Tree

Tree taxonomy

History of taxonomy

Modern taxonomy

Secondary growth

Cell layers in a tree trunk

Growth rings

Evolution

Forests

Ascent of sap

Cohesion-tension theory

Economic significance

Resources

A tree is a woody plant which has three principle characteristics: (a) the potential to grow to 20 ft (6.1 m) or more in height; (b) the formation of one or more trunks arising from the ground; and (c) the ability to stand on its own without support. Trees provide many products which are important to humans, such as timber, fruits, and nuts. They are also the dominant plants in the world’s forests, and thus provide critical habitats for the other species which live there.

Tree taxonomy

Taxonomy is the identification and classification of organisms. Dendrology is the identification and classification of trees, shrubs, and vines, and is a subdiscipline of taxonomy. Shrubs and vines are also woody plants. Shrubs are shorter than trees and have multiple stems arising from the ground. Vines generally rely upon another plant or other structure for physical support.

Some plant families consist entirely of trees and other types of woody plants. Other plant families have some species that are woody, and some that are herbaceous. This indicates that arborescence, the character of being tree-like, is not a reliable character for distinguishing families and higher taxonomic groups of plants.

History of taxonomy

Carl von Linnè of Sweden began the modern study of taxonomy in the mid-1700s. He classified trees and other plants according to the morphology of their reproductive structures, such as the flowers and fruits of Angiosperms, and the cones of Gymnosperms. Many religious leaders of his time considered it immoral to study the reproductive structures of plants. However, modern taxonomists still rely upon plants’ reproductive structures for conclusive identification of species.

Carl Von Linneè also advocated that all scientists refer to trees and other organisms by a Latinized name. He even Latinized his own name to Carolus Linneaus. Modern biologists continue to follow this convention. Thus, the tree which Americans call the white pine is known to biologists throughout the world as Pinus strobus, where “Pinus” is the pine genus and “strobus” is the specific epithet. There are about 90 other species of pine in the world, including red pine (Pinus resinosa), sugar pine (P. lambertiana ), and pitch pine (P. rigida ). Charles Darwin’s studies of evolution in the mid-1800s led taxonomists to group organisms hierarchically, according to their evolutionary relationships. For example, the pines (genus, Pinus ), spruces (genus, Picea ), and about seven other genera are grouped together in the family Pinaceae, because they are evolutionarily related. In turn, the Pineaceae and about six other families are grouped together in the order Coniferales (the conifers, or cone-bearing plants) because they are evolutionarily related.

Modern taxonomy

Traditionally, plant taxonomists have relied upon the morphology of plants’ reproductive structures to determine their evolutionary relationships. More recently, they have also used biochemical characteristics, DNA sequences, and additional features. Occasionally, there are disagreements about the relationships of different plant species. These disagreements are important in stimulating further research.

In identifying a tree in nature, dendrologists do not always rely upon its reproductive structures, because these are often only available for a brief time of the year. In practice, they typically rely upon features of a tree’s leaves, twigs, bark, wood, habit (general shape and appearance), and habitat as clues for identification.

Secondary growth

Most trees increase in thickness due to cell division in two special layers of undifferentiated tissues near the outside of their stems. This is known as secondary growth. The two tissues are referred to as the vascular cambium and the cork cambium. In contrast, herbs do not have secondary growth, and they stop growing once their primary tissues have matured.

Cell layers in a tree trunk

In a typical, sawed-off sector of a tree trunk, one encounters layers of different cells and a series of concentric, annual growth rings going from the outside toward the inside.

The cork of bark is on the external surface of the trunk, and consists of dead cells which are impregnated with suberin, a waxy substance which inhibits evaporation of water through the bark. The cork cambium lies just inside the cork. It produces cork cells on its outside face and secondary cortex on its inside face. Growth and division of the cork cambium differs among tree species, and this gives the bark of each species its own characteristic appearance.

Phloem cells lie just inside the secondary cortex. Phloem cells are elongated cells specialized for the transport of plant nutrients, such as the carbohydrates made during photosynthesis. The vascular cambium lies just inside the phloem cells. It produces phloem cells on its outside face, and xylem cells on its inside surface. Xylem cells are elongated cells specialized for transport of water and dissolved ions throughout the tree. Trees growing in places with a strongly seasonal climate typically contain thick layers of xylem cells with readily apparent, concentric growth rings. This thick layer of functional xylem cells is referred to as the sapwood.

Finally, the heartwood is in the center of the tree. This layer is typically darker than the sapwood and consists of dead cells that are very stiff and serve to strengthen the tree. The heartwood may also have readily apparent growth rings.

Growth rings

In most trees growing in a strongly seasonal climate, the vascular cambium produces wide, thin-walled cells in the spring, narrow thick-walled cells in the summer, and few or no cells in the autumn and winter. This seasonal regularity of cell production results in the formation of annual growth rings. The bristle-cone pine (Pinus aristata) is the world’s longest-lived tree species, and one specimen of this species has about 5,000 growth rings, indicating it is at least 5,000 years old.

