The roots of a tree are almost always underground, usually in a ball shaped region centered under the trunk, and extending no deeper than the tree is high. Roots can also be above ground, or deep underground. Some roots are short, some are meters long.
Roots provide support for the parts above ground, holding the tree upright, and keeping it from falling over in high wind.
Roots take in water, and nutrients, from the soil. Without help from fungus for better uptake of nutrients, trees would be small or would die. Most trees have a favorite species of fungus that they associate with for this purpose.
Above ground, the trunk gives height to the leaf-bearing branches, competing with other plant species for sunlight. In all trees the shape of the branches improves the exposure of the leaves to sunlight. Branches start at the trunk, big and thick, and get progressively smaller the farther they grow from the trunk. Branches themselves split into smaller branches, sometime very many times, until at the end they are quite small. The small ends are called twigs.
The leaves of a tree are held by the branches. Leaves are usually held at the ends of the branches. The, although some have leaves along the branches. The main functions of leaves are photosynthesis and gas exchange. A leaf is often flat, so it absorbs the most light, and thin, so that the sunlight can get to the green parts in the cells, which convert sunlight, carbon dioxide from the atmosphere, and water from the roots, into glucose and oxygen. Most of a tree's biomass comes from this process.
Most leaves have stomata, which open and close, and regulate carbon dioxide, oxygen, and water vapour exchange with the atmosphere.
Trees with leaves all year round are evergreens, and those that shed their leaves are deciduous. Deciduous trees and shrubs generally lose their leaves in autumn as it gets cold. Before this happens, the leaves change colour. The leaves will grow back in spring.
The word "tree" in English means a long lived plant having obvious main stem, and growing to a considerable height and size. Thus not all trees have all the organs or parts as mentioned above. For example, most (tree-like) palms are not branched, and tree ferns do not produce bark. There are also more exceptions.
Based on their general shape and size, all of these are nonetheless generally regarded as trees. Trees can vary a lot. A plant that is similar to a tree, but generally smaller, and may have multiple trunks, or have branches that arise near the ground, is called a "shrub", or a "bush". Since these are common English words there is no precise differentiation between shrubs and trees. Given their small size, bonsai plants would not technically be "trees", but are indeed "trees". Do not confuse the use of tree for a species of plant, with the size or shape of individual specimens. A spruce seedling does not fit the definition of a tree, but all spruces are trees.
A tree is a plant form that can be found in many different orders and families of plants. Trees show many growth forms, leaf type and shape, bark traits and organs.
The tree form has changed separately in classes of plants that are not related, in response to similar problems (for the tree). With about 100,000 types of trees, the number of tree types in the whole world might be one fourth of all living plant types. Most tree species grow in tropical parts of the world and many of these areas have not been surveyed yet by botanists (they study plants), making species difference and ranges not well 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. Most species of trees today are flowering plants (Angiosperms) and conifers.
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 very young tree is called a sapling.
Scientists in the UK and Malaysia say they have discovered the world's tallest tropical tree measuring more than 100m (328ft) high.
A coast redwood: , in Redwood National Park, California had been measured as tallest, but may no longer be standing.
The tallest trees in Australia are all eucalypts, of which there are more than 700 species. The so-called 'mountain ash'. with a slim, straight trunk, grows to over 300 feet.
The stoutest living single-trunk species in diameter is the African baobab: , Glencoe baobab (measured near the ground), Limpopo Province, South Africa. This tree split up in November 2009 and now the stoutest baobab could be Sunland Baobab (South Africa) with diameter 10.64 m and circumference of 33.4 m.
Some trees develop multiple trunks (whether from an individual tree or multiple trees) which grow together. The sacred fig is a notable example of this, forming additional 'trunks' by growing adventitious roots down from the branches, which then thicken up when the root reaches the ground to form new trunks; a single sacred fig tree can have hundreds of such trunks.
The life-span of trees is determined by growth rings. These can be seen if the tree is cut down or in cores taken from the edge to the center of the tree. Correct determination is only possible for trees which make growth rings, generally those which occur in seasonal climates. Trees in uniform non-seasonal tropical climates are always growing and do not have distinct growth rings. It is also only possible for trees which are solid to the center of the tree; many very old trees become hollow as the dead heartwood decays away. For some of these species, age estimates have been made on the basis of extrapolating current growth rates, but the results are usually little better than guesses or speculation. White proposed a method of estimating the age of large and veteran trees in the United Kingdom by correlation between a tree's stem diameter, growth character and age.
The verified oldest measured ages of living trees are:
Other species suspected of reaching exceptional age include European Yew "Taxus baccata" (probably over 2,000 years) and western redcedar "Thuja plicata". The oldest known European yew is the Llangernyw yew in the Churchyard of Llangernyw village in North Wales which is estimated to be between 4,000 and 5,000 years old.
The oldest reported age for an Angiosperm tree is 2,305 years for the Sri Maha Bodhi sacred fig ("Ficus religiosa") planted in 288 BC at Anuradhapura, Sri Lanka; this is said to be the oldest human-planted tree with a known planting date.
The earliest fossilised trees date to 386 million years ago in the Devonian period. They have been found at an abandoned quarry in Cairo, New York. The forest was so vast it originally stretched beyond Pennsylvania.
This discovery two or three million years older than the previous oldest forest at Gilboa, also in New York State.
Studies have shown that trees contribute as much as 27% of the appraised land value in certain markets.
These most likely use diameter measured at breast height (dbh), 4.5 feet (140 cm) above ground—not the larger base diameter. A general model for any year and diameter is:
assuming 2.2% inflation per year.
Tree climbing is an activity where one moves around in the crown of trees.