In North America, the word savanna is also used to describe the tall grass prairies that have scattered trees, typically oaks. This is a temperate climate (colder than the tropics). It is called an "oak savanna" since the oak tree is the main tree. At least half of the view of the sky must be open. If there are more trees, then it is called a grove or a woodland. If there are fewer trees, it is a prairie. (If there are many kinds of trees and brush and very little grass, then it is a forest). Oak savannas are found in California and Oregon on the west coast. In the southwest it is found in Arizona, New Mexico, and northern Mexico. In the midwest it is found in the tall-grass prairie. Often a grove or a savanna is east of a river, which stopped the fire often enough to let trees grow. Large oak trees will survive grass fires. The savannas in the midwest of the United States are kept open by regular fires, not because it is too dry for many trees.