Tree - biology. I INTRODUCTION Tree, woody plant with a distinct main stem, or trunk. At maturity, trees are usually the tallest of plants, and their height and single main stem differentiate them from shrubs, which are shorter and have many stems. Trees are perennials, plants that live for at least three years. Some species of tree only grow to 4 m (13 ft) in height, but the tallest species may reach heights of more than 112 m (more than 367 ft). The General Sherman Tree, a giant sequoia in California's Sequoia National Park, has a height of 84 m (275 ft) and a diameter of 11 m (37 ft). The largest trees, however, are not necessarily the oldest. For example, the bristlecone pine grows to a height of only about 9 m (about 30 ft), but one specimen has been dated as at least 4,600 years old. Trees grow throughout the world, from the extreme cold regions near the Arctic and the Antarctic to the hot tropical regions around the equator. They grow in both good and poor soil, in deserts and swamps, along shores, and at mountain elevations of several thousand feet. Although trees may grow singly, under natural conditions they more often grow in stands, which consist either of one species or of a mixture of species. A forest is a plant community made up of the trees, shrubs, and herbs that cover an area. Throughout much of North America, forests include only a few species of trees. In tropical forests, however, large numbers of different species can be found in very small areas. A survey of a 1-hectare (2.5-acre) plot in the Brazilian rain forest determined that it contained 476 tree species. II IMPORTANCE OF TREES Perhaps the most important ecological function of trees is protecting the land against erosion, the wearing away of topsoil due to wind and water. The trunks and branches of trees provide protection from the wind, and tree roots help solidify soil in times of heavy rain. In addition, trees and forests store water reserves that act as buffers for the ecosystem during periods of drought. In many areas the removal of forests has resulted in costly floods and subsequent droughts. Trees and forests also provide habitat, protection, and food for many plant and animal species. In addition, they play an important role in global climate and atmosphere regulation--the leaves of trees absorb carbon dioxide in the air and produce oxygen that is necessary for life. Trees have many economic uses. Lumber from trees is the most widely used material in the building of homes and other structures. Many trees yield edible fruits and nuts such as oranges, grapefruits, apples, avocados, peaches, pecans, hickory nuts, and almonds. Trees and their fruits are also the source of many commercial waxes and oils, including olive oil and coconut oil. Tree trunks are tapped for sap, which is used in making such products as maple syrup, rubber, and turpentine. The barks of certain trees are sources of cork and spices. Many trees yield important medicines, such as quinine. The bark of the yew tree is the source of the drug taxol, which in 1992 was approved for treating ovarian cancer. Chemical materials produced by trees are used in tanning leather and in the manufacture of inks, medicines, dyes, and wood alcohol. In addition, trees are used in landscaping homes, parks, and highways. In regions with extreme climates, they serve as windbreaks or as shade against the sun. III TYPES OF TREES There are two general types of trees, angiosperms and gymnosperms. Angiosperms are flowering plants in which the ovule, or seed, is encased in a protective ovary. With about 235,000 species, the angiosperm division is the largest and most diverse plant group. It is divided into dicots, plants with two cotyledons (seed leaf structures), that include the familiar broadleaf trees such as maple and oak; and the monocots, plants with one cotyledon that include the palms and lily trees. Unlike angiosperms, gymnosperms (Greek for "naked seed") do not bear flowers. Their seeds lie exposed in structures such as cones or fleshy cups called arils. The group includes about 500 tree species--including three major types, needle-leaf trees (or conifers), ginkgos, and cycads. A Angiosperms As angiosperms, all dicots produce flowers, and their seeds are always enclosed in a protective covering. In temperate regions, most of these broadleaf trees are deciduous, meaning that they shed their foliage each year. Some nontropical broadleaf trees, however, such as certain magnolias and hollies, are evergreen--that is, they retain foliage throughout the year. In warmer parts of the United States and in the tropics, most broadleaf trees are evergreen. Broadleaf trees include virtually all of the familiar trees and shrubs that are known to flower, such as the elm, chestnut, alder, birch, and willow, as well as tropical