Interspecific patterns underlying variations in allometric relationship of sympatric Sterculiaceae species in a Bornean rainforest

We compared aboveground tree forms among closely related species in two genera of the Sterculiaceae (Scaphium and Heritiera) in a Bornean mixed dipterocarp forest. Two significant allometric patterns were detected: a negative correlation between the height at the onset of branching and the slope of the species-specific Cr (crown width)-D (stem diameter) allometric relationship for juveniles (D<10 cm), and a negative correlation between H _max (observed maximum height) and the Cr-D slope. The slope of the Cr-D allometric relationship of branched trees was significantly steeper than that of monoaxial (unbranched) trees in most species. These results suggest that the branching growth habit is better adapted than the monoaxial growth habit to crown expansion, and that the morphology of short species is better adapted to crown expansion than that of tall species. We did not detected significant correlations between the height at the onset of branching and the slope of the H (height)-D allometric relationship for juvenile trees, and between H _max and the H-D slope. In addition, the monoaxial and branched juvenile of most species did not differ significantly in the allometric slopes of the H-D relationship. Therefore, the study does not support the hypotheses that a monoaxial growth habit favors rapid height growth and that tall species have allometries better adapted to height growth.