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Elevated CO2 Enhances the Effectiveness of a Bio-Energy Tree Plantation
Reference
Liberloo, M., Calfapietra, C., Lukac, M., Godbold, D., Luo, Z.-B., Polle, A., Hoosbeek, M.R., Kull, O., Marek, M., Raines, C., Rubino, M., Taylor, G., Scarascia-Mugnozza, G. and Ceulemans, R. 2006. Woody biomass production during the second rotation of a bio-energy Populus plantation increases in a future high CO2 world. Global Change Biology 12: 1094-1106.

What was done
The authors grew three well-watered and adequately fertilized poplar species - Populus alba L. clone 2AS-11 (white poplar), Populus nigra L. clone Jean Pourtet (black poplar), Populus x euramericana clone I-214 (robusta poplar) - for three years at the POPFACE (EUROFACE) facility in Central Italy near Viterbo, where the air's CO2 concentration was increased by approximately 180 ppm in half of the experimental plots, after which the trees were coppiced (cut to the bases of their stems some 5-8 cm above the ground) and allowed to sprout and grow again for another three years under the same, but even better fertilized, conditions. The results of the first 3-year growth period were reported by Liberloo et al. (2005), while Liberloo et al. (2006) describe the results of the second 3-year period.

What was learned
Interestingly, fertilization did not affect the growth of the second-rotation trees, "likely because of the high rates of fertilization during the previous agricultural land use," according to the 14 researchers involved with the experiment. "In contrast," in their words, "elevated CO2 enhanced biomass production by up to 29%, and this stimulation did not differ between above- and below-ground parts." They also report that the net rate of carbon assimilation was "on average for all species stimulated up to 30% during the third year of the second rotation," and that "after 6 years of fumigation, measurements of photosynthetic parameters along the canopy profile could not detect any clear sign of acclimation to elevated CO2" for the three species.

What it means
Liberloo et al. conclude that "poplar trees are able to optimally profit from future high CO2 concentrations, provided that they are intensively managed, planted in regions with high incident radiation and supplied with sufficient nutrients and water." Such "high-density poplar coppice cultures," in their opinion, "offer possibilities to mitigate the rise of atmospheric CO2 by producing renewable bio-energy in an economically feasible way, whereby the elevated CO2 stimulation might sustain over several rotation cycles."

Reference
Liberloo, M., Dillen, S.Y., Calfapietra, C., Marinari, S., Luo, Z.B., De Angelis, P. and Ceulemans, R. 2005. Elevated CO2 concentration, fertilization and their interaction: growth stimulation in a short-rotation poplar coppice (EUROFACE). Tree Physiology 25: 179-189.

Reviewed 23 August 2006