species such as teak and mahogany. The most recognizable trees in the monocot angiosperm class are the palm trees. There are about 2500 species of palms, including the coconut palm and date palm. Most palms lack branches and instead have enormous compound leaves (leaves composed of leaflets arranged along a central stem). Lily trees are another member of the monocot class. These trees are closely related to the familiar garden lily and include yucca trees such as the odd-looking Joshua tree, which is found in the deserts of the southwestern United States. B Gymnosperms The needle-leaf trees, or conifers, include such trees as the pine, spruce, fir, yew, redwood, and cypress. The seeds of these gymnosperms are usually produced in cones at the base of protective scales. As their name implies, needle-leaf trees have slender needle-shaped leaves. Because of their small surface area and structure, needle leaves are not seriously injured by very low temperatures or by severe droughts. For this reason, the forests of cold far northern latitudes and high-altitude arid regions consist mostly of needle-leaf trees. Most needle-leaf trees are evergreens and retain most of their foliage throughout the year. Deciduous members of the group include the bald cypress and the larch. The ginkgo, or maidenhair tree, is the sole living survivor of the ancient division of ginkgo gymnosperms. The ginkgo was abundant in the Mesozoic era (about 240 million to 65 million years ago). Virtually unchanged since that time, this tree is recognized by its fan-shaped leaves and the forking vein patterns on its leaves. A deciduous tree, the ginkgo grows slowly but may attain heights of more than 30 m (more than 98 ft). Ginkgos are native to Asia, where they have been preserved as sacred trees in Chinese temples since antiquity, but they are also cultivated in Europe and the United States. Cycads are palmlike gymnosperms found mainly in tropical and subtropical regions of Africa, Asia, and Central America. There are about 100 species, with only one native to the United States. The trunk has no branches, and the long leaves are clustered near the top. Cycads are evergreen and may grow to heights of more than 18 m (more than 59 ft). They first appeared 285 million years ago and were the dominant plant type during the age of the dinosaurs, which roughly paralleled the Mesozoic era. Because the cycad is a popular ornamental tree, most cycad species are in danger of extinction because so many of them have been collected from the wild. IV MAJOR PARTS OF A TREE The major parts of a tree are its roots, trunk, leaves, flowers, and seeds. These components play vital roles in a tree's growth, development, and reproduction. A Roots Trees are held in place by anchoring organs called roots. In addition to anchoring the tree, roots also absorb water and minerals through tiny structures called root hairs. From the roots the water and mineral nutrients are carried upward through the wood cells to the leaves. Although the internal structure of most kinds of roots is similar, there are often external differences. Pines, for example, have a strongly developed taproot, or main root, in addition to branching side roots. In maples, on the other hand, there is little or no central taproot, and the other roots are produced in great numbers near the surface of the soil. Roots grow constantly, and at the growing tip of each root is a region called the meristem, which is composed of special rapidly dividing cells. Just behind the meristem the cells become elongated, and farther from the tip the cells become differentiated into various kinds of plant tissue. In rapidly growing roots the root tip is covered by a root cap, a protective coat of loose cells that are constantly being rubbed off and replaced as the root grows. B Trunk Bark is the outer protective covering of tree trunks. Because bark varies so widely in color, texture, and thickness, its characteristics provide one of the most important means of identifying species of trees. Most of the total thickness of bark consists of outer bark, which is made up of dead cells. Outer bark may be very thick, as in the cork oak, or quite thin, as in young birches and maples. Openings in the outer bark allow the movement of carbon dioxide and oxygen to and from the inner tissues. The inner bark layer, called the phloem, consists of a thin layer of living cells. These cells act together to transport food in the form of sugars, which are made in the tree's leaves, through the trunk and stems to other parts of the tree. Phloem cells have thin walls, and their living contents are so interconnected that the sugar solutions can pass easily and rapidly from one end of the plant to the other. As old layers of outer bark are sloughed off, new ones are constantly being added from