Arizona-Sonora Desert Museum Digital Library

Grassland is a semiarid biome characterized by warm, humid summers with moderate rain and cold, dry winters. (The central valley of California is an exception; it is a winter-rainfall grassland at a lower than typical elevation.) Grass is the dominant life form; scores of species form a nearly continuous cover over large areas. Other well-represented life forms are annuals and geophytes (herbaceous perennials such as bulbs that die to the ground each year). Populations of trees, shrubs, and succulents are kept at low levels by periodic fires during the dry season.

Most of the grasslands in the western states are intermediate between the true prairies of the American Midwest and deserts. They are called semi-desert or desert grasslands. (Again the California grasslands are an exception. They are heavily influenced by the unique California floristic province and not much by the Midwest prairies.) Compared with prairie grassland, the grasses in desert grassland are shorter, less dense, and are more frequently interspersed with desert shrubs and succulents. Desert grassland or chaparral borders the northern Sonoran Desert on the east.