Recruitment Processes and Species Coexistence in a Sub-boreal Forest in Northern Japan

Recruitment Processes and Species Coexistence in a Sub-boreal Forest in Northern Japan

Annals of Botany 78 : 741–748, 1996 Recruitment Processes and Species Coexistence in a Sub-boreal Forest in Northern Japan Y A S U H I R O K U B O TA...

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Annals of Botany 78 : 741–748, 1996

Recruitment Processes and Species Coexistence in a Sub-boreal Forest in Northern Japan Y A S U H I R O K U B O TA* and T O S H I H I K O H A R A The Institute of Low Temperature Science, Hokkaido UniŠersity, Sapporo 060, Japan Received : 5 February 1996

Accepted : 28 June 1996

We investigated the recruitment of saplings (across the 2 m-height threshold) of six species, Picea jezoensis, Abies sachalinensis, Betula ermanii, Picea glehnii, Acer ukurunduense and Sorbus commixta, in a sub-boreal forest, northern Japan. Data were collected in a 2±48-ha plot over six growing seasons (1989–1994). We used path analysis to analyse the relationships between the recruitment rates of saplings and the stand structural attributes such as mother tree abundance, stand crowdedness, stand stratification, Sasa bamboo density on the forest floor, and fallen log abundance. The combination of stand structural attributes affecting recruitment rates of the six sub-boreal forest tree species differed markedly among the species and corresponded to species composition. It is suggested that the sizestructure dynamics of adult trees of the sub-boreal forest are regulated largely by different regeneration processes among the species and only slightly by interspecific competition between adult trees because interspecific competition between adult trees was not evident. The dynamics of species coexistence of the sub-boreal forest should be described as a process combining the diversity of recruitment processes of saplings of the component species and the diversity of interspecific competition between adult trees. We propose the boundary condition hypothesis for species coexistence in the sub-boreal forest, that the persistence of each component species is ascribed largely to the different recruitment processes of saplings (boundary conditions for adult tree growth dynamics) and only a little to interspecific adult tree competition. # 1996 Annals of Botany Company Key words : Climax forest, safe site, regeneration niche, mode of competition, species diversity.

INTRODUCTION The regeneration niche was proposed by Grubb (1977) as a key factor for species coexistence. The concept of regeneration niche is related to the idea of ‘ safe site ’ for individual plants in terms of spatial variation. In herbaceous species, Harper, Williams and Sagar (1965), Hartgerink and Bazzaz (1984) and Fowler (1988) showed that spatial heterogeneity such as soil surface, topography and litter, had a great influence on the emergence and establishment of seedlings. In tree species, Christy and Mack (1984), Collins and Good (1987), Taylor and Qin (1988 a, b), Collins (1989) and Nakashizuka (1989) showed that the establishment and demography of seedlings were affected by habitat gradients such as ground surface and litter. These studies suggested differentiation of regeneration niches among species in the early stage of life history. However, it has also been pointed out that the existence of regeneration niches is obscure (Hubbell and Foster, 1986 ; Wilson, Gitay and Agnew, 1987 ; Mahdi, Law and Willis, 1989 ; Welden et al., 1991). These studies did not detect specializations of species for specific sites of regeneration. It is therefore still controversial whether regeneration niches exist. Even in the studies that showed regeneration niches, few have demonstrated quantitatiŠely the effects of the differentiation of regeneration niches on the coexistence between species. In the present * Present address : Center for Ecological Research, Kyoto University, Kyoto 606-01, Japan.

