Defoliation and demography interact to affect oak survival in Southern New England

Abstract

Oak (Quercus) is a globally important genus, but its dominance is declining across the northeastern United States. Periodic outbreaks of an invasive insect, spongy moth (Lymantria dispar), can cause significant oak mortality. Additionally, oak forests may become more vulnerable to disturbance as they mature because biomass is concentrated in fewer trees and the same mortality rate results in larger biomass losses than in a younger stand comprising many small trees. We examined the interaction of population demography and defoliation from 1970–2020, contrasting a spongy moth outbreak that peaked in 1980–1981 with an outbreak that peaked in 2017, across a permanent plot network in a 23,000-ha forest landscape. We hypothesized that the 2010s outbreak resulted in larger losses of oak biomass because it affected an older forest composed of fewer, larger trees. The percentage of oaks that died during outbreaks was more than triple that of non-outbreak intervals. Smaller trees (<30 cm diameter) were more likely to die, but the largest trees (>60 cm) also had an elevated mortality risk. During the 2010s outbreak, mortality was higher in plots that experienced more years of defoliation. Finally, we found greater loss of trees and biomass during the 2010s outbreak than the 1980s outbreak, partially because the average tree that died between 2010–2020 was larger and contained much more biomass than the average tree that died between 1980–1990. Other stressors also likely contributed to the higher mortality risk in the 2010s, highlighting increased risks to oak in an era of accelerating global change.