Invasibility of mature and 15-year-old deciduous forests by exotic plants

High species richness, resource availability and disturbance are community characteristics associated with forest invasibility. We categorized commonly measured community variables, including species composition, topography, and landscape features, within both mature and 15-year-old clearcuts in West Virginia, USA. We evaluated the importance of each variable for predicting the degree of forest invasion by early-establishing exotic invasive plants. Biotic variables, including overall richness (excluding exotic invasive species) and mutually exclusive native and exotic non-invasive species richness, were the strongest indicators of invasibility. Sites that were located on northeast-facing slopes, more mesic conditions, or in clearcuts were more likely to be invaded by exotic invasive plants. Invasion of clearcut sites was more dependent on available microsites (e.g., lower solar radiation, northeast-facing slopes, and lower elevations) within each site than on the condition of the surrounding landscape, whereas invasion into the mature forests was dependent more on the surrounding landscape (e.g., proximity to paved roads). Our results indicate that exotic invasive plant species in our study area respond similarly as other plant species to resource availability and that competitive interactions are relatively unimportant. Current invasion into this landscape is more likely to be a passive reaction to site conditions instead of a driver of change.