A tree is a perennial woody plant. It is most often defined as a woody plant that has many secondary branches supported clear of the ground on a single main stem or trunk with clear apical dominance. A minimum height specification at maturity is cited by some authors, varying from 3m to 6m; some authors set a minimum of 10 cm trunk diameter (30 cm girth). Woody plants that do not meet these definitions by having multiple stems and/or small size are called shrubs. Compared with most other plants, trees are long-lived, some reaching several thousand years old and growing to up to 115 m (379 ft) high.
Trees are an important component of the natural landscape because of their prevention of erosion and the provision of a weather-sheltered ecosystem in and under their foliage. They also play an important role in producing oxygen and reducing carbon dioxide in the atmosphere, as well as moderating ground temperatures. They are also elements in landscaping and agriculture, both for their aesthetic appeal and their orchard crops (such as apples). Wood from trees is a building material, as well as a primary energy source in many developing countries. Trees also play a role in many of the world’s mythologies (see trees in mythology).
A tree is a plant form that occurs in many different orders and families of plants. Most species of trees today are flowering plants (Angiosperms) and conifers. Trees show a variety of growth forms, leaf type and shape, bark characteristics and reproductive organs. For the listing of examples of well-known trees and how they are classified, see List of tree genera.
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 estimate of 100,000 tree species, the number of tree species worldwide might total 25 percent of all living plant species. The majority of tree species grow in tropical regions of the world and many of these areas have not been surveyed yet by botanists, making species diversity and ranges poorly understood.
The earliest trees were tree ferns, horsetails and lycophytes, which grew in forests in the Carboniferous period; tree ferns still survive, but the only surviving horsetails and lycophytes are not of tree form. Later, in the Triassic period, conifers, ginkgos, cycads and other gymnosperms appeared, and subsequently flowering plants in the Cretaceous period.
A small group of trees growing together is called a grove or copse, and a landscape covered by a dense growth of trees is called a forest. Several biotopes are defined largely by the trees that inhabit them; examples are rainforest and taiga (see ecozones). A landscape of trees scattered or spaced across grassland (usually grazed or burned over periodically) is called a savanna. A forest of great age is called old growth forest or ancient woodland (in the UK). A young tree is called a sapling.