Polar deciduous forests were an important biome during much of the Mesozoic and Paleogene, occupying upwards of 40% of the total land surface. Little is known about their physiological ecology, however, because these types of forests do not exist for study today. Furthermore, the role of high atmospheric CO₂ levels in modulating the physiological response of ancient polar forests is poorly known. Here we report detailed measurements of whole-tree net carbon uptake over a full annual cycle for five tree species whose close ancestors were components of Cretaceous and Paleogene polar forests. Measurements were made on both evergreen and deciduous species after two years growth in a simulated Mesozoic polar (69°N) environment at either ambient (400 ppmv) or elevated (800 ppmv) levels of CO₂. The deciduous species exhibited a significant pulse in carbon uptake during the late summer and early autumn (August to mid-October) that enabled them to achieve annual carbon budgets similar to those of evergreen trees, despite incurring higher carbon losses through annual leaf shedding. Area-based photosynthetic rates dropped progressively in all species during the polar summer (June to mid-July), resulting in decreases in whole-tree carbon uptake late in the polar summer. The high-CO₂-grown trees were more strongly affected by this polar summer depression than the low-CO₂-grown trees. Our results indicate that, from a carbon balance perspective, deciduous taxa have no clear advantage over evergreens. Moreover, the seasonal patterns reported here suggest that at latitudes poleward of 69°, evergreens will be even more strongly favored. The consideration of factors not directly related to carbon budgeting is probably therefore required to fully understand the adaptive significance of the deciduous leaf habit in ancient polar forests.