Population dynamics of ash across the eastern USA following invasion by emerald ash borer

Abstract

Non-native insects and pathogens can alter the composition and successional trajectories of forests and, in exceptional cases, drive their host trees functionally extinct. Emerald ash borer has killed millions of ash trees in its invaded range across eastern North America. Predicting the future ecological role of ash requires information on the extent of ash regeneration and recruitment in the wake of invasion. Variable levels of ash regeneration have been observed in stands invaded by emerald ash borer, although recruitment rates of young ash trees into the overstory remain unclear. Here, we used forest inventory data to quantify ash recruitment and regeneration across the eastern United States, and we linked these observed patterns to emerald ash borer invasion history. Inventory data from 2013-2018 indicate there has been extensive ash regeneration in invaded regions, totaling > 1,200 seedlings per ha in counties invaded in 2002–2006. Plots in counties with the longest invasion histories contained the highest densities of ash seedlings and saplings, likely due to the high densities of overstory ash in these areas prior to invasion by emerald ash borer. Seedling densities also remained generally unchanged between inventory periods ending in 2007 and 2018 irrespective of invasion history. Despite this widespread regeneration, ash trees in the smallest overstory class died at faster rates than they were recruited from seedlings or saplings, resulting in negative population trajectories on plots that have been invaded for more than ~ 10 years. This trend suggests that ash will continue to decline in abundance and may become functionally extinct across the invaded range of emerald ash borer. However, the future ecological role of ash will ultimately be determined by the iterative production and survival of ash seeds to reproductive ages in the presence of emerald ash borer populations, along with effective management practices.