What was done
The authors grew Douglas fir (Psuedotsuga menziesii (Mirb.) Franco) seedlings in sunlit chambers programmed to track either ambient atmospheric CO2 concentration or ambient + 200 ppm CO2 plus either ambient air temperature or ambient +4°C over a 21-month period, while they measured light-saturated rates of net photosynthesis at approximately monthly intervals.

What was learned
Lewis et al. report that the extra CO2 they supplied to the seedlings "increased net photosynthetic rates by an average of 21% across temperature treatments during both the 1996 hydrologic year, the third year of exposure, and the 1997 hydrologic year," while "elevated mean annual temperature increased net photosynthetic rates by an average of 33% across CO2 treatments during both years." They also state that "between February and August 1996, the short-term temperature optima for photosynthesis shifted by approximately 10°C higher in both CO2 treatments," and that the elevated CO2 treatment "increased the short-term (minutes to hours) temperature optima for photosynthesis, as has been observed in other tree species (Idso and Idso, 1994; Eamus et al., 1995)."

What it means
The four researchers write that "an increase of 200 ppm above current atmospheric CO2 concentrations may shift temperature optima upward 3-4°C, paralleling the increase in mean annual temperatures predicted to occur during the next century," and that "by shifting temperature optima upward, elevated CO2 may 'acclimate' photosynthetic processes to future temperature regimes."

References
Eamus, D., Duff, G.A. and Berryman, C.A. 1995. Photosynthetic responses to temperature, light flux-density, CO2 concentration and vapour pressure deficit in Eucalyptus tetrodonia grown under CO2 enrichment. Environmental Pollution 90: 41-49.

Idso, K.E. and Idso, S.B. 1994. Plant responses to atmospheric CO2 enrichment in the face of environmental constraints: a review of the past 10 years' research. Agricultural and Forest Meteorology 69: 153-203.