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Photosynthetic Responses of Fast-Growing Poplar "Forests" to Atmospheric CO2 Enrichment
Reference
Wittig, V.E., Bernacchi, C.J., Zhu, X.-G., Calfapietra, C., Ceulemans, R., DeAngelis, P., Gielens, B., Miglietta, F., Morgan, P.B. and Long, S.P.  2005.  Gross primary production is stimulated for three Populus species grown under free-air CO2 enrichment from planting through canopy closure.  Global Change Biology 33: 644-656.

What was done
The authors grew stands of closely-spaced (1 m x 1 m) individuals of three Populus species - white poplar (P. alba), black poplar (P. nigra) and robusta poplar (P. x euramericana) - in a FACE study from the time of planting through canopy closure to coppice (aboveground tree harvest) at atmospheric CO2 concentrations of 370 and 550 ppm for three full years.  Based on measurements of meteorological conditions that were recorded at 30-minute intervals and biweekly measurements of leaf area index, as well as measurements of various photosynthetic parameters made at regular intervals throughout the entire period, the photosynthetic rates of different leaf classes were determined for monthly intervals and summed to obtain annual canopy photosynthesis or gross primary production (GPP) in each of the three years of the study.

What was learned
Wittig et al. report that "significant stimulation of GPP driven by elevated CO2 occurred in all 3 years, and was greatest in the first year (223-251%), but markedly lower in the second (19-24%) and third years (5-19%)."  Interestingly, this decline in CO2-induced growth stimulation was not due to photosynthetic acclimation, but simply a consequence of canopy closure and the increased shading of leaves that accompanies it.

Averaged across all species and plots, the CO2-induced stimulation of annual GPP was 234%, 22% and 11% in 1999, 2000 and 2001, respectively, while averaged over the entire three-year period, the GPP enhancements for P. alba, P. nigra and P. x euramericana were, respectively, 17%, 17% and 25%.  The authors also note that these results "were consistent with independent measurements of net primary production, determined independently from biomass increments and turnover."

What it means
In the concluding sentence of their paper, Wittig et al. say their results suggest that "with selection, nutrient and moisture supply, coppice managed plantation poplars have the potential for large and sustained increases in GPP."  To standardize their results to those obtained from the greater number of experiments where the air's CO2 concentration was increased by 300 ppm, as compared to their 180 ppm, for example, P. alba, P. nigra and P. x euramericana could have been expected to have registered GPP increases on the order of 28%, 28% and 42%, respectively, over the course of this 3-year study.

Reviewed 20 July 2005