Deserts are found where moisture is so lacking that a continuous, permanent vegetation cover cannot be maintained. Cenozoic Era (65.5 million years ago to the present).Indeed, the grass family itself (Poaceae or Gramineae) evolved only early in this era.
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.
In wet tropical regions these types of grasslands may be very dense, such as those in East Africa that are dominated by climates are varied, but all large regions of natural grassland are generally hot, at least in summer, and dry, though not to the extent that deserts are.
In general, tropical grasslands receive 500 to 1,500 millimetres (20 to 60 inches) of wet season.
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.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.They are also typical of the drier, colder parts of New Zealand and the southernmost regions of South America.Not all natural grasslands, however, arise from climate-related circumstances.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.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.Our editors will review what you’ve submitted and determine whether to revise the article.Join Britannica's Publishing Partner Program and our community of experts to gain a global audience for your work! Grasslands occur in environments conducive to the growth of this plant cover but not to that of taller plants, particularly trees and shrubs.