Effects of Tree Species Diversity on Litter Quality and Decomposition

Abstract

Inter- and intraspecific variation in leaf litter quality is substantial and substrate quality is of overriding importance\rfor rates of litter decay and mineralization within a forest site. Although this has long been recognized, the functional\rsignificance of litter mixtures for decomposition processes and ecosystem functioning has surprisingly been little explored.\rThe currently available literature summarized here shows that studies specifically addressing litter diversity effects on\rdecomposition rarely included more than two species and were carried out predominantly in temperate forests with other forest\recosystems outside the temperate zone (most importantly tropical forests) critically underrepresented.\r\rNevertheless, non-additive litter mixture effects on mass loss and/or nutrient mineralization have been observed in about\rhalf of all studies, suggesting that litter species richness and/or composition can have important implications for decomposition\rprocesses and ecosystem functioning. The relationship between litter species richness and process rate, however, does not\ryet appear to be predictable, and the currently available data suggest that the identity of species within a mixture is more\rimportant than the number of species. A comprehensive mechanistic approach in the analysis of diversity effects would substantially\rimprove our understanding of the functional significance of litter diversity for decomposition and should be of high priority\rfor future research. To get there, I think we first need to abandon the idea of a single, true correlation between diversity\rand process rate. The few studies separating diversity effects on decomposition of individual component species strongly suggest\rthat we have to focus beyond such simple correlations based on litter mixtures as a whole. Unraveling interactions across\rtrophic levels, identifying specific compounds such as polyphenols acting as inhibitors or stimulators in the process of decomposition,\rand the investigation of nutrient transfer among litter species are three promising areas of future, mechanistically oriented\rresearch.