The effects of elevated CO₂, elevated O₃, elevated temperature, and drought on plant leaf gas exchanges: a global...

Global change significantly influences plant leaf gas exchange, which affects the carbon-water cycle of terrestrial ecosystems. However, the magnitudes of the effects of multiple global change factors on leaf gas exchanges are currently lacking. Therefore, a global meta-analysis of 337 published articles was conducted to determine the effects of elevated CO₂ (eCO₂), elevated O₃ (eO₃), elevated temperature (eT), and drought on plant leaf gas exchanges. The results indicated that (1) the overall responses of photosynthesis rate (P_n) and instantaneous water use efficiency (WUE_i) to eCO₂ increased by 28.6% and 58.6%. But transpiration rate (T_r) and stomatal conductance (g_s) responded negatively to eCO₂ (- 17.5% and - 17.2%, respectively). Furthermore, all P_n, g_s, and WUE_i responded negatively to eO₃ (- 32.7%, - 24.6%, and - 27.1%), eT (- 23.2%, - 10.8%, and - 28.9%), and drought (- 53.6%, - 59.3%, and - 4.6%, respectively), regardless of functional groups and various complex experimental conditions. (2) Elevated CO₂ increased WUE_i combined with eO₃, eT, and drought (26.6%, 36.0%, and 58.6%, respectively, for eCO₂ + eO₃, eCO₂ + eT, and eCO₂ + drought) and mitigated their negative impacts on P_n to some extent. (3) Plant form and foliage type play an important role in the responses of leaf gas exchanges. Trees responded mostly to eCO₂, but responded least to eT in P_n, T_r, g_s, and WUE_i compared with shrubs and herbs. Evergreen broad-leaved species were more responsive to eCO₂ and drought. (4) The stress level of each factor can also significantly influence the responses of leaf gas exchanges to environment change. Hopefully, the quantitative results are helpful for the further assessments of the terrestrial carbon-water cycle.