Drivers Of Elevation Change In A Retreating Coastal Forest

With a diminishing sediment supply and accelerating rates of sea level rise, the future of coastal marshes increasingly depends on their ability to migrate to adjacent higher elevations. Future projections of global marsh extent suggest that marshes could expand by up to 60% or contract by up to 90% depending on the success of that migration. These projections assume a static topography, where the gradual inundation of a passive terrestrial landscape results in widespread conversion of uplands to marsh. However, coastal ecosystems dynamically respond to increases in inundation through vertical accretion and enhanced belowground productivity. Similarly, coastal upland forests may gain elevation through enhanced litter deposition and belowground productivity of transitional species such as Phragmites australis and Morella cerifera or subside due to flooding induced tree root zone collapse. In this study, I expand the coastal geomorphic framework into a salt impacted transitional forest using a series of Surface Elevation Tables (SETs) across a gradient of salinity and inundation stress to better understand the processes controlling coastal forest elevation. My study found that over short timescales, salt impacted forest soils are stable, exhibiting little to no total elevation change. However, this stability was a result of greater than expected, but offsetting rates of accretion and subsidence. Documented accretion rates in both high and mid forest zones exceeded nearby high marsh and were strongly correlated with the total cover of Morella shrubs. Subsidence, attributed to root zone collapse, was maximized in the high forest and minimized at lower elevations partially due to sub-root zone expansion. This study shows that conversion of forested upland to high marsh ultimately increases elevation change rate through both the minimization of subsidence and increase in surface accretion. Thus, the incorporation of a dynamic upland topography may better predict future marsh extent.