The effect of time since invasion on soil legacy effects of a widespread invasive shrub

Background/Question/Methods

Even after the removal of the invasive shrub Cytisus scoparius, standard reforestation efforts in previously invaded areas often fail.  In our previous work, we found that soils invaded by Cytisus harbored fewer ectomycorrhizal fungi than uninvaded forest soils, and that growth of Douglas-fir seedlings was linked to the abundance of these fungi.  These legacy effects of Cytisus may potentially be explained by soil nitrogen enrichment or by the exclusion of ectomycorrhizal host trees over long periods of time. To date, few studies have evaluated how soil legacy effects accumulate with time since invasion.  In 2012, we implemented a chronosequence study in Douglas-fir clearcuts invaded by Cytisus, to determine how soil legacy effects change with time since invasion. We identified 16 clearcuts harvested and invaded from 1 to 33 years previous to this study. Within each clearcut we collected soils from within and outside of invaded patches. Douglas-fir seeds were planted into each of these 32 soils in a randomized greenhouse (N = 2000) array.   We expected that EMF abundance, and consequently Douglas-fir growth, would decline with time since clearcutting, and the negative effect of time would be more pronounced in Cytisus-invaded soils.

Results/Conclusions

Contrary to our predictions, we found that time since invasion had no effect on Douglas-fir seedling growth in either the invaded or the uninvaded clearcut soils. Surprisingly, we also found that overall Douglas-fir grew better, not worse, in the Cytisus invaded  soils. We also evaluated the abundance of the ectomycorrhizal fungi and found that even in the absence of plant hosts for several decades both invaded and uninvaded clearcut soils had viable ectomycorhizal propagules that readily colonized Douglas-fir seedlings. These findings are inconsistent with the suppression of Douglas-fir that we have found in previous studies and suggests that Cytisus invasion can have both positive and negative soil legacy effects. There are several possible explanations for these results.  First, disruption of the ectomycorrhizal mutualism that can result from invasion may only have negative effects on Douglas-fir when N is not a limiting resource. It is also possible that the N fertilization effect we found in our greenhouse study could actually have negative effects on Douglas-fir survival under field conditions. This switch could occur if other species in the community, namely thatch forming grasses, gain a competitive advantage due to higher nitrogen use efficiencies. The balance of positive and negative legacy effects in field conditions may have important restoration implications.