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.

Plain Language Summary

As sea level rise (SLR) drives saltwater further inland, terrestrial ecosystems change to tidally controlled ecosystems. A common ecosystem transition is coastal forest conversion to marsh, which forms ghost forests, characterized as dead trees surrounded by marsh. Most projections of SLR assume that the boundary between forest and marsh can be defined simply by the furthest landward extent of the tide. However, forest to marsh conversion can be influenced by other physical processes and vegetation interactions. Here we analyze the location of the marsh‐forest boundary across the entire Chesapeake Bay, defined using 100,000 elevation points, alongside environmental variable data sets to determine drivers of coastal forest retreat. As the largest estuary in the U.S., the Chesapeake Bay provides a study area where the elevation of transition from forest to marsh varies substantially. We find this variation in elevation to be driven by not only tidal range, but also soil salinity and slope of the land, yet these variables explain <50% of the variability in elevation. This suggests that local factors unaccounted for in this study also strongly influence the retreat of coastal forests, even at regional scales. Therefore, projections of SLR that rely solely on tidal extents may misrepresent future land conversion.

Key Points

• Treeline elevations increase with tidal range, salinity and slope, but are not correlated with climate or marsh characteristics

• Macro‐scale drivers account for <50% treeline elevation variability indicating that local factors mediate estuary‐scale sea level responses

• Offsets between treeline elevation and tidal datums suggest that standard sea level rise projection methods may misrepresent land conversion