Forest Regeneration Following Emerald Ash Borer (Agrilus planipennis Fairemaire) Enhances Mesophication in Eastern Hardwood Forests

Emerald ash borer (EAB, Agrilus planipennis Fairemaire) is a phloem-feeding beetle that was introduced into North America in the late 20th century and is causing widespread mortality of native ash (Fraxinus) species. The loss of an entire genus from the forest flora is a substantial disturbance, but effects vary because of differences in Fraxinus dominance and remaining vegetation. At three sites near the center of the North American EAB range, we investigated the impacts of Fraxinus mortality on recruitment of woody and non-native vegetation in 14 permanent plots from 2012 to 2017. We used the change in relative Fraxinus basal area to determine the impact of EAB on density of woody species and non-native vegetation less than 2.5 cm diameter at breast height (dbh). Changes in canopy cover were not correlated with loss of Fraxinus from the overstory, and only the density of shade-tolerant shrubs and saplings increased with Fraxinus mortality. Both native and non-native shrub species increased in density at sites where they were present before EAB, but no new invasions were detected following Fraxinus mortality. These shifts in understory vegetation indicate that Fraxinus mortality enhances the rate of succession to shade-tolerant species.