ESTABLISHING RELATIONSHIPS BETWEEN VEGETATION DYNAMICS IN CONTEXT OF GEOLOGIC FRAMEWORK ALONG PADRE ISLAND NATIONAL SEASHORE, TEXAS, USA

Barrier islands provide valuable ecosystems and protective services to coastal communities. The longevity of barrier islands is threatened by sea-level rise, human impacts, and extreme storms. Beach and dune morphology can be viewed as free and/or forced behavior, where free systems are stochastic and the morphology is dependent on variations in the storm surge run-up, aeolian sediment supply, sediment transport potential, and vegetation dynamics. Forced systems are those where patterns in the coastal morphology are determined by some other structural control, such as the underlying and offshore framework geology. Previous studies have documented the effects of geologic framework or vegetation dynamics on the beach and dunes, although none have examined possible control by vegetation dynamics in context of the geologic framework (i.e. combined free and forced behavior). The purpose of this research is to evaluate how vegetation dynamics interact with the subsurface and offshore framework geology to influence the beach and dune morphology. Padre Island National Seashore (PAIS) was used to examine the interaction of free and forced morphology because the subsurface framework geology and surface beach and dune morphology are variable along the island. Vegetation dynamics were assessed by classifying historical aerial imagery into areas with vegetation and areas without vegetation. The subsurface geologic structure was assessed using a combination of geophysical surveys (i.e. electromagnetic induction, ground-penetrating radar). Comparison of the observed vegetation patterns and geologic framework leads to a series of questions surrounding how mechanistically these two drivers of coastal morphology are related. The long-term goal of this research is to examine and to predict how the island will change over time in response to continued climatic variability. This research will help us better understand how barrier islands have responded to environmental change in the past, and help coastal managers more accurately predict how the island is likely to change in the future.