Walker et al. (1998b) grew Ponderosa pine seedlings for an entire year in controlled environment chambers with atmospheric CO2 concentrations of either 350 (ambient), 525 or 700 ppm. In addition, low or high levels of nitrogen and phosphorus were supplied to determine the main and interactive effects of atmospheric CO2 enrichment and soil nutrition on seedling growth and fungal colonization of the seedlings' roots. After twelve months, they found that phosphorus supply had little impact on overall seedling growth, while high nitrogen increased nearly every parameter measured, including root, shoot and total biomass, as did atmospheric CO2 enrichment. Averaged over all nitrogen and phosphate treatments, total root dry weights at 525 and 700 ppm CO2 were 92 and 49% greater, respectively, than those observed at ambient CO2, while shoot dry weights were 83 and 26% greater. Consequently, seedlings grown at 525 and 700 ppm CO2 had total dry weights that were 86 and 35% greater, respectively, than those measured at ambient CO2. In addition, elevated CO2 increased the total number of ectomycorrhizal fungi on roots by 170% at 525 ppm CO2 and 85% at 700 ppm CO2 relative to the number observed at ambient CO2.

Walker et al. (1998a) grew Ponderosa pine seedlings for two growing seasons out-of-doors 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 juvenile tree growth. The elevated CO2 concentrations had little effect on most growth parameters after the first growing season, with the one exception of 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 soil 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.

Johnson et al. (1998) reviewed eleven of their previously published papers (including the two discussed above) in which they describe the results of a series of greenhouse and open-top chamber studies of the growth responses of Ponderosa pine seedlings to a range of atmospheric CO2 and soil nitrogen concentrations. These studies 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%. In addition, atmospheric CO2 enrichment mitigated the negative growth response of ponderosa pine to extremely high soil nitrogen in two separate studies.

Maherali and DeLucia (2000) grew Ponderosa pine seedlings for six months in controlled environment chambers maintained at atmospheric CO2 concentrations ranging from 350 to 1100 ppm, while they were subjected to either low (15/25°C night/day) or high (20/30°C night/day) temperatures. This study revealed that although elevated CO2 had no significant effect on stomatal conductance, seedlings grown in the high temperature treatment exhibited a 15% increase in this parameter relative to seedlings grown in the low temperature treatment. Similarly, specific hydraulic conductivity, which is a measure of the amount of water moving through a plant relative to its leaf or needle area, also increased in the seedlings exposed to the high temperature treatment. In addition, biomass production rose by 42% in the low temperature treatment and 62% in the high temperature treatment when the atmospheric CO2 concentration was raised from 350 to 1100 ppm.

Tingey et al. (2005) studied the effects of atmospheric CO2 enrichment (to approximately 350 ppm above ambient) on the fine-root architecture of Ponderosa pine seedlings growing in open-top chambers via minirhizotron tubes over a period of four years. This experiment showed that "elevated CO2 increased both fine root extensity (degree of soil exploration) and intensity (extent that roots use explored areas) but had no effect on mycorrhizae," the latter of which observations was presumed to be due to the fact that soil nitrogen was not limiting to growth in this study. More specifically, they report that "extensity increased 1.5- to 2-fold in elevated CO2 while intensity increased only 20% or less," noting that similar extensity results had been obtained over shorter periods of 4 months to 2 years by Arnone (1997), Berntson and Bazzaz (1998), DeLucia et al. (1997) and Runion et al. (1997), while similar intensity results had been obtained by Berntson (1994).

Last of all, Soule and Knapp (2006) studied Ponderosa pine trees growing naturally at eight different sites within the Pacific Northwest of the United States, in order to see how they may have responded to the increase in the atmosphere's CO2 concentration that occurred after 1950. In selecting these sites, they chose locations that "fit several criteria designed to limit potential confounding influences associated with anthropogenic disturbance." They also say they selected locations with "a variety of climatic and topoedaphic conditions, ranging from extremely water-limiting environments ... to areas where soil moisture should be a limiting factor for growth only during extreme drought years," additionally noting that all sites were located in areas "where ozone concentrations and nitrogen deposition are typically low."

