Constantly high tree species richness along an elevational gradient of Mt Bokor, a table shaped mountain in southwestern Cambodia
Land area per elevational zone usually decreases with altitude and this decrease may be a major determinant of the decrease of species richness with altitude. However, other factors such as temperature also change with altitude and it is usually difficult to separate effects of those confounding factors. Here, we test a hypothesis that species richness does not change with altitude in a table-shaped mountain where land area per elevational zone does not significantly change and neither temperature nor rainfall restricts species richness. The study was carried on Mt. Bokor, southwest Cambodia. We used two methods to record species richness; 20 plots of 100 m x 5 m placed from 266 to 1048 m, and general collection of herbarium specimens obtained from 200 m to 1048 m. For both datasets, we used rarefaction, extrapolation and Chao1 estimator to standardize the sampling efforts. We examined correlation between species richness (observed and estimated) with altitude. In addition, we used neutral models to infer the effect of evolutionary history and migration on species richness.
Species richness estimates obtained from both the plot data and the general collection data showed no significant correlations with elevation. Neutral model parameters showed that relatively high rates of speciation and immigration in Mt. Bokor. Unlike many previous observations on species richness distribution along elevational gradients, tree species richness of Mt. Bokor kept high and constant values, supporting the hypothesis that species richness does not change with altitude in a table-shaped mountain. Neither of temperature, precipitation, disturbance or stress may largely influence species richness along the elevational gradient, and habitat area is likely to be the major determinant of species richness.