Wayne et al. (2002) grew common ragweed plants from seed in controlled-environment glasshouses maintained at ambient (350 ppm) and enriched (700 ppm) atmospheric CO2 concentrations for 84 days, after which they sampled the pollen from the central plants of each stand, assessed the pollen's characteristics, and then harvested all mature seeds and above-ground shoot material. This work revealed, in their words, that "stand-level pollen production was 61% higher in elevated versus ambient CO2 environments," and that "CO2-induced growth stimulation of stand shoot biomass was similar to that of total pollen production." Although the researchers admitted it would be "challenging to accurately predict the future threat to public health caused by CO2-stimulated pollen production" -- since they say "it is likely that plant pollen production will also be influenced by factors expected to change in concert with CO2, including temperature, precipitation, and atmospheric pollutants," -- they nevertheless suggested that "the incidence of hay fever and related respiratory diseases may increase in the future."

Interestingly, in a guest editorial in the Annals of Allergy, Asthma & Immunology, Weber (2002) discussed the study of Wayne et al., which was published in the same issue of that journal. He began by saying that "one can always wonder whether such manipulations [i.e., those employed in Wayne et al.'s study] have any relationship to present reality, or indeed, conditions that one can expect in the near future," whereupon he proceeded methodically to get to his conclusion that "it would be premature to assume that increased pollen grain numbers necessarily lead to an increased aeroallergen exposure."

Elucidating some of the reasons for his assessment of the issue, Weber noted that "allergenic activity of short ragweed will vary from year to year, even from the same source and supplier (Maasch et al., 1987)," and he cited Lee et al. (1979) as having found "varying potency in plants at the same site from year to year, which [were] attributed to seasonal climatic differences, primarily of rainfall." In fact, the latter researchers found a four-fold range in the allergenic potency of ragweed pollen within a single county in Illinois (USA). Consequently, Weber concluded that "a constant relationship between pollen mass and allergenic protein content is not a given," and that it will remain speculative until it is determined whether "the increased pollen grains seen with the increased ambient CO2 levels maintain the same ratio of allergenic proteins."

A further demonstration of the tenuousness of the suggestion of Wayne et al. -- i.e., that "the incidence of hay fever and related respiratory diseases may increase in the future," due to the near-universal growth-promoting effects of atmospheric CO2 enrichment -- is provided by Rogers et al. (2006), who collected and vernalized ragweed seeds by sowing them in containers kept in a refrigerator maintained at 4°C, after which they transferred one third of the seeded containers at 15-day intervals to glasshouse modules maintained at atmospheric CO2 concentrations of either 380 or 700 ppm, where the seeds were allowed to germinate (also at 15-day intervals, with the middle germination date approximating that of plants currently growing naturally in the vicinity of where the seeds were collected), and where they remained under well-watered and fertilized conditions until they senesced and were harvested, at which time assessments of plant and allergenic pollen biomass were made.

As best we can determine from the graphical representations of Rogers et al.'s data, the end-of-season CO2-induced increase in aboveground plant biomass was about 16% for the date of emergence typical of the present, while the corresponding increase in pollen production was about 32%. However, for the 15-day earlier date of emergence, which was chosen to represent "anticipated advances of spring several decades into the future," based upon projected rates of future global warming, the end-of-season CO2-induced change in aboveground plant biomass was only about +3%, while the end-of-season CO2-induced change in pollen production was actually a negative 3%. The most meaningful way of viewing the results, therefore, is to determine the change in pollen production that would occur in going from today's atmospheric CO2 concentration and date-of-onset of spring (380 ppm, middle date of germination) to the elevated CO2 concentration and earlier date-of-onset of biological spring (700 ppm, 15-day earlier date of germination); and when this is done, the production of allergenic pollen is seen to rise by a less-than-whopping 1-2%, which is obviously totally insignificant.

