Probability of occurrence and habitat features for oriental bittersweet in an oak forest in the southern Appalachian mountains, USA
Oriental bittersweet (Celastrus orbiculatus), an introduced vine from southeast Asia, has become a serious threat to native forests in the eastern United States. It is typical of many exotic species in that quantitative ecological relationships are unavailable for assessment or management. We devised a rapid survey technique useful for hazard rating and modeled the probability of occurrence of oriental bittersweet in relation to environment, competition, and disturbance in stands of deciduous hardwoods in mountainous terrain. Oriental bittersweet was present on 39% of the study area, which has been managed by the selection system of silviculture and was recently disturbed by hurricane-force winds. Bittersweet was significantly associated with (1) topographic variables indicative of mesic environments, (2) density of midstory arborescent vegetation, (3) overstory canopy gaps, (4) past silvicultural harvests, (5) overstory canopy composition, and (5) scarification of the forest floor. Search distance from plot center to the first individual of bittersweet was significantly less (P = 0.04) on mesic than xeric sites. We developed a logistic regression model with five significant (P < 0.05) variables that classified correctly 87% of the sample plots. Variables in the model are biologically interpretable and indicate that the probability of occurrence of oriental bittersweet increases with (1) overstory canopy not dominated by oaks (Quercus spp.), (2) scarification of the forest floor, (3) concavity of the landscape around the site, (4) wind disturbance, and (5) increasing elevation. Using an independent data set from the same study area, the model classified correctly 88% of sample plots. Land management options in forests, where oriental bittersweet is present, are broadest on dry sites where its probability of occurrence is lowest and its growth response resulting from release should be least. Although herbicides can be effective in a program of intensive control, because of its biological characteristics we suggest that oriental bittersweet will present an increasing problem to iand managers throughout the eastern United States.