crown projection area in Sitka spruce (Picea sitchensis) increased with increasing tree diameter. Some recent research in the western United States suggests that older cohorts of ponderosa (Pinus ponderosa) and lodgepole pine may be more ef®cient users of their projected leaf area than smaller, younger cohorts (O'Hara, 1996; Kollenberg and O'Hara, 1999). Similarly, Maguire et al. (1998) found that growth ef®ciency in red spruce (Picea rubens) was highest among large and tall trees. Nevertheless, declines in ef®ciency with increasing tree size and age have been observed in lodgepole pine (Long and Smith, 1990), subalpine ®r (Roberts and Long, 1992), and red pine (Pinus resinosa; Larocque and Marshall, 1994a). Our results for hemlock suggest that GSE peaks between 40 and 50 cm DBH and then slowly declines. This range is similar to the range reported by Assmann (1970) for Norway spruce (Picea abies) and silver ®r (Abies alba) in Europe. Regardless of the measure of
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growing space used, if the occupied growing space increases more rapidly than volume production, ef®ciency declines. Thus, controls on crown and leaf area such as phenotypic differences between species, competition (Assmann, 1970; Larocque and Marshall, 1994b), crown architecture (Smith and Long, 1989), stand structure (Roberts et al., 1993), and stand density (Long and Smith, 1990; Jack and Long, 1991) may ultimately control GSE in large trees. Given that the shaded portion of a tree's crown had no signi®cant effect on volume increment, one might infer that once 100% crown closure is reached, no further increase in production would be gained by packing trees more tightly with overlapping crowns. However, there is evidence that shaded portions of crowns may have at least a minor positive impact on stand-level production. For example, Sterba and Amateis (1998) found that basal area increment per hectare of loblolly pine (P. taeda) increased until crown closure approached 130%. Additionally, if shade-tolerant species are grown as an understory beneath a canopy of shade-intolerant species, they can contribute an ``additive increment'' (Assmann, 1970), and in some cases, stands strati®ed in this fashion may be more productive than single-species stands of either species in the overstory (Kelty, 1989; Menalled et al., 1989; Man and Lieffers, 1999). This may help to explain why TCA in our study provided a better estimate of stand-level production than ECA (2.7% underestimate versus a 14.8% overprediction, respectively). However, it does not explain why use of ECA to extrapolate from tree-level to stand-level production resulted in an overestimate. Most of the overprediction was due to overestimating the volume production of hemlock. One possible explanation may be seasonal variation in ECA. For example, the ECA of a hemlock with mostly hardwood competitors may increase greatly following leaf fall, providing the tree with a few months of off-season photosynthesis over a larger than expected area. A mean ef®ciency value based on hemlocks with hardwood competitors could potentially lead to an overestimate of production. However, if a hemlock has mostly evergreen competitors, competition rather than seasonal factors would in¯uence the amount of crown area exposed to illumination. Therefore, to scale accurately from individual strata to the stand-level in mixed conifer± hardwood forests, it may be necessary to incorporate
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the species composition of neighboring trees into estimates of mean ef®ciency. Acknowledgements Financial support for this study was provided by the McIntire-Stennis Cooperative Forestry Research Program, project WISO4167. We gratefully acknowledge the following groups and individuals: the Menominee Nation for allowing us to conduct this research on their tribal lands; the staff of the Menominee Tribal Enterprises Forestry Center, especially Marshall Pecore, Dan Pubanz, and Matt Duvall, for logistical support; Erik Nordheim for advice on statistical analyses; Eric Kruger for helpful discussions and a review of an earlier draft of the manuscript; and Theran Stautz for assistance with data collection and entry. References Albert, D.A., 1995. Regional landscape ecosystems of Michigan, Minnesota, and Wisconsin: a working map and classi®cation. USDA Forestry Service General Technical Report NC-178. Assmann, E., 1970. The Principles of Forest Yield Study. Pergamon Press, Oxford. Avery, T.E., Burkhart, H.E., 1994. Forest Measurements, fourth ed. McGraw-Hill, Boston. Bates, D.M., Watts, D.G., 1988. Nonlinear Regression Analysis and its Applications. Wiley, New York. Burns, R.M., Honkala, B.H. (Tech. Coords.), 1990. Silvics of North America: (1) Conifers; (2) Hardwoods. USDA Forest Service Agricultural Handbook No. 654. Chapman, J.W., Gower, S.T., 1991. Aboveground production and canopy dynamics in sugar maple and red oak trees in southwestern Wisconsin. Can. J. For. Res. 21, 1533±1543. Chatterjee, S., Price, B., 1991. Regression Analysis by Example, second ed. Wiley, New York. Cloutier, E., 1948. Form-class Volume Tables. Canada Department of Mines and Resources, Ottawa. Crow, T.R., Erdmann, G.G., 1983. Weight and volume equations and tables for red maple in the Lake States. USDA Forestry Service Research Paper No. NC-242. Daniel, T.W., Helms, J.A., Baker, F.S., 1979. Principles of Silviculture, second ed. McGraw-Hill, New York. Fajvan, M.A., Seymour, R.S., 1993. Canopy strati®cation, age structure, and development of multicohort stands of eastern white pine, eastern hemlock, and red spruce. Can. J. For. Res. 23, 1799±1809. Gilmore, D.W., Seymour, R.S., 1996. Alternative measures of stem growth ef®ciency applied to Abies balsamea from four canopy positions in central Maine, USA. For. Ecol. Manage. 84, 209±218.
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