Tidal wetland vegetation in the San Francisco Bay Estuary: modeling species distributions with sea-level rise
Tidal wetland ecosystems are dynamic coastal habitats that, in California, often occur at the complex nexus of aquatic environments, diked and leveed baylands, and modified upland habitat. Because of their prime coastal location and rich peat soil, many wetlands have been reduced, degraded, and/or destroyed, and yet their important role in carbon sequestration, nutrient and sediment filtering, flood control, and as habitat requires us to further research, conserve, and examine their sustainability, particularly in light of predicted climate change. Predictions of regional climate change effects for the San Francisco Bay Estuary present a future with reduced summer freshwater input and increased sea levels, resulting in higher estuarine salinities throughout the growing season, increased saline influence in brackish and freshwater marshes, and increased depth and duration of inundation. Experimentally testing, monitoring across scales, and spatially modeling the responses of dominant wetland vegetation to the substantial predicted climate change effects are among the critical threads of knowledge needed to understand how this estuary and others along the Pacific coast might respond to significant changes in physical drivers and community interactions. My dissertation research focused on possibilities for wetland resilience in a changing climate in the San Francisco Bay Estuary across scales and using a suite of methodologies.
Tidal wetland resilience to predicted sea-level rise requires an understanding of both individual plant and community-level responses in addition their interactions with sediment supply and adjacent land uses. Through a large field experiment simulating sea-level rise, I found that wetland plants have a high tolerance for increases in inundation in the short term and that community interactions need to be incorporated into plant responses to increased sea-level rise. Scaling measurements of plant production up to the site level and across landscapes requires the integration of field measurements with remotely sensed measurements. Investigating remote sensing techniques of measuring carbon stock, I found that the presence of dense standing plant litter common in Pacific coast freshwater wetlands can hinder the ability to find a reliable way of measuring plant production remotely. Finally, I was able to successfully calibrate an ecogeomorphic mechanistic model for wetland accretion across four wetlands in the San Francisco Bay Estuary and examine potential wetland resiliency under a range of sea-level rise scenarios. At sea-level rise rates 100 cm/century and lower, wetlands remained vegetated. Once sea levels rise above 100 cm, marshes begin to lose ability to maintain elevation, and the presence of adjacent upland habitat becomes increasingly important for marsh migration. Results from this study emphasize that the wetland landscape in the bay is threatened with rising sea levels, and there are a limited number of wetlands that will be able to migrate to higher ground as sea levels rise. Despite these challenges, my dissertation presents a robust and new understanding of how tidal wetlands might respond to predicted climate change.