This famous but unsubstantiated hunch has pervaded the thinking of the world's climate-alarmists ever since it was first suggested; and it figures prominently in the never-ending doom-and-gloom prognostications of Al Gore and James Hansen. But is the highly-hyped contention correct? We herein broach this question as it pertains to CO2-induced changes in natural (i.e., non-agricultural) terrestrial vegetation, as revealed by a number of recent studies.

In introducing the report of their investigation of the subject, Rae et al. (2007) wrote that various studies "are beginning to identify genes that appear sensitive to elevated CO2 (Gupta et al., 2005; Taylor et al., 2005; Ainsworth et al., 2006)," noting that "leaf growth responses to elevated CO2 have been found in Populus," and that "quantitative trait loci (QTL) for this response [have been] determined (Rae et al., 2006)." In a continuation of this endeavor, they worked with a three-generation Populus pedigree generated by the hybridization of two contrasting Populus species -- where two full-siblings from the resulting F1 family were crossed to form an F2 family -- growing cuttings of the different generations for 152 days out-of-doors in open-top chambers maintained at either the ambient CO2 concentration or concentrations on the order of 600 ppm, while measuring various plant properties and physiological processes and determining QTL for above- and below-ground growth and genotype-by-environment interactions.

This work revealed, in the words of the four UK researchers, that "in the F2 generation, both above- and below-ground growth showed a significant increase in elevated CO2," and that "three areas of the genome on linkage groups I, IX and XII were identified as important in determining above-ground growth response to elevated CO2, while an additional three areas of the genome on linkage groups IV, XVI and XIX appeared important in determining root growth response to elevated CO2." Consequently, stating that their results "quantify and identify genetic variation in response to elevated CO2 and provide an insight into genomic response to the changing environment," they concluded that further work in this area "should lead to an understanding of micro-evolutionary response to elevated CO2," as has been further demonstrated by subsequent studies.

Working within controlled-environment chambers maintained at atmospheric CO2 concentrations of either 380 or 700 ppm, Springer et al. (2008) grew well watered and fertilized plants of two closely related out-crossed genotypes (SG and CG) of Arabidopsis thaliana that had been generated via artificial selection, where genotype SG was selected for high seed number at elevated CO2 over five generations, and where genotype CG was randomly selected and thus represented a non-selected control. For these plants, they measured time to flowering, number of leaves at flowering, and total biomass at flowering, as well as foliar sugar concentrations; and then -- in a second experiment conducted under the same growing conditions -- they characterized the expression patterns of several floral-initiation genes.

This work revealed, in the words of Springer et al., that "SG delayed flowering by 7-9 days, and flowered at a larger size (122% higher biomass) and higher leaf number (81 more leaves) when grown at elevated versus current CO2 concentration," but that "flowering time, size and leaf number at flowering were similar for CG plants grown at current and elevated CO2." In addition, they say that "SG plants had 84% higher foliar sugar concentrations at the onset of flowering when grown at elevated versus current CO2, whereas foliar sugar concentrations of CG plants grown at elevated CO2 only increased by 38% over plants grown at current CO2." Last of all, they report that "SG exhibited changes in the expression patterns of floral-initiation genes in response to elevated CO2, whereas CG plants did not."

Noting that "delayed flowering increases production of vegetative resources that can be subsequently allocated to reproductive structures," the researchers concluded that "such evolutionary responses may alter total carbon gain of annual plants if the vegetative stage is extended, and may potentially counteract some of the accelerations in flowering that are occurring in response to increasing temperatures." Hence, their results demonstrate the ability of elevated CO2 to alter the expression of plant genes in ways that may enable plants to take better advantage of the ongoing rise in the air's CO2 content.

Lau et al. (2008) measured the amount of pathogen damage caused by Pythium or Fusarium spp. to the common prairie legume Lespedeza capitata growing in ambient and elevated (560 ppm) CO2 treatments in the seventh and eighth full years (2004 and 2005) of the BioCON study (Reich et al., 2001) conducted at the Cedar Creek Natural History Area in Minnesota (USA), where the CO2 treatments were applied during the daylight hours of each growing season. In doing so, they found that disease incidence was lower in the elevated CO2 environment (down by 10% in 2004 and 53% in 2005). And "because disease caused major reductions in reproductive output," in the words of the five researchers, they concluded that "the effects of CO2 on disease incidence may be important for L. capitata evolution and population dynamics," which phenomena should significantly benefit this species in a high-CO2 world of the future. In addition, they note that Strengbom and Reich (2006), "working in the same experimental site ... also found that elevated CO2 ... reduced disease incidence on Solidago rigida."

Concluding this category of study, Kaligaric et al. (2008) investigated fluctuating asymmetry (FA), a descriptor of the magnitude of random deviations from perfect symmetry in morphological traits of both plants and animals that "offers a unique tool for comparative studies of developmental stability (Moller and Swaddle, 1997)," which in plants has been used as "an indicator of genetic and environmental stress (Martel et al., 1999; Cornelissen and Stiling, 2004)." This they did by measuring the degree of FA in "undamaged (not grazed, not visibly attacked by herbivores or pathogens) fully developed leaves" of the Mediterranean shrub Myrtus communis growing along an atmospheric CO2 gradient (570, 530, 490, 450, 410 and 370 ppm) moving away from a natural CO2 spring (I Borboi) near Lajatico (Pisa, Tuscany, Italy) at distances of 2, 18, 34, 50, 66 and 82 meters, respectively, from the CO2 source.

The four researchers say they found "a significant and negative correlation between CO2 concentration and leaf FA," such that "with increased CO2 concentration the leaf FA decreased," which result, in their words, "confirms what was obtained by Cornelissen et al. (2004) on Quercus myrtifolia and Quercus geminata (in a short-term experiment)." In addition, they note that "Myrtus communis, grown under elevated CO2 concentration at 'I Borboi,' showed a reduction in xylem embolism and an increase in hydraulic efficiency (Tognetti et al., 2001)," stating that "improved water relations could represent a good explanation for the observed reduction in leaf FA" as the air's CO2 content increased. Hence they concluded that "adaptation and selection could explain the tendency towards decreased leaf FA in plants from the CO2 spring relative to ambient conditions," since "the more symmetrical leaves under long-term elevated CO2 concentration were more developmentally stable in these conditions."

In conclusion, it would appear that the ongoing rise in the air's CO2 content is likely exerting significant selection pressure on earth's naturally-occurring terrestrial plants, which should improve their performance in the face of various environmental stressors via the process of micro-evolution.

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