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Transpiration (Woody Plants - Deciduous) - Summary
How do earth's deciduous trees respond to rising levels of atmospheric carbon dioxide? As people ponder the many potential consequences of the historical and ongoing rise in the air's CO2 content, this question is of more than academic interest, as it deals with one of an important group of phenomena that will ultimately determine what will be left of wild nature for future generations.  Hence, we here summarize what has been learned about the subject in several studies that have explored it experimentally.

Wayne et al. (1998) grew yellow birch seedlings in controlled environment chambers maintained at atmospheric CO2 concentrations of either 400 or 800 ppm and day/night air temperatures of either 26/21 or 31/26°C for two months.  They found that the elevated CO2 treatment stimulated net photosynthesis by 48% at both temperatures, while it increased seedling biomass by 60% and 227% at normal and high air temperatures, respectively.  In addition, because the extra CO2 reduced transpiration by 25 and 36% at the normal and high air temperatures, plant water-use efficiency rose by 52 and 94% in these two situations.  This reduction in transpiration and increase in water-use efficiency may allow yellow birch to expand into regions where high summer temperatures and limited rainfall currently discourage its presence.

In a FACE study established within a ten-year-old stand of sweetgum trees growing on a nutrient-rich soil in Tennessee, USA, Wullschleger and Norby (2001) measured CO2-induced sap-flow reductions that averaged 13% over the growing season.  The elevated CO2 (540 vs. 390 ppm) also reduced growing-season transpiration rates by about the same amount, leading to a 28% increase in stand-level water-use efficiency in the CO2-enriched trees.  Likewise, in a second year of measurements, Wullschleger et al. (2002) found that the CO2-enriched air reduced the stomatal conductances of individual leaves by an average of 23% across the growing season.  When extrapolated to the entire canopy, however, the reduction fell to 14%, and there was a 7% reduction in stand evapotranspiration.

Tognetti et al. (1998) studied the effects of naturally occurring elevated CO2 concentrations (500 to 1000 ppm) near a CO2-emitting spring in central Italy on summer water relations of mature oak trees by measuring leaf stomatal conductances and trunk sap flow rates (a measure of transpiration) over a period of two years and comparing the results with those obtained from similar-age oaks (15 to 25 years old) growing in air of ambient CO2 concentration at a site some three kilometers distant.  As both summers were characterized by severe drought, rates of water loss were relatively high in both sets of trees.  Leaf stomatal conductances, however, were significantly lower in the trees growing near the CO2 springs, as were trunk sap velocities.  These findings suggest that the trees in the vicinity of the CO2-emitting springs experienced less water loss and maintained a more favorable internal water status than the trees growing in non-CO2-enriched ambient air.

In an even more intriguing development, Tognetti et al. determined that the trees growing near the CO2-emitting springs possessed less foliage area than the control trees; and this reduction in transpirational surface area also allowed the CO2-enriched trees to maintain a better internal water status than the control trees during periods of drought.  In fact, the researchers say the reduction in foliage area was "equally, if not more, effective than stomatal closure in reducing transpiration and plant water use under elevated CO2."  The work of these scientists thus suggests that if drought situations continue to recur during future Italian summers, the rising CO2 content of the atmosphere should provide oak, and perhaps other trees, with at least two different mechanisms for sustaining their growth during protracted periods of reduced water availability.

Last of all, in a more broad-ranging study, Cech et al. (2003) enriched the air's CO2 concentration within the canopy of a 30-meter-tall species-rich forest just south of Basel, Switzerland -- via the web-FACE technique of Pepin and Korner (2002) -- to a mean value of 520 ppm for an entire growing season to test, in their words, "whether elevated CO2 reduces water use in mature forest trees," making sap flow measurements of 14 broadleaved trees (3 Fagus, 4 Quercus, 4 Carpinus, 1 Tilia, 1 Acer and 1 Prunus) and their ambient-treatment counterparts via the constant heat-flow technique of Granier (1985, 1987) from 5 June to 1 October 2001.  Over the entire growing season, the extra 150 ppm of CO2 reduced mean daily sap flow across all species by an average of 10.7%.  The reductions were high (22%) when the evaporative demand of the air was low (mean daily vapor pressure deficits less than 5 hPa) and small (2%) when the evaporative demand of the air was high (mean daily vapor pressure deficits greater than 10 hPa).  The researchers say their results "suggest that daily water savings by CO2-enriched trees may have accumulated to [produce] a significantly improved water status by the time when control trees were short of soil moisture," so that "CO2-enriched trees would enter drier periods with a higher soil moisture capital, permitting prolonged gas exchange (for a few days)."

In light of these several experimental observations, it would appear that most deciduous trees likely exhibit modest reductions in transpirational water loss in CO2-enriched air.  Consequently, if the planet continues to warm and experience enhanced dryness in some regions, we had better hope that the air's CO2 content continues to rise as well, so that earth's deciduous trees may more effectively deal with the increased water stress they would otherwise experience in these places.

References
Cech, P.G., Pepin, S. and Korner, C.  2003.  Elevated CO2 reduces sap flux in mature deciduous forest trees.  Oecologia 137: 258-268.

Granier, A.  1985.  A new method of sap flow measurement in tree stems.  Annales Des Sciences Forestieres 42: 193-200.

Granier, A.  1987.  Evaluation of transpiration in a Douglas fir stand by means of sap flow measurements.  Tree Physiology 3: 309-320.

Pepin, S. and Korner, C.  2002.  web-FACE: a new canopy free-air CO2 enrichment system for tall trees in mature forests.  Oecologia 133: 1-9.

Tognetti, R., Longobucco, A., Miglietta, F. and Raschi, A.  1998.  Transpiration and stomatal behaviour of Quercus ilex plants during the summer in a Mediterranean carbon dioxide spring.  Plant, Cell and Environment 21: 613-622.

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.

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.

Last updated 29 June 2005