Reconstructing Coastal Forest Retreat and Marsh Migration Response to Historical Sea Level Rise

Climate change assessments predict that rates of relative sea level rise will increase in the future, leading to enhanced inundation of low-lying coastal regions and a 20 – 50 % decline in salt marsh area by 2100. Global sea level rise began accelerating in the late 19th to early 20th century, and local rates along the U.S. mid-Atlantic coast are twice as fast as global estimates. Frequent flooding and salt stress associated with sea level rise lead to coastal transgression, and the survival of ecosystems depends on their ability to migrate inland faster than they erode and submerge. Here, I compared aerial imagery analyses and field measurements to test the hypothesis that marsh migration into retreating terrestrial forests is fundamentally tied to sea level rise, and that sea level rise does not necessarily lead to overall habitat loss. For my first chapter, I compared the areal salt marsh extent between historical topographic maps and modern aerial imageries across the entire Chesapeake Bay, and found that marsh migration into terrestrial forests largely compensated for marsh erosion at the seaward edge during the last century. This emphasizes that the location of coastal ecosystems changes rapidly on centennial timescales, and that sea level rise does not necessarily lead to overall habitat loss. For my second chapter, I reconstructed the position of coastal treelines through time at five study sites along the U.S. mid-Atlantic coast to identify long- and short- term drivers of coastal forest retreat. My findings suggest that 20th century migration rates greatly exceed pre-industrial rates (\u3c 1875 CE), and have generally accelerated throughout the last century in parallel with accelerating rates of relative sea level rise. Previous work predicts widespread marsh loss as a response to sea level rise, but underestimates the potential for marshes to migrate inland. Although anthropogenic barriers may locally prevent marsh migration into retreating coastal forests, my work finds that about 400 km2 (100,000 acres) of uplands have converted to marshes in the Chesapeake region since the late 1800s, and that this process was responsible for the formation of about 1/3 of all marsh area. Beyond the Chesapeake, my work reveals that forest retreat is fundamentally tied to the rate of sea level rise, and is accelerating through time. Therefore, management efforts that allow marshes to migrate into adjacent uplands may help preserve marshes by exploiting their ability to quickly adapt to environmental change