Paper Reviewed
Beidler, K.V., Taylor, B.N., Strand, A.E., Cooper, E.R., Schonholz, M. and Pritchard, S.G. 2015. Changes in root architecture under elevated concentrations of CO2 and nitrogen reflect alternate soil exploration strategies. New Phytologist 205: 1153-1163.

Working at the Duke Forest free-air carbon transfer and storage (FACTS-1) site in Orange County, North Carolina, USA, Beidler et al. (2015) studied the impacts of a 200-ppm increase in the air's CO2 concentration on the fine-root properties of the loblolly pine (Pinus taeda) trees that had been growing there over the prior 14 years, six of which years also saw the trees receiving extra nitrogen fertilization.

This work revealed, in their words, that "CO2 enrichment resulted in the development of a fine-root pool that was less dichotomous and more exploratory under N-limited conditions." As for the implications of this finding, the six scientists write that it emphasizes "the importance of architectural plasticity in response to environmental change and suggests that changes in root architecture may allow trees to effectively exploit larger volumes of soil, thereby pre-empting progressive nutrient limitations." Adding further emphasis to this finding, they also write that "the smaller effect of CO2 enrichment on fine-root biomass, relative to the larger increase in fine-root length density in soil, coupled with a shift toward a fine-root pool dominated by smaller diameter roots, may partly explain the enhanced N uptake efficiency observed in CO2-enriched plots in this study."

And so we have yet another demonstration of the fact that the once highly-touted Progressive Nitrogen Limitation Hypothesis clearly does not apply to the real world of nature; for as Beidler et al. write in the concluding paragraph of their paper, "these architectural changes are commonly considered more efficient in nutrient acquisition and may be allowing plants to effectively exploit larger volumes of soil, thereby pre-empting progressive nutrient limitations," while further citing the additional confirmatory work of Larigauderie et al. (1994), BassiriRad et al. (2001) and Drake et al. (2011)."

References
BassiriRad, H., Gutschick, V.P. and Lussenhop, J. 2001. Root system adjustments: regulation of plant nutrient uptake and growth responses to elevated CO2. Oecologia 126: 305-320.

Drake, J.E., Gallet-Budynek, A., Hofmockel, K.S., Bernhardt, E.S., Billings, S.A., Jackson, R.B., Johnsen, K.S., Lichter, J. , McCarthy, H.R., McCormack, M.I., et al. 2011. Increases in the flux of carbon belowground stimulate nitrogen uptake and sustain the long-term enhancement of forest productivity under elevated CO2. Ecology Letters 14: 349-357.

Larigauderie, A., Reynolds, J.F. and Strain, B.R. 1994. Root response to CO2 enrichment and nitrogen supply in loblolly pine. Plant and Soil 165: 21-32.