Grasslands are a major floralistic association of the Earth. They are characterized by grasses (Poaceae) and forbs (non-woody plants) of various heights and densities. On some grasslands, trees are interspersed, especially along watercourses. The climatic cause for widespread grasslands is a greater potential evapotranspiration than precipitation for the year. In such cases, the landscape cannot be dominated by trees because of the moisture stress. Grass species become shorter and sparser as potential evapotranspiration becomes progressively larger than precipitation. Ultimately, the grasslands grade into desert where the potential evapotranspiration is more than twice precipitation. There are also non-climatic reasons by which grasses can dominate the landscape, such as places in which the soils contain high concentrations of minerals such as nickel that are toxic to many tree species.
Geographers usually differentiate grasslands into two biomes dominated by grasses. The first is the tropical savanna and the second is the midlatitude grasslands. Their characteristics were given in the biomes article earlier in this Handbook. There are major grasslands on all the continents save Antarctica. The regional names are familiar to geographers: Examples include the Great Plains (North America), pampas (South America), steppe (Russia), veldt (Africa), and rangelands (Australia). Each continent has its own “flavor” of grassland floral and faunal species, yet the grassland formations are distinct from those in forests and other floralistic associations.
Within the last few thousand years, probably 40 percent of continental surfaces were grasslands but the percentage is declining as a result of the warming of the planet as it has emerged from the Pleistocene (ice age) and the unintentional destruction of the ecosystem by overgrazing (see desertification). A billion people live in the grasslands of the world, so these areas are quite susceptible to human modifications. Overall, grasslands have declined in their ability to sustain animals and people. For instance, the tallgrass prairie of the eastern Great Plains has all but disappeared. Agriculture is prolific in the more precipitation-rich grasslands with all the major grain crops having originated in grasslands and being variants of grasses. These crops include millet, sorghum, corn, rice, and rye. When considering the human influence, it is apparent that up to a third of the species of grasses in regions such as the Great Plains are not native to the region.
In general, there is lesser diversity of plant and animal species in grasslands compared to adjacent forests. However, this cannot be taken to mean a paucity of life. All sorts of bacteria, fungi, insects, and earthworms are at home in grassland soils. Moreover, plentiful large herbivores are one signature of native grassland conditions. In North America, the bison once populated the Great Plains in untold millions. In today’s African savanna, elephants, zebra, and wildebeests are still largely present but in declining numbers. The herbivores may, in fact, help keep grasses dominant in some places as herbivores are attracted to competing plant types such as young trees and bushes.
Grassland dominates in places where moisture is precious. A complete grass cover is very conservative of precipitation because of the slowing of surface runoff to the encouragement of moisture infiltration into the soil. Closely spaced root system mats help hold soil in place. A Great Plains thunderstorm dropping 2.5 cm (1 in) of precipitation may well not generate any significant runoff into streams. The increased infiltration leads to moist soil profiles under grasslands. Complex root systems greatly increase the length of storage and percentage uptake of soil moisture. So too is the leaching of soil nutrients slowed. This means that the mid-latitude grasslands of the world provide some of the most nutrient-rich soils when they are plowed for agriculture. Unfortunately, this is tempered by the fact that the soils dry out and are susceptible to wind erosion once the grass cover has been removed.
Of great importance to grasslands is the presence of fire. In dry seasons and droughts the aboveground portions of grasses brown out, allowing lightning-caused or human-caused fires to spread effectively, sometimes over millions of hectares. Fire represents a sudden change in the mass/energy pathways in grasslands but it does not represent the end of life. Fire releases plentiful nutrients into the top of the soil and the still-living underground portions of grasses are able to poke above the surface to tap this natural fertilizer. Additionally, fire kills trees and bushes helping grasses to dominate.
Native Americans were successful users of North America’s grasslands for thousands of years before European contact. Yet, as the United States enlarged to fulfill its “Manifest Destiny” during the 1800s, settlers neglected the Great Plains in search of opportunities on the Pacific Coast. The grasslands were viewed as so desolate compared to the woodlands of the eastern United States that the area was commonly known as the “Great American Desert.” With time, settlement accrued in this grassland realm. Three technological innovations played a key role: the invention of the steel plow, the invention of barbwire fence, and the building of railroads. As these innovations became widespread the vast grasslands were subdivided by farms and ranches and the crops and cattle could be shipped by rail to distant markets.
The Great Plains of the United States has a cautionary history that is mirrored in other places around the world. Grasslands are the climatic result of repeated moisture stress punctuated by relatively wet times. Grasslands usually exist between the deserts and the forests; people are tempted to use them as if they had dependable precipitation like the forests. As the Great Plains were fully settled, much grass cover was removed to grow dryland (unirrigated) crops like winter wheat. World War I saw rapid expansion of wheat agriculture as crop prices soared. Ironically, when wheat prices collapsed after the war, farmers opened up even more grassland to make enough money to pay mortgages. When the droughts of the 1930s arrived with the driest series of years in a century and a half in some places, the soil became so dry that much of it blew away, creating the Dust Bowl. The Dust Bowl’s droughts had a natural cause, but the social effects were staggering, depopulating entire areas by half or more. The ecological effects were also devastating, ruining grassland areas for centuries to come.
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