Dukes (2002) grew model serpentine grasslands common to California, USA, in competition with the invasive forb Centaurea solstitialis at atmospheric CO2 concentrations of 350 and 700 ppm for one year, determining that elevated CO2 increased the biomass proportion of this weedy species in the community by a mere 1.2%, while total community biomass increased by 28%.  Similarly, Gavazzi et al. (2000) grew loblolly pine seedlings for four months in competition with both C3 and C4 weeds at atmospheric CO2 concentrations of 260 and 660 ppm, reporting that elevated CO2 increased pine biomass by 22% while eliciting no response at all from either type of weed.  Likewise, in a study of pasture ecosystems near Montreal, Canada, Taylor and Potvin (1997) found that elevated CO2 concentrations did not influence the number of native species returning after their removal (to simulate disturbance), even in the face of the introduced presence of the C3 weed Chenopodium album, which normally competes quite effectively with several slower-growing crops in ambient air.  In fact, atmospheric CO2 enrichment did not impact the growth of this weed in any measurable way.

Ziska et al. (1999) also studied the C3 weed C. album, along with the C4 weed Amaranthus retroflexus, in glasshouses maintained at atmospheric CO2 concentrations of 360 and 720 ppm.  They determined that elevated CO2 significantly increased the photosynthetic rate and total dry weight of the C3 weed, but that it had no effect at all on the C4 weed.  Also, they found that the growth response of the C3 weed to a doubling of the air's CO2 content was approximately 51%, which is about the same as the average 52% growth response tabulated by Idso (1992), and that obtained by Poorter (1993) for rapidly-growing wild C3 species (54%), which finding suggests there is no enhanced dominance of the C3 weed over other C3 plants in a CO2-enriched environment.

Wayne et al. (1999) studied another agricultural weed, field mustard (Brassica kaber), which was sewn in pots at six densities, placed in atmospheric CO2 concentrations of 350 and 700 ppm, and sequentially harvested during the growing season.  Early in stand development, elevated CO2 increased aboveground weed biomass in a density-dependent manner; with the greatest stimulation of 141% occurring at the lowest density (corresponding to 20 plants per square meter) and the smallest stimulation of 59% occurring at the highest density (corresponding to 652 plants per square meter).  However, as stands matured, the density-dependence of the CO2-induced growth response disappeared, and CO2-enriched plants exhibited an average aboveground biomass that was 34% greater than that of ambiently-grown plants across a broad range of plant densities.  Moreover, this final growth stimulation was similar to that of most other herbaceous plants exposed to atmospheric CO2 enrichment (30 to 50% biomass increases for a doubling of the air's CO2 content), once again evidencing that atmospheric CO2 enrichment confers no undue advantage upon weeds at the expense of other plants.

In a study of a weed that affects both plants and animals, Caporn et al. (1999) examined bracken (Pteridium aquilinum), which poses a serious weed problem and potential threat to human health in the United Kingdom and other regions, growing specimens for 19 months in controlled environment chambers maintained at atmospheric CO2 concentrations of 370 and 570 ppm and normal or high levels of soil fertility.  They found that the high CO2 treatment consistently increased rates of net photosynthesis by 30 to 70%, depending on soil fertility and time of year.  However, elevated CO2 did not increase total plant dry mass or the dry mass of any plant organ, including rhizomes, roots and fronds.  In fact, the only significant effect of elevated CO2 on bracken growth was observed in the normal nutrient regime, where elevated CO2 actually reduced mean frond area.

Finally, in a study involving two parasitic species (Striga hermonthica and Striga asiatica), Watling and Press (1997) reported that total parasitic biomass per host plant at an atmospheric CO2 concentration of 700 ppm was 65% less than it was in ambient air.  And in a related study, Dale and Press (1999) observed that the presence of a parasitic plant (Orobanche minor) reduced its host's biomass by 47% in ambient air of 360 ppm CO2, while it only reduced it by 20% in air of 550 ppm CO2.

These several studies suggest that the ongoing rise in the air's CO2 content likely will not favor the growth of weedy species over that of crops and native plants.  In fact, it may well provide non-weeds greater protection against weed-induced decreases in their productivity and growth.  Thus, future increases in the air's CO2 content may actually increase the competitiveness of non-weeds over weeds.

References
Caporn, S.J.M., Brooks, A.L., Press, M.C. and Lee, J.A.  1999.  Effects of long-term exposure to elevated CO2 and increased nutrient supply on bracken (Pteridium aquilinum).  Functional Ecology 13: 107-115.

Dale, H. and Press, M.C.  1999.  Elevated atmospheric CO2 influences the interaction between the parasitic angiosperm Orobanche minor and its host Trifolium repens.  New Phytologist 140: 65-73.

Dukes, J.S.  2002.  Comparison of the effect of elevated CO2 on an invasive species (Centaurea solstitialis) in monoculture and community settings.  Plant Ecology 160: 225-234.

Gavazzi, M., Seiler, J., Aust, W. and Zedaker, S.  2000.  The influence of elevated carbon dioxide and water availability on herbaceous weed development and growth of transplanted loblolly pine (Pinus taeda).  Environmental and Experimental Botany 44: 185-194.

Taylor, K. and Potvin, C.  1997.  Understanding the long-term effect of CO2 enrichment on a pasture: the importance of disturbance.  Canadian Journal of Botany 75: 1621-1627.

Watling, J.R. and Press, M.C.  1997.  How is the relationship between the C4 cereal Sorghum bicolor and the C3 root hemi-parasites Striga hermonthica and Striga asiatica affected by elevated CO2?  Plant, Cell and Environment 20: 1292-1300.

Wayne, P.M., Carnelli, A.L., Connolly, J. and Bazzaz, F.A.  1999.  The density dependence of plant responses to elevated CO2.  Journal of Ecology 87: 183-192.

Ziska, L.H., Teasdale, J.R. and Bunce, J.A.  1999.  Future atmospheric carbon dioxide may increase tolerance to glyphosate.  Weed Science 47: 608-615.