TREE

woody plant that renews its growth every year (called a perennial). Most plants classified as trees have a single self-supporting trunk containing woody tissues, and in most species the trunk produces secondary limbs, called branches. To many, the word tree evokes images of such ancient, powerful, and majestic structures as the redwood and the giant sequoia, among the most massive and longest-living organisms in the world. Although the majority of the Earth's biomass is represented by trees, the fundamental importance of these seemingly ubiquitous plants for the very existence and diversity of life on Earth is perhaps not fully appreciated. Our very biosphere is dependent on the metabolism, death, and recycling of plants, especially trees. Their vast trunks and root systems store carbon dioxide and water and respire oxygen into the atmosphere. The organic matter of the soil develops primarily from plant residues (that is, from decayed leaves, twigs, branches, roots, and fallen trees), which release important nutrients, such as nitrogen, carbon, and oxygen. The tree is not an immutable biological category but rather a human concept based on visual criteria. Perhaps a general definition would describe a tree as a perennial woody plant that develops along a single main trunk to a height of at least 4.5 metres (15 feet) at maturity. This may be contrasted with a shrub, which might be loosely defined as a woody plant with multiple stems that is, in most cases, less than 3 metres tall. However, a species fitting the description of either in one area of the world might not necessarily do so in other regions, since a variety of stresses shape the habit of the mature plant. Thus, a given woody species may be a tree in one set of habitats within its range and a shrub elsewhere. For example, the spruce and fir may thrive in the tree form at the base of a mountain but assume a shrub form near the mountaintop, the variation due principally to stresses exerted by such environmental conditions as altitude, temperature, and oxygen tension. It is similarly true that the concept of tree is not a phylogenetic one, since trees are found among many plant families that also include shrubs and herbs. Further, there is no clear consensus as to whether the tree form is the advanced or primitive condition. Some paleobotanists suggest that trees are the most primitive members within these plant families. However, tree forms are found in all the vascular plants, from the club mosses and ferns to the gymnosperms and angiosperms. It is furthermore true that among the flowering plants trees are found not only among the most primitive members (Magnoliaceae) but also among the more specialized, or advanced, members, such as the rose family (Rosaceae). Consequently, from both a taxonomic and phylogenetic perspective, the tree is an artificial category. It is on an ecological basis that the tree can be recognized as a natural construct, representing an adaptive strategy by many different taxa to exploit and dominate the habitat above the ground. In the early stages of the development of terrestrial life, land plants were rootless and leafless; since they had their origins in aqueous environments, they did not require the specialized conducting and supporting tissues afforded by roots and stems, nor did they require localized regions of carbohydrate synthesis, since each cell was involved in metabolism, water and nutrient absorption, and respiration. Habitats farther from the water as well as aerial habitats represented available uninhabited environments. One key to exploiting these habitats is large size. This, however, requires physiological and morphological complexity. If all the tissues of massive tree trunks were alive, for example, the physiological cost of maintaining these structures in the living state would be enormous, and probably unattainable. The elegant solution came in the form of tremendous structural adaptations: new tissues and organs permitted localization of the functions of the plant body. The evolution of vascular tissues and localized regions of cell division (meristems) permitted the strengthening and conducting tissue, called wood, to be dead, hollow, thick-walled tubes at functional maturity. Roots provided anchorage and absorption of sufficient amounts of water and nutrients to support the huge biomass of the tree. Stems were not only strong enough to support the tree and project it into ever higher habitats but conductive enough to transport the water and nutrients to the leaves at the very top of the tree. The shape of a tree is an ecological construct as well, since its form is dependent on the habitat and the stresses of the environment. Open-grown trees, such as those in gardens and parks, generally have foliage extending along the length of the trunk (bole) for a considerable distance. Forest trees, on the other hand, compete for growing space and generally have an expanse of foliage-free bole below a more limited tree crown. The aggregate of the tree crowns constitutes the canopy of the forest, and this may be displayed in a single layer or stratified into several layers, depending on the number and kinds of trees that make up the forest. This article discusses the botanical and popular classifications of trees, their importance to humans, and their general structure and patterns of growth. For more information on specific plants, see fern, gymnosperm, and angiosperm. For general information on plants, see plant. woody plant that renews its growth every year (called a perennial); most plants classified as trees have a single self-supporting trunk containing woody tissues, and in most species the trunk produces secondary limbs, called branches. In modern plant-classification systems based on evolutionary and genetic relationships, trees do not constitute a group separate from shrubs, vines, and herbaceous plants. Instead, trees are among the plants in the botanical divisions and subdivisions of pteridophytes (which include tree ferns), gymnosperms (which include cycads, the ginkgo, and conifers), and angiosperms (which include all flowering plants). Gymnosperms and angiosperms bear seeds, whereas pteridophytes reproduce by means of spores. In angiosperms the