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Active Management Area Environmental Conditions - Vegetation

Information on ecoregions and biotic (vegetative) communities in the AMA Planning Area is shown on Figure 8.0-11.  The planning area contains five of the six ecoregions found in Arizona, most of which is within the Sonoran Desert ecoregion.  The Tucson and Santa Cruz AMAs also contain Chihuahuan desert with “sky islands” of Sierra Madre Occidental pine-oak forest. The northeastern portion of the Phoenix AMA and most of the Prescott AMA are within the Arizona Mountains Forests region, and the northern portion of the Prescott AMA includes part of the Colorado Plateau shrublands region.

Biotic communities range from Lower Colorado River Valley Sonoran desertscrub to Rocky Mountain (Petran) and Madrean montane conifer forest. Most of the planning area is covered by Lower Colorado River Valley and Arizona Uplands Sonoran desertscrub biotic communities.

Rocky Mountain and Madrean montane conifer forests occur at the highest elevations of the Tucson AMA in the Santa Catalina and Rincon mountains and in the Prescott AMA in the Bradshaw Mountains.  These forests commonly occur between about 7,200 to 8,700 feet.  Above 8,000 feet, in areas that receive from 25 to 30 inches of annual rainfall, the forest contains a mix of conifers that may include Douglas and White fir, Limber Pine, Blue Spruce, and White Pine, with Ponderosa Pine on warmer slopes. Aspen and Gambel Oak are prominent in these forests following disturbances.  Below 8,000 feet, in areas that receive about 18 to 26 inches of annual precipitation, the mix of species gives way to almost pure stands of Ponderosa Pine.  About half of the precipitation occurs during the growing season, which permits forests to exist on less than 25 inches of annual rainfall, making them some of the driest forests in North America (Brown, 1982).  Bark beetle infestations have killed large areas of Ponderosa Pine in the Prescott AMA within and in the vicinity of the City of Prescott.

Higher elevations in the Prescott AMA contain areas of Great Plains grassland and Great Basin conifer woodland not found in the other four AMAs.  Great Basin conifer (piñon-juniper) woodlands are found at elevations between about 5,000 and 7,500 feet that receive about 10 to 20 inches of annual precipitation. One of the most extensive vegetation types in the southwest, it is characterized by juniper and piñon pine trees.  Plains and Great Plain grasslands, primarily composed of mixed or short-grass communities, are located in the center of the AMA at elevations above about 4,000 feet that receive between 11 and 18 inches of annual precipitation. (Brown, 1982).

Madrean evergreen woodlands are found at higher elevations in the Tucson and Santa Cruz AMAs. This community occurs in the Santa Catalina, Baboquivari and Santa Rita Mountains and in the mountain ranges along the U.S.-Mexico border where the mean annual precipitation exceeds 16 inches. The woodland consists of evergreen oaks, Alligator Bark and One-seed Junipers, and Mexican Pinyon Pine, and transitions to semidesert grassland at lower elevations. Cacti of the semidesert grassland may extend into the woodland. (Brown, 1982)

Click to view Figure 8.0-11

Semi-desert grasslands occur predominantly in the Santa Cruz and Tucson AMAs with smaller areas in the Pinal AMA. These grasslands occur at elevations between 3,500 and 5,000 feet that receive annual precipitation of 10 to 17 inches.  The grasslands were originally covered with perennial bunch grasses with intervening areas of bare ground.  Where heavily grazed, these grasses have shifted to annual species where summer rainfall is low, or to low growing sod grasses where rainfall is moderate to heavy.  Shrubs, cacti and herbaceous plants are commonly found in the semi-desert grassland community. (Brown, 1982)

Southwest interior chaparral occupies mid-elevation foothill and mountain slopes in the Santa Rita Mountains in the Tucson AMA, the Superstition Mountains in the Phoenix AMA and the Bradshaw Mountains in the Phoenix and Prescott AMAs.  Southwest interior chaparral occurs in areas between about 3,500 and 6,000 feet that receive 15 to 25 inches of annual precipitation (Brown, 1982).  Typical shrubby species are mountain mahogany, shrub live oak, and manzanita. Chaparral plants are well adapted to drought conditions.

Two subdivisions of the Sonoran desertscrub region, the Lower Colorado River Valley subdivision and the Arizona Upland subdivision, dominate all but the Prescott AMA. The Lower Colorado River Valley subdivision is the hottest and driest of the two. There is intense competition for water, with plants widely spaced and more concentrated along drainage channels. Characteristic plants include creosote bush, bursage, saltbush, and mixed, more diverse vegetation along washes and other areas with more water.  These areas may include blue palo verde, ironwood and jojoba.  Also commonly found in the subdivision are several types of cholla and other cacti. (Brown, 1982)

The Arizona Upland subdivision borders the Lower Colorado River Valley subdivision and occurs primarily on slopes and sloping plains at elevations of 980 to over 3,000 feet where it merges with interior chaparral or semi-desert grassland. This subdivision receives more precipitation than the other Sonoran desertscrub subdivisions with average annual precipitation between 8 to 16 inches.  Vegetation is scrubland or low woodland in appearance with blue and foothill palo verde, ironwood, mesquite and cat-claw acacia as common tree species.  Cacti are extremely important in this subdivision including saguaro, organ pipe, cholla and barrel cacti. (Brown, 1982) 

Papago Park

Lower Colorado River Desertscrub in the Phoenix AMA.

