Wetlands provide humans with essential ecosystem services (e.g., water purification, climate regulation, and nutrient cycling); however they are among the most at-risk ecosystems for degradation and subsequent loss of biodiversity. While we know that plant traits such as productivity influence ecosystem functions (e.g., carbon cycling), little is known about how changes in plant assemblages impact wetlands. To address this gap in knowledge, I investigated how plant community composition and diversity affect carbon cycling in freshwater wetlands and assessed a potential mechanism for plant overyielding in diverse communities, resource partitioning of nitrogen. This research was accomplished in two stages 1) an across-site observational study and 2) a controlled mesocosm experiment varying the number and composition of 4 plant functional groups (facultative and obligate annuals, reeds and tussocks).
In the observational study, plant community composition explained more of the variation in carbon cycling processes (e.g., plant production and methane cycling) than diversity. My results indicated that the negative relationship between plant diversity and methane emission found under controlled conditions did not apply to natural wetlands. In the mesocosm experiment, plant biomass was a function of both community composition (facultative and obligate annuals < reeds < tussocks) and diversity. Plant biomass increased from 1 to 4 functional groups due to positive interactions among species. However, complementary use of inorganic nitrogen form was not the mechanism driving plant overyielding, because plant species did not switch their nitrogen form preference with increasing diversity. Rather, plants tended to take up more nitrate with increasing diversity, perhaps due to a positive relationship between nitrification and root biomass. Overall, soil processes in the mesocosms were not changed by plant community composition or diversity; however, methane production was inhibited by reeds and enhanced by tussocks. These results indicate that changes in wetland plant communities will alter plant production and methane dynamics depending on the functional traits of species which are either lost or introduced and by the interactions among species.