Tree species diversity indirectly affects nutrient cycling through the shrub layer and its high-quality litter

Tree species effects on biogeochemical cycles are well studied, but the interactive effects of tree species in mixtures remain poorly understood. We studied how tree species identity and species diversity affect nutrient cycling in mature forests. In a Belgian platform of 53 forest plots varying in tree species diversity and composition, we sampled the return of carbon, nitrogen and base cations via leaf litterfall and their stocks in the forest floor and topsoil. Tree species identity effects were clear; diversity effects were weak or absent. The leaf litter input from shrub species depended on the composition of the tree canopy and had a higher quality than the litter of the trees. Monocultures of pedunculate oak had the highest input of litter from shrub species; in mixtures with beech, however, this input was disproportionally low. We found indirect effects of tree species diversity on nutrient cycling, via effects of the tree species composition on the abundance and composition of the shrub layer. This is particularly important in forests consisting of tree species with low leaf litter quality, because nutrient cycling may benefit from the presence of shrub species with higher leaf litter quality.