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.

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

The Gale Encyclopedia of Science