Patterns of invasive plant abundance in disturbed versus undisturbed forests within three land types over 16 years.
Aim: Long-term monitoring of forest understorey species was used to describe changes in native and invasive exotic plant abundances over time within different disturbed or undisturbed forest types. This information was then used to determine the predictive invasion model (passenger/opportunist, driver or back-seat driver). Location: Cheat Ranger District of the Monongahela National Forest, West Virginia, USA. Methods: Understorey vegetation of mature and clear-cut forests was sampled every 5-6 years for 16 years. Stands were stratified across three land types following a moisture gradient. Invasive plant richness, frequency and abundance were evaluated across land type and disturbance type with general linear mixed models. Change in richness, diversity and abundance of both native and invasive non-native plants was evaluated as a measure of impact. Results: The mesic mature stands had the greatest invasive plant richness, frequency and abundance. The moderately mesic clear-cut stands initially showed the greatest invasive plant richness, frequency and abundance, but over time these values became greater for the drier clear-cuts. The mature forests showed no change in native species abundance in response to invasion. Clear-cut-drier stands showed a decrease, while the more mesic stands showed an increase in native plant richness, diversity and abundance in response to invasion. The drier clear-cuts, with increasing invasions and negative native species impacts, were indicative of the back-seat driver model. The hitchhiker model, a new term, described increasing plant invasions with no measurable impacts. Main Conclusions: The drier clear-cut stands exhibit a lack of biotic resistance to invasion, unlike the mesic clear-cut stands. Increasing invasion in the mature forests suggests that a threshold may be reached that results in impacts on the native vegetation, but with no increase in native plant abundance to help alleviate these effects.