The mediatory roles of species diversity and tree height diversity: Linking the impact of land-use intensity to soil erosion

Land-use intensification has increased soil erosion through biodiversity loss, which affects ecosystem properties and services owing to changes in species compositional and structural diversity. However, information is limited on how land-use intensity (LUI) affects soil erosion through species compositional diversity, structural diversity and ecosystem properties. In this study we monitored soil erosion in 15 plots over 12 consecutive months on tropical Hainan Island, where land-use changes and biodiversity loss have been severe. We quantified the direct and indirect effects of LUI, species compositional diversity (plant species diversity and evenness) and structural diversity (tree diameter and height diversity), and ecosystem properties on soil erosion using Bayesian structural equation models (BSEM). The optimal BSEM accounted for 69% of the variation in soil erosion. The LUI did not affect soil erosion directly but showed indirect impacts via species diversity, tree height diversity, and ecosystem properties. Both high species diversity and tree height diversity reduced soil erosion directly and indirectly by promoting root length density (an ecosystem property), but tree height diversity mediated the more important indirect effect of LUI on soil erosion than species diversity and ecosystem properties. We provide evidence for loss of species diversity and tree height diversity as a result of LUI leading to aggravated soil erosion, but the impact of the latter was more serious. Therefore, we emphasize that maintenance of tree height diversity through management practices may be an effective approach to control soil erosion in the context of drastic land-use changes in tropical areas.