Impacts of management methods and native and exotic propagule availability on restoration outcomes in a formerly invaded freshwater wetland after three years

Background/Question/Methods: 1. Habitat loss and invasive species are primary threats to global biodiversity and ecosystem function. Restoration and invasive species ecology can mitigate these threats, but because habitat degradation and invasions are intertwined, greater unification between the theories and applications of these disciplines is needed. 2. Chinese tallow tree (Triadica sebifera) is a major invader in the southeastern US that aggressively displaces native grassland, wetland and forest vegetation to form monocultures. Restorations of tallow-invaded habitats typically require long-term management, which can be expensive and/or negatively impact regenerating native plant communities. 3. How do propagule availabilities of native and invasive plants and invasive species control methods influence reinvasion pressure and restored plant communities? To address these issues, I am experimentally restoring a freshwater wetland in Texas that had been invaded by a dense monoculture of tallow trees. 4. After removing the extant tallow via mulching, I manipulated native and exotic seed densities and established treatments for common exotic control practices (mowing, grazing, and targeted herbicide application) in a full factorial design using 4 m by 4 m plots. I also manipulated seed addition density and native to exotic seed addition proportions in a full factorial design using 2 m by 2 m plots.

Results/Conclusions: 1. After the first growing season, native plant cover was significantly lower in plots subject to cattle grazing and targeted herbicide application. However, the effect of herbicide on native plant cover was modest and herbicide treatment also reduced exotic species richness and cover. Plots farther from the edge demonstrated reduced richness and cover among both native and exotic plant species. Neither mowing nor any seed addition treatment produced any detectable community-level effects. 2. Preliminary results from year three suggest that richness and cover patterns associated with plot distance from the edge have become far less pronounced and that the differences between grazed and fenced plots have increased. 3. NMS ordinations suggest that the plant communities in all treatments except those with grazing are converging strongly with the ungrazed target plant community. 4. Tallow has generally failed to recruit and is extremely rare even after three years (<1 stem per 2 m² in control plots). For this wetland, minimal ongoing management appears necessary. 5. Results from these and other experiments suggest that post-removal management is critical but the importance of initial propagule availability to both invasion and succession depends on environmental conditions – especially soil moisture – following exotic removal.