The southernmost part, the Rio Grande Valley, is a more subtropical region with more diverse vegetation. Historically the south Texas plains was covered by open grasslands with a scattering of trees, but extensive overgrazing prompted the encroachment of brush. Today, the primary vegetation consists of mesquite, acacias, blackbrush, guajillo, huisache, granjeno, persimmon, whitebrush, and prickly pear, mixed taller trees, such as live oak, hackberry, and cedar elm in the deeper, bottomland soils. Soils of the region are generally alkaline to slightly acidic clays and clay loams. The average annual rainfall of 20 to 32 inches increases from west to east with high summer temperatures and very high evaporation rates.
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