0ver the next century or so, C02 itself will not be the only environmental factor changing. Climate is likely to become substantially warmer in most parts of the world, because of the greenhouse effect of the C02 itself, as well as other greenhouse gases. So, to know how vegetation will really change, it may be helpful to try altering both factors simultaneously and see how they interact.
There have been a few short-term (one or two year) open-top chamber experiments in the temperate zone which artificially warmed plants growing in natural settings, while they were being fertilized by C02. Usually, this was done with either under-soil heating, or with infra-red lamps. For example, a study carried out in Tennessee by Rich Norby and colleagues studied young red maples (Acer rubrum) under this combination of conditions. The results were compared with controls at normal temperatures and C02, or under either only increased C02 or only increased temperatures. What studies such as this have tended to find is that there can be an additive effect in terms of growth rate of increasing both temperature and C02: the warming increases growth somewhat (at least in temperate climates, which tend to be heat-starved part of the year), and C02 also increases growth some more. Some studies seem to show a synergistic effect: the acceleration of growth under both
C02 and warmer temperatures is greater than the sum of the two separately. This might be, for example, because the benefit of high C02 in reducing wasteful photorespiration is greater at higher temperatures, so there is more to gain. In the warmer greenhouse world, this might then mean that biota will respond strongly to C02.
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