A biome is a major biotic community characterized by the dominant forms of plant life and the prevailing climate. The term grasslands refers to a main biome grouping that includes temperate grasslands and tropical savannahs. Both of these biome types are dominated by grasses, but differ in that temperate grasslands do not also support trees and shrubs, as savannahs do. Latitude is another key differentiator between tropical savannahs and temperate grasslands, with the grasslands occupying the middle latitudes in semiarid continental interiors far from the moderating influences of oceans, leading to tremendous temperature fluctuations, which have a large impact on growing seasons (Woodward).
Temperate grasslands can be found at both north and south middle latitudes across the globe and are generally categorized in four major regional expressions: 1. the South African veld, 2. the Eurasian steppes, 3. the South American pampas, and 4. the North American prairies ("Biomes of the World: Temperate Grassland”).
In the formation of temperate grasslands, rainfall amounts, fire, temperature fluctuations and the presence of grazing animals all contribute to creating conditions that discourage tree growth and allowing grasses to prevail.
Although any grassland is a mixture of many microclimates that produce a variety of plants, it is possible to make broad distinctions based on dominant grasses. Moving from west to east across the greater North American prairie, the corresponding increase in precipitation levels is reflected in grassland variations that can be divided into three formations: shortgrass, mixed grass and tallgrass (“About Prairies”).
Beginning in the hot, dry Rocky Mountains, the westernmost shortgrass prairie region receives an average of 10 to 12 inches of rainfall per year and consists mostly of bunch grasses that grow 12 to 18 inches high. The centrally positioned mixed grass prairie supports two layers of grasses – one reaching 12 inches and one 48 inches above the surface.
Easternmost in the North American prairie (where rainfall can reach 21 inches per year) was the tallgrass prairie, which once sprawled over 142 million acres, but has been reduced to less than 1% of that today (“Quick Facts: Tallgrass Prairie”). The Flint Hills in Kansas and Oklahoma include some of the largest tracts of intact tallgrass prairie still remaining (“Tallgrass Prairie Ecosystem”).
Tallgrass prairies are an extremely complex web of life (“A Complex Prairie Ecosystem”). The grass matrix is dominated (80% of the foliage is indeed grasses) by a suite of 40 to 60 species of tall grasses – big bluestem (so abundant that some prairies were called bluestem prairies), indiangrass, switchgrass, little bluestem and prairie dropseed. The remaining 20% of primary vegetation is made up of over 300 species of forb, such as sunflower-like Silphiums, blazing stars, mountain mint, starflowers and Lupines. The exact composition of any particular piece of prairie landscape is dictated by soil type and depth, moisture and slope.
Prairie grasses are hardy warm-season perennials with roots that often extend deeper into the ground than the stems rise above it. For example, the roots of big bluestem may be 7 feet or more deep, and switchgrass roots more than 11 feet deep (Robertson). This not only enables them to withstand hot dry summers and subzero winters, but as some of the roots die and decompose each year, they add large amounts of organic matter to the soil (Tilton).
In their natural state, before 19th century settlement, prairies were maintained by three important stresses – climate, fire and grazing.
The climate extremes already discussed removed competition from trees and shrubs and allowed the tall grasses to send out healthy green shoots each spring that were well-equipped with finely divided or narrow leaves that resisted the drying effects of sun and wind, as well as extensive root systems that were a steady source of moisture, even in times of extended drought (“About Prairies”). The most famous of these droughts occurred in the 1930s, when the prairies were called the “Dust Bowl” (“North American Prairie”).
Fire is as crucial to the health of the tallgrass ecosystem as water and air (Klinkenborg). On a regular basis (about every one to five years), fires started by lightning or intentional burning by Native Americans spread across an expanse of the prairie. Almost every American Indian tribe used purposeful ecosystem burns for a wide variety of reasons, including spring burns to improve grass for big game grazing, large burns to divert game into small unburned areas for hunting, crop management and more (Williams). Whatever the cause, regular burns aided the grasses by killing saplings and removing the thatch of dead grasses, which cleared the way for early flowering spring species. Prairie plants not only adapted to fire by growing underground storage structures and developing growth points slightly below the surface, they depended on it to help control competing species and as an aid to nutrient cycling. Today, fire is considered an essential tool for maintaining prairie remnants and establishing new tallgrass prairies.
An estimated 30-60 million bison roamed the prairies, along with astounding numbers of other browsers such as elk, deer, antelope, rabbits and grasshoppers. Together these species consumed a considerable amount of the above ground biomass of a tallgrass prairie each year. This grazing stress increased growth by recycling nitrogen through urine and feces, and their trampling opened up habitat for plant species that favor a disturbed soil (Robertson).
Today much of the land has been turned into agricultural uses, urban areas are moving in, the buffalo are all but wiped out, and fires are largely suppressed. The disruption of root systems combined with the removal of the stresses that worked so well to its advantage has greatly reduced the genetic and biological diversity of the plants in what little remains of the tallgrass prairie. But there is a strong movement to educate people about prairies, and many states are rehabilitating what is left of their prairies and reintroducing the native wildlife and plants.
The next section of this Discovery Guide surveys some of the research, programs and educational efforts aimed at restoring lost tallgrass prairies and conserving remnants that still exist.
Go To Restoration & Preservation