Trees improve air quality by producing oxygen. They also store carbon, offsetting harmful byproducts of fossil-fuel burning. They moderate the effects of sun and wind, and they clean the air by trapping dust, pollen and other pollutants.
Most importantly, trees sequester carbon, helping to remove carbon dioxide and other greenhouse gases from the air, which cools the earth. In fact, a mature canopy tree absorbs enough carbon and releases enough oxygen to sustain two human beings!