Stomatal sensitivity to CO2 diverges between angiosperm and gymnosperm tree species

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 CO(2 )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.