EVAPOTRANSPIRATION OF BEECH STANDS AND TRANSPIRATION OF BEECH LEAVES SUBJECT TO ATMOSPHERIC CO2 ENRICHMENT

Beech trees (Fagus sylvatica L.) show reduced stomatal conductance and increased leaf area index in response to increased atmospheric CO2 concentration. To determine whether the reduction in stomatal conductance results in lower stand evapotranspiration, we compared transpiration on a leaf-area basis and stand evapotranspiration on a ground-area basis in young European beech trees growing in greenhouses at ambient (360 +/- 34 mumol mol-1) and elevated (698 +/- 10 mumol mol-1) CO2 concentrations. Trees were grown in homogenized natural soil at constant soil water supply for two growing seasons. At light saturation, leaf transpiration rates were, on average, 18% lower in the elevated CO2 treatment than in the ambient C02 treatment. Mean transpiration coefficients (transpiration/net CO2 uptake) of leaves were 179 and 110 in the ambient and elevated CO2 treatments, respectively, indicating improved water use efficiency in trees in the elevated CO2 treatment. Total leaf conductance was decreased by 32% at light saturation. The elevated CO2 treatment resulted in a 14% reduction in stand evapotranspiration. In both CO2 treatments, evapotranspiration increased linearly at a rate of 0.2 kg H2O m-2 day-1 for each 1-degrees-C rise in air temperature between 14 and 25-degrees-C. We conclude that, under Central European conditions, water losses from deciduous forest stands will be reduced by a doubling of tropospheric CO2 concentration.