Ecological dynamics

Abiotic factors
This ecological site occurs on ballenas (steep, eroded alluvial fan remnants) and hillslopes at elevations between approximately 4500 to 5500 feet and slopes ranging from 8 to 30 percent. Mean annual precipitation ranges from 7 to 9 inches and the soil moisture regime is aridic bordering on ustic. Soils are shallow to bedrock or a petrocalcic horizon and formed in alluvium from limestone parent material or residuum and colluvium from mixed limestone and volcanic sources. Reference plants include Utah juniper, Jaeger’s Joshua tree, and black grama. Production reference value (RV) is 670 pounds per acre. This site occurs in the narrow elevation band in the eastern Mojave Desert where Utah juniper dominates over pinyon pine (Sawyer et al. 2009). Shallow droughty soils support relatively sparse, small-statured juniper trees. These soils also support relatively abundant purple needlegrass (Aristida purpurea), which can increase in cover with disturbance on this site. High summer precipitation supports black grama. Dry, rocky calcic soils support calciphiles such as roughseed cryptantha and slim tridens. Relatively steep slopes and a high cover of surface rock fragments increases run-on, and creates microsites for plant establishment, which supports a high diversity of shrubs and forbs. This site is in a transitional area between thermic to mesic soil temperature regimes, which also supports higher diversity, with a mixture of warm and cool desert species. At lower elevations north-facing aspects support higher densities of Utah juniper while south-facing aspects support higher densities of Jaeger’s Joshua tree, while at higher elevations this site may only be found on south-facing aspects.

Disturbance factors
The primary disturbances influencing this ecological site are fire, invasion by non-native annual plants, livestock grazing, and drought. Land clearing associated with ranching has also influenced this site.

Pre-European settlement fire return intervals for Utah juniper woodland in California are estimated at 100-200 years (Sawyer et al. 2009). The rocky dry soils of this ecological site support a low cover plant community without sufficient fuels to carry frequent fire. Utah juniper is generally killed by even light fire, and mature woodlands of this very slow growing species can take 85 to 90 years to develop (Zlatnik 1999).
The naturalization of non-native annual species such as red brome (Bromus rubens) and cheatgrass (Bromus tectorum) with European exploration and settlement from the 1860s through the 1900s (e.g. Brooks and Chambers 2011) caused a transition from the reference state with a long fire return interval and only native species, to a state that included non-natives and more frequent fire (State 2). Invasion by non-native annual grasses increases the flammability of these relatively sparse rocky habitats by providing a continuous fine fuel layer between trees and shrubs (e.g. Brown and Minnich 1986, Brooks et al. 2004, Brooks and Chambers 2011). After fire, these communities appear to be more susceptible to invasion by exotic grasses, which may lead to a grass-fire cycle (D'Antonio and Vitousek 1992). More frequent fires may be causing a reduction in the extent of Utah juniper woodlands in California (Sawyer et al. 2009). Although red brome is recorded in this site, the dry rocky soils of this ecological site do not appear to support high levels of non-native annual grasses.

Cattle grazing has influenced the community dynamics of this ecological site. Ranching was established in the eastern Mojave desert in approximately 1875 (Nystrom 2003). Grazing occurred unregulated in the area until the passage of the Taylor Grazing Act in 1934, which divided public land into allotments that were regulated by the Bureau of Land Management (BLM), and among other things, called for fenced ranges and multiple developed water sources (http://www.blm.gov/wy/st/en/field_offices/Casper/range/taylor.1.html). The Federal Land Policy and Management Policy Act of 1976 (FLPMA) brought further regulations, including 10-year grazing permits. In 1994 the California Desert Protection Act created the Mojave National Preserve, and the National Park Service took over management of grazing allotments in much of the eastern Mojave. All of the area occupied by this ecological site within the Mojave National Preserve was retired from grazing in 2000 (Lanfair Valley Allotment) (Kim 2004).

Heavy cattle grazing in the arid west has been shown to have numerous negative effects on vegetative communities, including decrease cryptogram crust cover, decrease seedling survival, decrease total biomass, decrease perennial grass and shrub cover, and decrease litter cover (Jones 2000). In addition, soils and hydrology may be impacted, with reduced infiltration, increased runoff and erosion (e.g. Rauzi and Hanson 1966, Rauzi and Smith 1973, Jones 2000) due to soil compaction, which increases bulk density and decreases pore space (e.g. Rauzi and Hanson 1966, Abdel-Magid et al. 1987). Grazing in juniper dominated communities can increase juniper dominance due to reduced competition (Zlatnik 1999). Grazing may cause shifts in species composition from more palatable species such as black grama to less palatable species such as purple needlegrass, and to species more tolerant of mechanical disturbance, or with a shorter life-cycle.

All tabular data listed for a specific community phase within this ecological site description represent a summary of one or more field data collection plots taken in modal communities within the community phase. Although such data are valuable in understanding the phase (kinds and amounts of ground and surface materials, canopy characteristics, community phase overstory and understory species, production and composition, and growth), they do not represent the absolute range of characteristics or an exhaustive listing of all species that may occur in that phase over the geographic range of the ecological site.