What was done
The authors grew ponderosa pine (Pinus ponderosa Dougl.) seedlings for five years in open-top chambers having atmospheric 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 the long-term growth response of this tree species.

What was learned
The moderate level of atmospheric CO2 enrichment (525 ppm) had the greatest effect on tree height and trunk diameter in the first three years of the experiment.  However, by the time years four and five were reached, trees grown at 700 ppm CO2 exhibited the greatest growth responses to elevated CO2.  At final harvest, for example, trees exposed to twice-ambient levels of atmospheric CO2 had heights that were 43, 64 and 25% greater than those of trees exposed to ambient air and conditions of high, medium and low levels of soil nitrogen, respectively.  Similarly, trunk diameters of trees fumigated with 700 ppm CO2 for five years were 24, 73, and 20% greater than trunk diameters of ambiently-grown trees exposed to high, medium and low levels of soil nitrogen.

What it means
This study demonstrates that ponderosa pine seedlings exhibit significant long-term increases in growth in response to atmospheric CO2 enrichment over a wide range of soil nitrogen fertility.  Indeed, even at the lowest soil nitrogen content investigated, trees grown at 700 ppm CO2 exhibited 25 and 20% greater height and trunk diameters, respectively, than trees grown at ambient CO2.  Thus, it is clear that regenerating forests of ponderosa pine will likely display significant increases in growth as the CO2 content of the air continues to rise.