Leaf litter derived from riparian vegetation can be a significant resource to aquatic ecosystems. Furthermore, 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 function more effectively than 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/measured traits and unknown interactions, which may be important to regulating ecosystem function. Here I present how FD and PD independently and interactively predict breakdown rate. Breakdown rates differed significantly across litter species and were correlated with each functional trait. Analysis of the independent role of FD and PD, revealed that breakdown of multi-species assemblages is significantly related to both FD and PD. As has been hypothesized for other systems, PD was an effective predictor of ecosystem function. 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. Based on these results, I offer that PD is an important dimension of biodiversity and should be considered when assessing how biodiversity loss under global change will influence important ecosystem function such as decomposition.