Architectural and allometric differences among Scaphium species are related to microhabitat preferences

1. Interspecific comparisons were made among Scaphium species (Sterculiacea e): S. borneense (Merr.) Kosterm., S. longipetiolatum (Kosterm.) Kostermans , and S. macropodum (Miq.) Beumee ex Heyne to test the hypothesis that vari ations in their architecture and allometry are related to the microhabitat conditions of stands where the species regenerate preferentially. 2. The species studied are shade-tolerant canopy trees. They occur in a tro pical rainforest in Lambir Hills National Park, Sarawak, East Malaysia (sym patric at the community level), where the study was conducted, but show an allopatric pattern of distribution among microhabitats due to localized eda phic conditions. Scaphium longipetiolatum occurs primarily in stands on cla y-rich soils, whereas S. borneense occurs primarily in stands on sandy soil s. Stands on clay-rich soils are more shaded, and gaps form less frequently than in stands on sandy soils. 3. Scaphium longipetiolatum initiated branching at a smaller tree size than S. borneense and S. macropodum, which did not produce branches until stems reached several centimeters in diameter. 4. The monoaxial saplings of S. borneense and S. macropodum expanded their crown area by producing bigger leaves. The branched saplings of S. longipet iolatum expanded their crown area by developing lateral branch systems. 5. An analysis of interspecific differences in above-ground allometries rev ealed that the growth pattern of monoaxial saplings, which emphasizes leade r growth, leads to rapid height growth. In contrast, that of the branching saplings of S. longipetiolatum prioritizes shade tolerance. 6. The architecture and allometry of each Scaphium species gave some potent ial advantage over the other Scaphium species in stands where the former re generates preferentially, that is, the advantages of each phenotype were co ntext-dependent.