The Impact of Atmospheric CO2 and Temperature Changes on Stomatal Density: Observation from Quercus robur Lammas Leaves

Abstract

A comparative study of leaves formed on shoots during the spring and summer (lammas) of Quercus robur from three contrasting geographical locations (Cardiff, Durham and London) gives a measure of the effect of temperature on stomatal density. This is of value in attempting to distinguish the effects of CO2 and temperature on observed stomatal density changes under different CO2 and temperature conditions through the Quaternary. These leaves of normal and lammas shoots will have developed under similar CO2 levels but different environmental temperatures. Our results demonstrate that leaves formed under the warmer summer temperatures had reduced stomatal densities and indices from all sites, compared with their spring counterparts. This trend was also detected from measurements of spring and summer leaves made upon herbarium material collected from the same tree in 1840. The results suggest that for Q. robur temperature overrides the influence of irradiance intensity and small seasonal (⩽ 10 ppmv) variations of CO2 concentration in determining stomatal density. In accordance with previous work we have also documented a decline in stomatal density since 1840 by examining herbarium leaf material in response to the rising atmospheric CO2 concentration determined from ice core studies. In conclusion, if we are to understand changes in stomatal density as a response to CO2 linked temperature changes (the 'greenhouse effect') it is important to distinguish the effects of these two environmental parameters on plants.