Multiyear defoliations in southern New England increases oak mortality

Abstract

After decades of multiyear defoliation episodes in southern New England, Lymantria dispar dispar (previously gypsy moth) populations diminished with the appearance of the L. dispar fungus in 1989. Multiyear defoliations did not occur again until 2015-2018. To assess the impact of the return of multiyear defoliations, we examined 3095 oaks on 29 permanent study areas in Connecticut and Rhode Island that were established at least eleven years before the latest outbreaks. Pre-defoliation stand level oak mortality averaged 2% (three-year basis). Post-defoliation mortality did not differ between managed and unmanaged stands, but was much higher in severely defoliated stands (36%) than in stands with moderate (7%) or low-no defoliation (1%). Pre-defoliation mortality of individual trees differed among species, was lower for larger diameter trees and on unmanaged than managed stands. Post-defoliation mortality on plots with no to moderate defoliation was similar to pre-defoliation mortality levels. Following multiyear defoliations, white oak mortality was higher than for northern red and black oak. There was weak evidence that mortality was elevated on stands with higher basal area following severe defoliation. Natural resource managers should not assume that oaks that survived earlier multiyear defoliations episodes will survive future multiyear outbreaks, possibly because trees are older.

Diplodia twig canker (Diplodia gallae) of northern red oak (Quercus rubra) in the northeastern United States

“…Prior to the spongy moth outbreak, defoliation from winter moth ( Operophtera brumata L.) ranged from ~2300 to ~36,000 ha per year from 2003–2015 in coastal portions of southern New England (Elkinton et al, 2021). In the years that followed the spongy moth outbreak, significant mortality of defoliated oaks occurred throughout the region (Ward et al, 2021). Flash droughts during the summer months in 2016 and 2020 (NOAA, 2022) served to predispose and further weaken oaks during and after this mass defoliation event.…”