Trees are large, perennial, woody plants, which continue growing until they die. They grow just about everywhere on Earth, from extreme cold regions near the Arctic and the Antarctic to hot tropical regions around the equator and even in deserts and the most diverse populations of trees are found in the tropical rain forests. In comparison to other plant forms, trees are considered long-lived. As perennial plants, they are classified as having a distinct, self-supporting main stem, or trunk, containing a woody tissue that produces secondary limbs and branches. Their height and single main stem differentiate them from shrubs, which are usually shorter and have many stems. With more than 20,000 types of trees growing around the world, a few species can grow really tall, like redwoods and eucalyptus that reach heights of more than 110 m. (360 ft.). Trees live anywhere from a few, to thousands of years, like the olive tree or a bristlecone pine growing in California's White Mountains that has been dated as at least 4700 years old.
Trees do come in a wide variety of shapes and sizes and can be divided into six basic groups according to the features they have in common. These are: broadleaf trees, needleleaf trees, palm and pandanus and lily trees, cycad trees, tree ferns, and ginkgo trees. All of the above groups have provided us with food, fibers, medicines, and many other substances valuable in every day life. If you are asked what product you can receive from trees, wood is the one that quickest comes to mind. Since ancient times, people have used wood to make tools and construct buildings as well as for forms of art and expression. Most importantly, wood has also been used as a source of fuel.
Furthermore, trees in the world's forests or plantations provide food and shelter for humans and wildlife. Trees provide us with fruits, nuts, roots and other tree parts which we opular and include avocados, grapefruits, mangoes, and oranges. Cooler, temperate regions have fewer kinds of fruit trees, but produce vast amounts of apples, cherries, and peaches. Almonds, pecans, walnuts, and macadamia nuts are a few easily recognized species of nut producing trees. The most important nut tree of warm regions is the coconut palm, which produces coconuts. Trees also supply chocolate, coffee, maple syrup, and such spices as cinnamon.
Finally, trees also have a role in conservation. Along lakes and rivers tree roots conserve soil, holding top soil in place, and water, holding water in the ground. Where they grow in open country, they act as windbreaks and keep the wind from blowing away topsoil, therefore aiding in stopping desertification.