A tree is a perennial woody plant. It is sometimes defined as a woody plant that attains diameter of 10 cm (30 cm girth) or more at breast height (130 cm above ground). However, there is no set agreement regarding minimum size, the term generally applies to plants that grow to at least 5-6 meters (15-20 ft) high at maturity[citation needed] and having secondary branches supported on a main stem or stems, called a trunk. Most trees exhibit clear apical dominance, though this is not always the case. Compared with most other plants, trees are long-lived, some of them getting to be several thousand years old and growing to up to 115 meters (375 ft) high.
Trees are an important component of the natural landscape due to their prevention of erosion and the provision of a specific weather-sheltered ecosystem in and under their foliage. Trees have also been found to play an important role in producing oxygen[citation needed] and reducing carbon dioxide in the atmosphere, as well as moderating ground temperatures. They are also significant elements in landscaping and agriculture, both for their aesthetic appeal and their orchard crops (such as apples). Wood from trees is a common building material. Trees also play an intimate role in many of the world's mythologies (see trees in mythology).
Classification
A tree is a plant form that occurs in many different orders and families of plants. Trees show a wide variety of growth forms, leaf type and shape, bark characteristics, and reproductive organs.
The earliest trees were tree ferns and horsetails, which grew in vast forests in the Carboniferous Period; tree ferns still survive, but the only surviving horsetails 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. The listing below gives examples of many well-known trees and how they are typically classified.
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.
Morphology
Leaves are an important feature of trees. These are Beech leaves.
Tree roots anchor the structure and provide water and nutrients. The ground has eroded away around the roots of this young Pine tree.
The basic parts of a tree are the roots, trunk(s), branches, twigs and leaves. Tree stems consist mainly of support and transport tissues (xylem and phloem). Wood consists of xylem cells, and bark is made of phloem and other tissues external to the vascular cambium. Trees may be broadly grouped into exogenous and endogenous trees according to the way in which their stem diameter increases. Exogenous trees, which comprise the great majority of modern trees (all conifers, and all broadleaf trees), grow by the addition of new wood outwards, immediately under the bark. Endogenous trees, mainly in the monocotyledons (e.g., palms), grow by addition of new material inwards.
As an exogenous tree grows, it creates growth rings. Also known as annual rings, each set of light/dark rings is equivalent to one year of growth in a small number of mostly temperate species. For the vast majority of tree species growth rings do not correspond to annual intervals and several may be laid down in a single year, or there may be intervals of several years between additions while in some tropical regions with constant year-round climate growth is continuous and distinct rings are not formed at all. In temperate climates, these rings are commonly visible due to changes in the rate of growth with temperature variation over an annual cycle. In temperate species these rings can be counted to determine the age of the tree, and used to date cores or even wood taken from trees in the past, a practice is known as the science of dendrochronology. Very few, if any, tropical trees can be accurately aged in this manner. Age determination is also impossible in endogenous trees.
The roots of a tree are generally embedded in earth, providing anchorage for the above-ground biomass and absorbing water and nutrients from the soil. It should be noted, however, that while ground nutrients are essential to a tree's growth the majority of its biomass comes from carbon dioxide absorbed from the atmosphere (see photosynthesis). Above ground, the trunk gives height to the leaf-bearing branches, aiding in competition with other plant species for sunlight. In many trees, the arrangement of the branches optimizes exposure of the leaves to sunlight.
Not all trees have all the plant organs or parts mentioned above. For example, most palm trees are not branched, the saguaro cactus of North America has no functional leaves, tree ferns do not produce bark, etc. Based on their general shape and size, all of these are nonetheless generally regarded as trees. Indeed, sometimes size is the more important consideration. A plant form that is similar to a tree, but generally having smaller, multiple trunks and/or branches that arise near the ground, is called a shrub. However, no sharp differentiation between shrubs and trees is possible. Given their small size, bonsai plants would not technically be 'trees', but one should not confuse reference to the form of a species with the size or shape of individual specimens. A spruce seedling does not fit the definition of a tree, but all spruces are trees.
