Stomatal pore length change in leaves of Eotrigonobalanus furcinervis (Fagaceae) from the Late Eocene to the Latest Oligocene and its impact on gas exchange and CO2 reconstruction

► Stomatal density/conductance is usually negatively correlated with atmospheric CO2. ► We obtained stomatal data for Eotrigonobalanus furcinervis from Eocene to Oligocene. ► Stomatal density increases while maximum stomatal conductance decreases. ► Application of an optimized gas exchange model to the data reveals a decrease of CO2. ► Use of stomatal data as CO2 proxies should consider gas exchange and climate.