We examined effects of a natural disturbance (hurricanes) on potential invasion of tree islands by an exotic plant (Old World climbing fern, Lygodium microphyllum) in the Arthur R. Marshall Loxahatchee National Wildlife Refuge, Florida. Three major hurricanes in 2004 and 2005 caused varying degrees of impacts to trees on tree islands within the Refuge. Physical impacts of hurricanes were hypothesized to promote invasion and growth of L. microphyllum. We compared presence and density of L. microphyllum in plots of disturbed soil created by hurricane-caused treefalls to randomly selected non-disturbed plots on 12 tree islands. We also examined relationships between disturbed area size, canopy cover, and presence of standing water on presence and density of L. microphyllum. Lygodium microphyllum was present in significantly more treefall plots than random non-treefall plots (76% of the treefall plots (N=55) and only 14% of random non-treefall plots (N=55)). Density of L. microphyllum was higher in treefall plots compared to random non-disturbed plots (6.0 stems per m2 for treefall plots; 0.5 stems per m2 for random non-disturbed plots), and L. microphyllum density was correlated with disturbed area size (P = 0.005). Lygodium microphyllum presence in treefall sites was significantly related to canopy cover and presence of water: it was present in five times more treefalls with water than those without. These results suggest that disturbances, such as hurricanes, that result in canopy openings and the creation of disturbed areas with standing water contribute to the ability of L. microphyllum to invade natural areas.
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