Black spruce (Picea mariana) is a dominant North American boreal species and its response to climate warming will determine the future of the boreal forest. We investigated how growth temperature affects photosynthesis, using CO₂ response curves, temperature response curves, O₂ sensitivity, and estimates of respiration for seedlings grown at cool (22/16°C) and warm (30/24°C) day/night temperatures. Below 30°C, warm seedlings had lower net and gross CO₂ assimilation rates than cool seedlings, due to reduced V_cmax and J_max. Above 30°C, warm seedlings had higher net CO₂ assimilation rates, but gross CO₂ assimilation rates were identical, because respiration acclimated in the warm seedlings, resulting in lower dark and day respiration rates. This study suggests that maintenance of positive carbon balance at high temperatures in black spruce is due mostly to acclimation of respiration, not photosynthesis. Higher growth temperatures will likely have direct, negative impacts on the growth rate and carbon uptake of black spruce, and may reduce the carbon sink potential of the North American boreal forest.