What was done
Saplings of Norway spruce (Picea abies Karst.) and beech (Fagus sylvatica L.) were grown for four years in open-top chambers receiving ambient and enriched (ambient plus 200 ppm) concentrations of atmospheric CO2 in combination with low and high levels of soil nitrogen supply to study the effects of these variables on water-use within these contrasting tree species.  In addition, saplings were grown on both calcareous and acidic soils to determine if soil type influences any CO2- or nitrogen-induced changes associated with water-use and gas exchange.

What was learned
Elevated CO2 reduced evapotranspiration by 7% in ecosystems established upon acidic soils; but it did not significantly affect ecosystem evapotranspiration rates on calcareous soils.  Nonetheless, elevated CO2 increased water-use efficiency (new leaf biomass produced per unit of water consumed by the ecosystem) on both soil types.  Indeed, in the low soil nitrogen regime, atmospheric CO2 enrichment increased ecosystem water-use efficiency by approximately 19%, regardless of soil type; while in the high soil nitrogen regime, it increased water-use efficiency by 14 and 25% on the acidic and calcareous soils, respectively.

What it means
As the atmospheric CO2 concentration increases, it is likely that ecosystems containing mixed stands of spruce and beech will exhibit significant increases in water-use efficiency, regardless of soil type and fertility.  Such increases in water-use efficiency may result from CO2-induced reductions in the rate of soil water depletion, or from CO2-induced enhancements of growth rates at similar rates of water consumption.  Thus, it is likely that such ecosystems will better deal with periods of drought and may even expand their ranges into areas where reduced soil moisture availability currently limits their existence.