0305-7364}96}120741­05 $25.00}0

paper, we investigate the pattern and process of species coexistence of a sub-boreal forest in relation to recruitment processes of saplings (! 2 m in height) and interspecific adult tree competition (& 2 m in height). Forest communities have enormous biomass and pronounced vertical stratification, particularly in climax forests consisting of different cohorts. Such stand structure is clear for forests, while it is not so obvious in other plant communities (Hara, 1994). The attributes such as size structure and spatial distribution are patterned by a collection of individuals, having an autonomic regulation system acting on its dynamics (e.g. Horn, 1971 ; Terborgh, 1985 ; Kohyama, 1994). To study the stability and diversity of forest communities, therefore, it is necessary to investigate the effects of stand structural attributes on tree population and community dynamics. Kubota and Hara (1995) assumed that the stand structure and species composition (Picea jezoensis, Abies sachalinensis, Betula ermanii, Picea glehnii, Acer ukurunduense and Sorbus commixta) of a sub-boreal forest in Hokkaido, northern Japan, were related to the mode of competition between individual trees. They showed, using diffusion models at the level of the individual tree & 2 m in height, that competition was almost entirely symmetric and that interspecific competition was almost absent between trees & 2 m high except between species belonging to upper- and lowercanopy layers. Finally, they concluded that the growth dynamics of each component species of the sub-boreal forest were mostly governed by the stochastic factors [D(t, # 1996 Annals of Botany Company

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Kubota and Hara—Recruitment and Species Coexistence in Sub-boreal Forest

x) function] and the boundary conditions [R(t)], rather than by the deterministic competitive interaction between component species [G(t, x) function]. In the present study, we focus on the boundary condition [R(t)] in the diffusion model studied by Kubota and Hara (1995, 1996), and recruitment of saplings (! 2 m in height) entering the vertical layer & 2 m over six growing seasons in the 2±48-ha stand. Using path analysis, we investigate the relationships between the recruitment rates of saplings and stand structural attributes such as mother tree abundance, stand crowdedness, stand stratification, Sasa bamboo density on the forest floor, and fallen log abundance. We propose the ‘ boundary condition hypothesis ’ for the species coexistence of sub-boreal forest, that the persistence of each component species and the species diversity of the subboreal forest are governed largely by the recruitment processes of saplings (! 2 m in height) (reported in the present study) and, to a much lesser extent, by interspecific competition between adult trees (" 2 m in height) [reported in detail by Kubota and Hara (1995)]. The boundary condition as a mathematical term is similar to Grubb’s regeneration niche. The regeneration niche can be divided into spatial and temporal aspects, and a difference in viewpoints among researchers brings about a large variation in the concept of the regeneration niche. The difference due to intuition of each researcher hampers testable arguments on regeneration dynamics of trees. Meanwhile, the boundary condition hypothesis we propose is a more explicit idea to argue on regeneration dynamics because this idea is attributable to the diffusion model describing plant size-structure dynamics proposed by Hara (1984 a, b). The boundary condition in the diffusion model represents a temporal aspect of regeneration. In the present study, furthermore, the spatial aspect such as stand structural attributes is incorporated into the boundary condition. In the present study, we think that the boundary condition hypothesis, based on both temporal and spatial aspects of regeneration, promotes more testable and quantitative work in the sub-boreal climax forest than the regeneration niche hypothesis does.

STUDY SITE AND MEASUREMENTS The study was conducted in Taisetsuzan National Park, Hokkaido, Japan (43°33« N, 143°11« E ; approx. 1000 m above sea level). Precipitation is approximately 1500 mm year−". Mean daily temperatures in the warmest month (August) and the coldest month (January) are 17±7 °C and ®10±7 °C, respectively. This region is located between the cool-temperate and sub-boreal zones. Snow covers the forest floor from Nov. to May. The soil is predominantly brown forest soil, with the local distribution of block streams by periglacial formation. The study site is dominated by Picea jezoensis Carr., Abies sachalinensis Masters, Betula ermanii Cham. and Picea glehnii Masters. Acer ukurunduense Trautv. et Mey. and Sorbus commixta Hedl. also occur where the canopy is sparse. The total basal area of all trees [sum of π(dbh)#}4, where dbh is the stem diameter at breast height, 1±3 m] in the study site was approximately 34 m# ha−" (tree density,