At each of the eight sites that met all of these criteria, Soule and Knapp obtained core samples from about 40 mature trees that included "the potentially oldest trees on each site," so that their results would indicate, as they put it, "the response of mature, naturally occurring ponderosa pine trees that germinated before anthropogenically elevated CO2 levels, but where growth, particularly post-1950, has occurred under increasing and substantially higher atmospheric CO2 concentrations." Utilizing meteorological evaluations of the Palmer Drought Severity Index, they thus compared ponderosa pine radial growth rates during matched wet and dry years pre- and post-1950.

So what did they find? Overall, the two researchers report finding a post-1950 radial growth enhancement that was "more pronounced during drought years compared with wet years, and the greatest response occurred at the most stressed site." As for the magnitude of the response, they determined that "the relative change in growth [was] upward at seven of our [eight] sites, ranging from 11 to 133%."

With respect to the meaning and significance of their observations, Soule and Knapp say their results "showing that radial growth has increased in the post-1950s period ... while climatic conditions have generally been unchanged, suggest that nonclimatic driving forces are operative." In addition, they say that "these radial growth responses are generally consistent with what has been shown in long-term open-top chamber (Idso and Kimball, 2001) and FACE studies (Ainsworth and Long, 2005)." Hence, they say their findings suggest that "elevated levels of atmospheric CO2 are acting as a driving force for increased radial growth of ponderosa pine, but that the overall influence of this effect may be enhanced, reduced or obviated by site-specific conditions."

Summarizing their findings -- which illustrate what one would expect from the results of the growth chamber and field studies described above -- Soule and Knapp recount how they had "hypothesized that ponderosa pine...would respond to gradual increases in atmospheric CO2 over the past 50 years, and that these effects would be most apparent during drought stress and on environmentally harsh sites," and they state in their very next sentence that their results "support these hypotheses." Hence, they conclude their paper by stating it is likely that "an atmospheric CO2-driven growth-enhancement effect exists for ponderosa pine growing under specific natural conditions within the [USA's] interior Pacific Northwest," providing yet another important real-world example of the ongoing CO2-induced greening of the earth.

References
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Arnone, J.A. 1997. Temporal responses of community fine root populations to long-term elevated atmospheric CO2 and soil nutrient patches in model tropical ecosystems. Acta Oecologia 18: 367-376.

Berntson, G.M. 1994. Modeling root architecture: are there tradeoffs between efficiency and potential of resource acquisition? New Phytologist 127: 483-493.

Berntson, G.M. and Bazzaz, F.A. 1998. Regenerating temperate forest mesocosms in elevated CO2: belowground growth and nitrogen cycling. Oecologia 113: 115-125.

DeLucia, E.H., Callaway, R.M., Thomas, E.M. and Schlesinger, W.H. 1997. Mechanisms of phosphorus acquisition for ponderosa pine seedlings under high CO2 and temperature. Annals of Botany 79: 111-120.

Idso, S.B. and Kimball, B.A. 2001. CO2 enrichment of sour orange trees: 13 years and counting. Environmental and Experimental Botany 46: 147-153.

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.

Maherali, H. and DeLucia, E.H. 2000. Interactive effects of elevated CO2 and temperature on water transport in ponderosa pine. American Journal of Botany 87: 243-249.

Runion, G.B., Mitchell, R.J., Rogers, H.H., Prior, S.A. and Counts, T.K. 1997. Effects of nitrogen and water limitation and elevated atmospheric CO2 on ectomycorrhiza of longleaf pine. New Phytologist 137: 681-689.

Soule, P.T. and Knapp, P.A. 2006. Radial growth rate increases in naturally occurring ponderosa pine trees: a late-20th century CO2 fertilization effect? New Phytologist: 10.1111/j.1469-8137.2006.01746.x.

Tingey, D.T., Johnson, M.G. and Phillips, D.L. 2005. Independent and contrasting effects of elevated CO2 and N-fertilization on root architecture in Pinus ponderosa. Trees 19: 43-50.

Walker, R.F., Geisinger, D.R., Johnson, D.W. and Ball, J.T. 1998a. Atmospheric CO2 enrichment and soil N fertility effects on juvenile ponderosa pine: Growth, ectomycorrhizal development, and xylem water potential. Forest Ecology and Management 102: 33-44.

Walker, R.F., Johnson, D.W., Geisinger, D.R. and Ball, J.T. 1998b. Growth and ectomycorrhizal colonization of ponderosa pine seedlings supplied different levels of atmospheric CO2 and soil N and P. Forest Ecology and Management 109: 9-20.