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Allometry and Competition between Saplings ofPicea jezoensisandAbies sachalinensisin a Sub-boreal Coniferous Forest, northern Japan
Annals of Botany 77 : 529–537, 1996
Allometry and Competition between Saplings of Picea jezoensis and Abies sachalinensis in a Sub-boreal Coniferous Forest, northern Japan Y A S U H I R O K U B O T A and T O S H I H I K O H A R A* Department of Biology, Tokyo Metropolitan Uniersity, Tokyo 192-03, Japan Received : 30 March 1995
Accepted : 7 December 1995
The crown shape and the mode of competition between saplings (! 2 m in height) of the two conifers, Picea jezoensis and Abies sachalinensis, of a sub-boreal forest, northern Japan, were investigated based on the diffusion model. A model for individual sapling growth considering both inter- and intraspecific competition was developed. The effect of species-specific crown shape on the sapling growth and competition of the two species were examined. Picea jezoensis and Abies sachalinensis saplings had deep conic and shallow flat crowns, respectively. Picea jezoensis had more foliage mass than Abies sachalinensis of the same sapling mass. It was suggested that the Picea jezoensis sapling has a high cost for assimilation–respiration balance under dark conditions of closed canopies, whereas the Abies sachalinensis sapling maintains effective assimilation even under suppressed conditions. Widely spaced saplings, such as gap successors, of Picea jezoensis had a greater relative growth rate (a ) than widely spaced Abies sachalinensis. The ! crown shape of saplings of the two species shows different adaptations for efficient persistence in the sub-boreal forest. Saplings of Picea jezoensis and Abies sachalinensis were not uniformly distributed, but aggregated in different sites as the saplings grew, indicating habitat segregation between the two species at the sapling stage. Intraspecific sapling competition was one-sided in each of the two conifers. Interspecific sapling competition was one-sided in the direction only from Abies sachalinensis to Picea jezoensis. Therefore, asymmetric competition prevailed at the sapling stage of the two species. These results contrast with weak symmetric competition or the almost absence of competition between trees (& 2 m in height) of the two species (Kubota and Hara, Annals of Botany 76 : 503–512, 1995). The mode of competition changed with the life-history stage from the sapling (intense and asymmetric) to the tree (weak and symmetric or almost absent). In conclusion (1) asymmetric and intense competition between saplings brought about habitat segregation between the dominant species, Picea jezoensis and Abies sachalinensis, in the early stage of life-history ; (2) therefore, the coexistence of Picea jezoensis and Abies sachalinensis of the sub-boreal forest was determined by the boundary conditions for the growth dynamics of the trees, as segregation of establishment sites resulting from asymmetric and intense competition between saplings ; (3) then the species composition of the forest was maintained by weak symmetric competition or the almost absence of competition between trees. # 1996 Annals of Botany Company Key words : Crown shape, growth dynamics, species coexistence, habitat segregation, diffusion model.
INTRODUCTION Competition between individual plants greatly affects the dynamics of plant populations (Koyama and Kira, 1956 ; Harper, 1977 ; Hutchings, 1986 ; Hara, 1988 ; Silvertown and Lovett Doust, 1993). Many researchers have investigated population dynamics, considering competitive effects on the growth of individual plants (Ford, 1975 ; Mack and Harper, 1977 ; Mithen, Harper and Weiner, 1984 ; Weiner, 1984 ; West, Jackett and Borough, 1989 ; Knox and Peet, 1989). However, most of these previous studies have dealt with even-aged monocultures. There have been few studies on the effects of competition on growth dynamics of multispecies plant communities (but see Kohyama, 1991, 1992). Co-occurring species in a plant community have different plant geometries (Horn, 1971 ; Kohyama, 1987 ; King, 1990 ; Kohyama and Hotta, 1990 ; Kohyama and Grubb, 1994). Significant interactions between growth form and competition have been suggested (Weller, 1987 ; Ellison and * Present address : The Institute of Low Temperature Science, Hokkaido University, Sapporo 060, Japan.
0305-7364}96}05052909 $18.00}0
Rabinowitz, 1989 ; Geber, 1989 ; Weiner, Berntson and Thomas, 1990). Hara, Kimura and Kikuzawa (1991) pointed out that the difference in the mode of competition between Abies eitchii, Abies mariesii and Betula ermanii, resulting from species-specific growth form, corresponds to the successional status of each species. Thus, plant form (crown architecture and allometry) may influence growth dynamics and species coexistence. Asymmetric (or one-sided) competition, where larger plants have a disproportionately larger suppressive effect on the absolute growth rates of smaller ones by shading, has been described by Kuroiwa (1960), Ford and Diggle (1981), Cannell, Rothery and Ford (1984), Takada and Iwasa (1986), Thomas and Weiner (1989) and Weiner (1990). Kohyama (1991, 1992, 1993) showed that one-sided competition (the most extreme case of asymmetric competition where only larger plants have a suppressive effect on the growth of smaller ones and not vice versa) governs the stand development of a warm-temperate rain forest, and suggested that one-sided competition contributes to the stable coexistence of Distylium racemosum, Illicium anisatum and Eurya japonica occupying different vertical layers of the # 1996 Annals of Botany Company
530
Kubota and Hara—Sapling Competition between Picea and Abies
warm-temperate forest. Theoretical studies by Hara (1992, 1993) suggest that one-sided or asymmetric competition promotes stability in size-structure dynamics of plants. Symmetric competition (where the suppressive effect on individual relative growth rate is size-independent), rather than one-sided or asymmetric competition, prevailed in some climax forests with multi-layered canopies (Kubota and Hara, 1995 ; Hara, Nishimura and Yamamoto, 1995). In these studies, either weak symmetric or little competition between dominant canopy trees was found in terms of DBH (stem diameter at breast height) growth as a substitute of tree mass. DBH is a good substitute of individual mass in trees, i.e. DBH growth dynamics are almost paralleled to those of individual tree mass (Nagano and Kira, 1978) and thus it is highly probable that competition is weak or almost absent even in mass growth if competition was weak or almost absent in terms of DBH growth in tree communities. Kubota and Hara (1995) and Hara et al. (1995) suggested that for dominant species occupying the same layer, competition between trees was either weak symmetric or almost absent. It is therefore suggested that size-structure dynamics of dominant species will be greatly influenced by the recruitment process for the growth dynamics of canopy trees, such as establishment processes and population dynamics of saplings. In the present paper, we show that the persistence of saplings (! 2 m in height) in the understorey, rather than the growth dynamics of trees (& 2 m in height), plays an important role for the coexistence of dominant species, Picea jezoensis Carr. and Abies sachalinensis Masters forming a sub-boreal coniferous forest in Hokkaido, northern Japan. In the same forest, the growth and competition between trees have already been investigated (Kubota and Hara, 1995). We examine how species-specific crown shape of saplings affects their growth dynamics and competition between species based on the diffusion model (Hara, 1984 a, b ; Yokozawa and Hara, 1992). Then, we discuss coexistence mechanisms of the two dominant species based on the mode of competition between saplings. STUDY SITE The study was carried out in the eastern region of Taisetsuzan National Park, Japan. Five study plots (2±3 ha in total) were set up in a Picea–Abies forest (850–1000 m above sea level) in 1989. The region is in a valley with low relief. Annual precipitation is approximately 1500 mm. Mean daily temperatures during the warmest month (Aug.) and the coldest month (Jan.) are 17±7 and ®10±7 °C, respectively. Snow cover on the forest floor is usually from Nov. to May. Soil is predominantly brown forest soil. The study site is dominated by Picea jezoensis Carr., Abies sachalinensis Masters, Betula ermanii Cham. and Picea glehnii Masters ; Acer ukrunduense Trautv. et Mey. and Sorbus commixta Hedl. also occur under relatively sparse canopies. The total basal area [π}4(DBH )#] of all trees & 2 m in height in the study site was 31±0 m# ha−" in 1989 (Kubota and Hara, 1995). The basal areas of the dominant canopy species, Picea jezoensis, Picea glehnii, Betula ermanii and Abies sachalinensis, were 8±4, 4±2, 6±0 and 12±3 m# ha−",
