Background
The authors write that "O3 stress often decreases carbon allocation to roots, leading to reductions in root biomass and growth," citing Andersen (2003) and Grantz et al. (2006), adding that "reduced carbohydrate stores in roots can lead to increased susceptibility to other stresses even after O3 exposure ends," citing Andersen et al. (1997). On the other hand, they note that CO2 tends to promote just the opposite behavior by promoting fine-root production and the benefits this phenomenon provides, citing Norby et al. (2004, 2005).

What was done
In a study designed to determine which of the two trace gases (CO2 or O3) has the greater impact on the growth and development of the fine roots of ponderosa pine trees, Phillips et al. grew Pinus ponderosa seedlings for three years in one-meter-deep containers filled with reconstructed pine-forest soil within sunlit controlled-environment chambers maintained at mean atmospheric CO2 concentrations of either 420 or 690 ppm, and at mean O3 conditions described by daily SUM06 index values of either 0 or 15.7 ppm h (representing the sum of hourly O3 concentrations >= 0.06 ppm), while images of fine roots growing along the upper surfaces of four minirhizotron tubes installed within each soil bin were collected every 28 days by a color video camera.

What was learned
The researchers report that "elevated CO2 increased both the number of fine roots produced and their life span," and that "increased O3 did not reduce the effect of elevated CO2." Consequently, they found that fine root biomass at the end of the study in the CO2-enriched treatment was consistently higher in each soil horizon and 16% higher in total. In addition, the greater fine-root survivorship in the elevated CO2 treatment was associated with increasing root depth and increasing fine-root diameter, as has also been observed by Eissenstat et al. (2000), Gao et al. (2008) and Joslin et al. (2006). Last of all, they report that averaged over the course of the experiment there was a slight (3.3%) decrease in soil respiration in the elevated CO2 treatment, as observed by Tingey et al. (2006).

What it means
In the words of the four U.S. Environmental Protection Agency scientists who conducted the work, "elevated O3 did not result in significant negative impacts on ponderosa pine seedling fine-root survival ... or in countering the increased survivorship caused by elevated CO2," as the good gas won and the bad gas lost, in a proxy representation of the much greater biospheric battle that will be played out in the years and decades to come, as the concentrations of the two trace gases of the atmosphere continue rising in tandem.

References
Andersen, C.P. 2003. Source-sink balance and carbon allocation below ground in plants exposed to ozone. New Phytologist 157: 213-228.

Andersen, C.P., Wilson, R., Plocher, M. and Hogsett, W.E. 1997. Carry-over effects of O3 on root growth and carbohydrate concentration of ponderosa pine seedlings. Tree Physiology 17: 805-811.

Eissenstat, D.M., Wells, C.E., Yanai, R.D. and Whitbeck, J.L. 2000. Building roots in a changing environment: Implications for root longevity. New Phytologist 147: 33-42.

Grantz, D.A., Gunn, S. and Vu, H.-B. 2006. O3 impacts on plant development: a meta-analysis of root/shoot allocation and growth. Plant, Cell and Environment 29: 1193-1209.

Guo, D., Mitchell, R.J., Han, W., Hendricks, J.J., Fahey, T.J. and Hendrick, R.L. 2008. Fine root heterogeneity by branch order: exploring the discrepancy in root turnover estimates between minirhizotron and carbon isotopic methods. New Phytologist 177: 443-456.

Joslin, J.D., Gaudinski, J.B., Torn, M.S., Riley, W.J. and Hanson, P.J. 2006. Fine-root turnover patterns and their relationship to root diameter and soil depth in a ¹⁴C-labeled hardwood forest. New Phytologist 172: 523-535.

Norby, R.J., Ledford, J., Reilly, C.D., Miller, N.E. and O'Neill, E.G. 2004. Fine-root production dominates response of a deciduous forest to atmospheric CO2 enrichment. Proceedings of the National Academy of Sciences, USA 101: 9689-9693.

Norby, R.J., DeLucia, E.H., Gielen, B., Calfapietra, C., Giardina, C.P., King, S.J., Ledford, J., McCarthy, H.R., Moore, D.J.P., Ceulemans, R., De Angelis, P., Finzi, A.C., Karnosky, D.F., Kubiske, M.E., Lukac, M., Pregitzer, K.S., Scarasci-Mugnozza, G.E., Schlesinger, W.H. and Oren, R. 2005. Forest response to elevated CO2 is conserved across a broad range of productivity. Proceedings of the National Academy of Sciences USA 102: 18,052-18,056.

Tingey, D.T., Johnson, M.G., Lee, E.H., Wise, C., Waschmann, R., Olszyk, D.M., Watrud, L.S. and Donegan, K.K. 2006. Effects of elevated CO2 and O3 on soil respiration under ponderosa pine. Soil Biology & Biochemistry 38: 1764-1778.