Evaluating the impact of climate change on the black spruce defoliation by spruce budworm

Abstract

Current ecological models predict profound climate change-related effects on the natural disturbance regimes of forests. Spruce budworm (Choristoneura fumiferana) (SBW) is the principal insect defoliator in eastern North America, and SBW outbreaks have a major impact on the structure and function of the Canadian boreal forest, as defoliation leads to decreased tree growth, increased mortality, and lower forest productivity. SBW outbreaks have become more severe over the last century with the changing climate; however, little is known about how the integrated effect of climate and SBW alters the growth of host species. Here we evaluate how climate and outbreak severity combined to affect black spruce (Picea mariana) growth during the SBW outbreak that occurred between 1968-1988 and 2006-2017. We compiled dendrochronological series (2271 trees), outbreak severity (estimated by observed aerial defoliation), and climate data for 164 sites in Québec, Canada. We used a linear mixed effect model to determine the impacts of climatic parameters, cumulative defoliation (of the previous five years), and their interaction effect on basal area growth. At maximum outbreak severity, basal area growth of black spruce was reduced by 14%–18% over five years due to SBW effect. This outbreak growth response was affected by climate: warmer previous summer minimum temperatures and a higher previous summer climate moisture index (CMI) further decreased growth by 11% and 4%, respectively. In contrast, the negative effect of defoliation was attenuated by 9% for a warmer minimum temperature in the previous spring and 7% for a warmer maximum temperature in the previous summer. This study improves our understanding of combined insect–climate effects on growth and helps in the predictions of future SBW-related damage to forest stands to bolster sustainable forest management. We also recommend that projections of boreal forest ecosystems include several classes of SBW defoliation severity coupled with multiple climatic scenarios.