What was done
At the Duke Forest FACE facility in the Piedmont region of North Carolina (USA), which was established in an unmanaged plantation of 13-year-old loblolly pine (Pinus taeda L.) trees, the authors (1) collected in ground traps the seeds released by the trees, (2) counted the number of seeds collected, and (3) analyzed various properties of the seeds over the twelve-year period 1997-2008, during which time the trees were growing in either ambient air or air enriched with an extra 200 ppm of CO2.

What was learned
Way et al. report that "the number of mature, viable seeds doubled per unit basal area in high-CO2 plots from 1997 to 2008 (P<0.001)," but they found that "there was no CO2 effect on mean seed mass, viability, or nutrient content," which they say "is consistent with observations from the few other studies examining reproductive responses to elevated CO2 in trees," citing the earlier work of LaDeau and Clark (2001, 2006), Stiling et al. (2004) and Kimball et al. (2007). In addition, they report that "the mass of male catkins collected in ground traps was more than doubled over two years in elevated CO2 plots compared with ambient plots."

What it means
On the basis of their own and others' observations, the seven scientists concluded that "increased production of high-quality seeds by woody species in response to rising CO2 would give them a reproductive advantage over herbaceous species that produce more seeds but cannot maintain seed quality," and they opine that this phenomenon "may facilitate woody encroachment into herbaceous communities, a wide-spread phenomenon that has already been linked to rising CO2 (Bond and Midgley, 2000; Davis et al., 2007)."

References
Bond, W.J. and Midgley, G.F. 2000. A proposed CO2-controlled mechanism of woody plant invasion in grasslands and savannas. Global Change Biology 6: 865f-869.

Davis, M.A., Reich, P.B., Knoll, M.J.B., Dooley, L., Hundtoft, M. and Attleson, I. 2007. Elevated atmospheric CO2: a nurse plant substitute for oak seedlings establishing in old fields. Global Change Biology 13: 2308-2316.

Kimball, B.A., Idso, S.B., Johnson, S. and Rillig, M.C. 2007. Seventeen years of carbon dioxide enrichment of sour orange trees: final results. Global Change Biology 13: 2171-2183.

LaDeau, S.L. and Clark, J.S. 2001. Rising CO2 levels and the fecundity of forest trees. Science 292: 95-98.

LaDeau, S.L. and Clark, J.S. 2006. Elevated CO2 and tree fecundity: the role of tree size, inter-annual variability and population heterogeneity. Global Change Biology 32: 822-833.

Stiling, P., Moon, D., Hymus, G. and Drake, B. 2004. Differential effects of elevated CO2 on acorn density, weight, germination, and predation among three oak species in a scrub-oak forest. Global Change Biology 10: 228-232.