In atmospheric CO2 enrichment experiments, nearly all plants almost always exhibit increases in photosynthetic rates and biomass production when environmental conditions are optimal for growth. Even when conditions are less than favorable (low soil moisture, poor soil fertility, high soil salinity, high air temperature), many plants still exhibit a CO2-induced growth enhancement; and that relative, i.e., percentage, enhancement is sometimes (more often than not, in fact) greater than what it is under ideal growing conditions. It is sometimes suggested, however, that experimental results obtained from CO2 -enrichment experiments conducted in growth cabinets, greenhouses and other enclosures may not reflect real-world plant responses to atmospheric CO2 enrichment due to perturbations in microclimate caused by the enclosures. Thus, FACE technology was developed as a means to enrich the air with CO2 around vegetation while having minimal effect on the surrounding microclimate. The following summary reviews the results of some of these experiments conducted on various grassland species, many of which were growing naturally in pastures.
In the study of Nitschelm et al. (1997), 18-m diameter circular plots of white clover were established at a field station of the Swiss Federal Institute of Technology near Zurich and exposed to atmospheric CO2 concentrations of 350 and 600 ppm. After one season of growth, the authors reported that elevated CO2 increased aboveground biomass production by 146%. In addition, the extra 250 ppm of CO2 increased carbon inputs to the soil by 50% while decreasing root decomposition by 24%, thereby enhancing the carbon sequestration capacity of the soils in the CO2-enriched plots.
In another Swiss study (Luscher et al., 1998), 9 to 14 genotypes of each of 12 native grassland species collected near Zurich were transplanted into FACE arrays receiving atmospheric CO2 concentrations of 350 and 700 ppm. Twice-ambient concentrations of CO2 generally increased aboveground biomass in all 12 species in this experiment, while showing no preferential effects on any specific genotype of a given species.
In the study of Rogers et al. (1998), swards of perennial ryegrass were grown as a frequently-cut herbage crop in a FACE experiment having atmospheric CO2 concentrations of 360 and 600 ppm. The researchers reported that photosynthetic rates were about 35% higher in the CO2-enriched plots than in the ambiently-grown plots, regardless of soil nitrogen content. Similarly, in a study of nutrient-poor chalk grassland swards, Bryant et al. (1998) noted that elevated CO2 increased photosynthetic rates in two of three perennial species by 28%.
Finally, Hamerlynck et al. (2000) established 25-m diameter circular FACE plots in the Mojave Desert of Nevada, USA, on land that contained a significant amount of grassland species. However, in this particular paper they only reported the response of an evergreen perennial shrub, creosote, to elevated CO2. They determined that a 52% increase in the air's CO2 content increased photosynthetic rates in this species by 100 and 80% during the wet and dry seasons, respectively. In addition, because elevated CO2 did not affect rates of stomatal conductance, the water-use efficiency of this species was similarly enhanced by 100 and 80%, respectively.
In summary, the results obtained from these FACE experiments demonstrate that the increasing CO2 content of the air will positively impact photosynthetic rates and biomass production in grassland plants, even if they are growing under stressful conditions imposed by poor soil fertility. Thus, as the air's CO2 content continues to increase, grassland productivity, growth and carbon sequestration should also increase. In addition, these FACE studies corroborate data previously obtained from CO2 enrichment studies on grassland species that were performed in growth chambers, greenhouses and various other enclosures, indicating that these other techniques are also valid approaches to learning about the various responses of plants to changes in the air's CO2 content.
References
Bryant, J., Taylor, G. and Frehner, M. 1998. Photosynthetic acclimation to elevated CO2 is modified by source:sink balance in three component species of chalk grassland swards grown in a free air carbon dioxide enrichment (FACE) experiment. Plant, Cell and Environment 21: 159-168.
Hamerlynck, E.P., Huxman, T.E., Nowak, R.S., Redar, S., Loik, M.E., Jordan, D.N., Zitzer, S.F., Coleman, J.S., Seeman, J.R. and Smith, S.D. 2000. Photosynthetic responses of Larrea tridentata to a step-increase in atmospheric CO2 at the Nevada Desert FACE Facility. Journal of Arid Environments 44: 425-436.
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.
Nitschelm, J.J., Luscher, A., Hartwig, U.A. and van Kessel, C. 1997. Using stable isotopes to determine soil carbon input differences under ambient and elevated atmospheric CO2 conditions. Global Change Biology 3: 411-416.
Rogers, A., Fischer, B.U., Bryant, J., Frehner, M., Blum, H., Raines, C.A. and Long, S.P. 1998. Acclimation of photosynthesis to elevated CO2 under low-nitrogen nutrition is affected by the capacity for assimilate utilization. Perennial ryegrass under free-air CO2 enrichment. Plant Physiology 118: 683-689.