Abstract We explored the effect of high-growth temperatures on a dominant North American boreal tree, black spruce [Picea mariana (Mill.) B.S.P.]. In 2004 and 2005, we grew black spruce at either 22 °C/16 °C day/night temperatures [low temperature (LT)] or 30°/24 °C [high temperature (HT)] and determined how temperature affected growth, leaf morphology, photosynthesis, respiration and thermotolerance. HT spruce were 20% shorter, 58% lighter, and had a 58% lower root : shoot ratio than LT trees. Mortality was negligible in the LT treatment, but up to 14% of HT seedlings died by the end of the growing season. HT seedlings had a higher photosynthetic temperature optimum, but net photosynthesis at growth temperatures was 19–35% lower in HT than LT trees. HT seedlings had both a lower apparent maximum ribulose-1,5-bisphosphate carboxylation capacity (Vcmax) and a lower apparent maximum electron transport rate (Jmax) than LT trees, indicating reduced allocation to photosynthetic components. Consistently, HT needles had 26% lower leaf nitrogen content than LT needles. At each measurement temperature, HT seedlings had 20–25% lower respiration rates than LT trees; however, this did not compensate for reduced photosynthetic rates at growth temperature, leading to a greater ratio of dark respiration to net carbon dioxide assimilation rate in HT trees. HT needles had 16% lower concentrations of soluble sugars than LT needles, but similar starch content. Growth at high temperatures increased the thermotolerance of black spruce. HT trees showed less PSII inhibition than LT seedlings and no increase in electrolyte leakage when briefly exposed to 40–57 °C. While trees that develop at high temperatures have enhanced tolerance for brief, extreme heat events, the reduction in root allocation indicates that seedlings will be more susceptible to episodic soil drying and less competitive for belowground resources in future climates of the boreal region.