Potential for persistence of blue ash in the presence of emerald ash borer in southeastern Michigan

Abstract

Since the introduction of the invasive bark beetle emerald ash borer (Agrilus planipennis Fairmaire, EAB) to southeastern Michigan, most native ash (Fraxinus spp.) species such as white ash (F. americana L.), black ash (F. nigra Marsh.), and green ash (F. pensylvanica Marsh.) have suffered mortality rates exceeding 99% after infestation in mixed stands. EAB-caused mortality has reduced or eliminated ash seed sources, resulting in a depleted seed bank and a reduced potential for regeneration that has led many researchers to predict a grim future for ash in North America. Blue ash (F. quadrangulata Michx.) is thought to be the most EAB-resistant of all North American ash species with observed mortality rates between 30% and 40%, but few data exist describing blue ash regeneration dynamics in the presence of EAB. We sampled blue ash-dominated stands at six different sites in southeastern Michigan, nearest the EAB introduction point. White ash stands at the same six sites were also sampled as the reference condition for ash in the presence of established EAB. Overstory mortality for blue ash ranged between 7 and 32% and was significantly lower than white ash across all sites. Post-EAB overstory blue ash was on average significantly older and larger than white ash across all sites and maintained a higher frequency of healthier crowns. Blue ash regeneration densities were found to be abundant at all sizes within all stands. New seedlings (<3yrs old) were significantly more abundant for blue ash than white ash across all sites, suggesting that blue ash has not experienced the same seed bank depletion as white ash. Blue ash seedling growth rates were greater than or equal to associated non-ash tree species at all size classes and at all sites, and equal to white ash seedling growth rates at the majority of sites and size classes. Our results suggest that blue ash is currently maintaining a relatively healthy and dominant position in the canopy of several forests in southeastern Michigan, and that blue ash is still regenerating in a capacity that shows a high probability of canopy self-replacement and thus persistence in the presence of EAB.