One of the most powerful ways to understand the biology of invasive plant species is to compare them to a non-invasive congener. Oriental bittersweet (Celastrus orbiculatus) is an invasive liana introduced into the Northeast from East Asia in the 1860s. In the Northeast, there is a native sympatric congener of C. orbiculatus, American bittersweet (C. scandens). While C. orbiculatus is still expanding its range in the Northeast and across the United States and Canada, populations of C. scandens appear to be declining in the Northeastern United States. By examining life-history traits, habitat tolerances, shade responses as well as competitive interactions of these two species, I sought to determine possible mechanisms for the success of C. orbiculatus as compared to C. scandens in the New England landscape. ^ Celastrus orbiculatus showed significantly higher germination percentages than C. scandens. In a three-year common garden experiment, C. orbiculatus had greater growth rates, higher aboveground biomass and lower mortality than C. scandens in both sun and shade conditions. In addition, C. orbiculatus showed higher photosynthetic rates than C. scandens in both the sun and shade treatments. When both species were transplanted into a variety of habitats that differed in light and moisture availability, C. orbiculatus had greater aboveground biomass and diameter and lower mortality than C. scandens across all habitats. ^ The were differing effects of altered R:FR on both species of Celastrus. Celastrus orbiculatus had increased height, aboveground biomass and total leaf mass in elevated FR compared to plants grown in neutral shade. Celastrus scandens had greater stem diameter, greater single leaf area and a greater leaf mass to stem mass ratio in elevated FR compared to plants grown in neutral shade. ^ Using Bayesian regression techniques with and without spatial effects, I determined that C. orbiculatus has the potential to spread even further in New England. The addition of a spatial random effect into the model resulted in a better fitting model than models using simple linear regression. ^ Finally, C. orbiculatus shows a positive response when it is surrounded by large (in terms of diameter, height and biomass) neighboring plants. In contrast, C. scandens did not show much of a positive or negative response to neighboring plants.