A tree is generally defined as an erect \Tplant\t with a single woody stem capable of reaching heights of at least 6-8 m (20-25 ft) at maturity. Depending on environmental conditions and age, however, some tree species may exhibit growth forms characteristic of \Lshrub\ls and \Lvine\ls. Trees dominate the ecosystem because they grow to large sizes and live for many years. For example, the coast redwood, Sequoia sempervirens, attains the greatest height (112 m/368 ft) of any tree species. The Sierran redwood, Sequoiadendron giganteum, has the largest diameter, reaching 9 m (30 ft). Although relatively short, the bristlecone pine, Pinus aristata, is one of the oldest trees known, attaining the age of about 4,600 years, which far exceeds the life span of 100 to 250 years of most trees. CLASSIFICATION Trees are classified in a variety of ways but are most commonly divided into two groups on the basis of their reproductive structures. The \Lgymnosperm\ls (meaning naked seeds), evolutionarily the more primitive group, bear seeds on modified leaf structures called scales, which are often aggregated into cones. They include such well-known trees as the pines (Pinus), firs (Abies), spruces (Picea), and hemlocks (Tsuga). The gymnosperms are often referred to as \Lconifer\ls, \Levergreen\ls, needle-bearing trees, or softwoods. These designations may be misleading, however. Cycads (Cycas, Zamia) are classified as gymnosperms but have cone-like reproductive structures, not true cones, and hence are not conifers. Some gymnosperms, such as the ginko, Ginko biloba, have broad leaves, instead of needles. Not all gymnosperms are evergreen; the larches, Larix, for instance, shed their leaves during the winter. Also, some gymnosperms have \TWood\t of considerable hardness. The other major group of trees, the \Langiosperm\ls, the most highly evolved land plants, have flowers and bear their seeds enclosed in a \Tfruit\t, which is the ripened ovary of a \Tflower\t. Representative flowering trees include the oaks (Quercus), beeches (Fagus), poplars (Populus), birches (Betula), cherries (Prunus), tulip trees (Liriodendron), elms (Ulmus), and maples (Acer). Angiospermous trees are sometimes referred to as deciduous trees (those that lose their leaves for part of the year) or hardwoods. Not all angiospermous trees are deciduous, however; many species in the genus Eucalyptus are evergreen. Some flowering trees have wood that is relatively soft. While the scientific classification of trees is based upon the evolutionary relationships between species, genera, and families, relying heavily on the characteristics of the reproductive structures described above, leaf, fruit, bud, and bark characteristics are also useful in tree identification. STRUCTURE AND FUNCTION Trees have four major structural components: the stem, or trunk, including branches and twigs; leaves; roots; and reproductive structures. Stem The stem supports the tree and provides pathways for the upward transport of water and nutrients and the downward movement of carbohydrates. Very little of the stem actually consists of elements of living cells. Elements of the stem are arranged concentrically in cross-section. The outer \Tbark\t consists mostly of dead cells and protects the living tissue of the stem. The inner bark, or phloem, is composed of living cells through which various organic materials are transported throughout the stem and root system. A thin cambial layer, which consists of meristematic tissue, or undifferentiated cells that are capable of reproducing themselves, is located at the interface of the bark and woody stem. This layer, called the cambium, generates phloem to the outside and xylem (wood) to the inside. The xylem is differentiated into \Tsapwood\t and heartwood. The sapwood, located next to the cambium, consists primarily of dead cells that are used to transport water and nutrients up through the stem to the crown, or the upper portion of the stem, branches, and leaves. In the older trees the inner core of wood consists of heartwood, which is composed of dead cells that contain deposits of various organic and inorganic chemicals and is no longer functional in the movement of water. Leaves Leaves are the sites of food production in most trees. Chlorophyll makes the leaves appear green in color. Leaves come in a wide variety of sizes and shapes, from large compound structures (as in walnuts, Juglans, and ashes, Fraxinus) to needles (as in pines) and tiny scales (as in junipers, Juniperus). The life span of leaves also varies greatly from less than one year to over 20 years in some species. Leaves are important as sites for gaseous exchanges such as the absorption of carbon dioxide, which is the gas used to produced carbohydrates in \Tphotosynthesis\t, and transpiration (loss of water vapor), which is an important process in maintaining the flow of water from the root system to the crown of the tree. Leaves have small openings or pores, called \Tstomata\t, which are the sites for most of these exchanges. Many tree species have a high degree of control over the size of the the stomatal openings and can reduce water losses during times of moisture stress within the plant. Waxy structures on the surfaces of some leaves are one example of an adaptation that reduces generalized moisture loss. Roots The root system consists of several large, woody roots and an extensive network of fine roots. The large roots anchor the tree, and the fine roots carry out the critical function of absorbing water and nutrients from the soil. Water absorbed by the fine root system moves through the coarse roots, stem, and branches to the leaves. The fine root system typically consists of both small, nonwoody root hairs and \Lmycorrhiza\le. Mycorrhizae are complex structures that incorporate both fungal strands, or hyphae, and root hairs. These structures greatly increase the absorptive capabilities of the tree's root system, and the fungus receives carbohydrates and other complex organic molecules manufactured by the tree. Many fungal species form this symbiotic relationship with trees, and, in turn, most tree species benefit by having mycorrhizae (see \Tsymbiosis\t). Symbiotic nitrogen-fixing bacteria living in the roots of some trees, such as alder, Alnus, and some legumes provide usable nitrogen compounds to the host plants; the bacteria obtain food from the tree in return. The absorptive portion of the tree's root