the inside, where new phloem is always being created. Most of a tree trunk is occupied by the wood, or xylem layer, which consists almost entirely of dead cells. The living xylem cells, however, act as the tree's plumbing system by transporting water and dissolved food through the trunk and stems. A layer of cells called the cambium separates the living xylem cells from the phloem. As the tree grows and develops, the cambium forms new phloem and xylem cells. The layers of xylem cells form rings; these rings can be counted to determine the age of the tree in areas with distinct growing seasons. In young trees the center of the woody column, inside the xylem, consists of soft thin-walled cells called the pith. The pith serves as a storage tissue for sugars and later as a reservoir for wastes. In older trees the pith is crushed by the xylem's woody tissue, and wastes are simply deposited in the wood cells near the center of the trunk. As a result, in some trees the cells within the pith become dark in color and form what is often called the heartwood. The lighter cells around them make up the sapwood. In monocot trees, such as palms, the xylem and phloem tissues are grouped into bundles, which are scattered through the mass of pith that makes up most of the tree's internal structure. In the stems of such trees, the yearly growth rings are not apparent, as they are in oaks, maples, and other trees of temperate regions. C Leaves In trees, as in other green plants, the principal function of the leaves is the manufacture of sugars by the process of photosynthesis. In this process, sugars are formed when carbon dioxide (from the air) and water (from the leaf cells) are combined in the presence of light and the green pigment chlorophyll. Oxygen is produced as a byproduct. Some of the newly formed sugar is used by the leaf cells for energy, but most is carried to other parts of the tree to provide energy for growth and development in those areas. The leaves are also the chief organs involved in the loss of water from the plant, called transpiration. Many of the tree's tissues cannot function without a constant supply of water, and water is necessary to prevent overheating or wilting of the leaves. Transpiration is responsible for the movement of water from the roots of the tree up to the top. As water is lost through the leaves, water that enters the roots is pulled upward through the xylem tissue to replace the lost moisture, ensuring a constant circulation of water through the tissues of the tree. D Flowers All angiosperms bear flowers, the trees' reproductive structures. In some trees, such as dogwoods, cherries, and some magnolias, the flowers are large and colorful. Oaks, willows, and other temperate forest trees, on the other hand, often bear small, pale, and inconspicuous flowers. In maples and many other trees the male and female reproductive parts are carried in separate flowers on the same tree. This arrangement is known as monoecism, and such trees are called monoecious. In oaks, for example, the male pollen-producing flowers are borne in long hanging tassels, and the short-stalked or stalkless female flowers are located on the twigs. In some trees, such as the hollies and willows, the male and female flowers are borne on separate trees. This is known as dioecism, and these trees are called dioecious. In needle-leaf trees the male and female reproductive structures may be produced in cones rather than flowers, and in such cases there are always separate male and female cones. The cones are often produced on the same tree, but frequently there are more female cones in the upper branches. In all reproductive arrangements, the flowers or female cones are fertilized when wind, birds, bats, mammals, or insects carry pollen from the male structures to the female ones. E Seeds Seeds, the ripened ovules of the plant that are capable of germination, are the product of fertilized flowers and are distributed in various ways. In pines, for example, each seed is surrounded by a winglike structure. As the winged seed falls from the cone, it floats down to the ground, riding air currents. Oak seeds are enclosed in acorns, which are either planted by squirrels or merely fall to the ground near the parent tree. Willow trees produce thin-walled, flask-shaped fruits that burst open, releasing the seeds. Each seed has a tuft of downy fibers, which enables it to be picked up by air currents and travel for considerable distances. Seeds of other tree species are dispersed by water, mammals, birds, and ants. V TREE GROWTH AND DEVELOPMENT Trees grow in both height and diameter. A tree increases in height through the elongation of its trunk or branches and the seasonal production of new branches. The growth of the tree in diameter results from the activity of the cambium, the