A cost‐effective climate change adaptation strategy for the forestry sector is to move seed sources to more northern and higher elevation planting sites as part of ongoing reforestation programs. This is meant to match locally adapted populations with anticipated environments, but adaptive traits do not always show population differences suitable to mitigate climate change impacts. For white spruce, drought tolerance is a critical adaptive trait to prevent mortality and productivity losses. Here, we use a 40‐year‐old provenance experiment that has been exposed to severe drought periods in 1999 and 2002 to retrospectively investigate drought response and the adaptive capacity of white spruce populations across their boreal range. Relying on dendrochronological analysis under experimentally controlled environments, we evaluate population differences in resistance, resilience, and recovery to these extreme events. Results showed evidence for population differentiation in resistance and recovery parameters, but provenances conformed to approximately the same growth rates under drought conditions and had similar resilience metrics. The lack of populations with better growth rates under drought conditions is contrary to expectations for a wide‐ranging species with distinct regional climates. Populations from the wettest environments in the northeastern boreal were surprisingly drought‐tolerant, suggesting that these populations would readily resist water deficits projected for the 2080s, and supporting the view that northeastern Canada will provide a refugium for boreal species under climate change. The findings also suggest that white spruce is sensitive to growth reductions under climate change in the western boreal. The study highlights that population differentiation in adaptive capacity is species‐ and trait‐specific, and we provide a counterexample for drought tolerance traits, where assisted migration prescriptions may be ineffective to mitigate climate change impacts. For resource managers and policy makers, we provide maps where planning for widespread declines of boreal white spruce forests may be unavoidable.

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Remote-sensing based vulnerability assessments to climate change are a research priority of critical importance for landscape-scale efforts to prioritize conservation and management of ecosystems. Limiting climatic factors can serve as a proxy for quantifying ecosystem vulnerability, since theory predicts that ecosystems close to critical climate thresholds will be more sensitive to interannual variation in limiting climate factors. Here, we analyze time series of enhanced vegetation index (EVI) data for continental-scale vulnerability assessments. The analytical approach is a lagged monthly correlation analysis that accounts for memory effects from the previous growing season. Mapping multivariate correlation coefficients reveals that drought vulnerabilities can be found across the continent, including a distinct geographic band across the western boreal forest. The analytical approach reveals climate dependencies at high spatial and temporal resolution, with the direction and strength of correlation coefficients indicating the risk of threshold transgressions at the edge of species and ecosystem tolerance limits. The approach is further useful for hypothesis testing of contributing non-climatic factors to climatic vulnerability, allowing locally targeted management interventions to address climate change.

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