When the air's CO2 content is increased, most plants exhibit increased rates of net photosynthesis and biomass production. Moreover, on a per-unit-leaf-area basis, they often lose less water via transpiration, as they tend to display lower stomatal conductances. Hence, the amount of carbon gained per unit of water lost per unit leaf area - or water-use efficiency - should increase significantly as the air's CO2 content rises. In this review, we summarize some recent experimental results pertaining to this phenomenon in trees.
The effect of elevated atmospheric CO2 concentrations on the water-use efficiencies of trees is clearly positive, having been documented in a number of different single-species studies of longleaf pine (Runion et al., 1999), red oak (Anderson and Tomlinson, 1998), scrub oak (Lodge et al., 2001), silver birch (Rey and Jarvis, 1998), beech (Bucher-Wallin et al., 2000; Egli et al., 1998), sweetgum (Gunderson et al., 2002; Wullschleger and Norby, 2001) and spruce (Roberntz and Stockfors, 1998). Likewise, in a multi-species study performed by Tjoelker et al. (1998), seedlings of quaking aspen, paper birch, tamarack, black spruce and jack pine, which were grown at 580 ppm CO2 for three months, displayed water-use efficiencies that were 40 to 80% larger than those exhibited by their respective controls grown at 370 ppm CO2.
Similar results are also obtained when trees are 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.
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. In addition, two species of oaks 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" (Blaschke et al., 2001; 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). In this case, the authors suggested that the greater atmospheric CO2 concentrations of the latter period allowed the trees to more quickly recover from water stress. Finally, 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.
On another note, Prince et al. (1998) demonstrated that rain-use efficiency, which is similar to water-use efficiency, slowly increased in the African Sahel from 1982 to 1990, while Nicholson et al. (1998) observed neither an increase nor a decrease in this parameter from 1980 to 1995 for the central and western Sahel.
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. It is thus 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 atmospheric CO2 concentration continues to rise.
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.
Bartak, M., Raschi, A. and Tognetti, R. 1999. Photosynthetic characteristics of sun and shade leaves in the canopy of Arbutus unedo L. trees exposed to in situ long-term elevated CO2. Photosynthetica 37: 1-16.
Beerling, D.J., McElwain, J.C. and Osborne, C.P. 1998. Stomatal responses of the 'living fossil' Ginkgo biloba L. to changes in atmospheric CO2 concentrations. Journal of Experimental Botany 49: 1603-1607.
Blaschke, L., Schulte, M., Raschi, A., Slee, N., Rennenberg, H. and Polle, A. 2001. Photosynthesis, soluble and structural carbon compounds in two Mediterranean oak species (Quercus pubescens and Q. ilex) after lifetime growth at naturally elevated CO2 concentrations. Plant Biology 3: 288-298.
Bucher-Wallin, I.K., Sonnleitner, M.A., Egli, P., Gunthardt-Goerg, M.S., Tarjan, D., Schulin, R. and Bucher, J.B. 2000. Effects of elevated CO2, increased nitrogen deposition and soil on evapotranspiration and water use efficiency of spruce-beech model ecosystems. Phyton 40: 49-60.
Centritto, M., Lee, H.S.J. and Jarvis, P.G. 1999. 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.
Duquesnay, A., Breda, N., Stievenard, M. and Dupouey, J.L. 1998. Changes of tree-ring d13C and water-use efficiency of beech (Fagus sylvatica L.) in north-eastern France during the past century. Plant, Cell and Environment 21: 565-572.
Egli, P., Maurer, S., Gunthardt-Goerg, M.S. and Korner, C. 1998. Effects of elevated CO2 and soil quality on leaf gas exchange and aboveground growth in beech-spruce model ecosystems. New Phytologist 140: 185-196.
Feng, X. 1999. Trends in intrinsic water-use efficiency of natural trees for the past 100-200 years: A response to atmospheric CO2 concentration. Geochimica et Cosmochimica Acta 63: 1891-1903.
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.
Gunderson, C.A., Sholtis, J.D., Wullschleger, S.D., Tissue, D.T., Hanson, P.J. and Norby, R.J. 2002. Environmental and stomatal control of photosynthetic enhancement in the canopy of a sweetgum (Liquidambar styraciflua L.) plantation during 3 years of CO2 enrichment. Plant, Cell and Environment 25: 379-393.
Knapp, P.A., Soule, P.T. and Grissino-Mayer, H.D. 2001. Post-drought growth responses of western juniper (Junipers occidentalis var. occidentalis) in central Oregon. Geophysical Research Letters 28: 2657-2660.
Lodge, R.J., Dijkstra, P., Drake, B.G. and Morison, J.I.L. 2001. Stomatal acclimation to increased CO2 concentration in a Florida scrub oak species Quercus myrtifolia Willd. Plant, Cell and Environment 24: 77-88.
Nicholson, S.E., Tucker, C.J. and Ba, M.B. 1998. Desertification, drought, and surface vegetation: An example from the West African Sahel. Bulletin of the American Meteorological Society 79: 815-829.
Prince, S.D., Brown De Colstoun, E. and Kravitz, L.L. 1998. Evidence from rain-use efficiencies does not indicate extensive Sahelian desertification. Global Change Biology 4: 359-374.
Rey, A. and Jarvis, P.G. 1998. Long-Term photosynthetic acclimation to increased atmospheric CO2 concentration in young birch (Betula pendula) trees. Tree Physiology 18: 441-450.
Roberntz, P. and Stockfors, J. 1998. Effects of elevated CO2 concentration and nutrition on net photosynthesis, stomatal conductance and needle respiration of field-grown Norway spruce trees. Tree Physiology 18: 233-241.
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.
Tjoelker, M.G., Oleksyn, J. and Reich, P.B. 1998. Seedlings of five boreal tree species differ in acclimation of net photosynthesis to elevated CO2 and temperature. Tree Physiology 18: 715-726.
Tognetti, R., Johnson, J.D., Michelozzi, M. and Raschi, A. 1998. Response of foliar metabolism in mature trees of Quercus pubescens and Quercus ilex to long-term elevated CO2. Environmental and Experimental Botany 39: 233-245.
Wayne, P.M., Reekie, E.G. and Bazzaz, F.A. 1998. Elevated CO2 ameliorates birch response to high temperature and frost stress: implications for modeling climate-induced geographic range shifts. Oecologia 114: 335-342.
Wullschleger, S.D. and Norby, R.J. 2001. Sap velocity and canopy transpiration in a sweetgum stand exposed to free-air CO2 enrichment (FACE). New Phytologist 150: 489-498.