SOIL ANALYSIS OF MID-ATLANTIC BARRIER ISLAND GHOST FORESTS ON ASSATEAGUE ISLAND, VIRGINIA: IMPLICATIONS FOR MODELING THE IMPACTS OF SEA-LEVEL RISE

Sand-formed barrier islands such as Assateague Island (AI) are common along passive margins and protect the mainland, estuaries, and salt marshes behind them. In doing so, these islands are vulnerable to the erosive effects of storms and sea level rise, especially when sparsely vegetated. When present, dense maritime forests are critical to stabilizing the core of barrier islands. They add organics to the soil, bind sediment with their roots, reduce wind velocity and baffle wind-blown sediment to help build elevation. Nevertheless, as storm processes, including surge and wave action, batter barrier islands sediment can be moved offshore where it is lost to the active littoral zone. On AI and neighboring islands, annual retreat of the ocean side shoreline ranges from 3 to 7 m/yr. In addition, storm surge has inundated backshore areas to elevations of up to 2.5 m above MSL and has flattened natural dunes. Surge waters have also seeped into upland soils and are here shown to have longer-lasting effects months to years after closely-spaced storm events.

Here we report on the loss of Loblolly pine forests on AI. Stands of dead and dying pine trees, “ghost forests,” continue to expand. Data from over 200 soil samples from four distinct areas taken since Hurricane Sandy reveal differences in soil chemistry between healthy and ghosted forests. Regardless of grain size, and proximity to open water, healthy forests are found at slightly higher elevations and are dominated by low pH (4 to 5.5). In contrast, ghosted areas are located at lower elevation (although not always) and have higher pH (5.5 to >7.0). These data suggest reduced activity of natural acids and increased roles of alkaline salt water are contributing to forest loss. In some areas on AI, tree loss may also be exacerbated by the effects of gallic acid toxicity from the roots of the invasive common reed Phragmites australis.

In the future, consequences of forest lost are still unknown, but it is likely that the impacts of sea level rise will be more pronounced and erosion may accelerate if maritime forests on barrier islands are lost permanently. Thus, coastal modeling, using SLOSH and SLAMM and other models, will need to consider the growing concern we have for rapid loss of maritime forests and potential accelerated erosion rates on both the beach and marsh environments.