Within a given growth ring, the large cells of springwood and the small cells of summerwood are often readily discernible with the naked eye. Light, temperature, soil moisture and other environmental factors affect the growth of trees, and therefore the width of their growth rings.

Evolution

most botanists believe that the first land plants were herbaceous. The first woody plants were probably Lycopsids, free-sporing plants which had narrow, tubular grass like leaves. Numerous Lycopsid fossils have been dated to the middle of the Upper Devonian period, more than 370 million years ago. The first known plant with a vascular cambium which exhibited true secondary growth was a species of Protopteridium, a free-sporing plant dated to about 370 million years ago.

Very few modern, free-sporing plants are arborescent. The only living relatives of the Lycopsids are the club mosses (Lycopodophyta), a group of simple, herbaceous, free-sporing plants.

Forests

Trees are the dominant organisms of forests. Climate and other factors determine which tree species grow in a forest. Forest ecologists have classified the forests of the world according to the species of trees that grow there. One classification scheme is described below.

Coniferous forests are characteristic of the boreal forests of cold regions of the northern hemisphere. The boreal forest is found in Canada, southern Alaska, northwestern America, northern Europe, and northern Russia. The dominant trees are conifers (cone-bearing plants) such as pines, spruces, firs, and larches. Many of these trees are important sources of wood used in construction, and of pulp for paper.

Broad-leaf forests are characteristic of temperate zones of central and eastern North America, central and southern Europe, and central Asia. Its dominant trees have broad leaves and are deciduous, in that they shed their leaves once a year. Oaks, hickories, maples, and sycamores are some of the trees commonly found in this forest type. Many broad-leaf trees are hardwoods, and are used for making some of our finest furniture.

Mediterranean forests are characteristic of regions with hot, dry summers and warm, humid winters. This forest type is found in southern California, northwestern Mexico, southern Europe, northern Africa, and southern Australia. The trees which grown in Mediterranean forests are adapted to minimizing water loss. Evergreen oaks and pines are well-known trees of this forest type. In Australia, about 600 different species of Eucalyptus grow in the Mediterranean forest type along the southern coast.

Savannas are characteristic of tropical regions with a very seasonal rainfall. The trees in a savanna are sparsely distributed and are specially adapted to minimize water loss. Savannas are found in northern Mexico, equatorial Asia, central Africa, and northern Australia. Acacias, Dracaenas, and the Baobab are well-known savanna trees.

Tropical rainforests are characteristic of regions with a relatively warm climate and a great deal of rainfall. This forest type is found in Central America, northern and central South America, central and western Africa, and the south Pacific region. Tropical rain forests have a great diversity of plant and animal species. The tropical rainforests of the world are currently threatened by people who are cutting down their trees at an increasing rate. At the same time, conservationists and tropical ecologists devote much effort to the preservation of tropical rainforests with their rich diversity of species.

Ascent of sap

It is vitally important for a tree to transport water from the soil to its upper-most leaves. This process has long fascinated plant physiologists, and has been studied for centuries. How does water move from the roots to the uppermost leaves of tall trees? The significance of this problem is best appreciated by considering the height of some of the world’s tallest trees. The world’s tallest tree was a Eucalyptus regnans of Australia, which was measured at 470 ft (143 m) in 1880. The tallest living tree is a coastal redwood (Sequoia sempervirens) of California, which is about 365 ft (111.3 m) in height. Its relative, the giant sequoia (Sequoidendron giganteum), is not quite as tall, but is the world’s most massive tree species.

Cohesion-tension theory

Most plant physiologists now accept the “cohesion-tension theory” as an explanation for the ascent of sap. According to this theory, water moves up the trunk of a tree in narrow, elongated cells near the periphery of the trunk, referred to as the xylem, and does not require the expenditure of metabolic energy. The movement of water only depends upon three important physical-chemical properties of water.

The first important property of water is that it always moves from a region with a more positive water potential, to a region with a more negative potential. Water potential is a measure of the energy available in a solution of water. Thus, water moves out of the leaves and into the air because the water potential of the air is more negative; water moves out of the tree trunk and into the leaves because the water potential of the leaves is more negative; water moves out of the roots and into the trunk because the water potential of the trunk is more negative; and water moves out of the soil and into the roots because the water potential of the roots is more negative.

The second important physical-chemical property of water is that it is a cohesive molecule. In other words, water molecules tend to bind to one another through the formation of hydrogen bonds. The cohesiveness of water molecules gives the thin water columns in a tree trunk a very great tensile strength. This prevents breakage of the water column when great longitudinal stresses are placed upon it as it is pulled out of the leaves and into the air.

The third important property of water is that it adheres very tightly to the walls of xylem cells in the tree’s transport pathway. Adhesion of water to these cell walls maintains the full hydration of the pathway for water transport. This prevents breakage of the water column, and allows water transport even when a tree is water-stressed in a dry environment.

