EFFECTS OF ELEVATED CO2 ON LEAF GAS-EXCHANGE IN BEECH AND OAK AT 2 LEVELS OF NUTRIENT SUPPLY - CONSEQUENCES FOR SENSITIVITY TO DROUGHT IN BEECH

Beech (Fagus sylvatica L.) and pedunculate oak (Quercus robur L,) were grown from seed for two whole seasons at two CO, concentrations (ambi ent and ambient + 250 mu mol mol(-1)) with two levels of soil nutrient supply, Measurements of net leaf photosynthetic rate (A) and stomatal conductance (g(s)) of well-watered plants were taken over both season s; a drought treatment was applied in the middle of the second growing season to a separate sample of beech drawn from the same population, The net leaf photosynthetic rate of well-watered plants was stimulated in elevated CO2 by an average of 75% in beech and 33% in oak; the eff ect continued through both growing seasons at both nutrient levels, Th ere were no interactive effects of CO2 concentration and nutrient leve l on A or g(s) in beech or oak, Stomatal conductance was reduced in el evated CO2 by an average of 34% in oak, but in beech there were no sig nificant reductions in g(s) except under cloudy conditions (-22% in el evated CO2), During drought, there was no effect of CO2 concentration on g(s) in beech grown with high nutrients, but for beech grown with l ow nutrients, g(s) was significantly higher in elevated CO2, causing m ore rapid soil drying, With high nutrient supply, soil drying was more rapid at elevated CO2 due to increased leaf area, It appears that bee ch may substantially increase whole-plant water consumption in elevate d CO2, especially under conditions of high temperature and irradiance when damage due to high evaporative demand is most likely to occur, th ereby putting itself at risk during periods of drought.