Much of what we know about the responses of desert plants to atmospheric CO2 enrichment has been derived from a long-term experiment that was begun in the Mojave Desert in Nevada, USA, in the spring of 1997, where several 25-m-diameter circular FACE rings were constructed so as to enclose natural plant communities that are dominated by the evergreen perennial creosote bush [Larrea tridentata D.C. (Cov.) (Zygophyllaceae)]. In what follows, we describe the results of a number of different studies that have utilized this facility, in which the various experimental plots are continuously maintained at either the ambient atmospheric CO2 concentration or approximately 1.5 times the ambient concentration.
Hamerlynck et al. (2000) report on photosynthetic and other gas exchange data collected during the first six months of differential CO2 fumigation. They determined that during wet and dry environmental conditions, respectively, the net photosynthetic rates of the CO2-enriched creosote plants were approximately 100 and 80% greater than those of the ambient-treatment plants; and because the elevated CO2 did not significantly impact stomatal conductance, the water-use efficiencies of the CO2-enriched plants were also 100 and 80% greater than those of control plants during the same wet and dry periods.
Billings et al. (2002) also studied Larrea tridentata, measuring plant nitrogen isotopic composition to determine if elevated CO2 affects nitrogen dynamics in this arid ecosystem. Over a seven-month sampling period, they found that the amount of 15N within ambiently-grown and CO2-enriched vegetation increased by 34 and 58%, respectively. They interpreted these results to imply that the larger CO2-induced enhancement of plant 15N concentration was the result of atmospheric CO2 enrichment helping soil microbes to overcome soil carbon limitations, thus enabling microbial activity to increase and enhance the availability of soil nitrogen to plants.
Housman et al. (2003) examined survival, growth, gas exchange and water potential responses of seedlings of Larrea tridentata and the drought-deciduous shrub Ambrosia dumosa A. Gray (Payne) (Asteraceae) that had germinated in the fall of 1997. They report that early survival of both species was greater under elevated CO2 in the initial wetter-than-normal year, but that this advantage disappeared in the following drier years. They thus concluded that "elevated CO2 may have its greatest effect on Mojave Desert shrub recruitment when accompanied by increased rainfall, which is predicted for this region (Taylor and Penner, 1994)."
Naumburg et al. (2003) report the results of five full years of photosynthetic and other gas exchange data collected from three different shrubs: Larrea tridentata, Ambrosia dumosa and the winter deciduous shrub Krameria erecta. On average, elevated CO2 increased rates of net photosynthesis in Larrea, Ambrosia and Krameria by 31, 32 and 63%, respectively. Moreover, the photosynthetic enhancements persisted over the entire duration of the five-year study. The highest absolute rates of photosynthesis were observed in CO2-enriched shrubs during times of comparatively high soil moisture following spring and sporadic summer rains. Surprisingly, atmospheric CO2 enrichment did not consistently affect stomatal conductance in any species, although it reduced conductances in Larrea by 25 to 50% during the summer dry season. These results led Naumburg et al. to conclude that "future elevated CO2 effects in desert ecosystems will strongly depend on concurrent climate changes, with above average precipitation years resulting in the greatest seasonal increase in carbon uptake."
Huxman and Smith (2001) measured seasonal gas exchange in an annual grass (Bromus madritensis ssp. rubens) and a perennial forb (Eriogonum inflatum) growing naturally within the plots of the Mojave Desert FACE facility during an unusually wet year characterized by abundant moisture delivered via rain showers spawned by an El Niņo regime. They found that atmospheric CO2 enrichment consistently increased net photosynthetic rates in the annual grass without inducing any signs of photosynthetic acclimation. Indeed, even as seasonal photosynthetic rates declined post-flowering, the decline was much less in CO2-enriched plants than in control plants. However, elevated CO2 had no consistent effect on stomatal conductance in this species. In contrast, Eriogonum plants growing in elevated CO2 displayed significant signs of photosynthetic acclimation, especially late in the season, which lead to similar rates of net photosynthesis in these plants in both CO2 treatments, while atmospheric CO2 enrichment did reduce stomatal conductance in this perennial species throughout most of the growing season. As a result, even though both desert plants exhibited quite different photosynthetic and stomatal conductance responses to elevated CO2, they both experienced significant CO2-induced increases in water use efficiency and biomass production, thus highlighting the existence of different species-specific mechanisms for responding positively to atmospheric CO2 enrichment.
