Implications of climate change for strategic conservation and restoration of tidal wetlands in the U.S. portion of the Salish Sea

Abstract

Coastal ecosystems are potentially at risk of sea level rise and other accelerated changes in climate. The overall goal of this thesis was to explore the potential influences of spatially varied climate change impacts on tidal wetlands in the U.S. portion of the Salish Sea and discuss implications for strategic conservation and restoration of current and future wetland areas. Since sediment accretion is a vital mechanism for tidal wetland persistence under sea level rise, the overall objective of Chapter 1 was to determine the relationship between sediment accretion rate and surface elevation in a restored and a natural tidal wetland in the Stillaguamish River delta. In the restored zone, there was a negative linear relationship between sediment accretion rates and surface elevation but a quadratic relationship in the reference zone. Vegetation, including dominant vegetation species and vegetation height, also helped explain the pattern of sediment accretion rates. The objective of Chapter 2 was to conduct a spatial analysis of potential tidal wetland responses to future climate change in the U.S. portion of the Salish Sea in order to simulate the (1) overall change in wetland area, (2) potential for tidal wetlands to persist locally, and (3) opportunity for transgressive migration between initial conditions and 2025, 2050, 2075, and 2100 under a low (0.5 m between 2000 and 2100) and high (1.4 m between 2000 and 2100) sea level rise scenario. Total tidal wetland area was projected to decline under both sea level rise scenarios, but some wetland types (e.g., emergent marsh) were projected to expand. Projected local persistence was greater for tidal flat and emergent marsh compared to transitional scrub-shrub and tidal swamp. While the projected area for transgressive migration was small, this process may serve as a buffer for wetland loss by providing dry land for the establishment of new wetland areas. Identifying variability in the adaptive capacity and opportunity for transgressive migration of tidal wetlands to climate change impacts is an important tool for prioritizing sites in order to protect wetlands and enhance their persistence and health into the future along with the ecosystem services they provide. The objectives of Chapter 3 were to model the projected changes in tidal wetlands in the U.S. portion of the Salish Sea without levee protection and to apply the findings of Chapter 2 to a framework of strategic conservation and restoration of tidal wetlands. The projected change in total wetland area between initial conditions and 2100 switched from a decline with levee protection to an expansion without levee protection in the San Juan and Whidbey sub-basins and the Skagit and Stillaguamish River deltas under both sea level rise scenarios. The Skagit, Stillaguamish, and Snohomish river deltas were identified as high priority for conservation and restoration based on historical potential and degradation level under a climate change context, followed by the Nooksack and Samish. In order for conservation and restoration efforts of tidal wetlands to be successful and persist into the future, this study shows that climate change should be considered to identify current and future tidal wetland areas that are projected to exist under the influence of accelerated sea level rise. Identifying priority deltas for tidal wetland conservation and restoration under a climate change framework will be beneficial for the allocation of resources in the short- and long-term.