respectively, in 1989. The gap ratio in the canopy, which was defined as the vertical projection area where no stems reached 10 m height or 10 cm DBH, ranged from 4±2 to 30±4 % (Kubota, 1995). The study site consists of stands of different maximal tree age ranging from 125 to 333 years (Kubota, 1995). The forest floor is characterized by two floristic types— Sasa dwarf bamboos and Carex. In the stands of low canopy tree densities, Sasa senaensis Franch. et Sav. and Sasa kurilensis Rupr. Makino, covered the forest floor. Consequently, the establishment site of saplings in such stands was restricted to raised surfaces such as fallen logs and stumps (Kubota, Konno and Hiura, 1994). FIELD MEASUREMENTS AND DATA ANALYSIS Field data Fallen logs with densely established saplings were selected as study quadrats. Fourteen quadrats were set up on 14 fallen logs in the study plots in Jun. 1990. Each quadrat on a fallen log was divided into several 1 m long subquadrats (31 in total for the 14 quadrats). Quadrat area was calculated as the product of the length and diameter of each fallen log, and ranged from 0±5 to 4±0 m# (16 m# in total for the 14 quadrats). Each sapling (branched individual and 5 cm ! height ! 200 cm) was tagged (n ¯ 1481) and identified by the species within each subquadrat. The shape of the crown projected onto the ground was assumed to be an ellipse with the lengths of major and minor axes, CW1 and CW2 (crown widths in two perpendicular directions). The crown depth, CD, was defined as the vertical distance from the lowest branch to the terminal leader. The stem diameter at ground level (DGL, mm), stem height (H, cm), crown depth (CD, cm) and crown widths (CW1 and CW2, cm) were measured in Jun. and Jul. 1990. The age of saplings was determined by counting whorls of branches and scars of the oldest whorls. The stem diameter at ground level (DGL) and height (H ) of tagged saplings were remeasured in Oct. 1993. Allometry of saplings Eight saplings of Picea jezoensis and ten saplings of Abies sachalinensis were harvested in 1990. The stem diameter at ground level (DGL), height (H ), crown depth (CD), crown widths (CW1 and CW2) and age for each harvested sapling were measured. Harvested saplings were divided into stem, branch and leaf, and then dried for 48 h at 80 °C, and weighed for each component. Allometry was obtained as a relationship between measured items (DGL, H, CW1, CW2 and CD) and component masses of harvested saplings. The DGL of harvested saplings ranged from 3 to 10 mm and the height from 20 to 50 cm. The observed sapling mass ranged from 3 to 40 g. The size range of the harvested saplings covered 97 % of that of the tagged saplings. The stem volume of a sapling, SV, was estimated as (DGL)#¬H (Ogawa and Kira, 1977). Projected horizontal crown area, CA, was calculated as an ellipse by (π}4)¬CW1¬CW2, and then crown volume, CV, was estimated by CD¬CA. The allometric relationship between
Kubota and Hara—Sapling Competition between Picea and Abies
531
T 1. Estimates of the coefficients, a, b, α and β, of allometric relationships : W®SV, ln (sapling mass) ¯ ab ln (stem olume) ; CD®SV, ln (crown depth) ¯ ab ln (stem olume) ; CA®SV, ln (crown area) ¯ ab ln (stem olume) ; CV®SV, ln (crown olume) ¯ ab ln (stem olume) ; CD®H, ln (crown depth) ¯ ab ln (stem height) ; CA®H, ln (crown area) ¯ ab ln (stem height) ; CV®H, ln (crown olume) ¯ ab ln (stem height) ; F®CV¬SV, ln (foliage mass) ¯ α ln (crown olume)β ln (stem olume) Picea jezoensis Allometry W®SV CD®SV CA®SV CV®SV CD®H CA®H CV®H F®CV¬SV
Abies sachalinensis
b
n
R#
P
®1±454 1±406 1±017 1±324 0±216 ®2±433 ®3±316
0±594 0±234 0±791 1±022 0±777 2±461 3±238
8 671 671 671 671 671 671
0±330 0±720 0±843 0±873 0±774 0±778 0±835
0±049 0±0001 0±0001 0±0001 0±0001 0±0001 0±0001
α 0±352
β ®0±146
n
R# 0±956
P 0±0001
a
8
sapling mass (stem massfoliage mass), W, and stem volume was estimated by ln W ¯ ab ln SV,
(1 a)
where a and b are species-specific parameters. In order to assess crown geometry of Picea jezoensis and Abies sachalinensis, allometric relationships between logtransformed sapling size, ln x (x ¯ SV, H ), and crown dimensions, ln y ( y ¯ CA, CD and CV ) were estimated as follows (Table 1) : ln y ¯ ab ln x,
(1 b)
where a and b are species-specific parameters. ANCOVA for ln y with the covariate ln x was used to test the difference in the intercept, a, and the slope, b, between the two species (Kohyama, 1987 ; King, 1990 ; Kohyama and Hotta, 1990 ; Kohyama and Grubb, 1994). The foliage mass, F, of each sapling was estimated by using multiple linear regression with sapling stem volume (SV ) and crown volume (CV ) as the explanatory variables : ln F ¯ α ln SVβ ln CV,
(2)
where α and β are species-specific parameters (Table 1). Sapling and foliage masses of each tagged sapling on fallen logs were estimated by using eqns (1 a) and (2). Diffusion model and the mode of competition The growth dynamics were analysed based on the diffusion model (Hara, 1984 a, b), ¦ 1 ¦# ¦ f (t, x) ¯ [D(t, x) f (t, x)]® [G(t, x) f (t, x) ¦t 2 ¦x# ¦x ®M (t, x) f (t, x)]
(3)
where f (t, x) is the size distribution function of sapling mass x at time t, and xmax and xmin are maximal and minimal sapling masses, respectively. The diffusion model consists of three functions describing the mean of absolute growth rate, G(t, x), the variance of absolute growth rates, D(t, x), and the mortality rate, M(t, x), of saplings of mass x (0 ! xmin % x % xmax) at time t.
b
n
R#
P
®3±003 1±343 1±382 1±627 0±184 ®1±708 ®2±623
0±842 0±214 0±773 0±987 0±737 2±322 3±059
10 810 810 810 810 810 810
0±330 0±548 0±855 0±873 0±609 0±766 0±788
0±048 0±0001 0±0001 0±0001 0±0001 0±0001 0±0001
α 0±546
β ®0±524
n 10
R# 0±923
P 0±0001
a
Considering the effects of the species composition of sapling subquadrats on the G(t, x) function of species k, Gk(t, x), we developed the equation of Yokozawa and Hara (1992) for investigating both inter- and intraspecific competition as follows :
(
*
# # Gk(t, x) ¯ x a ® 3 c , i Ci(t, x)® 3 c , i Ci(t, xmin) (4) " # ! i=" i=" where Ci(t, x) is the total foliage mass of individuals of sapling mass greater than x of species i (k, i ¯ 1, Picea jezoensis ; k, i ¯ 2, Abies sachalinensis) at time t, c , i and c , i # " are coefficients for species i, and xmin is the minimal sapling mass in a subquadrat. Thus, the Ci(t, x) value for an individual of sapling mass x of species i at time t(1990) was given as : xmax Fi(t, y) fi(t, y) dy (5) Ci(t, x) ¯
&
x
where xmax is the maximal sapling mass in a subquadrat, fi(t, y) is the distribution density of sapling mass y at time t of species i, and Fi(t, y) is the foliage mass of saplings of mass y at time t of species i calculated as eqn (2). A neighbourhood area for calculating Ci(t, x) was represented by subquadrat area. The Ci(t, x) and Ci(t, xmin) functions were calculated for individual saplings of the two species in each subquadrat in 1990 (¯ t). Competition can be described in terms of the mode and direction of effects. The mode of the competitive effect is symmetric (c , i ¯ 0, c , i " 0), asymmetric (c , i, # " " c , i " 0) or one-sided (c , i " 0, c , i ¯ 0) irrespective of inter" # # and intraspecific competition. In the case of interspecific competition, the direction of the competitive effect is symmetric (two-sided) between species i and k if ²c , i " 0 or " c , i " 0 for Gk(t, x)´ and ²c , k " 0 or c , k " 0 for Gi(t, x)´, or " # # one-sided from species i to species k if ²c , i " 0 or c , i " 0 # " for Gk(t, x)´ and ²c , k ¯ 0 and c , k ¯ 0 for Gi(t, x)´. For " # intraspecific competition, the competitive direction is identical to the competitive effect (symmetric or one-sided) (Kubota and Hara, 1995). Multiple linear regression analysis was conducted by using the forward stepwise method (independent variables are entered or removed stepwise one at a time), for the
532
Kubota and Hara—Sapling Competition between Picea and Abies
sapling mass increment of each individual sapling as the dependent variable for Gk(t, x) and ‘ x ’, ‘ xCi(t, x) ’ and ‘ xCi(t, xmin) ’ as the independent variables in eqn (4) for each of the two species. The F statistics to enter and remove an independent variable were both set at 2. The d.f.-adjusted R#-value for the regression model was calculated [R# ¯ 1®(1®r#) (n®1)}(n®q®1) ; n, sample size ; q, number of independent variables in the regression equation ; r, multiple correlation coefficient]. Residual analyses were conducted to examine the adequacy of the model.
60 50 40 30 20 10
RESULTS Allometry of saplings
100
A
0 45
10
20
30
10
20
30
40 35 Number of saplings
Table 1 shows allometric relationships between measures of crown shape, stem volume and stem height for Picea jezoensis and Abies sachalinensis. The allometric relationships were significantly different between the two species
30 25 20 15 10 5 0 30
10
Estimated foliage mass (g)
25 20 15 1 0.1 100
1
10
100
1000
B
10 5
0 10
30
F. 2. Frequency distributions of sapling age for Picea jezoensis (+) and Abies sachalinesis (*), in three representative subquadrats.
1
0.1
10 20 Age of saplings (years)
1 10 100 Estimated sapling mass (g)
1000
F. 1. Relationships between sapling and foliage estimated masses of Picea jezoensis (A), and Abies sachalinensis (B), in a sub-boreal forest, Hokkaido, northern Japan in 1990.
(ANCOVA with the covariate ln SV or ln H, P ! 0±001). Picea jezoensis showed a greater value of the intercept in the allometric relationship between crown depth and stem volume (or stem height) than Abies sachalinensis, whereas Abies sachalinensis showed a greater value of the intercept in crown area s. stem volume (or stem height) than Picea jezoensis (ANCOVA, P ! 0±001). These results indicate that Picea jezoensis had a conic crown with deep crown depth and small crown area, whilst Abies sachalinensis had a flat one with shallow crown depth and large crown area. Foliage mass of Picea jezoensis increased with sapling mass (R# ¯ 0±63), whereas that of Abies sachalinensis was almost size-indepenent (R# ¯ 0±01) (Fig. 1). Picea jezoensis
Kubota and Hara—Sapling Competition between Picea and Abies
533
0.25
A 300
0.2 M(t,x) per 4 years
Number of saplings per 16 m2
200
100
0 –1
1
2
3
0.15
0.1
0.05
B 300 0 –1
1 2 Log10 (estimated sapling mass) (g)
200
F. 4. The M(t, x) functions describing the relationship between estimated sapling mass x in 1990 (¯ t) and mortality rate during the four growing seasons from 1990 to 1993 for saplings of Picea jezoensis (E) and Abies sachalinensis (D), on fallen logs. The M(t, x) function was significantly different between the two species (one repeated measures MANOVA, P ! 0±05).
