IntroductionTree,
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 4600 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 andgymnosperms. 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 palmsand 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.
