Plant functional diversity mediates indirect effects of land-use intensity on soil water conservation in the dry season of tropical areas

Land-use intensification has led to biodiversity loss, which affects ecosystem properties and services by altering plant functional traits. However, the mechanistic pathways through which land-use intensity (LUI) affects ecosystem services across functional traits and ecosystem properties remain unclear. We studied the relationships among LUI, plant functional traits, ecosystem properties, and soil water conservation in tropical Hainan Island, China, where land-use changes, biodiversity loss, and seasonal drought have had severe impacts. Soil water conservation was represented by two complementary processes, namely soil water retention (SWR) and soil water capture (SWC). SWR and SWC were observed along a LUI gradient after 27 rainfall events (14 light, 10 moderate, and 3 heavy). We quantified the direct and indirect effects of LUI, the community-weighted mean (CWM) and functional divergence (FDvar) of water-related plant functional traits (tree height, leaf thickness, specific leaf area, and leaf dry matter content), and ecosystem properties on SWC and SWR using Bayesian structural equation models. The results showed that LUI did not affect SWC and SWR directly but presented indirect impacts via functional traits and ecosystem properties. Importantly, tree height FDvar mediated the most important indirect effect of LUI on both SWC and SWR. Tree height FDvar indirectly affected SWC through ecosystem properties, while the direction of the effect changed from negative to positive with increasing rainfall intensity, and promoted SWR directly and indirectly by increasing litter fall and soil organic matter. Our results further provide evidence of an indirect effect of LUI on soil water conservation primarily through tree height FDvar. Loss of functional diversity of plant height caused by LUI resulted in a decrease in SWR and SWC, indicating that seasonal drought causes increased impacts. The results emphasize that maintaining functional diversity of tree height in tropical land use is conducive for soil water conservation to mitigate increased intensity of seasonal drought predicted under climate change.