COS 26-3
Cross-island comparison of temporal variations in shrub-grassland bistability at the Virginia Coast Reserve
Coastal barrier systems may be among the most sensitive of all ecosystems to effects of global climate change as ecological processes are closely coupled to both atmospheric and oceanic drivers. Ecosystem state changes are large-scale, abrupt shifts to a new state that may not be as easily reversible. Recent worldwide phenomenon of shrub expansion into grassland has occurred so rapidly that underlying mechanisms are considered to be bistable with grass and shrub dominance representing alternate stable states. In coastal ecosystems, especially barrier islands, abrupt grass-to-shrub transitions are critical as these systems depend on soil freshwater lens to support vegetative communities. Woody dynamics may serve as sentinels to climate change on barrier islands considering impacts of predicted increases in sea level, storm frequency and intensity, and alterations in precipitation patterns on geomorphological/ecological processes. We examined island-scale conversion of land (i.e. sand to grassland to woody cover) while relating importance of climate variables on rate of woody expansion on seven barrier islands at the LTER Virginia Coast Reserve. Using Landsat imagery, we monitored changes in island size and vegetation classes at three time points: 1984, 1998, 2011.
Results/Conclusions
Across the Virginia barrier islands, each island differs in geomorphology and landscape community structure, yet similar patterns of change emerged. Over the study period, total terrestrial upland area was reduced by 32% across all islands, but varied from 3 to 62% for individual islands. From 1984 to 2011 bare sand was reduced by 41% and grassland by 29%, while woody cover increased by 40%. Comparisons among islands revealed non-linear dynamics in land cover changes. Of the seven islands studied, two had reductions in woody cover due to erosion along the long shoreline axis. Islands that experienced little relative change in shoreline position experienced the greatest transition from grassland to shrubland. The rate of change in woody cover was greatest between 1984 and 1998, which was related to significant increases in minimum winter temperature and precipitation. Increase in shrub dominated communities indicates stability across the barrier island landscape with both erosion and accretion taking place, while loss of shrubland is indicative of an island that is uniformly eroding. Shrub sensitivity to salinity and flooding may reveal bistability tipping points at which sea-level rise and changes in precipitation will lead to conversion back to grassland. Our results suggest that woody species are responsive to climate change, thus, serving as sentinels on Virginia barrier islands.