Plant communities of southeastern Arizona
— Daniel A. Dunham

Southeastern Arizona is an area of isolated mountain ranges and intermontane basins. Sonoran Desert flora on the eastern and northern edge of the Mexican Plateau follow the valleys of major drainages into grasslands of the highlands. Chihuahuan Desert flora characteristic of the lower Rio Grande Valley in New Mexico intermingle with desert grassland above the upper limits of the Sonoran Desert zone. These two distinct desert flora are grouped together along with Mohave desertscrub to comprise the Lower Sonoran Life Zone with an upper eleva-tion limit of 1,070 to 1,220 m (3,500 to 4,000 ft). The creosote-bush (Table 1) is everywhere conspicuous within this life zone and where abundant clearly delineates the boundary between the Lower and Upper Sonoran Life Zones. The Upper Sonoran Life Zone is comprised of grassland at its lower limit and evergreen woodland near its upper limit. The next elevation band of vegetation is the coniferous forest belt, extending from a minimum elevation of 2,000 to 2,150 m (6,500 to 7,000 ft), depending on slope exposure, to the tops of the highest mountain ranges (Chiricahuas, Huachucas, Santa Ritas, Santa Catalinas, Galiuros and Pinalenos).

Climate as well as topography has influenced the species composition in this part of Arizona. Two major continental climatic trends are present in this area, reflecting variations in climate to the east and west. In New Mexico summer precipitation dominates, while in California winter precipitation is dominant. In addition, there is a decrease in summer precipitation on a northward trend from tropical Mexico. Precipitation at Tucson is nearly bi-seasonal, grading to "California" precipitation (and vegetation) to the northwest. "Mexican" rains and flora follow a southeastern gradient from Tucson. Micro-climates are also present. Air drainage from cold mountain summits to the valleys create "thermal belts" where saguaro cactus are abundant below mid-elevation on mountain foot-slopes. Slope exposure affects the upper and lower elevation limits of plant species. For example, oak-pine woodland is first encountered 365 m (1,200 ft) higher on the southern slopes of the Santa Catalina Mountains than on the northern slopes.

 

Soils also affect plant distribution. In the arid southwest, topography is the most readily observable factor in the development of desert soils. Rockland is predominant in the foot-slopes. Rocky, gravelly soils are characteristic of upper alluvial fans and are developed from parent material eroded from a single mountain range. The alluvial soils of the basins, derived from a larger area encompassing several mountain ranges, are finely textured. The soils at the edge of the flood plains or playa lakes and in upland areas at the lower end of the fans are also derived from mixed sources, but grade from the fine soils of the flood plains to the coarse textured soils of the upper fans. Above the rocky footslopes increased soil moisture be-gins to influence soil weathering, and organic matter content increases toward higher elevations.