Ecological dynamics
The white bursage-creosotebush plant assemblage is extensive throughout the Mojave Desert and consists of both long-lived and short-lived perennial species. Surface disturbance on this site may reduce plant cover, density and diversity and increase erosion. These changes can be very subtle or extremely obvious depending on the intensity of use, rate of use and an assortment of environmental factors, such as topography, rainfall and soil type.
Vegetative cover is sparse across this ecological site. Patches of vegetation lead to greater water storage as well as increased organic carbon and nutrient inputs (Puidefabregas 2005). Patches of vegetation in semiarid ecosystems trap significantly higher amounts of runoff water as run-on (Ludwig et al. 2005). Run-on moisture enhances plant growth pulses. Depth of wetting increases under patches of vegetation contributing to overall greater biomass production.
White bursage and creosotebush occur together throughout the successional process, although their relative percentages may change (Marshall 1994). White bursage commonly occurs in open spaces away from other plants. Once established it serves as a nurse plant for creosotebush and other associated species. White bursage is a common colonizer of disturbed areas in the Mojave Desert. It is assumed that creosotebush-white bursage communities are adapted to relatively light disturbances, such as lightly shifting sand surfaces (Marshall 1994). Creosotebush is capable of sprouting from the root crown following disturbance. In stable, old communities, creosotebushes or clones may attain ages of several thousand years. Defoliation and death of branches of creosotebush may occur as a result of long periods of intense moisture stress (Marshall 1995).
White bursage leafs out in early spring and flowers in early summer. Seeds typically germinate following heavy September precipitation (Marshall 1994). White bursage has a dormancy period in the summer and may have a freeze induced dormancy period in the winter if temperatures drop enough to kill its leaves (Marshall 1994). Creosotebush leafs out in response to spring, summer or fall rains and usually flowers in May. Age distribution of creosotebush communities indicated that germination and survival is rare. Summer rains have been correlated to successful sexual reproduction and flower success of creosotebush is greatest with moderate rainfall (Marshall 1995).
Destructive impacts such as land clearing can reduce long-lived creosotebush. The opportunistic perennials such as rayless goldenhead, white burrobush, and wire lettuce will increase. With a loss of perennial cover, non-native annual grasses and forbs such as red brome, Mediterranean grass, and redstem filaree will readily invade this site.
Fire Ecology:
Pre-settlement fire regimes in Mojave Desert shrub communities are characterized by relatively infrequent, stand-replacement fires with return intervals in the range of 35 years to several centuries. Mojave Desert communities are usually unaffected by fire because of low fuel loads, although a year of exceptionally heavy winter rains can generate fuels by producing a heavy stand of annual forbs and grasses. When fires do occur, the effect on the ecosystem may be extreme due to the harsh environment and the slow rate of recovery. The loss of native vegetation can be followed by invasion of non-native annual forbs and grasses. The invasion of non-native annuals provides a continuous bed of fine fuels, promoting fires where they have historically been rare and decreasing the spatial variability of fire.
White bursage and creosotebush possess limited sprouting ability, thus can be killed by fire. White bursage, however, can rapidly re-establish from off-site seed. Range ratany resprouts from the crown after fire. Damage to big galleta from fire varies. If big galleta is dry, damage may be severe. However, when plants are green, fire will tend to be less severe and damage may be minimal, with big galleta recovering quickly.
State 1
Reference State
The reference state is representative of the natural range of variability under pristine conditions. Community phase changes are primarily driven by natural disturbances such as long-term drought and insect attack. Wildfire is infrequent and patchy due to low fuel loading and widely spaced shrubs. Timing of disturbance combined with weather events determines plant community dynamics.
Community 1.1
Reference Plant Community
The reference plant community is dominated by white bursage and creosotebush. Low woolygrass and big galleta are other important species associated with this site. Potential vegetative composition is about 10 percent grasses, 10 percent forbs and 80 percent shrubs. Approximate ground cover (basal and crown) is 5 to 20 percent.
