Leaf litter derived from riparian vegetation can be a significant resource to heterotrophic aquatic ecosystems. Furthermore, the loss of riparian biodiversity in these communities can greatly influence decomposition in streams, owing to interspecific variation in foliar chemical traits known to regulate carbon processing. Functional diversity (FD), which defines the distribution and range of functional traits for species in a community, has been shown to predict ecosystem processes, such as decomposition, more effectively than species richness alone. Phylogenetic diversity (PD), which accounts for evolutionary distinctiveness between species, is understudied in the context of carbon processing. It has been hypothesized that PD may take into account inadequately captured traits and unknown interactions, which may be important to regulating ecosystem function.
In this study we estimated litter breakdown rates of 16 riparian tree communities which were experimentally assembled to reflect high and low levels of FD and PD. To eliminate confounding taxonomic diversity with each of these dimensions, species richness and evenness were held constant. Leaf litter assemblages for each treatment were exposed to three forested, headwater streams in Patapsco State Park (Maryland, USA). Breakdown rates were estimated as mass loss over approximately 40 days. Breakdown was then related to FD and PD.
Overall breakdown rates differed significantly across litter species and were correlated with each functional trait measured. Despite the importance of functional traits for the decomposition of individual species, our analysis of the independent role of FD and PD revealed that breakdown of multi-species assemblages could be explained by PD only. The breakdown rate of mixtures increased significantly with PD, but did not vary with FD. These results suggest that the unknown interactions and traits that are encompassed in a dimension such as PD may play an important role in regulating carbon processing in aquatic habitats. Our study was novel in that it was the first to examine the role of PD in multiple riparian assemblages. We recommend that future studies consider PD as an important dimension of biodiversity, and in addition should incorporate PD when assessing how biodiversity loss will influence ecosystem functions such as decomposition.