As the air's CO2 content rises, most plants exhibit increased rates of net photosynthesis and biomass production. Moreover, on a per-unit-leaf-area basis, they typically lose less water via transpiration (Saxe et al., 1998; Seneweera et al., 1998; Sgherri et al., 1998; Smart et al., 1998; Tognetti et al., 1998; Wayne et al., 1998; Centritto et al., 1999a; Serraj et al., 1999), as they tend to display lower stomatal conductances at elevated atmospheric CO2 concentrations (Egli et al., 1998; Garcia et al., 1998; LeCain and Morgan, 1998; Tjoelker et al., 1998; Leymarie et al., 1999; Runion et al., 1999; Stanciel et al., 2000). Consequently, plant water use efficiency, or the amount of carbon gained per unit of water lost per unit leaf area, should increase dramatically as the air's CO2 content rises. In this review, we summarize the results of several recent studies that support this conclusion.
With respect to C3 agricultural crops, atmospheric CO2 enrichment nearly always increases plant water use efficiency. In the study of Serraj et al. (1999), soybeans grown at 700 ppm CO2 exhibited dry weight increases of as much as 33% over plants grown in ambient air, while using 10 to 25% less water, thereby boosting their water use efficiencies by 50 to 75%. In another study, Garcia et al. (1998) found that spring wheat grown at 550 ppm CO2 had a water use efficiency that was a third again as great as that of ambiently-grown plants; while Hunsaker et al. (2000) reported that the water use efficiencies of the same CO2-enriched plants rose by 20 and 10%, respectively, under high and low soil nitrogen regimes.
Sometimes, atmospheric CO2 enrichment increases the water use efficiencies of C3 agricultural crops by even greater amounts. De Luis et al. (1999), for example, demonstrated that alfalfa plants subjected to an atmospheric CO2 concentration of 700 ppm had water use efficiencies that were 2.6 and 4.1 times greater than those displayed by control plants growing at 400 ppm CO2 under water-stressed and well-watered conditions, respectively. Similarly, a 2.7-fold CO2-induced increase in water use efficiency was reported by Malmstrom and Field (1997) for oats infected with the barley yellow dwarf virus, when grown at an atmospheric CO2 concentration of 700 ppm.
Elevated CO2 also enhances the water use efficiencies of crops possessing alternate pathways of carbon fixation. Maroco et al. (1999), for example, demonstrated that maize - a C4 crop - grown for 30 days at an atmospheric CO2 concentration of 1100 ppm had an intrinsic water use efficiency that was 225% higher than that of similar plants grown at 350 ppm CO2. And Zhu et al. (1999) reported that pineapple - a CAM plant - grown at 700 ppm CO2 exhibited water use efficiencies that were always significantly greater than those displayed by control plants grown at 350 ppm CO2 over a wide range of growth temperatures.
Elevated CO2 concentrations also increase the water use efficiencies of various grassland species. In the study of Szente et al. (1998), for example, two C3 grasses and two C3 broad-leaved species grown at twice-ambient levels of atmospheric CO2 exhibited 72 and 366% increases in their respective water use efficiencies. Similarly, Clark et al. (1999) grew mixed grassland species (C3 and C4) from New Zealand at 700 ppm CO2 and observed that such plants exhibited significantly greater water use efficiencies than their counterparts grown at 350 ppm CO2. Likewise, LeCain and Morgan (1998) reported that six different C4 grasses all exhibited significant CO2-induced increases in water use efficiency, as did Seneweera et al. (1998) for the common C4 grass Panicum coloratum. Thus, there is little doubt the vast majority of all agricultural and grassland plants respond favorably to elevated concentrations of atmospheric CO2 by increasing their water use efficiencies.
