Diversity maintenance mechanism changes with vegetation type and the community size in a tropical nature reserve

Species‐abundance distribution (SAD) is an essential tool to explain the mechanism of diversity maintenance in ecological communities. Most of the studies on diversity maintenance in a specific forest dynamics plot just consider stems with a certain minimum size class to include into the tree community to be examined. However, the species in the juvenile stage are easily disturbed by a variety of factors; here, we define the minimum stem size to tag trees in a community as the community size (DBHmintag), which implies that the communities with different minimum diameter at breast height (DBH) sizes to tag trees are tree assemblages containing tree populations of different minimum DBH. We used data from 17 1‐ha forest dynamics plots across six old‐growth forest types in a tropical nature reserve to explore diversity maintenance mechanism by SAD curves (at three levels of DBHmintag) fitting to neutral model and niche preemption model. We found that the SADs of the two zonal vegetation types (tropical montane rain forest and tropical lowland rain forest) were best fitted by neutral model at each level of DBHmintag; meanwhile, the best fitted models for the four azonal vegetation types (tropical coniferous forest, tropical deciduous monsoon rain forest, tropical montane evergreen forest, and tropical montane dwarf forest) varied with DBHmintag levels, for communities with DBHmintag ≥ 1 cm and DBHmintag ≥ 5 cm, the fitting effect of neutral model was better than niche preemption model's for the forest dynamics plots in the four azonal vegetation types, and for communities with DBHmintag ≥ 10 cm, the four azonal vegetation types were all best fitted by the niche model. Our results suggest that species diversity maintenance mechanisms of the two zonal vegetation types derived from the neutral model increased the predictive accuracy at each level of DBHmintag, and meanwhile, the four azonal vegetation types derived from the neutral model increased the predictive accuracy at smaller community size; however, with the increase in DBHmintag, these communities derived from the niche theory model increased the predictive accuracy. Habitat heterogeneity might be the major constraints in determining the neutral or niche process for diversity maintenance of a specific forest community.