Turning our attention to some other noxious plants, Caporn et al. (1999) studied bracken, a weed that poses a potential threat to human health in the United Kingdom and other regions. Specimens of this plant were grown for 19 months in controlled-environment chambers maintained at atmospheric CO2 concentrations of 370 and 570 ppm and normal and high levels of fertilization; and by so doing, it was learned that the elevated CO2 consistently increased rates of net photosynthesis in bracken by some 30 to 70%, depending upon soil fertility and time of year. However, the elevated CO2 did not increase total plant dry mass nor the dry mass of any plant organs, including rhizomes, roots, and fronds. In fact, the only significant effect of the elevated CO2 on bracken growth was observed in the normal nutrient regime, where elevated CO2 actually reduced the area of bracken fronds.

Matros et al. (2006) grew tobacco plants in pots filled with quartz sand placed in controlled-climate chambers maintained at either 350 or 1000 ppm CO2 for a period of eight weeks, where they were irrigated daily with a complete nutrient solution containing either 5 or 8 mM NH4NO3. In addition, some of the plants in each treatment were mechanically infected with potato virus Y (PVY) when they were six weeks old. At the end of the study, the researchers reported that the plants grown at elevated CO2 and 5 mM NH4NO3 "showed a marked and significant decrease in content of nicotine in leaves as well as in roots," while at 8 mM NH4NO3 the same was found to be true of upper leaves but not of lower leaves and roots. In addition, with respect to the PVY part of the study, they found that the plants grown at high CO2 "showed a markedly decreased spread of virus."

Keeping the story simple, Matros et al. reported that "tobacco plants grown under elevated CO2 show a slight decrease of nicotine contents," and that "elevated CO2 resulted in reduced spread of PVY." Both of these impacts would likely be considered beneficial by most people, as potato virus Y is an economically important virus that infects many crops and ornamental plants throughout the world, while nicotine is nearly universally acknowledged to have significant negative impacts on human health (Topliss et al., 2002).

Contemporaneously, Mohan et al. (2006) investigated the effects of an extra 200 ppm of atmospheric CO2 on the growth and development of Toxicodendron radicans (commonly known as poison ivy), as well as its effect on the plant's toxicity, over a period of six years at the Duke Forest FACE facility, where the noxious vine grew naturally in a loblolly pine plantation's understory, and where clumps of it were surrounded by 4-cm plastic-mesh exclosures to protect them from damage by indigenous white-tailed deer. This long and detailed study revealed that atmospheric CO2 enrichment increased poison ivy photosynthesis by 77%, while boosting its water use efficiency by 51%. And at the end of the study's sixth year, the aboveground biomass of poison ivy plants in the CO2-enriched plots was 62% greater than that of poison ivy plants in the ambient-treatment plots. In addition, the researchers discovered that the high-CO2-grown plants produced "a more allergenic form of urushiol," which is the substance that produces the plant's allergic reaction in humans.

Not unsurprisingly, therefore, the seven scientists involved in the research said that their findings indicated that under future levels of atmospheric CO2, poison ivy "may grow larger and become more noxious than it is today." And so it may ... but the story is not quite that simple.

In a study of woody vines or lianas that focused on changes they had experienced over a period of 45 years in 14 temperate deciduous forests of southern Wisconsin (USA) -- during which time (1959-1960 to 2004-2005) the air's CO2 concentration rose by 65 ppm, while the mean annual air temperature of the region rose by 0.94°C, its mean winter air temperature rose by 2.40°C, but its mean annual precipitation (another important growth-altering factor) did not change -- Londre and Schnitzer (2006) found that contrary to their own initial hypothesis, "liana abundance and diameter did not increase in the interiors of Wisconsin (USA) forests over the last 45 years." In fact, they found that Toxicodendron radicans or poison ivy -- which they note "grew markedly better under experimentally elevated CO2 conditions than did competing trees" in the study of Mohan et al. (2006) -- actually decreased in abundance over this time period, and did so significantly.

