Similar results have also been obtained when trees were exposed to different environmental stresses. In a study conducted by Centritto et al. (1999), for example, cherry seedlings grown at twice-ambient levels of atmospheric CO2 displayed water-use efficiencies that were 50% greater than their 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 their respective controls, while simultaneously subjected to uncharacteristically low and high air temperature regimes, respectively.

In some parts of the world, perennial woody species have been exposed to elevated atmospheric CO2 concentrations for decades, due to their proximity to CO2-emitting springs and vents in the earth, allowing scientists to assess the long-term effects of this phenomenon. 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 local 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 around 465 ppm exhibited water-use efficiencies that were 100% greater than control trees growing at a lifetime CO2 concentration of 355 ppm. Also, two species of oak in central Italy that had been growing for 15 to 25 years at an atmospheric CO2 concentration ranging from 500 to 1000 ppm displayed "such marked increases in water-use efficiency under elevated CO2," in the words of the scientists who studied them, that this phenomenon "might be of great importance in Mediterranean environments in the perspective of global climate change," as suggested by the work of Blaschke et al. (2001) and Tognetti et al. (1998). Thus, the long-term effects of elevated CO2 concentrations on water-use efficiency are likely to persist and increase with increasing atmospheric CO2 concentrations.

In some cases, scientists have looked to the past and determined the positive 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 data derived from beech trees to determine that over the past century the water-use efficiency of such trees in north-eastern France increased by approximately 33%. Similarly, Feng (1999) used tree-ring chronologies derived from a number of trees in western North America to calculate a 10 to 25% increase in tree water-use efficiency from 1750 to 1970, during which time the atmospheric CO2 concentration rose by approximately 16%.

In another study, Knapp et al. (2001) developed tree-ring chronologies from western juniper stands located in Oregon, USA, for the past century, determining that growth recovery from drought was much greater in the latter third of their chronologies (1964-1998) than it was in the first third (1896-1930). And in this case, the researchers suggested that the greater atmospheric CO2 concentrations of the latter period allowed the trees to more quickly recover from water stress. Also, Beerling et al. (1998) grew Gingko saplings at 350 and 650 ppm CO2 for three years, finding that elevated atmospheric CO2 concentrations reduced leaf stomatal densities to values comparable to those measured on fossilized Gingko leaves dating back to the Triassic and Jurassic periods, implying greater water-use efficiencies for those times too.

Moving forward at a somewhat slower pace, but more descriptively, Tognetti et al. (2001) grew five-year-old seedlings of two olive cultivars in pots placed within Free-Air CO2 Enrichment (FACE) arrays maintained at atmospheric CO2 concentrations of 360 and 560 ppm for seven to eight months in a study designed to evaluate the effects of elevated CO2 on gas exchange in this economically important tree species. In doing so, they found that the elevated CO2 enhanced rates of net photosynthesis by an average of 38% in both cultivars, while reducing stomatal conductances by an average of 30%. Consequently, instantaneous water-use efficiency rose by approximately 80% in both cultivars, suggesting that as the air's CO2 content continues to rise, olive trees growing in semi-arid Mediterranean-type climates should be able to cope with recurring drought conditions that are common in such areas.

One year later, Centritto et al. (2002) reported that after having grown peach (Prunus persica) seedlings at atmospheric CO2 concentrations of 350 and 700 ppm for one full year in two "growth tunnels," they transferred them to pots and placed them in open-top chambers having the same CO2 concentrations for an additional three months, during the final four weeks of which half of the seedlings in each CO2 treatment were allowed to "dry-down," thus enabling the three researchers to investigate the interactive effects of elevated CO2 and water stress on photosynthesis and growth.

This work revealed that the elevated CO2 stimulated net photosynthesis rates by about 60% in the well-watered seedlings. Under drought conditions, however, the relative photosynthetic stimulation increased to as much as 180%, leading to CO2-induced increases in seedling dry mass of 33 and 31% for the well-watered and water-stressed seedlings, respectively. The larger percentage enhancement of net photosynthesis in the water-stressed seedlings essentially ameliorated the negative effect of water stress on growth. In addition, elevated CO2 increased whole-plant water-use efficiency by 51 and 63% in the well-watered and water-stressed seedlings, respectively. This enhancement resulted solely from CO2-induced increases in photosynthesis, and not from reductions in stomatal conductance or total water use. Thus, as the air's CO2 content increases, peach seedlings likely will exhibit increased rates of net photosynthesis and biomass production. In addition, they will likely be better able to deal with intermittent periods of water shortage, without compromising overall productivity and growth. And, consequently, peach production will likely increase as the atmospheric CO2 concentration continues to rise.

