Date Published: May 2, 2019
Publisher: Public Library of Science
Author(s): William S. Kearney, Arnold Fernandes, Sergio Fagherazzi, Vanesa Magar.
The retreat of coastal forests as sea level rises is well documented; however, the mechanisms which control this retreat vary with the physical and biological setting of the interface between tidal marsh and forest. Tidal flooding and saltwater intrusion as well as flooding and wind associated with storms can kill trees. Even if these processes do not kill stands, they may halt regeneration because seedlings are more sensitive to stress. We present a case study of a coastal pine forest on the Delmarva Peninsula, United States. This forest contains a persistent but nonregenerating zone of mature trees, the size of which is related to the sea level rise experienced since forest establishment. The transgression of coastal forest and shrub or marsh ecosystems is an ecological ratchet: sea-level rise pushes the regeneration boundary further into the forest while extreme events move the persistence boundary up to the regeneration boundary.
As sea-level rise accelerates [1–3] and regimes of precipitation [4–7] and storms [8–11] change, the spatial distribution of coastal ecosystems will also change. Exactly how the patterns of salt marshes, mangroves, freshwater marshes, tidal forests and upland environments evolve depends on complex interactions between physical forcings (sea level, precipitation and storms) and ecological processes (e.g. growth, competition, regeneration) in each of these ecosystems. The retreat of coastal forests as sea level rises is well documented [12–16]; however, the mechanisms which control this retreat vary with the geomorphological, hydrological, and ecological setting of the marsh-forest interface. Trees can be killed by increased tidal flooding, either because they are not flood-tolerant or because they are not tolerant to the salinity of flood waters. Saltwater intrusion in which the groundwater table becomes more saline, whether due to sea-level rise or groundwater withdrawals, can also lead to forest decline . Droughts can amplify these effects by reducing the supply of fresh water . Even if these processes do not kill trees, they may halt the regeneration of the forest because seedlings can be more sensitive to environmental change and variability [13, 18–20]. At the same time, there exist physical mechanisms, such as fresh groundwater inputs from the upland, and ecological mechanisms, such as competition for light, which can slow or even stop forest retreat [13, 21–25]. There is therefore a distinction between the regeneration niche , in which seedlings are able to establish and the persistence niche , in which mature plants that have already established maintain their position. These niches manifest on a heterogeneous landscape as demographic patterns in vegetation zonation.
The hydrology of the two sites is markedly different, despite being only 100 m apart. The hillslope site is inundated only by very large storms, and the lower part of the forest regularly stands over saline groundwater. The hollow site is inundated seasonally, and saline water is introduced only by very large storm surges. The hydrological processes that lead to demographic patterning in the pine forests at each of the sites are therefore very different. The hillslope site zonation is controlled by the dynamics of the subsurface freshwater-saltwater interface and by salinity introduced by storm surges. The hollow site zonation is controlled primarily by the seasonal fluctuations in water depth in the hollow with a possible secondary role for salinity during storm surges. The absolute elevations of the two regeneration boundaries are determined by the local hydrological setting. The lower elevation of the regeneration boundary at the hollow site indicates that it is more tolerant to flooding, perhaps because the water at the hollow site is consistently fresher than the water at the hillslope site. However, the similarity in the persistent zone size at both sites suggests that, while local processes control the absolute position of the marsh-forest boundary, a larger-scale driver, namely sea-level rise, controls the vegetation zonation observed in the boundary.