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Interaction of CO2 and Water Stress on Plant Growth (Woody Species) -- Summary
It is widely acknowledged that as the CO2 content of the air continues to rise, nearly all of earth's plants will exhibit increases in photosynthesis and biomass production; but climate alarmists periodically proclaim that future water stress will negate these benefits of atmospheric CO2 enrichment. In reviewing much of the pertinent scientific literature of the ten-year period 1983-1994, however, Idso and Idso (1994) determined that water stress will generally not negate the CO2-induced stimulation of plant growth. In fact, they found that the CO2-induced percentage increase in plant productivity was nearly always greater under water-stressed conditions than it was when plants were well-watered. This review thus discusses some of the subsequent relevant literature in this important research domain, in order to determine if these conclusions are still valid, particularly in the case of earth's woody species.

For starters, it is well known that during times of water stress, atmospheric CO2 enrichment often stimulates the development of larger-than-usual and more robust root systems in woody plants, which thus enable them to probe greater volumes of soil for scarce and much-needed moisture. Tomlinson and Anderson (1998), for example, found that greater root development in water-stressed red oak seedlings grown at 700 ppm CO2 helped them effectively deal with the reduced availability of moisture; and these trees eventually produced just as much biomass as well-watered controls exposed to air containing 400 ppm CO2. In addition, Polley et al. (1999) discovered that water-stressed honey mesquite trees exposed to an atmospheric CO2 concentration of 700 ppm produced 37% more root biomass than water-stressed seedlings in air of 370 ppm.

Elevated levels of atmospheric CO2 also tend to reduce the area of open stomatal pore space on leaf surfaces, thus reducing plant stomatal conductance. This phenomenon, in turn, reduces the amount of water lost to the atmosphere via transpiration; and, therefore, it was not surprising when Tognetti et al. (1998) determined that the stomatal conductances of mature oak trees growing near natural CO2 springs in central Italy were significantly lower than those of similar trees growing further away from the springs during periods of severe summer drought, which phenomenon allowed the CO2-enriched trees to better conserve what little water was available to them.

Working together, CO2-induced increases in root development and CO2-induced reductions in stomatal conductance often contribute to the maintenance of a more favorable plant water status during times of drought. In the case of three Mediterranean shrubs, for example, Tognetti et al. (2002) found that leaf water potential, which is a good indicator of plant water status, was consistently higher (less negative and, hence, less stressful) under twice-ambient CO2 concentrations. And in like manner, Polley et al. (1999)) found that leaf water potentials of water-stressed mesquite seedlings grown at 700 ppm CO2 were 40% higher than those of their water-stressed counterparts growing in ambient air, which is comparable to the values of -5.9 and -3.4 MPa observed in water-stressed evergreen shrubs (Larrea tridentata) exposed to 360 and 700 ppm CO2, respectively, as documented by Hamerlynck et al. (2000).

So if atmospheric CO2 enrichment thus allows plants to maintain a better water status during times of water stress, it is only logical to expect that plants growing under such conditions will exhibit CO2-induced increases in photosynthesis. And that is likely why Palanisamy (1999) observed water-stressed Eucalyptus seedlings grown at 800 ppm CO2 to display greater net photosynthetic rates than their ambiently-grown and water-stressed counterparts. In fact, Runion et al. (1999) found the CO2-induced photosynthetic stimulation of water-stressed pine seedlings grown at 730 ppm CO2 to be nearly 50% greater than that of similar water-stressed pine seedlings grown at 365 ppm CO2. Similarly, Centritto et al. (1999a) found that water-stressed cherry trees grown at 700 ppm CO2 displayed net photosynthetic rates that were 44% greater than those of water-stressed trees grown at 350 ppm CO2. And Anderson and Tomlinson (1998) found that a 300-ppm increase in the air's CO2 concentration boosted photosynthetic rates in well-watered and water-stressed red oak seedlings by 34 and 69%, respectively, demonstrating that the CO2-induced percentage enhancement in net photosynthesis in this species was essentially twice as great in water-stressed seedlings than it was in well-watered ones.

