Status of ash ( F raxinus spp.) in mixed hardwood post‐outbreak forests near the epicentre of the emerald ash borer invas

Emerald ash borer (EAB; Agrilus planipennis Fairmaire) has caused mortality of hundreds of millions of ash ( Fraxinus spp.), leaving an ‘orphaned’ cohort of immature ash seedlings and saplings in forests. As immature ash grow and become susceptible to EAB, their long‐term persistence in natural forests is unknown.

This study leveraged long‐term research sites established during the early stages of ash mortality near the epicentre of invasion to investigate the extent and severity of ash loss after two decades with a focus on the regenerating cohort of ash.

We found ash remained abundant in the understory of post‐outbreak forests, but their abundance and size class varied based on soil hydrology, with higher densities of larger, susceptible ash in wetland forests than in upland and riparian forests. While many ash of susceptible size (≥2.5 cm dbh) showed symptoms consistent with EAB infestation, the majority of larger ash (≥5 cm dbh) had healthy canopies. We found that silver maple ( Acer saccharinum ) has become dominant in the overstory in wetland forests and that populations of tamarack ( Larix laricina ) were associated with black ash regeneration.

These findings suggest ash regeneration has occurred differently among forest habitats. Ash in wetland forests have responded quickly to changes following EAB‐induced ash mortality and are closer to reaching the sizes required to become reproductively mature. However, EAB populations are present in low densities, and the persistence of ash in natural forests will depend on improved ash recruitment and the efficacy of introduced natural enemies.