Download Recruitment Processes and Species Coexistence

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
60
50
40
30
20
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
B
100
50
Number of 10×10 m stands
743
0
150
C
D
100
50
0
150
E
F
100
50
0
0
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
5000
10 000
15 000
0
2
Total basal area of living trees in a 10×10 m stand (cm )
2.0
0
3.0
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
40
20
20
0
0
100
200
300
Density of Sasa bamboos in a 2×2 m quadrat within a
10×10 m stand
0
0
10 000
20 000
30 000
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
746
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
747
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.
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