As the CO2 content of the air increases, plants exhibit modifications in their physiology. One common change is increased photosynthesis. With greater amounts of CO2 in the air – and greater amounts of CO2 thus diffusing into leaves – the primary carboxylating enzyme of most plants, i.e. rubisco, performs its biochemical functions more efficiently, leading to reductions in photorespiratory carbon losses and increases in carbohydrate synthesis; and with additional carbohydrate production, most plants exhibit enhanced rates of growth and biomass accumulation. In this summary we thus review the effects of elevated atmospheric CO2 concentrations on biomass production in grassland communities.
Navas et al. (1999) grew mixed communities of two grasses and two legumes across a range of soil nitrogen contents at ambient (357ppm) and enriched (712 ppm) atmospheric CO2 concentrations for a period of two months. Although soil nitrogen content had a much greater influence on community productivity than did atmospheric CO2 concentration, communities fumigated with CO2-enriched air tended to produce greater amounts of biomass than communities exposed to ambient air. Likewise, Jongen and Jones (1998) reported that an eight-month exposure to twice-ambient levels of atmospheric CO2 increased the community biomass of semi-natural grasslands characteristic of the Irish lowlands by 26%.
In a dual study investigating the responses of a species-rich turf growing over limestone and a species-poor turf growing over a peaty soil, Fitter et al. (1997) noted that enriching the air with an additional 250 ppm CO2 had no effect on shoot biomass production in either monolith after two years of fumigation. However, atmospheric CO2 enrichment significantly stimulated root biomass production by 40 to 50% in both grassland ecosystems.
In the two-year study of Stocklin et al. (1999), simulated low fertility Swiss grasslands were grown in glasshouses receiving atmospheric CO2 concentrations of 360 and 600 ppm, with the authors determining that elevated CO2 concentrations stimulated total biomass production by an average of 23% in these nutrient poor grassland communities. And in yet another two-year experiment, Niklaus et al. (1998) noted that swards of calcareous grasslands exposed to atmospheric CO2 concentrations of 600 ppm displayed total biomass values that were 25% greater than those exhibited by control swards grown in ambient air of 350 ppm CO2.
After growing microcosms of the C3 grass Danthonia richardsonii for four years in glasshouses receiving atmospheric CO2 concentrations of 360 and 720 ppm, Lutze and Gifford (1998) reported the elevated CO2 increased total microcosm biomass by an average of 24%. Similarly, in the four-year study of Leadley et al. (1999), species-rich Swiss grasslands exposed to atmospheric CO2 concentrations of 600 ppm in open-top and open-bottom chambers produced 29% more community biomass than control grasslands exposed to air of 350 ppm CO2. And in the longest CO2-enrichment study of grassland communities to date – lasting eight complete years – it was reported that tallgrass prairie ecosystems in Kansas, USA, exposed to twice-ambient levels of atmospheric CO2 displayed significant CO2-induced enhancements of biomass, but only during relatively dry years (Owensby et al., 1999).
A good summary of grassland community biomass responses to atmospheric CO2 enrichment can be found in the comprehensive review of Campbell et al. (2000), who compiled and analyzed over 165 peer-reviewed scientific journal articles dealing with pastures and rangelands. Although their review included many responses of individual species, it provides a conservative estimate of community responses as well: an average 17% increase for a doubling of the air’s CO2 content. Hence, as the atmospheric CO2 concentration continues to increase, it is likely that grassland communities will respond by exhibiting increases in photosynthesis and biomass production, which will invariably lead to enhanced carbon sequestration in the soils beneath them.
References
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.
Fitter, A.H., Graves, J.D., Wolfenden, J., Self, G.K., Brown, T.K., Bogie, D. and Mansfield, T.A. 1997. Root production and turnover and carbon budgets of two contrasting grasslands under ambient and elevated atmospheric carbon dioxide concentrations. New Phytologist 137: 247-255.
Jongen, M. and Jones, M.B. 1998. Effects of elevated carbon dioxide on plant biomass production and competition in a simulated neutral grassland community. Annals of Botany 82: 111-123.
Leadley, P.W., Niklaus, P.A., Stocker, R. and Korner, C. 1999. A field study of the effects of elevated CO2 on plant biomass and community structure in a calcareous grassland. Oecologia 118: 39-49.
Lutze, J.L. and Gifford, R.M. 1998. Carbon accumulation, distribution and water use of Danthonia richardsonii swards in response to CO2 and nitrogen supply over four years of growth. Global Change Biology 4: 851-861.
Navas, M.-L., Garnier, E., Austin, M.P. and Gifford, R.M. 1999. Effect of competition on the responses of grasses and legumes to elevated atmospheric CO2 along a nitrogen gradient: differences between isolated plants, monocultures and multi-species mixtures. New Phytologist 143: 323-331.
Niklaus, P.A., Leadley, P.W., Stocklin, J. and Korner, C. 1998. Nutrient relations in calcareous grassland under elevated CO2. Oecologia 116: 67-75.
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 CO2. Global Change Biology 5: 497-506.
Stocklin, J. and Korner, Ch. 1999. Interactive effects of elevated CO2, P availability and legume presence on calcareous grassland: results of a glasshouse experiment. Functional Ecology 13: 200-209.