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Central Highlands Planning Area Environmental Conditions - Vegetation

Environmental conditions reflect the effects of geography, climate and cultural activities and may be a critical consideration in water resource management and supply development. 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 5.0-10


Three of Arizona’s five ecoregions are included in the planning area: the Arizona mountains forests, which cover most of the area, the Sonoran Desert in the southwest, and an extension of the Colorado Plateau shrublands in the northern Verde River Basin.  Because of the wide elevation range in the planning area, there are many biotic communities, ranging from Sonoran desertscrub in the Upper Hassayampa Basin to subalpine grassland and subalpine conifer forest in the high elevations of the Salt River and Verde River basins. A very small area of alpine tundra is found above 12,000 feet on the San Francisco Peaks in the Verde River Basin (this small area is not distinguishable on Figure 5.0-10).  Much of the planning area is covered by Rocky Mountain and Madrean montane conifer forests, interior chaparral and Great Basin conifer woodlands.

Areas of subalpine grassland and subalpine forests are found at high elevations in the White Mountains and on the San Francisco Peaks.  The  subalpine conifer forests are limited to relatively small isolated mountaintop stands at elevations of 8,500 to almost 12,000 feet with annual precipitation from 30 to 40 inches a year.  These forests consist of dense stands of fir, spruce and aspen trees.  Bristlecone pine stands occur at elevations around 11,000 feet on the San Francisco Peaks (Brown, 1982).  Significant stands of aspen occur in places, especially in areas that have been burned.  Natural fires are relatively uncommon in subalpine conifer forests (Graham and Sisk, 2002).  Recent surveys of aspen sites show that low-elevation dry sites on the Coconino National Forest (<7,500 feet) experienced 95% mortality since 2000. Sites surveyed on the Apache-Sitgreaves National Forest above 7,500 feet showed 40% mortality in both mid-and high-elevation sites. Researchers found that while insects and disease were associated with the mortality, they appeared to be secondary agents on already drought-stressed trees (USDA, 2008)

Rocky Mountain (Petran) and Madrean montane conifer 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-fir, 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. The forest stretching from near Flagstaff along the Mogollon Rim to the White Mountains region is the largest ponderosa pine forest on the continent (Grahame and Sisk, 2002). 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). In the planning area these forests extend across the entire northern boundary and are also found at higher elevations in other locations in the planning area (Figure 5.0-10).

The high elevation subalpine and montane conifer forests receive much of their annual precipitation as snow.  Because of forest density, sunlight reaches the ground and snow melts slowly, releasing snowmelt gradually to streams.  Snowfall accumulations in this area of the state are critical to the Phoenix metropolitan area water supply.

Great Basin conifer (piñon-juniper) woodlands cover areas below the ponderosa pine forest at elevations between about 5,000 and 7,500 feet that receive about 10 to 20 inches of annual precipitation. Extensive stands exist throughout the planning area as shown on Figure 5.0-10.  Bark beetle infestations have affected large areas of piñon pine in the White Mountains in recent years although activity decreased in most areas in 2007 (USDA, 2008).

Madrean evergreen woodland occurs in small areas in the eastern part of the Tonto Creek and western part of the Salt River basins at elevations of about 5,000 to 6,000 feet. This mild winter-wet summer woodland consists of evergreen oak, juniper and piñon pine. This community is more commonly found in southeastern Arizona and the Sierra Madre of Mexico. In this northern reach it occurs above or within interior chaparral and below and along drainages within the Great Basin conifer woodland (Brown, 1982).

Plains and Great Basin grasslands, primarily composed of mixed or short-grass communities, occur in several parts of 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 Chino Valley and in small areas on the Fort Apache Indian Reservation south of Fort Apache. The piñon-juniper woodland is often intermixed with this grassland.

Great Basin conifer woodland

Great Basin conifer woodland in the Salt River Basin

Chaparral in the Salt River Basin

Interior chaparral in the Salt River Basin

Interior chaparral is found at lower elevations (4,000-6,000 feet) 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.

Semi-desert grasslands occur 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.  Semi-desert grasslands are found in the Upper Hassayampa and Agua Fria basins and south of Payson in the Tonto Creek Basin. Desert grasslands often contain a mixture of grasses, shrubs and small trees.

A small extension of Mohave desertscrub is found in the western part of the Upper Hassayampa Basin. While many of the same plants found in the other Arizona deserts occur here, some are indicative of the Mohave Desert such as the Joshua tree and certain cacti and endemic ephemeral plants, most of which are winter annuals (Brown, 1982).  The community is shrub-dominated and creosote bush and bursage are often dominant species. Mohave desertscrub is typically found at elevations below about 3,500 feet that receive 5 to 11 inches of annual rainfall.

