D3087: To grow or not to grow: How emerald ash borer affects tree regeneration in black ash forests

Emerald ash borer (EAB) (Agrilus planipennis Fairmaire) has become the most destructive forest insect pest to invade North America. Black ash (Fraxinus nigra), a foundational ecological and cultural keystone species that grows in wet-mesic forests of the northeast and Lake States regions of the US and in eastern Canada, is the most vulnerable and preferred EAB host in North America, facing functional extirpation by 2050. Despite the species significance, ecological effects of overstory black ash mortality in sites where these trees are abundant is largely unknown. In 2023, we surveyed overstory trees and regeneration in four Post-Invasion sites with extensive black ash mortality and four Pre-Invasion sites with little or no evidence of EAB. Additionally, two increment cores were collected from 67 black ash recruits (2.5-10 cm diam), and from 30 overstory black ash (>10 cm diam) (2 cores per tree). Black ash densities varied across stands, and average (±SE) proportion of dead black ash recruits was higher in Post-Invasion sites (0.49±0.1) than Pre-Invasion sites (0.21±0.05). On average, annual radial growth of black ash recruits in Post-Invasion sites was 46% higher than recruits in Pre-Invasion sites and increased after 2010. Presumably EAB-caused mortality of overstory trees increased light and resource availability, releasing recruits. In Pre-Invasion sites, recruit growth remained relatively consistent over time. Surveys and increment core collection will continue in 2024 at Pre-, Mid-, and Post-Invasion sites across Michigan. Results will further our understanding of black ash regeneration dynamics and long-term outlook for black ash.