The effect of invasive species management on post hurricane recovery patterns in a Louisiana bottomland forest

Background/Question/Methods

Louisiana has lost more than half of its bottomland hardwood forests since European settlement, making the remaining fragments a high conservation priority. However, even protected fragments are threatened by chronic hurricane disturbance and by non-native species invasion.  An example is the English Turn forest of southeastern Louisiana, which is one of the largest forested wetlands remaining between New Orleans and the Gulf of Mexico. A number of woody invasive species threaten the forest and there is evidence that disturbance from several tropical storms over the last decade have facilitated these invasions.  The degree to which chronic disturbance and invasion interact to drive forest dynamics and the degree to which management efforts can alter successional processes are poorly understood.  We set out to document long-term forest recovery patterns following hurricane disturbance and test the degree to which invasive species control efforts influence successional patterns.  We censused woody species composition and demography annually within replicated 0.04 ha permanent plots arrayed within areas that received intensive invasive species removal and areas where no management was applied. We report results from the first decade of recovery following the acute disturbance caused by Hurricane Katrina.

Results/Conclusions

In the ten years following Hurricane Katrina mean plot DBH and canopy height have steadily declined, a result both of the continuing loss of large trees and of a recruitment flush.  The composition of the recovering forest has been dominated by early successional species such as Acer negundo as well as woody invaders such as Triadica sebifera.  Invasive species removal efforts in the form of herbicide application and mechanical removal applied over a period of two years significantly reduced the abundance of T. sebifera seedlings and adults relative to untreated plots. The effects of treatment have been long lasting and appear to have contributed to divergent successional trajectories.  In the six years following treatment, T. sebifera abundance decreased in treated plots while it steadily increased in untreated plots.  However, treatment activities also caused a significant reduction in mean plot canopy cover and basal area relative to untreated plots that persisted over a period of five years.  These results suggest that short-term invasive species removal can influence longer term successional trajectories, but the potential impacts of treatment activities should be accounted for as part of a comprehensive recover strategy.