Reference
Johnson, D.W., Thomas, R.B., Griffin, K.L., Tissue, D.T., Ball, J.T., Strain, B.R. and Walker, R.F.  1998.  Effects of carbon dioxide and nitrogen on growth and nitrogen uptake in ponderosa and loblolly pine.  Journal of Environmental Quality 27:414-425.

What was done
The authors review eleven of their previously published papers in which they describe the results of a series of greenhouse and open-top chamber studies of the growth responses of ponderosa and loblolly pine tree seedlings to a range of atmospheric CO2 and soil nitrogen concentrations.

What was learned
When soil nitrogen levels were so low as to be extremely deficient, or so high as to be toxic, growth responses to atmospheric CO2 enrichment in both species were negligible.  For moderate soil nitrogen deficiencies, however, a doubling of the air's CO2 content sometimes boosted growth by as much as 1,000%.  In addition, atmospheric CO2 enrichment mitigated the negative growth response of ponderosa pine to extremely high soil nitrogen in two seperate studies.

What it means
Since the nitrogen status of most of earth's ecosystems falls somewhere between extreme deficiency and toxicity, these results suggest that much of earth's plant life -- especially trees, and pine trees in particular -- may experience large increases in growth as the air's CO2 content continues to climb.  They also suggest that rising levels of atmospheric CO2 may help to ameliorate the deleterious effects of excessive nitrogen deposition.