Habitat associations of Sterculiaceae trees in a Bornean rain forest plot

Questions: 1. Are trees in a Bornean tropical rain forest associated with a particular habitat? 2. Does the strength of habitat association with the species-specific optimal habitat increase with tree size? Location: A 52-ha plot in a mixed dipterocarp forest in a heterogeneous landscape at the Lambir Hills National Park, Sarawak, East Malaysia. Methods: Ten species from the Sterculiaceae were chosen as representative of all species in the plot, on the assumption that competition among closely related species is more stringent than that among more distantly related taxa. Their habitat associations were tested using data from a 52-ha plot by a torus-translation test. Results: The torus-translation test showed that eight out of the ten species examined had significant association with at least one habitat. We could not find negative species-habitat associations for rare species, probably due to their small sample sizes. Among four species small trees were less strongly associated with habitat than large trees, implying competitive exclusion of trees in suboptimal habitats. The other four species showed the opposite pattern, possibly owing to the smaller sample size of large trees. A habitat had a maximum of three species with which it was significantly positively associated. Conclusions: For a species to survive in population equilibrium in a landscape, habitats in which 'source' subpopulations can be sustained without subsidy from adjacent habitats are essential. Competition is most severe among related species whose source subpopulations share the same habitat. On the evidence of source subpopulations identified by positive species-habitat association, species-habitat association reduces the number of confamilial competitors. Our results therefore indicate that edaphic niche specialization contributes to coexistence of species of Sterculiaceae in the plot, consistent with the expectations of equilibrium hypotheses.