Tropical grasslands are found mainly in the Sahel south of the Sahara, in East Africa, and in Australia.Temperate grasslands principally occur in North America, Argentina, and across a broad band from Ukraine to China, but in most of these regions they have been substantially altered by agricultural activities.
Many grasslands formerly supposed to be natural are now recognized as having once been forests that grew in a marginally dry climate.
Early human disturbance is responsible for their transformation.
The most extensive natural grasslands can be thought of as intermediates in an environmental gradient, with forests at one end and deserts at the other.
Forests occupy the most favourable environments, where moisture is adequate for growth and survival of a tall, dense vegetation dominated by trees.
One of the best examples of a seasonally flooded subtropical grassland is the Pantanal in the Mato Grosso region of Brazil.
Across an area of 140,000 square kilometres (54,000 square miles), dry grasslands prevail for half of each year and shallow wetlands for the other, with small forest patches restricted to low rises that do not flood during the wet season.Woody plants may be prevented from growing in certain areas for other reasons, allowing grasses to dominate.One cause is seasonal waterlogging, which is responsible for the creation and maintenance of large grasslands in parts of the highly seasonal subtropics and in smaller areas of other regions.Changes in the severity or frequency of these factors can cause a change from one vegetation type to another.Other grassland types occur in places too cold for trees to grow—i.e., beyond the forest limits of high mountains or at high latitudes.The factors preventing establishment of such taller, woody vegetation are varied.Grasslands are one of the most widespread of all the major vegetation types of the world.A characteristic type of grassland in cool, moist parts of the Southern Hemisphere is tussock grassland, dominated by tussock or bunch grasses that develop pedestals of matted stems, giving the vegetation a lumpy appearance. In the tropics they are found above the forest limit on some high mountains—e.g., in New Guinea and East Africa.At the higher latitudes of the Southern Ocean they form the main vegetation of subantarctic islands.Small changes in management and usage can convert one to the other.Temperate grasslands are somewhat drier than tropical grasslands and also colder, at least for part of the year.