Arizona-Sonora Desert Museum Digital Library

This was Shreve's seventh subdivision of the Sonoran Desert. It has since been reclassified as foothills thornscrub community and is no longer considered part of the desert biome because of its greater rainfall, taller trees and cacti, and denser vegetation.

Shreve's delineation of the Sonoran Desert's boundary and subdivisions are the most widely accepted. There are at least five other major attempts to define this area with dramatically differing boundaries. One version excludes most of Baja California from the Sonoran Desert. Another includes the Mohave as part of the Sonoran Desert. (Indeed, it is difficult to distinguish the two along the currently accepted boundary.) These differences of interpretation reflect the great diversity of geography and biota found here.

The discussion is not yet over. Time will determine whether Arizona Upland and the Plains of Sonora will remain parts of the Sonoran Desert or be reclassified as thornscrub. Whatever they are called, all of these regions are fascinating places for nature lovers, whether they are classified as scientists or tourists.

Thornscrub is intermediate between the desert and tropical forest biomes. The vegetation consists largely of short trees, ten to twenty feet (3-6 m) tall, and shrubs, with cacti also being common in the “New World” communities. It is generally more dense and taller than desert vegetation, and many species are thorny. Annuals and herbaceous perennials are abundant, and vines — a primarily tropical life form — are well represented. During the dry season most perennial plants are drought-deciduous (as opposed to plants of more temperate regions which are cold-deciduous). In contrast, the rainy season, though short, is moderate and dependable and the vegetation grows lush. The climate is nearly frost-free, so temperature is not limiting; the vegetation is determined by the alternating dry and wet seasons.