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 we estimated litter breakdown rates of 16 riparian tree communities. Using a regional species pool, communities were randomly created and FD/PD were subsequently calculated for each. A subset of these communities was then chosen reflecting high/low levels of both FD/PD. To eliminate confounding taxonomic diversity with each of these dimensions, species richness and evenness were held constant. Leaf litter assemblages were created to reflect species composition in chosen communities and litter bags 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.
We learned that breakdown rates differed significantly across litter species and were correlated with each functional trait. Our analysis of the independent role of FD and PD, revealed that breakdown of multi-species assemblages is significantly related to both FD and PD. When the traits known to govern decomposition are measured and combined into a measure such as FD, the direction and magnitude of diversity effects can be predicted. Furthermore, 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. Our study was novel in that it was the first to examine the role of PD in multiple riparian assemblages. Based on these results, we propose that PD is an important dimension of biodiversity and should be considered when assessing how biodiversity loss will influence important ecosystem functions such as decomposition.