Nevertheless, plants sometimes suffer drastically when subjected to extreme water stress; but the addition of CO2 to the atmosphere often gives them an edge over plants growing in normal air. Tuba et al. (1998), for example, reported that leaves of a water-stressed woody shrub exposed to an atmospheric CO2 concentration of 700 ppm continued to maintain positive rates of net carbon fixation for a period that lasted three times longer than that observed for leaves of equally-water-stressed control plants growing in ambient air. Similarly, Fernandez et al. (1998) discovered that herb and tree species growing near natural CO2 vents in Venezuela continued to maintain positive rates of net photosynthesis during that location's dry season, while the same species growing some distance away from the CO2 source displayed net losses of carbon during this stressful time. Likewise, Fernandez et al. (1999) noted that after four weeks of drought, the deciduous Venezuelan shrub Ipomoea carnea continued to exhibit positive carbon gains under elevated CO2 conditions, whereas ambiently-growing plants displayed net carbon losses. In addition, Polley et al. (2002) reported that seedlings of five woody species grown at twice-ambient CO2 concentrations survived 11 days longer (on average) than control seedlings when subjected to maximum drought conditions. Therefore, in some cases of water stress, enriching the air with CO2 can actually mean the difference between a plant's growing or not growing. And if such conditions persist too long, that difference may translate into an actual life-or-death difference.

In view of the fact that elevated CO2 thus enhances photosynthetic rates during times of water stress, one would expect that tree and shrub biomass production would also be enhanced by elevated CO2 concentrations under drought conditions; and so it is, as demonstrated by Arp et al. (1998), who reported that six perennial plants common to the Netherlands increased their biomass under CO2-enriched conditions even when suffering from lack of water. In other cases, the CO2-induced percentage biomass increase is sometimes even greater for water-stressed plants than it is for well-watered plants. Catovsky and Bazzaz (1999), for example, reported that the CO2-induced biomass increase for paper birch was 27% and 130% for well-watered and water-stressed seedlings, respectively. Similarly, Schulte et al. (1998) noted that the CO2-induced biomass increase of oak seedlings was greater under water-limiting conditions than under well-watered conditions (128% vs. 92%), as did Centritto et al. (1999b) for basal trunk area in cherry seedlings (69% vs. 22%).

In another approach to the subject, Knapp et al. (2001) developed tree-ring index chronologies from western juniper stands in Oregon, USA, finding that the trees recovered better from the effects of drought in the 1990's, when the air's CO2 concentration was around 340 ppm, than they did from 1900-1930, when the atmospheric CO2 concentration was around 300 ppm. And in a loosely related study, Osborne et al. (2000) looked at the warming and reduced precipitation experienced in Mediterranean shrublands over the last century and concluded that primary productivity should have been negatively impacted in those areas. However, when the concurrent increase in atmospheric CO2 concentration was factored into their mechanistic model, a 25% increase in primary productivity was projected.

Looking to the future, Centritto (2002) grew peach seedlings for two growing seasons in pots within open-top chambers of either ambient or CO2-enriched air (350 or 700 ppm, respectively) inside a glasshouse, where all of the plants 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. Yet at the end of the study, there was 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 or in the maximum RuBP regeneration capacity mediated by electron transport.

Jumping ahead three years, Xiao et al. (2005) experimented with Caragana intermedia Kuanget H.C. Fu, a deciduous shrub of semi-arid northern China that occurs primarily in the country's Maowusu sandland, as well as parts of Inner Mongolia, where they say it is "used by local people as feed for livestock, and as shelter for protection of soils (Zhang, 1994)," while noting that "it is one of the dominant shrubs that fix soil and reduce wind speed, thus actively mitigating desertification." The five Chinese scientists grew seedlings of this species for 3.5 months in 10-cm-diameter by 10-cm-deep pots filled with sand and maintained at three different water regimes - well-watered (60-70% field capacity), moderate-watered (45-55% field capacity) and drought-stressed (30-40% field capacity) - in greenhouse compartments maintained at atmospheric CO2 concentrations of either 350 or 700 ppm, while near the end of this period, they measured leaf water potentials and a number of different plant growth parameters.

