Szente et al. (1998) grew four perennial C3 plants (two grasses and two broad-leaved species) common to loess grasslands of Budapest in open-top chambers maintained at atmospheric CO2 concentrations of 350 or 700 ppm for 231 days.  They found that this doubling of the air's CO2 content significantly enhanced rates of net photosynthesis in all species studied, with the two grasses and the two broad-leaved species exhibiting average increases of 136 and 486%, respectively.  In addition, the extra CO2 actually increased transpiration rates for one of the grasses and one of the broad-leaved species, but it did not affect the water loss rates of the remaining species.  Nevertheless, the two grasses exhibited an average CO2-induced increase in water-use efficiency of 72%, while the two broad-leaved species displayed an average increase of 366%, which large improvements should allow them to better cope with the hot dry summers they characteristically experience in this part of Hungary.

Engloner et al. (2003) removed grassland monoliths with their original soils to a depth of 40 cm from a xeric loess grassland and relocated them to open-top chambers outside Budapest, where they were exposed for seven years to either ambient air or air enriched to a CO2 concentration of 700 ppm. Measurements of leaf photosynthesis and transpiration rates of a dominant member of this ecosystem (couch-grass) were conducted throughout the experiment according to protocols described by Tuba et al. (1994, 1996), while measurements of starch and soluble sugars were made as described by Tuba et al. (1994).  This work revealed that rates of net photosynthesis increased by an average of 194% in response to the ~90% increase in atmospheric CO2 concentration, leading to starch and soluble sugar increases of approximately 50 and 72%, respectively.  At the same time, leaf transpiration rates declined by about 18%, leading to a whopping 345% increase in water use efficiency, which for a xeric grassland species has got to be like manna from heaven, greatly fortifying it against the rigors of its xeric environment.

Turning to C4 plants, Seneweera et al. (1998) grew a drought-resistant perennial grass (Panicum coloratum) for five weeks in controlled environment chambers having atmospheric CO2 concentrations of 350 or 1000 ppm and different vapor pressure deficits (VPDs) that were maintained by keeping the relative humidity of the air at either 50 or 80%, while the plants were watered daily to 65, 80 or 100% of their potting soils' field capacity.  They found that under favorable environmental conditions, characterized by a low VPD and high soil moisture (100% field capacity), atmospheric CO2 enrichment failed to cause any significant increases in leaf or stem dry weight.  However, when water-stressed conditions prevailed, due to either a high VPD, low field capacities of 65 or 80%, or combinations of both parameters, elevated CO2 caused large significant increases in growth.  At the high VPD, for example, the percentage increases in leaf dry weight at field capacities of 65 and 80% were 117 and 112%, respectively, while the growth responses for stems under these conditions were 50 and 57%.

These growth increases resulted in part from the ability of elevated CO2 to ameliorate the negative effects of water stress on growth.  Under the most extreme water-stressed condition, for example, leaf water potential values were about 3.5 times more negative, i.e., more stressful, for plants grown in air of 350 ppm CO2 than for plants grown in air of 1000 ppm, due to the fact that transpirational water loss was always less for plants grown in elevated CO2.  In fact, for the most water-stressed condition investigated, which resulted from a high VPD and a field capacity of 65%, the transpiration rates of plants grown in ambient CO2 were about 2.5 times greater than those of plants grown in elevated CO2.  Consequently, it can be appreciated that higher concentrations of atmospheric CO2 will likely allow C4 grasses to maintain better internal water relations by reducing transpirational water losses, which will result in greater water-use efficiencies and the likely expansion of the plants into hot arid regions that are commonly subjected to drought.

In a subsequent extension of this work, Seneweera et al. (2001) grew P. coloratum in controlled environment chambers maintained at atmospheric CO2 concentrations of 360 and 1000 ppm for three weeks before withholding water from half of the plants for ten days, after which the plants were re-watered for five days to promote recovery.  They found that at the onset of water stress, shoot dry mass in the CO2-enriched plants was 33% greater than that observed in the plants grown in ambient air.  Although water stress reduced shoot dry mass, the reductions were less severe for CO2-enriched than ambiently-grown plants.  Also, during the water stress treatment, leaf water potentials and leaf relative water contents dropped at much slower rates and to lesser degrees in the CO2-enriched plants than in the ambiently-grown plants.  Similarly, transpiration rates of the CO2-enriched plants were much less than those of plants growing in ambient air; and this phenomenon helped contribute to the greater soil moisture contents that were always present beneath the CO2-enriched plants.  At final harvest, therefore, the CO2-induced enhancement of shoot dry mass was determined to be 44 and 70% for plants that had been subjected to well-watered and water-stressed treatments, respectively.

