Comparison of forest stand characteristics and species diversity indices under different human impacts along an altitudinal gradient

Abstract

Various disturbances and ecological successional processes shape the structure of forests and affect biodiversity. We examined the forests over a 7400 km2 area in the Upper Min River watershed in Sichuan under different levels of human disturbance. This mountainous watershed represents a transitional landscape between the Qinghai-Tibet plateau and the Sichuan basin, with elevations ranging from 900 m to 5700 m. The watershed is degraded due to deforestation and soil degradation caused by natural and anthropogenic disturbances. According to different levels of human impact, the sample sites were divided into four classes: 1) near-natural forests, 2) selectively logged forests, 3) natural regeneration forests (after clear-cut), and 4) plantations. At different elevations, following quantitative characteristics were analysed: stand volume, basal area, weighted diameter, weighted height, and 7 biodiversity indices of tree species. The results showed that the structure of forest stands was characterized by a large number of seedlings and saplings. The near-natural forest had a significantly higher stand volume and basal area than the other managed stands. The four levels of human impact resulted in different abundance, evenness and richness of tree species along the altitudinal gradient. Explanatory variables, such as volume and weighted diameter, significantly contributed to the models for discrimination of stands under different human impacts. The result implies that near-natural forests, with their large stand volumes and biodiversity, can be used as references when developing strategies for forest restoration.