Quantifying Vegetation Recovery on Santa Rosa Island



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The rate of recovery on barrier islands after hurricanes is not well understood, because the majority of studies have focused on the geomorphic impact of storms on barrier islands. Dune vegetation recovery is a vital component of barrier island recovery because it promotes the deposition of the sand required for dune stabilization. Despite a paucity of studies that quantitatively characterize vegetation recovery following hurricanes, Escambia County, Florida, was advised to spend nearly half a million dollars on revegetation efforts following relatively minor damage during the 1998 hurricane cycle. This research seeks to better understand that process by quantifying changes in vegetation extent between 1994 and 2010 on Santa Rosa Island, Florida following Hurricanes Opal and Ivan. Multispectral airborne imagery is used to analyze vegetation patterns and recovery. If no vegetation is present, sediment will not be deposited consistently. The clumps of plants collect sand in their wind shadows that grow and merge to create dunes. Changing patterns of vegetation distribution over time are examined in a geographic information system. The Verhulst logistic growth model is used to describe post-hurricane vegetation recovery. The logistic growth models results indicate that given twenty-five years of observed data, vegetation recovery occurs in approximately ten years on the Fort Pickens portion of the island and after fifteen years on the Santa Rosa Island portion of the island. The Verhulst model reveals that vegetation growth rates (r) are higher in the overwashed transects. This is most likely the result of variations in the plant species found in an overwashed transect. The transects in which vegetation spread to a greater portion of the transect (K) were more commonly collision dominated. This was probably because greater portions of these transects were less vulnerable to disturbance from run-up. Results suggest that there is a time lag between beach recovery and dune recovery that can be explained by vegetation recovery. This scale dependence will be affected by high-magnitude, high-frequency disturbances because it increases the chance that the vegetation will not have recovered before the next disturbance event thereby changing the dominant geomorphic regime. Fourier transformation reveals that there was an alongshore variation in the vegetation coverage before and after Hurricanes Opal and Ivan that corresponds to the frequencies of the ridge and swale bathymetry, around 1400 meters.