Evidence regarding intermediate heterogeneous hypothesis on diversity maintenance trade-off to heterogeneity through a novel microhabitat partition under sheltered-canopy environments

Heterogeneity is ubiquitous and crucial to maintaining biodiversity due to variations in spatiotemporal resources. However, it is unclear which habitat heterogeneity can maintain greater species and functional diversity because of the difficulties in qualifying habitat heterogeneity, while the karst ecosystem provides a typical heterogeneous paradigm. We surveyed 50 plots of 20 m × 20 m divided into 200 subplots in the karst mountain forests. Information on microhabitats was collected innovatively for quantifying habitat heterogeneity through partitioning components of the stone hole, groove, trench, chink, stone surface, and soil surface under sheltering-canopy environments. We explored the relationship and effect of microhabitat components on species diversity (SD) and functional diversity (FD) through regression analysis, Mantel’s test, and structural equation model (SEM). These results showed that the microhabitat ratio significantly correlated with habitat heterogeneity, mainly through the soil surface and stone surface ratios. The microhabitat partition affected heterogeneity along with species and functional diversity by presenting a humped curve. It suggested that the moderate heterogeneity maintained greater SD of Patrick richness and Shannon-Wiener and FD of FRic and FEve in a heterogeneity range of 1.121 to 1.280, responding to the sum ratio of 0.778 to 0.863 of soil and stone surface. Mantel’s test indicated that SD and FD were affected by microhabitat components, mainly through stone chink, stone surface, and soil surface; meanwhile, the heterogeneity directly affected SD and FD and indirectly affected FD by affecting SD through SEM pathway analysis. This study has shown that the moderate heterogeneity maintained more incredible species and functional diversity depending on the trade-off from microhabitat partition to heterogeneity, providing evidence of intermediate heterogeneous hypothesis on the significance of understanding biodiversity maintenance.