Economic significance

Trees have great economic significance to humans as a source of food, building materials, and paper. Almond, coconut, cherry, prune, peach, pear, and many other tree species are grown in orchards for their fruits and nuts. The apple tree is the orchard tree of greatest economic significance, and there are several hundred different varieties of apples. (Many of North America’s best apples grow in New York state and Washington state.) Many trees are also useful for the wood they produce. Wood is used as a construction material and to make furniture. Wood is a valuable construction material because it is relatively inexpensive, easy to cut, and very strong relative to its weight. Many species of pines and other conifers are important sources of softwoods, and many broad-leaf trees are important sources of hardwoods.

KEY TERMS

Adhesion —Physical attraction between different types of molecules.

Cohesion —Physical attraction between molecules of the same type.

Cork cambium —Undifferentiated plant tissue which gives rise to cork cells and secondary cortex.

Dendrology —Identification and classification of woody plants.

Phloem —Plant tissue consisting of elongated cells which function in the transport of carbohydrates and other nutrients.

Vascular cambium —Undifferentiated plant tissue which gives rise to phloem and xylem.

Xylem —Plant tissue that transports water and minerals upward from the roots.

There are frequent conflicts between conservationists and loggers. In the United States, the coniferous rainforest of the Pacific northwest has been the site of one such conflict. In this region, loggers want to harvest coniferous trees from the forest just as they have done for many years, whereas conservationists seek to preserve the forest because it provides a habitat for the northern spotted owl, an endangered species.

See also Basswood; Citrus trees; Conifer; Deforestation; Dogwood tree; Ebony; Forestry; Horse chestnut; Magnolia; Mahogany; Mangrove tree; Nux vomica tree; Palms; Sapodilla tree; Screwpines; Spruce; Walnut family; Yew.

Resources

BOOKS

White, John, and David More. Illustrated Encyclopedia of Trees. Portland, OR: Timber Press, 2001.

PERIODICALS

Chaw, S. M., et al. “Seed Plant Phylogeny Inferred from All Three Plant Genomes: Monophyly of Extant Gymnosperms and Origin of Gnetales from Conifers.” Proceedings of the National Academy of Sciences of the United States of America. 97 (2000): 4086-4091.

Peter A. Ensminger

Tree

A tree is a woody plant which has three principle characteristics: (a) the potential to grow to 20 ft (6.1 m) or more in height; (b) the formation of one or more trunks arising from the ground; and (c) the ability to stand on its own without support. Trees provide many products which are important to humans, such as timber, fruits , and nuts. They are also the dominant plants in the world's forests , and thus provide critical habitats for the other species which live there.

Tree taxonomy

Taxonomy is the identification and classification of organisms. Dendrology is the identification and classification of trees, shrubs, and vines, and is a subdiscipline of taxonomy. Shrubs and vines are also woody plants. Shrubs are shorter than trees and have multiple stems arising from the ground. Vines generally rely upon another plant or other structure for physical support.

Some plant families consist entirely of trees and other types of woody plants. Other plant families have some species that are woody, and some that are herbaceous. This indicates that arborescence, the character of being tree-like, is not a reliable character for distinguishing families and higher taxonomic groups of plants.

History of taxonomy

Carl von Linné of Sweden began the modern study of taxonomy in the mid-1700s. He classified trees and other plants according to the morphology of their reproductive structures, such as the flowers and fruits of Angiosperms, and the cones of Gymnosperms. Many religious leaders of his time considered it immoral to study the reproductive structures of plants. However, modern taxonomists still rely upon plants' reproductive structures for conclusive identification of species.

Carl Von Linné also advocated that all scientists refer to trees and other organisms by a Latinized name. He even Latinized his own name to Carolus Linneaus. Modern biologists continue to follow this convention. Thus, the tree which Americans call the white pine is known to biologists throughout the world as Pinus strobus, where "Pinus" is the pine genus and "strobus" is the specific epithet. There are about 90 other species of pine in the world, including red pine (Pinus resinosa), sugar pine (P. lambertiana), and pitch pine (P. rigida). Charles Darwin's studies of evolution in the mid-1800s led taxonomists to group organisms hierarchically, according to their evolutionary relationships. For example, the pines (genus, Pinus), spruces (genus, Picea), and about seven other genera are grouped together in the family Pinaceae, because they are evolutionarily related. In turn, the Pineaceae and about six other families are grouped together in the order Coniferales (the conifers, or cone-bearing plants) because they are evolutionarily related.

Modern taxonomy

Traditionally, plant taxonomists have relied upon the morphology of plants' reproductive structures to determine their evolutionary relationships. More recently, they have also used biochemical characteristics, DNA sequences, and additional features. Occasionally, there are disagreements about the relationships of different plant species. These disagreements are important in stimulating further research.

In identifying a tree in nature, dendrologists do not always rely upon its reproductive structures, because these are often only available for a brief time of the year. In practice, they typically rely upon features of a tree's leaves, twigs, bark , wood , habit (general shape and appearance), and habitat as clues for identification.

