Decomposition in Mixed Beech Forests in the South-Western Alps Under Severe Summer Drought

Abstract

Climate and plant litter diversity are major determinants of carbon (C) and nitrogen (N) cycling rates during decomposition. Yet, how these processes will be modified with combined changes in climate and biodiversity is poorly understood. With a multisite field experiment, we studied the interactive effects of reinforced and prolonged summer drought (using rainout shelters) and tree species mixing on leaf litter decomposition in beech forests in the French Alps. Forests included monospecific stands of Fagus sylvatica, Abies alba and Quercus pubescens and two-species mixtures composed of beech and one of the other species. We hypothesized (1) slower C and N release during decomposition in response to experimentally prolonged summer drought, but (2) less so in mixed compared to monospecific tree stands, due to indirect canopy effects and direct litter mixing effects. Litter lost 35% of initial C and 15% of N on average across all sites and litter types over 30 months of decomposition. The experimentally prolonged summer drought led to weakly slower C loss but had no effect on N loss. Tree species mixing did not alter drought effects on decomposition but had non-additive effects on C and N loss, which were dominated by direct litter mixing rather than indirect plot-specific tree species composition effects. Our data suggest relatively small effects of reinforced and prolonged summer drought on decomposition, possibly because process rates are generally slow during summer and because microsite variability is larger than the effects of rainfall exclusion. The dominant contribution of litter mixing to the overall effect of plot-specific tree species mixtures on decomposition supports the importance of microsite conditions for C and N dynamics during decomposition, which should be accounted for more explicitly in climate and biodiversity change predictions.