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Weeds (Non-Parasitic - Competitiveness) -- Summary
Climate alarmists often suggest that weeds will become more aggressive as the air's CO2 content continues to climb, making them greater threats to the wellbeing of both natural ecosystems and farming operations. Does this claim have any merit? A brief review of the pertinent scientific literature pertaining to non-parasitic weeds suggests it does not.

Wayne et al. (1999) grew a common agricultural weed (field mustard) at six densities in pots exposed to atmospheric CO2 concentrations of 350 and 700 ppm, sequentially harvesting them during the growing season. In doing so, they found that early in stand development the extra CO2 increased above-ground biomass in a density-dependent manner; with the greatest stimulation (141%) occurring at the lowest weed density (20 plants per square meter) and the smallest stimulation (59%) occurring at the highest weed density (652 plants per square meter). However, as the stands matured, the density-dependence of the growth response disappeared; and the CO2-enriched weeds exhibited an average above-ground biomass that was 34% greater than that of the weeds grown in ambient air, which response is similar to that of most herbaceous plants (a 30 to 50% increase for a doubling of the air's CO2 content) and less than that of most woody species (50% and up). Consequently, in currently-farmed or abandoned agricultural fields, as well as in regenerating forests, it is unlikely that the competition provided by field mustard plants will be enhanced to any significant degree as the air's CO2 content continues to rise.

Caporn et al. (1999) grew bracken -- a serious weed and potential threat to human health in the United Kingdom and elsewhere -- for 19 months in controlled environment chambers maintained at atmospheric CO2 concentrations of 370 and 570 ppm and normal or high levels of fertilization, finding that the extra 200 ppm of CO2 increased rates of net photosynthesis by 30 to 70%, depending upon soil fertility and time of year. However, the elevated CO2 did not increase total plant biomass, nor did it increase the biomass of any plant organs, including rhizomes, roots and fronds. In fact, the only significant effect of elevated CO2 on plant growth was observed in the normal nutrient regime, where it actually reduced average frond area. Consequently, the competitiveness of bracken may well decline in a CO2-enriched world of the future.

Gavazzi et al. (2000) grew one-year-old loblolly pine seedlings for four months in pots that were seeded with a variety of C3 and C4 weeds and maintained at adequate and inadequate levels of soil moisture within growth chambers maintained at atmospheric CO2 concentrations of 360 and 660 ppm. They found that the elevated CO2 increased pine seedling biomass by 22%, but that it decreased total weed biomass by 22%, while reducing the percentage of weed biomass composed of C4 species, decreasing it from 53% to 35%. In addition, there was a CO2-induced increase in root-to-shoot ratio under water stressed conditions in the pine seedlings that Gavazzi et al. felt could "contribute to an improved ability of loblolly pine to compete against weeds on dry sites under elevated CO2 levels."

Ziska (2003) grew Canada thistle, which he describes as "the most frequently listed noxious weed species in surveys of the continental United States and southern Canada," in pots that were watered to the drip point daily with one of three complete nutrient solutions that differed only in nitrogen (N) concentration (3.0, 6.0 or 14.5 mM) in controlled environment chambers maintained at 287 and 373 ppm CO2 from seeding until flowering, which occurred at 77 days after seeding (DAS). He reports that "N supply did not affect the relative response to CO2 for any measured vegetative parameter up to 77 DAS." Hence, averaged across the three nitrogen treatments, the 86-ppm increase in atmospheric CO2 concentration employed in this study increased total plant biomass by 65.5%, which for the full 100 ppm CO2 increase experienced over the course of the Industrial Revolution (initial value ~275 ppm, current value ~375 ppm) translates into an approximate 76% biomass increase.

To assess the significance of this CO2-induced increase in weed biomass, it is necessary to compare it with what would have been expected for crops with which Canada thistle competes; and Mayeux et al. (1997) obtained data from which we have calculated that the 100-ppm increase in atmospheric CO2 concentration experienced over the course of the Industrial Revolution should have produced yield increases of 70 and 74% in the two wheat varieties they studied when grown under well-watered conditions comparable to those studied by Ziska. In addition, based on the voluminous data summarized by Idso and Idso (2000), these results can be scaled to derive comparable CO2-induced growth enhancements of 84% for other C3 cereals, 74% for legumes, and 80% for root and tuber crops. Hence, it would appear that the CO2-induced growth enhancement likely experienced by Canada thistle over the course of the Industrial Revolution was not much different from the growth enhancements experienced by most of the crops with which it competes, which suggests that the competitive ability of this noxious weed against these crops has remained largely unaffected by the historical increase in the air's CO2 content.

Three years later, Ziska and Goins (2006) grew genetically modified (Round-up Ready) soybean plants in the field within aluminum chambers maintained at ambient and ambient + 250 ppm atmospheric CO2 concentrations for two full growing seasons under conditions -- both environmental and managerial, including herbicide (glyphosate) application -- that led to a variety of different weed densities developing among the soybean plants. Based on linear relationships they thereby developed for soybean seed yield vs. weed biomass in the ambient and CO2-enriched treatments, it can be determined that for weed biomasses of 0, 200, 400, 600 and 800 gm-2, the seed yield enhancements provided by the extra 250 ppm of CO2 were on the order of 25, 33, 50, 90 and 250%, respectively, ultimately becoming infinite at the point where seed yield in the ambient-air treatment dropped to zero, i.e., at a weed density of approximately 920 gm-2. Also of note was the fact that soybean seed yield in the CO2-enriched treatment was calculated to not drop to zero until weed biomass reached a value of approximately 1250 gm-2. Consequently, as has been shown to be the case with certain other environmental stressors, such as plant diseases, lack of water and high temperatures, the atmospheric CO2 enrichment of this study boosted crop yield by an ever-increasing percentage as the stress of the expanding weed population grew ever larger, helping the soybean plants most when they needed it most.

