The Chihuahuan Desert
Main sources: Dimmit, Mark, "Biomes and Communities of the Sonoran Desert Region", in Phillips, Steven & Patricia Comus, eds., 2000, A Natural History of the Sonoran Desert, Tucson: Arizona-sonora Desert Museum Press, pp. 3-18; Ricketts, Taylor et al, 1999, Terrestrial Ecoregions of North America, Island Press: Washington, D.C.; Shreve, Forrest, 1951, Vegetation of the Sonoran Desert, Washington, D.C.: Carnegie Institute of Washington Publication 591; McClaran, Mitchell & Thomas Van Devender, eds., 1995, The Desert Grassland, Tucson: University of Arizona Press; Burgess, Tony, 1995, "Desert grassland, mixed shrub savannah, or semidesert scrub?", in McClaran & Thomas op. cit., pp. 31-67; Brown, David, ed., 1994, Biotic Communities: Southwestern United States and Northwestern Mexico, Salt Lake City: University of Utah Press.
Of the four major North American deserts, the Chihuahuan is the southernmost (which would seem to make it the most tropical), but it is also high in elevation and unprotected by barriers to Arctic air masses, so it suffers hard winter freezes. This (in addition of course to its aridity), limits its vegetation -- trees are rare -- though it receives more rainfall than our other deserts (more however in summer than in winter) and has a remarkable variety of plant species. According to some classifications, this desert stretches from central Mexico (where it is bounded on both sides by Sierra Madre), northward into west Texas, across southern New Mexico, and well into southeastern Arizona (see Ricketts map, below). But other classifications exclude it from our area. Its extension into the San Pedro River Drainage is disputed.
In the map below, adapted from the World Wildlife Fund's classification (Ricketts et al, cited above) pink represents the Sonoran Desert (without indicating subdivisions), the orange the Chihuahuan Desert, the blue-green the Madrean Sky Islands (or "Sierra Madre Occidental Pine-oak Forests"), and the green the Arizona Mountains Forest. (Ricketts et al, Figure 2.2.) (See this Link for discussion of the differences between these latter two zones.)
On this map, the approximate course of the San Pedro River is indicated in dark blue, and the Galiuro Mountains are located at the "G" in "Galiuros". The Chihuahuan Desert in this conception runs nearly the full length of the San Pedro River drainage, while Sonoran Desert does not enter our area. This sharply contradicts the geographical model of Shreve and others (see Arizona Uplands), which would extend the pink area considerably further southward along the San Pedro.
In fact, Shreve (cited above, p. 22) characterized the Chihuahuan Desert as "isolated" from the Sonoran, stating that in southeastern Arizona "the separating region is a high plain, studded with small mountains, on which the conditions are arid but not desert and the vegetation is a transition between desert and grassland" (p. 23). Such contradictions appear to arise out of the sporadic incidence of what McClaren and Van Devender (cited above) call "Desert Grasslands" throughout our area.
According to the World Wildlife Fund's criteria, vegetation of the Chihuahuan Desert is characterized primarily by shrubs (in Mexico) and grasses (in the United States). Creosotebush (Larrea divarecata) is the dominant species, along with Tarbush (Florensia cernua), shrubby growths of Mesquite (Prosopis articulata), and Acacia (Acacia spp). (Both Mesquite and Acacia are also characteristic of the Sonoran Desert, but not Tarbush.) Within the United States, grasses like Bush Muhly (Muhlenbergia porteri), Blue grama (Bouteloua gracilis), and (in the bottomlands) Big Sacaton (Sporobolus wrightii) are prominent (Ricketts et al, pp. ). Shreve, on the other hand, hardly mentions grasses in his classification of the Sonoran Desert (pp. 33-4), so in part these questions of classification for our area may depend on how much attention is being given to grasses in such schemes.
Burgess (cited above) deals with these problems in his essay, subtitled "The dilemma of coexisting growth forms". Burgess suggests that, in light of the widespread mixture of shrubs and grasses, terms such as "grassland" or "scrub" (which imply some kind of ideal geographic separation of the two general types of plant) are less suitable than concepts which designate the fact of their (probably long-term, if unstable) mixture -- for example, "mixed shrub savannah". Applying this notion to the general area formerly occupied by Apaches, he proposes the term "Apacherian mixed shrub savanna". The Nature Conservancy's classification appears to recognize a pattern of this kind as a broad intermediary zone between the Sonoran and Chihuahuan Deserts, which they label "Apache Highlands" -- see also the TNC map. TNC however includes in this ecoregion the "sky Islands" as well as the grasslands. For example, they include the Muleshoe Preserve in the "Apache Highlands", also referring to it as "a Sonoran/Chihuahuan transition zone, semi-desert grasslands, and 'Sky Island' mountain plant communities". Our Saguaro-Juniper areas include the first two components (but only a few remnant outliers of the third).
David Brown (1994, cited above) agrees with others in identifying the "Chihuahuan Desert Scrub" with a "distinctly summer rainfall regime" and with significant periods of freezing temperatures. He says that over hundreds of miles of Chihuahuan Desert, the diploid form of Creosotebush, Tarbush, and Whitethorn Acacia are the dominant shrub species. (But see our own local observations on creosotebush concerning the application of this Ecoregion designation to the Cascabel area.)
More links to this subject:
UTEP Lab for Environmental Biology
(useful pages on definition of this desert; includes discussion of various definitions)
WWF NA 1303 (World Wildlife Terrestrial Ecoregion)
The Chihuahuan Desert (a New Mexico State University link)
Terrestrial Ecoregions of the World (a World Wildlife Fund link)