In some usages, the definition of a tree may be narrower, including only woody plants, only plants that are usable as lumber, only plants above a specified height or only perennial species.
At its broadest, trees include the taller palms, the tree ferns, bananas and bamboo. Trees play a significant part in reducing erosion and moderating the climate. They remove carbon dioxide from the atmosphere and store large quantities of carbon in their tissues. Trees and forests provide a habitat for many species of animals and plants. Tropical rainforests are one of the most biodiverse habitats in the world. Trees provide shade and shelter, timber for construction, fuel for cooking and heating, and fruit for food as well as having many other uses. In parts of the world, forests are shrinking as trees are cleared to increase the amount of land available for agriculture. Because of their longevity and usefulness, trees have always been revered and they play a role in many of the world's mythologies. Trees are an evolutionary adaptation to competition for space, by growing taller trees are able to compete better for sunlight. They have modified structures that allow them to grow much taller and spread out their foliage, such as thicker stems that are composed of specialized cells that add structural strength and durability. They are long-lived perennial plants that can increase their size each year by producing woody stems. They differ from shrubs, which are also woody plants, by usually growing larger and having a single main stem; but the distin
tion between a small tree and a large shrub is not always clear, made more confusing by the fact that trees may be reduced in size under harsher environmental conditions such as on mountains and subarctic areas. The tree form has evolved separately in unrelated classes of plants in response to similar environmental challenges, making it a classic example of parallel evolution. With an estimated 100,000 species, the number of trees worldwide might total twenty-five percent of all living plant species. Their greatest number grow in tropical regions and many of these areas have not yet been fully surveyed by botanists, making tree diversity and ranges poorly known. Wood gives structural strength to a tree stem which is used to support the plant as it grows larger.
The vascular system of trees allows water, nutrients and other chemicals to be distributed around the plant, and without it trees would not be able to grow as large as they do. The three main parts of trees include the root, stem, and leaves; they are integral parts of the vascular system which interconnects all the living cells. In trees and other plants that develop wood, the vascular cambium allows the expansion of vascular tissue that produces woody growth. Because this growth ruptures the epidermis of the stem, woody plants also have a cork cambium that develops among the phloem. The cork cambium gives rise to thickened cork cells to protect the surface of the plant and reduce water loss. Both the production of wood and the production of cork are forms of secondary growth.