Secondary growth

Most trees increase in thickness due to cell division in two special layers of undifferentiated tissues near the outside of their stems. This is known as secondary growth. The two tissues are referred to as the vascular cambium and the cork cambium. In contrast, herbs do not have secondary growth, and they stop growing once their primary tissues have matured.

Cell layers in a tree trunk

In a typical, sawed-off sector of a tree trunk, one encounters layers of different cells and a series of concentric, annual growth rings going from the outside toward the inside.

The cork of bark is on the external surface of the trunk, and consists of dead cells which are impregnated with suberin, a waxy substance which inhibits evaporation of water through the bark. The cork cambium lies just inside the cork. It produces cork cells on its outside face and secondary cortex on its inside face. Growth and division of the cork cambium differs among tree species, and this gives the bark of each species its own characteristic appearance.

Phloem cells lie just inside the secondary cortex. Phloem cells are elongated cells specialized for the transport of plant nutrients , such as the carbohydrates made during photosynthesis . The vascular cambium lies just inside the phloem cells. It produces phloem cells on its outside face, and xylem cells on its inside surface. Xylem cells are elongated cells specialized for transport of water and dissolved ions throughout the tree. Trees growing in places with a strongly seasonal climate typically contain thick layers of xylem cells with readily apparent, concentric growth rings. This thick layer of functional xylem cells is referred to as the sapwood.

Finally, the heartwood is in the center of the tree. This layer is typically darker than the sapwood and consists of dead cells that are very stiff and serve to strengthen the tree. The heartwood may also have readily apparent growth rings.

Growth rings

In most trees growing in a strongly seasonal climate, the vascular cambium produces wide, thin-walled cells in the spring, narrow thick-walled cells in the summer, and few or no cells in the autumn and winter. This seasonal regularity of cell production results in the formation of annual growth rings. The bristle-cone pine (Pinus aristata) is the world's longest-lived tree species, and one specimen of this species has about 5,000 growth rings, indicating it is at least 5,000 years old.

Within a given growth ring, the large cells of spring-wood and the small cells of summerwood are often readily discernible with the naked eye . Light , temperature , soil moisture and other environmental factors affect the growth of trees, and therefore the width of their growth rings.

Evolution

Most botanists believe that the first land plants were herbaceous. The first woody plants were probably Lycopsids, free-sporing plants which had narrow, tubular grass-like leaves. Numerous Lycopsid fossils have been dated to the middle of the Upper Devonian period, more than 370 million years ago. The first known plant with a vascular cambium which exhibited true secondary growth was a species of Protopteridium, a free-sporing plant dated to about 370 million years ago.

Very few modern, free-sporing plants are arborescent. The only living relatives of the Lycopsids are the club mosses (Lycopodophyta), a group of simple, herbaceous, free-sporing plants.

Forests

Trees are the dominant organisms of forests. Climate and other factors determine which tree species grow in a forest. Forest ecologists have classified the forests of the world according to the species of trees that grow there. One classification scheme is described below.

Coniferous forests are characteristic of the boreal forests of cold regions of the northern hemisphere. The boreal forest is found in Canada, southern Alaska, northwestern America, northern Europe , and northern Russia. The dominant trees are conifers (cone-bearing plants) such as pines, spruces, firs , and larches. Many of these trees are important sources of wood used in construction, and of pulp for paper .

Broad-leaf forests are characteristic of temperate zones of central and eastern North America , central and southern Europe, and central Asia . Its dominant trees have broad leaves and are deciduous, in that they shed their leaves once a year. Oaks , hickories, maples , and sycamores are some of the trees commonly found in this forest type. Many broad-leaf trees are hardwoods, and are used for making some of our finest furniture.

Mediterranean forests are characteristic of regions with hot, dry summers and warm, humid winters. This forest type is found in southern California, northwestern Mexico, southern Europe, northern Africa , and southern Australia . The trees which grown in Mediterranean forests are adapted to minimizing water loss. Evergreen oaks and pines are well-known trees of this forest type. In Australia, about 600 different species of Eucalyptus grow in the Mediterranean forest type along the southern coast.

Savannas are characteristic of tropical regions with a very seasonal rainfall. The trees in a savanna are sparsely distributed and are specially adapted to minimize water loss. Savannas are found in northern Mexico, equatorial Asia, central Africa, and northern Australia. Acacias, Dracaenas, and the Baobab are well-known savanna trees.

Tropical rainforests are characteristic of regions with a relatively warm climate and a great deal of rainfall. This forest type is found in Central America, northern and central South America , central and western Africa, and the south Pacific region. Tropical rainforests have a great diversity of plant and animal species. The tropical rainforests of the world are currently threatened by people who are cutting down their trees at an increasing rate . At the same time, conservationists and tropical ecologists devote much effort to the preservation of tropical rainforests with their rich diversity of species.

