Effects of native vegetation on invasion success of Chinese tallow in a floating marsh ecosystem

1. Interactions between resident and exotic species have been shown to control the biotic resistance of communities to invasion. With different life stages of the exotic species, each sequential interaction may dampen or strengthen previous ones, thereby influencing invasion success. 2. We assessed the effects of resident vegetation type on the arrival and performance of Triadica sebifera (Chinese tallow), a widespread invader in freshwater floating marshes, using a combination of field studies and glasshouse experiments. 3. Our results indicated that Triadica abundance was positively associated with native woody species, particularly the native actinorhizal shrub Morella cerifera (wax myrtle). Seed dispersal and germination of Triadica were generally low but suggestive that Morella has weak, facilitative effects on the spread of Triadica by channelling its dispersal by birds into shrub thickets and promoting germination upon arrival. Triadica seedling growth was greatest where light and total inorganic N were higher. Growth of Triadica trees was greater in marsh sites without Morella and did not exhibit any detectable responses to elevated N levels in substrates from Morella thickets. 4. Morella may facilitate the spread of Triadica in floating shrub communities, yet inhibit its growth once established. The few individuals that establish in herbaceous marshes grow faster and may therefore reach maturity sooner than in Morella shrub thickets. 5. Synthesis. Our work suggests that the net effects of facilitation and inhibition by native, resident species on exotics can influence invasion success. In this case, weak, facilitative effects operating in the arrival and establishment phases of invasion are not completely negated by subsequent negative interactions between the same species, enabling the invader to persist. Models that quantify the relative strength and cumulative effects of these interactions are needed to improve predictions of community invasibility.