Results from experimental studies have established that it is no longer realistic to examine the effects of climate change on crop and forage plants without also accounting for the direct effects of rising atmospheric CO2 at the same time. The short-term responses to elevated CO2 of isolated plants grown in artificial conditions remain difficult to extrapolate to crops in the field (Körner, 1995). Even the most realistic free-air carbon dioxide enrichment (FACE) experiments yet undertaken impose an abrupt change in CO2 concentration and create a modified area (Kimball et al., 1993) analogous to a single irrigated field in a dry environment. Natural ecosystems and croplands are experiencing a gradual increase in atmospheric CO2. Nonetheless, a cotton crop exposed to FACE increased biomass and harvestable yields by 37% and 48% in elevated (550 ppm) CO2 (Mauney et al., 1994). At the same CO2 increase, spring wheat yields increased by 8% to 10% when water was nonlimiting (Pinter et al., 1996). A simple linear extrapolation of spring wheat FACE results to a doubling of CO2 produces a 28% increase in yields.
Several important breakthroughs in the understanding of how direct effects on crops via CO2 were accomplished since the Second Assessment Report (SAR). Most concern improvements in the understanding of how climate interacts with the physiology of CO2 direct effects. Horie et al. (2000) found that moderate temperatures accompanied by a doubling of CO2 increases the seed yield of rice by 30%. However, with each 1°C increase in temperature above 26°C, rice yields declined by 10% because of shortened growth period and increased spikelet sterility. This raises concerns that CO2 benefits may decline quickly as temperatures warm (established but incomplete). On the positive side, crop plant growth may benefit more from CO2 enrichment in drought conditions than in wet soil because photosynthesis would be operating in a more sensitive region of the CO2 response curve (Samarakoon and Gifford, 1995). Significantly, this effect was observed in C4 photosynthesis. The most likely explanation for this thus far is that drought-induced stomatal closure causes CO2 to become limiting in the absence of CO2 enrichment (established but incomplete). It is not clear how much this effect is likely to offset the overall effect of drought on crop yield. Also, the notable dearth of testing of tropical crops and suboptimal growth conditions (nutrient deficits, weed competition, pathogens) continues from the SAR as a major research gap.
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