Mechanisms of coexistence among Aporosa species in two Malaysian rain forests
Tropical forests contain the greatest biotic diversity of any biome, and those of South-east Asia are amongst the most diverse and tree species rich. I studied the speciose understorey tree genus, Aporosa (Euphorbiaceae), at two c. 50 ha plots in Malayisa, at Lambir Hills National Park, Sarawak, and at Pasoh Forest Reserve Peninsular Malaysia, where 17 and 12 species co-occur, respectively. The aim of this study was to determine which factors influence the spatial distribution of Aporosa species, what limits their abundance, and ultimately what are the mechanisms of coexistence among the species of this genus. Using a method of second-order spatial pattern analyses based on Ripley's K function to test for non-random spatial distributions, and to test for spatial associations between species (using a bivariate method) I found that Aporosa species formed spatially distinct assemblages at both sites. A randomization procedure suggested that these assemblages were explained by biases in the distribution of species in relation to habitat types. Soil type, as determined by parent material, was an important determinant of habitat preferences, although topography and forest stmcture also accounted for some variation. I found measurable effects of position on soil and especially topographic types on mortality and recruitment rates of many Aporosa species. Growth rates over a 5-8 year period varied between species, but, perhaps due to the short time interval and the correlation between abiotic variables, they were a poor indicator of habitat preference as defined by bias in a species' distribution. Aporosa species exhibit a range of morphological, anatomical and biochemical properties, and I found evidence that both habitat partitioning and density-dependent mortality of juveniles occurred. These are both mechanisms contributing to the maintenance of species richness. A severe drought had differential effects on mortality rates of Aporosa species and may be important in determining species' spatial distributions and abundance. Therefore disturbance events are also likely to be important mechanisms, as are non-equilibrium random population fluctuations, between ecologically similar species.