What was done
The authors raised ponderosa pine tree seedlings for two growing seasons in open-top chambers having CO2 concentrations of 350, 525 and 700 ppm on soils of low, medium and high nitrogen content to determine the interactive effects of these variables on juvenile tree growth.

What was learned
Elevated CO2 had little effect on most growth parameters after just one growing season, the one exception being belowground biomass, which increased with both CO2 and soil nitrogen.  After two growing seasons, however, elevated CO2 significantly increased all growth parameters, including tree height, stem diameter, shoot weight, stem volume and root volume, with the greatest responses typically occurring at the highest CO2 concentration in the highest soil nitrogen treatment.  Root volume at 700 ppm CO2 and high nitrogen, for example, exceeded that of all other treatments by at least 45%, as did shoot volume by 42%.  Similarly, at high CO2 and soil nitrogen, coarse root and shoot weights exceeded those at ambient CO2 and high nitrogen by 80 and 88%, respectively.

What it means
This study demonstrates the importance of conducting experiments on long-lived plants, such as trees, for more than a single growing season.  Based on its two-year results, which are far different from what was observed at the one-year point of the study, it seems likely that juvenile ponderosa pine trees will exhibit increases in both above- and belowground biomass as the air's CO2 content continues to rise.