What will rise from the ashes? Evaluating impacts of emerald ash borer on tree regeneration in black ash stands

Emerald ash borer (EAB) (Agrilus planipennis Fairmaire), the most destructive forest pest to ever invade North America, is expected to functionally extirpate black ash (Fraxinus nigra) by 2050. Black ash is both an ecological foundational and cultural keystone species that predominantly grows in wet-mesic forests in the Lake States and northeastern regions of the US and eastern Canada. Despite its significance to forested wetlands, the ecological impacts of losing black ash are uncertain. In 2023, we surveyed vegetation in 18 black ash stands representing varying stages of EAB invasion (eight Post-Invasion, five Mid-Invasion, and five Pre-Invasion) in Michigan. Additionally, we collected two increment cores from 99 overstory black ash trees (DBH >10 cm), and 115 black ash recruits (DBH >2.5< 10 cm). Unsurprisingly, black ash dominated regeneration across invasion strata. However, the proportion of black ash recruits that were dead was higher in Post-Invasion stands (0.64, 95% CI [0.45,0.79]) than in Mid-Invasion (0.12, 95% CI [0.079,0.37]) and Pre-Invasion stands (0.17, 95% CI [0.06, 0.43]) (c2 = 16.86, df = 2, P < 0.001). Radial growth of black ash recruits from 2015 to 2022 was higher in Post-Invasion (1.381 mm, 95% CI [0.910, 2.095]), than in Pre-Invasion stands (0.624 mm, 95% CI [0.411, 0.947]), but Mid-Invasion stands were intermediate (0.886 mm, 95% CI [0.584, 1.344]). Our results suggest that black ash recruits were often released following overstory tree mortality in Post-Invasion stands, but many became infested and were killed by EAB during or after release.