Local dominance of exotic plants declines with residence time: a role for plant–soil feedback?

Exotic plant species have shown boom-bust patterns, first becoming invasive, but then over a longer time period declining in dominance. Exotic plant species may escape from their native enemies, but might become increasingly exposed to enemies in the new range as time since introduction increases. We investigated whether soil interactions could explain a pattern in the Netherlands where exotic plant species with a longer residence time are less dominant, by performing a plant soil feedback experiment. We found no evidence that plant-soil interactions explain this pattern. Abstract Recent studies have shown that introduced exotic plant species may be released from their native soil-borne pathogens, but that they become exposed to increased soil pathogen activity in the new range when time since introduction increases. Other studies have shown that introduced exotic plant species become less dominant when time since introduction increases, and that plant abundance may be controlled by soil-borne pathogens; however, no study yet has tested whether these soil effects might explain the decline in dominance of exotic plant species following their initial invasiveness. Here we determine plant–soil feedback of 20 plant species that have been introduced into The Netherlands. We tested the hypotheses that (i) exotic plant species with a longer residence time have a more negative soil feedback and (ii) greater local dominance of the introduced exotic plant species correlates with less negative, or more positive, plant–soil feedback. Although the local dominance of exotic plant species decreased with time since introduction, there was no relationship of local dominance with plant–soil feedback. Plant–soil feedback also did not become more negative with increasing time since introduction. We discuss why our results may deviate from some earlier published studies and why plant–soil feedback may not in all cases, or not in all comparisons, explain patterns of local dominance of introduced exotic plant species.