100
1 2 Log10 (estimated sapling mass) (g)
3
F. 3. Frequency distributions of estimated sapling mass of Picea jezoensis (A), and Abies sachalinensis (B), in the total 16 m# quadrats on fallen logs. (*) Living and (+) dead saplings, respectively.
had a greater foliage mass than Abies sachalinensis for the same-sized individual in sapling mass (ANCOVA, P ! 0±0001). In particular, small-sized saplings of Abies sachalinensis showed larger variation in foliage mass than those of Picea jezoensis. Age and size structures The sapling age structures of Abies sachalinensis and Picea jezoensis were almost even-aged in each subquadrat (Fig. 2). Mean sapling age of each subquadrat ranged from 8 (³2 s.d.) to 26 (³5 s.d.) years. The sapling mass distributions of Abies sachalinensis and Picea jezoensis had positive skewness in all the subquadrats together (Fig. 3). Dead saplings of the two species were smaller than living ones (Kolmogorov-Smirnov test, P ! 0±0001). The mortality functions M(t, x) of the two species decreased with size. The values of M(t, x) of Picea jezoensis were larger than those of Abies sachalinensis (Fig. 4) (one repeated measures MANOVA, d.f. ¯ 1, F ¯ 5±87, P ! 0±05). Habitat segregation The species composition of sapling subquadrats was investigated based on the relationship between sapling biomasses per 1±0 m# of Picea jezoensis and Abies sachal-
10 000 Estimated total sapling biomass (g) of 2 A.sachalinensis in a 1-m subquadrat
0 –1
3
A
1000
100
B 10
100
1000
10 000
Estimated total sapling biomass (g) of P.jezoensis in a 1-m2 subquadrat F. 5. The relationship between the estimated sapling biomasses per 1±0 m# of Picea jezoensis (x) and Abies sachalinensis ( y) in each subquadrat. The solid line given as log y ¯®0±721 log x9±36 indicates habitat segregation between the two species at the sapling stage (PCA, P ! 0±001). The dashed arrows show that the sapling stand diverged to either Abies-dominated (arrow A) or Picea-dominated stands (arrow B) with sapling stand development.
inensis in each subquadrat (Fig. 5). Regression analysis was carried out by Principal Components Analysis (PCA). The sapling biomass of Picea jezoensis was negatively dependent on that of Abies sachalinensis (PCA, P ! 0±001). As the total biomass per 1 m# of saplings of the two conifers increased with sapling stand development (the arrows in Fig. 5), the sapling stand diverged to either Abies-dominated or Picea-
534
Kubota and Hara—Sapling Competition between Picea and Abies
T 2. The competitie effects of species i on the mass increment from 1990 to 1993 of indiidual saplings of mass x at time t (¯ 1990) of species k[Gk(t, x)]. The competitie effect is described as one-sided (c , i " 0, c , i ¯ 0), symmetric (c , i ¯ 0, " # " c , i " 0) or asymmetric (c , i " 0, c , i " 0). Statistical results are based on multiple linear regression analysis for eqn (4). " # # ****, significant at P ! 0±0001, ***, significant at P ! 0±001 ; **, significant at P ! 0±01 ; *, significant at P ! 0±05 ; ns, not significantly different from 0 (P & 0±05) P. jezoensis (i ¯ 1)
A. sachalinensis (i ¯ 2)
Gk(t, x)
n
a (s.e.)
c , (s.e.) ""
c , (s.e.) #"
c , (s.e.) "#
c , (s.e.) ##
P. jezoensis (k ¯ 1) A. sachalinensis (k ¯ 2)
663 801
0±708 (0±036)**** 0±661 (0±040)****
0±32 (0±033)**** n.s.
n.s. n.s.
0±11 (0±044)* 0±14 (0±028)****
n.s. n.s.
!
dominated stands (arrow A and arrow B in Fig. 5, respectively). There were no subquadrats with the two species co-dominant at a high biomass range. This indicates that saplings of Picea jezoensis and Abies sachalinensis were not uniformly distributed, but that the two species segregated establishment sites with sapling growth.
Growth dynamics and the mode of competition The results of multiple linear regression with forward stepwise method based on eqn (4) are shown in Table 2. Three transformations, ln (1y), oy and oyoy1, for dependent variable (sapling mass increment) y, were tried to have homoscedasticity (homogeneous variance of residuals). However, the statistical results were consistent with those of the untransformed case. Correlations between x and xCi(t, x), between x and xCi(t, xmin), and between xCi(t, x) and xCi(t, xmin) were all less than 0±4, and multicollinearity was not detected. The Durbin-Watson statistic and serial correlation of residuals were close to 2 (1±98, 1±86) and 0 (®0±01, ®0±07), respectively, indicating little autocorrelation between residuals. Normality of residuals were checked with normal plots, and outliers were also checked using residual-deleted residual plots and Cook’s distances. Outliers (Cook’s distances " 1) were not found in the regression analysis. Maximal values of Cook’s distance were 0±48 and 0±82 for Abies sachalinensis and Picea jezoensis, respectively. The size-dependent term, ‘ x ’, in eqn (4) was entered first in both the cases of Abies sachalinensis and Picea jezoensis (P ! 0±0001), representing a strongly size-dependent growth pattern. The mode of intraspecific and interspecific competition was different between the two species. Asymmetric competitive effects of Abies sachalinensis and Picea jezoensis saplings (c , , c , " 0) on the sapling growth of Picea "" "# jezoensis were detected. Asymmetric competitive effect of Abies sachalinensis saplings on the individual growth of Abies sachalinensis saplings was detected (Table 2). Abies sachalinensis suppressed one-sidedly the growth of Picea jezoensis but not vice versa (one-sided interspecific competition). These results indicate that an asymmetric competitive effect prevailed at the sapling stage of Abies sachalinensis and Picea jezoensis and that sapling competition between the two species was intense.
d.f.-adjusted R# for final model 0±57 0±61
DISCUSSION Allometry of saplings Tree geometry is an important ecological feature (Horn, 1971). Kohyama (1980) pointed out that Abies mariesii under suppressed conditions had a flat crown allowing for effective assimilation. Furthermore, Ku$ ppers (1989) classified the architectural pattern of woody plants into multi- and monolayer types. From the viewpoint of competition for light, Ku$ ppers (1989) suggested that costbenefit aspects of tree form reflects the successional status of species. The present results of the allometric relationships showed that Picea jezoensis and Abies sachalinensis saplings had conic and flat crowns, respectively (Table 1). Consequently, the Picea jezoensis sapling had more foliage mass and a greater mortality rate than the Abies sachalinensis sapling of the same sapling mass (Figs 1 and 4). These results suggest that at the sapling stage Picea jezoensis has a greater cost than Abies sachalinensis for assimilation-respiration balance under dark conditions of the closed canopy, whereas Abies sachalinensis maintains effective assimilation even under suppressed conditions (cf. Kohyama, 1980 ; Canham, 1988 ; Ku$ ppers, 1989). The difference in the crown form of saplings between the two species suggests different adaptations for efficient persistence in the sub-boreal forest (cf. King, 1990). Crown architecture is relevant to shade tolerance of tree species (Alvarez-Buylla and MartinezRamos, 1992). Kubota et al. (1994) pointed out that Abies sachalinensis can grow taller than Picea jezoensis under suppressed conditions, whereas Picea jezoensis requires canopy gaps for steady height growth. The present study also showed that widely spaced saplings of Picea jezoensis had a greater relative growth rate (a ) of sapling mass than ! widely spaced Abies sachalinensis (Table 2). This suggests that the regeneration pattern related to light environments is different between the two species having distinct crown shapes. Growth dynamics and the mode of competition Kubota and Hara (1995) showed that interspecific competition between trees (& 2 m in height) of the three dominant species, Picea jezoensis, Betula ermanii and Abies sachalinensis was either weak symmetric (P ¯ 0±05 for the
Kubota and Hara—Sapling Competition between Picea and Abies Maintenance of species diversity
Life stage
2 (P = 0.08) 2
Pj
Tree (>2 m in height)
As
2 (P < 0.001)
2 (P = 0.05)
(P < 0.001)
535
Stand development
Habitat segregation and stability of species composition
Sapling (<2 m in height) 1 (P < 0.0001)
Pj
As 1 (P = 0.01)
1 (P < 0.0001)
F. 6. The modes of intra- and interspecific competition (effect and direction) between saplings (below), and between trees (above), of Picea jezoensis (Pj) and Abies sachalinensis (As) of a sub-boreal forest, Hokkaido, northern Japan. The diagram of sapling competition is based on the results of Table 2. The diagram of tree competition is redrawn from Kubota and Hara (1995). The arrow indicates the direction of competitive effect : ‘ X U Y ’ represents that species X suppresses the individual growth of species Y (X ¯ Y, intraspecific competition ; X 1 Y, interspecific competition). ‘ 1 ’ and ‘ 2 ’ at the arrow represent one-sided and symmetric competitive effect, respectively (see Table 2). The P-level for the competition coefficient of the regression model is also given at the arrow ([[[", weak competition, 0±05 % P ! 0±1 ; U, intense competition, P ! 0±05).
competition coefficient of the regression model) or almost absent (P " 0±05), indicating that competitive interactions between trees cannot be a structuring force or relevant to the variation in species coexistence. On the contrary, asymmetric intense competition prevailed (Table 2, Fig. 6) in the saplings. The M(t, x) function for saplings of the two species was negatively size-dependent (Fig. 4), indicating intense competitive effects of larger saplings on smaller ones. The population of saplings on each fallen log is regarded as almost even-aged (Fig. 2) because the establishment of saplings on fallen logs is restricted to the early stage, 15–25 years in age, of fallen logs (Harmon, 1988). Furthermore, sapling density on fallen logs (1 045 625 ha−") is much higher than that of trees (1223 ha−" ; Kubota and Hara, 1995), suggesting that competitive interaction exaggerates size hierarchy of saplings. Horn (1971) pointed out that the geometry of tree crown affects light penetration into the understorey. Ellison and Rabinowitz (1989) showed that the difference in growth form between the two varieties of Pisum satium resulted in different size hierarchies. Weiner et al. (1990) investigated the effect of plant density on the vertical leaf arrangement of each Impatiens pallida individual in monocultures, and indicated that plant growth form and competitioin were correlated with each other. In the present results, the Picea jezoensis sapling with a conic and deep crown maintained a greater foliage mass than the Abies sachalinensis sapling
with a flat and shallow crown (Table 1, Fig. 1). The vertical foliage arrangement of Abies sachalinensis saplings is restricted to the upper layer. This point is also important for the mode of interspecific sapling competition between the two species. The competitive effect of Picea jezoensis saplings on Abies sachalinensis saplings was absent, whereas the competitive effect of Abies sachalinensis saplings on Picea jezoensis saplings was one-sided. The difference in the mode of interspecific sapling competition is ascribed to the difference in the crown shape of saplings between the two species, i.e. Picea jezoensis saplings are suppressed only by larger Abies sachalinensis saplings with a flat and shallow crown at the top of the stem (asymmetric interspecific competition). Hara (1993), Hara and Yokozawa (1994), and Hara and Wyszomirski (1994) pointed out that asymmetric competition is related to the stability of size-structure dynamics of plants, acting as a structuring force of the community, whereas symmetric competition cannot be a structuring force of the community and is irrelevant to the stability of size-structure dynamics. Asymmetric competition represents a deterministic interaction between individuals. A population undergoing asymmetric competition is mainly governed by the deterministic factor [the G(t, x) function], rather than by the stochastic factor [the D(t, x) function]. Therefore, the growth dynamics of saplings of Picea jezoensis and Abies sachalinensis, where asymmetric competition pre-
536
Kubota and Hara—Sapling Competition between Picea and Abies
vailed, were mainly governed by the deterministic factor [the G(t, x) function].
