Functional type changes over a disturbance gradient of overwash sites on a Virginia barrier island

Background/Question/Methods

Barrier islands are dynamic landscapes prone to disturbance including storm induced overwash events. Dune overwash physiographically alters the landscape and leads to displacement of resident species. Stability of barrier island systems depends on successful recolonization of overwash areas and development of distinct geomorphic features and gradients of plant zonation. Certain plant functional types reflect response to variation in environmental drivers of plant zonation and have an advantage in recovery and recolonization of disturbed zones. Our study aims to understand how functional type selection varies over a disturbance gradient of overwash sites. The objective is to determine if plant functional type reflects time since disturbance from overwash, indicating that stresses of overwash alter plant zonation.  Species composition and percent cover of active, intermediate, and reference dune sites on a Virginia barrier island (Metompkin) was collected over two summers in 2011 and 2012. Species composition data were regrouped into distinct functional types based on salinity and water tolerance. Nonmetric Multidimenisonal Scaling (NMDS) with Sorensen distance was used to identify patterns in site locations and relationships with plant functional types. 

Results/Conclusions

A total of 16 species were found across the six field sites on Metompkin Island and classified into 7 functional types. Grasses represented 80% of the total functional type composition of vegetative cover.  Dune reference sites reflected presence of vegetation specifically adapted to exposure of salt water and drought conditions. Intermediate sites had the most functional type diversity of habitat specialists including species adapted to a range of environmental conditions. Active sites contained habitat generalists that were able to tolerate a range of salinity and water availability. The recovery of plant communities following storm overwash is a complex process and is based on environmental drivers that select for species able to tolerate physiological changes in the environment. Using plant functional type revealed more information than species composition. This likely reflects limited dispersal in a physically dominated system as more than one species can occupy the same niche.  Future studies in barrier islands should focus on functional types to link pattern to process in dune and coastal succession.