# Why Is the Diversity of Tree Species in China’s Lowland Rainforests Higher than in Its Montane Rainforests?

## TL;DR

This study explores why lowland rainforests in China have more tree species than montane rainforests, finding that niche differentiation and negative density dependence play key roles.

## Contribution

The study provides new insights into the mechanisms driving higher tree diversity in lowland versus montane rainforests.

## Key findings

- Lowland rainforests had 154 tree species compared to 119 in montane rainforests.
- Lowland rainforests showed stronger negative density dependence and niche differentiation.
- Rare species were more abundant in lowland rainforests than in montane rainforests.

## Abstract

Despite extensive research on tree species diversity in tropical rainforests, the differences in diversity between lowland and montane rainforests, as well as the underlying mechanisms, remain unclear. This study utilizes tree inventory data from two dynamic monitoring sample plots, each with an area of 1 hm2, established in the lowland rainforest and montane rainforest regions of Diaoluo Mountain, Hainan Island. We analyzed the composition, diversity, spatial distribution patterns, and interspecific relationships within the tree communities. In total, 154 tree species with DBH > 3 cm were recorded in lowland rainforests, and 119 were recorded in montane rainforests, with lowland rainforests harboring 51 rare species compared to 40 rare species in montane rainforests. The distribution pattern of small trees (with DBH ≤ 7.5 cm) exhibited clustering at a small scale but random distribution at a larger scale. This phenomenon was more prevalent in tropical lowland rainforests, indicating that the negative density dependence effect is more pronounced in lowland rainforests compared to montane rainforests. Moreover, a higher proportion of negative associations and lower niche overlaps were observed in the lowland rainforest compared to the montane rainforest, suggesting that niche differentiation among tree species is more pronounced in the lowland rainforest. In conclusion, the more intense negative density dependence effect and niche differentiation are the primary factors contributing to the higher tree species diversity observed in lowland rainforests.