United States Department of Agriculture
Natural Resources Conservation Service

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Plant Communities

Ecological Dynamics of the Site

Abiotic Factors

This ecological site occurs on stabilized sandsheets on soils with a thermic soil temperature regime. Sandsheets are extensive, low relief accumulations of eolian sand deposits (Laity 2008). Stable, or dormant sandsheets are those where perennial vegetation cover is well-developed, and current rates of sand movement and deposition are low or absent, but may become active as a result of minor climate change or disturbance (Lancaster 1994). The stability of these landforms means that factors such as burial or abrasion by blowing sand does not restrict vegetation to psammophiles (plants restricted to active eolian environments).

The plant community is strongly dominated by perennial grasses. Big galleta is the dominant species; big gallleta is a highly drought-tolerant C4 grass that occurs on a range of soil types, but is dominant only on sandy soils where soil moisture is most readily available (McAuliffe 1994, Austin et al. 2004). Big galleta colonizes and stabilizes semi-stabilized eolian habitats with rhizomatous growth (Matthews 2000), and dominance by big galleta on these habitats is an indicator of eolian stability. Big galleta exhibits rapid growth and high productivity in response to temporal high moisture availability in these deep sands (Austin et al. 2004). Indian rice grass is a cool season perennial bunchgrass grass widely distributed throughout the western United States, but also reaches highest abundance on sandy soils (Tirmenstein 1999, Baldwin et al. 2002). It occurs in the thermic soil temperature regime of this ecological site, and disappears when the soil temperature regime transitions to hyperthermic. In arid regions, sand textured soils have greater water availability because water quickly infiltrates through sand to depths where it is not lost to evaporation, and because sandy surfaces form a physical crust that further reduces evaporation (Noy-Meir 1973, Hamerlynk et al. 2002). Thus, in desert regions, where the availability of soil water is the critical resource shaping plant communities in arid environments, productivity is highest on sandy soils (Noy-Meir 1973, McAuliffe 1994, Martre et al. 2002, Hamerlynk and McAuliffe 2002, Austin et al. 2004).

Creosote bush is a long-lived, deep-rooted evergreen shrub dominant across vast areas of the North American warm deserts. Creosote bush maintains its evergreen status by using water held in deep soil layers, and once established in this ecological site, individuals are large and productive. Creosote remains a secondary species in this site however, because of soil moisture restrictions and seedling sand abrasion during the establishment phase. Creosote bush establishes in response to warm season moisture; given limited warm season rain in this ecological site, the rapid infiltration of water, rapidly drying soil surfaces during the warm season, and increased erosion and abrasion during the summer, opportunities for successful establishment of creosote seedlings are rare.

Disturbance dynamics
Drought, invasion by nonnative species, and wind erosion are the primary disturbance affecting this ecological site.

Drought is an important shaping force in desert plant communities (Webb et al. 2003, Bowers 2005, Hereford et al. 2006, Miriti et al. 2007, Hamerlynk and McAuliffe 2008). The effects of drought may be particularly severe in deep sandy soils with little horizon development. High availability of soil moisture during normal to high precipitation conditions can lead to high growth rates and large individuals whose size cannot be sustained when water is no longer available (Hamerlynk and McAuliffe 2008). Short-lived shrubs and perennial grasses demonstrate the highest rates of drought-induced mortality (Webb et al. 2003, Bowers 2005, Hereford et al. 2006, Miriti et al. 2007), and annual species remain dormant in the soil seedbank (Beatley, 1974, 1976). Long-lived species are more likely to exhibit branch-pruning with limited recruitment during drought (Hereford et al. 2006, Miriti et al. 2007).

Non-native annual species such as red brome (Bromus rubens), Mediterranean grass (Schismus barbatus), redstem stork’s bill (Erodium cicutarium) and Asian mustard (Brassica tournefortii) have become naturalized throughout the Mojave Desert over the past century (Rickard and Beatley 1965, D'Antonio and Vitousek 1992, Brooks 1999, Reid et al. 2006, Norton et al. 2007). Asian mustard and prickly Russian thistle (Salsola tragus) are threats in eolian habitats, with prickly Russian thistle abundant on disturbed areas or active sand, and Asian mustard most abundant on stabilized sand (Barrows et al. 2009). Like native annuals, nonnative annual cover and production is directly related to winter precipitation (Beatley 1969, Brooks and Berry 2006, Barrows et al. 2009). In this ecological site, Asian mustard and prickly Russian thistle invasion may be severe.

Wind erosion and deposition is the driver of eolian dynamics. Wind strength, precipitation, vegetation cover and disturbance influence the degree to which sand depositional surfaces are active or stable (Cooke et al. 1993, Lancaster 1994, 1997, Musick 1999). Drought may cause a stabilized sand surface to become active, due to losses in vegetation cover, and the increased erodibility of dry soils (Cooke et al. 1993, Lancaster 1994, Breed and Reheis 1999, Musick 1999). Similarly, other disturbances that cause a decline in vegetation cover, such as off-road vehicle use, grazing, and fire can reactivate a stable sand surface.
 

