In botany, a tree is a plant with an elongated stem, or trunk, supporting leaves or branches.
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.
A tree typically has many secondary branches supported clear of the ground by the trunk. This trunk typically contains woody tissue for strength, and vascular tissue to carry materials from one part of the tree to another. For most trees it is surrounded by a layer of bark which serves as a protective barrier. Below the ground, the roots branch and spread out widely; they serve to anchor the tree and extract moisture and nutrients from the soil. Above ground, the branches divide into smaller branches and shoots. The shoots typically bear leaves, which capture light energy and convert it into chemical energy by photosynthesis, providing the food needed by the tree for its growth and development.
Flowers and fruit may also be present, but some trees such as conifers instead have pollen cones and seed cones, and others such as tree ferns produce spores instead.
Trees tend to be long-lived, some reaching several thousand years old. The tallest known specimen on Earth is 115.6 m (379 ft) and they have a theoretical maximum height of 130 m (426 ft). Trees have been in existence on the Earth for 370 million years. Trees are not a taxonomic group but are a number of plant species that have independently evolved a woody trunk and branches as a way to tower above other plants and make full use of the sunlight.
A forest, also referred to as a wood or the woods, is an area with a high density of trees. As with cities, depending on various cultural definitions, what is considered a forest may vary significantly in size and have different classifications according to how and of what the forest is composed. A forest is usually an area filled with trees but any tall densely packed area of vegetation may be considered a forest, even underwater vegetation such as kelp forests, or non-vegetation such as fungi, and bacteria. Tree forests cover approximately 9.4 percent of the Earth's surface (or 30 percent of total land area), though they once covered much more (about 50 percent of total land area). They function as habitats for organisms, hydrologic flowmodulators, and soil conservers, constituting one of the most important aspects of the biosphere.
A typical tree forest is composed of the overstory (canopy or upper tree layer) and the understory. The understory is further subdivided into the shrub layer, herb layer, and also the moss layer and soil microbes. In some complex forests, there is also a well-defined lower tree layer. Forests are central to all human life because they provide a diverse range of resources: they store carbon, aid in regulating the planetary climate, purify water and mitigate natural hazards such as floods. Forests also contain roughly 90 percent of the world's terrestrial biodiversity.