Introduction to Tree
Tree, a large, woody, perennial plant usually composed of one main leafless stem (called a trunk) and several leaf-bearing stems, or branches. Although the distinction between trees and shrubs is not sharp, trees are usually defined as being (upon maturity) at least 13 feet (4 m) in height with a trunk diameter of at least 3 inches (7.5 cm) at a point of 4 ½ feet (1.3 m) above the ground.
Trees surpass all other plants in size and can have extremely long life spans. A few kinds of trees, such as the redwood, grow more than 350 feet (100 m) tall. Bristlecone pines can live more than 4,500 years. Trees thrive in temperate and moist tropical climates but cannot grow in extremely cold or dry regions.
Each species of tree can be identified by its flowers and fruits, by the shape of its leaves, and by the texture of its bark. The way a tree branches is sometimes characteristic of a species. More than 50,000 species of trees exist worldwide. About 600 species are native to the United States and Canada.
The study of trees is called dendrology. Trees of many different lands and climates are often exhibited to the public in arboretums. .) The science of developing and caring for forests is called forestry.
Kinds of Trees
All trees (except tree ferns) bear seeds and are classified botanically according to seed type. Those that produce naked seeds are gymnosperms; those whose seeds are born inside a fruit are angiosperms (flowering plants). The remainder of this article is about seed-bearing trees.
The most familiar group of gymnosperms consists of the conifers (cone-bearing trees), so named because of the scaly structures called cones borne by most species. The seeds are in the cones, but are not enclosed in a protective structure (and hence are naked).
Yews bear fleshy cones, resembling berries, which are much different from the prickly pine cone.Most conifers are narrow-leaved. Some, such as the pine, spruce, and hemlock, have needlelike leaves; others, such as the juniper, have scalelike leaves. Most conifers are evergreens; they do not shed their leaves at the end of each growing season. Conifers are also called softwoods because, in general, their wood is softer than that of the angiospermous trees.
Trees that are angiosperms are broad-leaved and are usually deciduous in temperate zones, losing their leaves at the end of each growing season. Examples are the elm, poplar, and oak. Angiospermous trees are also called hardwoods.
Description
A tree has three main parts: roots, stems, and leaves. The top portion of a tree, including the branches and leaves, is called the crown. Trees with a single, straight trunk, such as conifers, usually have a tapering crown. The crown of a tree whose trunk is split into two or more large branches, such as an elm or oak, is usually more rounded and spreading.
Rootsserve primarily to absorb water and dissolved minerals from the soil. They also store food. In addition, roots anchor the tree to the soil, thereby helping also to prevent soil erosion. Most trees have one strong, central root and many smaller branch roots.
Stemsform the above-ground woody part of a treethe trunk and branches. The trunk supports the branches and provides the bulk of usable wood. It may rise straight up the full length of the tree, or it may rise to a certain height and then divide into two or more large branches.
Trunk and branches have similar structure. In most trees they are composed of the outer bark, inner bark, cambium, sapwood, and heartwood. The outer bark, which is made up of dead cells, protects the tree from injury. The inner bark (called phloem) carries food made in the leaves to the branches, trunk, and roots.
Under the phloem is the cambium. It is here that a stem grows in diameter. The cambium produces both bark and wood cells. Trees found in climates where there are definite seasons have definite periods of growth alternating with dormant periods. The formation of tree rings (also called annual, or growth, rings) is a result of this type of growth. In tropical climates where temperature and rainfall are fairly constant throughout the year, trees may grow continually and so have no tree rings.
The number of tree rings in the trunk of a tree is a measure of a tree's age. Scientists can date historical environmental events, such as droughts, floods, and volcanic eruptions, by determining the annual variations in the width of tree rings.
Under the cambium is the xylem, or wood, which is composed of two partsthe sapwood and the heartwood. Sapwood carries the sap (consisting of water and minerals) from the roots to the leaves. The center of the trunk is made up of heartwood, which imparts strength and stiffness to the trunk. Heartwood is composed of dead cells.
