Grasslands

Ecosystems in which grasses and grasslike plants such as sedges and rushes dominate the vegetation are termed grasslands. Grasslands occur on every continent except Antarctica. It is estimated that grasslands once covered as much as 25 to 40 percent of Earth's land surface, but much of this has been plowed and converted to crop production, such as corn, wheat, and soybeans. Prior to the European settlement of North America, the largest continuous grasslands in the United States stretched across the Great Plains from the Rocky Mountains and deserts of the southwestern states to the Mississippi River. Other extensive grasslands are (or were) found in Europe, South America, Asia, and Africa.

Grasslands can be broadly categorized as temperate or tropical. Temperate grasslands have cold winters and warm-to-hot summers and often have deep fertile soils. Surprisingly, plant growth in temperate grasslands is often nutrient limited because much of the soil nitrogen is stored in forms unavailable for plant uptake. These nutrients, however, are made available to plants when plowing disrupts the structure of the soil. The combination of high soil fertility and relatively gentle topography made grasslands ideal candidates for conversion to crop production. Grasslands in the Midwestern United States that receive the most rainfall (75 to 90 centimeters) and are the most productive are termed tallgrass prairies. Historically, these prairies were most abundant in Iowa, Illinois, Minnesota, and Kansas. The driest grasslands (25 to 35 centimeters of rainfall) and least productive are termed shortgrass prairies or steppes. These grasslands are common in Texas, Colorado, Wyoming, and New Mexico. Grasslands that are intermediate between these extremes are termed mid-grass prairies or mixed grass prairies. In tallgrass prairies, the grasses may grow to 3 meters tall in wet years. In shortgrass prairies, grasses seldom grow beyond 25 centimeters in height. In all temperate grasslands, production of root biomass belowground exceeds foliage production aboveground. Worldwide, other names for temperate grasslands include steppes, preferred for most of Europe and Asia, veld in Africa, and the pampas in South America. In North America, other names for temperate grasslands include prairies and steppes.

Tropical grasslands are warm throughout the year but have pronounced wet and dry seasons. Tropical grassland soils are often less fertile than temperate grassland soils, perhaps due to the high amount of rainfall (50 to 130 centimeters) that falls during the wet season and washes (or leaches) nutrients out of the soil. Most tropical grasslands have a greater density of woody shrubs and trees than temperate grasslands. Some tropical grasslands can be more productive than temperate grasslands. However, other tropical grasslands grow on soils that are quite infertile, or they may be periodically stressed by seasonal flooding. As a result, their productivity is reduced and may be similar to that of temperate grasslands. As noted for temperate grasslands, root production belowground far exceeds foliage production in all tropical grasslands. Other names for tropical grasslands include velds in Africa, and the compos and llanos in South America.

Although temperate and tropical grasslands encompass the most extensive grass-dominated ecosystems, grasses are present in most types of vegetation and regions of the world. Where grasses are locally dominant they may form desert grassland, Mediterranean grassland, subalpine and alpine grasslands (sometimes referred to as meadows or parks), and even coastal grassland. Most grasslands are dominated by perennial (long-lived) plants, but there are some annual grasslands in which the dominant species must reestablish each year by seed. Intensively managed, human-planted, and maintained grasslands occur worldwide as well.

It is generally recognized that climate, fire, and grazing are three primary factors that are responsible for the origin, maintenance, and structure of the most extensive natural grasslands. Although these factors will be described separately, their effects are not always independent of each other(e.g., grazing may reduce the fuel available for fire).

Climate

The climate of grasslands is best described as one of extremes. Average temperatures and yearly amounts of rainfall may not be much different from areas that are deserts or forested, but dry periods during which the plants suffer from water stress occur in most years in both temperate and tropical grasslands. The open nature of grasslands is accompanied by the presence of sustained high windspeeds. Windy conditions increase the evaporation of water from grasslands and this increases water stress in the plants and animals. Another factor that increases water stress is the high input of solar radiation in these open ecosystems. This leads to the convective uplift of moist air and results in intense thunderstorms. Rain falling in these intense storms may not be effectively captured by the soil, and the subsequent runoff of this water into streams reduces the moisture available to grassland plants and animals. In addition to periods of water stress within the growing season, consecutive years of extreme drought are more common in grassland than in adjacent forested areas. Such droughts may kill even mature trees, but the grasses and other grassland plants have extensive root systems and below-ground buds that help them survive and regrow after drought periods.

Fire

Historically, fires were a frequent occurrence in most large grasslands. Most grasslands are not harmed by fire. In fact, many benefit from fire and some depend on fire for their existence. When grasses are dormant , the moisture content of the foliage is low and the fine-textured fuel the grasses produce ignites easily and burns rapidly. The characteristic high windspeeds and lack of natural firebreaks in grasslands allows fires to cover large areas quickly. Fires may be started by lightning or set intentionally by humans in both tropical and temperate grasslands. Fires are most common in grasslands with high levels of plant productivity, such as tallgrass prairies, and in these grasslands fire is important for keeping trees and adjacent forests from encroaching into grasslands. Many tree species are killed by fire, or if they are not killed, they are damaged severely because their active growing points are aboveground. Grassland plants survive and even thrive after fire because their buds are belowground, where they are protected from lethal temperatures. In some highly productive grasslands, fire results in an increase in growth of the grasses and a greater production of plant biomass. This occurs because the buildup of dead biomass (mulch) from previous years inhibits growth, and fire removes this mulch layer. However, in drier grasslands, the burning of this dead plant material may cause the soil to become excessively dry due to high evaporation losses. As a result, plants become water-stressed, and growth is reduced after fire.

