Abstract: Aims Human impacts on natural ecosystems induce changes in their functioning through alterations in species richness, composition and evenness of plant communities. Most litter diversity–decomposition processes studies have only manipulated species richness, ignoring the role of evenness. Here, results from a field litterbag experiment are presented to test whether changes in evenness of species distribution in litter mixtures affected the strength of the litter-species richness–decomposition relationship.
Methods Ten herbaceous species abundant in Mediterranean grassland communities and representative of different genera and functional groups were used. Species richness was directly manipulated to produce litter mixtures of three and six plant species, as well as litter of each individual species used. Each level of species richness was replicated several times such that each repeat had a different species composition. Three- and six-species litter mixtures were also treated to vary in evenness (three levels). Decomposition rate was assessed by percentage dry weight loss over the 90 days of the experiment.
Important findings Decomposition rate was positively related to the linear increase in litter-species richness and was affected by the composition of the litter-species mixture. Decomposition rates differed significantly between evenness treatments and moreover, the strength of the positive relationship between litter-species richness and decomposition rate decreased notably in the low-evenness treatment. The effects of evenness on decomposition rate, at different richness levels, were partially explained by the differences in the initial litter mixture's carbon-to-nitrogen ratio within them. This study reveals that short-term decomposition rate is positively affected by both components of Mediterranean grassland litter-species diversity.

Key words: biodiversity, dominance, ecosystem functioning, old field, plant litter mixture