Last of all, we report on the study of Zavaleta et al. (2003), who in a two-year experiment in an annual-dominated California grassland delivered extra heating to a number of FACE plots (enriched with an extra 300 ppm of CO2) via IR heat lamps that warmed the surface of the soil beneath them by 0.8-1.0°C.  They determined that the individual effects of atmospheric CO2 enrichment and soil warming were of similar magnitude, and that acting together they enhanced mean spring soil moisture content by about 15% over that of the control plots.  The effect of CO2 was produced primarily as a consequence of its ability to cause partial stomatal closure and thereby reduce season-long plant water loss via transpiration.  In the case of warming, there was an acceleration of canopy senescence that further increased soil moisture by reducing the period of time (the length of the growing season) over which transpiration losses occur, all without any decrease in total plant production.

The six researchers note that their findings "illustrate the potential for organism-environment interactions to modify the direction as well as the magnitude of global change effects on ecosystem functioning."  In fact, whereas for the past several years we have been bombarded with climate-alarmist predictions of vast reaches of agricultural land drying up and being lost to profitable production in a CO2-enriched and warmed world of the future, this study suggests that just the opposite could well occur.  As Zavaleta et al. describe it, "we suggest that in at least some ecosystems, declines in plant transpiration mediated by changes in phenology can offset direct increases in evaporative water losses under future warming."

In conclusion, for both C3 and C4 grassland plants, the reductions in leaf transpirational water loss that result from increases in the air's CO2 concentration should significantly enhance their ability to withstand the rigors of periodic severe water stress in arid and semi-arid parts of the globe -- even in the face of significant warming, which may induce its own beneficial feedback -- while enhancing their productivities and thereby providing more forage for the various forms of animal life that inhabit these regions.  In addition, we may expect these plants to reclaim great tracts of desert as their water use efficiencies rise to levels not experienced for millions of years.

References
Engloner, A.I., Kovacs, D., Balogh, J. and Tuba, Z.  2003.  Anatomical and eco-physiological changes in leaves of couch-grass (Elymus repens L.), a temperate loess grassland species, after 7 years growth under elevated CO2 concentration.  Photosynthetica 41: 185-189.

Seneweera, S.P., Ghannoum, O. and Conroy, J.  1998.  High vapor pressure deficit and low soil water availability enhance shoot growth responses of a C4 grass (Panicum coloratum cv. Bambatsi) to CO2 enrichment.  Australian Journal of Plant Physiology 25: 287-292.

Seneweera, S., Ghannoum, O. and Conroy, J.P.  2001.  Root and shoot factors contribute to the effect of drought on photosynthesis and growth of the C4 grass Panicum coloratum at elevated CO2 partial pressures.  Australian Journal of Plant Physiology 28: 451-460.

Szente, K., Nagy, Z. and Tuba, Z.  1998.  Enhanced water use efficiency in dry loess grassland species grown at elevated air CO2 concentration.  Photosynthetica 35: 637-640.

Tuba, Z., Szente, K. and Koch, J.  1994.  Response of photosynthesis, stomatal conductance, water use efficiency and production to long-term elevated CO2 in winter wheat.  Journal of Plant Physiology 144: 551-668.

Tuba, Z., Szente, K., Nagy, Z., Csintalan, Z. and Koch, J.  1996.  Responses of CO2 assimilation, transpiration and water use efficiency to long-term elevated CO2 in perennial C3 xeric loess steppe species.  Journal of Plant Physiology 148: 356-361.

Zavaleta, E.S., Thomas, B.D., Chiariello, N.R., Asner, G.P., Shaw, M.R. and Field, C.B.  2003.  Plants reverse warming effect on ecosystem water balance.  Proceedings of the National Academy of Science USA 100: 9892-9893.