Relationship between species diversity and tree size in natural forests around the Tropic of Cancer

Abstract

Although numerous hypotheses have been proposed to explain the patterns of species diversity of forests at local and landscape levels, it is still difficult to predict the alpha diversity of species, especially in tropical and subtropical natural forests where trees of different sizes and shapes are highly mixed. Size might characterize species diversity, and the possible correlation between these variables may contribute to the development of easy-to-use growth indicators to predict diversity and to understand the status of trees within a stand. In this study, diameter classes were divided using the equal diameter class (EDC) and minimum measured diameter (MMD) methods, and five species diversity indices were calculated for each diameter class of 14 permanent plots in five national nature reserves surrounding the Tropic of Cancer (23.5°), southwest China. The results show that species richness, abundance, and spatial diversity indices decreased in a typical inverted J-shape pattern with increasing diameter class and MMD, and could be easily modeled by a negative exponential function. The Shannon–Wiener index showed a linear decrease while Pielou’s evenness index displayed a linear increase, with a small degree of instability. The results suggest that species diversity in subtropical forests is closely related to tree size, and the relationship is possibly independent of habitat. Measuring DBH of some trees in a stand could be informative regarding species diversity and contribute to the investigation and assessment of biodiversity.