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Climate change is expected to be most pronounced at high latitudes, but we have little data on how dominant boreal tree species will respond to rising temperatures and CO2 concentrations ([CO2]). We review the mechanisms through which elevated growth temperatures and atmospheric CO2 alter tree physiology and growth, focusing on the dominant species in northern forests. Water and nutrient availability, as well as day length, are likely to constrain the ability of these forests to respond positively to warmer, potentially longer growing seasons and higher CO2 levels. We also analyze published tree responses to future climate scenarios for key boreal tree species and show that ( i) high [CO2] increases biomass and net photosynthetic rates compared with ambient [CO2], under both current temperatures and warmer climates; ( ii) increases in temperature above current levels have little effect on growth or carbon gain; and ( iii) the combination of elevated [CO2] and elevated temperatures increases plant biomass, but this effect appears to have a threshold above a 5 °C increase in growth temperatures. While rising temperatures and [CO2], therefore, have the potential to increase the productivity of northern forest species (based on experiments that supply ample water and fertilizer), this response is likely to be limited by these soil resources and the photoperiod in the field, and may not occur under the more extreme warming conditions predicted for the future in this region. [ABSTRACT FROM AUTHOR]

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