Abstract
A comparison of tree architectures and allometries between two congeneric, light-demanding species, Pterospermum diversifolium and P. javanicum (Sterculiaceae), revealed that P. diversifolium is favored over P. javanicum at high light levels, but that the opposite is true at low light levels. Therefore, the abundance of these species within a landscape is likely to depend largely on habitat heterogeneities in understory light conditions. To verify this prediction, we compared the abundances and population size structures of these species among three 1-ha plots established on a ridge, a slope, and a flat plain within a tropical floodplain forest in East Kalimantan, Indonesia. We found a significant difference in understory light conditions among the plots: ridge > slope = flat plain. The abundances and size distributions of P. javanicum within the plots indicate that this species has a wide habitat range in the study area, from flat plains to slopes and ridges. We found no P. diversifolium in the flat plot; the habitat of this species may thus be restricted to ridges and slopes. Differences in the population size structures between the ridge and slope plots suggest that P. diversifolium regenerates more abundantly on ridges than on slopes. We discuss differences among plots in the abundance and size structure of P. diversifolium from the viewpoints of habitat heterogeneities in terms of differences in understory light and soil water conditions.