special cells lying at the inner surface of the bark. A Height All branches arise from buds, which are located at the tips of twigs and at the bases of leaves. Within each bud is a meristematic region, or growing zone, similar to that in the root tip. The growing tip is surrounded by many tiny leaves, and the entire bud is covered with a series of protective scales. The cells in the growing tip, like those of the root tip, divide rapidly, elongate, and become differentiated into the various kinds of stem tissue. B Diameter The part of the bark that causes the tree to grow in diameter is the cambium. During the growing season the cells of the cambium, like those of the stem tip and root tip, divide rapidly. They produce xylem, or wood cells, toward the center of the trunk, and they produce phloem, or food-conducting cells, toward the bark. At places the cambium also produces rows of thin-walled cells that run horizontally through the xylem and the phloem. Known as rays, these horizontal cells conduct and store water and nutrients. Among tropical woody plants, growth continues throughout the year. In trees of temperate regions, however, the annual growth is usually crowded into a short period of a few weeks in early spring. When the stem of a tree is cut fully across, one can observe each year's growth as a ring. By counting these rings, it is possible to determine the age of the tree as well as the growing conditions during its lifetime. In favorable conditions, when the tree experiences rapid growth, it forms wide rings, but in difficult times, when growth is slower, it forms narrower rings. Each annual ring consists of two parts. One part, which makes up most of the ring, is light-colored wood composed of relatively large cells. These cells were formed during the spring, when growing conditions were best. The second part of the ring is darker and is composed of small thick-walled cells, which were formed during the less favorable months. C Growth Rates The rate at which trees grow varies considerably. Young bamboo stems may increase at the rate of many centimeters a day until they eventually reach a height of 36 m (118 ft) with a diameter of more than 30 cm (more than 1 ft). Many needle-leaf trees, such as the longleaf pine, grow at a much faster rate than broadleaf trees like walnuts and oaks, especially in their early years. Some broadleaf trees, however, such as black locusts and cottonwoods, may grow rapidly in suitable soils. VI PLANTING AND CARING FOR TREES Whether trees are being planted for reforestation, ornament, shade, or fruit, the first step is selecting the species to be grown. The choice depends on such factors as the characteristics of the soil, the location of the site, and drainage. For example, sycamore and cottonwood trees will not grow on dry exposed slopes or ridges, or in fields with a thin topsoil over a heavy compact subsoil. Walnut trees will not grow in swampy places, and jack pines grow especially well on loose sandy soils with good drainage. A good rule of thumb is to plant native trees--trees that have demonstrated their ability to thrive in the local environment without harming other local species. The hole for a seedling should be deep enough to hold the fully expanded root system of the seedling. Larger plants should be placed in a hole 60 cm (2 ft) deep with a diameter 60 cm greater than that of the ball of the roots. In poor soil the hole should be 1.8 m (6 ft) wide and 60 cm deep for a 2.5-m (8-ft) tree, and proportionately wider for taller trees. After the tree is placed in the hole, the soil should be firmly pressed around the roots, and the ground should be thoroughly soaked with water. Mixing bone meal or well-rotted manure into the soil will help the tree become established quickly. Most deciduous trees should be planted in the fall when they are not growing, but evergreens are usually planted in the spring, at the beginning or middle of their period of vigorous growth. A Watering After planting, the soil around a tree should be kept moist, but not soaked. If artificial watering is not practical, a layer of mulch 7 to 15 cm (3 to 6 in) deep should be placed around the tree to conserve moisture and to discourage the growth of weeds. Because a transplanted tree does not adequately absorb water through its damaged roots, it is important to prevent water loss from the plant by pruning top limbs to limit transpiration. B Fertilizing In good soils it is less important to fertilize than in poor soils. However, all trees grow better and faster and are less likely to become diseased if fertilizer is supplied in the proper amounts. This may be done most easily by placing a large handful of fertilizer in holes made by a crowbar at the edge of the spread of the tree's limbs. The holes should be about 60 cm (2 ft) deep