Lewis et al. (2002) grew the common cocklebur (Xanthium strumarium L.) in controlled-environment growth chambers maintained at atmospheric CO2 concentrations of 365 and 730 ppm for 70 days post-emergence.  During the weed's vegetative growth phase, the photosynthetic rates of the CO2-enriched plants were 30% greater than those of the plants growing in ambient air.  In the flowering period, which ensued shortly thereafter, this stimulation was reduced to 10%, whereupon it increased to 20% during the fruiting period.

Gibeaut et al. (2001) grew the common weed Arabidopsis thaliana for seven weeks in controlled-environment chambers maintained at atmospheric CO2 concentrations of 360 and 1000 ppm, finding that the 640-ppm increase in the air's CO2 concentration increased the relative growth rate of the plants by about 20% during the first three weeks of the study.  Concomitantly, the extra CO2 increased the activity of the enzyme UDP-glucose dehydrogenase (an important enzyme involved in cell wall biosynthesis) by approximately 25%.  Thereafter, however, relative growth rate was the same in both treatments.  Nevertheless, by the end of the study the CO2-enriched plants had produced 2.3 times more biomass than the ambiently-grown plants.

Ziska (2002) grew Canadian thistle (Cirsium arvense L. Scop.) plants in controlled-environment chambers maintained at atmospheric CO2 concentrations of 280, 380 and 720 ppm for approximately two months.  They found that the first increment of extra CO2 enhanced rates of photosynthesis and total plant biomass production by 45 and 126%, respectively, while the second increment enhanced these two parameters by 49 and 69%.

Leishman et al. (1999) grew four weedy C3 plants common to European grasslands (Cardamine hirsute, Spergula arvensis, Senecio vulgaris, and Poa annua) from seed to senescence in glasshouses maintained at atmospheric CO2 concentrations of 350 and 550 ppm at two different light intensities: full light and 67% of full light.  Interestingly, the extra 200 ppm of CO2 did not significantly impact vegetative growth in three of the species.  For Spergula arvensis, however, it increased maximum leaf length by an average of 15%, regardless of light treatment, and total dry weight by 20 and 68% at full and reduced light levels, respectively.  Likewise, it significantly enhanced reproductive success in only one of the species, increasing the number of seeds in Poa annua by 50 and 26% at full and reduced light levels, respectively.

Nagashima et al. (2003) established even-aged stands of the summer annual Chenopodium album (a weed that is commonly found in open habitats, such as abandoned fields and flood plains) at ambient and twice-ambient atmospheric CO2 concentrations and low and high levels of soil nutrient availability in open-top chambers in the experimental garden of Tohoku University, Sendai, Japan, after which they monitored the growth of individual plants every week until flowering.  At the conclusion of the experiment, they could detect no significant effect of elevated CO2 on aboveground biomass in the low nutrient regime; but in the high nutrient regime, the extra CO2 increased aboveground biomass by 50%.  In addition, as time progressed throughout the study, the CO2-induced enhancement of growth in the high nutrient regime gradually waned and ultimately disappeared altogether in smaller subordinate individuals, but it continued in larger dominants throughout the whole experiment.

Ziska and Bunce (1999) grew four C4 plants in controlled-environment chambers maintained continuously at atmospheric CO2 concentrations of 350 and 700 ppm or at a nocturnal CO2 concentration of 700 ppm and 350 ppm during the day for approximately three weeks.  Continuous CO2 enrichment caused a significant increase in photosynthesis (+13%) and total dry mass (+21%) in only one of the four species, Amaranthus retroflexus.  However, there were no significant effects of nocturnal CO2 enrichment in this species, indicating that the CO2-induced increase in biomass was not facilitated by a reduction in dark respiration rate.  Also, plants exposed to continuous CO2 enrichment did not increase their biomass due to improved internal water balance, as leaf water potentials were not significantly different among plants of any CO2 treatment.

Ziska et al. (1999) grew broad-leaved C3 (Chenopodium album) and C4 (Amaranthus retroflexus) weeds in glasshouses maintained at atmospheric CO2 concentrations of 360 and 720 ppm.  In addition, both young and mature plants of each species were sprayed with one-tenth and full-strength solutions of the chemical glyphosate ("Roundup").  The elevated CO2 significantly increased the photosynthetic rate and total dry weight (by 51%) of the unsprayed C3 weed, regardless of maturity stage, but it had no effects on these parameters in the case of the C4 weed.  Also, spraying both young and mature A. retroflexus plants with full-strength herbicide resulted in their death, regardless of atmospheric CO2 concentration, while spraying C. album plants with full-strength glyphosate severely reduced, but did not eliminate, growth in the elevated-CO2 air, whereas chemically treated plants died in ambient CO2 air.  Consequently, farmers who use glyphosate to control A. retroflexus should not need to modify their current chemical practices in the future; but to better control C. album, they may need to apply glyphosate earlier in the season when the weeds are smaller or, if applied later, at higher concentrations, as elevated CO2 slightly increases the tolerance of this particular C3 weed to glyphosate.

In summary, it would appear that atmospheric CO2 enrichment tends to increase the growth of many weeds, but possibly not quite as many, percentage-wise, as is the case with non-weeds.  So what is the bottom line with respect to this subject?  You can read our opinion in our Editorial of 10 Apr 2002.

References
Gibeaut, D.M., Cramer, G.R. and Seemann, J.R.  2001.  Growth, cell walls, and UDP-glucose dehydrogenase activity of Arabidopsis thaliana grown in elevated carbon dioxide.  Journal of Plant Physiology 158: 569-576.

Leishman, M.R., Sanbrooke, K.J. and Woodfin, R.M.  1999.  The effects of elevated CO2 and light environment on growth and reproductive performance of four annual species.  New Phytologist 144: 455-462.

Lewis, J.D., Wang, X.Z., Griffin, K.L. and Tissue, D.T.  2002.  Effects of age and ontogeny on photosynthetic responses of a determinate annual plant to elevated CO2 concentrations.  Plant, Cell and Environment 25: 359-368.

Nagashima, H., Yamano, T., Hikosaka, K. and Hirose, T.  2003.  Effects of elevated CO2 on the size structure in even-aged monospecific stands of Chenopodium album.  Global Change Biology 9: 619-629.

Ziska, L.  2002.  Influence of rising atmospheric CO2 since 1900 on early growth and photosynthetic response of a noxious invasive weed, Canada thistle (Cirsium arvense).  Functional Plant Biology 29: 1387-1392.

Ziska, L.H. and Bunce, J.A.  1999.  Effect of elevated carbon dioxide concentration at night on the growth and gas exchange of selected C4 species.  Australian Journal of Plant Physiology 26: 71-77.

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.