In a paper published about the same time, Arneth et al. (2002) described how they developed twenty tree-ring ¹³C/¹²C chronologies from Pinus sylvestris (Scots pine) trees at five locations along a 1000-km north-south transect running through central Siberia that they converted into plant isotopic discrimination (δ¹³Cc) values. And based on these data, they concluded that in 17 of the 20 samples, the trees' δ¹³Cc had declined during the last 150 years, "particularly so during the second half of the twentieth century." And based on a model of stomatal behavior combined with a process-oriented photosynthesis model, they further deduced that "this trend indicates a long-term decrease in canopy stomatal conductance, probably in response to increasing atmospheric CO2 concentrations," which ultimately led them to conclude that their observations were suggestive of "increased water use efficiency for Scots pine in central Siberia over the last century."

Also with a paper appearing in the same year was Centritto (2002), who grew peach seedlings for two years in pots placed within open-top chambers of either ambient or CO2-enriched air (350 or 700 ppm, respectively) that were located inside a glasshouse, where they were continuously maintained at optimum soil fertility and, for the entire first growing season, at optimum soil water availability. In the second growing season, however, half of the seedlings had water withheld from them for a period of four weeks. And what did they learn?

At the end of the study, there were no CO2-induced differences in the basal diameters of the seedlings. In terms of total dry weight, however, the elevated CO2 treatment enhanced the growth of the water-stressed seedlings by 30% and the growth of the well-watered seedlings by 35%, which was largely a consequence of increased height growth. In addition, they reported no evidence of any downward acclimation of photosynthesis in the seedlings grown at elevated CO2; nor was there any downward acclimation in Rubisco carboxylation efficiency nor in the maximum RuBP regeneration capacity mediated by electron transport. Furthermore, there were also no significant effects of elevated CO2 on stomatal conductance in either of the two water treatments. But because of the CO2-induced increase in plant growth, there was a complementary increase in seedling water use efficiency, even though there was no difference in total water uptake between the two CO2 treatments. Therefore, based on these findings, it would appear that in a world of the future where atmospheric CO2 concentration is approximately doubled, young peach trees will likely produce about a third more growth on the same amount of water as they did at the turn of the century.

One year later - as part of the long-term (November of 1987 to January of 2005) sour orange tree study conducted at the U.S. Water Conservation Laboratory in Phoenix, Arizona (Idso and Kimball, 2001; Kimball et al., 2007) - Leavitt et al. (2003) reported the results of a multifaceted investigation of a phenomenon that had never before been assessed in this long-term experiment: the effects of a 75% increase in the air's CO2 content on the efficiency with which well-watered and fertilized sour orange trees utilize water. It was based, as the six scientists noted, "on the conceptual framework developed by Farquhar et al. (1982), who defined intrinsic water-use efficiency (iWUE) as the ratio of the photosynthetic uptake of CO2 through leaf stomata to the simultaneous transpirational loss of water vapor through the same [stomatal] openings."

This ratio may be experimentally evaluated by measuring stable-carbon isotopes of various plant tissues and the air to which those tissues were exposed during their development. In this study, the plant materials that were utilized were leaves that had been collected every two months throughout 1992 and on three occasions in 1994-95, plus wood samples that were extracted five years later from north-south- and east-west-oriented wood cores that passed through the centers of each of the eight trees' trunks at a height of 45 cm above the ground. The grand average result of these measurements, evaluated within the context described by Farquhar et al., was, as Leavitt et al. reported, "an 80% increase in [water use efficiency] in response to the [75%] increase in atmospheric CO2 concentration employed in the study."

This result is particularly interesting for a number of different reasons. First, it suggests that for a doubling of the air's CO2 content, there would likely be more than a doubling of the trees' water use efficiency. Second, in the words of the six scientists, "this increase in sour orange tree iWUE is identical to the long-term CO2-induced increase in the trees' production of wood and fruit biomass," as documented by Idso and Kimball (2001); and this observation suggests that a doubling of the air's CO2 content should produce more than a doubling of the trees' total productivity, which further suggests that land planted to sour orange trees, and perhaps many other tree species, will see its potential for carbon sequestration grow dramatically in a CO2-enriched world of the future.