This work revealed, in Xiao et al.'s words, that "elevated CO2 significantly increased leaf water potential" while also increasing tree height, basal diameter, shoot biomass, root biomass as well as total biomass, which was increased by 79% under the well-watered condition, by 61% under the moderate-watered condition, and by 53% under the drought-stressed condition. And they say that the Canopy Productivity Index (CPI, total growth per unit leaf area) was also "significantly increased by elevated CO2, and the increase in CPI became stronger as the level of drought stress increased." And so they concluded that the results of their study "confirmed the beneficial effects of elevated CO2 on C. intermedia seedlings exposed to drought-stressed conditions," and that these findings "suggest that elevated CO2 may enhance drought avoidance and improved water relations, thus weakening the effect of drought stress on growth of C. intermedia seedlings," all of which phenomena should help to fight desertification in the Maowusu sandland of China and parts of Inner Mongolia as the air's CO2 content continues to rise.

One year later, Soule and Knapp (2006) wrote that "two major environmental issues have arisen regarding the increasingly CO2-rich world of the late 20th and early 21st centuries: climatic change, and plant responses to the environment," and in their second sentence they wrote that "while the implications of atmospheric CO2 for potential climatic change have received the majority of attention, the potential role of atmospheric CO2 fertilization in plant growth and subsequent ecosystem dynamics may be equally important." Hence, they focused their attention on this latter topic in a study of ponderosa pine trees growing at eight different sites within the Pacific Northwest of the United States, in order to see how they may have responded to the increase in the atmosphere's CO2 concentration that occurred after 1950.

The two geographers say they carefully chose study sites that "fit several criteria designed to limit potential confounding influences associated with anthropogenic disturbance" and which also had "a variety of climatic and topo-edaphic conditions, ranging from extremely water-limiting environments ... to areas where soil moisture should be a limiting factor for growth only during extreme drought years." Also, they say that all of their study sites were located in areas "where ozone concentrations and nitrogen deposition are typically low."

At each of the eight sites that met all of these criteria, Soule and Knapp obtained core samples from about 40 mature trees that included "the potentially oldest trees on each site," so that their results would indicate, as they put it, "the response of mature, naturally occurring ponderosa pine trees that germinated before anthropogenically elevated CO2 levels, but where growth, particularly post-1950, has occurred under increasing and substantially higher atmospheric CO2 concentrations." And utilizing meteorological evaluations of the Palmer Drought Severity Index, they thus compared ponderosa pine (Pinus ponderosa Laws. var. ponderosa) radial growth rates during matched wet and dry years pre- and post-1950.

So what did they find? Overall, the two researchers report finding a post-1950 radial growth enhancement that was "more pronounced during drought years compared with wet years, and the greatest response occurred at the most stressed site." As for the magnitude of the response, they determined that "the relative change in growth [was] upward at seven of our [eight] sites, ranging from 11 to 133%."

With respect to the significance of their observations, Soule and Knapp say that their results, "showing that radial growth has increased in the post-1950s period ... while climatic conditions have generally been unchanged, suggest that nonclimatic driving forces are operative." In addition, they say that "these radial growth responses are generally consistent with what has been shown in long-term open-top chamber (Idso and Kimball, 2001) and FACE studies (Ainsworth and Long, 2005)." Hence, they say their findings suggest that "elevated levels of atmospheric CO2 are acting as a driving force for increased radial growth of ponderosa pine, but that the overall influence of this effect may be enhanced, reduced or obviated by site-specific conditions."

Summarizing their findings, Soule and Knapp recount how they had "hypothesized that ponderosa pine...would respond to gradual increases in atmospheric CO2 over the past 50 years, and that these effects would be most apparent during drought stress and on environmentally harsh sites," and in the following sentence they say that their results "support these hypotheses." Hence, they conclude their paper by stating it is likely that "an atmospheric CO2-driven growth-enhancement effect exists for ponderosa pine growing under specific natural conditions within the [USA's] interior Pacific Northwest," providing yet another important, specific and real-world example of the ongoing CO2-induced "greening of the earth."

