Evapotranspiration (ET) of model ecosystems of young spruce and beech trees, exposed to either ambient or elevated CO₂ (ambient + 200 µmol^-1 and high or low (65 vs. 6.5 kg N ha^-1 a^-1 wet nitrogen deposition for four years in open-top chambers was estimated from the water balance biweekly during the last growing season. The open-top chambers contained two lysimeter compartments, with two types of natural unfertilized forest soils taken from either an acidic or a calcareous site and were equipped with transparent roofs, which automatically closed at the onset of rain. After harvest the water use efficiency for the production of new leaf biomass (WUEp) was determined.
In the final year of the experiment, the CO₂ and N effects on water relations differed between the two soil types: on the acidic soil, elevated CO₂ reduced ET significantly by 7 % and increased soil moisture, whereas increased nitrogen deposition had opposite effects. In contrast, on the calcareous soil evapotranspiration was not significantly influenced by any treatment, but soil moisture and drainage water were reduced under elevated CO₂. WUEp was increased and ET per unit new foliage area decreased under elevated CO₂ on both soils. Under elevated CO₂ stomatal conductance was reduced in beech leaves, but not in spruce needles. Our results suggest that plants at high CO₂ either deplete the soil water more slowly at the same growth rate (as observed here under nutrient-poor conditions) or grow faster for the same rate of water 'consumption' (as we found under favourable growth conditions).