The results of C02 enrichment experiments are 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 is 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 in predicting the results of individual experiments 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.
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