Contemporaneously, Wang et al. (2006) conducted a similar type of study, wherein they sought to determine how the historical increase in atmospheric CO2 concentration had impacted the growth of trees in the real world, i.e., in non-contrived experimental settings. More specifically, they examined ring-width development in cohorts of young and old white spruce (Picea glauca) trees in a mixed grass-prairie ecosystem in southwestern Manitoba, Canada, where a 1997 wildfire killed most of the older trees growing in high-density spruce islands, but where younger trees slightly removed from the islands escaped the ravages of the flames. "Within each of a total of 24 burned islands," in the words of the three researchers, "the largest dominant tree (dead) was cut down and a disc was then sampled from the stump height," while "adjacent to each sampled island, a smaller, younger tree (live) was also cut down, and a disc was sampled from the stump height."

After removing size-, age- and climate-related trends in radial growth from the ring-width histories of the trees, Wang et al. plotted the residuals as functions of time for the 30-year periods for which both the old and young trees would have been approximately the same age: 1900-1929 for the old trees and 1970-1999 for the young trees. During the first of these periods, the atmosphere's CO2 concentration averaged 299 ppm, while during the second it averaged 346 ppm. Also, the mean rate-of-rise of the atmosphere's CO2 concentration was 0.37 ppm/year for first period and 1.43 ppm/year for the second.

The results of this exercise revealed that in comparison to the 1900-1929 period, the slope of the linear regression describing the rate-of-growth of the ring-width residuals for the 1970-1999 period (when the air's CO2 concentration was 15% greater and its rate-of-rise was 285% greater) was more than twice that of the linear regression describing the rate-of-growth of the ring-width residuals for the 1900-1929 period. And as the researchers describe it, these results show that "at the same developmental stage, a greater growth response occurred in the late period when atmospheric CO2 concentration and the rate of atmospheric CO2 increase were both relatively high," and they say that "these results are consistent with expectations for CO2-fertilization effects." In fact, they say that "the response of the studied young trees can be taken as strong circumstantial evidence for the atmospheric CO2-fertilization effect."

Another thing that Wang et al. learned was that "postdrought growth response was much stronger for young trees (1970-1999) compared with old trees at the same development stage (1900-1929)." And they add that "higher atmospheric CO2 concentration in the period from 1970-1999 may have helped white spruce recover from severe drought." In a similar vein, they also determined that young trees showed a weaker relationship to precipitation than did old trees, noting that "more CO2 would lead to greater water-use efficiency, which may be dampening the precipitation signal in young trees." Consequently, Wang et al.'s unique study provides an exciting real-world example of the tremendous benefits that the historical rise in the air's CO2 content has likely conferred on earth's long-lived woody species.

Hoping to shed still more light on the subject, Davi et al. (2006) used a meteorological model following "a moderate CO2 emission scenario" (B2 of the IPCC) to calculate a 1960-2100 average temperature increase of 3.1°C and a mean summer rainfall decrease of 27%, which they used as input to a physiologically-based multi-layer process-based ecosystem productivity model (which contained a carbon allocation sub-model coupled with a soil model) to evaluate net productivity changes of six French forest ecosystems representative of oceanic, continental and Mediterranean climates that are dominated, respectively, by deciduous species (Fagus sylvatica, Quercus robur), coniferous species (Pinus pinaster, Pinus sylvestris) and sclerophyllous evergreen species (Quercus ilex), which ecosystems, in their words, "are representative of a significant proportion of forests in western Europe."

"By comparing runs with and without CO2 effects," according to the researchers, they found that "CO2 fertilization is responsible from 1960 to 2100 for an NEP [net ecosystem productivity] enhancement of about 427 g(C) on average for all sites (= 3.05 g(C) m-2 year-1),"and they note that "the CO2 fertilization effect" actually turns a warming- and drying-induced "decrease of NEP into an increase." In addition, they report that "no saturation of this effect on NEP is found because the differences between the simulations with and without CO2 fertilization continuously increase with time." Consequently, even in the face of what truly would be an "unprecedented" global warming and drying scenario, the real-world physiological effects of atmospheric CO2 enrichment that are included in the ecosystem productivity model employed by Davi et al. are able to more than compensate for the deleterious effects of the dramatic climate-change scenario on the productivity of major European forests.