Species coexistence Large saplings of Picea jezoensis and Abies sachalinensis were aggregated in different sites on fallen logs, indicating habitat segregation with sapling stand development (Fig. 5). The growth dynamics of saplings of the two species were mainly governed by asymmetric competition. It is therefore suggested that deterministic interaction as asymmetric competition plays an important role for habitat segregation between Picea jezoensis and Abies sachalinensis saplings. A mixed sapling stand of the two species shifts with growth to a pure stand dominated by either Picea jezoensis or Abies sachalinensis saplings, suggesting that habitat segregation of the two species occurred at the sapling stage. In the mixed sapling stands of Picea jezoensis and Abies sachalinensis, Picea jezoensis is an inferior competitor under crowded competitive conditions but a superior competitor under widely spaced conditions (large a ), whilst Abies sachalinensis ! is a superior competitor under crowded competitive conditions but an inferior competitor under widely spaced conditions (small a ) (Table 2, Fig. 6). This tendency is ! related to the difference in crown shape between the two conifers. Therefore, in addition to the effects of asymmetric competition, the habitat segregation of sapling stands may have been promoted by the variation in seedling density of the two conifers due to a difference in the proportions of the amount of seeds between the two species at the establishment stage. Kubota and Hara (1995) investigated the growth dynamics and the mode of competition between trees (& 2 m in height) of the dominant species, Picea jezoensis, Picea glehnii, Betula ermanii and Abies sachalinensis and the subordinate species, Sorbus commixta and Acer ukurunduense of the sub-boreal forest, and indicated that the four dominant species showed either weak symmetric interspecific competition (P ¯ 0±05) between Abies sachalinensis and Picea jezoensis or almost no competition (P " 0±05 other cases). Therefore, Kubota and Hara (1995) suggested that the coexistence of the four dominant species is related to the boundary conditions for the growth dynamics of trees, such as seedling establishment and population dynamics of saplings. Especially in densely populated sites of saplings where the competitive effect is greatly exaggerated, it can be assumed that sapling competition between species determines a future coexistence pattern in the canopy layer. The present study showed that asymmetric competition prevailed in the population of saplings, and sapling competition between the species was relatively intense, in contrast to weak competition between canopy trees (Fig. 6). This implies that competitive exclusion occurred in the sapling stage. The regeneration niche has been regarded as an important factor for promoting species coexistence (Fox, 1977 ; Grubb, 1977 ; Knapp and Smith, 1982). Site-specific regeneration of constituent species in a plant community has been stressed (Forcier, 1975 ; Fox, 1977). Christy and Mack (1984)
investigated variations in the demography of conifers’ juveniles in relation to the substratum mosaic. Nakashizuka (1988) and Taylor and Qin (1992) showed that seedling establishment of Fagus crenata, Abies faxioniana and Betula utilis occurred after the simultaneous death of undergrowing Sasa dwarf bamboos. However, there have been few studies that investigated population dynamics of each component species in relation to interspecific competition processes in the early growing stages (seedlings and saplings) [but see Petersen (1988) and Petersen, Ning and Newton (1990) for monocultures]. The present study presents a process of habitat segregation based on competition between saplings of the two conifers. The mode of competition changed with the life-history stage from the sapling (intense and asymmetric) to the canopy tree (weak and symmetric or almost absent). In conclusion : (1) asymmetric and intense competition between saplings brought about habitat segregation between the dominant species, Picea jezoensis and Abies sachalinensis, in the early stage of life-history ; (2) therefore, the coexistence of Picea jezoensis and Abies sachalinensis of the sub-boreal forest was determined by the boundary conditions for the growth dynamics of trees, as segregation of establishment sites resulting from asymmetric and intense competition between saplings ; (3) then the species composition of the forest was maintained by weak symmetric competition or the state of almost no competition between trees. A C K N O W L E D G E M E N TS We thank Shin-ichi Niwa for help with field work. We are also grateful to a reviewer, Dr David King, for valuable comments and suggestions. This study was partly supported by a grant from the Ministry of Education, Science and Culture, Japan. LITERATURE CITED Alvarez-Buylla ER, Martinez-Ramos M. 1992. Demography and allometry of Cecropia obtusifolia, a neotropical pioneer tree—an evaluation of the climax–pioneer paradigm for tropical rain forests. Journal of Ecology 80 : 275–290. Canham CD. 1988. Growth and canopy architecture of shade-tolerant trees : response to canopy gaps. Ecology 69 : 786–795. Cannell MGR, Rothery P, Ford ED. 1984. Competition within stands of Picea sitchensis and Pinus contorta. Annals of Botany 53 : 349–362. Christy EJ, Mack RN. 1984. Variation in demography of juvenile Tsuga heterophylla across the substratum mosaic. Journal of Ecology 72 : 75–91. Ellison AM, Rabinowitz D. 1989. Effects of plant morphology and emergence time on size hierarchy formation in experimental populations of two varieties of cultivated peas (Pisum satium). American Journal of Botany 76 : 427–436. Forcier LK. 1975. Reproductive strategies and the co-occurrence of climax tree species. Science 189 : 808–810. Ford ED. 1975. Competition and stand structure in some even-aged plant monocultures. Journal of Ecology 63 : 311–333. Ford ED, Diggle PJ. 1981. Competition for light in a plant monoculture modelled as a spatial stochastic process. Annals of Botany 48 : 481–500. Fox JF. 1977. Alternation and coexistence of tree species. American Naturalist 111 : 69–89. Geber M. 1989. Interplay of morphology and development on size inequality : a Polygonum greenhouse study. Ecological Monographs 59 : 267–288.
Kubota and Hara—Sapling Competition between Picea and Abies Grubb PJ. 1977. The maintenance of species-richness in plant communities : the importance of the regeneration niche. Biological Reiew 52 : 107–145. Hara T. 1984 a. A stochastic model and the moment dynamics of the growth and size distribution in plant populations. Journal of Theoretical Biology 109 : 173–190. Hara T. 1984 b. Dynamics of stand structure in plant monocultures. Journal of Theoretical Biology 110 : 223–239. Hara T. 1988. Dynamics of size structure in plant populations. Trends in Ecology and Eolution 3 : 129–133. Hara T. 1992. Effects of the mode of competition on stationary size distribution in plant populations. Annals of Botany 69 : 509–513. Hara T. 1993. Mode of competition and size-structure dynamics in plant communities. Plant Species Biology 8 : 75–84. Hara T, Kimura M, Kikuzawa K. 1991. Growth patterns of tree height and stem diameter in populations of Abies eitchii, Abies mariesii and Betula ermanii. Journal of Ecology 79 : 1085–1098. Hara T, Nishimura H, Yamamoto S. 1995. Tree competition and species coexistence in a cool-temperate old-growth forest in southwestern Japan. Journal of Vegetation Science 6 : 565–574. Hara T, Yokozawa M. 1994. Effects of physiological and environmental variations on size-structure dynamics in plant populations. Annals of Botany 73 : 39–51. Hara T, Wyszomirski T. 1994. Competitive asymmetry reduces spatial effects on size-structure dynamics in plant populations. Annals of Botany 73 : 285–297. Harmon MK. 1988. Effects of bark fragmentation on plant succession on conifer logs in the Picea-Tsuga forests of Olympic National Park, Washington. American Midland Naturalist 121 : 112–124. Harper JL. 1977. Population biology of plants. London : Academic Press. Horn HS. 1971. The adaptie geometry of trees. New Jersey : Princeton University Press. Hutchings MJ. 1986. The structure of plant populations. In : Crawley MJ, ed. Plant ecology. Oxford : Blackwell Scientific Publications, 97–136. King DA. 1990. Allometry of saplings and understory trees of a panamanian forest. Functional Ecology 4 : 27–32. Knapp AK, Smith WK. 1982. Factors influencing understory seedling establishment of Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa) in southeastern Wyoming. Canadian Journal of Botany 60 : 2753–2761. Knox RG, Peet RK. 1989. Population dynamics in loblolly pine stands : changes in skewness and size inequality. Ecology 70 : 1153–1166. Kohyama T. 1980. Growth pattern of Abies mariesii saplings under condition of open-growth and suppression. Botanical Magazine Tokyo 93 : 13–24. Kohyama T. 1987. Significance of architecture and allometry of saplings. Functional Ecology 1 : 399–404. Kohyama T. 1991. Simulating stationary size distribution of trees in rain forests. Annals of Botany 68 : 173–180. Kohyama T. 1992. Size-structured multi-species model of rain forest trees. Functional Ecology 6 : 206–212. Kohyama T. 1993. Size-structured tree populations in gap-dynamic forest—the forest architecture hypothesis for the stable coexistence of species. Journal of Ecology 81 : 131–143. Kohyama T, Grubb PJ. 1994. Below- and above-ground allometries of shade-tolerant seedlings in a Japanese warm-temperate rain forest. Functional Ecology 8 : 229–236.
