San Pedro Valley Flora Today

At left, agaves bloom and ocotillos sport their leaves among Opuntia, upland shrubs, and rainy season desert grasses -- near Sierra Blanca at about 4,000 feet elevation, August 2001.






Sources: Shreve, Forrest, 1951, Vegetation of the Sonoran Desert, Washington, D.C.: Carnegie Institute of Washington Publication 591; Phillips, Steven & Patricia Comus, eds., 2000, A Natural History of the Sonoran Desert, Tucson: Arizona-Sonora Desert Museum Press; Kearny, Thomas & Robert Peebles, et al., 1960, Arizona Flora. Berkeley: University of California Press; Gould, Frank, 1951, Grasses of the Southwestern United States, Tucson: University of Arizona Press; For useful descriptions of many of the plants referred to below, see the US Forest Service Fire Effects Information System. "Feis" provides a database containing descriptions of almost 900 plant species, about 100 animal species, and 16 Kuchler plant communities found in North America. While these descriptions focus on the plant's relationship with fire, background descriptions and full bibliographies are also provided. A useful source for images is the Natural Resources Conservation Service Plants Database. On our own website, see also Hastings and Turner 1965 (and other links marked out below).

A Brief Overview of Our Defining Flora Today

[Note: for further descriptions of plants listed below, click on the underlined links associated with plant names.]

The limiting ecological factors in Western North America are available moisture and minimum winter temperatures. The warm desert of our area has more diverse species than the cool desert further north (the Great Basin). Our Sonoran (a “subtropical”) Desert rarely sustains freezing temperatures, while the Chihuahuan (a “warm-temperate”) Desert does have long freezes, with its higher elevations.

The one common important plant found in both of these warm deserts is creosote bush (Larrea tridentata) -- it’s both cold-hardy and drought-tolerant, found below 4,500'. See the link just given for varietal contrasts between the two areas.

(1) The Sonoran Desert has two subdivisions in our area:

(A) the Colorado Desert, which is true desert, located along the Colorado River, dominated by creosote bush, and

(B) our area, the Arizona Uplands portion:

At right, Arizona Uplands vegetation on the flanks of Lower Hot Springs Canyon -- in lower-left foreground, blooming Brittlebush and scattered grasses; behind these plants, the new green leaves of Mesquite trees with some Opuntia plants close by; behind them in the foothills, scattered Saguaro cacti and yellow-blooming Paloverde trees -- photographed in May 2001, elevation about 3,300 feet.



The Arizona Uplands is typified by the association of the following plants:

paloverde (Cerdicium microphyllum) (cold-hardy to 15 degrees -- so found up to 4,000') and

saguaro (Carnegia gigantea) (Mostly below 3,500'; can’t stand a sustained freeze)

Also common are:

mesquite (Prosopis velutina)(Below 4,500')

creosotebush (Larrea spp.) (Below 4,500') [See above.]

ocotillo (Fouquiera splendens) (cold-hardy to 10 degrees; needs summer rains)

white thorn acacia (Acacia constricta)  (2500-5,000')

"Desert" Hackberry (Celtis pallida): 1500-3500' in washes, canyons, open desert.

“Netleaf” Hackberry (Celtis reticulata ):  500-6,000' in moist soils, streams, but sometimes also on hillsides in upper desert and oak woodlands.

jojoba (Simondsia chinensis) (1,500-5,000')

bursage (triangle-leaf) (Ambrosia deltoidea) has triangle-to-lance-shaped leaves; this shrub is 1'+ high, rounded, found from 1000-3000'; hardy to 22 degrees, drought tolerant.

brittle bush (Encelia farinosa) (below 3,000').

Sometimes the defining association for our area is phrased as "Saguaro + leguminous trees" -- the latter referring to the family of beanpod-producing forbs/shrubs/trees that includes not only paloverde and mesquite but also the acacias, ironwood, and a variety of other important plants. See this link for an account of this family, all of which put scarce nitrogen into our soils: Fabaceae.

And see Ecoregions: The Sonoran Desert and The Arizona Uplands for more details on these two important divisions of our area. (This last provides a summary of the "characteristic vegetation" of various parts of our area, as delineated by Forrest Shreve.)

(2) Bounding this Arizona-Uplands complex:

Various terminologies are in use for describing adjacent (and in some places intermingling) zones. One major complex often defined as contiguous with the Arizona Uplands of the Sonoran Desert is:

The Northern Chihuahuan Desert (see this link for more details) -- its major distinctive character is that it rarely sustains arborescent plants, but many small succulents -- small agaves like shindagger (Agave schottii and lechuguilla)) -- are typical.

Our (Saguaro-Juniper) altitude elevations run from just under 3100' at Cascabel to 4356' at the peak of Sierra Blanca. Our Northeastern corner, with hills at 4300', has very few saguaros and trees (but lots of shindagger). In that sense, this particular corner is more like the Chihuahuan Desert than the Sonoran:

At left, a view in the northeast corner, where saguaros and trees are absent but the hills are covered with Shindagger agaves (left foreground), mixed in with Heteropogon contortus and other grasses, scattered shrubs, and patches of Yucca baccata.





