The basic responses to atmospheric CO2 enrichment of the higher plants that inhabit deserts are much the same as those of most other higher plants: as the air's CO2 content rises, their photosynthetic rates typically increase and their rates of transpiration often decrease, thereby enhancing their water use efficiencies, as demonstrated by a number of different studies.
Graham and Nobel (1996) grew the CAM plant Agave deserti Engelm. in environmental chambers maintained at atmospheric CO2 concentrations of 370 and 750 ppm for 17 months to study the effects of elevated CO2 on gas exchange and biomass production in this succulent Sonoran Desert species. The twice-ambient CO2 concentration increased plant net carbon uptake by nearly 50%, while reducing transpirational water loss by approximately 24%. Together, the two phenomena led to a 110% increase in plant water-use efficiency. At the same time, however, prolonged exposure to atmospheric CO2 enrichment induced measurable photosynthetic acclimation, as indicated by an 11% reduction in rubisco activity and a 34% reduction in the activity of PEP-carboxylase. Nevertheless, the plants grown in elevated CO2 produced 88% more biomass than the plants grown in ambient air.
BassiriRad et al. (1998) grew three desert plants -- the perennial C3 shrubs Larrea tridentata and Prosopis glandulosa and the perennial C4 grass Bouteloua eriopoda - for two and a half months in growth chambers maintained at atmospheric CO2 concentrations of 350 and 700 ppm to determine the effects of elevated CO2 on their rates of growth and nutrient uptake. They found that the doubled CO2 content of the air increased total biomass in all three species, with the largest responses of 55 and 69% being reported for Prosopis and Larrea, respectively. The growth response of Bouteloua was much smaller at 25%, as would normally be expected for a C4 plant. However, whereas elevated CO2 more than doubled the uptake rates of NO3- and PO43- in Bouteloua, it had no effect on PO43- uptake in Larrea nor on NO3- or PO43- uptake in Prosopis; and it actually decreased root NO3- uptake in Larrea by 55%. What are the implications of these diverse findings?
In a future world of higher atmospheric CO2 concentration, all three desert species will likely exhibit increased biomass and better growth rates, which may well lead to "reverse desertification" [see Deserts (Expanding or Shrinking?)]. The greater response of the C3 shrubs would tend to suggest they might out-compete the C4 grass in this regard; but the CO2-induced enhancement of the ability of Bouteloua to increase its uptake of NO3- and PO43- from the soil may ultimately put it on an equal footing with the C3 shrubs. Indeed, BassiriRad et al. think that "long-term CO2 exposure may favor Bouteloua as opposed to the C3 shrubs due to its greater capacity for nutrient acquisition." Hence, although many historical surveys indicate that woody plants have been gradually prevailing over grasses in common habitat areas [see Trees (Range Expansions)], the results of this study suggest that the ongoing rise in the air's CO2 content may enable desert-adapted grasses to coexist with woody plants to a greater degree in the future than they do currently.
Hamerlynck et al. (2000) conducted a FACE study of a Mojave Desert ecosystem in Nevada, USA, the dominant species of which was the perennial evergreen shrub known as creosote (Larrea tridentata). Over the first six months of differential CO2 fumigation (360 and 550 ppm), elevated CO2 positively impacted rates of net photosynthesis. In wet and dry conditions, for example, photosynthetic rates of CO2-enriched plants were about 100 and 80% greater, respectively, than those observed in ambiently-grown plants, due to the 53% increase in atmospheric CO2 concentration; and because 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.
Huxman and Smith (2001) measured seasonal gas exchange in an annual grass (Bromus madritensis ssp. rubens) and a perennial forb (Eriogonum inflatum) growing within the same set of FACE plots throughout an unusually wet year characterized by abundant moisture delivered via rain showers during an El Niņo regime. They found that elevated CO2 consistently increased net photosynthetic rates in the annual grass without inducing any signs of photosynthetic acclimation. In fact, even as seasonal photosynthetic rates declined post-flowering, the decline was much less in the CO2-enriched plants than in the control plants. However, elevated CO2 had no consistent effect on stomatal conductance in this species.
