text: As the concentration of CO 2 in the atmosphere continues to rise, yellow birch seedlings should exhibit increased rates of net photosynthesis, even at higher temperatures, thereby facilitating increased biomass production. With more CO 2 in the air, the beneficial reduction in transpiration and the concomitant rise in water-use efficiency may allow this species to expand its range southward into regions where high summer temperatures and limited rainfall currently discourage its presence. Similarly, yellow birch may also expand northwards, as the additional CO 2 may make seedlings more tolerant to freezing temperatures. Reviewed 1 November 1998