Climate, leaf litter chemistry and leaf litter decomposition in terrestrial ecosystems: A triangular relationship

Litter decomposition is an important component of the global carbon budget. Due to the strong climatic control of litter decomposition, climate change may significantly affect this pathway. This review quantifies the climatic influences on litter decomposition rates, both directly and indirectly through effects on litter chemistry. To this end, I analysed first-year leaf litter decomposition data from 44 locations, ranging from cool temperate sites to humid tropical sites. Actual evapotranspiration (AET) was used as an index for the climatic control on decomposition. As litter chemistry parameters I included N and P concentrations, C/N and C/P ratios, lignin concentrations, and lignin/N and lignin/P ratios. At a global scale, climate (expressed as AET) is the best predictor for the decomposition constants (ic-values) of the litter, whereas litter chemistry parameters have much lower predictive values. Path analysis showed that the control of AET on litter decomposability is partly mediated through an indirect effect of AET on litter chemistry. Thus, the relation between climate, leaf litter chemistry and leaf litter decomposition is a triangular relationship. Mean AET in the humid tropical region is three times as high as in both the temperate and the Mediterranean region and this results in a more than six-fold increase in mean k-values. However, due to the large variability in k-values within each region there is a considerable overlap in k-values between the tropics and the other climatic regions. Within a particular climatic region litter chemistry parameters are the best predictors of k-values, especially in the tropics, whereas the percentage of variance in k-values explained by AET is low or absent. In general, litters from the tropical sites have higher N concentrations and lower lignin/N ratios than litters from other climatic regions. In both the tropics and in the Mediterranean region, the lignin/N ratio is the best chemical predictor of litter decomposability. In the temperate region, however, there is no good chemical predictor of litter decomposability.