Various writers have characterized portions of our area as

(3) "Grassland",

but here we follow Forrest Shreve (in Gould, cited above, in an essay entitled "The role of grasses in the vegetation of Arizona", pp. 10-15), who views the true grasslands of our Arizona region as one kind of "transitional" zone between forest and desert -- the kind where pure stands of grasses form a closed cover. Shreve argues that in Arizona such grasslands are found only above 3,500 feet in elevation, in locations where rainfall is spread widely throughout the year, and where soils are present which possess considerable depth and uniform, medium texture (the exception to this being some localities of lower elevation which possess "small, poorly drained basins subject to seasonal flooding and known locally as 'swales'," where deep, fine-textured and moisture-retentive soils may suppport stands of coarse grasses like Hilaria mutica"). Our particular area, however -- though at certain times and places, grasses may flourish and become visually quite conspicuous -- is largely one "in which grasses and small perennial forbs and shrubs mingle but do not form a closed cover." (pp. 10-12) Exceptions occur in the bottom of washes, for example in the fairly well-watered wash below our Trail Tank, where at around 3800' elevation we sometimes find close stands of spruce-top grama grass:

Shreve does suggest that formerly (in the 19th century), bush muhly (Muhlenbergia porteri) was abundant in elevations below 3500' in our area (so much so, apparently, that in some locations it was harvested in substantial amounts as hay), but in recent times it is found mainly scattered and protected from grazers within shrubs like creosotebushes and cacti. Our grasses become more abundant as we rise from 3,500 to 4,000' in elevation, where perennial species appear as individuals or in clumps and even colonies. Often, good stands of grass cluster around the bushes and arborescent cacti, where these nurse-plants provide better shade and moisture and some protection from grazing. Grasses increase in density as you approach 5,000 feet in elevation in our area, but this elevation lies above the highest point of our Saguaro Juniper lands (which attain less than 4,400' elevation). The photograph two images above, which shows fairly dense stands of grass mingled with shindagger cacti, looks toward our highest hill of the far Northeast Corner of S-J lands, which stands at an elevation of 4,321 feet. Aside from elevation, in our area grassland development depends strongly on the nature of the soils: says Shreve, "it is best on the deep loam of level valleys or gentle slopes, and poor on shallow or stony soil. Limestone soils are poor in grasses at all elevations and in all situations." (p. 13) Since Saguaro Juniper lands have plenty of shallow or stony soils, and considerable areas of limestone soil, we do not lack in terrains that are "poor in grasses." For more on the subject of grasses in our area, see Apache Highlands Grasslands, and on grasses per se, see grasses.

While our area thus has no pure "grassland" as such, grasses are very important to our lands and the creatures who traverse them. The following are some of the most important of these grasses.

Our desert grasses have evolved a number of distinctive features, including their modes of photosynthesis (the distinction between C-3 and C-4 pathways), and their responses to drought/frost/fire/grazing -- in perennials, dying back to the crown or root; in annuals, dying to seed; (some of our grasses appear to be transitional between "perennial" and "annual"). These adaptations enable them to respond quickly to more propitious conditions when rains return.

C-3 and C-4 Grasses

To avoid technical discussions beyond the skill of the writer, let it suffice to say that in the 1970s scientists discovered that, whereas most plants create organic compounds with three carbon atoms through photosynthesis ("C-3"), a small number make compounds with four carbon atoms ("C-4"). These two types of grasses thus differ in conversion efficiency, which enables C-4 plants to hold their leaf pores closed longer (pores must open during photosynthesis), and thus they reduce their water loss through transpiration under conditions of high temperature and low humidity. C-4 grasses are therefore much more common in desert grasslands than elsewhere: more than 95% of our grass production is from grasses having the C-4 pathway (McClaran op. cit., p. 14-15). For a comprehensive listing of Arizona desert grasses, see the "Poaceae" at University of Arizona Agriculture. The following list includes our most important general types.

(Click on the blue, underlined links)

Bouteloua spp. (Grama)

Muhlenbergia spp. (Muhly; Deer Grass )

Digitaria (Arizona cottontop)

Setaria (Plains Bristlegrass)

Sporobolus wrightii (Sacaton)

Heteropogon (tanglehead)

Aristida spp.(three-awn)

Hilaria spp.  (Tobosa et al.) (Important in Arizona; second only to grama)

Sporobolus spp.(drop-seed)

Oryzopsis (rice grass)

Stipa (needle grass)

(4) Distinctive types of Vegetation are found in our Streamways:

The banks and margins of our streamways show differences in vegetation from other surfaces -- sometimes only in density and size of the common species (above), but sometimes in the presence of distinctive species. Usually both are present. Where deposits of sand deepen, vegetation becomes denser and taller. Where, as along the San Pedro River but also in some of our richer tributaries, the gradient is gradual, the floodplain has been built up by wide and slower moving floods, and soil is mostly a fine alluvial clay. Where our floodplains have a rich supply of water, they sometimes include a mingling of desert trees with those of higher elevations, for example Juniper, and these may mingle with the desert riparian trees: Cottonwoods (Populus fremontii), Goodding Willow (Salix gooddingi), Arizona Ash (Fraxinus velutina), Arizona Walnut (Juglans major), Arizona Sycamore (Platanus wrightii), and Netleaf Hackberry (Celtis reticulata), all of which are found along with Velvet Mesquite (Prosopis velutina).

In our larger and moister tributary washes, typically, further downstream where alluvial floodplains widen and the water table lowers, the number of trees and shrubs greatly lessens. Cottonwoods and Willows may continue to appear grow where water supply remains favorable. (For more on the ecologically important association of these trees in our area, see Cottonwood-Willow Gallery Forest,)

Where the floodplains are sandy, the dominant plants become riparian shrubs: Burro Bush (Hymenoclea monogyra) and Desert Broom (Baccharis sarothroides). Desert Willow (Chilopsis linearis) is often found with these two on the sandy floodplains, and all three of these riparian shrubs serve to catch silt and reduce erosion in these floodplains.

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