Persistence of a soil legacy following removal of a nitrogen-fixing invader

Abstract

Vast effort and resources are spent to control invasive plants. Often with the assumption that once these resources are spent and the invader 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, after the species itself is gone. Here we evaluate the persistence of soil legacy effects following the death of Cytisusscoparius, an invasive nitrogen-fixer. In a field experiment, we periodically killed C.scoparius with herbicide, so that by the end of 2 years the invader had been absent from plots for different durations of time (22, 10, and 1 month). After the final C.scoparius removal treatment, we measured available soil nitrogen and phosphorus as well as the abundance of native and exotic vegetation. We planted Douglas-fir seedlings into the removal plots and tracked seedling success. One month after C.scoparius removal, there was a soil legacy effect in the form of a large initial pulse of inorganic N, presumably as a result of rapid decomposition of N-rich C.scoparius biomass. In the 10-month removal plots, this initial pulse of N had declined dramatically and was 70 % less than the invaded state. However, over the following year, there was little additional decline of N. Time since C.scoparius removal also affected Douglas-fir seedling growth, where seedlings planted into areas where C.scoparius had been removed for 22 months were smaller than seedlings planted into areas where C.scoparius had been removed for 1 and 10 months. This pattern may be caused by competition from a second wave of exotic invaders, whose cover increased with time following C.scoparius removal. Rather than providing a lasting positive fertilization effect on native vegetation, our results suggest that increased N availability 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.