What drives leaf litter decomposition and the decomposer community in subtropical forests – The richness of the above-ground tree community or that of the leaf litter?

There is increasing concern regarding how plant diversity affects litter decomposition. However, few experiments have simultaneously investigated the independent effects of litter species composition and the richness of above-ground tree communities on litter decomposition. To elucidate the two facets of diversity effects on leaf litter decomposition, a field experiment with litterbags containing monospecific or mixed litter of Schima superba, Cyclobalanopsis glauca, and Castanopsis eyrie was conducted for 30 consecutive months in a subtropical forest. In addition, the fungal community composition of the decomposing litter was assessed in the later decomposition stage. Our results revealed that leaf litter species composition but not litter species richness per se affected decomposition rates, in which the decomposition rates were faster in the presence of S. superba litter but slower in the presence of C. eyrei litter. Soil properties showed stronger impacts on litter mass loss in the earlier stages, while the litter species composition became more important in the later stage. Moreover, above-ground tree species richness had a significant effect on leaf litter decomposition. As expected, the effects of above-ground tree species richness and litter species composition became much stronger with time, probably owing to the indirect effects of concomitant changes in the composition of the litter fungal community, which in turn depended on the changes of microenvironment or on accumulated recalcitrant substances in the late stage of decomposition. Overall, our findings demonstrate that not only litter species composition but also above-ground tree species communities play critical roles in regulating litter decomposition, particularly in the late stage. An investigation into the interactions between plant and decomposer communities is required to gain a full understanding of the mechanisms underlying litter mixture decomposition.