Ecoregions or Biomes of our Area
Main sources: Shreve, Forrest, 1951, Vegetation of the Sonoran Desert, Washington, D.C.: Carnegie Institute of Washington Publication 591; 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; Brown, David, ed., 1994, Biotic Communities: Southwestern United States and Northwestern Mexico, Salt Lake City: University of Utah Press; Brown, David, Frank Reichenbacher, & Susan Franson, 1998, A Classification of North American Biotic Communities, Salt Lake City: University of Utah Press; Bowers, Janice E, 1988, A Sense of Place: the Life and Work of Forrest Shreve, Tucson: University of Arizona Press; Whittaker, RH, & WA Niering, 1965, "Vegetation of the Santa Catalina Mountains, Arizona: a Gradient Analysis of the South Slope, Ecology Early Summer, Vol. 6 No. 4:429-452; Warshall, Peter, 1995, "The Madrean Sky Island Archipelago", in Debano, Leonard, ed., Biodiversity and the Management of the Madrean Archipelago, Diane Publishing; Gori, DF, and CAF Enquist, 2003, An Assessment of the Spatial Extent and Condition of Grasslands in Central and Southern Arizona, Southwestern New Mexico and Northern Mexico. Prepared by The Nature Conservancy, Arizona Chapter. 28 pp.
1) "Between Deserts and Sky Islands": Decisive Life Conditions in our Area
In our work as Saguaro Juniper Corporation, we start with a framework built around the area of Cascabel, Arizona along the Middle San Pedro River Valley. However, that work is set in a much broader ecological context, which we now discuss here below.
The concept of "Sky Island" most broadly denotes a type of continental or inland terrain made up of a sequence of valleys and mountains. For our area, the valleys are predominantly what ecologists call "deserts". The Biome situation of this complex is well summarized by Peter Warshall (see further in the link below): "All sky islands have a stack of biotic communities that allow vertical (as well as aspect) migration annually or during one of the planet's long-term climatic events. The valleys act as barriers or bridges to the colonization by new species that attempt to cross the intervening valley. The valleys become barriers when they contain an ecology alien to the migrating species." For more details on the implication of this duality, see this link:
2) Designations of North American Deserts based on Forrest Shreve's models:
To Forrest Shreve, the term "desert" denotes, geographically, "a region of low and unevenly distributed rainfall, low humidity, high air temperatures with great daily and seasonal ranges, very high surface soil temperatures, strong wind, soil with low organic content and high content of mineral salts, violent erosional work by water and wind, sporadic flow of streams, and poor development of normal dendritic drainage." (p. 26) Mark Dimmit (cited above) sees a desert as a place where climate is arid -- where water is "freely available only for short periods following rains" (Dimmit, 2000, p. 9). When this is combined with sustained high air temperatures, soil dryness, dry air, wind, and strong insolation, one may well feel (as Shreve put it, p. 18) as if "walking between two great fires." Today this condition of aridity is measured by the relationship of rainfall to "Potential Evapotranspiration" (PET), that is the water loss that would occur through evaporation and transpiration if water were continuously present (Dimmit, op.cit.). Placing a wide pan of water exposed to the weather and measuring the amount which evaporates provides a measure of the ratio of PET to rainfall; in Tucson, Arizona this ratio is 4.3 (4 1/3 times as much PET as rainfall annually, 100" to 12") -- barely a desert, since a ratio of 3.0 is classed as "semi-arid". Yuma, Arizona, on the other hand, fully qualifies as Desert with a ratio of 30, while Africa's interior Sahara Desert overwhelms the Tucsonan imagination with a ratio of 600 (ibid.). But the point is that water restrictions in deserts require their inhabitants to adapt in a variety of ways, in order to survive (and some even to thrive).
The North American Deserts are "Horse Latitude Desert", meaning they are in part products of the natural movement of global air masses which produces rather stable belts of dry air on the western sides of continents at the latitudes around 30 degrees, and in part of the cold ocean currents occuring along the western coasts of the continents at or near this latitude. In part these deserts are also a product of the rain-shadow effects of the mountain ranges which flank them. These arid air masses generate highly variable (and unpredictable) rainfall, high variations in temperature, and damaging effects of solar radiation, or "too much sun" (Dimmit, op. cit., pp. 10-12). All of these conditions place stresses on living things.
