Cross-scale interactions in barrier island land cover change

Abstract:

Coastal barrier systems naturally protect the mainland and may be among the most sensitive of all ecosystems to the effects of global climate change as ecological processes are closely coupled to both atmospheric and oceanic drivers. Large scale land cover change from grassland to woody vegetation has been documented at an unprecedented rate on Virginia barrier islands that have lost upland area to sea-level rise. In a new conceptual framework, feedbacks between vegetation and sediment result in cross-scale interactions in which vegetation cover influences broad scale island geomorphology. We quantified transitions between ecosystem states (bare, grass, woody) at the local scale to predict alternate states in broad scale geomorphology (stability domains) on seven Virginia barrier islands using Landsat imagery at ~ 10 year intervals. Over the study period (1984 – 2011), sea level rise ranged between 10.3 and 10.8 cm. Total terrestrial upland area of the seven islands was substantially reduced by 29% from 1984 to 2011, but varied from 3 to 62% for individual islands. Despite this rise in sea level and loss of upland area, we found a 40% increase in woody cover. At the same time, grassland declined by 30%. While island area was reduced in all islands, changes in woody cover and grassland were non-linear and variable with each island. Nonmetric multidimensional scaling was used to determine island level stability domains. Based on change vectors, three groups of islands emerged. “Low” islands exhibited directional change and were characterized by low levels of woody vegetation and active migration. “Transitional” islands were non-directional and had frequent ecosystem state transitions. “High” islands showed the opposite directional change with extensive woody expansion and relatively little change in upland area. These multi-scaled patterns affect the manner in which barrier islands respond to disturbance over different spatial scales and can inform us of the resiliency of an individual island and island chain to climate change effects as well as provide estimates of carbon and other nutrient fluxes through the system.