Shrubs

Shrubs grow in many shapes and sizes, ranging from climbers to small trees. The bearberry, oleaster, and broom (shown in silhouette, above) illustrate three common ways in which shrubs grow: they creep along the ground, branch from just beneath the surface of the ground, or branch from a central “trunk,” like trees.

Shrubs grow naturally in many of the world’s floras, from savanna to shrub, from the tundra to the tropical rain forest. Cultivated shrubs are common in gardens throughout the world.

There is no clear line of distinction between shrubs and trees or between semishrubs and subshrubs, but certain generalizations can be made. Plants that grow in the form of shrubs are perennials with woody stems but, unlike trees, have little or no trunk. They may grow as bushes or, in some cases, as climbers. Shrubs branch from near or just below the ground and reach a maximum height at maturity of between 1.5 and 16 feet (0.5 and 5 meters). In addition, they differ from trees in the way in which they allow their lateral buds to grow. A tree prevents these buds from growing except at special branch points, whereas a shrub lets as many grow as is practical. Some shrubs are evergreen and some are deciduous, but almost all are angiosperms—that is, most reproduce sexually by means of flowers.

Gorse (Uiex europaeus), below, is a spiny evergreen shrub growing up to 4 feet (1.2 meters) in height. Its seeds are contained in hairy pods, which explode loudly when ripe.

Types of shrubs

Different groups of shrubs have their own particular attributes. Some shrubs thrive in hot wet environments such as the tropical forests. Most of these are evergreen, for example, members of the genus Philodendron, which have big, broad leaves—a characteristic that makes them popular house plants.

The tropics and subtropics also support plants that are able to survive wide variations in rainfall. They are deciduous shrubs, which shed their leaves during a cool or dry season. When the new buds grow, at the arrival of warmth and rain, they are initially protected by scales, sometimes ieaflike in form. Buttropo-phytic shrubs are not confined only to the warmer parts of the world. They are also the dominant deciduous shrubs of temperate regions and extend as far as the Arctic.

In the warmer parts of the world’s temperate regions, conditions favor shrubs with large evergreen leaves, which have a thicker epidermis than some other types of shrubs. The Mediterranean sweet bay (Laurus nobiiis) is typical, as are the camellias from India, China, and japan.

Many dwarf species, such as the heathers (Erica spp.), are regarded as shrubs because of their woody, branching stems. They inhabit heath and moorland areas and grow to less than 1.5 feet (0.5 meter) tall, extending horizontally because high winds restrict vertical growth. Many leaves grow within the bush where they are protected from grazing animals.

Some groups of shrubs are categorized according to the particular adaptation that allows them to withstand extreme weather conditions, such as heat, drought, and gales. Examples include many of the acacias (Acacia spp.), which have small, rigid leaves. Because the leaves are small, water loss is minimized during drought, and their rigidity supports the leaf structure in dry, windy conditions.

The heathers are xerophytic; they conserve water by means of leaves that are rolled up at the edges, thereby sealing off the pores (stomata) through which water loss takes place during transpiration. Thorn shrubs—again including many of the acacias—have spiky leaves, which are reduced to needlelike structures. This adaptation helps to conserve water by presenting a smaller surface area from which evaporation can occur. Switch shrubs, such as the brooms (Cytisus spp.), prevent excess transpiration by having leaves of many minute, overlapping scales or in some cases by having no leaves at all. Tola shrubs are typified by their needle- or scalelike resinous leaves.

Two other groups of shrubs are also drought-resistant but work on a different principle: they store water as well as prevent its loss. Succulent shrubs of the salt marshes and salt deserts—halophytes—have fleshy water-retentive leaves borne on their woody stems. Other shrubs, such as some members of the genus Eucalyptus, store water in swollen, underground stems.

Colonization

When land is stripped by fire or human activity, such plants as annuals and herbaceous perennials are quick to colonize it (A). Shrubs eventually establish themselves (B), but when they are mature, shade the herbaceous plants from light. Similarly, after many years, trees come to dominate the land (C) and cut out the light that reaches shrubs and plants on the forest floor.

In the succession of plant species that occurs when land is colonized by plants, shrubs tend to follow annuals and herbaceous perennials, such as the grasses, but precede trees. When tree growth does occur, it is often at the expense of shrubs because the trees tend to deprive the shrubs of light and moisture, which they need for survival. In many forest habitats, however, both deciduous and coniferous shrubs benefit from gaps caused by fire or falling trees or from human interference. They colonize quickly, producing a large quantity of seed shortly after they start to grow. The cropping of forests, for example, leads to the establishment of shrubs, which dominate the habitat until succeeded by trees that have grown naturally from seeds or that are planted in reforestation programs.

Only in a few areas are shrubs the dominant form of plant life. Most notable of these are the shrublands, which are of two kinds— the chaparral and the scrublands found in continental interiors. Chaparral occurs in coastal regions of California. Similar shrublands occur in other Mediterranean climates, for example, France, the southwestern Cape of Africa, and southern and southwestern Australia. Thorny and succulent shrubs are typical of the scrublands of continental interiors. These areas, which occur, for example, south of the Sahara Desert and in the interior of Australia, are usually bounded by desert.

Yucca plants are a genus of the agave family and live in arid areas of the Southwestern United States and in Central and South America. Most yuccas rely on the “yucca” moth for pollination, which is attracted by the smell of the cream-colored flowers.