LAUSR

The response of tree leaf gas exchange to elevated CO₂ concentrations has been investigated in numerous experiments along the past 30 years. Stomatal regulation is a major plant control over leaf gas exchange, and the response to the increasing CO₂ will shape the biological activity of forests in the future. Here, we collected 144 records from 57 species on stomatal conductance in CO₂ manipulation experiments on trees (340–980 ppm CO₂). CO₂‐induced stomatal downregulation was calculated as the slope of the linear regression between stomatal conductance and [CO₂]. Among tree species, the slope (a) of change in stomatal conductance per 100 ppm CO₂ increase ranged between 0 and −151, indicating stomatal downregulation, and only four species showed upregulation. There was a significant divergence between evergreen gymnosperms (a = −3.6 ± 1.0), deciduous angiosperms (a = −16.3 ± 3.1) and evergreen angiosperms (a = −32.8 ± 7.1). Gymnosperms were less sensitive to CO₂ changes than deciduous angiosperms even when considering only field experiments. The significant role of tree functional group in predicting CO₂‐induced stomatal downregulation was detected in multiple mixed‐effect models, with p values ranging between 0.0002 and 0.0295. The significantly higher stomatal sensitivity to CO₂ of angiosperms versus gymnosperms might be related to the overall higher stomatal conductance of angiosperms; their thinner leaves, in turn losing water faster; and the decreasing atmospheric [CO₂] at the time of their taxa diversification. We conclude that species differences must be taken into account in forecasting future forest fluxes. A plain language summary is available for this article.