We carried two sampling methods to record species richness; plot-based with a uniform size (100 m x 5 m) from elevation 266 m to 1048 m and general collection of herbarium specimens in all elevational zones (0-99 m to 1000-1048 m). For both datasets, we proposed methods of rarefaction, extrapolation and Chao1 estimator to reduce the sampling limitation and bias. In addition, neutral model is used to infer the effect of evolutionary history on species richness. The rarefaction, extrapolation and Chao1 estimated richness from both datasets and Fisher’s alpha from plot dataset showed no significant correlations with elevation. Additionally there was no significant difference of d.b.h. distribution among all plots and the tree density and height distribution kept constantly along the whole elevation gradient except the top area. Neutral model estimated parameters indicated that high speciation and immigration rate in the Mt. Bokor region. Among the confounding factors (environment, climate or disturbance etc), the constant land area pattern of Mt. Bokor is the most determinate one for the constant tree species richness pattern.