We analyzed the relationship between species richness and biomass in natural forest communities at two similar sites on Mt. Xiaoiongshan, northwest China. At both sites, a wide range of tree layer biomass levels was available by local biomass estima-tion models. In order to identify underlying mechanism of the species richness-biomass relationship, we included different water resource levels and number of individuals in each plot in our analysis. We sampled 15 and 20 plots (20 m x 20 m), respectively, at both two sites. These plots were sampled equally on the sunny slope and the shady slope. Species richness, number of individuals of each species and diameter at breast height (DBH) as a substitute of biomass of tree layer were recorded in each sample. At one site,the relationship between species richness and biomass was significant on the sunny slope, and this relationship disappeared on the shady slope due to more environmental factors. The relations between species richness and number of individuals and between num-ber of individuals and biomass paralleled the species richness-biomass relation on both slopes. The difference in number of individu-als-biomass relationships on the sunny slope and the shady slope revealed "interspecific competitive exclusion" even though the species richness-biomass relationships were not hump-shaped. At the other site, species richness was not related to biomass or to number of individuals. Our study demonstrated the importance of environmental stress and succession of community in the under-standing of species diversity-productivity patterns.