Ascent of sap

It is vitally important for a tree to transport water from the soil to its upper-most leaves. This process has long fascinated plant physiologists, and has been studied for centuries. How does water move from the roots to the uppermost leaves of tall trees? The significance of this problem is best appreciated by considering the height of some of the world's tallest trees. The world's tallest tree was a Eucalyptus regnans of Australia, which was measured at 470 ft (143 m) in 1880. The tallest living tree is a coastal redwood (Sequoia sempervirens) of California, which is about 365 ft (111.3 m) in height. Its relative, the giant sequoia (Sequoidendron giganteum), is not quite as tall, but is the world's most massive tree species.

Cohesion-tension theory

Most plant physiologists now accept the "cohesion-tension theory" as an explanation for the ascent of sap. According to this theory, water moves up the trunk of a tree in narrow, elongated cells near the periphery of the trunk, referred to as the xylem, and does not require the expenditure of metabolic energy . The movement of water only depends upon three important physical-chemical properties of water.

The first important property of water is that it always moves from a region with a more positive water potential, to a region with a more negative potential. Water potential is a measure of the energy available in a solution of water. Thus, water moves out of the leaves and into the air because the water potential of the air is more negative; water moves out of the tree trunk and into the leaves because the water potential of the leaves is more negative; water moves out of the roots and into the trunk because the water potential of the trunk is more negative; and water moves out of the soil and into the roots because the water potential of the roots is more negative.

The second important physical-chemical property of water is that it is a cohesive molecule . In other words, water molecules tend to bind to one another through the formation of hydrogen bonds. The cohesiveness of water molecules gives the thin water columns in a tree trunk a very great tensile strength. This prevents breakage of the water column when great longitudinal stresses are placed upon it as it is pulled out of the leaves and into the air.

The third important property of water is that it adheres very tightly to the walls of xylem cells in the tree's transport pathway. Adhesion of water to these cell walls maintains the full hydration of the pathway for water transport. This prevents breakage of the water column, and allows water transport even when a tree is water-stressed in a dry environment.

Economic significance

Trees have great economic significance to humans as a source of food, building materials, and paper. Almond, coconut, cherry, prune, peach, pear, and many other tree species are grown in orchards for their fruits and nuts. The apple tree is the orchard tree of greatest economic significance, and there are several hundred different varieties of apples. (Many of North America's best apples grow in New York state and Washington state.) Many trees are also useful for the wood they produce. Wood is used as a construction material and to make furniture. Wood is a valuable construction material because it is relatively inexpensive, easy to cut, and very strong relative to its weight. Many species of pines and other conifers are important sources of softwoods, and many broadleaf trees are important sources of hardwoods.

There are frequent conflicts between conservationists and loggers. In the United States, the coniferous rainforest of the Pacific northwest has been the site of one such conflict. In this region, loggers want to harvest coniferous trees from the forest just as they have done for many years, whereas conservationists seek to preserve the forest because it provides a habitat for the northern spotted owl, an endangered species .

See also Basswood; Citrus trees; Conifer; Deforestation; Dogwood tree; Ebony; Forestry; Horse chestnut; Magnolia; Mahogany; Mangrove tree; Nux vomica tree; Palms; Sapodilla tree; Screwpines; Spruce; Walnut family; Yew.

Resources

books

Audubon Society and Staff. Familiar Trees of North America: Eastern Region. New York: Knopf, 1987.

Platt, R. One Thousand-and-One Questions Answered about Trees. New York: Dover Inc., 1992.

Raven, Peter, R.F. Evert, and Susan Eichhorn. Biology of Plants. 6th ed. New York: Worth Publishers Inc., 1998.

White, John, and David More. Illustrated Encyclopedia of Trees. Portland, OR: Timber Press, 2001.

other

Chaw, S. M., et al. "Seed Plant Phylogeny Inferred From All Three Plant Genomes: Monophyly of Extant Gymnosperms and Origin of Gnetales from Conifers." Proceedings of the National Academy of Sciences of the United States of America 97 (2000): 4086-4091.

Peter A. Ensminger

KEY TERMS

Adhesion

—Physical attraction between different types of molecules.

Cohesion

—Physical attraction between molecules of the same type.

Cork cambium

—Undifferentiated plant tissue which gives rise to cork cells and secondary cortex.

Dendrology

—Identification and classification of woody plants.

Phloem

—Plant tissue consisting of elongated cells which function in the transport of carbohydrates and other nutrients.

Vascular cambium

—Undifferentiated plant tissue which gives rise to phloem and xylem.

Xylem

—Plant tissue that transports water and minerals upward from the roots.

Trees

Trees are plants with an erect perennial stem at least 4 meters (13 feet) tall, a diameter measured at breast height (1.3 meters or 4.5 feet) of at least 7.5 centimeters (3 inches), and a distinct crown of leaves or leafy branches. This definition is widely used by foresters and forest ecologists to divide woody plants into trees, shrubs (smaller plants, often with clustered stems), and vines (plants not self-supporting and usually without a distinct crown). However, plant species are not constrained to fit definitions, thus plants of the same species may grow as trees in some areas and as shrubs in others, especially at the edge of the species' range where growing conditions are harsh. Thus lodgepole pine, valued by Native Americans for its tall, straight "lodge poles," becomes a sprawling shrub near timberline in the Rocky Mountains and on sandy beach dunes of the Pacific Coast.

