EFFECT OF ENVIRONMENTAL VARIATION ON THE INVASIVE SUCCESS OF A NONINDIGENOUS FOREST HERB

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Abstract

With the ever-burgeoning spread of nonindigenous plants, often facilitated by human activities, it is imperative to conduct case studies of particular invasive plants and the sites they invade in order to develop effective and efficient habitat management strategies. Alliaria petiolata is a native European biennial mustard that has become a serious invasive pest in many North American forests. In order to better characterize habitat vulnerability to invasion by A. petiolata, we conducted an experiment to test the effects of environmental heterogeneity in the form of habitat, microenvironment, and small-scale litter disturbance on A. petiolata germination, survival, growth, and reproduction. Treatments consisted of two habitats (upland and lowland), two microenvironments (forest edge and forest interior), and three small-scale litter disturbance treatments (control, litter completely removed, and litter partially removed). Seeds were sown into plots in November, and plots were monitored for two years. Lowland plots had greater soil moisture and less litter per unit area than upland plots. In general, forest edge plots had greater understory cover and light availability and lower overstory cover than forest interior plots. Rosette survival, mature plant survival, plant biomass, height, and fruit and seed production were significantly greater for plants in the lowland compared with plants in the upland. Germination, rosette survival, mature plant survival, and reproduction were significantly greater in the forest edge plots when compared with interior plots. Litter disturbance had no significant effect on germination, growth, or reproduction. It is apparent that site colonization by A. petiolata was not dependent on the creation of patches of bare soil by disturbance, since plants were capable of invading sites with an intact litter layer. Alliaria petiolata growth and reproduction was greatest in plots with adequate soil moisture and increased light availability. Therefore, mesic forests with a more open canopy structure and forest trails or edges may be the most vulnerable to invasion and the most difficult sites to manage. Drier upland forest interiors, which were more resistant to invasion, may be more responsive to management techniques.

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