Garlic mustard is among the most problematic invasive species in Eastern Deciduous Forests and is assumed to cause declines in native species abundance and richness. Additionally, garlic mustard was shown to suppress the mycorrhiza mutualism in the field and in greenhouse experiments.
We established 240 study plots in 2004 to evaluate intraspecific competition in garlic mustard and the effect of garlic mustard removal on native vegetation and mycorrhizae. From 2005 to 2009, second-year garlic mustard plants were annually removed from one-third of the plots early in the growing season (March 4-8) before garlic mustard seeds germinated and most native groundlayer species were actively growing or in mid May (15-18), after garlic mustard plants bolted and native plants were well established. The remaining plots were un-manipulated controls. Percent cover of all plant species in all plots was recorded in mid- to late-May from 2004 to 2009. Sampling was also conducted in mid-April in 2008 to assess the effect of garlic mustard removal on early spring flora. In 2009, mycorrhizal inoculum potential (MIP) for arbuscular mycorrhizal fungi (AMF) was analyzed in a subset of experimental plots.
Early removal of second-year garlic mustard plants resulted in increased cover of first-year plants compared to late removal and controls in 2006 indicating that second-year plants are strong competitors with first-year plants. This intraspecific competition leads to alternating dominance of first- and second-year plants within patches of garlic mustard. However, by 2007 early- and late-removal treatment plots had less cover of first-year plants than the control presumably resulting from reduced seed input due to removal of second-year plants and a decline in the seedbank on removal plots. From 2004 to 2007 no differences were found in cover of native species between control and garlic mustard removal plots. However, early surveys in 2008 (April 19-20) found increased percent cover of native plants in early removal plots (16.7±1.1%) compared to late removal (9.7±1.1%) and controls (10.3±1.1%). In 2009, summer dominant species increased in garlic mustard removal plots (104.5±2.6%) relative to controls (95.1±3.7%), and MIP was higher in removal plots than controls (25.7±2.3% and 18.3±2.0%, respectively). Our results provide the first evidence that mycorrhizae recover following garlic mustard removal and suggests recovery of native plant communities following management of garlic mustard invasion may take at least four years and may be associated with recovery of the mycorhiza symbiosis.