A third important fact noted by Leavitt et al. was that the CO2-induced increase in sour orange tree water use efficiency is also identical "to the increase in the mean iWUE reported for 23 groups of naturally occurring trees scattered across western North America that was caused by the historical rise in the air's CO2 content that occurred between 1800 and 1985," as documented by Feng (1999), who further noted that these iWUE trends in naturally occurring trees "are largely caused by the anthropogenic increase of the atmospheric CO2 concentration," concluding that this phenomenon "would have caused natural trees in arid environments to grow more rapidly, acting as a carbon sink for anthropogenic CO2." In addition, Leavitt et al. pointed out that "even greater water-use efficiency responses have been observed in European tree-ring studies," citing the work of Bert et al. (1997) with white fir and Hemming (1998) with beech, oak and pine trees.

What do these observations portend for the decades ahead? In addressing this subject at the conclusion of their paper, Leavitt et al. said "the ongoing rise in the air's CO2 content could continue to do the same for earth's trees in the future." And what is that? It is to dramatically increase their productivity and the efficiency with which they utilize water to achieve vastly enhanced growth rates.

How general could we expect this phenomenon to be? In a comprehensive review of the scientific literature pertaining to this subject, Saxe et al. (1998) determined that "close to a doubling" of the air's CO2 concentration leads to an approximate 50% increase in the biomass production of angiosperm trees and a 130% increase in the biomass production of coniferous species. With sour orange trees projected to experience just slightly more than a 100% increase in wood and fruit production in response to a doubling of the air's CO2 concentration, it is clear that the results of the Phoenix study fall well within the mid-range results typical of most other trees that have been similarly studied.

In light of these many empirical observations, therefore, one can confidently expect the growth rates of earth's trees to increase dramatically as the air's CO2 content continues to climb; and this phenomenon, in turn, should enable them to sequester increasingly greater amounts of carbon. In addition, as the planet's trees become ever more efficient at utilizing water, one could expect to see them rapidly expand into areas that are currently too dry to support their growth and reproduction; and this phenomenon should also increase the magnitude of carbon sequestration by earth's trees. Hence, as time progresses, the planet's trees, if not destroyed by mankind's cutting and burning them, should provide an ever-increasing brake upon the rate of rise of the air's CO2 content.

Moving ahead another year, Waterhouse et al. (2004) determined the intrinsic water use efficiency (iWUE) responses of three tree species growing across northern Europe - pedunculate oak (Quercus robur L.), common beech (Fagus sylvatica L.) and Scots pine (Pinus sylvestris L.) - to the increase in the air's CO2 concentration experienced between 1895 and 1994, using parameters derived from measurements of stable carbon isotope ratios of trunk cellulose. This work revealed, as they reported it, that "all species at all the sites show a long-term increase in their values of iWUE during the past century," and they opined that "the main cause of this common behavior is likely to be the increase in atmospheric CO2 concentration."

Linearly extrapolating these responses (which occurred over a period of time when the air's CO2 concentration rose by approximately 65 ppm) to what would be expected for the more common 300-ppm increase employed in the majority of atmospheric CO2 enrichment experiments, the iWUE increases they derived amount to +158 ± 14% for the oak trees (mean ± standard error for the five sites studied), +195% for the pine trees, and +220% for the beech trees, as best as can be determined from the graphs of their results. These responses are huge, and are probably not due to rising CO2 alone, but to the positive synergism that occurs when the air's CO2 content and temperature rise together, as these parameters have done over the past century or so.

Working and publishing concomitantly, Peterson and Neofotis (2004) grew velvet mesquite (Prosopis velutina Woot.) seedlings for six weeks from their time of planting (as seeds) in small pots within environmentally-controlled growth chambers that were maintained at atmospheric CO2 concentrations of 380 and 760 ppm and two levels of water availability (high and low). Although they did not see a significant CO2-induced increase in plant growth, they reported that by the end of their six-week study, there was a highly significant reduction of approximately 41% in the volume of water transpired by P. velutina in response to the experimental doubling of the air's CO2 content. "This large reduction in whole-plant water use," as they described it, "occurred because the reduction in transpiration per unit leaf area at elevated CO2 was not offset by a proportional increase in total leaf area."

The two scientists from the Biosphere 2 Center near Oracle, Arizona, USA, said their findings suggest that "under a future [high-CO2] scenario, seedlings may deplete soil moisture at a slower rate than they do currently," and that "this could facilitate seedling survival between intermittent rain events," noting that their work "corroborates the conclusions of Polley et al. (1994, 1999, 2003) that increasing levels of atmospheric CO2 may facilitate the establishment of mesquite seedlings through a reduction in soil water depletion." And that such has indeed occurred is suggested by the fact, again quoting Peterson and Neofotis, that "mesquites and other woody species in the semiarid southwestern United States have shown substantial increases in population density and geographic range since Anglo-American settlement of the region approximately 120 years ago," in support of which statement they cited the studies of Van Auken and Bush (1990), Gibbens et al. (1992), Bahre and Shelton (1993), Archer (1995), Boutton et al. (1999), Van Auken (2000), Ansley et al. (2001), Wilson et al. (2001) and Biggs et al. (2002).

