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Environmental Conditions of the Southeastern Arizona Planning Area - Vegetation

Environmental conditions reflect the impacts of geography, climate and cultural activities and may be a critical consideration in water resource management and supply development.  The sky island ecosystems of the planning area are relatively isolated from each other, and as a result there are a large number of endemic species in the planning area mountain ranges.  These ecosystems are of major interest to resource managers due to their biological diversity and distinct biogeography. (Warshall, 2006)  Discussed in this section is vegetation, riparian protection through the Arizona Water Protection Fund Program, instream flow claims, threatened and endangered species, protected public lands and unique waters.

Click to view Figure 3.0-9

Vegetation

Four of Arizona’s six ecoregions are included in the planning area: the Arizona Mountains Forests along the northern boundary; the Chihuahuan Desert, interspersed with Sierra Madre Occidental Pine-oak Forests, which covers most of the planning area; and the easternmost extension of the Sonoran Desert in the northwest. (Figure 3.0-9)  The Chihuahuan Desert region may have grown by as much as a third in the last few hundred years due to human activities including poor agricultural practices that have eroded grasslands (CDRI, 2008).  

Because of the wide elevation range in the planning area, many biotic communities are represented, ranging from sub-alpine forests at the highest elevations in the Pinaleño, Chiricahua and White mountains to Arizona Uplands Sonoran desertscrub.

As shown in Figure 3.0-9 high elevation subalpine and montane conifer forests, consisting of dense stands of fir, spruce and aspen trees, are found at the highest elevations in the planning area, primarily in the Morenci Basin.  These areas receive much of their annual precipitation as snow.  Because of the forest density, sunlight reaches the ground and snow melts slowly, releasing snowmelt gradually to streams.  Annual precipitation amounts are about 25 to over 30 inches a year in these areas.

Conifer woodlands consisting primarily of ponderosa pine occur at elevations between 6,000 and 9,000 feet that receive about 18 to 26 inches of annual precipitation. Piñon-juniper woodlands cover large areas below the ponderosa pine forest at elevations between 5,500 and 7,000 feet that receive 12 to 20 inches of precipitation. Plains and Great Plains grasslands occur in several locations in the planning area at elevations between 5,000 and 7,000 feet that receive between 11 and 18 inches of annual precipitation. These areas are located primarily in the Bonita Creek, Cienega Creek, San Rafael and Upper San Pedro basins. The piñon-juniper woodland and madrean evergreen woodland is often intermixed with this grassland in the planning area.

At lower elevations (4,000-6,000 feet), interior chaparral is found in areas that receive 13 to 23 inches of annual precipitation.  Chaparral consists of dense shrubs that grow around the same height with occasional taller shrubs or small trees.  Chaparral communities typically are a mix of several shrubby species such as mountain mahogany, shrub live oak, and manzanita and commonly include cactus, agave, and yucca. Chaparral plants are well adapted to drought conditions.  This community is found in the northwestern part of the planning area.

Semi-desert grasslands are found in all planning area basins except the San Rafael basin, occurring in valleys between the desert and woodlands or chaparral at elevations between 3,500 and 5,000 feet that receive annual precipitation of 10 to 15 inches.  This community is particularly predominant in the Douglas and Willcox basins. Desert grasslands often contain a mixture of grasses, shrubs and small trees.

The planning area contains the only Chihuahuan desertscrub community in Arizona. Found primarily in northeastern Mexico, its easternmost extension occurs extensively in the Duncan Valley, Safford, and Upper San Pedro basins, with smaller areas in the Cienega Creek, Douglas, Lower San Pedro and San Bernardino Valley basins.  In Arizona, this community occupies plains, low hills and bajadas generally above 4,000 feet in elevation.  Precipitation averages range from about 8 inches to more than 12 inches, much of which falls during the summer. Prominent plant species include creosotebush, lechuguilla, sotol, yucca, ocotillo, acacia and mesquite. (CDRI, 2008)

Arizona Uplands Sonoran desertscrub extends into the northwestern portion of the planning area below about 3,500 feet, in Aravaipa Canyon, Dripping Springs, Donnelly Wash, Lower San Pedro and Safford basins. Typical vegetation includes palo verde, mesquite, creosote, and cacti, including Saguaro cacti.

 

Mohave desertscrub in the Upper San Pedro Basin

Mohave desertscrub in the Upper San Pedro Basin

There are extensive reaches of riparian vegetation in some locations in the planning area.   The general location of riparian vegetation is shown in Figure 3.0-11.  Cultural water use has lowered groundwater levels and surface water diversions and impoundments have impacted streamflow in a number of areas.  On Bonita Creek, woodcutting for mines, overgrazing, beaver trapping and a water conveyance system to Safford has reportedly reduced topsoil as much as 50% and down cut the creek as much as 12 feet (Tellman, et al, 1997).

The Gila River, which once was perennial for most of its length in Arizona has been altered in the planning area by Coolidge Dam and farming activities.  However, groundwater levels along the river remain high. Floods have had significant impacts on riparian vegetation in a number of locations. Cottonwood has increased in narrow reaches of the river and in bedrock canyons but has decreased in the wide valleys where it once was common due to channel-widening floods in the early part of the 20th century.  Tamarisk and mesquite species have increased since the middle of the twentieth century, and large floods in the last third of the 20th century did not significantly reduce tamarisk. (Webb, et al 2007)

Gila River in the Dripping Springs Wash Basin

Gila River, Dripping Springs Wash Basin

The San Pedro River was a broad river of cienegas (marshes) when first observed by Spanish expeditions in the 1600s and 1700s.  Stream entrenchment began in the 1880s and by the early 1890s had spread along the length of the river. The San Pedro River channel began to stabilize during the 1950s (ADWR, 2005a). Riparian vegetation has generally increased along the river north of the international border despite notable floods in 1983 and 1993.  Gallery cottonwood forests exist along the upper San Pedro River, at scattered locations between Benson and San Manuel and near its confluence with the Gila River (Webb, et al., 2007).

Historically, the San Simon River was a broad intermittent stream that meandered through the San Simon Valley.  Settlers channelized the river in the 1880s to control flooding and direct its flow until it eventually became a 60 mile long, 600 to 800 foot wide river, 10 to 30 feet deep.  Restoration efforts began in the 1930s and numerous erosion control structures have been built on the river. (Tellman, et al, 1997)  Since then, riparian vegetation, primarily tamarisk, has increased while mesquite have increased on channel banks. Downstream, near Solomon, native riparian species are increasing including Fremont cottonwood and black willow. (Webb, et al., 2007)

Several large fires have occurred in the planning area since 2002 as shown in Figure 3.0-10.  The largest were the Nutall Complex fire in the Pinaleño Mountains, the Ryan Fire in the Huachuca Mountains and surrounding grasslands, and the Bullock and Aspen fires in the Santa Catalina Mountains. The Nutall Complex fire burned over 29,400 acres and threatened the Large Binocular Telescope Observatory on Mount Graham. The Aspen Fire burned for a month and destroyed much of the community of Summerhaven in the Tucson AMA.

 

Figure 3.0-10 Southeastern Arizona Planning Area Location of Major Wildfires, 2002-2006 (Source: USFS 2007)

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Santa Cruz River Flowers in San Rafael Valley