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 claim that water stress will negate these benefits of atmospheric CO2 enrichment. In reviewing the scientific literature of the ten-year period 1983-1994, however, Idso and Idso (1994) concluded that water stress will not negate the CO2-induced stimulation of plant productivity. 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. We here review some of the subsequent relevant literature in this area to determine if these conclusions are still valid, particular for earth's woody species.
During times of water stress, atmospheric CO2 enrichment often stimulates the development of larger-than-usual and more robust root systems in woody perennial species, which allows them to probe greater volumes of soil for scarce and much-needed moisture. Tomlinson and Anderson (1998), for example, report 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 ambient air containing 400 ppm CO2. In addition, Polley et al. (1999) notethat water-stressed honey mesquite trees subjected to an atmospheric CO2 concentration of 700 ppm produced 37% more root biomass than water-stressed control seedlings growing at 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. Tognetti et al. (1998), for example, determined that 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.
CO2-induced increases in root development together with 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, 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. Similarly, 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 (Polley et al., 1999), which is comparable to values of -5.9 and -3.4 MPa observed in water-stressed evergreen shrubs (Larrea tridentata) exposed to 360 and 700 ppm CO2, respectively (Hamerlynck et al., 2000).
If atmospheric CO2 enrichment thus allows plants to maintain better water status during times of water stress, it is only logical to expect that plants will exhibit CO2-induced increases in photosynthesis, even under conditions of low soil moisture availability. It is not surprising, therefore, that 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) observed 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 as in well-watered ones.
Sometimes, plants suffer drastically when subjected to extreme water stress. However, the addition of CO2 to the atmosphere often gives them an edge over ambiently-growing plants. 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. Thus, 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 plant 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%).
Finally, 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. In a loosely related study, Osborne et al. (2002) 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.
In summary, the conclusions of Idso and Idso (1994) are 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 woody species in the years and decades ahead, even in the face of stressful conditions imposed by less-than-optimal availability of soil moisture.
References
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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.
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.
Centritto, M., Magnani, F., Lee, H.S.J. and Jarvis, P.G. 1999a. Interactive effects of elevated [CO2] and drought on cherry (Prunus avium) seedlings. II. Photosynthetic capacity and water relations. New Phytologist 141: 141-153.
Centritto, M., Lee, H.S.J. and Jarvis, P.G. 1999b. Interactive effects of elevated [CO2] and drought on cherry (Prunus avium) seedlings. I. Growth, whole-plant water use efficiency and water loss. New Phytologist 141: 129-140.
Fernandez, M.D., Pieters, A., Azuke, M., Rengifo, E., Tezara, W., Woodward, F.I. and Herrera, A. 1999. Photosynthesis in plants of four tropical species growing under elevated CO2. Photosynthetica 37: 587-599.
Fernandez, M.D., Pieters, A., Donoso, C., Tezara, W., Azuke, M., Herrera, C., Rengifo, E. and Herrera, A. 1998. Effects of a natural source of very high CO2 concentration on the leaf gas exchange, xylem water potential and stomatal characteristics of plants of Spatiphylum cannifolium and Bauhinia multinervia. New Phytologist 138: 689-697.
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Polley, H.W., Tischler, C.R., Johnson, H.B. and Derner, J.D. 2002. Growth rate and survivorship of drought: CO2 effects on the presumed tradeoff in seedlings of five woody legumes. Tree Physiology 22: 383-391.
Polley, H.W., Tischler, C.R., Johnson, H.B. and Pennington, R.E. 1999. Growth, water relations, and survival of drought-exposed seedlings from six maternal families of honey mesquite (Prosopis glandulosa): responses to CO2 enrichment. Tree Physiology 19: 359-366.
Runion, G.B., Mitchell, R.J., Green, T.H., Prior, S.A., Rogers, H.H. and Gjerstad, D.H. 1999. Longleaf pine photosynthetic response to soil resource availability and elevated atmospheric carbon dioxide. Journal of Environmental Quality 28: 880-887.
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Tomlinson, P.T. and Anderson, P.D. 1998. Ontogeny affects response of northern red oak seedlings to elevated CO2 and water stress. II. Recent photosynthate distribution and growth. New Phytologist 140: 493-504.
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