1134 ha−") in 1989 (Kubota and Hara, 1995). The forest was generally multi-layered. The canopy was classified into four layers [layer I, the uppermost layer (approx. " 20 m in height) ; layer II, the second highest layer (approx. 10–20 m) ; layer III, the third highest layer (approx. 5–10 m) ; layer IV, the lowermost layer (approx. 2–5 m)]. Picea jezoensis, Picea glehnii and Betula ermanii reached the uppermost layer of the canopy layer I) ; Abies sachalinensis, and Sorbus commixta reached up to the second (layer II) and the third highest layer (layer III), respectively, and Acer ukurunduense occupied the lowermost layer (layer IV) (Kubota and Hara, 1995). The amount of area in canopy gaps ranged from 4±2 to 30±4 % (Kubota, 1995). The forest under study consisted of stands of different maximal tree age ranging from 125 to 333 years (Kubota, 1995). The history of major disturbances differs among the stands (Kubota, 1995). Stand structure is affected by either small-scale tree falls or large-scale disturbances by typhoons (particularly in 1954, ‘ Toyamaru ’ typhoon) (Kubota, 1995). Two types of forest floor vegetation were recognized ; Sasa and Carex on brown soil and Vaccinium, Rhododendron and Menziesia on the block stream. In the stands with lower densities of canopy trees, Sasa senaensis Franch. et Sav. and Sasa kurilensis Rupr. Makino, covered the forest floor. Consequently, the establishment site of saplings was restricted to fallen logs or stumps (Kubota, Konno and Hinra, 1994). The growth dynamics of saplings on fallen logs were studied by Kubota and Hara (1996). In order to investigate stand structural attributes (species composition, size structure etc.) of the sub-boreal forest at the landscape level in Taisetsuzan National Park, five study plots, 0±12, 0±16, 0±20 and 1±8 ha (2±48 ha in total), were located at five representative sites with a gentle slope at approx. 1000 m above sea level. The five plots were set up in 1989. The whole area was covered with a 10¬10 m grid system for field survey. All living and dead trees & 2 m in height were tagged and identified by species within each grid cell (n ¯ 2666 and 634, respectively). The dbhs of all tagged trees were measured in Jun. 1989. The decomposition of logs on the forest floor was classified into six categories, which varied from undecayed logs (class 1) with most of the bark and branches intact to nearly perfectly decayed ones (class 6), according to structural integrity and vegetation coverage on the logs (Kubota et al., 1994). The dbhs of these tagged living trees were remeasured in Oct. 1994, together with new recruits of live saplings (! 2 m in height) which had entered the vertical layer & 2 m height. The mortality rate of part of new recruits was assessed during 3 years. Culm density of Sasa bamboos on the forest floor was measured in a 2¬2 m quadrat at a grid intersection.

DATA ANALYSIS The stands consisted of two guilds—conifer (Picea jezoensis, Picea glehnii and Abies sachalinensis) and hardwood (Betula ermanii, Sorbus commixta and Acer ukurunduense). Thus, species diversity was assessed at the species level and at the guild level. Community diversity was described by the

Kubota and Hara—Recruitment and Species Coexistence in Sub-boreal Forest

relative frequency of individual trees of the i-th species (or guild) in each 10¬10 m grid cell. The number of new recruits of saplings (! 2 m in height) that had entered the vertical layer& 2 m during the six growing seasons from 1989 to 1994 within each grid cell was counted for each component species. The stand structure of each 10¬10 m grid cell was represented by the following structural attributes : (a) mother tree abundance, expressed by the total basal area of trees & 2 m in height of each species i(i ¯ 1, 2, …, 6),

Number of 10×10 m stands

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N

10 0

0

1.0 Shannon–Wiener index

2.0

F. 1. Frequency distribution of species diversity of 10¬10 m grid cells in a sub-boreal forest, northern Japan. Species diversity was assessed by the Shannon–Wiener index. 150

A

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0 150

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E

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5000 10 000 0 5000 Total basal area of mother trees in a 10×10 m stand (cm2)

10 000

F. 2. Frequency distributions of mother tree abundance. The mother tree abundance was expressed by the total basal areas of trees & 2 m in height of each species. A, Abies sachalinensis ; B, Picea glehnii ; C, Acer ukurunduense ; D, Picea jezoensis ; E, Betula ermanii ; F, Sorbus commixta.

