Disentangling the factors driving invasion of southern Appalachian oak and tulip poplar stands by Asiatic bittersweet (Celastrus orbiculatus): the role of land-use history

Background/Question/Methods   Although historic land use is often implicated in non-native plant invasion of forests, little is known about how land-use legacies might actually facilitate invasion. We conducted a two-year field seeding experiment at the Bent Creek Experimental Forest in western North Carolina, USA, to compare germination and first-year seedling survival of Asiatic bittersweet (Celastrus orbiculatus Thunb.) in stands dominated by tulip poplar (Liriodendron tulipifera L.), which had been cultivated and abandoned a century earlier, and in nearby stands dominated by oaks (Quercus spp.) that had never been cultivated. Our aim was to determine why tulip poplar stands had higher abundance of C. orbiculatus, as evidenced from previous work. Experiments were conducted at five sites, each with one tulip poplar and one oak stand, by varying litter mass (none, low, or high) and litter type (tulip poplar or oak). Each of the five treatments was assigned to three 1x1-m plots per stand that were seeded with C. orbiculatus in 2008 and 2009. To test for differential effects of site and soil type, we also performed reciprocal soil translocations using potted soils in which C. orbiculatus seeds were sown. Soil moisture and temperature were measured throughout the 2008 growing season.

Results/Conclusions   Germination and survival were consistently higher in tulip poplar stands than in oak stands. Germination was also higher in plots with low litter mass than in those with no litter or high litter mass. Seedling survival was lowest in plots with high litter mass. Soil moisture was higher in tulip poplar stands and under low-mass litter and thus may play a major role in determining establishment success. Differences in germination and survival among the pots were minimal, suggesting that soil type and site conditions unrelated to the forest floor were less important than litter conditions for C. orbiculatus establishment. The low litter mass and mesic soil conditions that are characteristic of tulip poplar stands may confer higher invasibility and explain the higher abundance of C. orbiculatus in areas with successional overstory communities associated with agricultural land-use history.