Predominance of abiotic drivers in the relationship between species diversity and litterfall production in a tropical karst seasonal rainforest

Abstract

Litterfall production is a major contributor to ecosystem net primary productivity and biogeochemical cycles. However, how various abiotic and biotic drivers influence litterfall production in heterogeneous natural forests is still debated. We used structural equation models (SEM) to test the direct and indirect effects of tree species diversity, tree diameter variation, stand basal area, and abiotic drivers (canopy exposure, elevation, slope, convexity, aspect, topographic wetness index and altitude above channel) on annual litterfall production in a heterogeneous tropical karst seasonal rainforest in Southern China. The SEM with tree species diversity, tree diameter variation, stand basal area, and abiotic drivers accounted for 43.4% of the variation in annual litterfall production. Tree species diversity and stand basal area had positive direct effects, while tree diameter variation had a negative direct effect on annual litterfall production. Tree species diversity had no significant effect on tree diameter variation nor stand basal area. Both annual litterfall production and tree species diversity decreased directly with water availability, while canopy exposure positively affected annual litterfall production but not tree species diversity. Our results indicate that the positive relationship between tree species diversity and litterfall production did not result from the effects of species diversity on canopy packing; instead, it appears that increasing soil water availability simultaneously reduce tree species diversity and annual litterfall production in the tropical karst seasonal rainforest.