The objects of the present study were:
1) to investigate the stomatal morphology of leaves of holm oak trees grown in a naturally CO2-enriched environment;
2) to compare it with the stomatal density of leaves of holm oak seedlings grown in an artificially CO2-enriched environment.
Among the stomatal morphology parameters we analysed, the only significant alteration we observed in trees which had grown by the CO2 spring was a reduction in stomatal density.
The rather special response of one tree might be related to the rock on which it stands, which may be causing the tree severe water stress. However, the reduction did not increase significantly as the concentrations of CO2 increased.
The experiment in artificially CO2-enriched atmospheres indicated that the stomatal density of holm oak leaves was reduced by an increase in CO2. The results are discussed with an analysis of herbarium holm oak leaves.
The effect on stomatal density of the increase of atmospheric CO2 is not yet fully clarified, although some experiments exposing plants to elevated CO2 regimes in small scale chambers and field enclosures indicate that stomatal density decreases with increasing CO2 concentrations (Madsen, 1973; O'Leary & Knetch, 1981; Thomas & Harvey, 1983; Imai, Coleman & Yanagisawa, 1984; Woodward, 1987), despite contradictory responses which were sometimes observed.
Criticisms have been levelled at this kind of experiment because of poor balance between energy supply and water loss from leaves (Morison, 1987). Conversely, natural vegetation is well coupled with the atmosphere, in terms of both its energy budget and its environmental feedback mechanisms.