Tree-ring chronologies reveal unexpected geographic patterns of drought vulnerability in western Canadian forests

Adaptive reforestation under climate warming will increasingly involve northward or upslope movement of tree species and seed sources. However, in western Canadian boreal and sub-boreal forests, release from cold constraints may be offset by emerging drought limitations, where low evapotranspiration has historically supported tree growth despite modest precipitation. We used 392 tree-ring chronologies from western Canada to compare drought resistance, recovery, and resilience among seven common conifer tree species across regional climatic gradients. Drought vulnerability metrics were not strongest at the southern forest-grassland transition, as might be expected from current moisture deficits alone. Instead, drought impacts were most pronounced further north in dry boreal mixedwood regions. Specifically, white spruce and jack pine showed lower post-drought resilience in northern boreal regions than in their southern boreal populations. We interpret this pattern as consistent with limited historical selection for drought tolerance in northern populations, where cold temperatures have historically moderated moisture stress. Similarly, interior Douglas-fir showed particularly low resilience in its sub-boreal distribution, near its northern range limit. These results suggest practical limits to northward planting of interior Douglas-fir, while southern boreal populations of white spruce and jack pine may be useful seed sources for improving drought resilience in central and northern boreal forests. More broadly, these results suggest that drought vulnerability in western Canadian forests is not confined to the southern boreal fringe, but also affects northern boreal and sub-boreal populations, where adaptive reforestation may be needed to maintain forest resilience under continued warming.