Shannon–Wiener information index, H«, using lntransformed data of the six component species and the two guilds : s

(1) H« ¯ ® 3 Pi loge Pi, i=" where s is the number of tree species (or guilds) and Pi the

(2) Fi(t) ¯ 3 π(dbhk,i)#}4, k=" where dbhk,i is dbh of k-th tree " 2 m high of species i at time t (¯ 1989), and N is the total number of trees of species i in a 10¬10 m grid cell, respectively ; (b) stand crowdedness at time t, C(t), expressed by the total basal area of trees & 2 m of all the six species, ' (3) C(t) ¯ 3 Fi(t), i=" which represents the stand development stage (Kubota and Hara, 1995) ; (c) stand stratification expressed by the CV (coefficient of variation) of the basal area size distribution of trees & 2 m in height in each 10¬10 m grid cell, which increases with stand developmental stage of successional status ; (d ) Sasa density on the forest floor, representing the shading effects of the foliage of Sasa on saplings in a 2¬2 m quadrat set up at each grid intersection (Kubota et al., 1994) ; (e) fallen log abundance expressed by the total basal area of fallen logs in each 10¬10 m grid cell, representing the safe site for conifer saplings in the sub-boreal forest (Kubota et al., 1994 ; Kubota and Hara, 1996). Correlation between these stand structural attributes was tested, and normality of frequency distributions of the variables of the stand structural attributes was assessed using the Kolmogorov–Smirnov goodness-of-fit test. In order to avoid over-estimating significances in iterated correlation tests between stand structural attributes, we carried out analysis of covariance between the variables. To investigate the causes for species diversity and regeneration patterns, path analysis was used. The causeand-effect relationships (n ¯ 248) were depicted by standard partial regression coefficients, i.e. path coefficients. Strength of the cause and effect relationship was estimated by the path coefficients (Sokal and Rohlf, 1995). We analysed the relationships between the species (or guild) diversity of the stands and the four stand structural attributes except for mother tree abundance, and investigated the relationships between the numbers of new recruits of saplings entering the vertical layer & 2 m in height on the 2±48-ha scale over the six growing seasons (recruitment rates) and the five stand structural attributes. Note that mother tree abundance was not used for path analysis between the species (or guild) diversity of the stands and the stand structural attributes because the species (or guild) diversity (in terms of density) was highly related to mother tree abundances (in terms of basal area) of the component species. The five stand structural attributes as the independent variables are the

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Kubota and Hara—Recruitment and Species Coexistence in Sub-boreal Forest

immediate causes of both the species diversity of the stands and the numbers of new recruits of saplings. The amount of unexplained influences on each dependent variable was represented by U(¯ 1®r#). Path diagrams were constructed from path coefficients for the species (or guild) diversity of the stands, the numbers of new recruits of saplings and the stand structural attributes.

causal components) to the criterion variables (species diversity, guild diversity, and recruitment rate of each component species). Multicolinearity was not detected. If a variable was not normally distributed, based on the Kolmogorov–Smirnov test, it was log-transformed to achieve normality. Community diŠersity and stand structure

RESULTS Correlations and covariates between the five stand structural attributes (as predictor variables) were close to 0 (approximately ®0±18–0±02), implying little indirect effect (causal components) or little spurious contribution (non-

Number of 10×10 m stands

50

The Shannon–Wiener information index of the stands had normal distributions (Kolmogorov–Smirnov test, Fig. 1). Mother tree abundance of each species in the stands had log-normal distributions, and differed between species (Fig. 2). Stand crowdedness and stand stratification (expressed by 80

A

B

40 60 30 40 20 20 10

0

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10 000

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Total basal area of living trees in a 10×10 m stand (cm )

2.0

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CV of size distribution of basal area in a 10×10 m stand

C

D 60

Number of 10×10 m stands

100

80 40 60

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Density of Sasa bamboos in a 2×2 m quadrat within a 10×10 m stand

0

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2 Total basal area of dead trees in a 10×10 m stand (cm )

F. 3. Frequency distributions of the five stand structural attributes. Stand structure was represented by the following structural attributes for each 10¬10 m grid cell : A, stand crowdedness expressed by the total basal area [C(t), eqn (2)] of individual trees & 2 m in height at time t (¯ 1989) in a 10¬10 m grid cell ; B, stand stratification expressed by the CV of basal area size distribution of trees in a 10¬10 m grid cell ; C, Sasa bamboo density in the forest floor in a 2¬2 m quadrat set up at each 10¬10 m grid intersection ; and D, fallen log abundance expressed as the total basal area of fallen logs in a 10¬10 m grid cell.

