PREDICTING THE RESPONSE OF LEAF STOMATAL FREQUENCY TO A FUTURE CO2-ENRICHED ATMOSPHERE - CONSTRAINTS FROM HISTORICAL OBSERVATIONS

The majority of the water flux from the earth's land surface to the at mosphere passes through the tiny pores (stomata) in the leaves of land plants. The maximum conductance to diffusion of the leaves, determine d by the number and geometry of stomata, has a profound effect on the terrestrial water and energy balance. Among tree species, there is eve r increasing evidence that anthropogenic increase in atmospheric CO2 c oncentrations results in a decrease in stomatal frequency. The rate of historical CO2 responsiveness of individual tree species can be used to calibrate empirical models of non-linear (sigmoid) stomatal frequen cy response to CO2 increase. Modelled response curves for European tre e birches (Betula pendula, Betula pubescens) and Durmast oak (Quercus petraea) predict different response limits to CO2 increase (similar to 350 and similar to 400 ppmv, respectively), indicating that non-linea r stomatal frequency responses may vary from one tree species to anoth er. Information on a wider selection of species is needed, but the mod els suggest that the maximum effect of anthropogenic CO2 increase on s tomatal frequency has already been reached. Further research is requir ed to establish the effect of rapidly declining response rates on futu re stomatal conductance of the ecologically contrasting trees of borea l, temperate, subtropical and tropical forests.