It has been suggested that impacts of invasive species may change in magnitude and even direction with increased invasion duration. With increased invader occupancy time, impacts could increase as the invader population increases, individual plants grow in size, and ecological changes accumulate. Alternatively, ecological or evolutionary change may temper the impacts of an invader over time. Here we used a chronosequence field experiment to characterize the development of soil-mediated impacts over time following the invasion of Cytisus scoparius, a widespread nitrogen-fixing shrub that limits reforestation success in Douglas-fir clearcuts. Planting Douglas-fir seedlings in the field, we evaluated how abundance of ectomycorrhizal fungal (EMF) and Douglas-fir growth changed across a chronosequence of invasion that ranged from 5 to 35 years. We used 11 sites and compared Cytisus-invaded and uninvaded areas in each site, to disentangle the impacts of the Cytisus invasion itself from disturbance and removal of Douglas-fir.
Results/Conclusions
Total EMF colonization was unaffected by invasion; that is, tree seedlings planted into invaded and uninvaded patches within clearcuts had the same level of EMF colonization overall. Further, there was no relationship between total EMF abundance and Douglas-fir survival. However, colonization by Cenococcum geophilum, an EMF species that putatively confers drought tolerance in host plants, was nearly twice as abundant on seedlings grown in the uninvaded areas within clearcuts. There was also a positive relationship between Cenococcum abundance and Douglas-fir survival.
There was no effect of invasion age on total EMF abundance or Douglas-fir seedling survival. Long-invaded sites did not show more extreme effects than recently-invaded sites. Our findings suggest that because the impacts of Cytisus invasion are not exacerbated over time following initial establishment, long invaded areas may not necessarily be more difficult to restore than recently invaded areas. In addition, inoculation with Cenococcum may improve reforestation success following Cytisus invasion.