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Invasive insects and pathogens have had major impacts on many forest tree species in North America that often affect forest structure and composition. Despite these effects, variation exists in the likelihood that some native species may persist following outbreaks and establishment of exotic insects and pathogens. Emerald ash borer ( EAB; Agrilus planipennis) has killed millions of trees near its introduction point in southeastern Michigan, and several recent studies in the area have predicted functional elimination of green ash ( Fraxinus pennsylvanica) from the landscape. Intensive management in yet unaffected stands that results from such predictions, however, demands data that examine the potential for persistence of the host species in the presence of the invasive. This study examined the potential for persistence of green ash in the presence of EAB by measuring surviving trees, regeneration, and seed rain characteristics in 17 small, near-pure stands of green ash in five consecutive growing seasons. Live trees experienced 58% mortality due to EAB, significantly less than that reported for ash in mixed stands. Approximately 20% of surviving trees exhibited signs of EAB, confirming that EAB mortality has slowed significantly but is ongoing. Sprouting was the dominant mode of ash regeneration in every year, and 27% of large sprouts produced seeds during a mast year in 2011. Advanced regeneration and new seedling establishment resulted in a sizable level of ash regeneration over the five years of the study even when sapling and seedling mortality was taken into account. Seed production was reduced considerably following EAB-caused mortality, but there was no evidence that seed dispersal limited seedling recruitment. These results suggest that the seed-producing ability of small trees and basal sprouts, as well as continued low-level mortality that will retain reduced host density, may allow green ash to persist in the presence of EAB. Although green ash populations and individual trees are unlikely to ever resemble the stature of those prior to EAB, their presence will continue to be an important component of forests on the landscape. [ABSTRACT FROM AUTHOR]

Abstract: The emerald ash borer (EAB) has killed millions of ash trees in Michigan and at least fourteen other states since its first detection near its introduction point in metropolitan Detroit in 2002. Despite overstory ash mortality near 100% in many areas, ash seedling and saplings remain unaffected and provide the potential for ash re-establishment into the canopy of deciduous forests of the region. We examined the potential for ash re-establishment in areas of heavy mortality by measuring the status and change of ash regeneration at 45 sites across southeastern Lower Michigan in 2007 and 2009. Ash regeneration was found to be abundant in all forest types, particularly in the smallest height classes, though it was more abundant in ash species of upland forests compared to those of lowland forests. New seedlings 1–2 years old were less common than other regeneration size classes, and declined substantially between 2007 and 2009, suggesting a depletion of the ash seed bank in these forests. Regeneration density was not explained well by the presence of overstory ash prior to EAB introduction, suggesting that competitive interaction with other tree species in higher canopy strata is an important driver of ash regeneration density. Regeneration is sufficient to replenish overstory ash to pre-EAB levels in upland forests, though ecological changes caused by overstory ash mortality in lowland areas may affect local hydrology in a way that reduces opportunities for tree regeneration. Despite abundant ash regeneration, it remains unclear whether ash will recover to the overstory of forests in southeastern Michigan because of uncertainties in the future dynamics of EAB in the region. [ABSTRACT FROM AUTHOR]

Emerald ash borer (EAB) is an invasive beetle that kills native North American ash species, threatening their persistence. A classical biological control program for EAB was initiated in 2007 with the release of three specialized EAB parasitoids. Monitoring changes in the health and regeneration of ash where EAB biocontrol agents have been released is critical for assessing the success of EAB biocontrol and predicting future changes to the ash component of North American forests. We sampled release and control plots across southern Michigan over a three-year period to measure ash health and recruitment to begin assessing the long-term impact of EAB biological control on ash populations. We noted a reduced mortality of larger trees between 2012 and 2015 in release plots compared to control plots and increases in ash diameter, but our results were otherwise inconsistent. Ash regeneration was generally higher in release plots compared to control plots but highly variable among sites, suggesting some protection of ash saplings from EAB by parasitoids. We conclude that EAB biocontrol is likely to have a positive effect on ash populations, but that the study duration was not long enough to definitively deduce the long-term success of the biocontrol program in this region. [ABSTRACT FROM AUTHOR]