Also with a paper published in the same year were Saurer et al. (2004), who by measuring carbon isotope ratios in the rings of coniferous trees from northern Eurasia - including the three genera Larix, Picea and Pinus - across a longitudinal transect covering the entire super-continent in the latitude range from 59 to 71°N, were able to determine the change in intrinsic water use efficiency (Wi, the amount of water loss at the needle level per unit of carbon gain) that was experienced by the trees between the two 30-year periods 1861-1890 and 1961-1990.

This work revealed, as they described it, that the concomitant "increasing CO2 in the atmosphere resulted in improved intrinsic water-use efficiency," such that "125 out of 126 trees showed increasing Wi from 1861-1890 to 1961-1990, with an average improvement of 19.2 ± 0.9%." As for the significance of this finding, the three Swiss scientists said their results suggest that the trees they studied "are able to produce the same biomass today [as they did 100 years ago] but with lower costs in terms of transpiration." This finding is highly significant, because some data had indicated that recent warming in other longitudinal segments of the same latitude belt "may be accompanied by increased drought stress (Lloyd and Fastie, 2002)." And the historical increase in the air's CO2 content may have been helping those trees to better cope with the newly established drought conditions.

Gradually moving forward in time, Syvertsen and Levy (2005) reviewed what was known about salinity stress in citrus trees and how it may be modified by atmospheric CO2 enrichment. They noted, for example, that rapidly growing plants almost always use more water than slower growing plants, and that, "in citrus, many vigorous rootstocks that produce fast-growing trees also tend to have poor salt tolerance (Castle et al., 1993)," possibly because they accumulate more salt in their tissues because of their greater uptake of water. When growing plants in CO2-enriched air, however, plant stomatal conductance and water use are often decreased at the same time that net photosynthesis and growth are increased, so that, in the words of the two scientists, "elevated CO2 almost always leads to higher water use efficiency as it disconnects rapid tree growth from high water use." And consequently, as they went on to explain, "if salt uptake is coupled with water uptake, then leaves grown at elevated CO2 should have lower salt concentrations than leaves grown at ambient CO2 (Ball and Munns, 1992)."

So, do things really work that way? "As expected," in the words of Syvertsen and Levy "all citrus rootstock species studied increased growth and water use efficiency in response to elevated CO2 that was twice ambient," and they said that generally, but not always, "the salinity-induced accumulation of sodium (Na⁺) in leaves was less when seedlings were grown at elevated CO2 than at ambient CO2." One exception, where Na⁺ accumulation was not affected by elevated CO2, was Rangpur lime (Citrus reticulata); but they noted that this citrus variety was relatively salt-tolerant, and that another variety of the same species (Cleopatra mandarin) had lower leaf chloride (Cl^-) concentrations in CO2-enriched air than in ambient air.

Therefore, all citrus trees that had been tested to that point in time had exhibited increased growth rates and water use efficiencies when growing in CO2-enriched air. In addition, they had generally experienced less salinity stress than when grown in lower-CO2 ambient air. And as a result, the ongoing rise in the atmosphere's CO2 concentration bodes well for the future vitality and productivity of earth's many varieties of citrus trees, which in turn bodes well for humanity.

Working concurrently, Hietz et al. (2005) collected samples of wood from 37 tropical cedar (Cedrela odorata L.) trees that were between 11 and 151 years old in 2001 and from 16 big-leaf mahogany (Swietenia macrophylla King) trees that were between 48 and 126 years old at that time from a rain forest in Aripuana, Brazil, after which they measured the wood samples' cellulose δ¹³C in 10-year growth increments. And in doing so, they found that the cellulose δ¹³C decreased by 1.3 per mil in Cedrela and by 1.1 per mill in Swietenia over the past century, with the largest changes occurring during the last 50 years. Based on these data and known trends in atmospheric CO2 and δ¹³CO2, they calculated that the intrinsic water-use efficiency of the trees increased by 34% in Cedrela and by 52% in Swietenia over this period, which they said was about the same as what had been deduced from similar measurements of the wood of temperate trees (Freyer, 1979; Bert et al., 1997; Feng, 1999).