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Kohyama T, Hotta M. 1990. Significance of allometry in tropical saplings. Functional Ecology 4 : 515–521. Kubota Y. 1995. Effects of disturbance and size structure on the regeneration process in a sub-boreal coniferous forest, northern Japan. Ecological Research 10 : 135–142. Kubota Y, Hara T. 1995. Tree competition and species coexistence in a sub-boreal forest, northern Japan. Annals of Botany 76 : 503–512. Kubota Y, Konno Y, Hiura T. 1994. Stand structure and growth patterns of understorey trees in a coniferous forest, Taisetsuzan National Park, Japan. Ecological Research 9 : 333–341. Ku$ ppers M. 1989. Ecological significance of above-ground architectural patterns in woody plants : A question of cost-benefit relationships. Trends in Ecology and Eolution 4 : 375–379. Kuroiwa S. 1960. Intraspecific competition in artificial sunflower communities. Botanical Magazine Tokyo 73 : 300–309. Mack RN, Harper JL. 1977. Interference in dune annuals : spatial pattern and neighbourhood effects. Journal of Ecology 65 : 345–363. Mithen R, Harper JL, Weiner J. 1984. Growth and mortality of individual plants as a function of ‘‘ available area ’’. Oecologia 62 : 57–60. Nagano M, Kira T. 1978. Primary production. In : Kira T, Ono Y, Hosokawa T, eds. Biological production in a warm-temperate eergreen oak forest of Japan. JIBP Synthesis 18. Tokyo : University of Tokyo Press, 69–82. Nakashizuka T. 1988. Regeneration of beech (Fagus crenata) after the simultaneous death of undergrowing dwarf bamboo (Sasa kurilensis). Ecological Research 3 : 21–35. Ogawa H, Kira T. 1977. Methods of estimating forest biomass. In : Shidei T, Kira T, eds. Primary productiity of Japanaese forest. JIBP Synthesis 16. Tokyo : University of Tokyo Press, 15–25. Petersen TD. 1988. Effects of interference from Calamagrostis rubescens on size distributions in stands of Pinus ponderosa. Journal of Applied Ecology 25 : 265–272. Petersen TD, Ning Z, Newton M. 1990. Dynamics of size structures in seedling stands of Fraxinus mandshurica in Northeast China. Annals of Botany 66 : 255–263. Silvertown J, Lovett Doust J. 1993. Introduction to plant population biology. Oxford : Blackwell Scientific Publications. Takada T, Iwasa Y. 1986. Size distribution dynamics of plants with interaction by shading. Ecological Modelling 33 : 173–184. Taylor AH, Qin Z. 1992. Tree regeneration after bamboo die-back in Chinese Abies-Betula forests. Journal of Vegetation Science 3 : 253–260. Thomas SC, Weiner J. 1989. Including competitive asymmetry in measures of local interference in plant populations. Oecologia 80 : 349–355. Weiner J. 1984. Neighbourhood interference amongst Pinus rigida individuals. Journal of Ecology 72 : 183–195. Weiner J. 1990. Asymmetric competition in plant populations. Trends in Ecology and Eolution 5 : 360–364. Weiner J, Berntson GM, Thomas SC. 1990. Competition and growth form in a woodland annual. Journal of Ecology 78 : 459–469. Weller DE. 1987. Self-thinning exponent correlated with allometric measures of plant geometry. Ecology 68 : 813–821. West PW, Jackett DR, Borough CJ. 1989. Competitive processes in a monoculture of Pinus radiata D.Don. Oecologia 81 : 57–61. Yokozawa M, Hara T. 1992. A canopy photosynthesis model for the dynamics of size structure and self-thinning in plant populations. Annals of Botany 70 : 305–316.
Allometry and Competition between Saplings of Picea jezoensis and Abies sachalinensis in a Sub-boreal Coniferous Forest, northern Japan Y A S U H I R O K U B O T A and T O S H I H I K O H A R A* Department of Biology, Tokyo Metropolitan Uniersity, Tokyo 192-03, Japan Received : 30 March 1995
Accepted : 7 December 1995
The crown shape and the mode of competition between saplings (! 2 m in height) of the two conifers, Picea jezoensis and Abies sachalinensis, of a sub-boreal forest, northern Japan, were investigated based on the diffusion model. A model for individual sapling growth considering both inter- and intraspecific competition was developed. The effect of species-specific crown shape on the sapling growth and competition of the two species were examined. Picea jezoensis and Abies sachalinensis saplings had deep conic and shallow flat crowns, respectively. Picea jezoensis had more foliage mass than Abies sachalinensis of the same sapling mass. It was suggested that the Picea jezoensis sapling has a high cost for assimilation–respiration balance under dark conditions of closed canopies, whereas the Abies sachalinensis sapling maintains effective assimilation even under suppressed conditions. Widely spaced saplings, such as gap successors, of Picea jezoensis had a greater relative growth rate (a ) than widely spaced Abies sachalinensis. The ! crown shape of saplings of the two species shows different adaptations for efficient persistence in the sub-boreal forest. Saplings of Picea jezoensis and Abies sachalinensis were not uniformly distributed, but aggregated in different sites as the saplings grew, indicating habitat segregation between the two species at the sapling stage. Intraspecific sapling competition was one-sided in each of the two conifers. Interspecific sapling competition was one-sided in the direction only from Abies sachalinensis to Picea jezoensis. Therefore, asymmetric competition prevailed at the sapling stage of the two species. These results contrast with weak symmetric competition or the almost absence of competition between trees (& 2 m in height) of the two species (Kubota and Hara, Annals of Botany 76 : 503–512, 1995). The mode of competition changed with the life-history stage from the sapling (intense and asymmetric) to the tree (weak and symmetric or almost absent). In conclusion (1) asymmetric and intense competition between saplings brought about habitat segregation between the dominant species, Picea jezoensis and Abies sachalinensis, in the early stage of life-history ; (2) therefore, the coexistence of Picea jezoensis and Abies sachalinensis of the sub-boreal forest was determined by the boundary conditions for the growth dynamics of the trees, as segregation of establishment sites resulting from asymmetric and intense competition between saplings ; (3) then the species composition of the forest was maintained by weak symmetric competition or the almost absence of competition between trees. # 1996 Annals of Botany Company Key words : Crown shape, growth dynamics, species coexistence, habitat segregation, diffusion model.
INTRODUCTION Competition between individual plants greatly affects the dynamics of plant populations (Koyama and Kira, 1956 ; Harper, 1977 ; Hutchings, 1986 ; Hara, 1988 ; Silvertown and Lovett Doust, 1993). Many researchers have investigated population dynamics, considering competitive effects on the growth of individual plants (Ford, 1975 ; Mack and Harper, 1977 ; Mithen, Harper and Weiner, 1984 ; Weiner, 1984 ; West, Jackett and Borough, 1989 ; Knox and Peet, 1989). However, most of these previous studies have dealt with even-aged monocultures. There have been few studies on the effects of competition on growth dynamics of multispecies plant communities (but see Kohyama, 1991, 1992). Co-occurring species in a plant community have different plant geometries (Horn, 1971 ; Kohyama, 1987 ; King, 1990 ; Kohyama and Hotta, 1990 ; Kohyama and Grubb, 1994). Significant interactions between growth form and competition have been suggested (Weller, 1987 ; Ellison and * Present address : The Institute of Low Temperature Science, Hokkaido University, Sapporo 060, Japan.