Most grassland animals are not harmed by fires, particularly if they occur during the dormant season. Animals living belowground are well protected, and most grassland birds and mammals are mobile enough to avoid direct contact with fire. Insects that live in and on the stems and leaves of the plants are the most affected by fire. But these animals have short generation times and populations recover quickly.

Grazing

Grazing is a form of herbivory in which most of the plant (leaves aboveground) or specific plant parts (small roots and root hairs belowground) are consumed by herbivores . Grazing, both above and belowground, is an important process in all grasslands. Many formerly natural grasslands are now managed for the production of domestic livestock, primarily cattle in North America, as well as sheep in Europe, New Zealand, and other parts of the world.

Grazing aboveground by large herbivores alters grasslands in several ways. Grazers remove fuel aboveground and may lessen the frequency and intensity of fires. Most large grazers such as cattle or bison primarily consume the grasses, thus the less-abundant forb species may increase in abundance and new species may invade the space that is made available. As a result, plant species diversity may increase in grazed grasslands. However, this effect is strongly dependent on the amount of grazing that occurs. Over-grazing may rapidly degrade grasslands to systems dominated by weedy and nonnative plant species.

Grazers may also accelerate the conversion of plant nutrients from forms that are unavailable for plant uptake to forms that can be readily used. Essential plant nutrients, such as nitrogen, are bound for long periods of time in unavailable (organic) forms in plant foliage, stems, and roots. Microbes slowly decompose these plant parts and the nutrients they contain are only gradually released in available (inorganic) forms. This decomposition process may take more than a year or two. Grazers consume these plant parts and excrete a portion of the nutrients they contain in plant-available forms. This happens very quickly compared to the slow decomposition process, and nutrients are excreted in high concentrations in small patches. Thus, grazers may increase the availability of potentially limiting nutrients to plants as well as alter the spatial distribution of these resources.

Some grasses and grassland plants can compensate for aboveground tissue lost to grazers by growing faster after grazing has occurred. Therefore, even though 50 percent of the grass foliage may be consumed by bison or wildebeest, when compared to ungrazed plants at the end of the season, the grazed grasses may be only slightly smaller, the same size, or even larger. This latter phenomenon, where grazed plants produce more growth than ungrazed plants, is called overcompensation and is somewhat controversial. However, the ability of grasses to compensate or make up partially or completely for foliage lost to grazers is well established. This compensation occurs for several reasons, including an increase in light available to growing shoots in grazed areas, greater nutrient availability (see above) to regrowing plants, and increased water availability. The latter occurs after grazing because the large roots system of the grasses is able to supply abundant water to a relatively small amount of regrowing leaf tissue.

Grassland Biota

By definition, grasses dominate grasslands in terms of plant numbers and biomass, but typically only a few species of grass account for most of the growth. By contrast, both temperate and tropical grasslands contain many more species of forbs than grasses. Forbs can be quite conspicuous when they have brightly colored flowers, and they are very important for maintaining high species diversity (biodiversity) in grasslands. The most conspicuous animals in grasslands are (or were) the large grazers, such as bison and antelope in North America and zebras, gazelles, and wildebeest in Africa. Although it may appear that the large herds of grazers in grasslands consume the most plant biomass, invertebrates such as grasshoppers aboveground can be important consumers of plants in some years, and nematodes and root-feeding invertebrates belowground are actually the most significant consumers of plant biomass.

Insect diversity can be great in grasslands. Even though most of the grasses are wind-pollinated, grassland forbs rely on a wide array of insect species for pollination. Grassland birds are unique in that many nest on the ground and some, such as the burrowing owl, nest belowground. Smaller mammals (e.g., mice, ground squirrels, prairie dogs, gophers) share the subterranean world and burrow extensively in some grasslands. These burrowing mammals may be important consumers of some plant parts and can alter soil nutrient availability to plants.

Conservation and Restoration

In North America, many grasslands are considered endangered ecosystems. For example, in some central Great Plains states that formerly had extensive tall grass prairies, up to 99 percent of these have been plowed and converted to agricultural use or lost due to urbanization . Similar but less dramatic losses of mixed and shortgrass prairies have occurred in other areas. Because grasslands have tremendous economic value as grazing lands and also serve as critical habitats for many plant and animal species, efforts to conserve the remaining grasslands and restore grasslands on agricultural land are underway in many states and around the world.

see also Biome; Grasses; Savanna.

Alan K. Knapp

Bibliography

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Knapp, A. K., J. M. Blair, J. M. Briggs, S. L. Collins, D. C. Hartnett, L. C. Johnson and E. G. Towne. "The Keystone Role of Bison in North American Tallgrass Prairie." BioScience 49 (1999): 39-50.

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