Trees are usually woody; that is, their stems are composed largely of densely packed, elongated, thick-walled cells (secondary wood) produced by a cylindrical growing center, the vascular cambium, that surrounds the stem underneath the bark. Typically the secondary cambium adds new wood throughout the life of the tree, gradually increasing the trunk diameter as the crown grows larger and taller. Notable exceptions occur in tree ferns, cycads, palms, and a few other plants that reach tree dimensions while producing little or no secondary wood. In these plants neither trunk diameter nor crown size increases much with age once the single apex of large compound leaves reaches mature size. However, mechanical support by the stem is often aided by an encircling band of tightly packed roots (e.g., tree ferns) or persistent leaf bases (e.g., cycads and palms).

Trees are regularly associated with certain plant groups such as the oak (oak, beech, chestnut) and pine (pine, spruce, fir, hemlock, etc.) families; however, most families of vascular plants contain some tree species. Residents of temperate climates are often surprised to find that tomatoes, violets, shrubby sumacs, and even grasses have close tropical relatives that are trees. Today's diminutive club mosses, spike mosses, and quillworts (order Lycopodiales) and horsetails (order Equisetales) are the remaining relatives of huge scale-trees (Lepidodendrales) and giant horsetails (Calamitales) that dominated swampy forests in the Carboniferous (coal-forming) period of Earth's history three hundred million years ago.

Trees are the giants of the plant world. Among the tallest ever measured was a Douglas-fir (Pseudotsuga menziesii ) in Washington that measured 117 meters (385 feet) tall. California claims the tallest living tree, a coast redwood (Sequoia sempervirens ) 112.6 meters tall, but competing for this distinction are trees of Eucalyptus in Australia, which are perhaps a few meters taller. In total weight or biomass , probably no other species has ever produced larger trees than the Big Tree redwoods (Sequoiadendron giganteum ) of the California Sierra Nevada Mountains. These colossal giants (to 88 meters) maintain their basal diameter of 5 to 10 meters with little taper to the base of their crown over 40 meters high.

Big Tree redwoods are also among the oldest known trees, reaching ages of more than three thousand years. However, the oldest trees in North America and perhaps in the world are quite modest in size (10×.6 meters). These belong to a species called bristlecone pine (Pinus aristata ). The oldest bristlecone pines (more than forty-six hundred years) grow in dry rocky soils near timberline in southern California where the harsh growing season is very short and annual growth in trunk diameter is often less than 0.2 millimeters. Age estimates of bristlecone pines are obtained by counting annual growth rings in studies collectively termed dendrochronology. The science of dendrochronology uses tree-ring information to reconstruct long records of climatic history and to date prehistoric wooden structures.

Trees and Forest Types

Trees greatly modify the habitat in which they live, by their shade and by their litter of fallen leaves. Typically light-loving tree species occupy the highest section of the forest, the overstory or canopy, with shade-tolerant species below. Species inhabiting these forest layers differ according to geographic and topographic location and associated climate and soils. Forest composition may also differ with age, and to some degree, by chance, that is, which species managed to get its seeds to the area first or survived the longest. Despite these causes of forest differences, it is possible to recognize common associations of particular plant species with particular habitats. For example, ecologists and foresters usefully refer to Oak-Hickory forests as tending to occur on dry upland areas in the east central United States, with Beech-Maple forests tending to occur on more moist north-facing slopes. Many such forest communities are recognized across the country and are often named for their two most common overstory trees.

Perhaps the greatest contrast between forest types in temperate regions is between deciduous broad-leaved and evergreen coniferous forests. In deciduous forests light penetrating to the forest floor in early spring supports a diverse array of spring wildflowers and ferns whereas the perennially dark floor of a dense coniferous forest supports many fewer species.

Tree Uses

It is hard to overestimate the importance of trees in their many uses for lumber, landscaping, shade, ornamental plantings, and windbreaks. In all of these uses diversity among trees is important. Wood types differ in strength, weight, hardness, color, figure, and other characteristics. Even taste can be important where cooking utensils or food storage is the use. Usually fast-growing trees such as cottonwood produce light, soft wood relative to that produced by slow-growing trees such as oaks and hickories. Conifers (often called softwoods) are preferred for construction lumber where ease of cutting, carrying, and nailing is important. The heavier, stronger, tougher wood of flowering trees (hardwoods) is preferred for railroad ties, strong crates, hardwood floors, tool handles, and sports equipment. Because of their attractive color and grain, hardwoods are also favored for fine furniture, cabinetry, and wall panels. All species and all sizes of trees are used for pulp and wood flakes for paper products and synthetic lumber.

