The relationship between functional dispersion of mixed-species leaf litter mixtures and species' interactions during decomposition

We tested the hypothesis that interactions between plant species during the process of mixed-species leaf litter decomposition increases with increasing functional diversity of leaves within the mixtures; specifically, there is a positive correlation between functional dispersion and the deviations from Grime's biomass-ratio hypothesis, with a null intercept. We measured decomposition rates (mg g(-1) d(-1)) of mixed-species leaf litter from two experimental designs: 1) a microcosm experiment with litterbags of species mixtures combining six tree species, alone and in 42 combinations, and 2) an in situ litterbag experiment with all possible mixture combinations of four herb species (from one to four species). Interaction strengths and directions were measured as deviations from community-weighted means (CWM) of monoculture decomposition values, following the biomass-ratio hypothesis (BRH). Functional diversity was measured as Laliberte and Legendre's functional dispersion (FDis), using leaf dry matter content (LDMC), leaf nitrogen and carbon contents, and proportions of water soluble compounds, cellulose, hemicellulose and lignin. Correlations between FDis and deviations from BRH varied strongly, depending upon the combination of functional traits, the plant type or the environmental conditions, and the way in which prediction error was expressed (absolute or actual deviation). For tree species, FDis that was based on a combination of water soluble compounds, hemicellulose concentration, and LDMC was negatively correlated with interaction strength but positively with its absolute value. For herbs, interaction strength (absolute or actual) decreased as FDis of the mixtures increased, based on cellulose and lignin contents. There was no positive correlation between functional dispersion and the deviations from Grime's biomass-ratio hypothesis, with a null intercept. Despite a relationship between litter interactions and functional divergence, this relationship was not generalisable. Other functional traits that were missing in our study might have played an important role.