In yet another important contribution from 2006 (it was a very good year for forest-CO2 studies), Pardos et al. (2006) grew seedlings of cork oak (Quercus suber L., described by them as "a typical Mediterranean species") that they germinated from acorns collected from trees near Toledo, Spain, and maintained for five months - one per each 3-L pot filled with a mixture of fine sand and peat maintained at either high (83%) or low (32-34%) growing medium moisture - under either high (600 Ámol m-2 s-1) or low (60 Ámol m-2 s-1) light intensity in growth chambers maintained at either ambient (360 ppm) or elevated (700 ppm) atmospheric CO2 concentrations. This work revealed, as the four Spanish researchers describe it, that "elevated CO2 caused the cork oak seedlings to improve their performance in dry and high light environments to a greater extent than under well-irrigated and low-light conditions, thus ameliorating the effects of soil water stress and high light loads on growth." Consequently, and because they believe these latter two stressful conditions are what "global change is likely to produce in the Mediterranean basin in the next decades," it can be appreciated that the ongoing rise in the air's CO2 concentration should help the cork oak species to successfully deal with those stresses, if and when they actually do occur.

One year later, Saleska et al. (2007) began their Science Express Brevia report by noting that "large-scale numerical models that simulate the interactions between changing global climate and terrestrial vegetation predict substantial carbon loss from tropical ecosystems, including the drought-induced collapse of the Amazon forest and conversion to savanna." Not being totally convinced of the likelihood of such a consequence, however, they used Terra satellite data - Enhanced Vegetation Index (EVI) derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) - to determine whether or not the widespread Amazon drought of 2005, which peaked during the dry season onset (July-September), did indeed reduce whole-canopy forest photosynthesis as predicted, which they say "should have been especially observable during this period, when anomalous interannual drought coincided with the already seasonally low precipitation."

Quite to the contrary of model predictions, however, the four researchers found that observations of intact forest "greenness" in the region were, in their words, "dominated by a significant increase (P<0.0001), not a decline." One phenomenon they offer to potentially explain this seemingly strange scenario is that the trees of the Amazon forest may be utilizing deep roots to "access and sustain" water availability during drought. Another possibility is that the historical increase in the air's CO2 content has significantly enhanced the trees' water use efficiency, enabling them to produce considerably more biomass per unit of water transpired and thereby conserve water. And, of course, yet another possibility is the phenomenon described in the set of four papers reviewed above that were published in 2006.

Moving on, Huang et al. (2007) compared, synthesized and evaluated the scientific literature describing (1) atmospheric CO2 enrichment experiments conducted on trees and (2) empirical tree-ring studies designed to determine if the growth-promoting effects of rising atmospheric CO2 concentrations occur in natural forests. This effort revealed, as they describe it, that numerous CO2-enrichment experiments have "demonstrated significantly positive physiological and growth responses of trees to CO2, providing strong evidence to support the direct CO2 fertilization effect (increased photosynthesis, water use efficiency, above- and below-ground growth) and thus allowing prediction of which ecosystems might be most responsive to CO2," with their thoughts in this regard being "warm, moderately drought-stressed ecosystems with an ample nitrogen supply," because, as they continue, "drought-stressed trees could benefit from increased water use efficiency to enhance growth." They note, however, that tree-ring studies on the cold and arid Tibetan Plateau also "showed significant growth enhancements as well as increased water use efficiency (24.7% and 33.6% for each species, respectively) in Qilian juniper and Qinghai spruce since the 1850s," citing in support of this statement the studies of Zhang et al. (2003), Shao et al. (2005), Liang et al. (2006), Huang and Zhang (2007) and Zhang and Qiu (2007).

