What was done
Four Sitka spruce genotypes were collected (two from 53.2° N latitude and two from 41.3° N latitude) and grown in open-top chambers near Edinburgh, UK (55.5° N latitude), for three growing seasons at atmospheric CO2 concentrations of 350 and 700 ppm to determine if genetic variation influences the photosynthetic response of spruce to elevated CO2.

What was learned
After three-years of atmospheric CO2 enrichment, saplings grown in elevated CO2 exhibited photosynthetic rates that were 62% greater than those displayed by ambiently-grown controls, despite an apparent reduction in rubisco activity of 36%.  In addition, total leaf chlorophyll and nitrogen contents were significantly lower in CO2-enriched saplings than they were in ambiently-grown saplings.  Nevertheless, because elevated CO2 significantly increased sapling dry mass and photosynthesis, it significantly enhanced nitrogen-use efficiency in the CO2-enriched saplings.

Few significant genotypic effects on photosynthesis and nitrogen-use, in response to atmospheric CO2 enrichment, were discovered.  Of importance, however, was the observation that southerly clones had an initial higher nitrogen-use efficiency than northerly clones, which may account for the higher growth stimulation observed in southerly clones, despite the lower absolute photosynthetic rates they displayed.

What it means
As the CO2 content of the air continues to rise, Sitka spruce trees will likely exhibit increased rates of photosynthesis and growth, regardless of genotype.  Moreover, nitrogen-use efficiency in this species should significantly increase in both southerly and northerly varieties.  Thus, it is likely that as the atmospheric CO2 concentration increases, Sitka spruce will require less nitrogen for growth, which should allow them to grow in environments where inadequate soil nitrogen is currently limiting their presence.