skip to the content of this page
AZ.gov Arizona's Official Website Arizona Department of Water Resources
Arizona Department of Water Resources AZ.gov Arizona's Official Web Site
Securing Arizona's Water Future

Environmental Conditions of the Eastern Plateau 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.  Discussed in this section is vegetation, riparian protection through the Arizona Water Protection Fund Program, instream flow claims, threatened and endangered species, public lands protected from development as national parks, monuments and wilderness and unique waters.

Click to view Figure 2.0-11

Vegetation

Information  on  ecoregions  and biotic  (vegetative) communities in the planning area are shown on Figure 2.0-11. Most of the Eastern Plateau Planning Area is located in the Colorado Plateau Shrublands ecoregion while higher elevation areas are located in the Arizona Mountains Forests  ecoregion.  Biotic communities range from Great Basin desertscrub at lower elevations to areas of subalpine grassland.  Plains and Great Basin grasslands are the predominant biotic community in the planning area. Due to grazing and fire suppression efforts, pre-settlement environmental conditions have been permanently altered in the region.  Woodland communities have expanded considerably and the increase in ponderosa pine density has led to both an increase in the severity and size of wildfires, and to a decrease in stream and spring flows due to less soil absorption of precipitation (Grahame and Sisk, 2002).

In Arizona, alpine tundra is found only at the highest elevations on the San Francisco Peaks, generally over 12,000 feet. (This small area is not distinguishable on Figure 2.0-11). Only specially adapted species can survive the harsh climate including small, ground-hugging mosses, lichens and herbs. An area of the San Francisco Peaks has been closed to travel to protect an endemic groundsel (Senecio franciscanus), a threatened species.  Areas of subalpine grassland are found at high elevations in the White Mountains, in the Chuska Mountains and on the San Francisco Peaks. (Grahame and Sisk, 2002).

High-elevation subalpine conifer forests are limited to relatively small isolated mountaintop stands on the San Francisco Peaks, White Mountains and Chuska Mountains 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.  Much of the precipitation is snow, but summer rainfall is also a substantial component of annual precipitation. 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)

Aspens on San Fransisco Peaks

Aspen stand in the San Francisco Peaks

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 give 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 southern boundary and are also found along the northeastern boundary in the Chuska and Lukachukai Mountains and the Defiance Plateau.

Great Basin Conifer (piñon-juniper) woodlands cover large 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 2.0-11.  Bark beetle infestations have affected large areas of piñon pine and juniper on the Navajo reservation and in the White Mountains in recent years although activity decreased in most areas in 2007 (USDA, 2008).

Plains and great basin grasslands, primarily composed of mixed or short-grass communities, are widespread in the planning area at elevations above about 4,000 feet that receive between 11 and 18 inches of annual precipitation.  These grasslands extend almost unbroken through the entire length and width of the planning area. Native bunchgrasses have been largely replaced by Eurasian annual species such as cheatgrass and shrubs have invaded the grasslands due to grazing and fire-suppression practices (Grahame and Sisk, 2002)

Great Basin Desertscrub

Great Basin desertscrub in the Eastern Plateau Planning Area

Great Basin desertscrub occurs in northern Arizona mostly at elevations of 4,000 to 6,500 feet where an average of about 7 to 12 inches of rainfall occurs.  This vegetative community is dominated by multi-branched, aromatic shrubs with evergreen leaves, primarily sagebrush, blackbrush and shadscale. Great Basin desertscrub is found throughout the planning area but primarily in the western portion. In addition to shrubs, vegetation consists primarily of grasses. Grazing has heavily impacted native grasses in this community, which have been replaced by exotic species including cheatgrass.  Cheatgrass is highly flammable, and where it is a significant component of sagebrush stands, the incidence of fire is greatly increased (Brown, 1982)

Riparian vegetation has been mapped along East Clear Creek, Chevelon Creek, the Little Colorado River, Chinle Creek and at a number of other locations in the planning area (see Figure 2.0-13). Using Arizona Game & Fish Department data, Parra and others (2006), identified approximately 5,226 acres of riparian vegetation and ten different riparian types in the Little Colorado River watershed. Wet meadow, conifer oak and tamarisk groups comprised the largest amount of riparian vegetation. The Little Colorado River headwaters area had the greatest amount of wetland vegetation. Less abundant were mixed broadleaf, mountain scrub and mesquite (Parra and others, 2006).  In the other planning area watersheds Russian olive and tamarisk are widely found. At higher elevations and along streams draining the Chuska Mountains and Defiance Plateau, conifer oak, wet meadow and mixed broadleaf occur (AZGF, 1997 & 1993).

Webb and others (2007) studied changes in riparian vegetation along a number of watercourses in the Southwestern United States. Watercourses studied in the Eastern Plateau Planning Area include the Little Colorado River and Moenkopi Wash. They noted that reaches of the Little Colorado River historically supported groves of cottonwood trees although the spatial distribution was not known. A series of floods and downcutting, and drainage of the alluvial aquifer, resulted in removal of most of this riparian vegetation. Woody riparian vegetation, primarily tamarisk but some native species, now populate terraces and parts of the channel. Moenkopi Wash was a wide, barren channel in the early 1930s but development of a low floodplain during the 1940s has allowed establishment of tamarisk and scattered cottonwood groves.

Click to view Figure 2.0-12

Several major wildfires occurred in the Eastern Plateau Planning Area during the severe drought years between 2002 and 2006 (see Figure 2.0-12).  The Rodeo-Chedeski fire in 2002, Arizona’s largest-ever, consumed about 462,600 acres in the Eastern Plateau and Central Highlands planning areas. The Jacket Fire, southeast of Flagstaff and the largest recorded fire in the Coconino National Forest, burned over 17,200 acres in 2006. 

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 peak flows, 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 Spring season and less snow cover could result in vegetative changes in the planning area with implications on runoff, infiltration and water supplies.

Extended drought combined with high tree densities resulted in the largest outbreak of pine bark beetle populations ever recorded in Arizona during 2002 – 2004 with massive mortality, particularly in the Kaibab National Forest in the Western Plateau Planning Area (USDA, 2006).  By 2007, bark beetle activity in Arizona had decreased substantially with the exception of the Apache-Sitgreaves National Forest, much of which is located in the Eastern Plateau Planning Area. Also noted in 2007 were large outbreaks of pine sawflies in several locations.  This outbreak defoliated ponderosa pines in an area between Pinedale and Overgaard where many trees had been previously damaged in the 2002 Rodeo-Chediski fire, and on Navajo tribal lands. Study plots were established in Arizona in 2003-2004 to monitor the impacts from bark beetle infestations on fuel loading and fire behavior.  Preliminary analysis shows that mortality plots have significantly higher fuel loads than areas with no mortality (USDA, 2008). 

 

water drop Click here to continue to Section 2.0.4 Environmental Conditions - Arizona Water Protection Fund Projects and Instream Flow Claims  

 

Arizona Water Atlas Home

Eastern Plateau Planning Area Home

Download pdf of entire Eastern Plateau Planning Area Download pdf of the Eastern Plateau Planning Area Overview References and Supplemental Reading for the Eastern Plateau Planning Area Overview
Colorado River Eastern Plateau Planning Area Arizona Water Atlas Volume 2 Download Little Colorado River Plateau Basin pdf References and Suggested Reading