What was done
The authors grew well-watered (via drip irrigation) 1.5-year-old ponderosa pine (Pinus ponderosa Dougl.) seedlings for four additional years in open-top chambers maintained at ambient atmospheric CO2 concentrations and at ambient + 175 ppm and ambient + 300 ppm CO2, while simultaneously imposing three levels of soil nitrogen (N) fertilization (0, 10, 20 g N m^-2 year^-1) upon the plants. Every two months throughout this period, they collected video images of roots on the surfaces of three minirhizotron tubes installed in each chamber.

What was learned
Phillips et al. report that yearly values of fine-root (1) standing crop, (2) production and (3) mortality were consistently higher in elevated CO2 treatments throughout the study. They also report that "in this same study, Johnson et al. (2000) found that elevated CO2 increased fine-root [4] life span." However, because elevated CO2 increased fine-root (5) length, they found that "the amount of root length dying per year was actually greater." Therefore, as they describe it, "the higher rates of mortality in absolute terms for elevated CO2 are driven by increased standing crop and not reduced life spans." In addition, they say that Tingey et al. (2005) found that "in the elevated CO2 treatments, fine roots explored the soil more [6] extensively and [7] deeper, and filled in the explored areas more [8] intensively."

With respect to the progressive nitrogen limitation (PNL) hypothesis, Phillips et al. note that "the increased fine-root length reported here explains how additional N was provided to support the increased whole plant growth in elevated CO2 treatments, and corresponds with the increased extent and intensity of the root system architecture discussed by Tingey et al. (2005)." This "mining of soil N," as they continue, "can in some cases go on for substantial lengths of time, and there is no evidence that PNL occurred during the course of this study (Johnson et al., 2006)."

What it means
Atmospheric CO2 enrichment enhanced a number of fine-root properties of juvenile ponderosa pines in this study, allowing them to better explore the soil in which they grew and find and acquire the nitrogen they needed to support enhanced whole-tree growth in this favorable aerial environment. Since many other studies report similar findings (see Roots (Trees) in our Subject Index), such will likely be the case with most of earth's woody plants as the air's CO2 content continues to rise.

References
Johnson, D.W., Hoylman, A.M., Ball, J.T. and Walker, R.F. 2006. Ponderosa pine responses to elevated CO2 and nitrogen fertilization. Biogeochemistry (in press).

Johnson, M.G., Phillips, D.L., Tingey, D.T. and Storm, M.J. 2000. Effects of elevated CO2, N-fertilization, and season on survival of ponderosa pine fine roots. Canadian Journal of Forest Research 30: 220-228.

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