Ecological site group description

Provisional. A provisional ecological site description has undergone quality control and quality assurance review. It contains a working state and transition model and enough information to identify the ecological site.

Physiography

This site occurs on pediments, structural benches, cuestas, valleys, basins, dunes, hillslopes, mesas, ridges, alluvial fans, fan terraces, and plateaus. Elevations range from 3,800 to 7,300 feet. Run off is low and can be influenced by site micro-topography. Slopes typically range from 1-12% but it have been mapped on slopes up to 30%.

Climate

The climate is characterized by hot summers and cool to warm winters, which can be slightly modified by local topographic conditions, such as aspect. Large fluctuations in daily temperatures are common. Average annual precipitation is 5 to 11 inches. Approximately 77 percent occurs as rain from March through October. On the average, February, May, and June are the driest months and August, September, and October are the wettest months. Soil temperatures are in the mesic regime with mean annual soil temperatures ranging from about 54 to 57 degrees F. Precipitation is extremely variable from month to month and from year to year. Much of the summer precipitation occurs as convection thunderstorms.

Soil features

The soils in this group range from moderately deep to very deep are moderately to well developed, they are well drained. They formed in eolian and alluvium deposits derived mainly from sandstone parent materials. Soils are in the coarse-loamy textural family and typically have a calcic horizon occurring at less than 24 inches. Runoff is typically slow but can be higher on some soils, permeability is moderate to rapid. Soils on the sites in the reference state generally have moderate wind and water erosion potential. Coppice mounding is common. The soil temperature and moisture regimes are mesic and ustic aridic respectively.

Surface and subsurface textures are generally sandy loams, sands, and loamy sands. Soils are nonsaline and the water holding capacity is moderate (2 to 3 inches). Surface rock fragments are rare, however where they do occur, they may show evidence of calcium carbonate deposits (small whiteish nodules and coatings).

Biological crust cover is characterized as weak, with light cyanobacteria and/or isolated moss clumps present with no continuity, or isolated pinnacles of lichen and moss with little continuity. In disturbed areas, rodent activity, water flow patterns, and blowout areas are more common.

Vegetation dynamics

These sites occur on the moderately deep to very deep, moderately developed soils. Plant species composition is generally dominated by blackbrush, however, native grasses and forbs are also commonly present.

An important natural disturbance regime consisted of infrequent fires that were likely ignited by both natural causes and Native Americans. When fire starts in this blackbrush community, it can spread easily when the right conditions are present due to the dense, close spaced nature of blackbrush. There are typically few forbs or grasses to provide fine fuels needed to carry fire in these communities, but it is still able to burn during extreme conditions such as high temperature, high wind velocity, and low relative humidity. Fire is more common in moister years when annual production of grasses of forbs is highest. Blackbrush communities have the highest cover of any desert shrub community, and fires typically result in stand replacement. Though historical documentation of fire return intervals are lacking, the historical fire regime is estimated to be from 35-100+ years.

Another natural disturbance mechanism consists of fluctuating weather which can influence soil/water/vegetation relationships. This fluctuation can facilitate the sites transition into different plant communities phases or cause the transition from one ecological state to another, depending on severity and duration.

These ecological sites have been grazed by domestic livestock since they were first introduced into the area, however, it is highly resistant to grazing due to the unpalatable nature of blackbrush. Therefore they have only minimally influenced the historic disturbance regime associated with this site. Improperly managed livestock grazing (continuous season long grazing, heavy stocking rates, etc.) over long periods of time however, may cause this site to depart from the reference plant community. When this occurs, native perennial grasses may decrease, allowing blackbrush increase in density often forming pure stands. Improper livestock grazing may also increase the chance of invasion by cheatgrass and invasive annual forbs however these species rarely increase to a point where they dominate in blackbrush communities.

Major Land Resource Area

MLRA 035X
Colorado Plateau

Subclasses

• R035XY121UT–Desert Sandy Loam (Blackbrush)
• R035XY218UT–Semidesert Sandy Loam (Blackbrush)

Stage

Provisional

Contributors

Vic Parslow
Keith Crossland