Cabeza Prieta Natural History Association
Plant life of the Tom Thumb Area (and beyond)

Observations of plant /habitat relationships in the Ajo region seem to indicate that in southwestern Arizona, the composition of plant communities mostly reflects the effects of topography rather than geology. Topography in the region is highly variable whereas geology is mostly igneous.

With that in mind, we would consider the overall species makeup of the plant species composition of the Sauceda Mountains area to be a function of the elevation and distribution of topographic classes within the framework of the Arizona Uplands subdivision of the Sonoran Desert. Mountains and their adjoining bajadas (alluvial fans) and washes, exemplified by the Sauceda Mountains, are considered to possess the conditions where species composition and diversity characteristic of the Arizona uplands will be found. These conditions would include higher precipitation and cooler daytime temperatures related to the elevation, high rates of runoff and erosion related to steep topography, and relatively rapid soil water percolation (bajadas) related to gradual slopes and coarse textured soils. The generally steep topography of the area also contributes to a condition of higher nighttime temperatures as contrasted to the adjacent low-lying desert plains, an important consideration, especially in winter.

Conditions and plant species makeup of the Sauceda Mountains in general are similar to the lower slopes of the Ajo Mountains of Organ Pipe Cactus National Monument, but with a lower abundance of high moisture- requiring species characteristic of the latter area. The Ajo Mountains are much higher and more massive, intercepting significantly more precipitation than the Saucedas, and actually project upward beyond the Sonoran Desert. Owing to a lack of climatic data in the Saucedas, we can only guess at what might be the average annual precipitation, but an estimate might be 9 to 10 inches.

Ryan's Canyon Wash This wash drains a basin in the southcentral Saucedas bounded by Coffeepot Mountain, Stimson Peak, Tom Thumb and a group of high, lavacapped plateaus to the north. Drainage is into Midway Wash crossing the Midway Plain of Area B.

Ryan's Canyon is a typical steep gradient drainage exiting a mountain watershed. Vegetation is typified as a riparian community comprising an assemblage of low trees, tall shrubs, perennial herbs, grasses, and annual herbs, most of which are not found in upland areas of the desert. The dominant species here appears to be blue paloverde (Parkinsonia florida) with some velvet mesquite (Prosopis velutina) occurring as a secondary tree species. Desert-willow (not a true willow) (Chilopsis linearis) is scattered sporadically along the wash. Some shrubs noted as being present along the wash were desert hackberry (Celtis pallida), bitter condalia (Condalia globosa), chuparosa (Justicia californica), graythorn (Ziziphus obtusifolia), Parish wolfberry (Lycium parishii). Brownfoot (Acourtia wrightii), a conspicuous perennial herb, was seen.

Bajadas Alluvial fans at the base of steep mountains are the site of soil deposition from the adjacent steep slopes. Such soils are relatively deep, rocky and gravelly and permeable. Moisture tends to soak into these areas more so than on the adjacent steep slopes, making them somewhat more favorable for vegetation growth. Also, as are the steep mountain slopes, these areas are sloping and elevated above the washes and valley bottoms. Heavy cold air tends to slide off and collect in the low areas at night. As a result of this, nighttime temperatures, especially in winter, become much colder in valley bottoms becoming a limiting factor for the survival of cactus species and other frost sensitive plants. Warmer winter nighttime temperatures coupled with greater soil permeability and aeration on bajadas contrasted to valleys make them relatively more favorable for the survival of many types of plants, especially cacti. In the Tom Thumb area, stands of saguaros (Carnegia gigantea) are much thicker than on either the valley bottoms, washes or mountainsides. This is basically true throughout the Sonoran Desert. Also on the bajadas, chain-fruit cholla (Cylindropuntia fulgida), teddy-bear cholla (Cylindropuntia bigelovii), buckhorn cholla (Cylindropuntia acanthocarpa), foothill paloverde (Parkinsonia microphylla), creosote bush (Larrea tridentata) (although not as abundant as on the valley bottoms), triangle-leaf bursage (Ambrosia deltoidea), strawberry hedgehog (Echinocereus engelmannii), brittlebush (Encelia farinosa), and jojoba (Simmondsia chinensis). Some less abundant, although fairly common perennials seen were pink fairy duster (Calliandra eriophylla), Emory and mountain barrel cactus (Ferocactus emoryi and F. cylindraceus) (never real abundant anywhere), white ratany (Krameria bicolor), and ironwood (Olneya tesota). A thick crop of ephemerals emerging as a result of the heavy November rain was evident. Observed were very abundant desert hyacinth (Dichelostemma capitatum), Mexican gold poppy (Eschscholtzia californica ssp mexicana), and popcorn flower (Cryptantha angustifolia). Somewhat less abundant was bare-stem larkspur (Delphinium scaposum).

Mountainsides Steep slopes support most or many of the species abundant on the bajadas with the addition of some that are absent or not often see in the latter area. Plant density is not as dense on mountainsides as on bajadas simply because the relatively large amount of area occupied by bare rock and cliffs reduces the amount of area that can be occupied by plants. Species that tend to grow near rocks and cliffs and in cracks of rocks are observed more on mountainsides than anywhere else. Because of the large number and variability of microhabitats and niches on mountainsides, species number and diversity are higher overall than in other areas. Because of the extremely contorted and chaotic nature of felsic extrusive deposits such as is found in the Sauceda Mountains, habitat diversity may be higher here than most anywhere else.

In addition to many of the species found on bajadas, some of the plants favored by steep rocky slopes and cliffs observed on Tom Thumb included flat-top buckwheat (Eriogonum fasciculatum), desert hopbush (Dodonaea viscosum), oreganillo (Aloysia wrightii), desert agave (Agave deserti), trixis (Trixis californica), emoryi rock daisy (Perityle emoryi), fishhook pincushion (Mammillaria grahamii), Indian mallow (Abutilon incanum), jumping bean (Sebastiania bilocularis) and scaly cloak fern (Astrolepis cochisensis). As is typical for rocky slopes, these species were not abundant but widely scattered.

Tom Thumb Introduction

Tom Thumb Geology


Return to Desert Destinations Page



Copyright Creative Commons

HTML & Programing by
Thomas R. Powell