Tissue et al. (1997) grew loblolly pine tree seedlings for a period of four years in open-top chambers maintained at atmospheric CO2 concentrations of either 350 or 650 ppm in a study of the long-term effects of elevated CO2 on the growth of this abundant pine species. This experiment indicated there was a mean biomass accumulation in the seedlings grown in CO2-enriched air that was 90% greater than that attained by the seedlings grown in ambient air.

Johnson et al. (1998) reviewed eleven of their previously published papers, describing the results of a series of greenhouse and open-top chamber studies of the growth responses of loblolly pine seedlings to a range of atmospheric CO2 and soil nitrogen concentrations. This work indicated that 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 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%. Consequently, since the nitrogen status of most of earth's ecosystems falls somewhere between extreme deficiency and toxicity, these results suggest that loblolly pine trees may experience large increases in growth as the air's CO2 content continues to climb.

Naidu and DeLucia (1999) described the results of working one full year in 30-meter-diameter circular FACE plots maintained at atmospheric CO2 concentrations of either 350 or 560 ppm in an originally 13-year-old loblolly pine plantation in North Carolina, USA, where they determined the effects of the elevated CO2 treatment on the productivity of the trees, which were growing in soil that was characteristically low in nitrogen and phosphorus. After the first year of atmospheric CO2 enrichment in this Duke Forest Face Study, the growth rate of the CO2-enriched trees was about 24% greater than that of the trees exposed to ambient CO2, in spite of the likelihood of soil nutrient limitations and a severe summer drought (rainfall in August 1997 was about 90% below the 50-year average).

After four years of work at the Duke Forest Face Site, Finzi et al. (2002) reported that the extra 200 or so ppm of CO2 had increased the average yearly dry matter production of the CO2-enriched trees by 32%, while at the eight-year point of the experiment Moore et al. (2006) reported there had been a sustained increase in trunk basal area increment that varied between 13 and 27% with variations in weather and the timing of growth. What is more, they say "there was no evidence of a decline in the relative enhancement of tree growth by elevated CO2 as might be expected if soil nutrients were becoming progressively more limiting," which many people had expected would occur in light of the site's low soil nitrogen and phosphorus content. In addition, at the six-year point of the study Pritchard et al. (2008) determined that the extra CO2 had increased the average standing crop of fine roots by 23%.

Returning to other types of studies, Gavazzi et al. (2000) grew one-year-old loblolly pine seedlings for about four months in pots placed within growth chambers maintained at atmospheric CO2 concentrations of either 360 or 660 ppm and adequate or inadequate levels of soil moisture, while the pots were seeded with a variety of C3 and C4 weeds. In the course of this experiment, they found that total seedling biomass was always greater under well-watered as opposed to water-stressed conditions, and that elevated CO2 increased total seedling biomass by 22% in both water treatments. In the elevated CO2 and water-stressed treatment, however, they also found that seedling root-to-shoot ratios were about 80% greater than they were in the elevated CO2 and well-watered treatment, due to a 63% increase in root biomass. In the case of the weeds, total biomass was also always greater under well-watered compared to water-stressed conditions. However, the elevated CO2 did not increase weed biomass; in fact, it reduced it by approximately 22%. Consequently, in assessing the effects of elevated CO2 on competition between loblolly pine seedlings and weeds, the seedlings were definitely the winners, with the researchers concluding that the CO2-induced increase in root-to-shoot ratio under water-stressed conditions may "contribute to an improved ability of loblolly pine to compete against weeds on dry sites."

Working with data obtained from stands of loblolly pine plantations at 94 locations scattered throughout the southeastern United States, Westfall and Amateis (2003) employed mean height measurements made at three-year intervals over a period of 15 years to calculate a site index related to mean growth rate for each of the five three-year periods, which index would be expected to increase monotonically if growth rates were being enhanced above "normal" by some monotonically-increasing growth-promoting factor. This protocol indicated, in their words, that "mean site index over the 94 plots consistently increased at each remeasurement period," which would suggest, as they phrase it, that "loblolly pine plantations are realizing greater than expected growth rates," and, we would add, that the growth rate increases are growing larger and larger with each succeeding three-year period.

