The persistence of soil legacy effects after the removal of Cytisus scoparius: implications for reforestation

Background/Question/Methods

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 two years of removal treatments Cytisus had been absent from plots for 22 months, 11 months, and 1 month. 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 measured the abundance of native and exotic vegetation over time following Cytisus removal. 

Results/Conclusions

After Cytisus removal, there was a soil legacy effect in the form of a large initial pulse of N, presumably a result of decomposition of N-rich Cytisus tissues.  This pulse of available N was 35% higher relative to areas where Cytisus was left intact.  However, after 10 months following removal N availability was 30% less than areas where Cytisus was left intact, and by 22 months following removal, N availability decreased another 6%.  Despite this sharp decrease in available N with time following Cytisus removal, these N concentrations in the soil are still very high relative to local uninvaded Douglas-fir forest soils. Time since Cytisus removal also affected Douglas-fir seedling growth.  Seedlings planted 22 months after Cytisus removal were smaller than seedlings planted 1 and 10 months post removal. This pattern may be explained by differences in exotic grass cover, which increased with time following Cytisus removal. Rather than providing a lasting positive fertilization effect on native vegetation, our results suggest that N enrichment instead favors the invasion of fast-growing, nitrophyllic exotic grasses and forbs and that these species limit colonization and growth of native vegetation, including the locally dominant tree Douglas-fir.