Using functional and phylogenetic diversity of riparian tree communities as predictors for decomposition in streams

Background/Question/Methods:

 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. Functional diversity, which defines the distribution and range of functional traits in a community, has been shown to effectively predict ecosystem function. Phylogenetic diversity, which accounts for evolutionary distinctiveness between species, is understudied in the context of decomposition. It has been hypothesized that phylogenetic diversity may take into account inadequately captured traits, and unknown interactions, which may be important to regulating ecosystem function.

 Here we estimated litter breakdown rates of 16 riparian tree communities. Litter assemblages were estimated using riparian community composition and species-specific foliar biomass from the Forest Inventory and Analysis dataset. Functional and phylogenetic diversities were calculated for each riparian plot and a subset chosen based on observed levels of each index. Leaf litter assemblages were created using community-level foliar biomass, 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 taxonomic, functional and phylogenetic diversity.

Results/Conclusions:

We learned that breakdown rates differed significantly across litter species and were correlated with each functional trait. Our analysis of the independent and interactive role of taxonomic, phylogenetic and functional diversity revealed that breakdown of multi-species assemblages could be explained by taxonomic diversity only. Breakdown of mixtures declined significantly with Shannon diversity, but did not vary with either phylogenetic or functional diversity. We offer that indices of functional and phylogenetic diversity do not effectively characterize rates of carbon processing in the streams we studied.  While certain foliar chemical traits did explain breakdown, diversity of these functional traits did not emerge as important.  Furthermore, as has been hypothesized for other systems, phylogenetic diversity was not a predictor of carbon processing.  Our study was novel in that is was the first to examine litter diversity effects from multiple riparian assemblages, with one caveat being that taxonomic diversity here was positively correlated with both functional and phylogenetic diversity. This limited our inference about the roles these two dimensions play in carbon processing.  That stated, neither measure alone explained significant variation in decomposition, suggesting taxonomic diversity of regional floral assemblages plays a significant role in regulating carbon processing in aquatic habitats.