As for what these results signify, the three researchers noted that since "water is probably not a strong limiting factor in tropical rain forest trees," the increase in water use efficiency that they discovered likely "translates mostly to increased carbon assimilation, which may explain the observed increase in tree growth and turnover (Phillips, 1996; Laurance et al., 2004)" in such forests. Thus, as these reports indicate, evidence continues to accumulate for a worldwide stimulation of tree growth over the course of the Industrial Revolution, which has been driven primarily by the historical increase in the air's CO2 concentration. And this being the case, one could logically expect the upward trend in tree growth to continue, as long as the burning of fossil fuels that is required to sustain the growth of the world's developing economies continues to release ever more CO2 to the atmosphere.

Jumping ahead two more years, and defining intrinsic water-use efficiency (iWUE) as the ratio of the photosynthetic uptake of CO2 through leaf stomata to the simultaneous transpirational loss of water vapor through the stomata, Liu et al. (2007) evaluated this parameter based on δ¹³C measurements of cellulose extracted from the wood of tree-ring cores taken from living Qilian juniper (Sabina przewalskii Kom.) and Qinghai spruce (Picea crassifolia Kom.) trees, focusing on the period AD 1850-2000 at time resolutions of three years for juniper from the semi-arid Qilian Mountains, two years for juniper from the arid Qaidam Basin and one year for spruce from both of the northwest China sites. And what did they thereby learn?

Overall, and based on means for the first and last decades of the study period, the seven Chinese researchers found that "the iWUE values of the two species both showed long-term increases, by 33.6 and 37.4% for spruce in the aird and semi-arid areas, respectively, and by increases of 24.7 and 22.5% for juniper," noting that "the main cause of this behavior is likely to be an increase in atmospheric CO2 concentration," which for the start and end decades of the study period rose from approximately 285 ppm to 362 ppm, or by about 27%.

Clearly, increases in the water use efficiencies of trees in arid and semi-arid regions must be considered a significant benefit. And in the case of the two species studied by Liu et al., they stated that Qinghai spruce, in particular, "plays an important role in preventing soil erosion, regulating climate, and retaining ecological stability," citing the work of Zhou and Li (1990) in this regard. Thus, this phenomenon is undoubtedly one of the chief reasons for the concomitant "greening of the earth" that has been so evident in many historical studies of China and other parts of Asia.

Continuing to move towards the present, Leal et al. (2008) obtained cores from 8 to 20 black pine (Pinus nigra) trees growing at each of 28 sites within the Vienna basin of Austria in the European Eastern Alps during the summers of 1996 and 1997, focusing on trees possessing umbrella-like crowns (indicative of water-limited conditions) growing on shallow and poor soils, in order to maximize their ring-width response to moisture availability. In doing so, they discovered "a very clear change in the sensitivity of the growth rate of tree stems to water availability in the late 20th century," noting that "trees previously sensitive to spring-summer drought show a lack of response to this climatic parameter in recent decades." That is to say - as they actually did say - that "tree-ring indices were larger in the second half of the 20th century than predicted given prevailing spring-summer drought conditions and the previous sensitivity of growth to these conditions." In addition, they found "a decrease in correspondence between the occurrence of extreme events in precipitation and rate of change of growth," such that "in the second half of the century this correspondence was not significant," and that "recent extreme droughts did not result in the formation of very narrow rings, which means the droughts were not as limiting to tree growth as they had been in the past."

In light of these observations, the five researchers concluded their paper by suggesting that the greater atmospheric CO2 concentrations of the latter decades of the 20th century "induced improved water-use efficiency enabling P. nigra growing in the Vienna basin to avoid the impact of recurrent dry conditions," which phenomenon has also been observed in many other parts of the world in a number of different tree species, which is but another indication of the propensity of the ongoing rise in the air's CO2 content to promote a greening of the earth.

Inching ahead another year, while describing Araucaria angustifolia as "an indigenous conifer tree restricted to the southern region of South America that plays a key role in the dynamics of regional ecosystems where forest expansion over grasslands has been observed," Silva et al. (2009) worked with various types of tree-ring data obtained from A. angustifolia trees growing in both forest and grassland sites of southern Brazil, comparing changes in intrinsic water use efficiency (iWUE, defined as the ratio of the rate of CO2 assimilation by the trees' needles to their stomatal conductance) with historical changes in temperature, precipitation and atmospheric CO2 concentration that occurred over the past century.

