Probably the most generally held principle in forecasting raised C02 effects is that plants using the more water-efficient and C02-efficient C4 photosynthetic system will lose out by competition to the "normal" C3 plants which have more to gain from raised C02. When C4 plants growing alone are fertilized with extra C02, they tend to show little gain from it. There is almost no enhancement of photosynthesis, although they do lose a little less water because they can get the C02 they need quicker and then shut their stomatal pores. A FACE-type experiment on a corn (maize) field in the USA showed that, much as expected, it did not grow any bigger or faster under doubled C02. Various experiments growing wild C3 and C4 plants in competition in chambers under raised C02 have shown that, as expected, C4 plants tend to lose out to C3 species, which benefit much more strongly from the extra C02. However, it is rather puzzling that closed-chamber experiments growing apparently realistic combinations of prairie plants under raised C02 do not support this (see above). In one case, C3 and C4 species responded about equally, and in another experiment C4 species actually did better than the C3 species under raised C02!
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