As for what might be causing the monotonically increasing growth rates of loblolly pine trees over the entire southeastern region of the United States, the two researchers say that in addition to rising atmospheric CO2 concentrations, "two other likely factors that could affect growth are temperature and precipitation." However, they report that a review of annual precipitation amounts and mean ground surface temperatures showed no trends in these factors over the period of their study. They also suggest that if increased nitrogen deposition were the cause, "such a factor would have to be acting on a regional scale to produce growth increases over the range of study plots." Hence, they are partial to the aerial fertilization effect of atmospheric CO2 enrichment explanation. What is more, they note that "similar results were reported by Boyer (2001) for natural stands of longleaf pine, where increases in dominant stand height are occurring over generations on the same site."

Could it be that the results of these comprehensive studies are manifestations of the greening of the earth phenomenon, which has been postulated by Idso (1982, 1986, 1995) to result from the historical increase in the air's CO2 content? The many observations filed under the several sub-sections of the Greening of the Earth heading of our Subject Index suggest that such may well be the case. Such is also suggested by the results of the studies reported here, which indicate that as the CO2 content of the air continues to rise, loblolly pine trees will likely experience significant increases in biomass production, even on nutrient-poor soils, during times of drought, and in competition with weeds.

References
Boyer, W.D. 2001. A generational change in site index for naturally established longleaf pine on a south Alabama coastal plain site. Southern Journal of Applied Forestry 25: 88-92.

Finzi, A.C., DeLucia, E.H., Hamilton, J.G., Richter, D.D. and Schlesinger, W.H. 2002. The nitrogen budget of a pine forest under free air CO2 enrichment. Oecologia 132: 567-578.

Gavazzi, M., Seiler, J., Aust, W. and Zedaker, S. 2000. The influence of elevated carbon dioxide and water availability on herbaceous weed development and growth of transplanted loblolly pine (Pinus taeda). Environmental and Experimental Botany 44: 185-194.

Idso, S.B. 1982. Carbon Dioxide: Friend or Foe? IBR Press, Tempe, Arizona, USA.

Idso, S.B. 1986. Industrial age leading to the greening of the Earth? Nature 320: 22.

Idso, S.B. 1995. CO2 and the Biosphere: The Incredible Legacy of the Industrial Revolution. Department of Soil, Water & Climate, University of Minnesota, St. Paul, Minnesota, USA.

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.

Moore, D.J.P., Aref, S., Ho, R.M., Pippen, J.S., Hamilton, J.G. and De Lucia, E.H. 2006. Annual basal area increment and growth duration of Pinus taeda in response to eight years of free-air carbon dioxide enrichment. Global Change Biology 12: 1367-1377.

Naidu, S.L. and DeLucia, E.H. 1999. First-year growth response of trees in an intact forest exposed to elevated CO2. Global Change Biology 5: 609-613.

Pritchard, S.G., Strand, A.E., McCormack, M.L., Davis, M.A., Finzi, A.C., Jackson, R.B., Matamala, R., Rogers, H.H. and Oren, R. 2008. Fine root dynamics in a loblolly pine forest are influenced by free-air-CO2-enrichment: a six-year-minirhizotron study. Global Change Biology 14: 588-602.

Tissue, D.T., Thomas, R.B. and Strain, B.R. 1997. Atmospheric CO2 enrichment increases growth and photosynthesis of Pinus taeda: a 4-year experiment in the field. Plant, Cell and Environment 20: 1123-1134.

Westfall, J.A. and Amateis, R.L. 2003. A model to account for potential correlations between growth of loblolly pine and changing ambient carbon dioxide concentrations. Southern Journal of Applied Forestry 27: 279-284.