This effort revealed, in their words, that during the past several decades, "iWUE increased over 30% in both habitats [forests and grasslands]," and that "this increase was highly correlated with increasing levels of CO2 in the atmosphere." However, tree growth remained rather stable over this latter period, due to lower-than-normal precipitation and higher-than-normal temperatures, which would normally tend to depress the growth of this species, as Katinas and Crisci (2008) describe A. angustifolia as being "intolerant of dry seasons and requiring cool temperatures." And, therefore, Silva et al. concluded that "climatic fluctuations during the past few decades," which would normally be expected to have been deleterious to the growth of A. angustifolia, seemed to have had their growth-retarding effects "compensated by increases in atmospheric CO2 and changes [i.e., increases] in iWUE." And it would appear that this phenomenon is what has helped to sustain the historical-and-still-ongoing greening of the earth that is gradually transforming the terrestrial face of the planet.

One year later, Wyckoff and Bowers (2010) wrote that "with continued increases in global greenhouse gas emissions, climate models predict that, by the end of the 21st century, Minnesota [USA] summer temperature will increase by 4-9°C and summer precipitation will slightly decrease," citing in this regard the work of Kling et al. (2003) and Christensen et al. (2007); and they added that certain "forest models and extrapolations from the paleoecological record suggest that, in response to increased temperature and/or drought, forests may retreat to the extreme north-eastern parts of the state," citing the work of Pastor and Post (1998), Hamilton and Johnson (2002) and Galatowitsch et al. (2009).

Therefore, working with bur oak (Quercus macrocarpa) trees, Wyckoff and Bowers explored the likelihood of the latter of these two projections coming to pass by: "(i) using tree rings to establish the relationship between drought and Q. macrocarpa growth for three sites along Minnesota's prairie-forest border, (ii) calculating the current relationship between growth and mortality for adult Q. macrocarpa and (iii) using the distributions of current growth rates for Q. macracarpa to predict the susceptibility of current populations to droughts of varying strength." In addition, they looked for "temporal trends in the correlation between Q. macrocarpa growth and climate, hypothesizing that increases in CO2 may lead to weaker relationships between drought and tree growth over time," because of the fact that atmospheric CO2 enrichment typically leads to increases in plant water use efficiency, which phenomenon generally makes them less susceptible to the deleterious impact of drought on growth.

In thus exploring this subject, the two University of Minnesota researchers discovered that "the sensitivity of annual growth rates to drought has steadily declined over time as evidenced by increasing growth residuals and higher growth rates for a given PDSI [Palmer Drought Severity Index] value after 1950 [when the atmosphere's CO2 concentration rose by 57 ppm from 1950 to 2000] compared with the first half of the century [when the CO2 increase was only 10 ppm]." And as a result, they concluded that "for Q. macrocarpa, declining sensitivity of growth to drought translates into lower predicted mortality rates at all sites," and that "at one site, declining moisture sensitivity yields a 49% lower predicted mortality from a severe drought (PDSI = -8, on a par with the worst 1930s 'American Dust Bowl' droughts)." Hence, they further concluded that "the decreasing drought sensitivity of established trees may act as a buffer and delay the movement of the prairie-forest ecotone for many decades even in the face of climate change." In fact, the bur oak forests may be so significantly benefited by continued increases in the air's CO2 content that they need never retreat, especially in light of the likely over-estimation of warming predicted by most climate models for the remainder of the 21st century.

In a paper published the following year, Brienen et al. (2011) wrote as background for their study that water use efficiency is the ratio of photosynthesis (A) to transpiration (E), or the amount of carbon gained per unit of water used in the process of acquiring the carbon; and they defined A/gs - where gs is stomatal conductance - to be intrinsic water use efficiency (Wi), stating that "an increase in Wi in response to increasing CO2 since the industrial revolution has been found in nearly all temperate trees that have been studied," citing the work of Feng (1999), Saurer et al. (2004) and Nock et al. (2010). And they thus decided to see if such was also the case for tropical trees.

Noting that "increases in Wi have been observed in short-term experiments of tree responses to elevated CO2 (Norby et al., 1999), and over long-time periods using records of δ¹³C in tree rings that reflect the global increase in atmospheric CO2 (Feng, 1999; Waterhouse et al., 2004)," Brienen et al. "analyzed carbon isotope ratios over the last 40 years in tree rings of Mimosa acantholoba, a tropical dry forest pioneer species," in a study conducted "on the Pacific slope of the isthmus of Tehuantepec, close to the village of Nizanda in the state of Oaxaca, South Mexico (16°39'N, 95°00'W)." And what did they find? The three researchers, representing Austria, Mexico and the United Kingdom, reported that the dry-forest tropical M. acantholoba trees "responded strongly to the increase in atmospheric CO2 over the last four decades," as their "Wi increased dramatically by 40%."