State-and-Transition Diagram




State 1: Historic State

State 1 represents the historic range of variability for this ecological site. This state no longer exists due to the ubiquitous naturalization of non-native species in the Mojave Desert. Periodic drought and rare fire were the natural disturbances influencing this ecological site. Fire would have been a very rare occurrence due to the lack of a continuous fine fuel layer between shrubs.

Data for this State does not exist, but dynamics and composition would have been similar to State 2, except with only native species present. See State 2 narrative for more detailed information.

 

State 2: Reference State

State 2 represents the current range of variability for this site. Non-native annuals, including red brome, Mediterranean grass, red-stem stork’s bill, and Asian mustard are naturalized in this plant community. Their abundance varies with precipitation, but they are at least sparsely present (as current year’s growth or present in the soil seedbank).
 
Community Phase 2.1: Reference Plant Community


Community Phase 2.1

The reference plant community is characterized by an open two-tiered canopy less than 2 meters tall with creosote bush in the upper tier over a dense stand of big galleta and Indian ricegrass. Sand dropseed (Sporobolus cryptandrus) may also be present at low levels. Secondary shrubs may include burrobush (Ambrosia dumosa), white ratany (Krameria grayi), California ephedra (Ephedra californica), Wiggins’ cholla (Cylindropuntia echinocarpa), Plummer’s baccharis (Baccharis plummerae), and rayless goldenhead (Acamptopappus sphaerocephalus). The subshrubs California croton (Croton californicus) and desert globemallow (Sphaeralcea ambigua) are typically present. A spectacular display of annual forbs occurs during years of above-average precipitation. Common species typically include Esteve’s pincushion (Chaenactis steviodes), smooth desertdandelion (Malacothrix glabrata), bristly fiddleneck (Amsinckia tessellata), and birdcage evening primrose (Oenothera deltoides).

Non native species that may be present include Asian mustard, prickly Russian thistle, redstem stork's bill, and Mediterranean grass.

Community Phase Pathway 2.1a
This pathway occurs with prolonged or severe drought.

Community Phase Pathway 2.1b
This pathway occurs with moderate to severe fire.

Reference Plant Community Plant Species Composition
Grass/Grasslike Annual Production
(pounds per acre)
Foliar cover
(percent)
Group Group name Common name Symbol Scientific name Low High Low High
1 -Perennial Grasses 490 980
Indian ricegrassACHYAchnatherum hymenoides100150
big galletaPLRI3Pleuraphis rigida500650
sand dropseedSPCRSporobolus cryptandrus2050


Shrub/Vine Annual Production
(pounds per acre)
Foliar cover
(percent)
Group Group name Common name Symbol Scientific name Low High Low High
2 -Native shrubs 105 210
rayless goldenheadACSPAcamptopappus sphaerocephalus20100
burrobushAMDU2Ambrosia dumosa20100
Plummer's baccharisBAPLBaccharis plummerae0200
Wiggins' chollaCYEC3Cylindropuntia echinocarpa20100
California jointfirEPCA2Ephedra californica20100
white ratanyKRGRKrameria grayi20100
creosote bushLATR2Larrea tridentata50150


Forb Annual Production
(pounds per acre)
Foliar cover
(percent)
Group Group name Common name Symbol Scientific name Low High Low High
3 -Native Forbs 0 210
bristly fiddleneckAMTE3Amsinckia tessellata0170
milkvetchASTRAAstragalus0200
browneyesCACLC3Camissonia claviformis subsp. claviformis0200
Esteve's pincushionCHSTChaenactis stevioides0200
California crotonCRCA5Croton californicus2080
cryptanthaCRYPTCryptantha0200
smooth desertdandelionMAGL3Malacothrix glabrata060
birdcage evening primroseOEDE2Oenothera deltoides0200
desert globemallowSPAM2Sphaeralcea ambigua2050


Grass/Grasslike Annual Production
(pounds per acre)
Foliar cover
(percent)
Group Group name Common name Symbol Scientific name Low High Low High
4 -Native Annual Grasses 0 50
sixweeks gramaBOBA2Bouteloua barbata050


5 -Non-native annual grasses 0 110
common Mediterranean grassSCBASchismus barbatus0110


Forb Annual Production
(pounds per acre)
Foliar cover
(percent)
Group Group name Common name Symbol Scientific name Low High Low High
6 -Non-native annual forbs 0 200
redstem stork's billERCI6Erodium cicutarium0100
prickly Russian thistleSATR12Salsola tragus0100


Annual Production by Plant Type
Annual Production (lbs/ac)
Plant type Low Representative value High
Grass/Grasslike 490 700 980
Forb 0 150 210
Shrub/Vine 105 150 210

Total

595

1000

1400

Structure and Cover
Ground Cover
 
Vegetative cover Minimum Maximum
Grasses/grasslikes 7% 14%
Forbs 2% 3%
Shrubs/vines 2% 3%
 
Forest Overstory

 
Forest Understory

 
Plant Growth Curve
Growth curve number: CA3015
Growth curve name: Creosote bush XB
Growth curve description: Growth starts in early spring with flowering and seed set occurring by July. Dormancy occurs during the hot summer months. With sufficient summer/fall precipitation, some vegetation may break dormancy and produce a flush of growth.
 
