Do roads or streams explain plant invasions in forested protected areas?

Giving land managers the ability to predict invasion patterns can provide planning tools for acquisition and management of protected areas. We compared the effects of roads and streams, two substantial pathways for delivery of invasive plant propagules and sources of disturbance that may facilitate invasions, on the abundance of non-native invasive plants in 27 protected areas in the Appalachian Mountains of the eastern United States. As an extension of our road analysis, we also evaluated specific road type as a predictor of protected area invadedness. We found that road and stream predictors did not improve on a model that included only other covariates (e.g., distance to an urban area, average canopy cover, average slope, edge-to-interior ratio, percent agricultural land, and percent developed land). In this model, only percent agricultural land was marginally significant in predicting parcel invadedness. However, we found that four-wheel drive (4WD) roads did predict protected area invadedness well relative to other road types (primary, secondary, and local) and better than a covariates-only model. The role of 4WD road density in predicting protected area invadedness may be explained by their relation to recreation, the unmaintained nature of 4WD roads, or the accumulation of mud and plant materials on 4WD vehicles. Although we found overall streams and roads in general to be poor predictors of invadedness of protected areas by invasive plants, we do propose that our finding of a relationship between plant invasions and density of 4WD roads merits further investigation in the future.