Even after the removal of the invasive shrub Cytisus scoparius, standard reforestation efforts in previously invaded areas often fail. In our previous work, we have found that soils invaded by Cytisus harbor less ectomycorrhizal fungi, and that growth of Douglas-fir seedlings is linked to the abundance of these fungi. In 2011, we implemented a soil transplant experiment in five Cytisus-invaded clearcuts to determine if soils transplanted from forests can improve the survival and growth of Douglas-fir seedlings in the field. We also asked whether proximity to intact forest edges ameliorates the impacts of Cytisus invasion. We used transects along forest edges paired with transects 15-25 m into the invaded clearcut. Along each transect, we manually removed Cytisus before planting Douglas-fir seedlings and three liters of soil collected from either an adjacent uninvaded forest or from within the Cytisus-invaded clearcut. We further tested the soil-mediated effect of Cytisus on Douglas-fir growth with a soil conditioning experiment in the greenhouse. We collected soil from a recently clearcut Douglas-fir forest, then conditioned the soil with either Douglas-fir or Cytisus seedlings, with an unplanted control. We removed the plants at 8 mo and quantified Douglas-fir germination in the three soil treatments.
Results/Conclusions
In the field, Douglas-fir seedlings survived better and grew larger when planted into forest soils compared to Cytisus-invaded soils. Seedlings planted near the forest edge survived better and grew twice as large as seedlings planted out in the invaded clearcuts. The positive effect of amendment with forest soil was irrespective of distance from the forest edge. In the soil conditioning experiment, the germination success of Douglas-fir seeds was three-fold higher in soils conditioned by Douglas-fir than in soils conditioned by Cytisus. Relative to the unconditioned control soils, conditioning by Douglas-fir increased germination success, and conditioning by Cytisus decreased success. Our findings suggest that Cytisus invasion leaves behind soil legacy effects that should be considered when restoring Cytisus-invaded sites. These legacy effects are likely to be mediated by the disruption of the Douglas-fir ectomycorrhizal mutualism or other important soil microbial interactions. The implication for forest management is that soil rehabilitation may be a critical step in reforestation after Cytisus invasion.