Ecological dynamics
This ecological site occurs at the base of mountains on drainageways, inset fans and fan aprons. This site is distinguished by a small drainage size, on first and second order streams, low sediment transport capacity, and the absence of deep rooted phreatophytes, such as desert willow or catclaw acacia. This site receives flow from first and second order drainageways, but the flow infiltrates into the sandy soils in the divergent channels, leaving the distal areas without surface flow except in extreme precipitation events. Soil disturbance from flash flood events is the primary driver of plant community dynamics within this ecological site. The soils are hyperthermic, very deep, and composed of sands and gravels. Shrub diversity is higher than the surrounding less flooded landforms on the fan piedmont slope. Schott's dahlia, creosote bush and burrobush dominate the site.
Ephemeral streams lack permanent flow except in response to rainfall events (Bull 1997, Levick et al. 2008). These ephemeral streams are characterized by extreme and rapid variations in flooding regime, and a high degree of temporal and spatial variability in hydrologic processes (Bull 1997, Stanley et al. 1997, Levick et al. 2008, Shaw and Cooper 2008). The drought-tolerant vegetation that exists on ephemeral streams and drainageways is referred to as xeroriparian vegetation. It is distinct from the surrounding landforms due to a difference in species composition, size, and production (Johnson et al. 1984, Levick et al. 2008). Xeroriparian vegetation is present because of the increased availability of water and flood disturbances in these drainageways. Deep rooted phreatophytes, that rely primarily rely on a deep water source such as desert willow, smoketree, and catclaw acacia are generally absent. Collectively, the plant communities along the drainageways are considered xeroriparian vegetation, and provide xeroriparian habitat.
This site can experience channel avulsion (defined as the “diversion of the majority of the surface flow to a different channel, with total or partial abandonment of the original channel” [(Field 2001)]). As sediment deposits in the active drainageway the likelihood of channel avulsion increases because of decreased drainageway volume. Cycles of channel avulsion on alluvial fans are an ongoing and a long-term process in the development of alluvial fans, and can occur after any substantial overland flow event when existing channel capacity is rapidly and dramatically exceeded.
The active channels are small, and are composed of bare sand and gravels. In less active areas creosote bush, Schott’s dalea, and burrobush are present. Creosote bush and burrobush are larger and more productive in the wash than in neighboring alluvial fans, without the extra run-on. Creosote bush and burrobush are community dominates on adjacent fan aprons. Schott's dalea depends on flood water for dispersal and seed germination (Vogl and McHargue 1966), and occurs on hyperthermic soils in habitats receiving additional run-on such as rocky hill-slopes, washes, and alluvial fans with regular sheet-flow. This site is at the northern most distribution of Schott’s dalea. It is more common further south in California and Northern Mexico. Data is lacking on the ecology of this species.
Other disturbances such as drought, fire, and human hydrologic alterations can affect the community composition and/or hydrologic process of this site. Drought is common in the desert, and can cause mortality or die-back of vegetation. Decreased vegetative cover can lead to increased erosion and change sediment deposition patterns, possibly increasing the chance of channel migration.
Historically fire was very uncommon in these ephemeral drainages; however the presence of continuous and flashy fuels from non-native grasses in adjacent upland sites can increase the possibility of fire. Invasion by non-native annual grasses has increased the flammability of desert vegetation communities (Brooks 1999 2004), and after fire, Mojave Desert ecosystems appear to be more susceptible to invasion by exotic grasses, leading to a grass-fire cycle (D'Antonio and Vitousek 1992). Very wet (El Nino) years followed by severe drought produce conditions where large areas where creosote scrub burn (Brown and Minnich 1986, DeFalco et al. 2010).
A properly functioning ephemeral drainage will provide some similar hydrologic functions as perennial streams. Ephemeral streams maintain water quality by allowing energy dissipation during high water flow. They transport nutrients and sediments, store sediments and nutrients in deposition zones, provide temporary storage of surface water, and longer duration storage of subsurface water. The structure and forage provided by diverse vegetation, and the availability of water (although brief), significantly increases animal abundance along ephemeral streams relative to upland areas. The open channels provide important migration corridors for wildlife (Levick et al. 2008).
When modifications affect the hydrologic function of this ephemeral stream system, this ecological site has the potential to transition to a hydrologically altered state (State 3). Once this threshold is crossed, it is extremely difficult to repair the hydrologic system.
Modifications to hydrology such as surface flow alterations, ground water depletion, and loss of vegetative cover can have irreversible impacts on hydrologic processes (Nishikawa et al. 2004, Levick et al. 2008). An increase in cover of impermeable surfaces (such as pavement, homes, malls, etc.) reduces the amount of runoff that can infiltrate into the soil creating higher surface runoff and greater peak flows. The runoff is collected in ditches, culverts, and drainage networks, and diverted to the nearest ephemeral stream. In some areas, ephemeral streams are armored to reduce damage to property from flood events. These confined channels reduce the ability for the stream to spread out and decrease flow velocity to allow sediment deposition. As a result, the channels will generally incise, with a higher volume of concentrated flows. These processes eventually cause higher peak flows due to increased runoff and concentrated flows. Higher flow velocities may cause uprooting, stem breakage or scour under the roots of vegetation. This loss of root structure along the stream increases scour potential, and the loss of above ground vegetation will increase flow velocity. When the vegetative community is degraded, important animal species dependent upon this community may be lost from the area as well. Ground water drawdown from household wells (Nishikawa et al. 2004) can deplete the water source for deep rooted species.
