How does a tree species influence litter decomposition? Separating the relative contribution of litter quality, litter mixing, and forest floor conditions

Litter quality is often considered the main driver of decomposition rate. The objective of this study was to investigate the relative contribution of two other tree-driven mechanisms, litter mixing and forest floor conditions, to foliar litter decomposition and nutrient dynamics for trembling aspen (Populus tremuloides Michx.) and black spruce (Picea mariana (Mill.) BSP) using a microcosm approach. Results based on mixed linear models show that the greater influence over these processes was obtained through litter quality followed by forest floor conditions and litter mixing. Specifically, the results indicate that significantly more C and nutrients were mineralized (i) from aspen than from spruce litter, (ii) from spruce litter in mixture with aspen litter than from spruce litter applied singly, and (iii) from litter incubated on forest floor from the aspen stand rather than from the spruce stand, except for nutrients in the spruce litter. Collectively, our results show that the litter and forest floor material from aspen both favour decomposition and nutrient mineralization processes. Hence, we provide evidence that the effect of tree species on litter decomposition may not only be caused by the properties of its litter but also, indirectly, by the specific conditions and the decomposer community that tree species develop in their forest floor.