Background
Climate alarmists typically claim that CO2-induced increases in air temperature will have severe negative consequences for many of earth's plants, reducing their growth rates and leading to the deaths of many individual plants and, in some cases, the actual extinctions of certain species of plants. But is this really so? The authors note, for example, that for many plants, growth at high temperature increases the optimal temperature (Topt) of light-saturated net photosynthesis (Asat), citing Berry and Bjorkman (1980), while noting that Topt was increased by 0.34-0.54°C per °C of warming in some eucalyptus species exposed to large seasonal temperature fluctuations (7-19°C) in the field, citing the work of Battaglia et al. (1996). So what happens to Topt and Asat when the air's temperature and CO2 concentration increase together.

What was done
Ghannoum et al. grew individual well watered and fertilized plants of two species of Australian eucalypts -- faster-growing Eucalyptus saligna and slower-growing E. sideroxylon -- from seed in 10-L pots filled with 9 kg of loamy-sand in naturally-lit glasshouse compartments maintained at either ambient or ambient + 4°C air temperature and three different CO2 concentrations (280, 400 or 650 ppm) for a period of 140 days, while measuring various plant responses throughout the experiment.

What was learned
The seven scientists report that Asat increased by ~50% with each step-increase in the air's CO2 concentration -- i.e., going from 280 to 400 ppm, and going from 400 to 650 ppm -- and that at the higher of the two temperature treatments, the optimal temperature for Asat increased by 2-7°C across the three CO2 treatments.

What it means
The Australian and U.S. researchers conclude, as they describe it, that "when water and nutrient supplies are non-limiting, photosynthesis of eucalypt seedlings is expected to strongly increase with rising CO2 concentration and temperature," while noting that their results "partly explain the strong growth responses to elevated CO2 and temperature observed in a previous study with the same eucalypt seedlings," citing the work of Ghannoum et al. (2010b).

References
Battaglia, M., Beadle, C. and Loughhead, S. 1996. Photosynthetic temperature responses of Eucalyptus globulus and Eucalyptus nitens. Tree Physiology 16: 81-89.

Berry, J.A. and Bjorkman, O. 1980. Photosynthetic response and adaptation to temperature in higher plants. Annual Review of Plant Physiology and Plant Molecular Biology 31: 491-543.

Ghannoum, O., Phillips, N.G., Conroy, J.P., Smith, R.A., Attard, R.D., Woodfield, R., Logan, B.A., Lewis, J.D. and Tissue, D.T. 2010a. Exposure to preindustrial, current and future atmospheric CO2 and temperature differentially affects growth and photosynthesis in Eucalyptus. Global Change Biology 16: 303-319.