745

Kubota and Hara—Recruitment and Species Coexistence in Sub-boreal Forest Guild diversity

Picea glehnii

Species diversity 0.25***

–0.14*

–0.15*

–0.17**

Abies sachalinensis

–0.31***

Stand crowdedness Stand stratification

–0.26***

Fallen log abundance

Betula ermanii

Sasa density F. 4. Path diagram of the effects of the four stand structural attributes (Fig. 3) on community diversity of the 10¬10 m grid cell (Fig. 1). Note that mother tree abundance was not used for path analysis between the species (or guild) diversity and the stand structural attributes because the species (or guild) diversity (in terms of density) is highly related to mother tree abundances (in terms of basal area) of the component species. Community diversity at both the species level and the guild level, conifer (Picea jezoensis, Picea glehnii and Abies sachalinensis) and hardwood (Betula ermanii, Sorbus commixta and Acer ukurunduense) guilds was assessed by the Shannon–Wiener information index, H« (Fig. 1). The four stand structural attributes (Fig. 3) as the independent variables were the immediate causes of the species diversity. Cause-andeffect relationships were depicted by the standard partial regression coefficients, i.e. path coefficients. Strength of the relationship between cause and effect was estimated by the path coefficients. Significant path coefficients are indicated by asterisks : ***, significant at 0±0001 % P ! 0±01 ; *, significant at 0±01 % P ! 0±05.

the CV of basal area size distributions) showed normal distributions. Fallen log abundance also showed a lognormal distribution. Sasa density on the forest floor was not uniformly distributed, and was extremely different among the stands (Fig. 3). These results indicate that the sub-boreal forest under study consisted of various patches showing different species diversity and different structural attributes within the 2±48-ha area. Direct effects of the four structural attributes on species and guild diversities of the patches are shown in the path diagram of Fig. 4. Species diversity increased with stand stratification and decreased with an increase in fallen logs (P ! 0±001, U ¯ 0±98, Fig. 4). The direct effect of fallen log abundance was greater than that of stand stratification. Guild diversity of the stands also decreased with an increase in fallen log abundance (P ! 0±05, U ¯ 0±98, Fig. 4). Recruitment processes The numbers of new recruits (n ¯ 427 per 2±48 ha) of the six component species during the six growing seasons (recruitment rates) were related to different stand structural attributes (Fig. 5). The mortality of new recruits during three growing seasons was zero. The recruitment rate of Picea jezoensis increased with fallen log abundance (n ¯ 20, P ! 0±01, U ¯ 0±96). That of Abies sachalinensis decreased with an increase in both stand crowdedness and Sasa density (n ¯ 137, P ! 0±05, U ¯ 0±93). Recruitment rates of Betula ermanii and Sorbus commixta increased with development of stand stratification (n ¯ 25 and 9, P ! 0±05, U ¯ 0±95 and 0±97, respectively). The recruitment rate of Acer

–0.15*

Acer ukurunduense

Sorbus commixta

0.31*** 0.20** Stand crowdedness

0.15*

Picea jezoensis 0.17**

Stand stratification Fallen log abundance

Sasa density Mother tree abundance F. 5. Path diagram of the effects of five stand structural attributes on the numbers of new recruits of saplings ! 2 m in height which entered the layer & 2 m during six growing seasons (recruitment rates) for the six component tree species of the sub-boreal forest on the 2±48-ha scale. The shaded ellipse represents the hardwood guild (Betula ermanii, Sorbus commixta and Acer ukurunduense). The five stand structural attributes (independent variables) were the immediate causes of the recruitment rates of the six component tree species. Arrows indicate the direction of cause-and-effect relationship. Significant path coefficients are indicated by asterisks : ***, significant at 0±0001 % P ! 0±001 ; **, 0±001 % P ! 0±01 ; *, 0±01 % P ! 0±05.

