This is an interesting question and the answer might be quite long. It is hard to imagine our life and our planet without trees, because trees have had a great influence on the shaping of the ecology and in determining the present arrangements of life on earth.

Of particular importance for us has been the role of trees in the evolution of mankind and the development of human cultures and communities. Just after the demise of the dinosaurs, a small rat-like species of mammal (now known as a prosimian) left the ground and took to life in the trees. Eventually after 50 million years had passed, this creature returned to the ground as the ancestor of man. The period spent in the environment of the trees was of great formative importance because it promoted many physical changes. These changes included a massive increase in body size, the development of paws into hands and 3D color vision. The physical changes were mapped by an increase in the size and the capability of the brain. Thus prosimian developed into simian. It was these changes which led to the descent from the trees and eventually to Homo sapiens. So it can be seen that without trees the evolution of prosimian into man would never have taken place. Without trees we would not be here.

On the other hand, trees are of continued importance to the environment. Tropical rain forests have of particular significance; although they now occupy less than 6 per cent of the land surface of the earth they probable sustain more than half of the biological species on the planet.

Trees provide us with the following:
1. Building materials
2. Fuel
3. Sap
4. Food such as fruit and nuts
5. Rubber
6. Alcohol
7. Coal from decayed remains and
8. Pulp for paper ( to name just a few)

Trees also:
1. Slow down runoff by holding or absorbing water
2. Provide shelter for people, animals, and other plants
3. Reduce the devastating effects of flooding
4. Provide nutrients and shelter for a variety of organisms while decomposing
5. Protect the worlds climate by absorbing carbon dioxide (too much contributes to the Greenhouse effect)
6. Provide certain habitats with the stable conditions necessary for life, and offer us beautiful scenery.

In order for you to know more about the importance of trees, try to read the next link on the internet:

benefits of trees