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Transpiration - Summary
Most plants respond to increases in the air's CO2 content by displaying reduced stomatal conductances, which typically leads to reduced rates of transpirational water loss.  This water savings often results in greater soil moisture contents in CO2-enriched ecosystems, which positively feeds back to increase plant growth.  In this summary, we review a few papers that treat various aspects of this phenomenon.

In a review of studies conducted over the prior decade, Pospisilova and Catsky (1999) compiled over 150 individual plant water use responses to atmospheric CO2 enrichment.  They found that elevated CO2 increased rates of net photosynthesis in about 85% of the reported studies, while reducing stomatal conductances and rates of transpiration in approximately 75% of the cases analyzed.  Consequently, atmospheric CO2 enrichment increased plant water-use efficiency in more than 90% of the experiments that were conducted; and it reduced total water uptake in more than 50% of the studies, while slowing the development of water stress as indicated by plant water potential data.  As a result Pospisilova and Catsky concluded that plants growing in future atmospheres of higher CO2 concentration "will probably survive eventual higher drought stress and some species may even be able to extend their biotope into less favourable sites."

In a subsequent experiment, Wullschleger et al. (2002) studied sweetgum trees growing in FACE plots maintained at an atmospheric CO2 concentration of 540 ppm and found them to display a 14% reduction (relative to trees growing in ambient air) in seasonal stomatal conductance at the canopy level, which significantly reduced their rates of transpiration during the growing season.  In fact, Wullschleger and Norby (2001) quantitatively demonstrated that elevated CO2 reduced season-long stand transpiration by approximately 10%.

Apple et al. (2000) studied Douglas fir seedlings (Pseudotsuga menziesii Mirb. Franco) that were grown for three years in environmental chambers maintained at atmospheric CO2 concentrations of 350 or 530 ppm and ambient or elevated (ambient plus 3.5C) air temperatures to study the impacts of these treatments on needle stomatal function.  They found that the extra CO2, when applied alone, reduced stomatal conductance and transpiration by 8 and 12%, respectively; but that the elevated air temperature alone increased these parameters by 100 and 66%, respectively.  Together, however, their combined influence produced no significant change in either parameter.

In a subsequent study of the same seedlings a year later, Lewis et al. (2002) determined that the elevated CO2, applied by itself, reduced both stomatal conductance and needle transpiration by 12%.  In contrast, the elevated air temperature alone increased these parameters by 17 and 37%, respectively.  In combination, however, exposure to elevated temperature and CO2 had no significant impact on stomatal conductance and increased transpiration rate by only half as much (19%) as it increased it in the presence of elevated temperature alone (37%).  Hence, since most climate models would suggest that a 3.5C increase in air temperature implies a nominal doubling of the air's CO2 concentration, or twice the amount of CO2 enrichment provided in this experiment (360 as opposed to 180 ppm), the temperature-alone-induced 37% increase in needle transpiration could be expected to be reduced all the way to zero via the concomitant impact of the doubled CO2 concentration, since a 180-ppm increase in CO2 reduced the temperature-alone-induced 37% increase in transpiration to only 19% in this study.

Working with another conifer, Kellomaki and Wang (1998) found that mature Scots pines growing at twice-ambient atmospheric CO2 concentrations displayed a 14% reduction in cumulative sap flow, which also suggests significant CO2-induced reductions in transpirational water loss.  And in a study of another woody species, Dugas et al. (2001) observed that Acacia plants grown for one year in air of 385 ppm CO2 reduced their rates of transpiration by about one-half upon transfer to air containing 980 ppm CO2.  In addition, plants grown for a year at 980 ppm CO2 and measured at that same concentration exhibited transpiration rates that were only one-fourth of those of control plants grown and measured at 385 ppm CO2.

Shifting to grasses, Niklaus et al. (2003) enriched the air above plots of a nutrient-poor species-rich calcareous grassland in northwestern Switzerland for six full years with an extra 240 ppm of CO2 via a set of novel windscreens that "operated around the clock," except during mid-winter (December-February).  Among a number of other findings, they report there was an increase in soil moisture due to CO2-induced reductions in plant transpiration that may have helped to increase the above-ground plant biomass of the CO2-enriched plots by the average observed value of 21%.

Last of all, Dong-Xiu et al. (2002) grew spring wheat (Triticum aestivum L. cv. Gaoyuan 602) in open-top chambers maintained at atmospheric CO2 concentrations of 350 and 700 ppm and three levels of soil moisture (40, 60 and 80% of field capacity) to study the interactive effects of these environmental variables on productivity and growth in this important agricultural crop.  Elevated CO2 increased rates of net photosynthesis by 48, 120 and 97% at low, medium and high soil water capacities, respectively, while it reduced rates of transpiration by 56, 53 and 63% in the same order.  Consequently, these physiological changes lead to CO2-induced increases in plant water-use efficiency of approximately 25, 15 and 30% under low, medium and high soil moisture conditions, respectively.

These several results, as well as those obtained from many other studies, suggest that as the air's CO2 content continues to rise, earth's plants will likely display reductions in stomatal conductance, which should reduce their rates of transpirational water loss.  As a result, most plants should be able to better deal with periodic water shortages and warmer temperatures, possibly even expanding their ranges into areas where it was too dry for them to successfully live and reproduce in the recent past.

References
Apple, M.E., Olszyk, D.M., Ormrod, D.P., Lewis, J., Southworth, D. and Tingey, D.T.  2000.  Morphology and stomatal function of Douglas fir needles exposed to climate change: elevated CO2 and temperature.  International Journal of Plant Science 161: 127-132.

Dong-Xiu, W., Gen-Xuan, W., Yong-Fei, B., Jian-Xiong, L. and Hong-Xu, R.  2002.  Response of growth and water use efficiency of spring wheat to whole season CO2 enrichment and drought.  Acta Botanica Sinica 44: 1477-1483.

Dugas, W.A., Polley, H.W., Mayeux, H.S. and Johnson, H.B.  2001.  Acclimation of whole-plant Acacia farnesiana transpiration to carbon dioxide concentration.  Tree Physiology 21: 771-773.

Kellomaki, S. and Wang, K.-Y.  1998.  Sap flow in Scots pines growing under conditions of year-round carbon dioxide enrichment and temperature elevation.  Plant, Cell and Environment 21: 969-981.

Lewis, J.D., Lucash, M., Olszyk, D.M. and Tingey, D.T.  2002.  Stomatal responses of Douglas-fir seedlings to elevated carbon dioxide and temperature during the third and fourth years of exposure.  Plant, Cell and Environment 25: 1411-1421.

Niklaus, P.A., Alphei, J., Ebersberger, D., Kampichlers, C., Kandeler, E. and Tscherko, D.  2003.  Six years of in situ CO2 enrichment evoke changes in soil structure and soil biota of nutrient-poor grassland.  Global Change Biology 9: 585-600.

Pospisilova, J. and Catsky, J.  1999.  Development of water stress under increased atmospheric CO2 concentration.  Biologia Plantarum 42: 1-24.

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.

Wullschleger, S.D., Gunderson, C.A., Hanson, P.J., Wilson. K.B. and Norby, R.J.  2002.  Sensitivity of stomatal and canopy conductance to elevated CO2 concentration - interacting variables and perspectives of scale.  New Phytologist 153: 485-496.