Large, infrequent disturbances are discrete disturbance events that occur over a large spatial extent and with great intensity and severity. Such a fire disturbance occurred during 2007 in and around the Okefenokee National Wildlife Refuge (~161,000 ha) in Southeast Georgia, USA, a wetland ecosystem comprised of mature floodplain forest, shrub lands, and aquatic prairies. The fire ignited in April during a regional drought, burned ~257,000 ha (including the Refuge), and was extinguished by rainfall in June. We investigated the effects of the 2007 fire on vegetation structure and community composition in the Refuge landscape, evaluating a long-standing succession/disturbance model for the Okefenokee Swamp (D.B. Hamilton, 1984, Plant Succession and the Influence of Disturbance in the Okefenokee Swamp). We also examined influences of vegetation composition, patch arrangement, and hydroperiod on fire severity. Vegetation communities were classified from satellite imagery collected May 2008, adding to vegetation maps previously developed with satellite imagery from 1990 and 2001. Vegetation classes in the three maps were coded to nine common types to compare vegetation composition in the 2001 and 2008 maps and burn severity (light, moderate, severe, no burn) mapped with differenced normalized burn ratio data. We also modeled integrated hydrological profiles for the common vegetation classes with five years of water depth and hydroperiod measurements collected throughout the Refuge.
Results/Conclusions
The 2007 fire dramatically altered the overall vegetation structure of the Refuge, creating two new vegetation communities, mixed scrub/shrub and shrubs with sparse mid-canopy trees, which were not mapped in 2001. Communities with complex vertical structure, such as mixed mature forest (29% of Refuge in 2001) comprised of mature forest intermixed with shrubs, was reduced by 85% and replaced with shrub-dominated communities after the fire. A vegetation class of canopy slash pine (P. elliottii) followed a similar pattern. Much of the mature forest community, with the wettest long term hydroperiod, remained unchanged. Shrub communities had the shortest hydroperiod and promoted moderate and severe fire intensities. Areas of shrubs, canopy pines, and herbaceous vegetation types that burned severely or moderately were more likely to be adjacent to like vegetation, whereas, mature forest and aquatic prairie that burned were more likely adjacent to non-like vegetation. Overall, the trend was a landscape with less local variation, as vegetation patches become larger, less interspersed, and less complex in shape. Our findings generally support vegetation changes of Hamilton’s succession model, however, those changes occurred with different burn severities than originally proposed.