Changing Sonoran Desert Vegetation Patterns
From the Mid-19th Century to 1960
Note: This essay is based almost entirely on the book by Hastings, James & Raymond Turner, 1965, The Changing Mile: An Ecological Study of Vegetation Change with Time in the Lower Mile of an Arid and Semiarid Region. University of Arizona Press. Bear in mind that Hastings and Turner's research dates to the early 1960s, and therefore the "present" in this essay ends with conditions obtaining at about that time. 1960 had seen a previous period of both high temperatures and low precipitation, according to the Southeast Arizona Climate History provided by the Arid Lands Institute at the University of Arizona, Tucson -- see especially Climate Data Tables 1 and 6.
Today during most of the year the San Pedro River in Arizona presents -- except in parts of its upper reaches or where rock formations force water to the surface -- mostly dry, sandy streambeds which support little vegetation. Above these beds (which have cut abrupt vertical banks as much as 30 feet deep), are 19th-century floodplains now covered mostly with mesquite bosque. When the summer rains come, these streambed channels are filled bank to bank with raging, muddy torrents, which actively further the process of cutting apart and into the old floodplain. After the floods flows decline and -- until the next flash flood -- are likely to cease completely after a brief time.
But the San Pedro River Valley was very different before the time of the American Civil War. American travelers through the area during the mid-19th century provide the best descriptive accounts of the San Pedro River Valley before the large changes of 1890 and thereafter, which made the river what it is today. These mid-19th century accounts describe a river which was overall rather little incised by channels (though there were some stretches where degrading erosion did cut substantial channels), and the river flowed perennially (though the flow did go underground sometimes at various stretches). The flow was mostly quite slow, and "about every five miles there is a beaver dam this is a great Country for them" (1858). "Fish are abundant in this pretty stream. Salmon trout [perhaps Colorado River Squawfish?] are caught by the men in great numbers". In some places, "the bottom lands of the San Pedro... were covered with a dense growth of sacaton grass...." (1858), and below one cut, the river "widens out, and from beaver dams and other obstructions overflows a large extent of bottom land, forming marshes, densely timbered with cottonwood and ash...."(1859) Elsewhere, there were treeless marshes -- "cienegas" -- for example a large one extended from near modern Benson to old Tres Alamos, with adjacent areas dominated by grasses. A high degree of marshiness along the River in our area is indicated by the prevalence of malaria, which became quite serious in the late 1860s at Camp Grant near the junction of Arivaipa Creek with the River. On the other hand, in some places "the river bottom is a dense thicket of bramble bush, mostly muskeet...."
From the time of their earliest incursions the Spaniards brought sheep, horses, and cattle down the San Pedro River Valley, and in the 1690s Fr. Eusebio Kino gave out cattle and other livestock to the Native Americans he was seeking to missionize. By 1700 the intensity of grazing to the south, in Sonora, was high, and yet arroyo cutting did not occur in that region until nearly two centuries later -- the same time it began in our area. While Mexican settlers did large-scale cattle-raising in the 1820s-30s, really massive cattle ranching only began with Anglo-American stock-raising after the Civil War -- especially following the U.S. Army's reduction of Apache raiding after 1870, and the arrival of the transcontinental railroad in Tucson in 1880. Trail drives from Sonora and Texas even earlier brought in thousands of head -- Hooper and Hooker alone brought in four herds totaling 15,500 animals in 1872. Where there had been only 5,000 cattle in all of Arizona Territory in 1870, by 1880 the San Pedro Valley alone supported perhaps 8,000 head of cattle and 10-12,000 sheep. A ballyhoo campaign holding that "grass is gold" brought an inflow of capital and entrepreneurs. By 1890 one census showed over a million range cattle in Arizona as a whole, and the Southwestern Stockman of January 1891 conceded that "the malady of overcrowding is with us in an aggravated form...."
