Perennial plant with a woody stem, usually a single stem (trunk), made up of wood and protected by an outer layer of bark. It absorbs water through a root system. There is no clear dividing line between shrubs and trees, but sometimes a minimum achievable height of 6 m/20 ft is used to define a tree.
Angiosperms A treelike form has evolved independently many times in different groups of plants. Among the angiosperms, or flowering plants, most trees are dicotyledons. This group includes trees such as oak, beech, ash, chestnut, lime, and maple, and they are often referred to as broad-leaved trees because their leaves are broader than those of conifers, such as pine and spruce. In temperate regions angiosperm trees are mostly deciduous (that is, they lose their leaves in winter), but in the tropics most angiosperm trees are evergreen. There are fewer trees among the monocotyledons, but the palms and bamboos (some of which are treelike) belong to this group.
Gymnosperms The gymnosperms include many trees and they are classified into four orders: Cycadales (including cycads and sago palms), Coniferales (the conifers), Ginkgoales (including only one living species, the ginkgo, or maidenhair tree), and Taxales (including yews). Apart from the ginkgo and the larches (conifers), most gymnosperm trees are evergreen.
Tree ferns There are also a few living trees in the pteridophyte group, known as tree ferns. In the swamp forests of the Carboniferous era, 300 million years ago, there were giant treelike horsetails and club mosses in addition to the tree ferns.
Oldest trees The world's oldest living trees are found in the Pacific forest of North America, some more than 2,000 years old.
Conservation According to a 1998 report by the World Conservation Monitoring Centre (WCMC) and the World Conservation Union, approximately 8,750 tree species (around 10% of all known tree species) are in danger of extinction. In the 20th century 77 species became extinct and 7 more species were reduced to fewer than a dozen specimens.
Roots The great storm of October 1987 destroyed some 15 million trees in Britain, and showed that large roots are less significant than those of 10 cm/4 in diameter or less. If enough of these are cut, the tree dies or falls.
British Trees Home Page
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