What was done
The authors grew 15 genotypes of ponderosa pine, collected from different geographic locations throughout California, USA, for 28 months in open-top chambers receiving atmospheric CO2 concentrations of 350, 525, and 700 ppm to study the effects of elevated CO2 on their photosynthesis and growth.

What was learned
After 28 months of differential treatment exposure, the average rates of net photosynthesis in current-year needles of seedlings exposed to 525 and 700 ppm CO2 were 19 and 49% greater, respectively, than the mean photosynthetic rates of seedlings fumigated with air containing 350 ppm CO2.  In addition, while average values of stomatal conductance were similar for seedlings grown at 350 and 525 ppm CO2, they were 18% lower for those grown at 700 ppm CO2.  Thus, atmospheric CO2 enrichment effectively increased the water-use efficiencies of all seedlings exposed to elevated CO2.

For assessing the impact of elevated CO2 on aboveground seedling growth, the authors collected non-destructive data on seedling height and stem diameter and volume, all of which parameters were significantly increased by atmospheric CO2 enrichment in seven of the 15 genotypes studied.  Because all seedlings exhibited significant increases in photosynthesis, it is thus likely that the other eight genotypes experienced increases in belowground growth, which unfortunately was not assessed in this study.

What it means
As the CO2 content of the air rises, it is likely that ponderosa pine will exhibit increased rates of photosynthesis throughout their entire range in the state of California and elsewhere, regardless of genotype.  In addition, with greater carbohydrate supplies, resulting from enhanced photosynthetic rates, it is likely that ponderosa pines will also increase their growth and biomass accumulation as the atmospheric CO2 concentration continues to increase in the future.