Leaf-litter overyielding in a forest biodiversity experiment in subtropical China

The production and subsequent turnover of aboveground litter is an important process in the ecosystem carbon (C) cycle. Litterfall links above- and belowground processes by transferring organic material to the soil where it becomes available to heterotrophs, fueling nutrient cycling. Little is known about how litter fluxes respond to experimental manipulation of tree species richness. We sampled litterfall in a large-scale forest biodiversity experiment in subtropical China. Litter was collected at monthly intervals during peak senescing season for two years and throughout the whole year in 2016, using 0.75 m × 0.75 m litter traps, in plots with a tree species richness of 1, 2, 4, 8 and 16 species. Cumulated annual litter production increased with tree species richness from an average of 1.96 ± 0.25 Mg∙ha − 1 ∙yr. − 1 in monocultures to 4.39 ± 1.15 Mg∙ha − 1 ∙yr. − 1 in 16-species mixtures. At site B, the doubling of species richness resulted in a positive effect with peak litter production increasing from 0.09 Mg∙ha − 1 when trees were 5 years old in 2015, to 0.14 Mg∙ha − 1 in 2016. The intra-annual distribution of litter production varied strongly among species, leading to a larger community niche for seasonal distribution of litter in species-rich than in species-poor plots. Community-niche size was positively correlated with litter production, thus providing an explanation for the species richness effects. Different species had complementary temporal dynamics of litterfall, which led to a more or less constant litter supply in species-rich stands over the whole year. This caused positive richness effects on litter production which in turn may positively affect mineralization and subsequent tree growth.