Drivers of Elevation Change in a Retreating Coastal Forest

Abstract The future of coastal marshes depends in part on their ability to migrate inland with sea level rise into coastal forests, but projections of land conversion commonly assume that migration occurs over a static terrestrial topography. Here, we assess the assumption of a static topography in a rapidly retreating coastal upland forest by measuring surface accretion, subsurface expansion or subsidence, and net elevation change across a gradient of salinity and inundation stress. We found that salt-affected forest soils were largely stable over the 5-year study period, exhibiting limited elevation change (–0.2 to 1.5 mm yr −1 ) in most stages of forest health. This stability was achieved by relatively equal and offsetting rates of accretion and subsidence that were each greater than expected. Surface accretion rates were positively correlated with the total cover of Morella cerifera shrubs rather than Phragmites australis , an invasive reed that dominates retreating coastal forests in the mid-Atlantic. Subsidence was greatest in higher elevation forest areas and attributed to root zone collapse that occurred prior to ongoing tree mortality. Net elevation-change rates were highest in the marsh-forest transition zone (2.9 mm yr −1 ), which is characterized by dead trees and fully developed marsh vegetation. Though this study measured relatively low elevation change rates in most portions of a retreating coastal forest, similarity between transition zone and adjacent salt marsh rates suggests that elevation change rates respond quickly to marsh migration and are elevated by the time of extensive tree mortality.