Predicting variable reinvasion pressure in restorations of exotic-plant invaded ecosystems: Field-testing a new model emphasizing propagule pressure and abiotic variation

Background/Question/Methods: When restoring ecosystems dominated by exotic plants, the rate of new exotic recruitment following mature exotic removal (reinvasion pressure) can vary broadly between similarly invaded habitats. Due to the diverse effects invasive species can have on ecosystems and the costs and ecological impacts associated with controlling exotics, reinvasion pressure strongly influences restoration outcomes and costs. Greater understanding of what determines reinvasion pressure is essential to predicting restoration outcomes and costs, and maximizing management efficacy and efficiency. Ontogenetic niche shifts in exotic species paired with interannual variation in abiotic factors in a habitat may decouple mature exotic abundance and reinvasion pressure. We hypothesize reinvasion pressure is driven by exotic propagule pressure and availability of realized recruitment windows, which are periods of time permitting exotic establishment from seed that are determined by abiotic conditions and interspecific interactions with the recipient community. To test this hypothesis and investigate the factors driving reinvasion pressure, we are conducting replicated restoration experiments in 11 habitats along a moisture gradient that were previously dominated by exotic Chinese tallow trees (Triadica sebifera). At each site we are independently manipulating soil moisture and both native and tallow seed density in mesocosms, and exotic management at a plot scale.

Results/Conclusions: In the second growing season post-removal, restoration site identity has heavily influenced native plant community composition and abundance and all quantified metrics of tallow reinvasion pressure. Nested moisture treatments have significantly impacted tallow germination and native plant community composition and abundance. Tallow germination and abundances of different seedling age classes were strongly affected by tallow seed addition. Native seed addition has significantly influenced tallow survival and native community structure and abundance, and the native seed*moisture interaction had a significant effect on total tallow abundance. To identify useful a priori predictors of reinvasion pressure we tested for significant correlations between numerous initial site characteristics and reinvasion pressure, but no reliable predictors have yet emerged. Overall, moisture regimes and propagule availability of native and exotic species influence reinvasion pressure in predictable ways. More exotic propagules and intermediate moisture levels have generally increased reinvasion pressure. However, relatively extreme average moisture conditions appear tolerable if adequately long windows of opportunity occur, and competition from native plants may only impede reinvasion in cases where tallow is experiencing water stress. Current a priori estimates of reinvasion pressure will have limited accuracy until better initial environmental or ecological predictors are identified.