What was done
Sitka spruce (Picea sitchensis) seedlings were grown for two years in pots placed within open-top chambers receiving atmospheric CO2 concentrations of 355 and 700 ppm.  For the last year of this study, half of the seedlings received one-tenth of the optimal soil nitrogen supply recommended for this species, while the other half received twice the optimal amount.  Thus, the authors investigated the interactive effects of elevated CO2 and soil nitrogen supply on photosynthesis and growth in this tree species.

What was learned
Elevated CO2 increased light-saturated rates of net photosynthesis by 19 and 33% in seedlings grown at low and high soil nitrogen contents, respectively, in spite of the fact that photosynthetic acclimation was induced, as determined by biochemical modeling, in seedlings grown at low, but not high, soil nitrogen.  In addition, atmospheric CO2 enrichment increased seedling total biomass by 37%, but only when grown at high nitrogen.  Although no significant CO2-induced growth response was detected in seedlings grown at low soil nitrogen supply, there was an observable reallocation of biomass from aboveground organs (leaves and stems) into roots.  And this phenomenon, in the words of the authors, "may provide a long-term mechanism by which Sitka spruce could utilize limited resources both more efficiently and effectively."  In other words, although low soil nitrogen precluded a short-term CO2-induced growth response in this tree species, it is possible that this observation may be overcome after long-term CO2 enrichment.

What it means
As the atmospheric CO2 concentration increases, Sitka spruce seedlings growing on fertile soils will likely display enhanced rates of photosynthesis and biomass accumulation.  However, seedlings growing on infertile soils, particularly those deficient in nitrogen, will likely display increases in photosynthesis without any concomitant increase in aboveground biomass - in the short-term.  In the long-term, however, it is conceivable that CO2-induced increases in root growth and exudations of organic carbon compounds could increase soil nutrition and plant nutrient acquisition, thereby helping the plants to overcome the reduced growth imposed by nutrient limitations.