Functionally diverse stands promote leaf litter decomposition irrespective of litter species' resource acquisition strategy

Stand functional diversity (RaoQ) and stand functional composition in terms of the functional traits of the component species (CWM trait) are critical for ecosystem processes and functioning, such as litter decomposition and associated nutrient cycling. Litter decomposition rates could differ among stands varying in functional diversity and composition because of interspecific variation of resource economic traits in litter quality as well as differences in microenvironmental conditions. However, whether and how functionally diverse stands with different microenvironmental conditions change the litter decomposition rates among functionally contrasting species is poorly understood. Here, we hypothesized that stand functional diversity would promote litter decomposition of resource acquisitive, intermediate and conservative species but with inconsistent strengths, because of the potential positive effects of higher probability for litter nutrient exchange or facilitative microclimate in diverse stands. Conservative species' litter might benefit less from the positive stand diversity effect than intermediate species' litter, and then acquisitive species' litter (Hypothesis 1). Meanwhile, we hypothesized two alternative scenarios for the effect of stand functional composition: First, acquisitive species' litter would decompose faster in stands dominated by acquisitive species, while conservative species' litter would decompose relatively faster in stands dominated by conservative species (Hypothesis 2a). Second, any species would decompose faster in stands halfway between ‘conservative’ and ‘acquisitive’ functional composition with a steeper humpback in acquisitive species' litter than in intermediate species' litter, and then in conservative species' litter, as a consequence of high functional diversity (Hypothesis 2b). We tested our hypotheses through a litter decomposition study in a tree diversity experiment in subtropical China. We quantified the leaf litter mass losses for acquisitive, intermediate and conservative species and measured microclimates across 30 stands that represent a range of tree functional diversity and composition over five decomposition periods over 394 days. Increasing stand species richness and stand functional diversity were significantly associated with higher leaf litter decomposition rates across all three functionally contrasting litter species. This positive effect did not change with the litter species' resource acquisition strategies, in partial support of our Hypothesis 1. Increasing stand functional composition along an axis from resource conservative to acquisitive had rather similar humpback relations with leaf litter decomposition rates for all three litter species, which partly supported our Hypothesis 2b but rejected our Hypothesis 2a. Our study provides mechanistic insights into the potential direct and indirect roles of functional trait diversity and composition in driving litter decomposition, and these insights are relevant to many ecosystem types globally. Read the free Plain Language Summary for this article on the Journal blog.