ukurunduense increased with development of stand stratification (n ¯ 229, P ! 0±001), and decreased with an increase in stand crowdedness (P ! 0±001) and Sasa bamboo density on the forest floor (P ! 0±05, U ¯ 0±83). Stand stratification had the greatest total direct effect on the recruitment processes of the component species. The recruitment rate of Picea glehnii (n ¯ 7) was independent of the five stand structural attributes. Mother tree abundance had no effects on recruitment rates of the six species. DISCUSSION Recruitment process of sub-boreal forest The present study demonstrated that the recruitment processes of the six tree species of the sub-boreal forest were governed by different combinations of the five stand structural attributes. The recruitment of Picea jezoensis was governed by the abundance of fallen logs, which confirms the fact that the establishment site of saplings of Picea jezoensis was restricted to fallen logs (Kubota et al., 1994 ; Kubota and Hara, 1996). The recruitment process of Abies

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Kubota and Hara—Recruitment and Species Coexistence in Sub-boreal Forest

sachalinensis was governed by the shading effects of both canopy trees and Sasa on the forest floor, indicating that Abies sachalinensis has a lower shade tolerance than Picea jezoensis (Kubota et al., 1994). It is also suggested that Abies sachalinensis regenerates in early-successional stands after disturbances, whereas Picea jezoensis can regenerate irrespective of the stand developmental stage if there are many fallen logs. The recruitment process of Picea glehnii was independent of the five stand structural attributes, probably due to a scarcity of new recruits. Although it is suggested that the occurrence of Picea glehnii was determined by other site factors, such as local distribution of block streams and serpentine soil conditions (Tatewaki and Igarashi, 1971 ; Suzuki et al., 1987 ; Takahashi, 1994), the main cause of regeneration is still unknown. Stand stratification had the greatest positive effect on the recruitment of the three hardwood species, Acer ukurunduense, Betula ermanii and Sorbus commixta, whereas it had no effects on the recruitment processes of the three coniferous species, Picea jezoensis, Abies sachalinensis and Picea glehnii. The three hardwood species regenerated mostly in welldeveloped patches, suggesting that the recruitment processes of the three hardwood species were favoured by environmental factors (e.g. nutrient or water conditions in the soil) stabilized long after disturbances. We should investigate the effects of environmental fluctuation caused by disturbances on the regeneration process of the hardwood species, to examine whether they establish well after disturbances or not. Furthermore, it seems that the habitat segregation among the species is also affected by seed rains related to spatial distribution of mother trees and dispersal of reproductive propagules (Shibata and Nakashizuka, 1995). Our results show that the species composition of the subboreal forest is correlated with at least five structural attributes through the recruitment processes. The present study statistically showed the differentiation of recruitment processes among species of the sub-boreal forest in terms of structural attributes. However, structural attributes fluctuate in space and time. Kubota (1995) showed, based on the disturbance history of the past 300 years in the same forest stands as in the present study, that size structure and regeneration process have changed with the natural disturbance regime. Spies, Franklin and Thomas (1988) and Arthur and Fahey (1989) pointed out that fallen log abundance tended to be high in either very young (60–80 years old) or very old (" 200 year) stands according to stand development. Furthermore, it has been known that understorey Sasa, preventing the regeneration of tree species, die simultaneously over a wide area (Nakashizuka, 1988 ; Makita, 1992 ; Taylor and Qin, 1992 ; Makita et al., 1993). These studies suggest that the stand structural attribute is an all-inclusive parameter at some scale of space and time and that the regeneration processes of sub-boreal tree species change with fluctuation in the stand structural attributes. Therefore, the coexistence pattern of species involves stochastic processes. In our present study, the minimum U value was 0±83 (Acer ukurunduense ; Fig. 5), indicating that the deterministic factor in the recruitment process was at most 17 %, and that the remaining 83 % was due to either stochastic factors or other site variables (e.g.

