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CO2 Enrichment "Levels Playing Field" in Ragweed Study
Stinson, K.A. and Bazzaz, F.A. 2006. CO2 enrichment reduces reproductive dominance in competing stands of Ambrosia artemisiifolia (common ragweed). Oecologia 147: 155-163.

What was done
The authors grew well-watered stands of common ragweed (Ambrosia artemisiifolia L.) out-of-doors in open-top-chambers maintained at either 360 or 720 ppm CO2 from the seedling stage to the onset of senescence, after which the plants were harvested and the dry masses of their shoots, roots and reproductive structures determined. In addition, at 14, 33 and 52 days after the start of the experiment, the heights and numbers of leaves of all plants were measured.

What was learned
Doubling the air's CO2 concentration increased the mean stand-level biomass of the shoots of the ragweed plants by 44%, while it increased the biomass of their roots and reproductive structures by 46% and 94%, respectively, for a total CO2-induced biomass increase of 70%. Of perhaps even greater interest, however, Stinson and Bazzaz report that the extra CO2 "reduced the coefficients of variation for all aspects of plant growth, especially reproductive biomass," such that the CO2-induced growth enhancements were "more pronounced in small, rather than large plants." That is to say, as they rephrase their findings, "growth enhancements to smaller plants diminished the relative biomass advantages of larger plants in increasingly crowded conditions," or as they state in yet another place in their paper, "CO2-induced growth gains of subordinate A. artemisiifolia plants minimize differences in the reproductive output of small and large plants."

What it means
The Harvard University scientists conclude that "more homogeneous reproduction between subordinates and dominants also implies that a larger number of individuals will contribute propagules to future generations," which phenomenon, in their words, "could in turn affect evolutionary and population dynamics." We agree, while wondering if what they found to be true for within-species subordinates and dominants might also be true for among-species subordinates and dominants. If it is, this preferential stimulation of growth responses to atmospheric CO2 enrichment in subordinate species, could well help those that are endangered to better withstand whatever forces might be pushing them towards extinction.

Reviewed 12 April 2006