What’s Going on Down There? Impacts of Long-Term Elevated CO2 and Community Composition on Components of Below-Ground Biomass in a Chesapeake Bay Saltmarsh

Roots and rhizomes play diverse roles in the response of coastal wetland ecosystems to climate change through hydrobiogeomorphic and biogeochemical processes. The accumulation of living and dead belowground biomass contributes significantly to surface elevation gain, redox status through root oxygen loss and exudates, and plant transport of greenhouse gases to the atmosphere. Yet, responses of belowground biomass to global climate stressors are difficult to measure and remain poorly understood. Here, we report on the response of individual components of belowground biomass to 12 years of CO2 enrichment in a temperate tidal marsh. In both a community initially dominated by the C3 species Schoenoplectus americanus and another initially dominated by the C4 species Spartina patens, elevated CO2 increased total belowground biomass and subtly altered depth distributions of some components. In the Spartina community, this effect was the result of the direct effects of CO2 on plant biomass allocation, while any direct response in the Schoenoplectus community was difficult to detect because of changes in the relative abundance of C3 versus C4 species. In the Schoenoplectus community, belowground biomass was positively related to S. americanus stem density. Compared to the C4 community, the Schoenoplectus community had higher root and rhizome biomass and deeper rhizomes. These results highlight the importance of community composition and plant functional traits in understanding ecosystem- and community-scale responses to elevated CO2 and their potential impacts on marsh elevation.