The Smithsonian Institution’s Global Change Research Wetland (GCREW) is a tidal brackish marsh on the Chesapeake Bay that supports three long-term experiments (5-29 years) and a sea level rise mesocosm facility. The site is dedicated to understanding the ecological processes that confer elevation stability on coastal marshes as they respond to global environmental change. We present a new conceptual model for the site that unifies the ecological interactions between plant traits, eCO2, nitrogen loading, and relative sea level (RSL).
The model recognizes plant traits as the single most important biological feature regulating carbon sequestration and elevation gain, largely controlling microbial processes governing decomposition. The model identifies sea level as a master variable that regulates the response of the system to elevated CO2 (eCO2) and N loading. Sea level acts through plant production and community composition to regulate soil surface elevation. The model is supported by a variety of observations. The current decadal RSL trend at the site is three times higher than the long-term trend, and plant communities have shifted towards the more flood-tolerant sedge species. Previous results from the site show that eCO2 can increase elevation gain in communities dominated by a C3 sedge; we now show that this effect is transient and may fade during periods of rapid RSL rise due to flooding stress. We previously demonstrated that nitrogen addition favors the C4 grasses in this system, but this effect is also transient during the recent period of rapid RSL rise. Rates of decomposition in this continuously saturated system appear to be largely regulated by plant traits that control the delivery of labile carbon and molecular oxygen to otherwise anaerobic soils, and that even old pools of soil organic matter are subject to increased rates of decomposition in the presence of live roots. Soil elevation data show that the native marsh community is not keeping pace with RSL rise under any treatment, a result that may indicate the early stages of marsh collapse. Simultaneously, non-native Phragmites australis is expanding rapidly in the marsh. Our data indicate that this expansion will accelerate due to eCO2 and nitrogen loading, and that the only experimental plots keeping pace with RSL rise are Phragmites-dominated areas to which nitrogen fertilizer has been added.