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. Community phase changes are primarily driven by long term drought. 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).
On undisturbed sites, blackbrush dominates the landscape and species diversity is generally low. 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). Communities are characterized by a flammable shrub architecture allowing fire to easily spread, thus these communities experience stand replacing fire regimes. The short-lived seed of blackbrush is readily destroyed by fire and it may take upwards of 60 years for blackbrush to reestablish. There is frequently 100 percent mortality of blackbrush following fire (Brooks and Matchett 2003).
Fire Ecology:
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 response of woody vegetation 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 saltbush 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).
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 seed (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.
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. The majority of its vegetative growth occurs in the spring and early summer. Senescence begins in late summer, making the seasonality of disturbance very important. Desert needlegrass will likely survive disturbances and fires that occur in the late summer and fall, but early summer fires cause high mortality (Pavek 1993). Fire effects on Utah agave vary depending on season and frequency of burn. Fire can minimize competition for water and nutrients, but also exposes young agave to harsh conditions.
State 1
Reference State
The reference state represents the natural range of variability under pristine conditions. This state is dominated by long-lived evergreen shrub communities with an understory of cool and warm season perennial bunchgrasses. Plant community phase changes are primarily driven by fire, long-term drought and/or insect attack. Historically, fire is rare in this system, but does impact long-term plant community dynamics.
Community 1.1
Reference Plant Community
The plant community is dominated by blackbrush with a variety of native shrubs. Utah agave is conspicuous on the site. Potential vegetative composition is about 20 percent grasses, 10 percent annual and perennial forbs and 70 percent shrubs. Approximate ground cover (basal and crown) is less than 15 percent.
Table 5. Annual production by plant type
| Plant type |
Low
(lb/acre) |
Representative value
(lb/acre) |
High
(lb/acre) |
| Shrub/Vine |
70 |
105 |
175 |
| Grass/Grasslike |
20 |
30 |
50 |
| Forb |
10 |
15 |
25 |
| Total |
100 |
150 |
250 |
Community 1.2
Plant Community 1.2
This plant community is characteristic of a post-disturbance plant community. Herbaceous biomass initially increases. Sprouting shrubs, including Apache plume, yucca and ephedra, quickly recover and provide favorable sites for the germination and establishment of other shrub seedlings. Post-disturbance plant community composition varies depending on season of disturbance. This plant community is at risk of invasion by non-natives. Non-native species take advantage of increased availability of critical resources following a disturbance.
Pathway 1.1a
Community 1.1 to 1.2
Prolonged drought, wildfire, disease and/or insect attack.
Pathway 1.2a
Community 1.2 to 1.1
Absence from disturbance and natural regeneration over time.
State 2
Invaded State
The invaded state is characterized by a the presence of non-native species in the understory. Introduced annuals such as red brome, cheatgrass and redstem filaree have invaded the reference plant community and have become a dominant component of the herbaceous cover. A biotic threshold is crossed, with the introduction of non-native annuals that are difficult to remove from the system and have the potential to alter disturbance regimes significantly from their natural or historic range of variation. These non-native annuals are highly flammable and promote wildfires where fires historically have been infrequent.
Community 2.1
Plant Community 2.1
Compositionally this plant community is similar to the reference plant community with the presence of non-native species in the understory. Ecological processes have not been compromised at this time, however, ecological resilience is reduced by the presence of non-natives. This plant community may respond differently following a disturbance, when compared to the reference plant community. Management focused on protecting intact blackbrush communities is important to ensure seed sources are available for regeneration in the future.
Community 2.2
Plant Community 2.2
This plant community is characteristic of a post-disturbance plant community, herbaceous biomass initially increases, which may or may not be dominated by non-native annuals. Sprouting shrubs recover quickly and provide favorable sites for the establishment of other shrubs. Further disturbance may result in increased bare ground and increased soil erosion. This plant community is considered at risk, due to the increased fuel loading from non-native annuals. Management should be focused on managing non-native fuel loading and reducing anthropogenic impacts to protect soil and ecological resources.
Pathway 2.1a
Community 2.1 to 2.2
Prolonged drought, wildfire, disease or insect attack.
Pathway 2.2a
Community 2.2 to 2.1
Absence from disturbance and natural regeneration over time. Many years with NO fire, minimal disturbance, the presence of a blackbrush seed source, ideal climatic conditions and multiple recruitment pulses blackbrush seedlings will establish and recruit into the stand.
State 3
Burned With No Blackbrush
This state is characterized by the inability of blackbrush to return to the site following wildfire or other disturbance. A biotic threshold has been crossed due to insufficient climatic conditions, the lack of an available seed source or both which prevent the reestablishment of blackbrush in the plant community. Plant community phases consist of fire tolerant shrubs with high growth rates and high reproductive capacities, that were present in smaller quantities in the reference plant community.
Community 3.1
Plant Community 3.1
This plant community is characteristic of a post-disturbance plant community. Initially, this community phase is heavily dominated by herbaceous biomass, which may or may not be non-native. Sprouting shrubs recover quickly and provide a favorable environment for the establishment of other shrubs. Blackbrush is absent from the plant community. This plant community phase is at-risk of wildfire due to increased fuel loading from herbaceous vegetation and short lived perennials.
Community 3.2
Plant Community 3.2
This plant community is dominated by a variety of shrubs that were present in smaller quantities in the reference plant community, such as Mojave buckwheat, range ratany, ephedra and fourwing saltbush. Blackbrush continues to be excluded from this site due to the lack of available seed source and the climatic conditions required for recruitment and establishment.
Pathway 3.1a
Community 3.1 to 3.2
Absence from disturbance and natural regeneration over time.
Pathway 3.2a
Community 3.2 to 3.1
Wildfire or other disturbance removes woody vegetation and promotes growth of non-native annuals.
Transition T1
State 1 to 2
Introduction of non-native species due to a combination of factors including: 1) surface disturbances, 2) changes in the kinds of animals and their grazing patterns, 3) drought, and 4) changes in fire history.
Transition T2
State 2 to 3
Wildfire, insect attack or other disturbance resulting in the removal of blackbrush, in combination with insufficient climatic conditions for germination and establishment of blackbrush. Blackbrush requires specific climatic conditions for germination and survival.