Arizona Upland Sonoran desertscrub covers parts of the planning area below about 3,500 feet in the Upper Hassayampa, Agua Fria, Tonto Creek and Salt River basins.  The community occurs primarily on slopes and sloping plains at elevations of 980 to over 3,000 feet where it merges with interior chaparral or semidesert grassland 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, cholla and barrel cacti. (Brown, 1982) 

Extensive reaches of riparian vegetation occur throughout the planning area.  Areas that have been mapped along perennial streams are shown in Figure 5.0-12. Along the Verde River and several of its tributaries, riparian vegetation is composed of mixed broadleaf, cottonwood-willow, mesquite and strand vegetation (riparian obligate plants adapted to periodic flooding, scouring, or soil deposition). Conifer-Oak riparian obligate habitat is found at higher elevations along West Clear Creek and the East Verde River.  Mixed broadleaf, mesquite and strand vegetation is found along three perennial reaches of the Agua Fria River.  Two tributaries to the Agua Fria River, Little Ash Creek and Sycamore Creek also contain significant mixed broadleaf vegetation (NEMO, 2006).  In the higher elevation headwaters area of the Black River, riparian habitat is composed of wet meadow, mountain scrub and conifer-oak vegetation.  At lower elevations mixed broadleaf and strand vegetation are found along the Black River.  Along the Salt River, riparian vegetation is composed of mesquite, strand and tamarisk.  In the Tonto Creek Basin, mixed broadleaf, cottonwood-willow, strand and mesquite vegetation are found along Tonto Creek.  Along the Hassayampa River at Wickenburg, riparian vegetation consists of cottonwood-willow, mesquite and strand while conifer-oak and mixed broadleaf are found at the Hassayampa River headwaters.

Agua Fria River

Riparian vegetation along the Agua Fria River.  Extensive reaches of riparian vegetation occur throughout the planning area.

In their study of the change in riparian vegetation in the southwest, Webb and others (2007) remarked that “The Verde River….has the largest number of species of woody riparian vegetation that we observed…” They found that riparian vegetation had generally increased along the entire length of the Verde River and its tributaries, following a series of large floods between 1891 and 1940.  They noted that riparian vegetation along the Salt River had increased somewhat upstream from Roosevelt Dam despite a number of severe floods between 1978 and 1995. Riparian vegetation also increased along the Agua Fria River upstream from New Waddell Dam.  Riparian vegetation along the Hassayampa River was also found to have increased at several sites although the impact of drought, resulting in mortality of young trees, was noted near the downstream end of the Hassayampa River Preserve south of Wickenburg.

Several years of drought combined with high tree densities resulted in the largest outbreak of pine bark beetle populations ever recorded in Arizona during 2002 – 2004.  This outbreak killed millions of piñon and ponderosa pine trees.  In 2003 bark beetle mortality was detected on about 763,000 acres in Arizona and New Mexico, with most of the mortality occurring in Arizona (USFS, 2003).  Areas most affected were trees at the lower end of their elevational range. Drought conditions improved in 2004 and 2005, and mortality decreased substantially as a result of both higher precipitation and because many of the trees in the most susceptible areas were already dead. 

Based on aerial surveys conducted in 2004 by the U.S. Forest Service, there were several areas of ponderosa pine infestation in the planning area.  Areas with substantial bark beetle-caused ponderosa pine mortality occurred on parts of the Fort Apache Indian reservation, on lands west and north of the reservation, areas southwest of Bellemont, and areas west of Interstate 17 in the vicinity of Crown King.  Data from aerial surveys recorded 2.1 million acres of piñon-juniper woodland and 1.3 million acres of ponderosa pine were affected in Arizona and New Mexico during 2002 – 2004 (USDA, 2007).

Click to view Figure 5.0-11

Figure 5.0-11 Wildfires in the Central Highlands Planning Area 2002-2006

Wildfire risk increases with the number of dead trees in the landscape, which provide fuel for fires.  There were several major wildfires in the Central Highlands Planning Area during the severe drought years between 2002 and 2005 (see Figure 5.0-11).  The Rodeo-Chediski fire in 2002, Arizona’s largest ever, consumed about 462,600 acres, much of it in the north-central part of the Salt River Basin. The Willow Fire (2004) burned almost 120,000 acres southwest of Payson in the Tonto Creek and Verde River basins and the Cave Creek Complex Fire (2005) burned 243,800 acres in the east-central part of the Agua Fria Basin and adjacent areas in the Verde River Basin and Phoenix AMA. 

In the Southwest, fire can be among the most significant watershed disturbance agents, particularly to peak stream flows.  In areas severely burned by the Rodeo-Chedeski Fire, peak flows were as much as 2,350 times greater than previously measured, the highest known post-fire peak flow in the Southwest.  Increased peak flows can degrade stream channels and make them unstable, increase sediment production and cause flood damage. (Neary and others, 2003)  Drought, wildfire and long-term climate change involving warmer temperatures with earlier springs and less snow cover could result in vegetative changes in the planning area with implications for runoff, infiltration and water supplies.


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