The legacy of invasion: Effects of garlic mustard (Alliaria petiolata) and its removal on native woodland herb restoration

Background/Question/Methods

The nature and mechanisms of plant invasion impacts have important implications for management outcomes and post-invasion restoration.  Failure of native plant communities to recover following invasion control may be due to insufficient control levels, legacy effects, dispersal limitation, or other environmental stressors driving native plant declines and preventing recovery.  Garlic mustard is an invasive biennial herb that appears to impact woodland communities via multiple mechanisms; removal of garlic mustard often does not lead to native plant recovery. In this study, we investigated the impact of garlic mustard on native herbs; the potential mechanisms of impact; and whether impacts depend on the presence of living garlic mustard or persist as a soil legacy effect.  We further tested whether impacts persist at low garlic mustard densities (i.e. incomplete removal), and whether they persist following multiple years of garlic mustard control.  In two oak woodlands in Minnesota, twelve species of native woodland herbs were planted into invaded and non-invaded field plots to which full, partial or multi-year garlic mustard removal treatments were applied.  Impacts were assessed by measuring above-ground biomass two and three years following planting. Mechanisms examined included root colonization by arbuscular mycorrhizal fungi (AMF) and availability of light and soil resources. 

Results/Conclusions

Garlic mustard presence did not negatively impact herb biomass as hypothesized; in many cases herbs had higher biomass in invaded plots than in non-invaded plots.  Light availability and root colonization by AMF were not affected by garlic mustard relative to native cover, but soil resource availability was generally higher in invaded plots.  Most herbs benefitted from vegetation removal, indicating release from competition, and growth was not inhibited by the legacy of garlic mustard, either at low garlic mustard densities or after multiple years of removal.   Both herb biomass and soil nutrient availability remained high, suggesting a potential positive legacy effect due to nutrient enrichment. Garlic mustard’s fertilization effect may be a more important pathway of impact in woodland herbs than reductions in mycorrhizal associations.  These findings are consistent with the “backseat driver” model of invasion in which the invader is not a primary driver of native decline but does contribute to ecosystem change.  Restoration plantings may be successful following garlic mustard control, provided other environmental stressors (e.g. deer herbivory) are managed.