Species-specific growth, morphological and physiological responses of Abies faxoniana and Picea purpurea under elevated temperature and CO2

Although many studies have evaluated plant ecophysiological responses to increasing atmospheric carbon dioxide concentration (CO2) and increasing temperature, few studies have addressed the interactive effects of these two factors, especially on high-altitude trees that are more sensitive. To address this issue, we used Abies faxoniana and Picea purpurea seedlings to evaluate the effects of elevated CO2 (CeTa, 700 ppm), elevated temperature (CaTe, 2 degrees C above ambient temperature) and elevated CO2 combined with elevated temperature (CeTe) on plant growth, morphology and physiological responses. We found that CaTe increased conifer total dry mass, specific root length, net photosynthesis rate and translocation rates of (NH4+)-N-15 and (NO3-)-N-15, but CeTe had stronger responses (except net photosynthesis rate of A. faxoniana). These results indicate that the effect of elevated temperature on the growth and physiological responses is enhanced by elevated CO2. Furthermore, effect of CeTe on physiological traits was higher in P. purpurea, which possessed a higher total dry mass, specific leaf area, water use efficiency (delta C-13), delta (NO3-)-N-15-N level, translocation rates of (NH4+)-N-15 and (NO3-)-N-15 and total nonstructural carbohydrates than A. faxoniana. Overall, these findings suggest that the interactive effects of CO2 x temperature should be considered when assessing conifer responses to future climates.