Colorado Plateau Blackbrush Desert Shrubland

This ecological system occurs in the Colorado Plateau on benchlands, colluvial slopes, pediments or bajadas. Elevation ranges from 560-1650 m. Substrates are shallow, typically calcareous, non-saline and gravelly or sandy soils over sandstone or limestone bedrock, caliche or limestone alluvium. It also occurs in deeper soils on sandy plains where it may have invaded desert grasslands. This is an evergreen, microphyllous scrub with succulents, half-shrubs, and scattered deciduous shrubs. The vegetation is characterized by extensive open shrublands dominated by Coleogyne ramosissima often with Ephedra viridis, Ephedra torreyana, or Grayia spinosa. Sandy portions may include Artemisia filifolia, Eriogonum leptocladon, Poliomintha incana, or Quercus havardii var. tuckeri (relict populations) as codominant. The herbaceous layer is sparse and composed of graminoids such as Achnatherum hymenoides, Pleuraphis jamesii, or Sporobolus cryptandrus.

This ecological system is dominated by sparse to moderately dense shrubs. Dominant shrubs include Coleogyne ramosissima, Ephedra nevadensis, and Ephedra viridis (which may codominate with Grayia spinosa, Salvia dorrii, and Lycium andersonii). There is usually a sparse herbaceous layer with some perennial grasses and forbs. Annual grasses and forbs are present seasonally. Some characteristic species associated with this system include the shrubs Gutierrezia sarothrae, Chrysothamnus viscidiflorus, Yucca baccata, and Krameria grayi, succulents such as Ferocactus cylindraceus (= Ferocactus acanthodes), Opuntia spp., Echinocereus spp., Echinocactus spp., and Agave spp., the graminoid Pleuraphis rigida, and perennial forbs such as Machaeranthera pinnatifida and Sphaeralcea ambigua.

This shrubland ecological system occurs in the Colorado Plateau at elevations ranging from 580 to 1650 m (1903-5413 feet) (Bowns and West 1976). Climate: This shrubland system occurs in an arid to semi-arid climate with annual precipitation in the form of summer monsoons and winter storms is generally less than 30 cm, averaging approximately 20 cm.

Physiography/landform: Stands occur on gentle to steep, bouldery or rocky colluvial and alluvial slopes of mountains, plateaus, canyons, washes, valley bottoms, and mesas with varying aspects (Anderson 2001a).

Soils/substrates/hydrology: Substrates are shallow, well-drained, typically calcareous, non-saline and gravelly or sandy soils over sandstone or limestone bedrock, caliche or limestone alluvium, but may include other parent materials such as shale, gneiss, quartzites, and igneous rocks (Anderson 2001a). Effective soil moisture appears to be primarily controlled by regolith depth and position in relation to the water table. This brushland system occupies most sites where regolith is uniformly shallow. In association with blackbrush (Coleogyne ramosissima) sites, the soil moisture is concentrated on top of impermeable bedrock at a shallow depth. This perching effect allows for gradual uptake of moisture by the plants roots (Loope and West 1979). This permits growth of plants with more mesic habitat requirements (Warren et al. 1982). On sites with deep soil, blackbrush may occur in almost pure occurrences with only a few associated species (Warren et al. 1982). Dark-colored biological soil crusts, composed of lichens, mosses, fungi, and algae, are often present in this system in fairly undisturbed areas. Sandy soils may have more biological soil crusts than clayish or silty soil surfaces.

Blackbrush is a slow-growing, long-lived, drought-tolerant, evergreen shrub with a diffuse and shallow root system (Bowns 1973, Anderson 2001a). It may lose older leaves during the dry summer season (drought-deciduous) to reduce water stress and become dormant during dry periods. Unlike many rosaceous species, Coleogyne ramosissima is wind-pollinated and largely self-incompatible (Pendleton et al. 1995, Pendleton and Pendleton 1998). Blackbrush is a mast species. The resulting fruit crop is a function of available stored energy, producing abundant crops of seeds every few to several years (Pendleton and Meyer 2004).

In general, seed germination and establishment are rare as seedings are uncommon (Anderson 2001a). The germination rate is low, except after a wet spring when soils remain moist for two weeks (Lei 1997). Seeds also require cold stratification (6 weeks) without light to break dormancy (Lei 1997, Meyer and Pendleton 1990). Seeds appear to remain viable for a long time in seed bank. Meyer and Pendleton (2005) observed 80% germination from 15-year-old seeds. Abundant seedlings have been observed in clumps from rodent caches (Bowns and West 1976, Lei 1997) or after heavy spring rains, which suggests adaptions to seed caching by small mammals or large runoff events that bury seeds. Kangaroo rats are the main seed dispersers, caching large numbers during mast years (Meyer and Pendleton 2005). Fruits are large and require small mammals or large storm runoff for dispersal (Anderson 2001a).

Blackbrush also provides fair forage for desert bighorn sheep (Ovis canadensis nelsoni) and mule deer (Odocoileus hemionus) during the winter, and it can tolerate heavy browsing (USFS 1937, Mozingo 1986, Anderson 2001a). Herbaceous forage from understory is generally low.

