Sprouting and seed production may promote persistence of green ash in the presence of the emerald ash borer

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.