Taking another leap forward in time, Wyckoff and Bowers (2010) set the stage for their exploration of the issue at hand by stating 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 say 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). And thus it came to be that working with bur oak (Quercus macrocarpa) trees, they explored the likelihood of this scenario occurring 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. macrocarpa 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.

Proceeding as they had planned, 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]." In addition, they discovered that "for Q. macrocarpa, declining sensitivity of growth to drought translates into lower predicted mortality rates at all sites," and they note that at one such 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)." So Wyckoff and Bowers thus 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 huge over-estimation of warming predicted by most climate models for the remainder of the 21st century.

One year later, and noting that climate models "consistently project significant increases in temperature and decreases in precipitation in the Mediterranean basin," Keenan et al. (2011) noted that these changes may have a large impact on current Mediterranean forests and the related ecosystem services they provide. In addition, they state that niche-based models - also known as bioclimatic envelope models or habitat models - are by far the most commonly used method for predicting potential species distribution responses to future climatic changes. And these models typically predict significant negative consequences for terrestrial plants and animals in the face of continued increases in atmospheric CO2 concentrations.

Keenan et al., however, prefer process-based models, which describe eco-physiological processes ranging from purely empirical relationships to mechanistic descriptions based on physical laws. And they write that these models - supported by experiments and growth and yield surveys - "suggest that global warming will have a positive impact on forest productivity (van der Meer et al., 2002; Nigh et al., 2004; Norby and Luo, 2004; Brice˝o-Elizondo et al., 2006; Gaucharel et al., 2008), due to the direct fertilization effect of increased CO2 and indirect effects such as lengthening of the growing period."

To demonstrate the difference in results obtained by employing these two approaches to divining the future, the five researchers assessed and compared the projections of each of them when they were applied to stands of three common forest species (Quercus ilex, Pinus halepensis and Pinus sylvestris) with widely contrasting distributions in continental Spain; and this procedure revealed, as they describe it, that "CO2 fertilization through projected increased atmospheric CO2 concentrations is shown to increase forest productivity in the mechanistic process-based model (despite increased drought stress) by up to three times that of the non-CO2 fertilization scenario by the period 2050-2080, which is in stark contrast to projections of reduced habitat suitability from the niche-based models by the same period."

And so it was that the Spanish and U.S. scientists wrote that their results show that "previous reports of species decline in continental Spain (e.g. Benito-Garzon et al., 2008) may be overestimated due to two reasons: the use of only one predictive niche-based model, and the failure to account for possible effects of CO2 fertilization." And they add that "similar studies in other regions, which do not consider these two aspects, are also potentially overestimating species decline due to climate change." In addition, they suggest that "niche-based model results also likely overestimate the decline in [habitat] suitability," and they therefore conclude that "an organism's niche must be modeled mechanistically if we are to fully explain distribution limits," additionally citing the work of Kearney (2006) in this regard.

Also publishing in the same year were Osorio et al. (2011), who wrote that "water deficits and high temperature are major abiotic stress factors restricting plant growth and productivity in many regions," and who said that "the impact of climate change on temperature and rainfall patterns is of great importance in determining the future response of tree crops to new environmental conditions," which is what they proceeded to do for the Carob or St. John's tree (Ceratonia siliqua) that grows in the Mediterranean area, where they state that water stress will be the most important factor limiting plant growth throughout the remainder of this century. And thus it was that the impacts of drought and high-temperature stresses on photosynthesis, energy partitioning and membrane lipids, as well as the potential ability of Carob trees to attenuate oxidative damage, were investigated by them in young seedlings growing within controlled-environment chambers, where they were rooted in 3-dm3 pots filled with a 2:1 mixture of a fertilized substrate and natural soil, and where they were maintained under two different thermal regimes - low and high temperature (LT: 25/18°C; HT: 32/21°C) - and three soil water conditions (control, water stress and rewetting), under which sets of conditions numerous physiological and biochemical plant properties and processes were repeatedly monitored.

