Spatially detailed tree-ring analysis exposes widespread forest growth decline throughout Canada

Environmental changes across Canada's forests highlight the need to understand long-term growth dynamics and identify areas of decline, essential for predicting ecosystem vulnerability to future vegetation shifts. Here, we analyzed basal area increment trends using tree-ring data from 4410 sites spanning 1950–2018, organized into 647 1° × 1° grid cells. We then analyzed spatial patterns of these changes in relation to the long-term averages of mean annual temperature (MAT) and mean annual precipitation (MAP), the rates of change in MAT and MAP, and tree species dominance. Significant tree growth declines occurred in 42.3% of grid cells, while only 8.3% showed increases. Declines were concentrated in the boreal and montane forests of British Columbia, Alberta, and the southern Northwest Territories, with additional declines in southern Quebec, southern Labrador, and Ontario's boreal mixedwood forest transition zones. The strength of growth declines was moderately dependent on MAT, with cooler regions having more negative trends for Pseudotsuga menziesii, Picea engelmannii, and Picea glauca. Additionally, Abies lasiocarpa and Pseudotsuga menziesii exhibited declining growth in areas of their geographic range experiencing the most rapid warming. These widespread growth declines could signal early stages of forest degradation and highlight strategically timed and targeted need for adaptive forest management.