soil conditions). This suggests that the deterministic effects on the recruitment process were relatively small even in our case where these effects were statistically significant (at least P ! 0±05). The relative degree of deterministic factors in the recruitment processes may vary depending on the type of plant communities and environmental conditions. This may be part of the reason why some researchers found regeneration niches in some plant communities, while others did not (see Introduction). It is meaningless to question whether the regeneration niche exists or not. We should instead ask about the relative degree of deterministic factors (or stochastic factors) in regeneration processes, and investigate the effects of exogenous factors such as disturbance on species diversity. Wilson, Sykes and Peet (1995) suggest that fluctuation of safe sites in the time course can be a niche for most of the species comprising a grass community, which is essentially the same as the lottery model. The present results suggest that regeneration of the sub-boreal forest is more deterministic in the spatial scale than in the time scale, although the spatial process also contains stochastic factors (see large U values). Therefore, the dynamics of the sub-boreal forest in the course of time can be understood by the lottery model, where natural disturbance is an important stochastic factor. Species coexistence in a sub-boreal forest The boundary condition hypothesis. The competitive effects of density and neighbours’ size on individual performances have been investigated extensively in monospecific stands (Ford, 1975 ; Mack and Harper, 1977 ; Mithen, Harper and Weiner, 1984 ; Weiner, 1984 ; Pacala and Silander, 1985 ; Firbank and Watkinson, 1985, 1987 ; Smith and Goodman, 1986 ; Hara, 1988 ; Weiner, 1990). The effect of competition on species coexistence has also been regarded as an ecologically important theme. Kohyama (1991, 1992 a, b) concluded from continuity equation modelling, that the dynamics of major tree species, Eurya japonica, Illicium anisatum and Distylium racemosum, of a warmtemperate rain forest in Japan were governed by one-sided competition for light. Kohyama (1993) proposed in the forest architecture hypothesis that the functional relationship between gap dynamics and one-sided competition gives rise to stable species coexistence. Hara (1994) showed, with a diffusion model, that asymmetric (or one-sided) competition contributes to the stability of plant communities. Only a few studies, however, have looked at the relationship between interspecific adult plant competition and recruitment of each species for species coexistence ; no studies have demonstrated concrete quantitative relationships based on actual data of multi-species plant communities. Kohyama (1991) and Hara (1992) showed that size-structure dynamics under either symmetric competition or no competition are greatly affected by the recruitment rate (i.e. the boundary condition for adult plant growth dynamics), whereas those under one-sided or strongly asymmetric competition are independent of the recruitment rate. Therefore, except under asymmetric competition, diversity in the recruitment process leads directly to diversity in the size-structure dynamics.

Kubota and Hara—Recruitment and Species Coexistence in Sub-boreal Forest Stand structural attributes

Recruitment diversity (Boundary conditions)

Interspecific competition diversity

Species diversity F. 6. The maintenance mechanism of species coexistence in the subboreal forest. Competition diversity : interspecific competition was scarce at the level of the individual adult tree & 2 m in height on the 2±48ha scale (Kubota and Hara, 1995), suggesting low competition diversity. Recruitment diversity : recruitment processes were determined by species-specific combinations of stand structural attributes, suggesting high recruitment diversity. The relationship between species diversity and stand structural attributes is shown in Fig. 4. The dynamics of species coexistence in the sub-boreal forest are described as a process combining the diversity of recruitment processes of saplings ! 2 m in height of the component species and the diversity of interspecific competition between adult trees & 2 m in height. The boundary condition hypothesis (recruitment rate is regarded as a boundary condition for adult tree growth dynamics) states that the former corresponds to the species composition of the sub-boreal forest much more than the latter. Solid and dashed arrows indicate relatively large and small effects, respectively.

