Ecological dynamics

Blackbrush communities are most prevalent in the transitional zone between the Mojave Desert and Great Basin and are commonly associated with creosotebush. Blackbrush is a paleoendemic species as originally postulated by Stebbins and Major (1965). Blackbrush is a transitional species that occupies a boundary that has shifted in recent geologic time. Analysis of packrat middens suggests a 50–100-m downward movement of the blackbrush zone along elevational gradients in the Mojave (Cole and Webb, 1985; Hunter and McAuliffe, 1994). The plant communities of this site are dynamic in response to changes in disturbance regimes and weather patterns.
Undisturbed blackbrush communities are fairly resistant to invasion by non-natives (Brooks and Matchett 2003). Mature blackbrush plants are well adapted to persist under less than optimal conditions, and individuals’ may live as long as 400 years (Pendleton and Meyer 2004). Reproduction and recruitment are episodic, based on favorable environmental conditions (Pendleton and Meyer 2004). Very old stands of blackbrush may have established hundreds to thousands of years ago under very different climatic conditions and will take a considerable amount of time to recover following disturbances.
Blackbrush is a long-lived and generally considered a climax species. It is a non-sprouter; regeneration depends on wind pollinated seed and heavy winter precipitation, and is therefore slow to re-colonize burned areas (Anderson 2001). Blackbrush recruitment is episodic, like many shrubs in arid systems, when conditions are favorable large seed crops are produced and the rest of the time is characterized by minimal seed output (Pendleton and Meyer 2004). Blackbrush seeds are frequently cached away by rodents, until conditions are conducive for germination. Typically, germination occurs during the winter and early spring, given the proper moisture conditions and cool soil temperatures (Pendleton 2008). Seeds require cold stratification before germination and the survival of seedlings following germination is dependent on the availability of spring time moisture (Pendleton 2008).
Plant community phases of this ecological site have a significant amount of perennial bunchgrasses. Desert grasses are intensive exploiters; which means they extract a large portion of their moisture from shallow soil horizons (Burgess 1995). Intensive exploiters are very effective competitors for limited soil moisture and tend to be very resilient. The topographic position of this ecological site results in increased run-on moisture contributing to the increased annual production. Big galleta, a warm season grass, is the dominant grass on this ecological site. Warm season grasses have higher light and temperature requirements to begin photosynthesis, therefore grow most actively during the summer. Big galleta generally undergoes two major growth periods, coinciding with the bimodal rainfall pattern (Matthews 2000). Big galleta typically flowers from February through June in the Mojave Desert and is the most drought tolerant of the Pleuraphis species. Its shallow root system extends radially from the base of the plant, maximizing the area covered by the roots (Matthews 2000).
Long-term surface disturbance or reoccurring wildfire will reduce native plant cover, density and species diversity. As ecological condition deteriorates big galleta and perennial forbs decrease, short-lived perennials initially increase. Blackbrush may also decrease. Non-native annual grasses and forbs may increase as native perennials are lost.

Fire Ecology:
Blackbrush communities are characterized by a flammable shrub architecture allowing fire to easily spread, thus these communities experience stand replacing fires. The short-lived seed of blackbrush is readily destroyed by fire and it may take upwards of 60 years for blackbrush to reestablish. Plant succession varies widely following fire and blackbrush communities can be replaced by undesirable species, like redstem filaree, snakeweed (Gutierrezia spp), and Bromus spp. (Anderson 2001).
The vegetation response post-fire largely depends on site history, species present prior to the fire, as well as, fire severity and frequency. Common plant species include those that are known to sprout, are fire resistant, and/or prolific seed producers. Mojave buckwheat, creosotebush, Ephedra spp., Encelia spp., and white bursage are all found on burned blackbrush sites. However, it is uncommon to see blackbrush recruitment under the current climatic conditions, especially at the lower extent of its elevational range. The traits that allow established blackbrush communities to persist for centuries, even after environmental conditions have changed are now precluding seedling establishment under the current climatic regime (Pendleton and Meyer 2004).
Big galleta commonly sprouts from the root crown following low severity fire. Damage from fire varies; depending on whether big galleta is dry when burned. Indian ricegrass can be killed by fire, depending on the seasonality of the burn. It easily reestablishes through seed dispersed from adjacent unburned areas. Desert needlegrass is a facultative seral species and is often one of the first perennials to appear following a disturbance. It is highly susceptible to top kill by fire due to the persistent leaf bases. The root crown will resprout following low to moderate intensity fires.
Under current environmental conditions in the Mojave Desert it is common to see disturbed blackbrush sites dominated by the semi-erect, evergreen, Mojave buckwheat. Eriogonum species are frequently pioneering species following natural disturbance (Meyer 2008). Following severe fires, resprout success of Mojave buckwheat is limited. Most regeneration is from seeds (Montalvo 2010). The seedbank of Mojave buckwheat will not persist under a frequent fire regime. Under an unnaturally high fire frequency herbaceous communities are favored over woody dominated plant communities, which cause habitat degradation. Dramatic changes have occurred in the middle elevation shrublands dominated by blackbrush, where most fires have occurred in recent times. This zone is more susceptible than other areas of the Mojave Desert to increased fire size related to the flush of non-native annual grasses following years of high rainfall (Brooks and Matchett 2006). A non-native annual grassland state would develop with frequent reoccurring fire causing the plant community to cross an irreversible threshold.

The blackbrush ecotone experiences long-term species composition changes post wildfire. Non-native annual species like red brome and cheatgrass benefit from the reduced competition for moisture and nutrients from native woody species and readily establish. The rapid life cycles and efficient use of sunlight exhibited by annuals allows them to achieve great reproductive success on previously burned sites. Additional biomass provided by non-native annuals, leads to an accumulation of flammable fuels and a shorter fire return interval. This process contributes to the loss of blackbrush shrublands. Researchers have shown that species diversity is greater in burned, than unburned blackbrush communities (Engel and Abella 2011). This process leads to long-term changes in desert plant communities, because dominant late-successional species reduced by disturbance are often nurse plants for other species (Engel and Abella 2011).