ELEVATED CO2 AMELIORATES BIRCH RESPONSE TO HIGH-TEMPERATURE AND FROSTSTRESS - IMPLICATIONS FOR MODELING CLIMATE-INDUCED GEOGRAPHIC RANGE SHIFTS

Despite predictions that both atmospheric CO2 concentrations and air t emperature will rise together, very limited data are currently availab le to assess the possible interactive effects of these two global chan ge factors on temperate forest tree species. Using yellow birch (Betul a alleghaniensis) as a model species, we studied how elevated CO2 (800 vs. 400 mu l l(-1)) influences seedling growth and physiological resp onses to a 5 degrees C increase in summer air temperatures (31/26 vs. 26/21 degrees C day/night), and how both elevated CO2 and air temperat ure during the growing season influence seedling ability to survive fr eezing stress during the winter dormant season. Our results show that while increased temperature decreases seedling growth, temperature-ind uced growth reductions are significantly lower at elevated CO2 concent rations (43% vs. 73%). The amelioration of high-temperature stress was related to CO2-induced reductions in both whole-shoot dark respiratio n and transpiration. Our results also show that increased summer air t emperature, and to a lesser degree CO2 concentration, make dormant win ter buds less susceptible to freezing stress. We show the relevance of these results to models used to predict how climate change will influ ence future forest species distribution and productivity, without cons idering the direct or interactive effects of CO2.