Trees

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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 meterstaller. 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 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.

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.

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 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.

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.

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.

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