Biophysical Drivers of Coastal Treeline Elevation

Abstract

Sea level rise is leading to the rapid migration of marshes into coastal forests and other terrestrial ecosystems. Although complex biophysical interactions likely govern these ecosystem transitions, projections of sea level driven land conversion commonly rely on a simplified “threshold elevation” that represents the elevation of the marsh‐upland boundary based on tidal datums alone. To determine the influence of biophysical drivers on threshold elevations, and their implication for land conversion, we examined almost 100,000 high‐resolution marsh‐forest boundary elevation points, determined independently from tidal datums, alongside hydrologic, ecologic, and geomorphic data in the Chesapeake Bay, the largest estuary in the U.S. located along the mid‐Atlantic coast. We find five‐fold variations in threshold elevation across the entire estuary, driven not only by tidal range, but also salinity and slope. However, more than half of the variability is unexplained by these variables, which we attribute largely to uncaptured local factors including groundwater discharge, microtopography, and anthropogenic impacts. In the Chesapeake Bay, observed threshold elevations deviate from predicted elevations used to determine sea level driven land conversion by as much as the amount of projected regional sea level rise by 2050. These results suggest that local drivers strongly mediate coastal ecosystem transitions, and that predictions based on elevation and tidal datums alone may misrepresent future land conversion.

Horizontal Rates of Wetland Migration Appear Unlikely to Keep Pace with Shoreline Transgression under Conditions of 21st Century Accelerating Sea Level Rise along the Mid-Atlantic and Southeastern USA

“…The results of this investigation also suggest that the one-to-one replacement of habitat or ecogeomorphologic cross-shore profile translation [31,33] is unlikely under conditions of 21st century accelerating sea level rise within the study domain, as has recently been reported in the Delaware and Chesapeake Bay regions [54]. The data assembled during this investigation-albeit limited-suggest that the transition will not simply progress inland over time as a well-defined ecotone (see also [33,54,55]). This can be demonstrated through an inspection of the paired rates of marsh migration and forest retreat measured in the studies conducted in Delaware Bay and Northwest Florida (Table 1).…”