Non-negligible contribution of subordinates in community-level litter decomposition: Deciduous trees in an evergreen world

Subordinate species have relatively low abundance compared to dominant species; however, they may contribute significantly to functional diversity and ecosystem functionality, particularly if they differ strongly from the dominants in key traits. Here we investigated whether this phenomenon might be applied to litter decomposition as a key carbon and nutrient cycling process. We hypothesized that species litter mass-weighted predictions of community-level litter decomposition based on dominant rates only, would deviate strongly from observed community-level rates, and that predictions would improve as subordinates with strongly contrasting traits were combined with those of their dominant counterparts. We tested this hypothesis through a 1-year field decomposition experiment across a chronological sequence in a subtropical evergreen broadleaved forest. The experiment included the single-species litter of evergreen dominants, evergreen subordinates and deciduous subordinates, respectively, as well as community-level litter mixtures. The expected community-weighted mean decomposition rates based on the evergreen dominants alone, with or without the addition of evergreen subordinates, deviated strongly from those of observed community litter mixtures at the middle and late succession stages, but not at the early stage. When adding deciduous subordinates to this expectation, there was no longer any difference to the observed community litter decomposition rate across succession stages. Deciduous subordinates alone explained 7%, 21% and 15% of the total variation in community litter mixture decomposition rate for early, middle and late successional stages, respectively, which is more than would be expected from their litter mass fraction. Synthesis. Deciduous subordinates with strongly contrasting nutritional and water-storage traits compared to the dominant evergreens significantly impacted litter decomposition at the community-level in spite of their low abundance. This study highlights the importance of ‘being different’ for subordinates to be influential in ecosystem carbon cycling.