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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.
The tropical grassland climate overlaps very broadly with that of savanna.
As previously stated, these vegetation types differ little from each other, a savanna being merely a grassland with scattered trees.
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.
A dynamic balance commonly exists between grasslands and related vegetation types.
Droughts, fires, or episodes of heavy grazing favour grassland at some times, and wet seasons and an absence of significant disturbances favour woody vegetation at others.
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.
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.