Abstract
Barrier islands are complex and dynamic systems that provide critical ecosystem services to coastal populations. Stability of these systems is threatened by rising sea level and the potential for coastal storms to increase in frequency and intensity. Recovery of dune-building grasses following storms is an important process that promotes topographic heterogeneity and long-term stability of barrier islands, yet factors that drive dune recovery are poorly understood. We examined vegetation recovery in overwash zones on two geomorphically distinct (undisturbed vs. frequently overwashed) barrier islands on the Virginia coast, USA. We hypothesized that vegetation recovery in overwash zones would be driven primarily by environmental characteristics, especially elevation and beach width. We sampled species composition and environmental characteristics along a continuum of disturbance from active overwash zones to relict overwash zones and in adjacent undisturbed environments. We compared species assemblages along the disturbance chronosequence and between islands and we analyzed species composition data and environmental measurements with Canonical Correspondence Analysis to link community composition with environmental characteristics. Recovering and geomorphically stable dunes were dominated by Ammophila breviligulata Fernaud (Poaceae) on both islands while active overwash zones were dominated by Spartina patens (Aiton) Muhl. (Poaceae) on the frequently disturbed island and bare sand on the less disturbed island. Species composition was associated with environmental characteristics only on the frequently disturbed island (p = 0.005) where A. breviligulata was associated with higher elevation and greater beach width. Spartina patens, the second most abundant species, was associated with larger sediment grain size and greater sediment size distribution. On the less frequently disturbed island, time since disturbance was the only factor that affected community composition. Thus, factors driving the abundance of dune-building grasses and subsequent recovery of dunes varied between the two geomorphically distinct islands.
Highlights
Barrier islands host critically important ecosystems that provide a number of ecosystem services to coastal communities
We addressed three questions relating to community development in overwash zones: 1) How do plant communities vary among active, recovering and relict overwash zones and adjacent, undisturbed communities? 2) How does plant community recovery following overwash vary between two geomorphically distinct barrier islands? and 3) How does community composition relate to elevation, distance to shoreline, beach width and sediment characteristics? While we addressed these questions for the whole community, we give additional attention to two common dune species—A. breviligulata and Spartina patens (Aiton) Muhl. (Poaceae)—that have previously been shown to have important effects on dune building
We found a total of 29 species across all sites with the most common families being Poaceae and Asteraceae (Table 2)
Summary
Barrier islands host critically important ecosystems that provide a number of ecosystem services to coastal communities. Storm overwash events, where wave runup and storm surge combine to overtop dunes, are important in driving both community composition and island topography [3,6,7,8,9,10]. Overwash may reduce or temporarily eliminate vegetative cover by exposing plants in lowelevation areas (relative to wave height) and/or areas unprotected by dunes to severe, acute stresses such as saltwater flooding, abrasion by water-borne sand and sand burial [4,11,12,13]. Overwash may alter local topography by flattening dunes, a primary source of protection for many barrier island plants [6,7,8,9,14,15]
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