Black spruce growth under climate extremes: Genetic insights for managing a key resource production species

Understanding the influence of intraspecific genetic variation on the response of tree species to stress events—like heatwaves, droughts, and growing season frost—is crucial. This genetic variation is linked to species adaptive potential and plays a pivotal role in shaping the resilience and long-term adaptability of species to climate change. Furthermore, genetic variation can affect populations’ responses to stress events, thereby influencing forest productivity and carbon sequestration potential. We combined dendroecological and genomic approaches to analyze the growth response of 61 black spruce (Picea mariana) populations, grown for over 40 years in four common gardens, to daily extreme vapor pressure, soil moisture deficits, and growing season frosts. Our objectives were to 1) assess the effects of stress events defined from physiological thresholds on annual biomass production and 2) explore the potential influence of standing genetic variation on trees’ responses to stress events at the population scale. The growth response of black spruce to those events was site-specific but with an important influence of soil or atmospheric drought at most sites. This response was also nonlinear, with a negative effect on growth occurring only after several stress events a year. Additionally, an interaction between this response and the geographic origin of the population indicated an influence of standing genetic variation, notably linked to local adaptation. These results may provide insight into tree resilience to stress events and help estimate the adaptive potential of the Canadian boreal forest. Moreover, this knowledge can help guiding forest management, for example assisted gene flow.