Interacting effect of habitat and conspecific density on tree mortality in a Bornean rain forest

Background/Question/Methods Habitat divergence and conspecifec-density-dependent mortality are two important mechanisms in maintenance of the high tree diversity of tropical rain forests. We have been monitoring mortality and growth of all trees (> 1 cm dbh) of a 52-ha plot at Lambir National Park in Borneo since 1992. Previous studies provided independent evidence suggesting that both mechanisms are working at Lambir plot. However, the two mechanisms seem to make contradicting predictions about species’ spatial distributions. Habitat divergence would promote aggregation of conspecific trees to their preferable habitats, while density-dependent mortality would reduce the conspecific aggregation. To date, we could not get good evidence for habitat divergence in demographic data; mortalities of habitat specialists were not necessarily smaller in their "home habitats". To resolve the contradicting results, we analyzed interaction of habitat and conspecific density effects in the mortality of Lambir trees. We divided the species into three habitat groups based on their spatial distributions along an elevation gradient, which was a major axis in habitat associations in our plot. Then, we analyzed the main and interaction effects of habitat preference and conspecific density on 10-year mortality (1992–2002) of Lambir trees by logistic regression analysis.

Results/Conclusions Results of model selection with AIC showed that the best fitting model included both of main and interaction effects. The selected model predicted that mean mortality was smallest in "home-habitat species” than “away-habitat specie” if conspecific densities were higher than 5–10 trees per 100 m² at any of the three habitats. In contrast, if conspecific densities were lower than these values, mean mortality was always smallest for the species whose habitats were on high elevations or ridges. This was consistent with previous studies that showed no “home advantage” possibly because that many individuals were growing under low conspecific densities in our plot. Based on these results, we hypothesize that habitat specialist species in the Lambir forest can become more abundant than other species at a certain habitat because they are less likely to suffer the conspecific-density effect at that habitat through yet unidentified mechanisms.