In another paper from the same year, Chen et al. (2011) wrote that "Idso (1998) suggested that elevated CO2 affects plant growth dependent upon plant water status: it has less effect on plants in the well-watered optimal growth phase, but exerts more effect under non-lethal dry conditions, and is most beneficial to plants under severe drought conditions." And in a further assessment of this phenomenon, Chen et al. measured leaf transpiration rate (E) and net photosynthetic rate (PN) in Populus euphratica trees growing just within the northern edge of the Taklimakan Desert in Xinjiang, northwestern China, where the riparian trees dominate the indigenous vegetation because of their tolerance of severe drought and the high salinity and alkalinity of the region's soils. This they did in four different locations, where mean soil water contents at groundwater depths of 4.12, 4.74, 5.54 and 7.74 meters were 10.9, 9.5, 3.5 and 1.3%, respectively, making their measurements at atmospheric CO2 concentrations of either 360 or 720 ppm, after which they calculated the trees' water use efficiencies (WUE = PN/E) when measured under the two atmospheric CO2 concentrations.

So what did they learn? In the case of each CO2 concentration, there was no statistical difference between the leaf water use efficiencies of the first three groundwater depths; but the mean WUE at the higher of the two CO2 concentrations was 44% greater than the mean measured at the lower CO2 concentration. However, the WUE of the lowest and driest of the four groundwater depths, was statistically different from the WUEs of the other three groundwater depths; and the mean WUE of the trees growing under this most stressful condition when measured at the higher of the two CO2 concentrations was 86% greater than the mean measured at the lower CO2 concentration. And in light of these several findings, Chen et al. concluded that with respect to the plant water use efficiency of Populus euphratica trees, those growing "under a mild water stress show a weak responsiveness, and those under a moderate drought stress display a strong responsiveness to CO2 enrichment."

In another concurrent study, Soule and Knapp (2011) wrote that "in 2008, atmospheric CO2 concentrations from the Mauna Loa, Hawaii, Observatory records exceeded 385 ppm, representing a 22% increase since 1959," and they indicated that "as CO2 has increased, most tree species have been able to use water more efficiently," as their "leaf stomatal apertures narrow during photosynthesis," resulting in "less transpirational water loss per biomass gained."

The parameter that represents this phenomenon is generally referred to as intrinsic water-use efficiency or iWUE, which is defined as the ratio of net CO2 assimilation to leaf or needle stomatal conductance; and the two researchers employed it in their study of changes in - and relationships among - radial growth rates and the iWUE of ponderosa pine (Pinus ponderosa) trees, climate and atmospheric CO2 concentration in the western United States since the mid-nineteenth century, developing tree-ring chronologies for eight sites in three climate regions, while using carbon isotope data to calculate pentadal values of iWUE, after which they examined relationships among radial growth, climate, iWUE and CO2 via correlation and regression analyses.

In following these procedures, Soule and Knapp reported finding significant upward trends in iWUE at all sites; and they said that "despite an absence of climate changes that would favor growth," upward radial growth trends occurred at five sites. In addition, they discovered that the highest iWUE values "were recorded in the last pentad at six of eight sites and follow a positive quadratic progression at all sites, suggesting that future increases in iWUE are likely for ponderosa pine within our study regions as CO2 levels increase." And they further remarked that they found "significant improvements in radial growth rates during drought years after 1950," when the air's CO2 content rose at an accelerating rate.

In discussing their findings, the two U.S. researchers suggested that "increased iWUE associated with rising CO2 can positively impact tree growth rates in the western United States and are thus an evolving component of forest ecosystem processes." And they concluded that "if potential climate changes lead to increasing aridity in the western United States, additional increases in iWUE associated with future increases in CO2 might ameliorate growth declines associated with drought conditions."

