Buying Time: Preliminary Assessment of the Potential Role of Biocontrol in the Recovery of Native Forest Vegetation Following the Invasion of the Emerald Ash Borer

Abstract

Introduced forest pests have become one of the major threats to forest ecosystems in North America. Once the spread phase is underway, biological control is one of the few environmentally acceptable and sustainable practices available for the management of destructive invasive pests in natural ecosystems. Assessing the impact of a biocontrol program progresses from evaluating the establishment of biocontrol agents, to control of the target pest, to impacts on the affected organism, and ultimately, to the indirect impacts that biocontrol may have on the whole community. In our study, we assessed the recovery of forest vegetation following the mortality of overstory ash trees caused by the emerald ash borer (EAB) invasion and ongoing management of EAB using biological control. We collected data on the forest structure and composition of stands affected by this pest and where biocontrol agents were released or not (biocontrol and no-biocontrol plots). We then used a multilevel modeling framework to evaluate the potential indirect effects of a biocontrol agent on native tree seedling forest regeneration. We found that the impacts of biocontrol on ash saplings had community-level effects by protecting native seedlings from invasive and weedy saplings. Our results showed a higher number of ash saplings with increasing numbers of the dominant EAB biocontrol agent T. planipennisi, while the number of invasive and weedy saplings was negatively associated with number of ash saplings. Density of native seedlings was negatively associated with invasive and weedy saplings. As disturbance events produce gaps in the canopy, the protection of ash saplings by the biocontrol agent may help native recruitment during forest transition by supporting the growth of native hardwood seedlings over invasive and weedy saplings. We found that evaluating the efficacy of the ash biocontrol program will need to include varied ash size classes and the community dynamics of the co-occurring species.