Sea levels in California, south of Cape Mendocino, are predicted to rise between 90 and 95 cm between 2000 and 2100, with a range as high as 167 cm and as low as 42 cm, according to a 2012 report by the National Academy of Sciences.

Whatever the ultimate rise in sea level is, it will largely control the fate of tidal marsh habitats in the San Francisco Bay-Delta.

Tidal marshes are wetlands periodically inundated by tidal flows and, unlike mudflats, are vegetated. They provide overwintering and foraging habitat for resident and migratory birds, and are a buffer against floods and storm surge. They also filter and remove pollutants.

Only about 5 to 10 percent of the delta’s historic tidal marshes remain. Most fringe an aging collection of levees. Former wetlands— now diked, behind levees and dry—are sinking and below sea level by as much as 20 m in some places.

Plans are underway to restore thousands of acres of tidal marshes. Whether these marshes are sustainable in the long term will be deter- mined by their ability to build elevation or migrate to higher ground.

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.

Early restoration efforts in San Francisco Bay focused primarily on establishing appropriate elevations for plant recruitment, based on plant distributions in natural wetlands. Sites were graded and planted, and tidal connections were re-established with the expectation that restored wetlands would quickly resemble natural ecosystems. Over time, restoration efforts have evolved, with the realization that natural development of restoration sites is preferable, including a dense channel network and the accumulation of soils of appropriate texture. Bay restoration efforts also have grown substantially in size and scope. Whereas projects of 50 hectares were considered large in the 1980s, now many projects are 100s of hectares. Larger projects are on the scale of 1000s of hectares, with the largest approximately 6000 hectares (the South Bay Salt Pond Restoration Project). This massive increase in scale has brought enormous restoration opportunities, but it also has increased the complexity of restoration projects and highlighted the necessity of large-scale public involvement. Awareness of non-native plants at restoration sites is just one example of factors that have increased restoration complexity. Potential impacts of climate change also have moved to the forefront of restoration design, as sea-level rise and potential shifts in salinity are critical factors for long-term restoration planning.

Climate change will affect tidal wetlands with higher rates of sea-level rise and higher concentrations of salt in brackish and freshwater tidal systems, in addition to causing increases in atmospheric CO2 concentration, warmer temperatures, and shifts in precipitation. In the San Francisco Bay–Delta, the areas most likely to be affected—brackish and freshwater tidal wetlands—are also the sites with the majority of endemic plant species and the greater biodiversity and productivity. Effects on the San Francisco Bay– Delta estuary are complex and difficult to predict, but a few things are clear. Biodiversity of the tidal wetland system in the San Francisco Bay–Delta region will decline, with subsequent effects on ecosystem functioning and services. Altered plant production, physiological tolerances, and shifts in rates of mortality will modify wetland plant communities in ways not yet predictable. Lower ecosystem productivity from salinity increases will affect both primary and detrital-based food webs. Such changes will cascade via the food webs into invertebrate, bird, and pelagic systems. Tidal wetlands are especially sensitive to processes that climate change will alter. Several of these altered processes are exacerbated by water diversions from the Delta.

We analyzed tidal wetland vegetation patterns in the San Francisco Bay–Delta estuary (Bay–Delta) and discuss their significance for future conservation. Our objective was to conduct a balanced, random sample of six historic “remnant” tidal wetlands along a salinity gradient that ranged from southern San Pablo Bay to the lower Delta. We also wanted to compare diversity among these sites at five different scales, ranging from small subplots to total species per site. We randomly established twenty 0.1-ha plots at each site, and sampled ten 7-m2 circular subplots (subplot scale) in each plot for species presence and cover. We calculated total species per plot (plot scale) and total species per site (site scale) based on these subplot data. We calculated importance values for each species found in subplots based on frequency of occurrence and proportion of total cover at each site. In addition, we recorded species found within the 0.1-ha plot but not sampled in subplots (plot_all scale), and calculated the total species for each site based on these plot_all data (site_all scale). Thus, richness at each site was evaluated at five different scales that ranged from 7 m2 to 20,000 m2. We conducted a one-way ANOVA that compared mean richness among three scales with continuous data: subplot, plot, and plot_all. At lower estuary (San Pablo Bay) sites, richness values at these three scales were significantly lower than upper estuary (Suisun–Delta) sites. In Suisun–Delta sites, significant differences in richness varied, depending on sampling scale. Rush Ranch, a brackish wetland, had the highest average number of species per plot, whereas Sand Mound Slough, a freshwater wetland, had the highest average number of species at the subplot scale and the most total species at the site_all scale. Sand Mound Slough also had the highest number of unique species (22) compared to the other sites. We found a strong negative correlation (r2 = 0.99) between total species number and salinity (the most in the Delta at the lowest salinity, and the least at China Camp with the greatest salinity). Our analysis suggests the following conservation recommendations: (1) restore habitat for freshwater plant species diversity in the Delta; (2) restore transitional brackish wetland habitat in salt marshes of the San Pablo Bay and San Francisco Bay; and (3) focus on control of pepperweed (Lepidium latifolium) in brackish marshes.