It is typically assumed that once an invasive species is successfully removed, the impact of that species on the community is also eliminated. However, invasive species may change the environment in ways that persist, as legacy effects, long after the species itself is gone. To evaluate the persistence of soil legacy effects following the removal of Cytisus scoparius, an invasive N-fixer, we implemented a randomized blocked field experiment. We killed Cytisus using herbicide in one randomly selected plot per block, so that by the end of 5.5 years of removal treatments Cytisus had been absent from plots for 64, 43, 17, 15, 12, 10 and 7 months. We also had a treatment where Cytisus was left unmanipulated. After the final Cytisus removal treatment, we planted Douglas-fir seedlings into all plots, measured available nitrogen and phosphorus and implemented a competitor removal treatment to determine if the competitive effects of secondary invaders change over time following Cytisus removal.
Douglas-fir seedlings planted into sites where Cytitus had been removed longer ago had lower survival. The negative effect of time following Cytisus removal on seedling survival was small when the secondary invaders were removed. However, where the secondary invaders we left intact the negative effect of time following Cytisus removal is greatly exacerbated. Overall seedling survival was 26% higher in the absence of secondary invaders. Soil nitrogen availability decreased with time following Cytisus removal, and was likely used by the fast growing secondary invaders, whose abundance increased with time following Cytisus removal. Rather than providing a lasting positive fertilization effect on native vegetation, our results suggest that N enrichment associated with Cytisus instead favors the invasion of fast-growing, nitrophyllic exotic grasses and forbs and that these species limit reforestation success in areas invaded by Cytisus scoparius.