and about 5 cm (2 in) in diameter, and they should be spaced about 90 cm (3 ft) apart. After the fertilizer has been introduced, the holes should be filled in with soil. Organic fertilizers such as manure and mulch are preferable to chemicals that may replace or destroy natural organisms in the soil. C Pruning Pruning of ornamental trees maintains the form of the tree, removes weak or sickly branches, and rejuvenates old or unhealthy plants. If performed during a period of vigorous growth, pruning often also results in an increased production of flowers. In pruning small branches, cuts are made just above the buds that point in the direction branches are desired. When it is necessary to remove a large branch, cuts are made just in front of a swollen area at the base of the branch called the branch collar. Protective chemicals contained in the branch collar prevent insect infestation and decay from harming the tree after a large branch has been removed. VII UNUSUAL TREES There are many types of unusual trees around the world that exhibit a variety of interesting characteristics. Among these are prehistoric relics, trees that exhibit bizarre growth patterns or formations, and trees with interesting strategies for obtaining the air, water, and nutrients necessary for growth. Like the ginkgo, the dawn redwood is old enough to be considered a living fossil. Its fossilized remains were studied and identified in the 1800s, and scientists believed that the tree had become extinct about 20 million years ago. In 1941, however, a Chinese botanist discovered living trees in an isolated valley in central China. Since then, the tree has been found in other places in China, and the fossil record indicates that millions of years ago it grew in Greenland and in North America as far south as California. Like other sequoias, the dawn redwood grows well from seeds, and seedlings have been planted throughout the Pacific Northwest and in Alaska. Specimen trees have also been successfully grown in many of the milder parts of the East Coast of the United States. Unlike other sequoias, the dawn redwood is deciduous and loses its leaves in the fall. Although the banyan tree begins life with a single trunk, aerial roots grow down from its spreading limbs and take root. These roots enlarge, eventually becoming trunklike, and in old age a single tree may have the appearance of a small forest. The largest banyan, found on the island of Sri Lanka, has 350 major trunks and several thousand smaller ones. The name of the tree comes from a Hindu word for trader, because in many regions where banyans are found, traders and merchants use the tree's shade while displaying their wares. The baobab tree, the closely related bottle tree, and certain cactus trees have trunks with fleshy centers. These trunks store large amounts of water, enabling the trees to survive the arid conditions under which they grow. The trunk of the baobab may measure 9 m (30 ft) in diameter. The bottle tree has a bulging bottle-shaped trunk. Cacti, although frequently covered with spines, can yield water to a lost hiker even in the driest desert. Although it is native to Africa, the sausage tree is cultivated as an ornamental in warm areas such as southern Florida and Hawaii. The sausage tree has large deep red flowers, which are replaced eventually by giant sausage-shaped fruits about 60 cm long. Both the flowers and fruits are borne on long stems hanging from the limbs of the tree. The pencil tree, a native of Asia, is grown as an ornamental in Hawaii and elsewhere because of its odd twigs, which are swollen, pencil-like, and bright green. The twigs serve the tree in place of leaves, which are almost totally lacking. The pencil tree has strongly caustic sap. On contact with the skin the sap causes a rash, and it may even cause temporary or permanent blindness if it touches the eye. Bald cypress trees, deciduous needle-leaf trees that grow in swampy areas, are unusual because of special breathing organs known as cypress knees. These organs are extensions from the root system that project above the usual water level and absorb oxygen from the air for use by the roots. Strangler fig trees are an example of a parasitic method of development that occurs in several plant families. A seed is deposited by a bird, monkey, or other animal in a depression or a crotch of a limb. The seed sprouts and begins to develop, and roots eventually find their way to the ground, sometimes traveling 25 m (82 ft) or more. There the roots take hold and begin to enlarge toward each other, enclosing the trunk of the host tree. Finally, the roots join side by side to restrict the growth of the original tree, which eventually rots away, leaving the parasite standing as an independent plant. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.