Table 5. Annual production by plant type
| Plant type |
Low
(lb/acre) |
Representative value
(lb/acre) |
High
(lb/acre) |
| Shrub/Vine |
80 |
200 |
280 |
| Grass/Grasslike |
10 |
25 |
35 |
| Forb |
10 |
25 |
35 |
| Total |
100 |
250 |
350 |
Community 1.2
Plant Community 1.2
This plant community is characteristic of a post-disturbance plant community. Herbaceous biomass initially increases, sprouting shrubs recover quickly and provide favorable sites for the establishment of shrub seedlings. Post-disturbance plant community composition may vary depending on season of disturbance. This plant community is ‘at-risk’ of invasion by non-native species. Non-natives are able to take advantage of increased availability of critical resources following disturbance.
Pathway 1.1a
Community 1.1 to 1.2
Prolonged drought, insect attack and possibly low intensity, patchy wildfire.
Pathway 1.2a
Community 1.2 to 1.1
Absence from disturbance and natural regeneration over time
State 2
Invaded
The invaded state is characterized by the presence on non-native species. 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 historic range of variability. Theses non-native annuals are highly flammable and promote wildfire where fires historically have been infrequent. A 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 historic range of variation.
Community 2.1
Plant Community Phase 2.1
This plant community is characterized by the presence of non-native species. Species composition and ecological function is similar to the reference plant community. However, ecological resilience is reduced by the presence of non-natives. This plant community will respond differently following disturbance, when compared to non-invaded plant communities.
Community 2.2
Plant Community Phase 2.2
This plant community is characterized by the reduction of native perennials and the increase of non-native annuals. Native shrubs will persist through the invasion but will experience reduced vigor and seedlings recruitment. This plant community is identified as “at-risk”. The decreased native perennial vegetation and dominance by non-native annuals reduces the soil stability and leaves the site vulnerable to erosion from wind and water.
Pathway 2.1a
Community 2.1 to 2.2
Prolonged drought, wildfire, insect/disease attack or other localized disturbance.
Pathway 2.2a
Community 2.2 to 2.1
Absence from disturbance and natural regeneration over time. Non-natives persist in the plant community.
State 3
Eroded State
The eroded state is characterized by reduced cover of perennial vegetation. Increasing the amount of bare ground, leading to higher levels water erosion, decreased infiltration rates, and loosening of the soil surface causing channeling. Feedbacks keeping this state stable include reduced perennial vegetative cover causing increased runoff, decreased infiltration and reduced run-on moisture preventing establishment of desirable perennial vegetation.
Community 3.1
Plant Community Phase 3.1
This plant community is characterized by decreased perennial native vegetation and increased soil erosion. Patches of vegetation increase water storage capacity and increase organic carbon and nutrient inputs. The loss of patches of vegetation negatively affects soil hydrology, nutrient cycling and vegetation establishment. Careful management is needed to ensure further degradation and loss of ecosystem function.
Community 3.2
Plant Community 3.2
This plant community is characterized by the total loss of perennial vegetation. Soil and soil nutrients are being relocated down slope. All ecological processes have been significantly altered, even truncated in some cases. Non-native annuals persist in the plant community.
Pathway 3.1a
Community 3.1 to 3.2
Prolonged surface disturbance, prolonged drought or both.
Pathway 3.2a
Community 3.2 to 3.1
Changes in management and/or periods of favorable climatic conditions allow for natural regeneration over time.
Transition T1
State 1 to 2
Introduction of non-native species due to a combination of factors including: 1) surface disturbance, 2) changes in the kinds of animals and their grazing patterns, 3) drought and/or 4) changes in fire history.
Transition T2
State 2 to 3
Large scale disturbance removes native perennial vegetation.
Restoration pathway 3
State 3 to 2
Restoration pathway. Possible restoration techniques, to stabilize the site and reestablish native perennials, include flattening and terracing hill slopes, closing roads, vertical, horizontal and rock mulching, as well as, planting container stock. Non-native species remain in the plant community.
| Range Planting |
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| Native Plant Community Restoration and Management |
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