Ecological disaster followed: to quote further from Hastings and Turner's account, "The summer rains of 1891 were well below normal. In the arid foresummer of 1892 stock began to die. The summer rains of 1892 again were scanty, and by the late spring of 1893 the losses were 'staggering' (Report of the Governor 1896:22). 'Dead cattle lay everywhere. You could actually throw a rock from one carcass to another".... Hastings and Turner then summarize the impact of the overgrazing on regional ecology: "Thousands of square miles of grassland, denuded of their cover, lay bared to the elements. The cropping unquestionably weakened the old plant communities, leaving them open to invasion; it unquestionably upset the balance between infiltration and runoff, in favor of the latter." (page 41)
However, Hastings and Turner go on to say that flooding in this region can be documented well back into the Spanish period, and that flooding has likely been occurring as long as arid and semiarid conditions have prevailed, due to chronically sparse ground cover coupled with occasionally intense rainfalls. Still, before the 1880s, floods tended to spread out over flood plains in shallow sheets, a pattern which changed only in the mid-1880s: In 1886, "'the water in the San Pedro River was higher than it was ever known to be...." (H&T p. 42) Floods occurred in 1888, in 1889, and then in 1890 large channels were carved upstream by the floods in both the San Pedro and the Santa Cruz Rivers. "Of the country down the San Pedro, from Tres Alamos to the Gila [Captain Van Alstine]... says, 'all of it is gone, destroyed, torn up, 'vamosed down with high water.' He never saw such a destruction in all his life."
Hastings and Turner argue that removal of ground cover -- by whatever agency (human, drought, or cattle) --will unfailingly increase runoff and surface erosion. Human agency such as roads, trails, and drainage ditches definitely tend to become runoff points which rapidly enlarge because of their barrenness. However, geological evidence also indicates that a number of erosion cycles have occurred in the American Southwest since late Pleistocene times, and Kirk Bryan has argued that "Arroyos similar to and even larger than the recent arroyos were cut in past time. As these ancient episodes of erosion antedate the introduction of grazing animals, they must be independent of that cause.... It seems reasonable that the present arroyo is essentially climatic in origin... [and that overgrazing has been] merely the trigger pull which timed the arroyo cutting in the thirty years following 1880." (H&T, p. 43; bold print added)
Hastings and Turner's own research (published in 1965) largely supports this view. Comparing photographs of various parts of the Sonoran Desert region taken from the 1880s into the early years of the twentieth century with those taken at the same sites during the 1960s, they found some striking changes in vegetation. First, at the higher elevations of the the Oak Woodland zone (roughly speaking, 4,000-6,000') they found definite decline in the presence of oaks below 4,500', whereas at their upper elevations oaks have increased. So the Oak Woodland zone has retreated upwards. Lower levels of former Woodland have been invaded by mesquite, one-seed juniper, Ocotillo, Sotol, and a number of other plants.
At elevations of 3,000 to 4,000' in the San Pedro River Valley Desert Grassland near Tombstone and Charleston, they found that invasions of mesquite, acacias (especially Acacia vernicosa, related to Whitethorn), burroweed and other woody species from the Chihuahuan Desert have largely replaced the grassland.
Throughout the Arizona Uplands portion of the Sonoran Desert ecoregion, the Paloverdes (Blue and Foothill) have increased in the upper elevations, but decreased in the lower elevations. Mesquite has increased, including spreading into higher elevations --but in the harsh, arid habitats of the Lower Colorado River Valley portion of the Sonoran Desert, near the limits of of its tolerance, Mesquite has declined. So the evidence suggests that these distinctively Arizona Uplands plants are "migrating upward, away from the low, hot, dry elevations and into the higher, cooler, moister ones." (H&T p 270)
Summarizing their research, Hastings and Turner say that "the uniform onset of erosion [in the Sonoran Desert region] points to the operation of a broad, regional factor like climate." "There have been striking trends toward a more arid vegetation, the dominant pattern being an upward displacement of plant ranges" (oak retreating upward, paloverdes and mesquite retreating from dry, hot habitats while advancing upward). "These movements are consistent with the hypothesis of drier, warmer conditions at all elevations." (p. 287) The evidence for these trends is summarized in the Figure shown below (p. 282).