12. Functional Differentiation and Positive Feedback
187
cruitment rate per species basal area) both in a warm-temperate rain forest in southern
Japan (Fig. 6) and in a mixed dipterocarp forest in west Kalimantan (T. Kohyama
et al. in preparation, the pattern is very similar to Fig. 6). Thus the functional
differentiation among species along a forest profile is likely to play an important
role in the coexistence of species in these forests. The question then arises why
there exists such a large difference in tree species diversity between these two forest
types. It is worth paying attention to the fact that the above analysis was only done
in terms of trunk diameter as a proxy for individual size.
Light-resource competition in a one-sided manner is a result of vertical crown
stratification, so tree height is a better size dimension to use than trunk diameter.
The relationship between diameter and height was quite different between these
two forests (Fig. 7), even though the distribution in trunk diameter was similar.
Trees of the warm-temperate forest approached their asymptotic height at a smaller
diameter, above which differentiation among species in maximum size is fairly
difficult. By contrast in the mixed dipterocarp forest, differentiation in terms of
maximum height is still possible in larger size classes. Therefore, not only the
biomass and the turnover rate of biomass, but also a measure of vertical differentiation among tree crowns such as the asymptotic height H* of D-H allometry (Fig. 2)
increase the possibility of coexistence. These attributes are co-related with each
other (Fig. 3). Therefore, it is not necessary to assume any single factor amplifying
species packing in wet tropical lowland, but these ecosystem attributes are likely to
multiplicatively widen the resource axis for the coexistence of species (Fig. 8).
5 Positive Feedback Between
Ecosystem Efficiency and Biodiversity
The results of the model competition between species (Fig. 4) suggest not only a
condition of coexistence but also of exclusive replacement by superior species in
terms of resource use efficiency. For instance, for the parameter space defined by
reproductive capacity and asymptotic size (where other parameters arc identical
between species), species with larger asymptotic size in growth performance always wins over smaller species when the reproductive capacity is kept identical
between the two species. When keeping the asymptotic size identical, species with
higher reproductive capacity always wins. If there is only one species with superior
performance beyond the tradeoff belt shared by many species, this species eventually excludes all other species. Therefore, succession or evolution is a series of
replacement by superior species, which does not necessarily enhance species diversity.
However, when we examine the range of coexistence defined by the new, superior species in the monodominant ecosystem, it becomes clear that this species
offers a wider domain of coexistence than the replaced less-efficient species does
(Fig. 9). The pattern of widened domain of coexistence is also true in a series of
replacements changing other parameters such as initial size growth rate. If there is
187
cruitment rate per species basal area) both in a warm-temperate rain forest in southern
Japan (Fig. 6) and in a mixed dipterocarp forest in west Kalimantan (T. Kohyama
et al. in preparation, the pattern is very similar to Fig. 6). Thus the functional
differentiation among species along a forest profile is likely to play an important
role in the coexistence of species in these forests. The question then arises why
there exists such a large difference in tree species diversity between these two forest
types. It is worth paying attention to the fact that the above analysis was only done
in terms of trunk diameter as a proxy for individual size.
Light-resource competition in a one-sided manner is a result of vertical crown
stratification, so tree height is a better size dimension to use than trunk diameter.
The relationship between diameter and height was quite different between these
two forests (Fig. 7), even though the distribution in trunk diameter was similar.
Trees of the warm-temperate forest approached their asymptotic height at a smaller
diameter, above which differentiation among species in maximum size is fairly
difficult. By contrast in the mixed dipterocarp forest, differentiation in terms of
maximum height is still possible in larger size classes. Therefore, not only the
biomass and the turnover rate of biomass, but also a measure of vertical differentiation among tree crowns such as the asymptotic height H* of D-H allometry (Fig. 2)
increase the possibility of coexistence. These attributes are co-related with each
other (Fig. 3). Therefore, it is not necessary to assume any single factor amplifying
species packing in wet tropical lowland, but these ecosystem attributes are likely to
multiplicatively widen the resource axis for the coexistence of species (Fig. 8).
5 Positive Feedback Between
Ecosystem Efficiency and Biodiversity
The results of the model competition between species (Fig. 4) suggest not only a
condition of coexistence but also of exclusive replacement by superior species in
terms of resource use efficiency. For instance, for the parameter space defined by
reproductive capacity and asymptotic size (where other parameters arc identical
between species), species with larger asymptotic size in growth performance always wins over smaller species when the reproductive capacity is kept identical
between the two species. When keeping the asymptotic size identical, species with
higher reproductive capacity always wins. If there is only one species with superior
performance beyond the tradeoff belt shared by many species, this species eventually excludes all other species. Therefore, succession or evolution is a series of
replacement by superior species, which does not necessarily enhance species diversity.
However, when we examine the range of coexistence defined by the new, superior species in the monodominant ecosystem, it becomes clear that this species
offers a wider domain of coexistence than the replaced less-efficient species does
(Fig. 9). The pattern of widened domain of coexistence is also true in a series of
replacements changing other parameters such as initial size growth rate. If there is