system typically must have a high rate of turnover (growth and mortality) in order to exploit the moisture and nutrient resources of the soil; hence, much photosynthate is required for maintenance of the roots. Reproductive Structures In most gymnosperms the seed-bearing structure is the cone-like strovbilus. Male, or pollen-producing, and female, or ovuliferious, strobili are borne on different branches of the same tree. In angiospermous trees, the ovary of the flower contains the ovules that become seeds following pollination and fertilization. Perfect flowers have both male and female parts; imperfect flowers are either male or female. Many trees, such as beeches, which have male and female flowers on the same plant, are monoecious. Dioecious trees, such as willows, Salix, have male and female flowers on separate individuals. GROWTH AND DEVELOPMENT Growth of a tree occurs from division of cells in the meristematic tissue in the cambium and the tips of the branches and roots. Cambial cell division results in an increase in tree diameter. The xylem cells added to the stem usually differentiate into thin-walled springwood and thick-walled summerwood that produce the annual rings visible in cross-sections of most tree trunks from temperate areas. The meristems associated with the crown produce elongation of the branches, resulting in increases in tree height. Trees elongate only at these growing tips; hence a point on a tree trunk, such as a forking of branches, will always remain at the same height above the ground. The life cycle of a tree begins with the development of a seed that may have been dispersed by wind or animals. When the seed begins to germinate, or grow, it becomes a seedling. Many tree seedlings are eaten at this state of development or die because of excessive heat or drought. Those that survive and grow into larger sizes will, in turn, begin to produce seed, typically after 10 to 30 years. Trees are potentially immortal since meristematic tissues are retained throughout the life of the tree. Death usually results from some environmental agent, such as fire, wind, lightning, drought, or human cutting, or from a biological cause, such as disease or insect attack. As a tree ages it may become more susceptible to insect pests or diseases, such as bark beetles or fungal root rots, which contribute directly or indirectly to its death. Most trees actually die of several causes. Insects or diseases introduced from outside of a species' natural range can cause very high levels of mortality, as exemplified by the white pine blister rust, chestnut blight, balsam woolly aphid, and Dutch elm disease; all of these diseases were introduced to North America from Eurasia by humans and have caused catastrophic death of affected tree species. Environmental pollution in the form of \Tacid rain\t can also seriously weaken or kill trees. ECOLOGY AND DISTRIBUTION Moisture, temperature, and nutrient conditions are the most important environmental factors affecting the establishment and growth of tree species. Forests are widely distributed in the temperate and tropical regions of the world and are a reflection of favorable moisture and temperature regimes in these areas (see \Tforests and forestry\t). Some notable forest regions in North America are the pine forests of the southeastern states, the deciduous hardwood forests of the northeastern and Great Lakes region, the mixed coniferous forests of the Rocky Mountains and Sierra Nevada, and the dense coniferous forests of the Pacific Coast. Two of the most widespread forest formations in the world are the taiga, a coniferous evergreen forest of boreal and subarctic regions, and the pine-juniper savannas of dry regions (see \Ttaiga climate\t: \Tsavanna life\t). Africa, South and Central America, and southeast Asia are sites of extensive tropical rainforests that are being rapidly logged, partially for agriculture (see \TJungle\t and RAINFOREST). Tree species tolerate different environmental conditions. Trees grow more slowly, attain smaller dimensions, and are often widely spaced in cold or arid regions. Cold temperatures, short growing seasons, and heavy snows prevent the growth of trees at high elevations and high latitudes. Moisture stress typically limits tree growth at lower timberlines, such as those adjacent to grasslands or deserts. Ecological differences among trees result in changes in forest composition across a landscape. IMPORTANCE Trees are valuable to mankind for many products and amenities. Many trees are major sources of food, primarily as fruits and \Tnuts\t; also, sugar is derived from the sap of some trees. Wood is a major source of fuel for heating and cooking, particularly in developing counties. Construction materials from wood include \Tlumber\t, \Tplywood\t, and particle board. Wood is the major source of fiber for the production of pulp and \Tpaper\t. Trees are a primary or secondary source for many chemical products. Some fibers, such as \Trayon\t, are produced from wood pulp. The bark of some tree species is the major source of tannins. Other chemicals are harvested directly from living trees, such as \Trubber\t and various resins, which are then refined to such products as \Tturpentine\t. Trees also provide numerous services. They protect soils from erosion and help maintain high quality water supplies. Tree root systems make a major contribution to soil stability. Living trees create valuable wildlife habitats. Standing dead trees, often called snags, also serve as animal habitats. Downed trees are important in conserving and cycling nutrients, in reducing soil erosion, as wildlife habitats, and as nursing sites for establishment of other plants. Trees create shelterbelts in agricultural regions and attractive and effective barriers in urban areas, and also contribute aesthetically to many natural and domesticated landscapes. Jerry F. Franklin Bibliography: Archer, R. R., Growth Stresses and Strains in Trees (1986); Clapham, A. R., The Oxford Book of Trees (1975); Davis, B., The Gardener's Illustrated Encyclopedia of Trees and Shrubs (1987); Harlow, W. M., Textbook of Dendrology, 6th ed. (1979); Kozlowski, T. T., Tree Growth and Environmental Stresses (1979); Mitchell, A., The Trees of North America (1987); Walker, L. C., Trees: An Introduction to Trees and Forest Ecology for the Amateur Naturalist (1984); Zimmerman, M. H., and Brown, C. L., Trees: Structure and Function (1975).