0305-7364}96}05052909 $18.00}0
Rabinowitz, 1989 ; Geber, 1989 ; Weiner, Berntson and Thomas, 1990). Hara, Kimura and Kikuzawa (1991) pointed out that the difference in the mode of competition between Abies eitchii, Abies mariesii and Betula ermanii, resulting from species-specific growth form, corresponds to the successional status of each species. Thus, plant form (crown architecture and allometry) may influence growth dynamics and species coexistence. Asymmetric (or one-sided) competition, where larger plants have a disproportionately larger suppressive effect on the absolute growth rates of smaller ones by shading, has been described by Kuroiwa (1960), Ford and Diggle (1981), Cannell, Rothery and Ford (1984), Takada and Iwasa (1986), Thomas and Weiner (1989) and Weiner (1990). Kohyama (1991, 1992, 1993) showed that one-sided competition (the most extreme case of asymmetric competition where only larger plants have a suppressive effect on the growth of smaller ones and not vice versa) governs the stand development of a warm-temperate rain forest, and suggested that one-sided competition contributes to the stable coexistence of Distylium racemosum, Illicium anisatum and Eurya japonica occupying different vertical layers of the # 1996 Annals of Botany Company
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Kubota and Hara—Sapling Competition between Picea and Abies
warm-temperate forest. Theoretical studies by Hara (1992, 1993) suggest that one-sided or asymmetric competition promotes stability in size-structure dynamics of plants. Symmetric competition (where the suppressive effect on individual relative growth rate is size-independent), rather than one-sided or asymmetric competition, prevailed in some climax forests with multi-layered canopies (Kubota and Hara, 1995 ; Hara, Nishimura and Yamamoto, 1995). In these studies, either weak symmetric or little competition between dominant canopy trees was found in terms of DBH (stem diameter at breast height) growth as a substitute of tree mass. DBH is a good substitute of individual mass in trees, i.e. DBH growth dynamics are almost paralleled to those of individual tree mass (Nagano and Kira, 1978) and thus it is highly probable that competition is weak or almost absent even in mass growth if competition was weak or almost absent in terms of DBH growth in tree communities. Kubota and Hara (1995) and Hara et al. (1995) suggested that for dominant species occupying the same layer, competition between trees was either weak symmetric or almost absent. It is therefore suggested that size-structure dynamics of dominant species will be greatly influenced by the recruitment process for the growth dynamics of canopy trees, such as establishment processes and population dynamics of saplings. In the present paper, we show that the persistence of saplings (! 2 m in height) in the understorey, rather than the growth dynamics of trees (& 2 m in height), plays an important role for the coexistence of dominant species, Picea jezoensis Carr. and Abies sachalinensis Masters forming a sub-boreal coniferous forest in Hokkaido, northern Japan. In the same forest, the growth and competition between trees have already been investigated (Kubota and Hara, 1995). We examine how species-specific crown shape of saplings affects their growth dynamics and competition between species based on the diffusion model (Hara, 1984 a, b ; Yokozawa and Hara, 1992). Then, we discuss coexistence mechanisms of the two dominant species based on the mode of competition between saplings. STUDY SITE The study was carried out in the eastern region of Taisetsuzan National Park, Japan. Five study plots (2±3 ha in total) were set up in a Picea–Abies forest (850–1000 m above sea level) in 1989. The region is in a valley with low relief. Annual precipitation is approximately 1500 mm. Mean daily temperatures during the warmest month (Aug.) and the coldest month (Jan.) are 17±7 and ®10±7 °C, respectively. Snow cover on the forest floor is usually from Nov. to May. Soil is predominantly brown forest soil. The study site is dominated by Picea jezoensis Carr., Abies sachalinensis Masters, Betula ermanii Cham. and Picea glehnii Masters ; Acer ukrunduense Trautv. et Mey. and Sorbus commixta Hedl. also occur under relatively sparse canopies. The total basal area [π}4(DBH )#] of all trees & 2 m in height in the study site was 31±0 m# ha−" in 1989 (Kubota and Hara, 1995). The basal areas of the dominant canopy species, Picea jezoensis, Picea glehnii, Betula ermanii and Abies sachalinensis, were 8±4, 4±2, 6±0 and 12±3 m# ha−",
respectively, in 1989. The gap ratio in the canopy, which was defined as the vertical projection area where no stems reached 10 m height or 10 cm DBH, ranged from 4±2 to 30±4 % (Kubota, 1995). The study site consists of stands of different maximal tree age ranging from 125 to 333 years (Kubota, 1995). The forest floor is characterized by two floristic types— Sasa dwarf bamboos and Carex. In the stands of low canopy tree densities, Sasa senaensis Franch. et Sav. and Sasa kurilensis Rupr. Makino, covered the forest floor. Consequently, the establishment site of saplings in such stands was restricted to raised surfaces such as fallen logs and stumps (Kubota, Konno and Hiura, 1994). FIELD MEASUREMENTS AND DATA ANALYSIS Field data Fallen logs with densely established saplings were selected as study quadrats. Fourteen quadrats were set up on 14 fallen logs in the study plots in Jun. 1990. Each quadrat on a fallen log was divided into several 1 m long subquadrats (31 in total for the 14 quadrats). Quadrat area was calculated as the product of the length and diameter of each fallen log, and ranged from 0±5 to 4±0 m# (16 m# in total for the 14 quadrats). Each sapling (branched individual and 5 cm ! height ! 200 cm) was tagged (n ¯ 1481) and identified by the species within each subquadrat. The shape of the crown projected onto the ground was assumed to be an ellipse with the lengths of major and minor axes, CW1 and CW2 (crown widths in two perpendicular directions). The crown depth, CD, was defined as the vertical distance from the lowest branch to the terminal leader. The stem diameter at ground level (DGL, mm), stem height (H, cm), crown depth (CD, cm) and crown widths (CW1 and CW2, cm) were measured in Jun. and Jul. 1990. The age of saplings was determined by counting whorls of branches and scars of the oldest whorls. The stem diameter at ground level (DGL) and height (H ) of tagged saplings were remeasured in Oct. 1993. Allometry of saplings Eight saplings of Picea jezoensis and ten saplings of Abies sachalinensis were harvested in 1990. The stem diameter at ground level (DGL), height (H ), crown depth (CD), crown widths (CW1 and CW2) and age for each harvested sapling were measured. Harvested saplings were divided into stem, branch and leaf, and then dried for 48 h at 80 °C, and weighed for each component. Allometry was obtained as a relationship between measured items (DGL, H, CW1, CW2 and CD) and component masses of harvested saplings. The DGL of harvested saplings ranged from 3 to 10 mm and the height from 20 to 50 cm. The observed sapling mass ranged from 3 to 40 g. The size range of the harvested saplings covered 97 % of that of the tagged saplings. The stem volume of a sapling, SV, was estimated as (DGL)#¬H (Ogawa and Kira, 1977). Projected horizontal crown area, CA, was calculated as an ellipse by (π}4)¬CW1¬CW2, and then crown volume, CV, was estimated by CD¬CA. The allometric relationship between
Kubota and Hara—Sapling Competition between Picea and Abies
531
T 1. Estimates of the coefficients, a, b, α and β, of allometric relationships : W®SV, ln (sapling mass) ¯ ab ln (stem olume) ; CD®SV, ln (crown depth) ¯ ab ln (stem olume) ; CA®SV, ln (crown area) ¯ ab ln (stem olume) ; CV®SV, ln (crown olume) ¯ ab ln (stem olume) ; CD®H, ln (crown depth) ¯ ab ln (stem height) ; CA®H, ln (crown area) ¯ ab ln (stem height) ; CV®H, ln (crown olume) ¯ ab ln (stem height) ; F®CV¬SV, ln (foliage mass) ¯ α ln (crown olume)β ln (stem olume) Picea jezoensis Allometry W®SV CD®SV CA®SV CV®SV CD®H CA®H CV®H F®CV¬SV
Abies sachalinensis
b
n
R#
P
®1±454 1±406 1±017 1±324 0±216 ®2±433 ®3±316
0±594 0±234 0±791 1±022 0±777 2±461 3±238
8 671 671 671 671 671 671
0±330 0±720 0±843 0±873 0±774 0±778 0±835
0±049 0±0001 0±0001 0±0001 0±0001 0±0001 0±0001
α 0±352
β ®0±146
n
R# 0±956
P 0±0001
a
8
sapling mass (stem massfoliage mass), W, and stem volume was estimated by ln W ¯ ab ln SV,
(1 a)
where a and b are species-specific parameters. In order to assess crown geometry of Picea jezoensis and Abies sachalinensis, allometric relationships between logtransformed sapling size, ln x (x ¯ SV, H ), and crown dimensions, ln y ( y ¯ CA, CD and CV ) were estimated as follows (Table 1) : ln y ¯ ab ln x,
(1 b)
where a and b are species-specific parameters. ANCOVA for ln y with the covariate ln x was used to test the difference in the intercept, a, and the slope, b, between the two species (Kohyama, 1987 ; King, 1990 ; Kohyama and Hotta, 1990 ; Kohyama and Grubb, 1994). The foliage mass, F, of each sapling was estimated by using multiple linear regression with sapling stem volume (SV ) and crown volume (CV ) as the explanatory variables : ln F ¯ α ln SVβ ln CV,
(2)
where α and β are species-specific parameters (Table 1). Sapling and foliage masses of each tagged sapling on fallen logs were estimated by using eqns (1 a) and (2). Diffusion model and the mode of competition The growth dynamics were analysed based on the diffusion model (Hara, 1984 a, b), ¦ 1 ¦# ¦ f (t, x) ¯ [D(t, x) f (t, x)]® [G(t, x) f (t, x) ¦t 2 ¦x# ¦x ®M (t, x) f (t, x)]
(3)
where f (t, x) is the size distribution function of sapling mass x at time t, and xmax and xmin are maximal and minimal sapling masses, respectively. The diffusion model consists of three functions describing the mean of absolute growth rate, G(t, x), the variance of absolute growth rates, D(t, x), and the mortality rate, M(t, x), of saplings of mass x (0 ! xmin % x % xmax) at time t.
b
n
R#
P
®3±003 1±343 1±382 1±627 0±184 ®1±708 ®2±623
0±842 0±214 0±773 0±987 0±737 2±322 3±059
10 810 810 810 810 810 810
0±330 0±548 0±855 0±873 0±609 0±766 0±788
0±048 0±0001 0±0001 0±0001 0±0001 0±0001 0±0001
α 0±546
β ®0±524
n 10
R# 0±923
P 0±0001
a
Considering the effects of the species composition of sapling subquadrats on the G(t, x) function of species k, Gk(t, x), we developed the equation of Yokozawa and Hara (1992) for investigating both inter- and intraspecific competition as follows :
(
*
# # Gk(t, x) ¯ x a ® 3 c , i Ci(t, x)® 3 c , i Ci(t, xmin) (4) " # ! i=" i=" where Ci(t, x) is the total foliage mass of individuals of sapling mass greater than x of species i (k, i ¯ 1, Picea jezoensis ; k, i ¯ 2, Abies sachalinensis) at time t, c , i and c , i # " are coefficients for species i, and xmin is the minimal sapling mass in a subquadrat. Thus, the Ci(t, x) value for an individual of sapling mass x of species i at time t(1990) was given as : xmax Fi(t, y) fi(t, y) dy (5) Ci(t, x) ¯
&
x
where xmax is the maximal sapling mass in a subquadrat, fi(t, y) is the distribution density of sapling mass y at time t of species i, and Fi(t, y) is the foliage mass of saplings of mass y at time t of species i calculated as eqn (2). A neighbourhood area for calculating Ci(t, x) was represented by subquadrat area. The Ci(t, x) and Ci(t, xmin) functions were calculated for individual saplings of the two species in each subquadrat in 1990 (¯ t). Competition can be described in terms of the mode and direction of effects. The mode of the competitive effect is symmetric (c , i ¯ 0, c , i " 0), asymmetric (c , i, # " " c , i " 0) or one-sided (c , i " 0, c , i ¯ 0) irrespective of inter" # # and intraspecific competition. In the case of interspecific competition, the direction of the competitive effect is symmetric (two-sided) between species i and k if ²c , i " 0 or " c , i " 0 for Gk(t, x)´ and ²c , k " 0 or c , k " 0 for Gi(t, x)´, or " # # one-sided from species i to species k if ²c , i " 0 or c , i " 0 # " for Gk(t, x)´ and ²c , k ¯ 0 and c , k ¯ 0 for Gi(t, x)´. For " # intraspecific competition, the competitive direction is identical to the competitive effect (symmetric or one-sided) (Kubota and Hara, 1995). Multiple linear regression analysis was conducted by using the forward stepwise method (independent variables are entered or removed stepwise one at a time), for the
532
Kubota and Hara—Sapling Competition between Picea and Abies
sapling mass increment of each individual sapling as the dependent variable for Gk(t, x) and ‘ x ’, ‘ xCi(t, x) ’ and ‘ xCi(t, xmin) ’ as the independent variables in eqn (4) for each of the two species. The F statistics to enter and remove an independent variable were both set at 2. The d.f.-adjusted R#-value for the regression model was calculated [R# ¯ 1®(1®r#) (n®1)}(n®q®1) ; n, sample size ; q, number of independent variables in the regression equation ; r, multiple correlation coefficient]. Residual analyses were conducted to examine the adequacy of the model.
