Tree Species Diversity Affects Litter Decomposition via Modification of the Microenvironment

Abstract

Decomposition is a key determinant of forest functioning, controlling nutrient and carbon cycling. Although litter-mixing effects on decomposition (that is, using mixtures of litter of different species) have been studied extensively, less is known about the indirect effects of modified microenvironments via overstory tree species mixing. To investigate the effects of tree species diversity on decomposition, we installed 384 standardized litterbags, filled with leaf litter of four broadleaved tree species with contrasting litter quality, in a large, 10-year-old tree diversity experiment. To quantify microenvironments, we used microclimate sensors, below-canopy rain gauges and measured soil characteristics. We then analysed indirect tree species diversity effects, that is, tree species richness effects on mass loss rates via tree species-induced alterations in the microclimate, throughfall and soil characteristics. We found that understory microenvironmental conditions indeed affect mass loss rates, with the main drivers differing among incubation stages. Predominantly soil phosphorus, but also vapour pressure deficit and throughfall amounts, was negatively associated with mass loss rates across litter types during the first 2 months of the decomposition process. After 6 months of the decomposition, soil moisture was found to be the key determinant positively affecting mass loss rates. In sum, our research contributes to a better understanding of the determinants of decomposition and shows an important pathway in which tree species diversity affects decomposition, via modified microenvironmental conditions acting via the soil, microclimate and throughfall.