Wednesday, February 3, 2016

Decadal change in Virginia barrier islands

By: Julie Zinnert

It is easy to see how dynamic the landscape is working at the Virginia barrier islands. Dramatic changes to shoreline, dunes, and interior swales can happen in a matter of weeks. One offshore hurricane (like Hurricane Joaquin) can completely destroy a master's dune grass experiment. New shrub thickets can develop over three years in what was previously grassland. Seeing this change is exciting for students and myself after working on the islands over the last ten years.
Recently, we analyzed several of the Virginia islands to see how the islands and communities are changing at the decadal scale. Our results were astonishing! We knew from previous work that woody vegetation has been expanding on many of the islands. In fact, shrub loss only occurred on islands that had lost island area. In a soon to be published paper in Ecosystems, we took a closer look at the upland communities. What is happening with grassy swales? Are shrubs only expanding? Are there any patterns of change consistent across the islands?
What we found was astounding! From the period between 1984 to 2011, all of the islands lost upland area - some more than others (ranging from 12% on Cedar to 62% on Cobb; 29% total area loss from all islands). This is similar to the amount of ice loss from the Arctic over the timeframe. Surprisingly, this amount of land loss was seen over 16 decades in the Gulf Coast barrier islands (not 3 as in Virginia)!

Shoreline migration is how barrier islands respond to sea-level rise. Shoreline hardening to protect economic infrastructure poses a challenge for the future of barrier islands. We first need to understand how barrier islands naturally adapt and change to rising sea level so that we can implement more green solutions in developed islands. The Virginia Coast Reserve provides a rare and unique site for research as the islands are undeveloped.
In our analysis of barrier island change, we found three major patterns of land cover transitions: complete loss of vegetation (these islands are actively migrating onto the marsh platform), frequent transitions between grass and woody vegetation, and increase in woody vegetation. It appears that islands with extensive woody vegetation do not migrate, but rather lose area through shoreline erosion. Woody vegetation may decrease resilience of barrier islands in the same way that human infrastructure does. This is exemplified in Cobb Island (see picture below). Cobb Island had very low woody cover in 1984. Between 1984 and 2011, woody area increased (over 100 ha) while the island lost over 400 ha. Extensive shrub death can be seen at the shoreline. This vegetation prevents overwash of sediments onto the backbarrier marsh. Cobb Island has had virtually no migration onto the marsh during the timeframe.
Shrub death is visible at the shoreline as Cobb Island erodes (a). Loss of upland area and increase in woody vegetation (red) is visible from satellite imagery on Cobb Island in 1984 (b) and 2011(c).
We are beginning to document linkages that occur across island processes. This is an important step as we scale from local level measurements to island and island-chain level response. The relationship between species and dune building is important to determine the species composition of interior swales. Interior swale communities affect sediment transport, island migration, and marsh dynamics. Thus, the ecological function of barrier island upland communities is highly relevant beyond the barrier island landscape.