What was done
The authors rooted eight saplings (of different genotypes) of beech (Fagus sylvatica) and Norway spruce (Picea abies) directly into calcareous or acidic soils in open-top chambers and exposed them to atmospheric CO2 concentrations of 370 or 570 ppm and low or high soil nitrogen contents to determine the effects of elevated CO2 and soil quality on their photosynthesis and growth.

What was learned
Elevated CO2 generally stimulated light-saturated rates of photosynthesis under all conditions, with the greatest stimulations of 24 and 35% occurring for beech and spruce, respectively, regardless of genotype.  Elevated CO2 caused down regulation of photosynthesis in both species, with spruce exhibiting less of a photosynthetic reduction than beech.  However, rates of leaf photosynthesis still remained higher for plants grown in elevated CO2, despite the occurrence of this phenomenon.  This increase in photosynthesis ultimately contributed to greater instantaneous water-use efficiencies for plants grown in elevated CO2.  The CO2-enriched trees also exhibited lower rates of stomatal conductance to water vapor.

Increased photosynthetic carbohydrate production consistently led to increased aboveground biomass production in spruce, regardless of genotype, soil type and nitrogen content, while beech generally exhibited increased biomass only on the calcareous soil, regardless of soil nitrogen content and genotype.

What it means
As the CO2 content of the air continues to rise, it is likely that the current genetic diversity of beech and spruce will be maintained, as atmospheric CO2 concentration did not act as a selective factor among species genotypes.  All genotypes of both species tended to exhibit increased rates of photosynthesis in response to atmospheric CO2 enrichment, regardless of soil quality, suggesting that they will have greater supplies of carbohydrate available to facilitate their growth and development.  Such a response was clearly observed in spruce saplings, which consistently exhibited aboveground growth increases, while the inconsistent aboveground growth response to elevated CO2 in beech suggests that their additional carbohydrates may have been used more in the belowground environment.