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Western Plateau Planning Area Environmental Conditions - Vegetation

Environmental conditions reflect the geography, climate and cultural activities in an area 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 areas and managed waters.

Click to view Figure 6.0-11


Information on ecoregions and biotic (vegetative) communities in the planning area are shown on Figure 6.0-11.  Three of Arizona’s six ecoregions are included in the planning area: the Colorado Plateau Shrublands, which covers most of the area, the Mojave Desert in the western portion, and the Arizona Mountains Forests ecoregion in the eastern section. Biotic communities range from Mohave desertscrub in the western part of the planning area and along the Colorado River to a small area of alpine tundra in the Coconino Plateau Basin.  Much of the planning area is covered by Great Basin conifer woodland and plains and Great Basin grassland.

Alpine tundra communities are found only at the highest elevations on the San Francisco Peaks, generally over 12,000 feet. The Peaks are the southernmost climatic alpine area in the United States. Because of the relatively harsh climate, only specially-adapted species can survive.  Plants are commonly small and ground-hugging and include mosses, lichens and herbs.  An area of the Peaks has been closed to travel to protect an endemic groundsel (Senecio franciscanus), a threatened species.  Small areas of subalpine grassland are also found on the San Francisco Peaks and on the Kaibab Plateau at elevations above 8,500 feet that receive from 30 to 45 inches of annual rainfall (Grahame and Sisk, 2002).

High elevation subalpine conifer forests are limited to relatively small isolated mountaintop stands on the Kaibab Plateau and the San Francisco Peaks area 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 and receive much of their annual precipitation as snow.  Summer precipitation 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 with patchy crown fires occurring about every several hundred years, and surface fires occurring every 15 to 30 years (Graham and Sisk, 2002).

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, particularly on the Kaibab Plateau and at the south rim of the Grand Canyon. 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).

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 6.0-11.  Piñon pine dominates at higher elevation while junipers are the dominant species at lower and drier areas that may include open grasslands. Bark beetle infestations have killed large areas of piñon pine southeast of Valle and smaller areas south of the South Rim in the Coconino Plateau Basin.

Jacob Lake Area

Rocky Mountain (Petran) and Madrean Montane conifer forest near Jacob Lake, Kanab Plateau Basin.

Plains 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 areas are located primarily in the Coconino Plateau, Kanab Plateau and Shivwits Plateau basins.  On the Arizona Strip, Great Plains grassland, which is drier and receives a larger percentage of annual rainfall in the winter and spring, transitions with plains grasslands (Brown, 1982). Native bunchgrasses have been largely replaced by Eurasian annual species such as cheatgrass due to grazing and fire-suppression practices (Grahame and Sisk, 2002).

Interior chaparral occupies mid-elevation foothill, mountain slopes and canyons in the Virgin Mountains in the Virgin River and Grand Wash basins, and in several isolated locations in the southern part of the Shivwits Plateau Basin.  It is found in areas between about 3,500 and 6,000 feet in elevation that receive 15 to 25 inches of annual precipitation (Brown, 1982). Chaparral consists of dense shrubs that grow around the same height with occasional taller shrubs or small trees.  Typical shrubby species are mountain mahogany, shrub live oak, and manzanita. Chaparral plants are well adapted to drought conditions.

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 in all basins in the Western Plateau Planning Area except the Paria Basin. 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).

Mohave Desertscrub

Mohave desertscrub in the Virgin River Basin.

Mohave desertscrub covers a transitional zone between the higher and cooler Great Basin desert and the lower, hotter Sonoran desert.  It is found along the Colorado River and in the western part of the planning area at elevations below about 3,500 feet. While many of the same plants found in the other deserts occur here, some are found only in the Mohave Desert such as the Joshua tree.  The Mohave Desert is rich in endemic ephemeral plants, most of which are winter annuals (Brown, 1982).

There are reaches of riparian vegetation along the major watercourses in the planning area including the Colorado River, Kanab Creek, Paria River and Virgin River.  Prior to construction of Glen Canyon Dam, the Colorado River supported a “sparse” riparian ecosystem above the high water zone of 100,000 cfs.  Following construction, the new high water zone lowered and there was die-off in the upslope areas. However a mix of native and non-native species has increased in the new zone under the more stable conditions and higher year-round low flows. Riparian vegetation has also increased in tributary canyons. (Webb and others, 2007)

Within the alluvial reaches of Kanab Creek, thick stands of coyote willow that historically grew have been replaced by a mix of native and non-native trees growing in the channel. Downstream in Kanab Canyon large floods and low baseflow precludes establishment of significant riparian vegetation. Along the Paria River upstream of its confluence with the Colorado River tamarisk, coyote willow and scattered cottonwood are found. Historically, willow and some cottonwood were present in this reach (Webb and others, 2007). Downstream from the Virgin River Gorge to Lake Mead, extensive stands of native and non-native vegetation exist along the Virgin River. Tamarisk is predominant downstream of Littlefield (Webb and others, 2007).  Dixon and Katzer (2000) estimated that nearly 10,000 acre-feet of water is used by phreatophytes along the Virgin River from the Littlefield gage to the state line.

Several years of drought combined with high tree densities resulted in the largest outbreak of pine bark beetle populations ever recorded in Arizona forests during 2002 – 2004.  Based on aerial surveys conducted in 2004 by the U.S. Forest Service, substantial bark beetle-caused ponderosa pine mortality occurred in a swath of forest stretching northeast from Williams and on forest lands south of the South Rim of the Grand Canyon. While drought conditions improved in 2004 and 2005, Ponderosa pine mortality due to Ips beetles increased in 2006, with 6,850 acres infested on the Kaibab National Forest.  Other beetle species have also attacked trees on the Kaibab Plateau and on the San Francisco Peaks (USDA, 2006).  By 2008, bark beetle activity had decreased substantially with only 560 affected acres in the Kaibab National Forest. However, almost 67,000 acres of aspen damage by defoliating insects was observed on the Kaibab in that year.  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)

Mortality rates of 60 to 95 percent in low elevation aspen groves, around 7,000 feet, has been observed on the Kaibab and Coconino national forests. Sudden Aspen Death (SAD) is believed caused by a combination of factors including drought and warmer temperatures that make trees more vulnerable to pests and pathogens. Fire exclusion is also thought to be a factor in longer term decline of aspen throughout the western U.S.  Research is being conducted on the Kaibab National Forest to determine the cause of SAD and determine whether aspen are permanently disappearing from its lower elevation range. (Stevens, 2009)

Figure 6.0-12 Wildfires in the Central Highlands Planning Area 2002-2005

Click to view Figure 6.0-

A number of major wildfires occurred in the Western Plateau Planning Area during the severe drought years between 2002 and 2006 (see Figure 6.0-12).  The largest was the lightning-caused Warm Fire, which consumed about 40,000 acres on the central Kaibab Plateau in 2006.  Of the area burned, about 30 percent was identified as having high burn severity related to soil and watershed conditions (USFS, 2007b). In the Southwest, fire can be among the most significant watershed disturbance agents, particularly to peak stream flows.  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.


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