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Biodiversity (C3 vs C4 Plants) -- Summary
C3 plants typically respond better to atmospheric CO2 enrichment than do C4 plants in terms of increasing their rates of photosynthesis and biomass production [see C4 Plants (Biomass and Photosynthesis)].  Hence, it has periodically been suggested that in a world of rising atmospheric CO2 concentration, C3 plants may out-compete C4 plants and displace them, thereby decreasing the biodiversity of certain ecosystems.  However, the story is much more complex that what is suggested by this simple scenario.

Wilson et al. (1998) grew 36 species of perennial grass common to tallgrass prairie ecosystems with and without arbuscular mycorrhizal fungi, finding that the dry matter production of the C3 species that were colonized by the fungi was the same as that of the non-inoculated C3 species, but that the fungal-colonized C4 species produced, on average, 85% more dry matter than the non-inoculated C4 species.  This finding is of pertinence to the relative responsiveness of C3 and C4 plants to atmospheric CO2 enrichment; for elevated levels of atmospheric CO2 tend to enhance the mycorrhizal colonization of plant roots [see Fungi (Grasses and Herbaceous Plants)], which is known to make soil minerals and water more available for plant growth (see Nutrient Acquisition).  Hence, this CO2-induced fungal-mediated growth advantage, which from this study appears to be more readily available to C4 plants, could well counter the inherently greater biomass response of C3 plants relative to that of C4 plants, leveling the playing field relative to their competition for space in any given ecosystem.

Another advantage that may come to C4 plants as a consequence of the ongoing rise in the air's CO2 content was elucidated by BassiriRad et al. (1998), who found that elevated CO2 enhanced the ability of the perennial C4 grass Bouteloua eriopoda to increase its uptake of NO3- and PO43- considerably more than the perennial C3 shrubs Larrea tridentata and Prosopis glandulosa.  Hence, it is not surprising that in an eight-year study of the effects of twice-ambient atmospheric CO2 concentrations on a pristine tallgrass prairie in Kansas, Owensby et al. (1999) found that the elevated CO2 did not affect the basal coverage of its C4 species or their relative contribution to the composition of the ecosystem.

Then, of course, there is the well-known antitranspirant effect of atmospheric CO2 enrichment (Pospisilova and Catsky, 1999), which is often more strongly expressed in C4 plants than in C3 plants and that typically allows C4 plants to better cope with water stress.  In a study of the C3 dicot Abutilon theophrasti and the C4 dicot Amaranthus retroflexus, for example, Ward et al. (1999) found that Amaranthus retroflexus exhibited a greater relative recovery from drought than did the C3 species, which suggests, in their words, that "the C4 species would continue to be more competitive than the C3 species in regions receiving more frequent and severe droughts," which basically characterizes regions where C4 plants currently exist.

Two years later, Morgan et al. (2001) published the results of an open-top chamber study of a native shortgrass steppe ecosystem in Colorado, USA, where they had exposed the enclosed ecosystems to atmospheric CO2 concentrations of 360 and 720 ppm for two six-month growing seasons.  In spite of an average air temperature increase of 2.6C, which was caused by the presence of the open-top chambers, the elevated CO2 increased aboveground biomass production by an average of 38% in both years of the study; and when 50% of the standing green plant biomass was defoliated to simulate grazing halfway through the growing season, atmospheric CO2 enrichment still increased aboveground biomass by 36%.  It was also found that the communities enriched with CO2 tended to have greater amounts of moisture in their soils than communities exposed to ambient air; and this phenomenon likely contributed to the less negative and, therefore, less stressful plant water potentials that were measured in the CO2-enriched plants.  Last of all, the elevated CO2 did not preferentially stimulate the growth of C3 species over that of C4 species in these communities.  Hence, elevated CO2 did not significantly affect the percentage composition of C3 and C4 species in these grasslands; and they maintained their original level of vegetative biodiversity.

In light of these several observations, we believe it to be highly unlikely that the ongoing rise in the air's CO2 content will lead to C3 plants replacing C4 plants in the vast majority of earth's ecosystems.  This would also appear to be the take-home message of the study of Wand et al. (1999), who in a massive review of the scientific literature published between 1980 and 1997 analyzed nearly 120 individual responses of C3 and C4 grasses to elevated CO2.  On average, they found photosynthetic enhancements of 33 and 25%, respectively, for C3 and C4 plants, along with biomass enhancements of 44 and 33%, respectively, for a doubling of the air's CO2 concentration.  These larger-than-expected growth responses in the C4 species led them to conclude that "it may be premature to predict that C4 grass species will lose their competitive advantage over C3 grass species in elevated CO2."

Further support for this conclusion comes from the study of Campbell et al. (2000), who reviewed research work done between 1994 and 1999 by a worldwide network of 83 scientists associated with the Global Change and Terrestrial Ecosystems (GCTE) Pastures and Rangelands Core Research Project 1, which resulted in the publication of over 165 peer-reviewed scientific journal articles.  After analyzing this great body of research, they concluded that the "growth of C4 species is about as responsive to CO2 concentration as [is that of] C3 species when water supply restricts growth, as is usual in grasslands containing C4 species."  Hence, the work of this group of scientists also provides no evidence for the suggestion that C3 plants may out-compete C4 plants and thereby replace them in a high-CO2 world of the future.

References
BassirRad, H., Reynolds, J.F., Virginia, R.A. and Brunelle, M.H.  1998.  Growth and root NO3- and PO43- uptake capacity of three desert species in response to atmospheric CO2 enrichment.  Australian Journal of Plant Physiology 24: 353-358.

Campbell, B.D., Stafford Smith, D.M., Ash, A.J., Fuhrer, J., Gifford, R.M., Hiernaux, P., Howden, S.M., Jones, M.B., Ludwig, J.A., Manderscheid, R., Morgan, J.A., Newton, P.C.D., Nosberger, J., Owensby, C.E., Soussana, J.F., Tuba, Z. and ZuoZhong, C.  2000.  A synthesis of recent global change research on pasture and rangeland production: reduced uncertainties and their management implications.  Agriculture, Ecosystems and Environment 82: 39-55.

Morgan, J.A., Lecain, D.R., Mosier, A.R. and Milchunas, D.G.  2001.  Elevated CO2 enhances water relations and productivity and affects gas exchange in C3 and C4 grasses of the Colorado shortgrass steppe.  Global Change Biology 7: 451-466.

Owensby, C.E., Ham, J.M., Knapp, A.K. and Auen, L.M.  1999.  Biomass production and species composition change in a tallgrass prairie ecosystem after long-term exposure to elevated atmospheric CO2Global Change Biology 5: 497-506.

Pospisilova, J. and Catsky, J.  1999.  Development of water stress under increased atmospheric CO2 concentration.  Biologia Plantarum 42: 1-24.

Wand, S.J.E., Midgley, G.F., Jones, M.H. and Curtis, P.S.  1999.  Responses of wild C4 and C3 grass (Poaceae) species to elevated atmospheric CO2 concentration: a meta-analytic test of current theories and perceptions.  Global Change Biology 5: 723-741.

Ward, J.K., Tissue, D.T., Thomas, R.B. and Strain, B.R.  1999.  Comparative responses of model C3 and C4 plants to drought in low and elevated CO2Global Change Biology 5: 857-867.

Wilson, G.W.T. and Hartnett, D.C.  1998.  Interspecific variation in plant responses to mycorrhizal colonization in tallgrass prairie.  American Journal of Botany 85: 1732-1738.