Freshwater marshes, transition areas between land and water, are critically important in the functioning of watersheds and estuaries. The existence of these ecologically important areas is threatened by rapid environmental changes that disrupt the feedbacks between the physical and biological processes critical to marsh formation. Remote sensing of marsh surface elevations, extensive field observation of the interactions between elevation, sediment dynamics and vegetation, and spatially explicit modeling will be used to forecast the impact of global environmental change on freshwater marsh plant diversity, sediment deposition and erosion, and, hence, the maintenance of complex marsh surfaces in a changing world. The development of coupled geomorphic-ecological models of marsh surface evolution, models which are still uncommon and do not explicitly consider the biological complexity of marsh ecosystems, will be advanced by the interdisciplinary nature of the research. In addition, consideration of spatial processes (landscape heterogeneity, disturbances, and local dispersal of plant propagules), and their impacts on plant biodiversity, will contribute to biodiversity theory. The simultaneous consideration of biodiversity theory and geomorphology is a unique integration of two traditionally separate disciplines that should offer tremendous advances in both fields.
Using a tidal freshwater marsh in the Potomac River estuary as a natural laboratory, results will be used to inform ongoing efforts to restore the study site and freshwater marshes in general. Resource managers and restoration practitioners working to preserve and restore an important marsh ecosystem near the nation's capital will be engaged as partners in this project. Additional outreach will result from participation in a teacher internship program, and training will be provided for graduate students and for young scientists working with senior investigators.
Tidal marshes maintain their position within the tidal frame through the deposition of mineral and organic matter that counterbalances sea level rise and elevation losses. To effectively protect and restore marshes within the estuarine landscape, this ecogeomorphic balancing act needs to be firmly understood to identify which environmental factors are most likely to destabilize it. This project closely examined ecogeomorphic feedbacks in tidal freshwater marshes by working across scientific disciplines (ecology, geology) and approaches (observations, experiments, modeling). The rate of surface elevation change in marshes is attributed to the length of time a marsh surface is inundated during a tidal cycle. Under this model, sediment supply is greater to low-lying areas, resulting in faster elevation gain at lower elevations. We tested this hypothesis using repeated real time kinematic GPS field surveys combined with a digital elevation model from a Light Detection and Ranging (LiDAR) aerial survey. As expected, these data showed that the average rate of elevation change was similar to sea level rise rate and that elevation gain indeed decreased at higher elevations. However the relationship was not monotonic. Rather, two local maxima of elevation gain were observed at -0.2m and 0.3m (NAVD88 using GEOID09) elevation. Whereas the observed –0.2m local maximum can be attributed to greater sediment supply and deposition in low marsh areas, we hypothesized that the observed 0.35m local maximum is influenced by the biophysical complexity of the plant community. We tested this hypothesis by measuring sediment deposition at 27 locations at monthly (sediments deposited on tiles) and seasonal (7Be ) time scales and relating observed deposition rates to the structure of marsh plant communities. Tile and 7Be-derived sedimentation rates showed similar spatial trends with highest rates near tidal channels and lowest rates in the marsh interior. At 0.35m elevation, the marsh plant community is the most diverse in species and function, as it transitions between two species-poor assemblages at low elevations (dominated by Nuphar lutea) and high elevations (dominated by Typha angustifolia). Within the transition zone, stem density and surface roughness are high, allowing sediments to be filtered and to settle before reaching higher elevations. If inorganic sediments are deposited before reaching high marsh elevations, other processes must allow higher elevations to maintain their elevations within the tidal prism. We therefore tested the hypothesis that plant litter contributes organic matter to soil development at high elevations. We conducted a field and remote sensing analysis of plant litter height, vertical cover and stem density (collectively called structure) and observed that plant litter structure increased with elevation. These spatial patterns lead to corresponding patterns in soil organic matter, revealed by measuring loss on ignition of surface sediments. The amount of mineral material embedded within the plant litter decreased with elevation, suggesting that, while mineral sediment deposition is important at low elevations, plant litter deposition and incorporation into the soil surface is an important process at high marsh elevations to maintain elevation in tidal freshwater marshes. We established that elevation and biophysical complexity influences sediment accumulation and elevation change, but does sediment deposition and elevation change feed back on plant communities? We tested the hypothesis that elevation change and sedimentation (as inorganic and organic material) affects plant community structure using repeat field observations of vegetation communities at 57 locations and replicated greenhouse experiments. Throughout all studies we consistently observed strong effects of sediment and litter deposition, and resulting elevation change, on the germination of individuals, abundance of species, and, hence, community turn-over. Thus, as the elevation landscape shifts in space and time, so does the plant community, with feedbacks on marsh surface evolution. We modeled the observed ecogeomorphic feedbacks using a spatially-explicit simulation model of tidal marsh plant communities, MCAP. MCAP includes species-specific life-history functions that are parameterized with empirical data of 16 tidal freshwater marsh plant species. Model experiments manipulate habitat, sediment deposition, weather, and wind or animal disturbances to explore patterns of plant abundance, diversity, and persistence. The MCAP modeling approach results in stable and realistic communities that do not rely on specific trade-offs for the development of species-rich communities. The study system is slated for restoration as half of the marsh was lost to sand and gravel dredging between 1930 and 1980. Marsh loss is ongoing in the form of shoreline erosion. During the project period, we have interacted with NPS managers by sharing data and results, communicating findings, participating in a management workshops, and serving as scientific advisers. Through these efforts, the NPS has established a target marsh elevation. In addition, we have trained high school students and their teachers, and undergraduates through REU and RET funds. We have also mentored a postdoctoral research scientist now employed at an academic institution.
