Plastic responses of Populus yunnanensis and Abies faxoniana to elevated atmospheric CO2 and warming

To examine whether deciduous and evergreen tree species differ in their performance and plastic responses to elevated atmospheric CO2 and temperature, we investigated growth, leaf area ratio (LAR), specific root length (SRL), nitrogen uptake efficiency (NUpE) and nitrogen use efficiency (NUE) of Populus yunnanensis and Abies faxoniana grown in environment-controlled chambers. Our results showed that temperature stimulated growth more than did CO2 in both species. The magnitude of temperature and CO2 effects varied between the species, as greater stimulation was detected in A. faxoniana than in P. yunnanensis. Greater responses of A. faxoniana were associated with higher LAR and higher NUE. However, NUpE did not differ between species and could not explain this advantage. On the other hand, the studied species did not differ in their mean overall plasticity (calculated for each species by averaging the indices of plasticity obtained for each of the 17 variables), which, however, was achieved in different ways. Across CO2 and temperature treatments, A. faxoniana exhibited greater plasticity in stomatal conductance, LAR, SRL and NUE, whereas P. yunnanensis exhibited greater plasticity in net photosynthesis rate, leaf respiration and root/shoot ratio. Our results suggested that plasticity of key traits, such as LAR, SRL and NUE, may have important implications for the superiority of A. faxoniana in the response to climate change. Our results highlight the importance of exploring the functional significance of traits rather than the extent of plasticity as potential determinants of interspecific differences in responses to climate change.