Effects of atmospheric CO2 concentration on wheat yield:: review of results from experiments using various approaches to control CO2 concentration

Fifty studies (156 experiments) of effects of CO2 concentration ([CO2]) on wheat (Triticum aestivum L.) yield (grain mass at maturity) were analyzed (24 were out-of-doors studies). Only studies controlling [CO2] during all (or most) of the wheat life cycle were considered. Studies were divided into five categories based on the method of controlling [CO2]: laboratory-chamber, glasshouse (greenhouse), closed-top field chamber, open-top field chamber (OTC), and a free-air (chamberless) field CO2 enrichment (FACE) system. Only three studies, all conducted in glasshouses, included subambient-[CO2] treatments ([CO2] < 330ppm), with yield positively related to [CO2] in all three. In superambient-[CO2] experiments with ample water and nutrients and with favorable temperature, [CO2] up to about 2000 ppm increased yield, with a maximum effect (+37%) at about 890 ppm CO2 (according to curve fitting with data from all methods pooled). On average, doubling [CO2] from 350 to 700 ppm increased yield about 31 %. Differences in effects of [CO2] on yield between methods of controlling [CO2] could not be judged (or did not exist) because of large variation in yield across chamber (including glasshouse) experiments and too few FACE experiments. Side-by-side comparisons of different methods of controlling [CO2] in which yield was measured were notably lacking. The large variation in effect of [CO2] on yield, even with ample water and nutrients, probably reflected interactions between [CO2] and other factors. With mineral nutrient limitations, effects of [CO2] on yield were small, and with severe nutrient limitations increased [CO2] sometimes reduced yield, With ample nutrients and [CO2] greater than 2000 ppm, yield was also reduced, but this may be of limited significance to field crops for at least the next 100 years. Elevated [CO2] stimulated yield of water-stressed wheat, but usually did not fully compensate for water shortage, though few data were available. Elevated [O-3] sometimes reduced positive effects of elevated [CO,] on yield, though again, few data were available. Usually, modest warming (1-4 degreesC) counteracted positive effects of doubled [CO2] on yield. Combinations of rising temperature, [CO2]. and [O-3] may result in positive or negative effects on wheat yield, though the [CO2]-effect per se will normally be positive. Predictions of effects of rising [CO2] on wheat yield carry with them intrinsic uncertainty. Published by Elsevier Science B.V.