Like many natural areas in human-disturbed landscapes, those in the Midwest are susceptible in varying degrees of invasion from non-indigenous species. Approximately one-fourth of the plant species occurring in the Midwest are non-indigenous. Of these, garlic mustard, Alliaria petiolata (Brassicaceae), is considered among the most problematic. This study examines the distribution and abundance of A. petiolata at Allerton Park, a natural area in East Central Illinois, with respect to previous land use, plant community traits, and distance from paths. There was no significant difference in A. petiolata density among land cover categories and between forested areas having different land use histories in 1997 and 1998. However, A. petiolata density was significantly greater within a 35 m band from footpaths in forest, the land cover category with the least disturbance. Plant density (excluding A. petiolata) per m² was positively correlated with species richness and density of A. petiolata rosettes. The density per m² of A. petiolata rosettes was negatively correlated with the density of mature A. petiolata plants.

Non-indigenous species comprise about one-fourth of the plant species occurring in the Midwest (Myers and Henry, 1979). For instance, in Illinois, approximately 28% of its 2,853 vascular plant species are non-indigenous (Iverson and Schwartz, 1994). Although mature forests are often assumed to be more resistant to invasions than early successional forests, recent evidence suggests forests also can be readily invaded even in the absence of disturbance. As discussed by Luken (1997), non-indigenous species having negative impacts in Midwestern forests are those that (1) change the general physiognomy of the forest, (2) compete successfully with understory seedlings and saplings, or (3) retard succession. Based on these criteria, garlic mustard (Alliaria petiolata [Bieb.] Cavara and Grande), a biennial herb native to Europe, is among the most significant of the non-indigenous species currently impacting Illinois and other Midwestern forest natural areas (Brothers and Spingarn, 1992).

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