An inverse stomatal frequency–pCO₂ correlation for Quercus glauca

The inverse correlation between atmospheric CO₂ partial pressure (pCO₂) and stomatal frequency in many plants has been widely used to estimate palaeo-CO₂ levels. However, apparent discrepancies exist among the obtained estimates. Hu et al. attempted to find a potential proxy for palaeo-CO₂ concentrations by analysing the stomatal frequency of Quercus glauca (section Cyclobalanopsis, Fagaceae), a dominant species in East Asian sub-tropical forests with abundant fossil relatives.

An inverse correlation between stomatal frequency and pCO₂ was found for Q. glauca through cross-validation of the three material sources. The combined calibration curves integrating data of extant altitudinal samples and historical herbarium specimens improved the reliability and accuracy of the curves. However, materials in the climatic chambers exhibited a weak response and relatively high stomatal frequency possibly due to insufficient treatment time.

A new inverse stomatal frequency–pCO₂ correlation for Q. glauca was determined using samples from three sources. These three material types show the same response, indicating that Q. glauca is sensitive to atmospheric pCO₂ and is an ideal proxy for palaeo-CO₂ levels. Quercus glauca is a nearest living relative (NLR) of section Cyclobalanopsis fossils, which are widely distributed in the strata of East Asia ranging from the Eocene to Pliocene, thereby providing excellent materials to reconstruct the atmospheric CO₂ concentration history of the Cenozoic. Quercus glauca will add to the variety of proxies that can be widely used in addition to Ginkgo and Metasequoia.