What was done
The authors write that an unusual heat wave and drought occurred in northern Wisconsin (USA) during the summer of 2006, leading to maximum temperatures that varied between 30 and 38°C and exacerbated the heat stress experienced by the leaves of most trees. Taking advantage of this situation, they measured leaf gas exchange characteristics and several other plant physiological properties of the leaves of paper birch (Betula papyrifera) trees and two clones (271 and 42E) of quaking aspen (Populus tremuloides Michx) trees exposed to either ambient or elevated (ambient + 200 ppm) atmospheric CO2 concentrations at the Aspen FACE facility before, during and after the highest air temperatures recorded during that period of extreme heat.

What was learned
The four researchers report that "elevated CO2 protected photosynthesis of both species against moderate heat stress" by increasing "carboxylation capacity, photosynthetic electron transport capacity and triose phosphate use." In addition, they say they "observed significant increases in transpiration rates in both aspen clones and the birch trees under elevated CO2," the cooling effect of which decreased the leaf temperatures of the aspen trees by 1.9°C in clone 42E and by 2.7°C in clone 171, while the leaf temperatures of the birch trees were decreased by 3.1°C.

As for how this cooling could have occurred, Darbah et al. note that under normal less stressful conditions, atmospheric CO2 enrichment typically does just the opposite; it reduces transpiration and increases leaf temperature. And this type of behavior earlier in the growing season in the elevated CO2 treatment may have left more water in the soil, enabling the trees to transpire at a greater rate and thereby cool their leaves when it was most needed (in this case, during the subsequent heat wave and drought), which phenomenon they describe as being "one part of the increased thermotolerance of the plants under high CO2."

As a result of these various phenomena, Darbah et al.'s gas exchange measurements indicated that whereas the CO2-induced stimulation of net photosynthesis in aspen clone 42E was about 31% over the leaf temperature range 32-35°C, it was approximately 218% over the temperature range 36-39°C, while for aspen clone 171 the corresponding enhancements were 38 and 199%, and for the birch trees they were 95 and 297%.

What it means
In discussing their findings, Darbah et al. state that they agree with those of Veteli et al. (2007), who "reported that elevated CO2 ameliorated the negative effects of high temperature in three deciduous tree species," as well as those of Wayne et al. (1998), who "reported that elevated CO2 ameliorated high temperature stress in yellow birch trees," and that all of these observations are "in agreement with Idso and Kimball (1992), who reported that elevated CO2 (ambient + 300 ppm) increased net photosynthetic rate in sour orange tree leaves exposed to full sunlight by 75, 100 and 200% compared to those in ambient CO2 concentration at temperatures of 31, 35 and 42°C, respectively, suggesting that elevated CO2 ameliorates heat stress in tree leaves." Hence, they conclude that "in the face of rising atmospheric CO2 and temperature (global warming), trees will benefit from elevated CO2 through increased thermotolerance."

References
Idso, S.B. and Kimball, B.A. 1992. Effects of atmospheric CO2 enrichment on photosynthesis, respiration and growth of sour orange trees. Plant Physiology 99: 341-343.

Veteli, T.O., Mattson, W.J., Niemela, P., Julkunen-Tiitto, R., Kellomaki, S., Kuokkanen, K. and Lavola, A. 2007. Do elevated temperature and CO2 generally have counteracting effects on phenolic phytochemistry of boreal trees? Journal of Chemical Ecology 33: 287-296.

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.