A tree is a large, woody perennial plant with one main trunk and many branches. Trees are the most visible component of forests, a crucial component of the world's ecology.

Closely related plants may grow into large trees, or into much smaller plants. For example, the western yew (Taxus brevifolia) grows into a tree taller than 60 feet (20 m) in the Pacific rainforests of western North America, while the closely related Canada yew ( T. canadensis) of the northeastern forests is a shrub only 3-4 ft. tall (1 m). Many kinds of plants are capable of reaching tree size. This includes plants that do not develop true woody (or xylem) tissues, such as tree-ferns and palms, as well as many species of coniferous and angiosperm trees.

Foresters and ecologists define trees strictly on the basis of their size. In some forests, for example, a tree might be defined as being taller...