Positive pCO₂–stomatal frequency relationship in Quercus

The inverse relationship between atmospheric pCO₂ and stomatal frequency in plants has been widely used to estimate palaeo-CO₂ levels. However, Hu et al. determine a new positive relationship in Quercus guyavifolia, a dominant element of sclerophyllous forests in the Himalayas, based on data from extant samples collected along an altitudinal gradient combined with examination of herbarium specimens. The correlation between pCO₂ and stomatal frequency is applied to Q. preguyavaefolia fossils from two late-Pliocene floras and produces palaeo-CO₂ estimates of approximately 180–240 ppm, which is consistent with most other previous estimates.