One year later, Wang et al. (2012) introduced their treatment of the subject by writing that intrinsic water-use efficiency (iWUE) "represents the ratio of photosynthetic assimilation (A) to stomatal conductance (gw)," while noting that "higher iWUE can result from reducing gw, increasing A, or a combination of the two responses." They also indicated that "empirical evidence from lab studies with a controlled CO2 concentration and from free-air CO2 enrichment (FACE) experiments have revealed significantly increased iWUE in response to rising CO2," as demonstrated by the studies of Luo et al. (1996), Ainsworth and Rogers (2007) and Niu et al. (2011). And they also noted that "tree-ring stable carbon isotope ratios (δ¹³C) have proven to be an effective tool for evaluating variations in iWUE around the world," citing Farquhar et al. (1989), Saurer et al. (2004), Liu et al. (2007) and Andreu et al. (2011). What is more, they reported that "during the past 100-200 years, most of the sampled forests demonstrated a trend of increasing iWUE, which paralleled the increasing atmospheric CO2," citing Penuelas et al. (2011) and references therein.

Getting down to business, Wang et al. described how, while working at a site in the Xinglong Mountains in the eastern part of northwestern China (35°40'N, 104°02'E), they extracted two cores from the trunks of each of 17 dominant living Qinghai spruce (Picea crassifolia) trees in November 2009, from which they obtained precise ring-width measurements that they used to calculate yearly mean basal area growth increments, after which they used subsamples of the cores to conduct the stable carbon isotope analyses needed to obtain the δ¹³C data required to calculate iWUE over the course of their study period: 1800-2009. Also, by calibrating the δ¹³C data against climatic data obtained at the nearest weather station over the period 1954-2009, they were able to extend the histories of major meteorological parameters all the way back to 1800. And by comparing these weather data with the tree growth and water use efficiency data, they were able to interpret the impacts of climate change and atmospheric CO2 enrichment on spruce tree growth and water use efficiency.

These efforts revealed that for the arid region of northwestern China in which the spruce trees they studied were growing, iWUE increased by approximately 40% between 1800 and 2009, rising very slowly for the first 150 years, but then more rapidly to about 1975, and then faster still until 1998, whereupon it leveled off for the remaining eleven years of the record. And in commenting on the main cause of the increasing trend in iWUE from 1800 to 1998, they said it "is likely to be the increase in atmospheric CO2," because "regression analysis suggested that increasing atmospheric CO2 explained 83.0% of the variation in iWUE from 1800 to 1998 (p<0.001)." Thereafter, however, they noted that a substantial drought at the end of the record is probably what caused the leveling off of iWUE, which was also strong enough to cause a decline in yearly basal area growth increment, much as what occurred between 1923 and 1934, which period they described as "the most severe drought since 1800," citing Fang et al. (2009).

The ultimate knowledge gained from Wang et al.'s study would thus appear to be that the historical increase in the air's CO2 content over the course of the Industrial Revolution gradually but greatly enhanced the intrinsic water use efficiency of Qinghai spruce trees in northwest China, as well as their growth rates. However, during times of very severe drought stress, even this added help can fall short of what is needed to keep the trees from maintaining an exemplary rate of growth. Nevertheless, the continually-rising atmospheric CO2 concentration sees them through their times of real distress to where they can once again grow exceedingly well once the drought is past.

Finally - and almost reaching the present - Battipaglia et al. (2013) combined tree ring analyses with carbon and oxygen isotope measurements made at three Free Air CO2 Enrichment (FACE) sites - POP-EUROFACE in Italy, Duke FACE in North Carolina (USA), and ORNL in Tennessee (USA) - in order "to cover the entire life of the trees," which feat they accomplished using δ¹³C to assess carbon isotope discrimination and changes in water-use efficiency, while direct CO2 effects on stomatal conductance were explored using δ¹⁸O as a proxy."

This work revealed, in the words of the seven scientists who conducted it, that "across all the sites, elevated CO2 increased ¹³C-derived water-use efficiency on average by 73% for Liquidambar styraciflua [POP-EUROFACE, +200 ppm CO2], 77% for Pinus taeda [Duke FACE, +200 ppm CO2] and 75% for Populus sp. [ORNL, +153 ppm CO2], but through different ecophysiological mechanisms." Thus, they went on to say that their findings provided "a robust means of predicting water-use efficiency responses from a variety of tree species exposed to variable environmental conditions over time, and species-specific relationships that can help modeling elevated CO2 and climate impacts on forest productivity, carbon and water balances." And their results indicated that the CO2-induced increases in forest WUEi that they documented were huge.

In summary, it is clear that as the CO2 content of the air continues to rise, nearly all of earth's trees will respond favorably by exhibiting increases in water-use efficiency. And it is thus quite likely that as time progresses, earth's woody species will expand into areas where they previously could not exist due to limiting amounts of available moisture. Therefore, one can expect the earth to become a greener biospheric body with greater carbon sequestering capacity as the air's CO2 concentration continues to rise.

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