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Deserts (Higher Plants -- Stress Reduction by CO2) - Summary
Atmospheric CO2 enrichment often helps to ameliorate the adverse consequences of various environmental stresses on plant growth and development.  Here we review a number of scientific studies that have investigated this phenomenon with respect to desert vegetation.

Hamerlynck et al. (2000) grew seedlings of Larrea tridentata, a perennial evergreen shrub, in glasshouses maintained at atmospheric CO2 concentrations of 360, 550 and 700 ppm for a period of one year.  In addition, half of the seedlings had water withheld from them for three months prior to a nine-day high-temperature treatment, in order to determine the interactive effects of elevated CO2, soil moisture and heat stress on gas exchange in this woody inhabitant of the Mojave Desert.  The scientists found that elevated CO2 largely offset the detrimental effects of both drought and high temperature.  Midday leaf water potentials, for example, were similar in all well-watered plants; but in plants experiencing drought, internal plant water status was much improved at the higher atmospheric CO2 concentrations, with leaf water potentials registering -5.9, -4.6 and -3.4 MPa in plants grown at 360, 550 and 700 ppm CO2, respectively.  Also, measurements of photosynthetic efficiency indicated that heat-shocked seedlings grown at 700 ppm CO2 ultimately recovered to exhibit pre-heat-shock-treatment values, while seedlings grown at 360 and 500 ppm exhibited lasting photosynthetic efficiency reductions of up to 20%.  Averaged across the entire experiment, in fact, photosynthetic rates of seedlings grown at 550 and 700 ppm CO2 were 31 and 90% greater, respectively, than rates displayed by plants grown in ambient air.

Polley et al. (2002) grew seedlings of five woody leguminous species (Acacia farnesiana, Gleditsia triacanthos, Leucaena leucocephala, Parkinsonia aculeata and Prosopis glandulosa) for close to one month in greenhouses maintained at atmospheric CO2 concentrations of 390 and 700 ppm under well-watered and water-stressed conditions to determine the effects of elevated CO2 on plant survivorship in the face of drought.  They found that the seedlings grown in air of 700 ppm CO2 exhibited greater photosynthetic rates and more favorable leaf water potentials than plants grown in air of 390 ppm CO2.  In addition, the extra 310 ppm of CO2 increased seedling biomass from 11 to 43%, except in Leucaena, whose biomass was indifferent to atmospheric CO2 concentration.  Also, the CO2-enriched seedlings reached the 50% drought-survivorship point four days later than seedlings grown in ambient air, and they survived 11 days longer than control seedlings when subjected to maximum drought conditions.

Working at the Nevada Desert FACE Facility, Housman et al. (2003) documented the survival, growth, gas exchange and water potential responses of seedlings of the evergreen shrub Larrea tridentata and the drought-deciduous shrub Ambrosia dumosa over a number of years.  They report that early survival of both species was greater under elevated CO2 (550 ppm as opposed to the ambient concentration of 350 ppm) in the initial wetter-than-normal year, but that this advantage disappeared in the following drier years.  Hence, they 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)."

In another study conducted at the Nevada Desert FACE Facility, Billings et al. (2002) measured plant nitrogen isotopic composition to see if elevated CO2 affected the arid ecosystem's nitrogen dynamics.  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, which suggests that the larger CO2-induced enhancement of plant 15N concentration was due to 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.

This finding is rather important, since in many desert areas, the productivity of natural ecosystems is limited by low soil carbon concentrations.  As the CO2 content of the atmosphere increases, however, greater inputs of carbon to soils via enhanced plant root exudation and litter production will likely stimulate soil microbial activities; and this enhanced microbial activity should increase the amount of soil nitrogen that is available to plants.  This phenomenon, in turn, should allow plants to produce even more biomass.  Hence, the productivity of carbon-limited ecosystems, such as deserts, will likely rise significantly as the CO2 concentration of the atmosphere continues its upward trajectory.

Loik et al. (2000) grew three Yucca species (brevifolia, schidigera, and whipplei) native to the Mojave Desert in pots placed within glasshouses maintained at atmospheric CO2 concentrations of 360 and 700 ppm and day/night air temperatures of 40/24C for seven months.  After that time, some of the plants were further subjected to a two-week day/night air temperature treatment of 20/5C; and leaves from each species were removed and placed in a freezer that reduced temperatures by 3C per hour until a minimum value of -15C was reached.  Thus, they investigated the effects of elevated CO2 exposure on low-temperature tolerance in these native desert plants.

The experimental protocol demonstrated that elevated CO2 enhances low-temperature tolerance in all three Yucca species.  Specifically, the near-doubled CO2 concentration lowered the air temperature at which 50% low-temperature-induced cell mortality occurs by 1.6, 1.4 and 0.8C in brevifolia, schidigera and whipplei, respectively.  Hence, as the air's CO2 content continues to rise, the three Yucca species will likely become more adept at surviving periods of low-temperature exposure, which occur quite frequently in the Mojave Desert.  This CO2-enhanced low-temperature tolerance may also enable them to expand their ranges both poleward in latitude and upward in elevation, where air temperatures are cooler than what they experience in their current ranges.  As Loik et al. describe it, "increases in atmospheric air temperatures and concentrations of CO2 may allow seedlings to have a greater likelihood of surviving lower temperature and thereby establishing at higher elevations and latitudes in the future."

In concluding this summary, we report the results of Dole et al. (2003), who modeled potential changes in the geographic distribution of the Joshua Tree (Yucca brevifolia) based on (1) climate changes predicted to accompany a doubling of the air's CO2 content and (2) the experimental observation that a doubling of atmospheric CO2, as they describe it, "enhances the low-temperature tolerance of Y. brevifolia seedlings, such that the lethal low temperature is lowered by 1.6C," as reported by Loik et al. (2000).

In describing their results, Dole et al. state that "the increase in freezing tolerance caused by doubled CO2 would increase the potential habitat of this species by 14%, independent of any climate change."  On the other hand, if only the predicted warming effect of the doubled CO2 is considered, the total area occupied by Y. brevifolia would decline by 25%.  Nevertheless, they indicate that when both effects are considered together, "the model predicts a different and slightly larger future distribution."

Dole et al. say their results indicate "the importance of including the physiological effects of CO2 in studies that try to predict the biological effects of climate change."  This admonition is most appropriate, for many global-warming scare stories of impending biological catastrophe are based on studies that fail to comply with this important and necessary condition.  Also, they note that an "interesting implication of this study is that anthropogenic CO2 increases will drive ecosystem change even in the absence of significant climate change."

We couldn't agree more.

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.

Dole, K.P., Loik, M.E. and Sloan, L.C.  2003.  The relative importance of climate change and the physiological effects of CO2 on freezing tolerance for the future distribution of Yucca brevifoliaGlobal and Planetary Change 36: 137-146.

Hamerlynck, E.P., Huxman, T.E., Loik, M.E. and Smith, S.D.  2000.  Effects of extreme high temperature, drought and elevated CO2 on photosynthesis of the Mojave Desert evergreen shrub, Larrea tridentataPlant Ecology 148: 183-193.

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.

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 CO2Journal of Arid Environments 46: 43-56.

Polley, H.W., Tischler, C.R., Johnson, H.B. and Derner, J.D.  2002.  Growth rate and survivorship of drought: CO2 effects on the presumed tradeoff in seedlings of five woody legumes.  Tree Physiology 22: 383-391.

Taylor, K.E. and Penner, J.E.  1994.  Response of the climate system to atmospheric aerosols and greenhouse gases.  Nature 369: 734-737.