The effects of elevated CO, elevated O, elevated temperature, and drought on plant leaf gas exchanges: a global meta-analysis of experimental studies.

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 CO (eCO), elevated O (eO), elevated temperature (eT), and drought on plant leaf gas exchanges. The results indicated that (1) the overall responses of photosynthesis rate (P) and instantaneous water use efficiency (WUE) to eCO increased by 28.6% and 58.6%. But transpiration rate (T) and stomatal conductance (g) responded negatively to eCO (- 17.5% and - 17.2%, respectively). Furthermore, all P, g, and WUE responded negatively to eO (- 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 CO increased WUE combined with eO, eT, and drought (26.6%, 36.0%, and 58.6%, respectively, for eCO + eO, eCO + eT, and eCO + drought) and mitigated their negative impacts on P to some extent. (3) Plant form and foliage type play an important role in the responses of leaf gas exchanges. Trees responded mostly to eCO, but responded least to eT in P, T, g, and WUE compared with shrubs and herbs. Evergreen broad-leaved species were more responsive to eCO 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.