Assistant Professor Virginia Commonwealth University Richmond, Virginia, United States
Background/Question/Methods
Fronting 15% of coastlines globally, barrier islands protect >41.5 million people and highly productive wetlands along the North American Atlantic coastline from storms. These landforms are vulnerable to drivers such as storm erosion, sediment loss, and sea level rise (SLR). Ecological processes including plant growth, competition and facilitation interact with these drivers influencing barrier-island ecosystem state change. This state change determines island structure and function, including island migration with SLR. On the Virginia barrier islands, a native shrub, Morella cerifera, is encroaching into grasslands. Most of what is known about Morella cerifera encroachment has been discovered by studying the adult stage of development. This current research addresses a research gap; understanding the factors that affect shrub seedling recruitment and survival. New grassland swales on the south end of Hog Island have provided a unique opportunity to understand how and where shrub encroachment occurs. We test the hypothesis that both dune elevation and grasses influence the establishment of M. cerifera shrubs in the early life stages by experimentally planting M. cerifera seedlings behind three different dune elevations at two densities of grass cover. Abiotic (i.e., temperature and soil chlorides) and biotic (i.e., seedling growth and survival) variables were quantified.
Results/Conclusions
In swales where M. cerifera seedlings naturally occur, distance from the ocean and dune height were the best predictors of where seedlings establish. The most protected swale was the only location where seedlings were found. Soil chlorides were higher in this swale during the summer months, but values were within the tolerance range of M. cerifera. Most seedlings established in areas with grass cover < 40% and no seedlings were found with grass cover >95%, demonstrating grass density and dune elevation strongly impact the naturally occurring spatial distribution of shrub encroachment. M. cerifera plantings exhibited greater height growth and growing season survival behind the tallest dune ( >2m tall). There was no significant difference in survival found between high- and low-density grass, suggesting that grass cover may be more important during the winter months. Previous studies show a warming effect in microhabitats covered by grasses and M. cerifera is sensitive to colder temperatures. Future work will evaluate temperature modification by grasses and shrub survival over the winter. Results show the importance of dune height and grass cover to M. cerifera establishment. The increase in the spatial distribution of M. cerifera has significant consequences for Virginia barrier islands with sea-level rise.