Most longer-lived perennial plants also have their water use efficiencies enhanced by higher concentrations of atmospheric CO2. Arp et al. (1998), for example, reported that five of six perennial plants common to The Netherlands that were subjected to an atmospheric CO2 concentration of 566 ppm exhibited greater water use efficiencies than control plants fumigated with air of 354 ppm CO2. Likewise, in a study performed by Tjoelker et al. (1998), seedlings of quaking aspen, paper birch, tamarack, black spruce and jack pine grown at 580 ppm CO2 for three months all displayed increases in water use efficiency, ranging from 40 to 80%. Also, in a study conducted by Centritto et al. (1999b), cherry seedlings grown at twice-ambient levels of atmospheric CO2 displayed water use efficiencies that were 50% greater than those of ambient controls, regardless of soil moisture status. And in the study of Wayne et al. (1998), yellow birch seedlings grown at 800 ppm CO2 had water use efficiencies that were 52 and 94% greater than control plants subjected to low and high air temperatures regimes, respectively. Other trees that have been found to be benefited by extra carbon dioxide are longleaf pine (Runion et al., 1999), red oak (Anderson and Tomlinson, 1998), silver birch (Rey and Jarvis, 1998), beech (Egli et al., 1998) and spruce (Roberntz and Stockfors, 1998).
In some parts of the world, perennial plants have been exposed for decades to elevated CO2 concentrations, due to their proximity to CO2-emitting springs and vents in the earth's surface. In studying such plants, scientists have been able to assess the long-term effects of elevated CO2 concentrations on water use efficiency. In Venezuela, for example, the water use efficiency of a common tree exposed to a lifetime atmospheric CO2 concentration of approximately 1,000 ppm rose 2-fold and 19-fold during the wet and dry seasons, respectively (Fernandez et al., 1998). Similarly, Bartak et al. (1999) reported that 30-year old Arbutus unedo trees growing in central Italy at a lifetime atmospheric CO2 concentration of approximately 465 ppm exhibited water use efficiencies that were 100% greater than those of control trees growing at a lifetime CO2 concentration of 355 ppm. And two species of mature oak trees growing for 15 to 25 years at an atmospheric CO2 concentration ranging from 500 to 1000 ppm in central Italy displayed "such marked increases in water use efficiency under elevated CO2" that the authors concluded it "might be of great importance in Mediterranean environments in the perspective of global climate change" (Tognetti et al., 1998).
In some cases, scientists have looked to the past and determined the impact the historic rise in the air's CO2 content has already had on plant water use efficiency. Duquesnay et al. (1998), for example, used tree-ring carbon isotope data derived from beech trees to determine that the water use efficiency of such trees in northeastern France increased by approximately 33% over the past century. Similarly, Feng (1999) used tree-ring carbon isotope data derived from western North America to infer a 10 to 25% increase in forest water use efficiency from 1750 to 1970, during which time the atmospheric CO2 concentration rose by approximately 16%. In another interesting study, Beerling et al. (1998) grew Gingko saplings at 350 and 650 ppm CO2 for three years and reported that elevated CO2 reduced leaf stomatal densities to values comparable to those measured on fossilized Gingko leaves dating back to the Triassic and Jurassic periods, but that it did not affect photosynthesis, which suggests that at those earlier times of greater atmospheric CO2 concentration, these plants were much more efficient at utilizing water than they are today. Finally, Nicholson et al. (1998) found that rain use efficiency, which is similar to water use efficiency, neither increased nor decreased from 1980 to 1995 for the central and western Sahel, contrary to the popular view supported by many international agencies; while Prince et al. (1998) demonstrated that rain use efficiency actually increased, on average, over the whole of the African Sahel from 1982 to 1990.
So what do these many studies imply? They suggest that as the CO2 content of the air continues to rise, nearly all of earth's plant life should exhibit increases in water use efficiency. It is thus likely that as time progresses, more and more of the planet's vegetation will expand into areas that have been too dry to support much life in the recent past. Therefore, one can expect the earth to become ever greener as time marches on and more CO2 accumulates in the atmosphere.
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