In contrast, Eriogonum plants growing at 550 ppm CO2 displayed significant signs of photosynthetic acclimation, especially late in the season, which led to similar rates of net photosynthesis in both CO2 treatments. In this species, however, atmospheric CO2 enrichment did reduce stomatal conductance throughout most of the growing season. Hence, even though Bromus and Eriogonum exhibited different photosynthetic and stomatal conductance responses to elevated CO2, they both experienced significant CO2-induced increases in water use efficiency and, consequently, biomass production, thus highlighting the existence of different species-specific mechanisms for responding positively to atmospheric CO2 enrichment.
Also in the Mojave Desert FACE array, Hamerlynck et al. (2002) studied the effects of elevated CO2 on the drought-deciduous shrub Lycium andersonii, determining that plants grown in air of elevated CO2 exhibited stomatal conductances that were about 27% lower than those observed in ambiently-grown plants. However, elevated CO2 had little impact on midday leaf water potential values until the last month of the spring growing season, during which time the plants in the elevated CO2 plots displayed values that were more positive (less stressful) than those exhibited by control plants. In contrast, elevated CO2 did not significantly impact rates of photosynthesis, and plants grown in elevated CO2 experienced photosynthetic acclimation, displaying maximum rates of rubisco activity that were 19% lower than those of plants growing in ambient air.
These observations suggest that as the CO2 content of the air continues to rise, Lycium andersonii will likely respond by increasing its ability to more effectively deal with the highly variable precipitation and temperature regimes of the Mojave Desert. Indeed, the data indicate it will likely enhance its water-use efficiency due to persistent CO2-induced reductions in stomatal conductance. Moreover, acclimation within the shrub's photosynthetic apparatus should allow it to reallocate more resources to producing and sustaining greater amounts of biomass, even under conditions of limited soil moisture. Thus, it is likely that future increases in the air's CO2 content will favor a greening of the Mojave Desert much like that expected to occur most everywhere else [see Greening of the Earth in our Subject Index].
Yet another study conducted in the Mojave Desert FACE array was that of Naumburg et al. (2003), who analyzed photosynthetic data collected from three different shrubs -- the dominant evergreen perennial Larrea tridentata, the drought-deciduous Ambrosia dumosa, and the winter-deciduous Krameria erecta -- during five years of growth in air of either 360 or 550 ppm CO2. On average, they found the extra 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; and the highest rates of photosynthesis were observed in the CO2-enriched shrubs during times of comparatively high soil moisture following spring and sporadic summer rains. Hence, they concluded 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."
All in all, these several studies suggest that earth's desert plants should fare well indeed in a future world of higher atmospheric CO2 concentration, even if it is also a warmer world [see, in this regard, Growth Response to CO2 with Other Variables (Temperature) in our Subject Index, plus our first Major Report: The Specter of Species Extinction: Will Global Warming Decimate Earth's Biosphere?].
References
BassiriRad, 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.
Graham, E.A. and Nobel, P.S. 1996. Long-term effects of a doubled atmospheric CO2 concentration on the CAM species Agave deserti. Journal of Experimental Botany 47: 61-69.
Hamerlynck, E.P., Huxman, T.E., Charlet, T.N. and Smith, S.D. 2002. Effects of elevated CO2 (FACE) on the functional ecology of the drought-deciduous Mojave Desert shrub, Lycium andersonii. Environmental and Experimental Botany 48: 93-106.
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.
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.
Loik, M.E., Huxman, T.E., Hamerlynck, E.P. and Smith, S.D. 2000. Low temperature tolerance and cold acclimation for seedlings of three Mojave Desert Yucca species exposed to elevated CO2. Journal of Arid Environments 46: 43-56.
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.