The role of riparian functional and phylogenetic diversity on carbon processing in rivers

Background/Question/Methods

Biodiversity is the foundation for nearly all ecosystem services. How species loss confers changes to the rates of ecosystem functions/services has been the focus of research for decades. Ecologists are embracing different dimensions of biodiversity. Loss of phylogenetic diversity (PD) in a community and subsequent effects on ecosystem processes provides a more nuanced interpretation than loss of taxonomic diversity alone. However, such effects need to be interpreted through the lens of functional diversity (FD) since it is likely species traits explain patterns of ecosystem function better than taxonomic richness or PD alone. In temperate ecosystems, the majority of terrestrial primary production enters the detrital pool. Breakdown and decomposition of senesced carbon, much in the form of leaf litter, and release of nutrients are ecosystem processes responsible for the transfer of energy to higher trophic levels in both aquatic and terrestrial environments. Further, there exists strong interspecific variation in leaf litter foliar chemistries (e.g., functional traits), that are known to drive the decay process. We performed two studies in three streams. The first examined existing riparian community structure, while the second was an experimental manipulation of FD and PD to explore their independent and interactive roles in driving decomposition rates of leaf litter. Armed with this information, we asked the following questions: 1) What are the patterns in functional traits and decomposition for a large suite of riparian species? 2) Does taxonomic, functional or phylogenetic diversity of naturally occurring riparian communities drive rates of decomposition across multiple streams? and 3) by experimentally creating treatments with phylogenetic and functional diversities uncorrelated and holding species richness constant, does functional and phylogenetic diversity explain variation in breakdown rates?

Results/Conclusions

Decomposition rates in mixed species treatments were significantly faster than single species treatments across streams (p<0.001). When decomposition rates were examined in existing riparian communities, multiple linear regression analyses revealed that Shannon diversity of leaf litter assemblages was significantly correlated with both FD and PD (p-<0.001), preventing us from determining whether multi-species breakdown is related to FD and PD. The relationship between FD and PD was addressed with the second study, where FD and PD were independently manipulated and regressed against litter decomposition. PD was the only significant predictor of breakdown (p=0.012), and despite a positive relationship with FD, the correlation was not significant (p=0.182). We conclude that PD can provide further insight into how biodiversity explains carbon processing in rivers.