Savannas are characterized by a primary cover of perennial grasses, often one to two meters tall. There are tree, park, shrub, and grass savannas. Tree savannas are also known as woodland savannas. Shrubs and trees, if present, are sparse. Woodland savannas are often identified by the dominant tree found, such as the acacia, palm, and pine savannas of Earth.
Savannas form when some element of the environment restricts the growth of trees. This may be due to extreme drought, damage due to browsing animals, frequent fires, toxic soil, or particularly thin soil. If the savanna is accompanied by scattered trees, the trees are species unusually resistant to the hazard. Grasses have evolved to thrive in regions where trees falter. These factors often change over time, causing dramatic shifts in the ecosystem.
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