Focusing on the biological definition of "deserts", Shreve was struck by his first experiences there that, in contrast to the rain forests of Jamaica where he had worked, wherevegetation canopies were continuous and crowded plants competed for light, in the deserts near Tucson closed canopies were absent and one could walk freely through open spaces between the plants. Rejecting the dominant views of plant ecologists at the time he was working -- who emphasized competition between plants as the prime factors determining vegetation patterns -- he argued that in deserts, plants must primarily adapt to their physical conditions -- temperature, rainfall, soil moisture, and the like. As he put it, in the desert, "the make-up of the vegetation, the relative abundance of the different types or species, and even, to a large extent, the density of the stand itself are products of the conditions which control germination and the activities of the seedling during its first twelve months." Consequently, deserts may accommodate a certain kind of diversity (unlike, for example, tropical rain forests where all the dominant plants are evergreen broadleaved trees, or grasslands where one species may completely dominate a stand), diverse types of desert plants may develop succulence in response to aridity. Due to the harsh conditions desert plants face, species tend to be low but unequal in stature, tend to form open stands, tend to mix dissimilar forms; hence there is a scarcity of truly "subordinate plants" - those involved for example in rain forest layering (liana vines, epiphytes, etc). The relative scarcity of plant litter (what there is, is rapidly consumed by termites, not much by fungi/bacteria) prevents such material from permanently enriching the earth where it falls.
Forrest Shreve spelled out the relationships among the four great North American deserts -- the Great Basin, the Mohave, the Sonoran, and the Chihuahuan -- designating each by a name reflecting where more of its area lies, and defining each as a biological unity based on the distinctive features of its plant life, each part of the boundary being set by the limit of some particular type of vegetation and of a group of species. The boundary of the regions is more sharply defined where topography is abrupt, and more ill defined in level or rolling regions. (pp. 10-11) Here we will focus on the Sonoran Desert, with its "Arizona Uplands" subdivision, comparing and contrasting it with the Chihuahuan Desert. This comparison has importance because of current controversy over the status of our area in relation to these Ecoregions.
Briefly put, Shreve drew the contrast between the Sonoran Desert and the Chihuahuan Desert as follows:
Sonoran Desert: mild winters (freezing temps no more than 24 hours); maximum elevation around 3450 feet (1050 meters); biseasonal rainfall (approaching equal quantities of summer & winter rains); vegetation (above 1000') "many small trees and a strong representation of cacti of many types" (p. 33).
Chihuahuan Desert: cold winters (occasional 72 hours of freezing temperatures); elevation mostly above 3500 feet (1070 meters); rainfall 70-80% in summer (mid-june to mid-september); prevailing vegetation is mainly low shrubs. (p. 31)
This contrast is significant in light of continuing disagreement over the Ecoregional designation of our part of the San Pedro River Valley.
The differences: briefly put, disagreements arise over the designation of the country in and around our Saguaro Juniper area (the vicinity of Cascabel, Arizona) as "Arizona Uplands series of the Sonoran Desert" (our understanding), versus as "Transitional zone" (Shreve's apparent designation of Cascabel), versus as "Chihuahuan Desert" (designations by the World Wildlife Fund and by Brown & Lowe (1994). The following links examine the issues from the perspective of Shreve's designations, followed by that of the World Wildlife Fund. Then, in part (2), we will focus on Brown & Lowe's recent models in relation to the problem.
See the following links for detailed discussion of the contrasts:
In recent years, Brown et al. (1994, 1998) have developed a systematic classification of the "Biotic Communities" of all of North America. Their classification of Biomes of the Southwest builds on the work of Shreve and others, but (like the World Wildlife Fund) designates the Cascabel area as part of "The Chihuahuan Desert":
3) Biomes constituting and contiguous with our Deserts, in relation to Brown & Lowe's 1994 models:
Brown & Lowe, as well as the World Wildlife Fund, draw substantially different Biome boundaries than those of Forrest Shreve, though their Ecoregion criteria appear to be very similar to his. Here we first present summaries of the Browne and Lowe models for our area (focusing around Cascabel, Arizona), and then we will consider the issues further in relation to our own observations.
4) Some Preliminary Conclusions on Cascabel Area designation:
Our overall sense of these patterns is, first (and perhaps most important), that the vicinity of Cascabel is definitely a "Transitional Zone" with regard to Ecoregion designations, if we take vegetation types rather than rainfall patterns as diagnostic. Shreve of course argued for such transitionality here (though we continue to wonder at his placement of the southern limits of Saguaro plants near Redington, even if that placement dated from early in the 1930s). Second, grounds do exist for considering this area a part of "Chihuahuan Desert", if we regard the presence of such plants as Yucca elata and Whitethorn Acacia as diagnostic of that designation. However, the more definitive Chihuahuan Desert diagnostics -- Tarbush and Sandpaperbush -- are rare around Cascabel, while diagnostics of the Arizona Uplands division of the Sonoran Desert are now much more common here. So it would seem that most of the area around Cascabel, AZ is tending toward plants characteristic of the Arizona Uplands Series of the Sonoran Desert Biome (with the immediate floodplain zone of the San Pedro River fitting a "Wetland" designation, perhaps transitional between a [Sub-tropical] Sonoran Riparian Zone and a "Warm-temperate [Chihuahuan] Interior Strand" (Brown & Lowe pp. 265-79). Chihuahuan Desert plants may be in retreat in our immediate area.
5) "Apache Highlands Grasslands":