Everything on the earth needs water to live; animals, people and even trees get water through a process called the "water cycle." Water vapor changes to rain that falls to the ground, some of which, in turn, eventually evaporates back into the atmosphere. Trees are also integral in the water cycle in that they produce water vapor via transpiration -- the process whereby the leaves of trees essentially emit water vapor. The effects of global warming often cause leaves to fall from trees too early, which inhibits trees' transpiration.
Air pollution is a contributing factor in global warming, which injures trees substantially. Substances in the air such as sulfur dioxide, commonly the result of burning fossil fuels, exacerbate the effects of global warming on trees by chemically burning a tree, compromising its ability to produce foliage or even grow fully. The deficiency may even promote the spread of disease between tree species. This also adversely affects the surrounding animal and insect life, which in turn inhibits proper tree development yet again. Further, the injury or disease may also stop photosynthesis, a process that is extremely important in the removal of carbon dioxide from the earth's air. Too much carbon dioxide causes the earth to essentially stay overheated, which is thought to intensify the greenhouse effect of global warming.
Acid rain is an aftereffect of air pollution that is thought to promote global warming, which paradoxically enough, causes more acid rain. When chemical oxidants are released into the air, they mix with water vapor; tainted water vapor changes to rain which then falls to the ground. A certain pH level must be maintained in both the water and soil for trees to grow properly. Acid rain contains pH levels that are often too high. Such levels may allow bacteria to spread, which may cause trees to die prematurely -- thereby inhibiting transpiration yet again.
Trees grows where the environment is healthy for them to do so; where it isn't they stop growing entirely. Global warming causes tree redistribution, meaning that trees that typically grew over certain landscapes essentially go elsewhere to survive. Carbon dioxide stays in the air, causing gases to remain trapped in certain pockets all over the earth. This may actually enhance the greenhouse effect, which is the process whereby chemistry and sunlight contribute to a rise in the earth's overall temperatures.