Emerald ash borer (EAB, Agrilus planipennis) has decimated ash (Fraxinus spp.) across eastern hardwood forests of North America since its discovery in Michigan in 2002. Prior to its detection on the Allegheny National Forest (ANF) in 2013, 193 plots across a range of ash densities were established in 2010 on the ANF to monitor white ash (F. americana) decline and mortality. In 2015, 27 plots (3.14 ha) were established where 20 randomly selected ash trees started a treatment regime with the injected insecticide emamectin benzoate to conserve ash genetic diversity, test treatment efficacy, and evaluate associational protection for untreated trees. In this study, we measured ash canopy health, tree growth, canopy cover, shrub and small stem (<2.5-cm dbh) density, and cover of understory vegetation across treated and untreated plots with low, medium, and high ash densities. For analyses, data are pooled by plot and analyzed by two-way ANOVA to detect effects of treatment and ash density.
Results/Conclusions
Insecticides can be used to conserve white ash genetic diversity across the landscape. Four years following treatment, insecticide-treated ash trees have maintained canopy health, while untreated trees have declined. For ash trees in the most degraded canopy condition, epicormic shoots were higher on the bole in treated than in untreated trees, suggesting resilience to EAB. Meanwhile, patterns of associational protection are beginning to emerge as canopies of untreated ash trees intermixed with treated ash remain stable or decline at a slower rate relative to untreated ash in control environments.
Despite these conservation efforts, the post-disturbance ecosystem is undergoing change. We observed increased canopy cover on average associated with ash decline, highlighting infilling of the canopy and suggesting resilience of these forests. However, in high ash density plots, we observed reductions in forb and woody seedling diversity as canopy cover increased, indicating long-term species diversity and resilience may be threatened. Such changes in diversity in high ash density plots may be linked to increasing forb cover. Both ash genetic diversity and understory plant diversity may be conserved by protecting ash trees through insecticidal treatment. Furthermore, results suggest that management activities should be focused on high-density ash stands to mitigate large-scale ecosystem ramifications.