Ecological dynamics

This ecological site is located in a transition zone between the Mojave Desert and the Southern Nevada Basin and Range major land resource areas (MLRA 30 and MLRA 29, respectively). The dominant species are blackbrush (Coleogyne ramosissima), creosote bush (Larrea tridentata), and white bursage (Ambrosia dumosa). Blackbrush is commonly found in cooler environments, such as at higher elevations and in areas more affected by the climate of MLRA 29. Creosote bush and white bursage are commonly found in warmer environments. These species form two plant communities within this ecosite. The most widespread community is dominated by blackbrush and creosote bush, with little to no white bursage present. A less extensive plant community within this ecosite is dominated by blackbrush and white bursage (Ambrosia dumosa), with little to no creosote bush present. The plant community dominated by blackbrush and white bursage is found closer to the MLRA 29 "line" which is mainly at the upper elevations of this ecological site.

The dominant species on this ecological site are late seral species. Blackbrush is shallow-rooting, slow-growing species commonly found on stable soils and on soils shallow to weathered bedrock. The stable soils typically have an argillic horizon at shallow depth. At this depth and with its higher available water capacity, an argillic horizon may aid blackbrush survival by supplying more water to blackbrush’s shallow roots. The presence of blackbrush on shallow granitic soils may also be a function of water availability. Due to the way in which granite weathers, the higher available water capacity of the bedrock and the ability of blackbrush to access the water through numerous cracks may be important for supporting the community. It is thought to be a paleoendemic species, once having a wider range but now limited to its present extent by environmental conditions (Stebbins and Major 1965). It has infrequent germination events and low seedling survival (Anderson 2001).

Creosote bush (Larrea tridentata) is a long-lived species that reproduces largely by cloning (Marshall 1995). It is slow to establish from seed due to infrequent masting and germination (Barbour 1968). Creosote bush has a deep, expansive root system that can extract water from a large volume of soil. In addition, it is highly effective at extracting water at very low water potentials. White bursage is a drought-deciduous species often found with late seral species such as blackbrush and creosote bush, but its seeding ability allows it to colonize disturbed sites (Marshall 1994).

The successional status of the dominant species suggests that disturbances to this area are not common and/or are not intense. Flooding and ponding on this ecosite are unlikely. Wildfire has historically been a rare occurrence in the desert, but its effects may be severe. Widely spaced shrubs and discontinuous fuels prevented fires from spreading easily. Spread of invasive annual species in the Mojave Desert creates a more continuous and easily ignitable fuel bed, particularly after heavy rains, and increases the fire frequency and the size of the area disturbed (Clarke 2006, Howard 2006). Invasive annual plants such as red brome (Bromus rubens) often re-colonize these disturbed sites (Brooks et al. 2003). Red brome is currently present in small amounts on this ecosite. Its spread would increase the risk and frequency of fire on this ecosite. Non-native forbs such as redstem filaree (Erodium cicutarium) are also present and can produce a large amount of biomass on this ecosite.

Blackbrush communities are likely to be significantly altered by fire or other widespread disturbance. The ability of blackbrush to recolonize a disturbed site is severely limited by infrequent seedling establishment and an inability of existing plants to resprout following a disturbance. Fires in blackbrush communities were often stand-replacing (Brooks et al. 2003), but historical fire return intervals of more than 100 years allowed for slow re-establishment (Anderson 2001). Burning or scorching often causes high mortality in creosote bush, but creosote bush has some ability to resprout following removal of aboveground stems if the root crown is not killed (Marshall 1995). White bursage is generally killed by disturbance. Its ability to resprout is low, but it can re-establish on a site if a seed source is available (Marshall 1994).

Early and mid-successional shrubs are present in small amounts and would become more common if this ecosite were disturbed. These include spiny hopsage (Grayia spinosa), Cooper’s goldenbush (Ericameria cooperi), burrobrush (Hymenoclea salsola), and California buckwheat (Eriogonum fasciculatum). These species are adapted to a wide range of environmental conditions. Ample seed production, easy seed dispersal, and rapid growth help these species establish on disturbed areas. Perennial grasses such as Sandberg bluegrass (Poa secunda) also have the ability to resprout following a disturbance. Reduced competition from late seral species for light, water, and nutrients facilitates plant growth.

Several forb species are present in one community but not the other. The two communities were sampled in different years. The difference in species composition may be a reflection of factors such as interannual variability in the precipitation and temperature that allowed some species to grow better than others in that year. Production by and percent cover of annual herbaceous species relative to perennial species may also differ as a result of this variability. In years of above-average rainfall, annual herbaceous species may contribute relatively more to production and cover than in drier years. In below-average rainfall years, many annual species will not grow, and perennial species will contribute more to production and to cover.