Tree species diversity affects decomposition through modified micro-environmental conditions across European forests

• Different tree species influence litter decomposition directly through species-specific litter traits, and indirectly by distinctly modifying the local decomposition environment. Whether these indirect effects on decomposition are influenced by tree species diversity is presently not well known. • We addressed this question by studying the decomposition of two common substrates, cellulose paper and wood sticks, in a total of 209 forest stands of varying tree species diversity across six major forest types at the scale of Europe. • Tree species richness showed a weak but positive correlation with the decomposition of cellulose but not with that of wood. Surprisingly, macroclimate had only a minor effect on cellulose decomposition and none on wood decomposition despite the wide range in climatic conditions among sites from Mediterranean to boreal forests. Instead, forest canopy density and stand-specific litter traits affected the decomposition of both substrates. • Our study suggests that species richness and functional composition of tree canopies modify decomposition indirectly by changes in micro-environmental conditions. These canopy-induced differences in the local decomposition environment control decomposition more than continental-scale differences in macroclimatic conditions