Recruitment of three non-native invasive plants into a fragmented forest in southern Illinois

Plant invasions are a current threat to biodiversity conservation, second only to habitat loss and fragmentation. Density and heights of three invasive plants, Rosa multiflora, Lonicera japonica, and Elaeagnus umbellata, were examined between edges and adjacent interiors of forest sites in southern Illinois. Density (stems m⁻²) and heights (cm) of invasive plants were obtained in plots along transects at edge and interior sampling locations within forest sites. The effect of species, sampling location, and site shape index on invasive plant density was investigated, as well as differences in heights of invasive plants in edge vs. interior sampling locations.

Species, sampling location, and fragment shape index were significant factors influencing invasive plant density at study sites. Density for all three species ranged from 0 to 18 stems m⁻². All three species invaded interiors of sites, however, R. multiflora and L. japonica had significantly greater densities in edge as opposed to interior transects. These two species also had significant differences in density among site shape indices. Density of E. umbellata was not significantly different between edge and interior sampling locations or among site shape indices. Mean heights of all three invasive plants were higher in edge transects, however, this relationship was only significant for L. japonica.

These findings suggest that (1) preservation of tracts of forest with less edge can minimize invasion by these three plants in this area. Interiors of forest sites can be protected from invasion by having less edge habitat for invasive species to enter and become established; and (2) E. umbellata, unlike the other two species, is established in both interior and edges of study sites. Therefore, E. umbellata may pose the most ecological damage because it is well established throughout sites, and should be given a high priority when implementing management and control efforts in southern Illinois forest ecosystems.