The size of big trees and their appetites mean a big effect
The big trees in established forests, like the tropical rainforests of the world, have enormous appetites for sunshine, rainfall and nutrients. Scientists agree that without the correct environmental conditions, it is difficult for big trees to grow to maturity. But there is disagreement about whether a warming world will increase the growth of trees or slow it down.
There is some evidence that tree growth might slow if the planet warms up and that this effect could be more extreme in locations that are already hot. Some scientists argue that when it gets too hot, the photosynthesis in trees shuts down, then, during the night, they use up more energy than usual since their metabolic rates are higher. If less energy is being produced from photosynthesis and yet more is being consumed, the logic is that there will be less energy available for plant growth.
If this is true, even our biggest tropical forests might shrink in the face of global warming. With forests dying back, there would be a double negative impact; more carbon would be released to the atmosphere and there would be fewer trees to absorb carbon emissions.
Some scientists think differently, arguing that tree growth is accelerating in forests of the world. The probable cause for this, they suggest, is increased levels of carbon dioxide in the atmosphere, a gas which provides nutrition for plant growth. There is evidence from Brazil that trees are getting bigger in the Amazon for this reason.
Source: The Economist and Cool Earth