At about the same time, Kao-Kniffin and Balser (2007) grew invasive reed canary grass from seed for four months in well-watered mesocosms located within greenhouses maintained at atmospheric CO2 concentrations of either 365 or 600 ppm in soils of either low or high nitrogen (N) supply (5 mg N l-1 or 30 mg N l-1) under conditions where the invading species was either dominant (high invasion: >90% cover) or sub-dominant (low invasion: <50% cover), and where the remaining surface portions of the mesocosms were covered with native graminoids (grasses, sedges and bulrushes) and native forbs that were also grown from seed. In doing so, they learned that elevating the air's CO2 content only increased belowground biomass in the plant communities moderately invaded by reed canary grass; and that the only plants to show a significant increase in aboveground biomass were the native graminoids in the moderately invaded low N treatment. Hence, they concluded that "when CO2 concentrations rise in the future, wetland plant communities comprised of native graminoids may be better able to hinder reed canary grass invasion, particularly under low N environments," which most biologists would likely judge to be a positive outcome.

Working at the Tasmanian Free-Air CO2 Enrichment (TasFACE) facility, which is located in a native lowland grassland in the southern midlands region of Tasmania, Australia, Williams et al. (2007) studied the impacts of a 170-ppm increase in atmospheric CO2 concentration and a 2C rise in air temperature over the period stretching from the spring of 2003 to the summer of 2006, during which time they documented annual seed production, seedling emergence, seedling survival and adult survival of four abundant perennial species, including the two most dominant invading weeds: Hypochaeris radicata L. and Leontodon taraxacoides (Vill.) Merat, which are members of the Asteraceae family. This effort led to their finding that there were no significant CO2-induced differences in the population growth rates of either weed species; but they found that the population growth rates of both of them "were substantially reduced by warming." The six researchers thus concluded that "global warming may be a more important determinant of the success of invasive species than CO2 concentration," and that both of the invading weed species they studied "are likely to be excluded from the grassland community by increasing temperatures."

In concluding this brief review, we end with a study of the infamous dandelion, wherein McPeek and Wang (2007) collected seeds from a single plant in Speedway, Indiana (USA), which they allowed to sprout and grow until reaching reproductive maturity in pots placed within each of two controlled-environment chambers, one continually flushed with ambient air of 370 ppm CO2 and the other maintained at an elevated atmospheric CO2 concentration of 730 ppm. Then, after harvesting the plants and measuring numerous parameters related to their reproductive prowess, they conducted a second similar experiment where they measured various parameters related to the germination of the seeds produced in the two CO2 treatments, along with the physical characteristics of the second-generation plants 35 days after planting.

Following this protocol, McPeek and Wang found that the dandelion plants "produced 83% more inflorescences and 32% more achenes, i.e., single-seed fruits, per plant at elevated than at ambient CO2," and that the "seeds from elevated CO2-grown plants were significantly heavier and had a higher germination percentage, leading to larger seedlings and earlier establishment in the subsequent generation." Furthermore, they say that "achenes from plants grown at elevated CO2 had characteristics, such as higher stalks at seed maturity, longer beaks, and larger pappi, which would increase the distance of seed dispersal by wind."

In light of these findings, the two researchers were forced to conclude that "dandelion can potentially become more widespread and noxious as atmospheric CO2 continues to rise because of human activities." And so it can; but so also can the desirable plants of the earth be benefited by the ongoing rise in the air's CO2 content -- and they are! Consequently, and especially in light of the several results we have discussed above, we conclude that non-parasitic weeds will likely be no more competitive in a high-CO2 world of the future than they are today. And the bulk of the evidence we have reviewed suggests than many of them could actually end up being a little less competitive.

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.

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.

Idso, C.D. and Idso, K.E. 2000. Forecasting world food supplies: The impact of the rising atmospheric CO2 concentration. Technology 7S: 33-56.

Kao-Kniffin, J. and Balser, T.C. 2007. Elevated CO2 differentially alters belowground plant and soil microbial community structure in reed canary grass-invaded experimental wetlands. Soil Biology & Biochemistry 39: 517-525.

Mayeux, H.S., Johnson, H.B., Polley, H.W. and Malone, S.R. 1997. Yield of wheat across a subambient carbon dioxide gradient. Global Change Biology 3: 269-278.

McPeek, T.M. and Wang, X. 2007. Reproduction of dandelion (Taraxacum officinale) in a higher CO2 environment. Weed Science 55: 334-340.

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.

Williams, A.L., Wills, K.E., Janes, J.K., Vander Schoor, J.K., Newton, P.C.D. and Hovenden, M.J. 2007. Warming and free-air CO2 enrichment alter demographics in four co-occurring grassland species. New Phytologist 176: 365-374.

Ziska, L.H. 2003. The impact of nitrogen supply on the potential response of a noxious, invasive weed, Canada thistle (Cirsium arvense) to recent increases in atmospheric carbon dioxide. Physiologia Plantarum 119: 105-112.

Ziska, LH. and Goins, E.W. 2006. Elevated atmospheric carbon dioxide and weed populations in glyphosate treated soybean. Crop Science 46: 1354-1359.

Last updated 30 July 2008