From the beginning of life on our planet, animals, including the human, has sought a place in the shade in the heat of day. Look at the cows clustered together under the trees. Watch your dog in the summer out on the deck. He will have a sunbath, then move to a shady spot under your umbrella table, which you are sitting at yourself for protection from the heat.
We have all heard of their ecological importance, and take it very much for granted. But do we know that trees are as important to human beings as food and water? To keep city air cool and clean, trees should cover at least 40% of city land. This is the canopy we hear about now and then, and pay so little attention to. One tree can clean the toxic emissions from the dirty air exhausted from an average car driven 4,000 miles.