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.
