Ecological dynamics

Abiotic factors
This site occurs on gently sloping alluvial fan remnants at elevations of approximately 3300 to 3900 feet. Soils have loamy to coarse loamy textures, and are shallow to moderately deep to a petrocalcic horizon. The soil moisture regime is typic aridic and the soil temperature regime is thermic. Creosote bush and winterfat are the dominant shrubs of the reference plant community, and big galleta is an important grass. Burrobush is an important secondary shrub that becomes more abundant as perennial grasses are lost with grazing. This species is more competitive on the shallow soils of this site than on adjacent deeper soils. Production reference value (RV) is 492 pounds per acre, and ranges from 332 to 691 pounds per acre depending on annual precipitation. Winterfat is more abundant on adjacent higher elevations, but does well on these well-drained moderately alkaline soils. Creosote bush is a dominant on most surrounding landforms at this elevation. Big galleta is less abundant on this site than on adjacent deeper soils with higher water holding capacity, but becomes dominant with disturbance that removes shrub competition.

Disturbance dynamics
The disturbances impacting this plant community are fire, invasion by non-native annual species, livestock grazing, and land-clearing for dryland farming.

Prior to European settlement, fire in this ecological site was likely relatively rare due to low abundance of fine fuels between shrubs, and patchy perennial grasses (e.g. Brown and Minnich 1986, Brooks and Matchett 2006, Brooks et al. 2007). The low moisture holding capacity in these shallow to moderately deep soils do not support an abundant annual community, but one or multiple years of heavy winter precipitation such as occurs during El Nino events (Hereford et al. 2006), or heavy summer precipitation could lead to a relatively heavy standing crop of native annuals in intershrub spaces, providing a continuous fine fuel layer that could carry a fire (Brown and Minnich 1986, Minnich 2003). Years of heavy summer or early fall precipitation that lead to high production in big galleta, also increasing fire likelihood (Minnich 2003). The dominant shrubs, creosote bush and winterfat, are killed by moderate to high severity fire (Carey 1995, Brooks et al. 2007), and typically take many years to recolonize after a stand-clearing fire (e.g Brown and Minnich 1986, Brooks et al. 2007, Engel and Abella 2011, Steers and Allen 2011). Following fire, big galleta gains dominance in this site, and annuals greatly increase in abundance with release from competition with shrubs, and this increase in fine fuels increases the likelihood of repeat burning.

Naturalization of non-native annual species such as red brome (Bromus rubens), common Mediterranean grass (Schismus barbatus), cheatgrass (Bromus tectorum), and redstem storks bill (Erodium cicutarium) with European exploration and settlement from the 1860s through the 1900s (e.g. Brooks and Chambers 2011) caused a transition from the reference state, which contained only native species, to a state that included non-native species (State 2). Invasion by non-native annual grasses is thought to have increased flammability of invaded Mojave Desert shrublands by providing higher levels of fine fuels (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). In this ecological site, abundance of native annuals and big galleta was found to be more significant after disturbance than invasion by non-native annuals, although non-native annuals certainly have the potential to become a problem. Winterfat re-establishment is greatly suppressed by the presence of abundant annual grasses and can also be suppressed by native perennial grasses, which typically recover well after fire (Woodmansee and Potter 1971, Hild et al. 2007); thus recurrent fire can prevent winterfat recovery.

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 Desert where this site occurs. All of the area occupied by this ecological site within the Mojave National Preserve was retired from grazing in 2000 (Lanfair Valley and Kessler Spring Allotments) (Kim 2004).

Heavy cattle grazing in the arid west has been shown to have numerous negative effects on vegetative communities, including decreases in cryptogram crust cover, seedling survival, total biomass, perennial grass and shrub cover, and 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), and soil compaction (Rauzi and Hanson 1966, Abdel-Magid et al. 1987). Grazing in winterfat dominated communities has been demonstrated to increase soil compaction, lower soil aggregate stability, increase bare ground cover and decrease winterfat, biological crust, and litter cover (Rasmussen and Brotherson 1986, Matney 2010). Grazing may cause shifts in species composition from more palatable to less palatable species, and to species more tolerant of mechanical disturbance, or with a shorter life-cycle (Rasmussen and Brotherson 1986). Ten years of monitoring (1970 to 1980) of a grazing system where sites were rested every other year in the Arizona strip of the Mojave Desert, which has similar climatic and environmental characteristics as this site, found an upward trend in winterfat and burrobush cover, and typically (but not always) downward trends on big galleta cover (Hughes 1982). Higher utilization, e.g. above 50 percent, caused loss of native perennial grasses (Hughes 1982), potentially causing a permanent shift to a degraded state. This state has not been observed in this ecological site, and is not included in the state-and-transition model; however, land managers should be aware that the potential for a permanent transition with sustained heavy grazing exists.

Large portions of this ecological site were cleared for dryland farming and homesteading in the early 1900s to 1920s (Carpenter et al. 1986, Sharp and Moore 2004). Existing vegetation was chained and soils were plowed, but most were abandoned by 1930 (Carpenter et al. 1986). Many of these cleared areas were subsequently grazed until 2000. Recovery of these cleared areas resembles recovery after fire; big galleta is strongly dominant, the dominant shrubs are greatly reduced, overall diversity is higher, and annual species are more abundant.

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.