What was done
Among a number of other things, the authors studied the effects of long-term exposure of birch (Betula papyrifera) trees to elevated CO2 (an extra 200 ppm) on flower and pollen production at the Aspen FACE site in Rhinelander, Wisconsin (USA) in the eighth and ninth years (2006 and 2007) of the experiment.

What was learned
Darbah et al. report they recorded "an increase of 140% and 70% for 2006 and 2007, respectively, in the total number of trees that produced male flowers under elevated CO2 and an increase of 260% in 2006 and 100% in 2007, respectively, in the quantity of male flowers produced under elevated CO2."

What it means
The six researchers state that "the increases in the number of trees and in the quantity of male flowers produced under elevated CO2 implies that more birch pollen will be produced," noting that "these results support the findings of Curtis et al. (1994, 1996), Johnson and Lincoln (2000), Edwards et al. (2001), Jablonski et al. (2002), Bunce (2005) and Ladeau and Clark (2006a,b) that elevated CO2 increases reproductive potential through increased pollination, and hence, fertilization and viable seed formation," in harmony with the hypothesis of Herms and Mattson (1992) that "birch trees under adequate carbohydrate status [such as provided by atmospheric CO2 enrichment] tend to favor male flower production." Last of all, they conclude by noting that "since sexual reproductive development is an important stage in the life cycle of plants, any change in the processes involved might have significant implications for the productivity of the plants and their survival," which implications in the case of birch trees and atmospheric CO2 enrichment would clearly be positive.

References
Bunce, J.A. 2005. Seed yield of soybeans with daytime or continuous elevation of carbon dioxide under field conditions. Photosynthetica 43: 435-438.

Curtis, P.S., Snow, A.A. and Miller, A.S. 1994. Genotype-specific effects of elevated CO2 on fecundity in wild radish (Raphanus raphanistum). Oecologia 97: 100-105.

Curtis, P.S., Klus, D.J., Kalisz, S. and Tonsor, S.J. 1996. Intraspecific variation in CO2 responses in Raphanus raphinistum and Plantago lanceolata: assessing the potential for evolutionary change with rising atmospheric change. In: Korner, C. and Bazzaz, F.A. (Eds.), Carbon Dioxide, Populations and Communities. Academic Press, New York, NY, USA, pp. 13-22.

Edwards, G.R., Clark, H. and Newton, P.C.D. 2001. The effects of elevated CO2 on seed production and seedling recruitment in a sheep grazed pasture. Oecologia 127: 383-394.

Jablonski, L.M., Wang, X. and Curtis, P.S. 2002. Plant reproduction under elevated CO2 conditions: a meta analysis of reports on 79 crop and wild species. New Phytologist 156: 9-26.

Johnson, S.L. and Lincoln, D.E. 2000. Allocation responses to CO2 enrichment and defoliation by a native annual plant Heterotheca subaxillariis. Global Change Biology 6: 767-778.

LaDeau, S.L. and Clark, J.S. 2006a. Elevated CO2 and tree fecundity: the role of tree size, interannual variability, and population heterogeneity. Global Change Biology 12: 822-833.

LaDeau, S.L. and Clark, J.S. 2006b. Pollen production by Pinus taeda growing in elevated atmospheric CO2. Functional Ecology 20: 541-547.