The gasoline we burn and other fossil fuels add more carbon dioxide to the atmosphere than trees and oceans can absorb. This is the main cause of global warming. The more trees there are, the more carbon dioxide is absorbed, and the less global warming we have. Trees drive the Hydrological Cycle, I read. They do this by absorbing vast amounts of water through their roots and leaves.
Trees are what make our climate livable.
Of course, trees are also a food source. Nuts and fruits come from trees. And they are an energy source, as well. Many people around the world still use wood as their primary source of heat. Pencils, paper, lumber, chemicals and garden mulch are made from trees, even some medicines, not to mention the fact that trees are known to reduce stress.