60 50 40 30 20 10
RESULTS Allometry of saplings
100
A
0 45
10
20
30
10
20
30
40 35 Number of saplings
Table 1 shows allometric relationships between measures of crown shape, stem volume and stem height for Picea jezoensis and Abies sachalinensis. The allometric relationships were significantly different between the two species
30 25 20 15 10 5 0 30
10
Estimated foliage mass (g)
25 20 15 1 0.1 100
1
10
100
1000
B
10 5
0 10
30
F. 2. Frequency distributions of sapling age for Picea jezoensis (+) and Abies sachalinesis (*), in three representative subquadrats.
1
0.1
10 20 Age of saplings (years)
1 10 100 Estimated sapling mass (g)
1000
F. 1. Relationships between sapling and foliage estimated masses of Picea jezoensis (A), and Abies sachalinensis (B), in a sub-boreal forest, Hokkaido, northern Japan in 1990.
(ANCOVA with the covariate ln SV or ln H, P ! 0±001). Picea jezoensis showed a greater value of the intercept in the allometric relationship between crown depth and stem volume (or stem height) than Abies sachalinensis, whereas Abies sachalinensis showed a greater value of the intercept in crown area s. stem volume (or stem height) than Picea jezoensis (ANCOVA, P ! 0±001). These results indicate that Picea jezoensis had a conic crown with deep crown depth and small crown area, whilst Abies sachalinensis had a flat one with shallow crown depth and large crown area. Foliage mass of Picea jezoensis increased with sapling mass (R# ¯ 0±63), whereas that of Abies sachalinensis was almost size-indepenent (R# ¯ 0±01) (Fig. 1). Picea jezoensis
Kubota and Hara—Sapling Competition between Picea and Abies
533
0.25
A 300
0.2 M(t,x) per 4 years
Number of saplings per 16 m2
200
100
0 –1
1
2
3
0.15
0.1
0.05
B 300 0 –1
1 2 Log10 (estimated sapling mass) (g)
200
F. 4. The M(t, x) functions describing the relationship between estimated sapling mass x in 1990 (¯ t) and mortality rate during the four growing seasons from 1990 to 1993 for saplings of Picea jezoensis (E) and Abies sachalinensis (D), on fallen logs. The M(t, x) function was significantly different between the two species (one repeated measures MANOVA, P ! 0±05).
100
1 2 Log10 (estimated sapling mass) (g)
3
F. 3. Frequency distributions of estimated sapling mass of Picea jezoensis (A), and Abies sachalinensis (B), in the total 16 m# quadrats on fallen logs. (*) Living and (+) dead saplings, respectively.
had a greater foliage mass than Abies sachalinensis for the same-sized individual in sapling mass (ANCOVA, P ! 0±0001). In particular, small-sized saplings of Abies sachalinensis showed larger variation in foliage mass than those of Picea jezoensis. Age and size structures The sapling age structures of Abies sachalinensis and Picea jezoensis were almost even-aged in each subquadrat (Fig. 2). Mean sapling age of each subquadrat ranged from 8 (³2 s.d.) to 26 (³5 s.d.) years. The sapling mass distributions of Abies sachalinensis and Picea jezoensis had positive skewness in all the subquadrats together (Fig. 3). Dead saplings of the two species were smaller than living ones (Kolmogorov-Smirnov test, P ! 0±0001). The mortality functions M(t, x) of the two species decreased with size. The values of M(t, x) of Picea jezoensis were larger than those of Abies sachalinensis (Fig. 4) (one repeated measures MANOVA, d.f. ¯ 1, F ¯ 5±87, P ! 0±05). Habitat segregation The species composition of sapling subquadrats was investigated based on the relationship between sapling biomasses per 1±0 m# of Picea jezoensis and Abies sachal-
10 000 Estimated total sapling biomass (g) of 2 A.sachalinensis in a 1-m subquadrat
0 –1
3
A
1000
100
B 10
100
1000
10 000
Estimated total sapling biomass (g) of P.jezoensis in a 1-m2 subquadrat F. 5. The relationship between the estimated sapling biomasses per 1±0 m# of Picea jezoensis (x) and Abies sachalinensis ( y) in each subquadrat. The solid line given as log y ¯®0±721 log x9±36 indicates habitat segregation between the two species at the sapling stage (PCA, P ! 0±001). The dashed arrows show that the sapling stand diverged to either Abies-dominated (arrow A) or Picea-dominated stands (arrow B) with sapling stand development.
inensis in each subquadrat (Fig. 5). Regression analysis was carried out by Principal Components Analysis (PCA). The sapling biomass of Picea jezoensis was negatively dependent on that of Abies sachalinensis (PCA, P ! 0±001). As the total biomass per 1 m# of saplings of the two conifers increased with sapling stand development (the arrows in Fig. 5), the sapling stand diverged to either Abies-dominated or Picea-
534
Kubota and Hara—Sapling Competition between Picea and Abies
T 2. The competitie effects of species i on the mass increment from 1990 to 1993 of indiidual saplings of mass x at time t (¯ 1990) of species k[Gk(t, x)]. The competitie effect is described as one-sided (c , i " 0, c , i ¯ 0), symmetric (c , i ¯ 0, " # " c , i " 0) or asymmetric (c , i " 0, c , i " 0). Statistical results are based on multiple linear regression analysis for eqn (4). " # # ****, significant at P ! 0±0001, ***, significant at P ! 0±001 ; **, significant at P ! 0±01 ; *, significant at P ! 0±05 ; ns, not significantly different from 0 (P & 0±05) P. jezoensis (i ¯ 1)
A. sachalinensis (i ¯ 2)
Gk(t, x)
n
a (s.e.)
c , (s.e.) ""
c , (s.e.) #"
c , (s.e.) "#
c , (s.e.) ##
P. jezoensis (k ¯ 1) A. sachalinensis (k ¯ 2)
663 801
0±708 (0±036)**** 0±661 (0±040)****
0±32 (0±033)**** n.s.
n.s. n.s.
0±11 (0±044)* 0±14 (0±028)****
n.s. n.s.
!
dominated stands (arrow A and arrow B in Fig. 5, respectively). There were no subquadrats with the two species co-dominant at a high biomass range. This indicates that saplings of Picea jezoensis and Abies sachalinensis were not uniformly distributed, but that the two species segregated establishment sites with sapling growth.
Growth dynamics and the mode of competition The results of multiple linear regression with forward stepwise method based on eqn (4) are shown in Table 2. Three transformations, ln (1y), oy and oyoy1, for dependent variable (sapling mass increment) y, were tried to have homoscedasticity (homogeneous variance of residuals). However, the statistical results were consistent with those of the untransformed case. Correlations between x and xCi(t, x), between x and xCi(t, xmin), and between xCi(t, x) and xCi(t, xmin) were all less than 0±4, and multicollinearity was not detected. The Durbin-Watson statistic and serial correlation of residuals were close to 2 (1±98, 1±86) and 0 (®0±01, ®0±07), respectively, indicating little autocorrelation between residuals. Normality of residuals were checked with normal plots, and outliers were also checked using residual-deleted residual plots and Cook’s distances. Outliers (Cook’s distances " 1) were not found in the regression analysis. Maximal values of Cook’s distance were 0±48 and 0±82 for Abies sachalinensis and Picea jezoensis, respectively. The size-dependent term, ‘ x ’, in eqn (4) was entered first in both the cases of Abies sachalinensis and Picea jezoensis (P ! 0±0001), representing a strongly size-dependent growth pattern. The mode of intraspecific and interspecific competition was different between the two species. Asymmetric competitive effects of Abies sachalinensis and Picea jezoensis saplings (c , , c , " 0) on the sapling growth of Picea "" "# jezoensis were detected. Asymmetric competitive effect of Abies sachalinensis saplings on the individual growth of Abies sachalinensis saplings was detected (Table 2). Abies sachalinensis suppressed one-sidedly the growth of Picea jezoensis but not vice versa (one-sided interspecific competition). These results indicate that an asymmetric competitive effect prevailed at the sapling stage of Abies sachalinensis and Picea jezoensis and that sapling competition between the two species was intense.