And what did they find? Among a number of other pertinent observations, Osorio et al. report that the decrease in net photosynthesis (PN) caused by drought was 33% in the LT chamber and 84% in the HT chamber. However, they found that "the negative effects of soil drying on PN and stomatal conductance of HT plants were no longer detected 36 hours following rewatering." And the five Portuguese scientists thus add that "although C. siliqua seedlings exhibit clear signs of oxidative stress under drought and high temperature, they retain a remarkable ability to quickly restore normal physiological activity upon rehydration, which let us believe that they can satisfactorily deal with predicted climate warming and increased soil drying in the Mediterranean area."

Rounding out the brief synopses of papers contained in this topical summary is the study of Soule and Knapp (2011), who write that "in 2008, atmospheric CO2 concentrations from the Mauna Loa, Hawaii, Observatory records exceeded 385 ppm, representing a 22% increase since 1959," and they say 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 representing this phenomenon is referred to as intrinsic water-use efficiency or iWUE (defined as the ratio of net CO2 assimilation to stomatal conductance; and it has been documented, as they describe it, "for various tree species in many parts of the world," citing in support of this statement the findings of Bert et al. (1997), Feng (1999), Tang et al. (1999), Arneth et al. (2002), Saurer et al. (2004), Waterhouse et al. (2004) and Liu et al. (2007). And thus they too examined changes in - and relationships between - 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.

As a result of this massive undertaking, Soule and Knapp report finding significant upward trends in iWUE at all sites; and they say that "despite an absence of climate changes that would favor growth," upward radial growth trends occurred at five sites. In addition, they say 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 additionally remark 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 further discussing their work and its significance, the two researchers say their findings suggest 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 conclude 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."

In analyzing the findings of the papers discussed in this section, the earlier conclusions of Idso and Idso (1994) are found to be well supported by the recent peer-reviewed scientific literature, which indicates that the ongoing rise in the air's CO2 content will likely lead to substantial increases in photosynthetic rates and biomass production in earth's many woody species in the years and decades ahead, even in the face of stressful conditions imposed by less-than-optimal soil moisture availability.

Ainsworth, E.A. and Long, S.P. 2005. What have we learned from 15 years of free-air CO2 enrichment (FACE)? A meta-analytic review of the responses of photosynthesis, canopy properties and plant production to rising CO2. New Phytologist 165: 351-372.

Anderson, P.D. and Tomlinson, P.T. 1998. Ontogeny affects response of northern red oak seedlings to elevated CO2 and water stress. I. Carbon assimilation and biomass production. New Phytologist 140: 477-491.

Arneth, A., Lloyd, J., Santruckova, H., Bird, M., Girgoryev, S., Kalaschnikov, Y.N., Gleixner, G. and Schulze, E. 2002. Response of central Siberian Scots pine to soil water deficit and long-term trends in atmospheric CO2 concentration. Global Biogeochemical Cycles 16: 10.1029/2000GB001374.

Arp, W.J., Van Mierlo, J.E.M., Berendse, F. and Snijders, W. 1998. Interactions between elevated CO2 concentration, nitrogen and water: effects on growth and water use of six perennial plant species. Plant, Cell and Environment 21: 1-11.

Benito-Garzon, M., Sanchez de Dios, R. and Sainz Ollero, H. 2008. Effects of climate change on the distribution of Iberian tree species. Applied Vegetation Science 11: 169-178.

Bert, D., Leavitt, S. and Dupouey, J.-L. 1997. Variations of wood δ13C and water-use efficiency of Abies alba during the last century. Ecology 78: 1588-1596.

Brice˝o-Elizondo, R., Garcia-Gonzalo, J., Peltola, H., Matala, J. and Kellomaki, S. 2006. Sensitivity of growth of Scots pine, Norway spruce and silver birch to climate change and forest management in boreal conditions. Forest Ecology and Management 232: 152-167.

Catovsky, S. and Bazzaz, F.A. 1999. Elevated CO2 influences the responses of two birch species to soil moisture: implications for forest community structure. Global Change Biology 5: 507-518.

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Last updated 30 April 2014