Cross-island comparison of temporal variations in woody-grassland bistability at the Virginia Coast Reserve

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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.  The recent worldwide phenomenon of woody expansion into grassland has occurred so rapidly that underlying mechanisms are considered to be bistable.  In coastal ecosystems, especially barrier islands, abrupt grass-to-woody transitions are critical as these systems depend on a soil freshwater lens to support vegetative communities.  Woody vegetation 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 and associated ecological processes.   We examined island-scale conversion of land (i.e. sand to grassland to woody cover) while relating the importance of climate variables on rate of woody expansion on seven barrier islands at the LTER Virginia Coast Reserve (VCR).  Using Landsat TM imagery, we monitored changes in island size and vegetation classes at three time points: 1984, 1998, 2011. Our analysis revealed that upland area was reduced by 29 % while woody vegetation increased 40%.  Conversions between grassland and woody vegetation occurred at variable rates and indicated patterns of island compositional stability.  Over time, these patterns can be used to predict future island behavior.  At the VCR, two main patterns emerged: directional and non-directional change. Increase in woody dominated communities indicates short-term stability across the barrier island landscape with both erosion and accretion taking place, while loss of woody vegetation is indicative of an island that is uniformly eroding. Evaluation of temporal dynamics in barrier upland is needed to characterize underlying processes including island resilience or chronic stress.  Linking these changes to island topography is a prerequisite to sustainable coastal management and resilience based planning.