d.f.-adjusted R# for final model 0±57 0±61
DISCUSSION Allometry of saplings Tree geometry is an important ecological feature (Horn, 1971). Kohyama (1980) pointed out that Abies mariesii under suppressed conditions had a flat crown allowing for effective assimilation. Furthermore, Ku$ ppers (1989) classified the architectural pattern of woody plants into multi- and monolayer types. From the viewpoint of competition for light, Ku$ ppers (1989) suggested that costbenefit aspects of tree form reflects the successional status of species. The present results of the allometric relationships showed that Picea jezoensis and Abies sachalinensis saplings had conic and flat crowns, respectively (Table 1). Consequently, the Picea jezoensis sapling had more foliage mass and a greater mortality rate than the Abies sachalinensis sapling of the same sapling mass (Figs 1 and 4). These results suggest that at the sapling stage Picea jezoensis has a greater cost than Abies sachalinensis for assimilation-respiration balance under dark conditions of the closed canopy, whereas Abies sachalinensis maintains effective assimilation even under suppressed conditions (cf. Kohyama, 1980 ; Canham, 1988 ; Ku$ ppers, 1989). The difference in the crown form of saplings between the two species suggests different adaptations for efficient persistence in the sub-boreal forest (cf. King, 1990). Crown architecture is relevant to shade tolerance of tree species (Alvarez-Buylla and MartinezRamos, 1992). Kubota et al. (1994) pointed out that Abies sachalinensis can grow taller than Picea jezoensis under suppressed conditions, whereas Picea jezoensis requires canopy gaps for steady height growth. The present study also showed that widely spaced saplings of Picea jezoensis had a greater relative growth rate (a ) of sapling mass than ! widely spaced Abies sachalinensis (Table 2). This suggests that the regeneration pattern related to light environments is different between the two species having distinct crown shapes. Growth dynamics and the mode of competition Kubota and Hara (1995) showed that interspecific competition between trees (& 2 m in height) of the three dominant species, Picea jezoensis, Betula ermanii and Abies sachalinensis was either weak symmetric (P ¯ 0±05 for the
Kubota and Hara—Sapling Competition between Picea and Abies Maintenance of species diversity
Life stage
2 (P = 0.08) 2
Pj
Tree (>2 m in height)
As
2 (P < 0.001)
2 (P = 0.05)
(P < 0.001)
535
Stand development
Habitat segregation and stability of species composition
Sapling (<2 m in height) 1 (P < 0.0001)
Pj
As 1 (P = 0.01)
1 (P < 0.0001)
F. 6. The modes of intra- and interspecific competition (effect and direction) between saplings (below), and between trees (above), of Picea jezoensis (Pj) and Abies sachalinensis (As) of a sub-boreal forest, Hokkaido, northern Japan. The diagram of sapling competition is based on the results of Table 2. The diagram of tree competition is redrawn from Kubota and Hara (1995). The arrow indicates the direction of competitive effect : ‘ X U Y ’ represents that species X suppresses the individual growth of species Y (X ¯ Y, intraspecific competition ; X 1 Y, interspecific competition). ‘ 1 ’ and ‘ 2 ’ at the arrow represent one-sided and symmetric competitive effect, respectively (see Table 2). The P-level for the competition coefficient of the regression model is also given at the arrow ([[[", weak competition, 0±05 % P ! 0±1 ; U, intense competition, P ! 0±05).
competition coefficient of the regression model) or almost absent (P " 0±05), indicating that competitive interactions between trees cannot be a structuring force or relevant to the variation in species coexistence. On the contrary, asymmetric intense competition prevailed (Table 2, Fig. 6) in the saplings. The M(t, x) function for saplings of the two species was negatively size-dependent (Fig. 4), indicating intense competitive effects of larger saplings on smaller ones. The population of saplings on each fallen log is regarded as almost even-aged (Fig. 2) because the establishment of saplings on fallen logs is restricted to the early stage, 15–25 years in age, of fallen logs (Harmon, 1988). Furthermore, sapling density on fallen logs (1 045 625 ha−") is much higher than that of trees (1223 ha−" ; Kubota and Hara, 1995), suggesting that competitive interaction exaggerates size hierarchy of saplings. Horn (1971) pointed out that the geometry of tree crown affects light penetration into the understorey. Ellison and Rabinowitz (1989) showed that the difference in growth form between the two varieties of Pisum satium resulted in different size hierarchies. Weiner et al. (1990) investigated the effect of plant density on the vertical leaf arrangement of each Impatiens pallida individual in monocultures, and indicated that plant growth form and competitioin were correlated with each other. In the present results, the Picea jezoensis sapling with a conic and deep crown maintained a greater foliage mass than the Abies sachalinensis sapling
with a flat and shallow crown (Table 1, Fig. 1). The vertical foliage arrangement of Abies sachalinensis saplings is restricted to the upper layer. This point is also important for the mode of interspecific sapling competition between the two species. The competitive effect of Picea jezoensis saplings on Abies sachalinensis saplings was absent, whereas the competitive effect of Abies sachalinensis saplings on Picea jezoensis saplings was one-sided. The difference in the mode of interspecific sapling competition is ascribed to the difference in the crown shape of saplings between the two species, i.e. Picea jezoensis saplings are suppressed only by larger Abies sachalinensis saplings with a flat and shallow crown at the top of the stem (asymmetric interspecific competition). Hara (1993), Hara and Yokozawa (1994), and Hara and Wyszomirski (1994) pointed out that asymmetric competition is related to the stability of size-structure dynamics of plants, acting as a structuring force of the community, whereas symmetric competition cannot be a structuring force of the community and is irrelevant to the stability of size-structure dynamics. Asymmetric competition represents a deterministic interaction between individuals. A population undergoing asymmetric competition is mainly governed by the deterministic factor [the G(t, x) function], rather than by the stochastic factor [the D(t, x) function]. Therefore, the growth dynamics of saplings of Picea jezoensis and Abies sachalinensis, where asymmetric competition pre-
536
Kubota and Hara—Sapling Competition between Picea and Abies
vailed, were mainly governed by the deterministic factor [the G(t, x) function].
Species coexistence Large saplings of Picea jezoensis and Abies sachalinensis were aggregated in different sites on fallen logs, indicating habitat segregation with sapling stand development (Fig. 5). The growth dynamics of saplings of the two species were mainly governed by asymmetric competition. It is therefore suggested that deterministic interaction as asymmetric competition plays an important role for habitat segregation between Picea jezoensis and Abies sachalinensis saplings. A mixed sapling stand of the two species shifts with growth to a pure stand dominated by either Picea jezoensis or Abies sachalinensis saplings, suggesting that habitat segregation of the two species occurred at the sapling stage. In the mixed sapling stands of Picea jezoensis and Abies sachalinensis, Picea jezoensis is an inferior competitor under crowded competitive conditions but a superior competitor under widely spaced conditions (large a ), whilst Abies sachalinensis ! is a superior competitor under crowded competitive conditions but an inferior competitor under widely spaced conditions (small a ) (Table 2, Fig. 6). This tendency is ! related to the difference in crown shape between the two conifers. Therefore, in addition to the effects of asymmetric competition, the habitat segregation of sapling stands may have been promoted by the variation in seedling density of the two conifers due to a difference in the proportions of the amount of seeds between the two species at the establishment stage. Kubota and Hara (1995) investigated the growth dynamics and the mode of competition between trees (& 2 m in height) of the dominant species, Picea jezoensis, Picea glehnii, Betula ermanii and Abies sachalinensis and the subordinate species, Sorbus commixta and Acer ukurunduense of the sub-boreal forest, and indicated that the four dominant species showed either weak symmetric interspecific competition (P ¯ 0±05) between Abies sachalinensis and Picea jezoensis or almost no competition (P " 0±05 other cases). Therefore, Kubota and Hara (1995) suggested that the coexistence of the four dominant species is related to the boundary conditions for the growth dynamics of trees, such as seedling establishment and population dynamics of saplings. Especially in densely populated sites of saplings where the competitive effect is greatly exaggerated, it can be assumed that sapling competition between species determines a future coexistence pattern in the canopy layer. The present study showed that asymmetric competition prevailed in the population of saplings, and sapling competition between the species was relatively intense, in contrast to weak competition between canopy trees (Fig. 6). This implies that competitive exclusion occurred in the sapling stage. The regeneration niche has been regarded as an important factor for promoting species coexistence (Fox, 1977 ; Grubb, 1977 ; Knapp and Smith, 1982). Site-specific regeneration of constituent species in a plant community has been stressed (Forcier, 1975 ; Fox, 1977). Christy and Mack (1984)
investigated variations in the demography of conifers’ juveniles in relation to the substratum mosaic. Nakashizuka (1988) and Taylor and Qin (1992) showed that seedling establishment of Fagus crenata, Abies faxioniana and Betula utilis occurred after the simultaneous death of undergrowing Sasa dwarf bamboos. However, there have been few studies that investigated population dynamics of each component species in relation to interspecific competition processes in the early growing stages (seedlings and saplings) [but see Petersen (1988) and Petersen, Ning and Newton (1990) for monocultures]. The present study presents a process of habitat segregation based on competition between saplings of the two conifers. The mode of competition changed with the life-history stage from the sapling (intense and asymmetric) to the canopy tree (weak and symmetric or almost absent). In conclusion : (1) asymmetric and intense competition between saplings brought about habitat segregation between the dominant species, Picea jezoensis and Abies sachalinensis, in the early stage of life-history ; (2) therefore, the coexistence of Picea jezoensis and Abies sachalinensis of the sub-boreal forest was determined by the boundary conditions for the growth dynamics of trees, as segregation of establishment sites resulting from asymmetric and intense competition between saplings ; (3) then the species composition of the forest was maintained by weak symmetric competition or the state of almost no competition between trees. A C K N O W L E D G E M E N TS We thank Shin-ichi Niwa for help with field work. We are also grateful to a reviewer, Dr David King, for valuable comments and suggestions. This study was partly supported by a grant from the Ministry of Education, Science and Culture, Japan. LITERATURE CITED Alvarez-Buylla ER, Martinez-Ramos M. 1992. Demography and allometry of Cecropia obtusifolia, a neotropical pioneer tree—an evaluation of the climax–pioneer paradigm for tropical rain forests. Journal of Ecology 80 : 275–290. Canham CD. 1988. Growth and canopy architecture of shade-tolerant trees : response to canopy gaps. Ecology 69 : 786–795. Cannell MGR, Rothery P, Ford ED. 1984. Competition within stands of Picea sitchensis and Pinus contorta. Annals of Botany 53 : 349–362. Christy EJ, Mack RN. 1984. Variation in demography of juvenile Tsuga heterophylla across the substratum mosaic. Journal of Ecology 72 : 75–91. Ellison AM, Rabinowitz D. 1989. Effects of plant morphology and emergence time on size hierarchy formation in experimental populations of two varieties of cultivated peas (Pisum satium). American Journal of Botany 76 : 427–436. Forcier LK. 1975. Reproductive strategies and the co-occurrence of climax tree species. Science 189 : 808–810. Ford ED. 1975. Competition and stand structure in some even-aged plant monocultures. Journal of Ecology 63 : 311–333. Ford ED, Diggle PJ. 1981. Competition for light in a plant monoculture modelled as a spatial stochastic process. Annals of Botany 48 : 481–500. Fox JF. 1977. Alternation and coexistence of tree species. American Naturalist 111 : 69–89. Geber M. 1989. Interplay of morphology and development on size inequality : a Polygonum greenhouse study. Ecological Monographs 59 : 267–288.
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