Potential synergistic interactions between Celastrus orbiculatus (oriental bittersweet) and Agrilus planipennis (emerald ash borer): Implications for northeastern forests

Background/Question/Methods:

The increasing number of species introductions across the United States raises the potential for interactions between invasive species.  This study looked at the potential for synergistic interactions between Celastrus orbiculatus, an invasive vine, and Agrilus planipennis, an invasive insect. Once it invades a region, A. planipennis causes the mortality of Fraxinus spp. and creates gaps in the forest canopy.  We examined the likelihood that these gaps would facilitate the invasion of C. orbiculatus by increasing light resources to the forest floor. C. orbiculatus employs a ‘sit and wait’ strategy in shaded environments. The seedlings germinate and grow slowly in the understory and then increase growth rates dramatically when light becomes available.  They are then able to reach the forest canopy, overtop trees, and alter forest structure.  A. planipennis has been found nearby, but has not yet invaded our study site, providing a unique opportunity to study interactions between C. orbiculatus and A. planipennis as the invasion occurs. To determine whether the invasion of A. planipennis has the potential to release suppressed C. orbiculatus we used GIS interpolation techniques to map density levels of Fraxinus americana and both climbing C. orbiculatus plants as well as C. orbiculatusseedlings.

Results/Conclusions:

We found two main areas of high-density C. orbiculatus climbing plants and seedlings. We performed a Pearson’s Chi-squared test on presence or absence of seedling and climbing C. orbiculatus and found indications of a positive association between climbing plants and seedlings (x² value=11.137, p-value=0.0017). In addition, two high-density areas of C. orbiculatus overlap directly with high-density areas of F. americana indicating that there is a high potential for synergistic interactions when A. planipennis invades. One area in particular has moderate densities of C. orbiculatus seedlings and F. americana trees but few mature C. orbiculatus plants. In this area there is a high probability that the light gaps created when the F. americana trees die will release the C. orbiculatus seedlings, currently suppressed in the dark understory, and allow them to mature and alter the forest structure. Our management suggestions are to cut the climbing C. orbiculatus plants to decrease host tree mortality and seed input in areas found to have high densities of mature vines.  In addition we will monitor understory plant and vine growth in areas with high C. orbiculatus and F. americana densities to assess the impact of A. planipennis on forest dynamics.