Barrier islands are at the forefront of global climate change, vulnerable to both sea level rise and the threat of increasingly frequent and stronger oceanic storms. These dynamic landforms provide valuable ecosystem services ranging from protection of the mainland from storms and wave erosion, creating and sheltering back bays and estuaries that support productive oyster and fishing industries, to providing critical habitat for many endangered organisms. A key feature of barrier islands is the capacity to “rollover” (i.e. overwash of sediment onto existing marsh platform) and migrate landward due to sea level rise. Recent expansion of woody vegetation into historic grassland may block sediment transfer, limiting rollover and influencing island resilience. Our objective was to assess state change from marsh to barrier island upland at the Virginia Coast Reserve, LTER over 32 years (1984-2016). Using ten islands, we performed classification of Landsat imagery in 1984, 1998, 2011 and 2016. We quantified transitions between classes (bare, grassland, woody, marsh and ocean) in island subsections, comparing rates of marsh change to upland with and without woody vegetation. We also quantified multiple correlating variables including island width, elevation, and amount of vegetation cover.
Over the time period back barrier marsh area was reduced by 20%. Of total marsh, more converted to ocean (21%) than to upland (16%), likely driven by increased sea level rise. During the same time period, woody vegetation cover increased by 40%. Across all islands, logistic regression showed a significant relationship between movement of the marsh upland boundary and presence of woody vegetation (p<0.001). Presence of woody vegetation significantly reduced marsh transition when accounting for island width (p=0.02). Although state change varied from island to island, several major trends were highlighted across the entire system; overall marsh and island area was lost, an increasing portion of marsh loss could be attributed to the combined effects of woody expansion and sea level rise rather than overwash. Measuring these changes are key to understanding and modeling response of barrier islands to sea level rise and storm events. Expansion of woody vegetation reduces barrier island resilience by limiting conversion of marsh to upland. Woody vegetative cover and the proportion of marsh area lost to ocean are indicators that barrier islands may be unable to keep up with the high rate of sea level rise in coastal Virginia.