Stomatal numbers are sensitive to increases in CO₂ from pre-industrial levels

Abstract

Recent measurements of atmospheric CO₂ levels in ice cores¹ have shown that global CO₂ has increased by about 60 µmol mol⁻¹ over the past 200 years. Evidence for the response of plants in the field to this change in CO₂ levels is here presented in the form of a significant change in stomatal density—an anatomical response of considerable ecophysiological importance. A 40% decrease in density of stomata was observed in herbarium specimens of leaves of eight temperate arboreal species collected over the last 200 years. This decline was confirmed for some of the species observed as herbarium specimens by experiments under controlled environmental conditions. In these an increase in the mole fraction of CO₂ from 280 μmol mol⁻¹ to the current ambient level of 340 µmol mol⁻¹ was found to cause a decrease in stomatal density of 67%. Experiments have shown that the combination of this previously unreported response of stomatal density to the level of CO₂, with the known responses of stomatal aperture², cause water use efficiency to be much lower than expected at low levels of CO₂ and over a wide range of humidities.