Tree Diversity

To meet the need for diverse wood products, naturally diverse forests must be maintained. More than 600 species of native trees occur in North America north of Mexico, but much greater diversity occurs in tropical forests. A single hectare (2.5 acres) of Amazon forest may contain more than 200 different species whereas the most diverse U.S. forests contain about 20 species per hectare. In addition to variety in wood products, tree diversity aids forests in resisting diseases and pests and promotes wildlife diversity through the variety of foods, dens, and perching and nesting sites they maintain. Tropical rain forests' tree canopies support an array of epiphytic mosses, ferns, and flowering plants and hundreds of insect and larger animal species that scientists are just beginning to explore.

Tree Identification

Tree identification is necessary for forest and tree management and a rewarding hobby enjoyed by many nonprofessionals. Books aiding identification and describing trees, tree uses, and tree care are available for nearly all areas, ranging in coverage from local regions to entire countries and continents. Leaf size, shape, and arrangement on the stem are often sufficient to identify a tree, but twigs, bark, fruits, cones, and sometimes flowers may be required. Winter identification of deciduous trees by twigs and bark presents a special challenge, but one that can be mastered by the dedicated student.

see also Chestnut Blight; Coniferous Forests; Conifers; Deciduous Forests; Dendrochronology; Dutch Elm Disease; Forester; Forestry; Gymnosperms; Palms; Rain Forest Canopy; Rain Forests; Record-Holding Plants; Sequoia; Tree Architecture; Wood Anatomy; Wood Products.

Donald R. Farrar

Bibliography

Brockman, C. Frank. Trees of North America. New York: Golden Press, 1986.

Farrar, John Laird. Trees of the Northern United States and Canada. Ames, IA: Iowa State University Press, 1995.

Harlow, William. M., Donald J. Leopold, and Fred M. White. Harlow and Harrar's Textbook of Dendrology, 9th ed. New York: McGraw-Hill Inc., 1996.

Little, Elbert L. The Audubon Society Field Guide to North American Trees. New York: Chanticleer Press, 1980.

Preston, Richard J., Jr. North American Trees, 4th ed. Ames, IA: Iowa State University Press, 1989.

Tree

A tree is a woody perennial plant that has a single trunk arising from the ground (typically without branches near the base) and that usually grows to 20 feet (6 meters) or more in height. Branches and twigs grow from the trunk of a tree to form its characteristic leafy crown. Trees are the dominant plants in the world's forests, providing critical habitats for the other species that live there. Trees also provide many products that are important to humans, such as fruits, nuts, timber, and medicine.

Trees may be classified into two major groups: deciduous and evergreen (conifer). Deciduous trees shed their broad leaves at the end of the growing season—typically each fall. Examples of deciduous trees are maples, oaks, and elms. Evergreen or coniferous trees typically have needle-shaped leaves that remain for several years before being replaced. Pines, firs, and spruces are examples of evergreens.

Structure of a tree trunk

Bark is the protective external covering of the stems (roots, trunk, and branches) of trees. The waterproof outer layer is known as cork. Composed of dead cells, cork can be as thick as several inches or more and serves to protect the internal living tissues from insects, animals, fungi, fire, and dehydration (the loss of water).

Underneath the cork is a layer of living tissue called the phloem (pronounced FLOW-em). Phloem cells are elongated cells that transport plant nutrients, such as the carbohydrates made during photosynthesis, from the leaves to all other parts of the plant.

When young, all trees have smooth bark. As trees mature, the outer surface of their bark begins to change in appearance, varying greatly among different species. For example, the bark of a mature American beech is distinctively grey and smooth while the bark of a mature sugar maple is rough and scaly, with deep fissures (long narrow cracks).

Beneath the layers of bark is the cambium layer, a living sheath or covering of cells. On its outside face, the cambium produces phloem cells.

On its inside face, it produces xylem (pronounced ZEYE-lem) cells. These thick-walled cells carry water and mineral nutrients from the roots to the farthest branches. The broad layer of xylem tissue is known as the sapwood.

Finally, the heartwood is in the center of the tree. As a tree ages and increases in diameter, dead xylem cells are used to store waste products, such as resin and other compounds. These waste-filled xylem cells form the heartwood. Typically darker than the sapwood, the heartwood is very stiff and serves to strengthen the tree.

Words to Know

Bark: Protective external covering of the stems (roots, trunk, and branches) of trees and other woody plants.

Cambium: Plant tissue that produces phloem and xylem cells.

Cork: Waterproof outer layer of bark composed of dead cells.

Growth ring: Layer of wood produced in a single growing season.

Heartwood: Dead central portion of wood in a tree.

Perennial: Any plant that lives, grows, flowers, and produces seeds for three or more consecutive years.

Phloem: Plant tissue consisting of elongated cells that transport carbohydrates and other nutrients.

Photosynthesis: Process by which light energy is captured from the Sun by pigment molecules in plants and algae and converted to food.

