Insect defoliation modulates influence of climate on the growth of tree species in the boreal mixed forests of eastern Canada

Abstract

Increasing air temperatures and changing precipitation patterns due to climate change can affect tree growth in boreal forests. Periodic insect outbreaks affect the growth trajectory of trees, making it difficult to quantify the climate signal in growth dynamics at scales longer than a year. We studied climate‐driven growth trends and the influence of spruce budworm (Choristoneura fumiferana Clem.) outbreaks on these trends by analyzing the basal area increment (BAI) of 2058 trees of Abies balsamea (L.) Mill., Picea glauca (Moench) Voss, Thuja occidentalis L., Populus tremuloides Michx., and Betula papyrifera Marsh, which co‐occurs in the boreal mixedwood forests of western Quebec. We used a generalized additive mixed model (GAMM) to analyze species‐specific trends in BAI dynamics from 1967 to 1991. The model relied on tree size, cambial age, degree of spruce budworm defoliation, and seasonal climatic variables. Overall, we observed a decreasing growth rate of the spruce budworm host species, A. balsamea and P. glauca between 1967 and 1991, and an increasing growth rate for the non‐host, P. tremuloides, B. papyrifera, and T. occidentalis. Our results suggest that insect outbreaks may offset growth increases resulting from a warmer climate. The observation warrants the inclusion of the spruce budworm defoliation into models predicting future forest productivity.

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

“…In addition, our study demonstrated that the SBW effect on growth is modulated significantly by climate (Figure 2.5 and Annex E). Climatic variation influences outbreak severity and the extent of defoliation (Gray, 2013), and, the degree of defoliation also determines the extent to which climate affects host tree growth (Boakye et al, 2022). Both the direct climate effect on growth and how climate mediates the effect of defoliation depend on the specific climate variable undergoing change (e.g., spring-summer temperature and precipitation) and time of the year.…”

The Use of Basal Area Increment to Preserve the Multi-Decadal Climatic Signal in Shrub Growth Ring Chronologies: A Case Study of Betula glandulosa in a Rapidly Warming Environment

“…BAI is already widely used in studies aiming to analyze the relationship between climate and tree growth [33,35,37,40,[55][56][57]. In fact, many studies have demonstrated that the use of BAI instead of RW generates higher correlations to tree biomass and wood volume gain [39,56,58].…”

“…The conversion of ring width (RW) to ring area, or basal area increment (BAI), is a robust detrending approach that compensates for the ever-increasing perimeter of the cambium [33,34]. BAI has been used in studies aiming to characterize tree responses to environmental changes, and some of these studies confirm the effectiveness of converting RW into BAI as a detrending method [35][36][37][38]. Leblanc [39] demonstrated that RW measurements of pine and spruce species tend to cloud the relationship between a growth trend and its potential cause, and that BAI measurements are more closely related to the stem volume increment than RW data.…”