Percent Production by Month
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
0 5 20 30 20 10 0 10 5 0 0 0
 
 
Plant Growth Curve
Growth curve number: CA3022
Growth curve name: Indian ricegrass
Growth curve description: Growth begins in late winter, flowering and fruiting finished by the hot summer months. Early fall rains can trigger a flush of new growth.
 
Percent Production by Month
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
0 5 15 30 35 5 0 0 5 5 0 0
 
 
Plant Growth Curve
Growth curve number: CA3024
Growth curve name: Big galleta
Growth curve description: Some green up in spring; dormant May and June; most growth occurs after summer rains.
 
Percent Production by Month
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
0 5 20 10 0 0 15 40 10 0 0 0
 
 
Community Phase 2.2: Drought Response

This community phase is characterized by an overall decline in cover due to branch-pruning and lack of recruitment of longer-lived species, mortality of shorter-lived perennials, and lack of emergence of annual forbs and grasses. Big galleta and Indian ricegrass are likely to decline due to drought-induced mortality, while creosote bush remains stable.

Big galleta may suffer very high rates of drought-induced mortality (Webb et al. 2003; Hereford et al. 2006); however, big galleta can respond very quickly to brief, intermittent rain during rare summer monsoonal events, which can buffer big galleta populations in the absence of more predictable winter rains. Creosote bush is an evergreen species capable of utilizing moisture at any time of the year. This ability buffers populations from the effects of drought that occur as the absence of the winter rains (the primary source of moisture for this ecological site). Further, creosote bush germinates in response to moisture during the warm season, so may still recruit if warm season rains occur during winter drought (Hereford et al. 2006). Creosote bush exhibits branch-pruning during severe drought, but mortality during drought in the Mojave Desert is very low (Webb et al. 2003, Griffeths et al. 2006). Nevertheless, during severe drought, creosote bush mortality may occur.

This is an at-risk community. Reduced cover in this eolian landscape increases the risk of erosion, which can trigger a transition to State 3.

Community Phase Pathway 2.2a
This pathway occurs with a return to average or above average precipitation.

Community Phase Pathway 2.2b
This pathway occurs with moderate to severe fire.

Community Phase 2.3: Fire regeneration community

This community phase is characterized by increased dominance by big galleta and Indian rice grass, severe declines in creosote bush, and an increase in shrub diversity. Fire damage to big galleta varies depending on whether plants are dormant when burned; if plants are dry, damage may be severe because the live center may be burned out (Matthews 2000). However big galleta often increases after fire (Minnich 2003). Indian rice grass is highly fire tolerant, and also increases after fire (Tirmenstein 1999). Creosote bush is generally killed by fire, and is slow to re-colonize burned areas due to specific recruitment requirements (Brown and Minnich 1986, Brooks et al. 2007, Steers and Allen 2011). The timing and severity of fire, as well as post-fire climate conditions determines trajectories of recovery (Brown and Minnich 1986, Steers and Allen 2011).

Community Phase Pathway 2.3a
This pathway occurs with time without fire, adequate precipitation, and no other significant disturbances (e.g. grazing).

Transition 2
This transition occurs with a loss of vegetation cover, in combination with drought and/or extreme wind conditions and/or anthropogenic disturbance such as grazing or off-road vehicle use that increases wind erosion beyond the threshold that will sustain the reference plant community. It is difficult to pinpoint the precise combination of these factors that will trigger this conversion (Cooke et al. 1993).


State 3: Eroded State

This State is characterized by the loss of sandsheet stability, with increased rates of wind erosion leading to deflation. This state has been significantly altered from the natural range of variability found in States 1 and 2. Increased wind erosion decreases the suitability of this ecological site for vegetation, killing established or recruiting individuals by abrasion and burial (Okin et al. 2001). Ongoing disturbance could result in complete loss of vegetation cover. Sand deflation could result in the accumulation of surface rock fragments, dramatically altering the soil and hydrological characteristics of this ecological site, and decreasing site suitability for annual species and big galleta.

We do not have data for this State, and further research is necessary to describe the community phases and successional pathways that may exist within the state.

 

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