Sapwood: Newly formed, outer layer of wood between the heartwood and bark that contains the living elements of the wood.

Xylem: Plant tissue consisting of thick-walled cells that transport water and mineral nutrients.

Tree growth

The periodic formation of layers of xylem cells results in the diameter growth of trees. New sapwood is created during each growing season, but within two to three years these cells become part of the heartwood.

In most trees growing in a climate with definite seasons, the cambium produces wide, thin-walled cells in the spring, narrow thick-walled cells in the summer, and few or no cells in the autumn and winter. This seasonal cell production results in the formation of annual growth rings. The bristle-cone pine is the world's longest-lived tree species. One specimen of this species has about 5,000 growth rings, indicating it is at least 5,000 years old.

Within a given growth ring, the large cells of springwood and the small cells of summerwood are often readily seen with the naked eye. Light, temperature, soil moisture, and other environmental factors affect the growth of trees, and therefore the width of their growth rings.

Human use of trees

Humans use trees as a source of food, building materials, and paper. Almond, coconut, cherry, prune, peach, pear, and many other tree species are grown in orchards for their fruits and nuts. The wood of many trees is a valuable construction material because it is relatively inexpensive, easy to cut, and very strong relative to its weight. Many species of pine and other conifers are important sources of softwoods, while deciduous trees are important sources of hardwoods.

Chemicals in the bark of specific trees are used for medicinal or commercial purposes. Quinine, found in the bark of the South American cinchona tree, has been used for many years to treat malaria. More recently, anticancer chemicals have been discovered in the bark of the Pacific yew. Tannic acid, extracted from the bark of North American oak, hemlock, and chestnut trees, is used in tanning (the process of turning animal hides into leather).

The bark of certain trees is itself used to create products. The light, spongy outer bark of the European cork oak is used for bottle stoppers and in life rafts, insulation, and flooring. The bark of certain conifers, such as Douglas-fir and redwood, is used as a mulch in landscaping.

[See also Forestry; Forests; Rain forest ]

Tree

A tree is a tall, woody plant with one main trunk and many branches. It is perennial (grows for many years) and distinguished from a shrub by its size (usually 15 feet [4.6 meters] or taller) as well as by its single woody stem or trunk. A shrub is a short, woody plant that usually divides at its base. Trees are the most visible and dominate part of a forest. They also play an important role in the world's environment and economy.

Trees vary greatly in size, with the tallest being the redwood trees of coastal California that reach well over 350 feet (106.7 meters). Trees began their evolution more than 400,000,000 years ago. The oldest known living trees are the bristlecone pine (Pinus aristata) growing in the Rocky Mountains which are believed to be more than 4,500 years old. The widest tree is a giant sequoia (Sequoia gigantea) growing in central California. Nicknamed the "General Sherman," it has a circumference (measured completely around) of 115 feet (35.1 meters). Trees also make up the largest living organism in the world, with some stands or groups of trembling aspen trees covering as many as 100 acres (404,700 square meters). These trees reproduce by sending out roots that send up new sprouts that in turn become trees. Since the trees are all genetically identical and actually connected,

they can be said to represent a single organism. Trees are found in every region of the world except deserts, the Arctic, and Antarctica.

Woody plants, like trees, have tough stems covered by bark that do not die when the growing season has ended. It is their tough, woody stems that allow them to grow as tall as they do. Trees can be divided by the type of wood they form. Conifers (produced in cones) like pine and spruce are called softwoods, while most others are considered hardwood. A tree is like any other green plant in that it draws in water from its roots and makes food in its leaves through photosynthesis (the process by which plants use light energy to make food from simple chemicals). Deciduous trees, like maples and oaks, lose their leaves for a season and grow new, flat broadleaves each spring. Evergreen trees, like pines, are always covered with leaves (which have evolved into sharp needles), although they are constantly losing them and replacing them in small numbers. As with all plants, trees are either angiosperms (producing flowers and seeds with coverings), or gymnosperms (producing naked seeds). Conifers are gymnosperms since they produce seeds on modified leaf structures called scales that are on their cones. Most other trees produce flowers and are angiosperms. While spring flowers on a fruit tree are obvious and beautiful, the flowers on some trees do not resemble anything like a flower. The age of a tree can be determined by counting its rings after it has been cut down. Each year new wood is formed in a layer that is outside that of the previous year's wood growth. Very dry years result in thin rings, and years of good rainfall result in thicker rings.

Forests of trees have been called the "lungs of the world," since they provide an enormous amount of oxygen to the environment as a by-product of photosynthesis. Besides providing oxygen, trees and the forests they make up form the habitat for many animals, and virtually every part of a tree has been put to some use by humans—even a tree's sap and bark. Trees supply wood for fuel as well as for lumber, paper, and plastic products. Humans dependence on trees for products has begun to have a negative impact on forests. Today, deforestation is a growing problem, especially in less developed countries where trees are being cut down faster than they are able to grow.

[See alsoForest; Plants; Rain Forests ]