Fire does not appear to play a role in maintenance of shrublands within this system. Topographic breaks dissect the landscape, and isolated pockets of vegetation are separated by rock walls or steep canyons that protect it from spreading fire. Blackbrush is fire-intolerant (Loope and West 1979). It does not sprout after fire and is slow to re-colonized burned sites (Wright 1972). In shallow regolith situations, secondary succession, in the sense of site preparation by seral plants, may not occur at all (Loope and West 1979). In Coleogyne ramosissima mixed shrub stands, fire will favor more fire-tolerant shrubs such as Artemisia filifolia, Ephedra viridis, Grayia spinosa, Quercus havardii var. tuckeri, or ruderal species (Tirmenstein 1999j, Anderson 2001a, 2001b, Gucker 2006d).

Biological soil crusts associated with the system are negatively affected by fire, as burning reduced biological soil crusts from 9% cover to less than 1% of total cover, and there was little evidence of recovery postburn after 19 years (Callison et al. 1985). Biological soil crusts are critically important for soil fertility, soil moisture, and soil stability in the many semi-arid ecosystems in the western U.S. (Belnap and Lange 2003). Biological soil crusts fix large amounts of soil nitrogen (mostly by cyanobacteria) and soil carbon, they protect soils from wind erosion, and rough surface texture slows runoff and allows for more infiltration (Evans and Belnap 1999, Belnap et al. 2001, Belnap and Lange 2003, Johansen 2001). Fires in desert scrub are typically patchy and vary in severity, leaving patches of biological crust organisms to recolonize. Recover rates for biological soil crust organisms vary, e.g., green algae (~2 years), cyanobacteria (2-6 years), mosses (3-8 years); however, lichens may take decades (Johansen 2001).

Burning blackbrush stands should be minimized because of the unpredictability of successive vegetation, accelerated soil erosion, long-term or permanent removal of blackbrush, and damage to biological soil crusts (Wright 1980, West 1983d, 1988, Callison et al. 1985).

LANDFIRE (2007a) VDDT model for this system (BpS 2310780) has three classes:
A) Early Development 1 Open (5% of type in this stage): Shrub cover is 0-5%. Dominated by grasses, shrub seedlings and post-fire associated forbs. This type typically occurs where fires burn relatively hot in classes B and C. Shrubs (Coleogyne ramosissima, Ephedra viridis, Ephedra torreyana, and Grayia spinosa) will generally be re-established after 20-30 years.

B) Late Development 2 Closed (shrub-dominated - 30% of type in this stage): Shrub cover (Coleogyne ramosissima, Ephedra viridis, Ephedra torreyana, and Grayia spinosa) 21-100%. Greater than 15% shrub cover and 10-20% herb cover; generally associated with more productive soils. Effects of cumulative drought can cause a shift from this class to class C.

C) Late Development 1 Closed (shrub-dominated - 65% of type in this stage): Shrubs (Coleogyne ramosissima, Ephedra viridis, Ephedra torreyana, and Grayia spinosa) are the dominant lifeform with canopy cover of 10-20%. Less than 15% shrub cover and <10% herb cover generally associated with less productive cobbly and gravelly soils. Effects of cumulative drought can cause a shift from class B to this class.

LANDFIRE modelers emphasized that blackbrush is fire-intolerant, may be slow to re-establish following fire such that grasses may dominate immediately following fire. Invasion of non-native annual grasses following fire is likely under current conditions (LANDFIRE 2007a). LANDFIRE modelers state that generally, the mean fire interval is approximately 75 years with high variability due to annual variation in drying of shrub foliage, shrub mortality and grass and forb production related to drought and moisture cycles (LANDFIRE 2007a). There is also high variation in ignitions and associated fire weather (LANDFIRE 2007a). Fire years are typically correlated with wet years that produce high herbaceous biomass/fine fuel amounts. In areas with high summer moisture from monsoon season rains there are many chances for lightning strikes (LANDFIRE 2007a). Fire-return intervals would have been much longer in drier geographic areas with return intervals over 200 years (LANDFIRE 2007a). Fire size would have been small because of the discontinuous fuel; frequent topographic breaks that dissect the landscape creating isolated pockets of vegetation are separated by rock walls or steep canyons (LANDFIRE 2007a).

Altered fire regime and invasive species are the biggest threats to this system. These are brought on by activities that disturb vegetation and biological soil crusts and include livestock grazing, mining, utility rights-of-way, ORVs and other dispersed recreation. Conversion of this type has commonly come from burning. Burning blackbrush stands is not recommended due to the unpredictability of successive vegetation, accelerated soil erosion, long-term or permanent removal of blackbrush, and damage to biological soil crusts (Wright 1980, West 1983d, 1988, Callison et al. 1985). Following fires, these communities are often colonized by non-native grasses, such as Bromus rubens and Bromus tectorum which serve to encourage recurrent fires and delay shrub regeneration. Where non-native annual grasses have invaded, fire may be much more frequent than the reference condition and can cause a rapid decline in ecological function (and a higher Fire Regime Condition Class) (LANDFIRE 2007a).

Human development and land use have impacted many areas. Fragmentation from transportation infrastructure (roads, railways, pipelines and transmission lines) leads to dispersal of invasive non-native species and altered hydrological processes such as surface flow when excessive runoff from roads creates gullies. Additionally, increased mortality from road kill affects wildlife populations. Other developments that have large impacts include high- and low-density urban and industrial such as energy (renewable wind/solar, oil/gas), mining and landfills. Human land-use impacts from recreation (ORVs, mountain biking, hiking) and agriculture (livestock grazing/browsing) can also be significant (West 1983d).

Occurs in the Colorado Plateau on benchlands, colluvial slopes, pediments or bajadas. Elevation ranges from 560-1600 m.