Shrub Land

I

Introduction

Shrub Land, ecosystem in which relatively small (up to 6 m/20 ft high), woody, usually multistemmed, densely branched vegetation dominates. Shrub lands are distinguished by stable (climax) or successional plant communities (see Ecology).

II

Stable Shrub Lands

Naturally stable shrub-land ecosystems cover large portions of the semiarid and arid parts of the earth. The most extensive shrub lands are found in a zone between 32° and 40° latitude north and south of the equator. These shrub lands include parts of semiarid southwestern North America, the Mediterranean region, central Chile, parts of Brazil, the Cape region of South Africa, and southern and southwestern Australia. For the most part the climate is characterized either by hot, dry summers and cool, moist winters or by pronounced wet and dry seasons. The shrub vegetation is dominated either by small-leafed evergreen shrubs able to survive fire and limited nutrients, or by shrubs that lose their leaves during the dry season. Among these shrub lands are the Mediterranean-type chaparral (see Chaparral), the caatinga of northeastern Brazil, and the mallee of Australia, which is dominated by low-growing eucalyptus. The cool, semiarid Great Basin of North America (see Desert) supports a northern desert shrub community dominated by sagebrush (Artemisia) and shad scale (Atriplex).

Other stable shrub communities result from the degradation of forest or grassland ecosystems due to overgrazing or deforestation. Among such shrub lands in the Mediterranean region are the garigue, the low, open shrub land of oak species resulting from the degradation of pine forests; and the maquis, a taller, more dense growth that replaced the cork-oak forest. In Scotland, heather moors resulted from the cutting of Scotch-pine forests.

III

Successional Shrub Land

Most shrub lands throughout the temperate regions are successional; that is, they are at one of the stages in which once-disturbed land is returning to forest. On uplands, shrubs usually grow in scattered clumps among grasses, herbs, and tree saplings. Among these shrubs are hazels, sumacs, chokecherry, shrub dogwoods, and blackberries. Shrub thickets often are well developed along fencerows, roadsides, and forest edges. On low, wet ground, stands of shrubs dominated by dogwoods, spireas, and willows make up shrub communities. Along streams, lakes, and marshes, alder and willows may form extensive, relatively stable communities that persist for years.

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IV

Shrub Life

Shrub communities, especially in semiarid regions, are more successful than grass and trees. Shrubs put more of their nutrients and energy into the growth of roots than into the growth of above-ground stems; these extensive root systems enable shrubs to reach moisture deep in the ground. Some shrub species inhibit competing herbs by secreting substances toxic to other plants.

Shrub lands support their own distinctive animal life. Frequently, however, these lands are considered worthless and are converted to other uses, which eliminates wildlife and plant species associated with them. In semiarid regions, shrub lands that are important for livestock and wildlife forage are being overgrazed, misused as farmland, and uprooted for firewood, allowing deserts to encroach on them.

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