State 1
Historic State
State 1 represents the historic-natural condition for this ecological site. It is similar to State 2, but has only native species. If we were to include dynamics for this state it would be the same as displayed in State 2. The presence of non-native species is minimal in State 2, and has not altered the hydrology or fire frequency.
State 2
Reference State
This state represents the most common and most ecologically intact condition for this ecological site at the present time.
Community 2.1
Reference Phase
Figure 5. Community Phase 2.1
Figure 6. Community Phase 2.1
This community phase is dependent upon unimpaired hydrologic function and average to above average precipitation. At any given point along the stream the following community components are generally present. The relative spatial extent of these communities varies as the channel morphology fluctuates from flash flood events. Areas with sheet flow have a higher area of surface disturbance and will have more disturbance dependent species.
Three community components are present, including:
Community Component 1 (CC1):
Small active drainageway with occasional surface flow.
Community Component 2 (CC2):
This community is present on inset fans and depressions on alluvial fans. Sheet floods provide additional moisture and soil disturbance, which encourage the establishment of Schott's dalea, bladderpod spiderflower (Cleome isomeris), and burrobrush (Hymenoclea salsola). Creosote bush is larger and burrobrush is more abundant on this site than on the adjacent non-flooded alluvial fans. The presence and abundance of annual forbs is dependent upon precipitation but whitemargin sandmat (Chamaesyce albomarginata), cryptantha (Cryptantha sp.), western Mojave buckwheat (Eriogonum mohavense), smooth desertdandelion (Malacothrix glabrata), birdcage evening primrose (Oenothera deltoides), desert Indianwheat ((Plantago ovata), and chia (Salvia columbariae) are usually present in spring.
Community Component 3 (CC3):
This community is a transitional community that resembles adjacent non-flooded alluvial fans. It is not subject to active flooding, but is within the inset fan and is subject to flooding if channel avulsion occurs, and may have been in an active zone in the past. Creosote bush and burrobush dominate, but other species may include, Wiggins’ cholla (Cylindropuntia echinocarpa), branched pencil cholla (Cylindropuntia ramosissima), brittlebush (Encelia farinosa) and brownplume wirelettuce (Stephanomeria pauciflora), The forbs listed in CC2 are likely in this community as well.
The non-native annual forb redstem stork’s bill (Erodium cicutarium), and the non-native annual grasses red brome (Bromus rubens) and Mediterranean grass (Schismus sp.) arepresent in all community components but with low cover.
Table 5. Annual production by plant type
| Plant type |
Low
(kg/hectare) |
Representative value
(kg/hectare) |
High
(kg/hectare) |
| Shrub/Vine |
45 |
280 |
392 |
| Forb |
– |
11 |
45 |
| Grass/Grasslike |
– |
1 |
6 |
| Total |
45 |
292 |
443 |
Community 2.2
Drought Response
This community develops with prolonged or severe drought. It is difficult to determine the exact duration or intensity of drought that will cause this change, but a one to two year severe drought (of approximately 60 percent or less of average annual precipitation) can cause severe mortality in short lived perennials (Bowers 2005, Hereford et al. 2006, Miriti et al. 2007). During drought years, flood events are unlikely. The plant community components remain similar to those described in Community Phase 2.1, but show a decline in overall health, cover and production due to drought. Shorter lived species in CC2 (such as burrobush, burrobrush, and brittlebush) may suffer high mortality while longer lived species (creosote bush and Schott’s dalea) may have severe branch die back.
Community 2.3
Post-fire Regeneration
Initially annuals and short-lived perennials will dominate. Short-lived shrubs capable of quickly colonizing after fire include burrobush, burrobrush, and brittlebrush. These species produce seeds that are easily wind dispersed to the burn after fire. 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).
Pathway 2.1a
Community 2.1 to 2.2
This pathway is caused by a prolonged or severe drought.
Pathway 2.1b
Community 2.1 to 2.3
This pathway is caused by moderate to severe fire.
Pathway 2.2a
Community 2.2 to 2.1
This pathway occurs with the return of average to above average precipitation and associated flood events.
Pathway 2.2b
Community 2.2 to 2.3
This pathway occurs as a result of fire. Given low cover of annuals during drought, this pathway is unlikely except in periods immediately following heavy precipitation years.
Pathway 2.3a
Community 2.3 to 2.1
This pathway occurs in response to the passing of time with average to above average precipitation and associate flood events.
Pathway 2.3b
Community 2.3 to 2.2
This pathway occurs in response to the passing of time with drought conditions and absence of flooding.
State 3
Hydrologically altered
State 3 represents altered hydrological conditions typical of similar watersheds. Data is needed to develop a successional diagram for this state.
Community 3.1
Hydrologically Altered
Alterations in surface flow or changed in sediment transport capacity alter the stream hydrology. Surface flow can either be diverted away from or to an area. The diversion of flow from an area will cause a decline in disturbance dependent species such as Schott’s dalea and burrobrush and subsequent increase of creosote dominated plant communities. Increased flow to an area can cause erosion with channel widening or incision.
Transition T2A
State 2 to 3
Triggers that can cause a transition to State 3 include surface flow alterations and prolonged drought.
Any of the community phases from this state can cross the threshold to State 3, but community phase 2.3 and the later stages of 2.2 are especially vulnerable because decreases in vegetation density (and upland vegetation density) leave soils more susceptible to erosion (Bull 1997).
Restoration pathway R3a
State 3 to 2
Restoration from State 3 back to State 2 would be an intensive task. Individual site assessments would be required to determine proper restoration methods. Some hydrological modifications are not feasible restored, such as ground water depletion. However, road diversions can be redesigned to allow proper stream alignment and flow. Seeds or plants of appropriate species may need to be reintroduced to the restored channels.