So how did it happen that what seemed to be so logical turned out to be so wrong? The two researchers say that their study suggests that "lianas are limited in the interiors of deciduous forests of Wisconsin by factors other than increased levels of CO2." It is likely, for example, that the growth of interior-forest lianas was limited by the enhanced tree growth provided by the CO2 increase, which resulted in the trees becoming more competitive with the vines because of CO2-induced increases in tree leaf numbers, area and thickness, all of which factors would have led to less light being transmitted to the lianas growing beneath the forest canopy, which phenomenon apparently negated the enhanced propensity for growth that was provided the vines by the historical increase in the atmosphere's CO2 concentration, but which was not realized due to the negative influence of the competing factor of declining light intensity.

Support for this reasoning is provided by Londre and Schnitzer's finding that "compared to the forest interior, lianas were >4 times more abundant within 15 m of the forest edge and >6 times more abundant within 5 m of the forest edge," which "strong gradient in liana abundance from forest edge to interior," in the words of the two researchers, "was probably due to light availability." In addition, they say their results "are similar to findings in tropical forests, where liana abundance is significantly higher along fragmented forest edges and within tree fall gaps," and, we might add, where the interior tropical trees have also not suffered what some had claimed would be the negative consequences of CO2-induced increases in liana growth rates.

In conclusion, there appears to be little reason to expect any significant CO2-induced increases in human-health-harming substances that are produced by earth's plants as the atmosphere's CO2 concentration continues to rise.

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.

Lee, Y.S., Dickinson, D.B., Schlager, D. and Velu, J.G. 1979. Antigen E content of pollen from individual plants of short ragweed (Ambrosia artemisiifolia). Journal of Allergy and Clinical Immunology 63: 336-339.

Londre, R.A. and Schnitzer, S.A. 2006. The distribution of lianas and their change in abundance in temperate forests over the past 45 years. Ecology 87: 2973-2978.

Maasch, H.J., Hauck, P.R., Oliver, J.D. et al. 1987. Allergenic activity of short ragweed pollen (Ambrosia elatior) from different years and/or suppliers: criteria for the selection of an in-house allergen reference preparation. Annals of Allergy 58: 429-434.

Matros, A., Amme, S., Kettig, B., Buck-Sorlin, G.H., Sonnewald, U. and Mock, H.-P. 2006. Growth at elevated CO2 concentrations leads to modified profiles of secondary metabolites in tobacco cv. SamsunNN and to increased resistance against infection with potato virus Y. Plant, Cell and Environment 29: 126-137.

Mohan, J.E., Ziska, L.H., Schlesinger, W.H., Thomas, R.B., Sicher, R.C., George, K. and Clark, J.S. 2006. Biomass and toxicity responses of poison ivy (Toxicodendron radicans) to elevated atmospheric CO2. Proceedings of the National Academy of Sciences, USA 103: 9086-9089.

Rogers, C.A., Wayne, P.M., Macklin, E.A., Muilenberg, M.L., Wagner, C.J., Epstein, P.R. and Bazzaz, F.A. 2006. Interaction of the onset of spring and elevated atmospheric CO2 on ragweed (Ambrosia artemisiifolia L.) pollen production. Environmental Health Perspectives 114: 665-669.

Topliss, J.G., Clark, A.M., Ernst, E. et al. 2002. Natural and synthetic substances related to human health. Pure and Applied Chemistry 74: 1957-1985.

Wayne, P., Foster, S., Connolly, J., Bazzaz, F. and Epstein, P. 2002. Production of allergenic pollen by ragweed (Ambrosia artemisiifolia L.) is increased in CO2-enriched atmospheres. Annals of Allergy, Asthma, and Immunology 88: 279-282.

Weber, R.W. 2002. Mother Nature strikes back: global warming, homeostasis, and implications for allergy. Annals of Allergy, Asthma & Immunology 88: 251-252.