Apple et al. (2000) grew two-year-old Douglas fir seedlings for three additional years in controlled environment chambers maintained at atmospheric CO2 concentrations of either 350 or 550 ppm and ambient or elevated (ambient plus 4°C) air temperature.  They found that the 200-ppm increase in the air's CO2 concentration and the 4°C increase in air temperature, when applied together, did not significantly affect seedling transpiration.  When applied alone, however, the extra CO2 reduced transpiration by 12%, while the elevated air temperature increased it by 66%.

After a one-year continuation of the same experiment, Lewis et al. (2002) reported results obtained over its final 21 months.  Once again, the extra CO2 decreased transpiration by 12%; but the elevated air temperature increased it by 37%, which was considerably less than the 66% increase reported by Apple et al.  Adding more confusion to the story, the combination of the identical CO2 effect and a weaker temperature effect yielded a 19% increase in transpiration, whereas in the first three years of the study the identical CO2 effect and a stronger temperature effect produced no change in transpiration.

In a one-year study where closed-top chambers were constructed around 30-year-old Scots pine trees growing in Finland, Kellomaki and Wang (1998) applied doubled CO2 to half of the trees while they too studied the effect of a 4°C increase in air temperature.  In this case, the elevated CO2 reduced cumulative sap flow (a measure of transpiration) by 14% in the trees maintained at ambient temperatures, but when both air temperature and CO2 concentration were increased together, cumulative sap flow once again exhibited no change.

In the studies cited above, atmospheric CO2 enrichment consistently produced a small reduction in tree transpiration rates at ambient air temperatures, while the added effect of warming was inconsistent and confusing.  Hence, it is instructive to consider the study of Saurer et al. (2004), who measured carbon isotope ratios in the rings of coniferous trees of northern Eurasia (including the genera Larix, Picea and Pinus) across a longitudinal transect that covered the entire super-continent in the latitude range 59-71°N.  Between the two 30-year periods 1861-1890 and 1961-1990, when air temperature and CO2 concentration both rose significantly, they found that intrinsic water use efficiency (Wi, the amount of carbon gain at the needle level per unit of water loss) also rose significantly, such that "125 out of 126 trees showed increasing Wi from 1861-1890 to 1961-1990, with an average improvement of 19.2 ± 0.9%."

The three Swiss scientists say their real-world in situ results suggest that the trees they studied "are able to produce the same biomass today [as they did 100 years ago] but with lower costs in terms of transpiration."  This finding is very important, because recent warming in other longitudinal segments of the same latitude belt, according to them, "may be accompanied by increased drought stress (Lloyd and Fastie, 2002)," and the ongoing rise in the air's CO2 content may be helping the trees in those areas to better cope with this environmental challenge ... even in the face of increasing air temperatures.

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.

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.

Lloyd, A.H. and Fastie, C.L.  2002.  Spatial and temporal variability in the growth and climate response of treeline trees in Alaska.  Climatic Change 52: 481-509.

Saurer, M., Siegwolf, R.T.W. and Schweingruber, F.H.  2004.  Carbon isotope discrimination indicates improving water-use efficiency of trees in northern Eurasia over the last 100 years.  Global Change Biology 10: 2109-2120.