Effects of elevated CO2 on leaf gas exchange in beech and oak at two levels of nutrient supply: consequences for sensitivity to drought in beech

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Abstract
Beech (Fagus sylvatica L.) and pedunculate oak (Quercus robur L.) were grown from seed for two whole seasons at two CO2 concentrations (ambient and ambient + 250 μmol mol−1) with two levels of soil nutrient supply. Measurements of net leaf photosynthetic rate (A) and stomatal conductance (gs) of well‐watered plants were taken over both seasons; 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 effect continued through both growing seasons at both nutrient levels. There were no interactive effects of CO2 concentration and nutrient level on A or gs in beech or oak. Stomatal conductance was reduced in elevated CO2 by an average of 34% in oak, but in beech there were no significant reductions in gs except under cloudy conditions (–22% in elevated CO2). During drought, there was no effect of CO2 concentration on gs in beech grown with high nutrients, but for beech grown with low nutrients, gs was significantly higher in elevated CO2, causing more rapid soil drying. With high nutrient supply, soil drying was more rapid at elevated CO2 due to increased leaf area. It appears that beech may substantially increase whole‐plant water consumption in elevated CO2, especially under conditions of high temperature and irradiance when damage due to high evaporative demand is most likely to occur, thereby putting itself at risk during periods of drought.