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 mixed-species grassland communities.
Luscher et al. (1998) collected 9 to 14 genotypes for each of 12 native grassland species from two permanent meadows near Zurich, Switzerland, and transplanted them into artificial gaps created within well-fertilized swards of Lolium perenne growing within FACE arrays maintained at 350 and 700 ppm CO2 to determine inter- and intraspecific species growth responses to elevated CO2 by harvesting aboveground biomass several times over a three-year period. CO2-induced biomass increases varied with plant type, being greatest for legumes followed by non-legume dicotyledonous species and lastly monocotyledonous grasses. However, there were no significant differences in CO2 responsiveness within genotypes for any of the 12 species.
Zaller and Arnone (1999) established open-top chambers in a species-rich grassland located near Basal, Switzerland, and fumigated them continuously with atmospheric CO2 concentrations of 350 and 600 ppm, except during winter months, for nearly one and a half years to see whether graminoids, non-legume forbs and legumes growing near earthworm surface casts were more responsive to atmospheric CO2 enrichment than those growing further away from these nutrient-rich microsites. Generally, plants growing in close proximity to earthworm casts produced more biomass than similar plants growing further away, regardless of CO2 concentration. When assessing the influence of earthworm casts on plant responsiveness to atmospheric CO2 enrichment, however, no statistically significant results were detected. Nonetheless, the average growth response of graminoids to elevated CO2 was greater for those plants that were growing closer to earthworm casts, in comparison to those that were growing further away from them. Thus, it is conceivable that plant growth responses to atmospheric CO2 enrichment may be increased if local plant niches are closely associated with earthworm casts, which provide limiting nutrients to facilitate greater plant growth.
Reich et al. (2001) grew sixteen perennial grassland species as monocultures within FACE plots maintained at atmospheric CO2 concentrations of 360 and 560 ppm and low and high levels of soil nitrogen for two years. Interestingly, there were no interactions between atmospheric CO2 concentration and soil nitrogen for any measured plant parameter. Nonetheless, elevated CO2 increased total plant biomass for forbs, legumes and C3 grasses by 31, 18 and 9%, respectively, while it decreased the growth of C4 grasses by 4%.
Teyssonneyre et al. (2002) grew three C3 grasses (Lolium perenne, Festuca arundinacea and Holcus lanatus) as monocultures and two-species mixtures for five months within plastic tunnels maintained at atmospheric CO2 concentrations of 350 and 700 ppm, cutting the grasses either frequently or infrequently to stimulate competition for light. In monoculture, the high-CO2 treatment increased total aboveground biomass by 22%, 22% and 4% in Festuca, Holcus and Lolium, respectively. In two-species mixtures, however, elevated CO2 caused a 22% reduction in the amount of Lolium in the Lolium x Festuca mixture under the infrequent cutting regime; while it caused 30% and 67% reductions in the amount of Lolium in the Lolium x Holcus mixture under the frequent and infrequent cutting regimes, respectively.
The results of these several studies would seem to suggest that the ongoing rise in the air's CO2 content will likely stimulate the productivity of earth's grasslands and that it may also alter their species composition. The latter implication, however, must be received with caution, as temporal differences in responsiveness to elevated CO2 among species depend on numerous competitive and cooperative interactions that are not yet well understood.
References
Luscher, A., Hendrey, G.R. and Nosberger, J. 1998. Long-term responsiveness to free air CO2 enrichment of functional types, species and genotypes of plants from fertile permanent grassland. Oecologia 113: 37-45.
Reich, P.B., Tilman, D., Craine, J., Ellsworth, D., Tjoelker, M.G., Knops, J., Wedin, D., Naeem, S., Bahauddin, D., Goth, J., Bengtson, W. and Lee, T.A. 2001. Do species and functional groups differ in acquisition and use of C, N and water under varying atmospheric CO2 and N availability regimes? A field test with 16 grassland species. New Phytologist 150: 435-448.
Teyssonneyre, F., Picon-Cochard, C. and Soussana, J.F. 2002. How can we predict the effects of elevated CO2 on the balance between perennial C3 grass species competing for light? New Phytologist 154: 53-64.
Zaller, J.G. and Arnone III, J.A. 1999. Interactions between plant species and earthworm casts in a calcareous grassland under elevated CO2. Ecology 80: 873-881.