Hurricanes have powerful effects on ecosystems resulting in a mosaic of microhabitats favoring establishment of native pioneers and invasive species. In the Florida Everglades, invasive exotic plants are prevalent and Lygodium microphyllum is one of the most serious threats to the ecosystem. In 2004 and 2005 three major hurricanes struck southeast Florida causing impacts to natural communities. We conducted a series of studies to: assess impacts to tree islands; assess colonization of L. microphyllum post-hurricane; relate patterns of colonization to both hurricane impacts and environmental factors.
Spatial extent of L. microphyllum and magnitude of hurricane impact were examined by sampling 73 tree islands and recording density of L. microphyllum and hurricane impact class based on type and quantity of impact to trees and amount of canopy openness. Tree islands were resampled one year later. On eight additional tree islands, we measured location, height, and area of all L. microphyllum patches along transects sampled biannually for 2.5 years. Hierarchical models were used to identify spatial and temporal factors related to patch parameters. We also compared L. microphyllum colonization in treefall and non-treefall areas and in 1 m2 plots in open and closed canopy areas on 12 tree islands.
Results/Conclusions
All sampled tree islands showed hurricane impact ranging from defoliation to uprooted trees. Tree islands that experienced heavy impacts had greater L. microphyllum cover one year post-hurricane than tree islands that experienced light impacts. Number and occurrence probability of patches logarithmatically increased over time. Lower water depth was associated with larger number of patches while higher depth was associated with greater mean patch height. Patches spread from outer areas of tree islands to the interior. Occurrence probability was lower in sawgrass-dominated locations at island edges compared to locations dominated by woody plants. Lygodium microphyllum was present in significantly more treefall plots (76%) than non-treefall plots (14%). Presence in treefall plots was significantly related to canopy cover and presence of water. We also observed a significant difference in presence of L. microphyllum in canopy gaps compared to closed canopy areas.
These results suggest that colonization and growth of L. microphyllum are related to extent of impact, location on a tree island, water depth, and time. Disturbances, such as hurricanes, that result in canopy openings and creation of disturbed areas with standing water contribute to the ability of L. microphyllum to invade natural areas.