Kubota and Hara (1995) investigated the effect of the mode of competition on the size-structure dynamics of individual adult trees & 2 m in height in the same subboreal forest studied here, with diffusion models. They demonstrated that interspecific competitive effects on the individual growth of adult trees & 2 m in height of Picea glehnii, Picea jezoensis, Betula ermanii and Abies sachalinensis, which reached up to the upper canopy layer of the stands (layers I and II), were weak or absent except for symmetric interspecific competition between Betula ermanii and Abies sachalinensis. The present study showed that the recruitment processes of Abies sachalinensis, Picea jezoensis and Picea glehnii were governed by different combinations of five stand structural attributes. Therefore, the result that interspecific competition was scarce between these four species at the level of the adult individual tree & 2 m in height is ascribed to habitat segregation due to different recruitment processes among the species, but is not an outcome of competitive exclusion between adult trees. The individual growth of the two lower-canopy species (Sorbus commixta and Acer ukurunduense) was regulated asymmetrically by the four upper-canopy species (Abies sachalinensis, Picea jezoensis, Picea glehnii and Betula ermanii), suggesting that interspecific competition between adult trees plays an important role for species coexistence only between species belonging to totally different vertical layers (Kubota and Hara, 1995). Based on Kubota and Hara (1995, 1996), we think that the degree and mode of intra- and interspecific competition differ among species and change with life history stage, bringing about the diversity of competition in the subboreal forest. Therefore, the dynamics of species coexistence

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in the sub-boreal forest should be described as the diversity of interspecific competition between adult trees & 2 m in height and a process combining the diversity of recruitment processes of saplings ! 2 m high among the component species (Fig. 6). The present study shows that the combination of stand structural attributes affecting the recruitment processes of each sub-boreal component species was markedly different among the species and corresponded to the species composition. The result of Kubota and Hara (1995) suggests that interspecific competition among adult trees & 2 m in height was almost irrelevant to species composition of the sub-boreal forest except between upperand lower-canopy species. These two results indicate that the size-structure dynamics of adult trees of the sub-boreal forest were regulated largely by different regeneration processes among the species under little interspecific competition between adult trees. Therefore, we propose the boundary condition hypothesis for species coexistence in the sub-boreal forest, that the persistence of each component species is ascribed more to the differentiated recruitment processes of saplings ! 2 m in height (boundary conditions for adult tree growth dynamics) than to interspecific competition between adult trees & 2 m in height. A C K N O W L E D G E M E N TS We thank Nobuyuki Watanabe, Osamu Watanabe and Shin-Ichi Niwa for help with field work. We also thank Dr Makoto Kimura and Dr EA Johnson for helpful comments. We thank two anonymous reviewers for valuable comments on the manuscript. This study was partly supported by a grant from the Ministry of Education, Science and Culture, Japan. LITERATURE CITED Arthur MA, Fahey TJ. 1989. Mass and nutrient content of decaying boles in an Engelmenn spruce-subalpine fir forest, Rockey Mountain National Park, Colorado. Canadian Journal of Forest Research 20 : 730–737. Christy EJ, Mack RN. 1984. Variation in demography of juvenile Tsuga heterophylla across the substratum mosaic. Journal of Ecology 72 : 75–91. Collins SL. 1989. Habitat relationships and survivorship of tree seedlings in hemlock-hardwood forest. Canadian Journal of Botany 68 : 790–797. Collins SL, Good RE. 1987. The seedling regeneration niche : habitat structure of tree seedlings in an oak-pine forest. Oikos 48 : 89–98. Firbank LG, Watkinson AR. 1985. A model of interference within plant monocultures. Journal of Theoretical Biology 116 : 291–311. Firbank LG, Watkinson AR. 1987. On the analysis of competition at the level of the individual plant. Oecologia (Berlin) 71 : 308–317. Ford ED. 1975. Competition and stand structure in some even-aged plant monocultures. Journal of Ecology 63 : 311–333. Fowler NL. 1988. What is a safe site ? Neighbour, litter, germination date, and patch effects. Ecology 69 : 947–961. Grubb PJ. 1977. The maintenance of species-richness in plant communities : The importance of the regeneration niche. Biological ReŠiew 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 EŠolution 3 : 129–133.

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