When tundra ecosystems in northern Alaska were exposed to increased C02 at nearly 600 ppm in a FACE experiment, there was an initial increase in growth which disappeared after 2 3 years. This particularly short-lived response is thought to occur because tundra ecosystems are highly nutrient-limited: they cannot respond well to the extra carbon supply by increasing plant tissues because the nutrients they also need to build their tissues are in such short supply.
Other FACE studies have included salt marshes, and tallgrass prairies in the Midwestern USA. These too have tended to show an initial burst of growth, followed by decline.
In contrast to the results from natural systems, C02 fertilization FACE experiments in well-fertilized agricultural grassland in the Midwestern USA and in Switzerland suggest that the increase in growth rate persists over time, with growth rates at least 25% faster than "normal C02" plots adjacent to the experimental plots.
Both FACE and closed-chamber experiments show that the greatest benefits from CO: fertilization come when the crops arc well supplied with mineral fertilizers, which enable them to construct more tissues using the extra carbon that they fix. This response will probably increase global food production, but it will be concentrated in certain parts of the world. The increased yield is thus likely to favor richer farmers who can already afford plenty of fertilizer to put on their crops, rather than the poorest farmers in the Third World.
The results of C02 enrichment experiments arc diverse, confusing and sometimes contradictory. Increases in growth rate of plants are often reported during the early part of an experiment, so at the end of the experimental period they are bigger than they would otherwise be (though this relative gain tends to lessen during the experiment). It is important to realise however that this does not necessarily translate into a long-term equilibrium change in biomass. For all we know the plants might just grow more quickly to maturity then fall over and rot, with no change in their long-term biomass. Even less is known about this than the question of whether growth rate of the world's vegetation will be significantly speeded up.
It would be scientifically risky to try to make predictions that scale up from such isolated experiments, operated for such a brief period of time, to predict the response of the whole world's vegetation. But, one general lesson that the experiments do give us is that C02 responses are very complex and are not always what we would expect from a physiological model. Models that extrapolate from cell and leaf-level processes, to forecast global-scale responses in vegetation to increased C02, seem to score some successes but also many significant failures. Even the successes are rather ambivalent if one looks at the details of the response. For example, the C02-fertilized sweetgum forest in Tennessee showed about the "right" amount of increase in NPP predicted by the models, but the way in which the NPP showed up (in fine-root turnover) is rather bizarre. The expectation of most of the modelers, even if not clearly stated, is that the extra growth put on by plants under increased C02 will show up in the form of wood. This will then form a negative feedback in terms of taking up C02. In contrast, it is not at all clear what an increase in fine-root turnover would do for overall carbon storage.
8.10.1 Will a high CO> world favor C3 species over C4 species?
Probably the most generally held principle in forecasting raised C02 effects is that plants using the more water-efficient and C02-eificient 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 C'02. Various experiments growing wild C3 and C4 plants in competition in chambers under raised C'02 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 closcd-chamber experiments growing apparently realistic combinations of prairie plants under raised CO: do not support this (sec 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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