The occurrence and composition of riparian vegetation has changed along many of the watercourses in the AMA Planning Area, including the Santa Cruz River in the Santa Cruz and Tucson AMAs, the Gila River in the Pinal and Phoenix AMAs, and the Salt and Verde rivers in the Phoenix AMA. 

Along the Santa Cruz River riparian vegetation has increased in most reaches upstream from Tucson that have perennial flow from either base flow or sewage effluent, while it has been largely eliminated within Tucson.  North of Nogales below the International WWTP the Santa Cruz River is line with Cottonwood and Willow.  In the late 1990s and early 2000s, die-off of riparian trees occurred at Nogales and near Rio Rico respectively, and may be related in part to groundwater pumping. North of Tucson, effluent discharge supports a relatively newly established riparian ecosystem. North of Marana, the Santa Cruz River is ephemeral and there is little historic evidence of riparian vegetation with the exception of tamarisk.  Tamarisk density may be increasing at some locations. (Webb and others, 2007)

Sabino Canyon

Sabino Canyon , Tucson AMA

Riparian vegetation along the Gila River has significantly declined between Florence in the Pinal AMA and its confluence with the Salt River in the Phoenix AMA due to surface water diversion and groundwater pumpage. This reach historically supported lush, woody riparian vegetation, but now mostly tamarisk and mesquite are found. However, cottonwood has returned along the Gila River near its confluence with the Salt River due to rising groundwater levels and changes in the flow regime of the Salt River.  Current groundwater levels are high at the confluence and support a cottonwood-willow forest surrounded by “a sea of tamarisk” (Webb and others, 2007).  Effluent discharge from the City of Phoenix and agricultural return flow have created perennial flow and also increased riparian vegetation below the confluence, where vegetation is primarily tamarisk and mesquite with small stands of cottonwood-willow (AZGF, 1993).

The reservoir system on the Salt River has largely stabilized the channel in the Phoenix AMA below the dams (except during large flood events) and allowed establishment of native and nonnative (primarily tamarisk) riparian vegetation.  Below its confluence with the Verde River and Granite Reef Dam, most surface flow in the Salt River is diverted, and the riparian vegetation declines and disappears downstream to the effluent-dependent section near the confluence of the Salt and Gila rivers.  Downstream of Bartlett Dam, native and nonnative riparian vegetation has increased along the Verde River due to relatively steady release of water.  (Webb and others, 2007)  Vegetation includes cottonwood-willow, tamarisk and mesquite (AZGF, 1993).

Concerns about receding riparian areas at some locations have resulted in restoration projects in the Phoenix and Tucson metropolitan areas, including the Rio Salado project in downtown Phoenix in the Phoenix AMA; and the San Xavier Riparian Restoration project on the Tohono O’odham Reservation, south of Tucson in the Tucson AMA.

Many of the natural biotic communities in the planning area are threatened by invasive species that interfere with ecosystem function through altering natural fire, nutrient flow and flooding regimes.  The most problematic invasive species include buffel grass, fountain grass, natal grass, onionweed, Sahara mustard and tamarisk. Numerous agencies and interest groups throughout the planning area are cooperating to control the spread of these species where feasible, and to educate the public about the threat of these species to ecosystem function. (ASDM, 2008) 

Figure 8.0-12

Figure 8.0-12 AMA Planning Area Location of Major Wildfires 2002-2006

Although not necessarily caused or exacerbated by invasive species, several major wildfires occurred in the AMA Planning Area during the drought years between 2002-2006 (see Figure 8.0-12).  The 2003 Aspen fire in the Tucson AMA burned 85,000 acres in the Santa Catalina Mountains, including much of the Town of Summerhaven.  The 2005 Cave Creek Complex fire, of which a portion is located in the Phoenix AMA, burned 243,950 acres and is the second largest fire in Arizona to date.  Both of these fires occurred in areas with perennial streams and have documented impacts on peak-flow events.  Rainfall two months after the Aspen fire caused runoff to increase three-fold over pre-burn runoff in the Sabino Creek watershed.  (Reed and Schaffner, 2007)  Increased peak flows can degrade stream channels and make them unstable, increase sediment production, and cause flood damage (Neary and others, 2003). 


water drop  Continue to Section 8.0.4 Environmental Conditions - Instream Flow Claims and Threatened and Endangered Species


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