Abstract
Disturbances occur in all ecosystems, and can strongly influence their structure and function, but the synergy of interactions between multiple disturbance types is underexamined and poorly understood. This is certainly true of the interactions between fire and hurricanes. Both fire and wind can be a primary structuring force in forest ecosystems. Both disturbances reduce vertical physical structure and biomass, and may influence dispersal or establishment of new propagules. However, fires, particularly controlled burns, tend to affect lower strata of vegetation and reduce fuel loads, whereas hurricanes most directly impact the upper vegetation layers, canopy trees, and significantly increase fuel loads. The focus of this study is the interaction between fire regimes and hurricanes. The location selected for the study is the xeric sand pine scrub ecosystem at the Archbold Biological Station. There were two specific objectives for this study: l.To investigate the interactions between previous fires and subsequent hurricane impacts; and 2. To establish plots in unburned areas that could be used for a future study of the post-hurricane impact of controlled burns. On the Archbold Biological Station (ABS), we selected four long-unburned sand pine scrub sites, one sand pine scrub site burned in 1986, and one sand pine scrub site burned in 1998 for the study sites. We established 100 m long by 5 m wide belt transects in each area. We assessed vegetation physical structure, species composition, and hurricane damage within each transect. We quantified vegetation structure by canopy closure and height, vegetation density, and determination of coverage by different vegetation classes. We assessed the vegetation community composition by estimating plant coverage by species. Hurricane damage quantification focused on the canopy trees in, or near, each transect. We identified and estimated percent cover for 27 species. We identified, measured, and assessed 189 trees greater than 10 cm in diameter at breast height (dbh). The sand pine (Pinus clausa) comprised 68.25% of the trees measured. We found vegetative debris coverage averaged 17.5 % overall on all transects. Coverage for 10,000 hour fuel loads (logs> 20 cm diameter) averaged 3.8 %. Vegetative composition by species did vary some among transects. The degree and type of tree cover varied considerably among transects. Data analyses indicate that the bum history may have influenced the severity of hurricane impacts. The most recent, 1998 bum area, consistently showed more open canopy, a lower canopy, lower average tree heights, lower cover in the vertical structure analysis, and lower percent cover of canopy and subcanopy vegetative layers. Litter cover and fuel loads show similarities between the older (1986) bum area and the unburned plots. The 1986 bum and the unburned plots showed very similar damage severity, with the 1998 burn area lower in both compositional (mortality) and structural damage, the latter significantly so. This study indicates that physical structure may be more important than species composition in predicting damage following subsequent disturbances. This study further indicates that 20 years following a controlled burn, the legacy of fire disturbance may be lost. However, seven years after a controlled burn, the fire may still impact the severity of hurricane disturbance.