Leavesmake food for the tree. Although in summer most trees have green leaves, some trees have leaves of some other color. The copper beech and plum, for example, have purple leaves.
Reproduction and Growth
All trees (except tree ferns) have flowers, by which they reproduce, in one form or another. The egg, in the pistil, is fertilized by sperm, in the pollen of the stamen, to produce a seed. Most tree flowers are bisexual, or perfect, possessing both stamens and pistils. Other flowers are of one sex only; depending on the species, the flowers of the two sexes appear either on the same tree, in which case the tree is called monoecious, or on separate trees, in which case it is dioecious. Pines are examples of monoecious trees; willows, hollies, and poplars of dioecious ones. A few tree species, such as the red maple, have both unisexual and bisexual flowers on the same plant.
Except for the tree ferns, trees begin from seeds that have dropped to the ground from mature trees. When conditions are favorable, the seed germinates, or sproutsthat is, the cells in the seed become active and start multiplying. Rudimentary organs such as roots, stems, and leaves take form. The roots grow downward into the soil and the stem and its leaves break out above the ground.
At the tip of the new stem, which will become the main trunk of the mature tree, is a terminal bud from which growth in height of the seedling (young tree) will take place. Other buds on the stem will give rise to branches and to flowers and leaves. At the tip of each branch is a terminal bud where growth in the length of the branch takes place. Growth in breadth of the branches and trunk takes place by division and enlargement of the cells in the cambium.
The basic food materials needed by trees to make new parts during the growth periods are carbohydrates. The leaves make carbohydrates through a process known as photosynthesis.
The numerous species of trees differ greatly in size, rate of growth, and life span. For example, both a red alder and an Alaska cedar can attain a height of 100 feet (30 m), but the Alaska cedar can live up to 600 years while the red alder's maximum age is about 100 years. Some trees, such as the gray birch and black willow, grow rapidly but do not live long. Others, such as the redwood, grow rapidly and live for hundreds or even thousands of years.
Trees, like other living things, eventually die of old ageif they are not destroyed by fire, drought, insects, or disease or cut down by humans. Insects destroy trees by eating the leaves, by boring into the trunk, or by spreading disease-causing organisms. Tree diseases are caused by bacteria, fungi, parasitic seed plants, and viruses. Insects and disease-causing organisms are controlled by spraying, by adding chemicals to the soil, and by tree surgery.
In thick forests, trees can grow very tall as they reach toward the sunlight. In a forest, however, a tree often loses its lower branches because sunlight cannot get through to them.
A tree's growth can be affected by the wind. Wind constantly blowing against a tree from one direction can prevent branches from growing on that side, or can cause the tree to grow leaning away from the wind.
Uses of Trees
Much of the wildlife on earth could not exist without trees. In addition to releasing oxygen into the air for animals to breathe, trees provide homes and food for many animals. Trees are also important because they provide shelter from the wind, aid in preventing soil erosion, and enrich the soil with their decaying leaves.
Trees have many commercial uses. Their wood yields thousands of products, including paper, medicines and other chemicals, and lumber. Trees also provide food, such as fruits, spices, and nuts. Bark from the roots of the sassafras yields a tea and oils, and various chemicals are derived from the roots of the longleaf pine. Some tree bark yields such substances as cork, tannins, and cinnamon, as well as various kinds of drugs and dyes. Some leaves, such as those of the palmyra palm, provide fibers that are woven into twine, rope, and mats. Fluids from trees yield many useful products, including rubber, maple syrup, and turpentine.
Trees are also valuable for ornamentation. They line streets and adorn gardens, making them cooler and more comfortable in summer by providing shade. Among favorite shade trees in the United States are the locust, oak, elm, beech, linden, maple, birch, willow, ash, and sweet gum. Trees cultivated for their ornamental flowers include the tulip tree, horse chestnut, locust, crab apple, and catalpa.