What was done
In the summer of 2006, the authors measured the effects of a natural and prolonged heat wave on the photosynthetic rates of quaking aspen (Populus tremuloides Michx) and paper birch (Betula papyrifera) trees that had been growing from the seedling stage for an additional nine years in the Free-Air CO2-Enrichment (FACE) facility near Rhinelander, Wisconsin (USA), where from 0700 to 1900 hours each day throughout the growing season, half of them were exposed to an extra ~190 ppm of CO2.

What was learned
Darbah et al. report that the aspen trees "showed no visible symptoms of stress," but that the birch trees exhibited "leaf curling and then yellowing of leaves and finally leaf shedding," with trees in the control treatment dropping 33% of their leaves, while trees in the CO2-enriched treatment dropped only 20%. In terms of rates of photosynthesis at saturating light intensities, aspen clone 42E exhibited a 30% increase in the 32-35°C temperature range, but a whopping 218% in the 36-39°C range. Similarly, aspen clone 271 exhibited a 38% increase in the 32-35°C range, but a 199% increase in the 36-39°C range. Contemporaneously, the birch trees exhibited a 95% in increase in photosynthetic rates in the 32-35°C range, but a 297% increase in the 36-39°C range.

What it means
The four researchers state that their findings are "in agreement with Idso and Kimball (1992), who reported that elevated CO2 (ambient + 300 ppm) increased net photosynthetic rate in sour orange tree (Citrus aurantium L.) 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." In addition, they say their observations "agree with Veteli et al. (2007), who reported that elevated CO2 ameliorated the negative effects of high temperature in three deciduous tree species," and that "Wayne et al. (1998) reported that elevated CO2 ameliorated high temperature stress in yellow birch trees (Betula alleghaniensis)." Hence, they say 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.