seeds are enclosed within fruits. Most of the Earth's trees are angiosperms, of which there are two classes: monocotyledons, including palms, aloes, and yuccas; and the more numerous dicotyledons, including oaks, poplars, apples, and birches. Trees provide many valuable products, especially wood, which remains one of the world's chief building materials and fuels, and wood pulp, which is used in papermaking. Trees are also an important source of edible fruits and nuts. In addition, they help to clean the air by taking in carbon dioxide and releasing oxygen during photosynthesis (the process by which plants use the energy of sunlight to convert water and carbon dioxide into food). The root systems of trees help to conserve water and deter floods and soil erosion. Trees provide homes and food for a wide variety of animals, and they beautify both natural and man-made landscapes. The tallest trees in the world are the Pacific coast redwoods (Sequoia sempervirens), specimens of which are found in Redwood Creek valley, Calif. They reach heights of more than 110 m (360 feet). El Gigante, a Mexican swamp cypress (Taxodium mucronatum) in Tule, Oaxaca, holds the record for circumference: its base measures 46 m (150 feet). The oldest known living tree is a bristlecone pine (Pinus aristata) in Nevada; it is believed to be about 4,900 years old. Trees are part of the natural vegetation of regions that have an adequate growing season and receive at least 76 cm (30 inches) of rainfall a year. A few species have become adapted to desert and semidesert climates. Features such as branching, bark, leaves, flowers, fruit, and thorns, as well as colour, texture, and shape, vary among species. The reproductive organs of a mature tree are the flowers in angiosperms, the strobili (cones and related structures) in gymnosperms, and the spore-producing sporangia in pteridophytes. A tree develops when a male reproductive cell (sperm) fertilizes a female reproductive cell (egg). Among gymnosperms and angiosperms fertilization is preceded by pollination, in which sperm-bearing pollen grains are transferred to egg-bearing ovules by wind, insects, or other agents. Roots and stems (both of which remain for the life of a tree) and leaves make up the tree's nonreproductive organs. Roots anchor the tree and take in water and nutrients. Stems, which include trunks, branches, and branchlets, expose the reproductive parts to pollinating and dispersing agents and the leaves to light for photosynthesis. Stems also move water and nutrients from the roots to the sites of photosynthesis, and, once the food is manufactured, carry it back to storage and growth tissues. Tree growth occurs when new cells are formed or old ones are increased in size. The ends of stems and roots contain meristematic tissue, in which cell division produces new cells and thereby causes a tree to increase in height and length. Growth in diameter is caused by secondary meristems, or cambium layers, which lie between the wood and bark in conifers and dicotyledons, and are scantier in monocotyledons and cycads. A cross section of a conifer or dicotyledon reveals concentric rings that signify each year's growth. Treelike plants first appeared during the Devonian Period (from 408 to 360 million years ago). Early trees reproduced by spores, as do ferns today, and had no flowers or seeds. At the end of the Devonian, gymnosperms (plants whose seeds are not enclosed by a ripened ovary or fruit) appeared; these were the first seed plants. The warm, moist Carboniferous Period (from 360 to 286 million years ago) produced enormous trees and dense, lush forests. The decaying remains of these forests were transformed over time into vast coal deposits. Most of the plants of the Carboniferous forests died out during the colder, drier Permian Period (from 286 to 245 million years ago). Angiosperms appeared during the Early Cretaceous Period (144 to 97.5 million years ago). Additional reading Information on trees is included in general botanical reference works such as Liberty Hyde Bailey et al., Hortus Third: A Concise Dictionary of the Cultivated Plants in the United States and Canada (1976); and D.J. Mabberley, The Plant-Book (1987); and in the more specific works by Bayard Hora (ed.), The Oxford Encyclopedia of Trees of the World (1981); and United States Dept. Of Agriculture, Trees (1949), part of the department's Yearbook of Agriculture series. Tree types and distribution are presented in E. Lucy Braun, Deciduous Forests of Eastern North America (1950, reissued 1985); William B. Critchfield and Elbert L. Little, Jr., Geographic Distribution of the Pines of the World (1966); W. Dallimore and A. Bruce Jackson, A Handbook of Coniferae and Ginkgoaceae, 4th ed., rev. by S.G. Harrison (1967); Thomas S. Elias, The Complete Trees of North America: Field Guide and Natural History (1980); Samuel J. Record and Robert W. Hess, Timbers of the New World (1943, reissued 1972); and Frederic L. Steele, At Timberline: A Nature Guide to the Mountains of the Northeast (1982), on identifying New England Alpine flora and fauna. Discussions of the structure and growth of trees include F. Herbert Bormann and Graeme Berlyn (eds.), Age and Growth Rate of Tropical Trees (1981); William M. Harlow et al., Textbook of Dendrology: Covering the Important Forest Trees of the United States and Canada, 7th ed. (1991); Theodore T. Kozlowski, Paul J. Kramer, and Stephen G. Pallardy, The Physiological Ecology of Woody Plants (1991); Donald Culross Peattie, A Natural History of Western Trees (1953, reissued 1991), and a companion volume, A Natural History of Trees of Eastern and Central North America (1950, reissued 1991); W. Tranquillini, Physiological Ecology of the Alpine Timberline: Tree Existences at High Altitudes with Special Reference to the European Alps (1979); Brayton F. Wilson, The Growing Tree, rev. ed. (1984); and Martin H. Zimmermann, Claud L. Brown, and Melvin T. Tyree, Trees: Structure and Function (1971). A.J. Panshin and Carl De Zeeuw, Textbook of Wood Technology: Structure, Identification, Properties, and Uses of the Commercial Woods of the United States and Canada, 4th ed. (1980), focuses on economic aspects. Thomas H. Everett Lillian M. Weber Graeme Pierce Berlyn