Weatherly et al. (2003) collected naturally-senesced litter from each of the FACE plots in both a wet year (1998, 306 mm precipitation) and a dry year (1999, 94 mm precipitation). Five different types of litter samples (from different plants) were placed in "litterbags" and returned to the locations from which the litter had been collected. Then, after four months and twelve months, the litterbags were retrieved and a number of different measurements made on their contents. Although CO2-induced reductions in litter quality were observed (increased C:N), the reductions were too small to materially affect rates of litter decomposition. Year-to-year variations in precipitation, on the other hand, influenced both the quantity and quality of litter production, as well as species composition; and these changes, in sharp contrast to the inconsequential CO2-induced changes, profoundly affected litter decomposition, as well as litter-mediated ecosystem N and C cycling.
These observations led Weatherly et al. to conclude that "decomposition in arid shrub-dominated systems will remain largely unchanged in a CO2-richer world," which is essentially what has been determined to be the case for mesic ecosystems (Franck et al., 1997; Hirschel et al., 1997; Norby et al., 2001). Hence, we can confidently conclude, as did Billings et al. (2003) in yet another study conducted at the Mojave Desert FACE facility, that "potential increases in desert productivity with elevated CO2 thus may not be limited by reduced leaf litter quality," which finding could well place the world's deserts, in their words, "among the most responsive ecosystems to elevated CO2, with increases in productivity leading to potential increased C sequestration," which is a fitting note upon which to conclude this Summary.
References
Billings, S.A., Schaeffer, S.M., Zitzer, S., Charlet, T., Smith, S.D. and Evans, R.D. 2002. Alterations of nitrogen dynamics under elevated carbon dioxide in an intact Mojave Desert ecosystem: evidence from nitrogen-15 natural abundance. Oecologia 131: 463-467.
Billings, S.A., Zitzer, S.F., Weatherly, H., Schaeffer, S.M., Charlet, T., Arnone III, J.A. and Evans, R.D. 2003. Effects of elevated carbon dioxide on green leaf tissue and leaf litter quality in an intact Mojave Desert ecosystem. Global Change Biology 9: 729-735.
Franck, V.M., Hungate, B.A., Chapin III, F.S. and Field, C.B. 1997. Decomposition of litter produced under elevated CO2: dependence on plant species and nutrient supply. Biogeochemistry 36: 223-237.
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.
Hirschel, G., Korner, C. and Arnone III, J.A. 1997. Will rising atmospheric CO2 affect leaf litter quality and in situ decomposition rates in native plant communities? Oecologia 110: 387-392.
Housman, D.C., Zitzer, S.F., Huxman, T.E. and Smith, S.D. 2003. Functional ecology of shrub seedlings after a natural recruitment event at the Nevada Desert FACE facility. Global Change Biology 9: 718-728.
Huxman, T.E. and Smith, S.D. 2001. Photosynthesis in an invasive grass and native forb at elevated CO2 during an El Niņo year in the Mojave Desert. Oecologia 128: 193-201.
Naumburg, E., Housman, D.C., Huxman, T.E., Charlet, T.N., Loik, M.E. and Smith, S.D. 2003. Photosynthetic responses of Mojave Desert shrubs to free air CO2 enrichment are greatest during wet years. Global Change Biology 9: 276-285.
Norby R.J., Cotrufo, M.F., Ineson, P., O'Neill, E.G. and Canadell, J.G. 2001. Elevated CO2, litter chemistry, and decomposition: a synthesis. Oecologia 127: 153-165.
Taylor, K.E. and Penner, J.E. 1994. Response of the climate system to atmospheric aerosols and greenhouse gases. Nature 369: 734-737.
Weatherly, H.E., Zitzer, S.F., Coleman, J.S. and Arnone III, J.A. 2003. In situ litter decomposition and litter quality in a Mojave Desert ecosystem: effects of elevated atmospheric CO2 and interannual climate variability. Global Change Biology 9: 1223-1233.
Last updated 21 November 2007