Ecological dynamics

This site occurs on large sized (typically order 3) ephemeral drainageways with braided channels at elevations of approximately 3,000 to 4,500 feet. Slopes are typically 1 to 4 percent. These drainageways occur on the middle portions of alluvial fans, where depositional forces are more prominent than erosive forces. Soils are very deep sands formed from mixed rock, and they generally have a typic aridic soil moisture regime. The site is a complex of landforms and vegetation communities that are dictated by flooding intensity and frequency. This site encompasses very rarely to rarely flooded stream terraces, rarely to occasionally flooded inset fans, bars and stream margins, and frequently flooded stream margins and channels. Frequently flooded stream margins are occupied by patches of desert willow and catclaw acacia. Frequently to occasionally flooded channels and bars are occupied by a Mojave rabbitbrush and woolly fruit bur ragweed community. Very rarely to rarely flooded terraces are occupied by mixed shrub community dominated by burrobrush and creosote bush with a high diversity of other shrubs. These large drainages provide a relatively consistent deep-water source, which supports desert willow communities.

Although ephemeral stream processes are much more variable than perennial streams, a properly functioning ephemeral drainageway will provide similar hydrological and biological functions as perennial streams (Hild et al. 2007, Levick et al. 2008, Vyverberg 2010). 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. They also support a disproportionate share of biodiversity (Hild et al. 2007, Levick et al. 2008, Vyverberg 2010). The drought-tolerant vegetation that occurs within ephemeral streams 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. This vegetation protects soils from erosion and influences flow by providing bank and channel roughness, and initiates formation and maintenance of channel bars ((Levick et al. 2008, Vyverberg 2010, Stein et al. 2011). The structure and forage provided by xeroriparian 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).

Soil disturbance from flash flood events is the primary driver of plant community dynamics within this ecological site. Ephemeral streams flow only in response to rainfall events, and flow may last only minutes or days (Bull 1997, Levick et al. 2008, Vyverberg 2010). Extreme and rapid variations in flooding regime, and a high degree of temporal and spatial variability in hydrologic processes is characteristic (Bull 1997, Stanley et al. 1997, Levick et al. 2008, Shaw and Cooper 2008, Vyverberg 2010). Episodic high magnitude events that may occur only a few times a decade or century function to ‘reset’ vegetation and channel form (Levick et al. 2008, Stein et al. 2011). Smaller more frequent flood events deposit sediment, leading to channel infilling and eventually channel avulsion dynamics. Channel avulsion is 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 main channel of the depositional zone, and as vegetation colonizes stream channels, banks and bars, the likelihood of channel avulsion increases because of decreased channel volume (Levick et al. 2008). Channel avulsion is frequently observed in the depositional environment of this ecological site.

Extreme temporal and spatial variability in disturbance events, water availability, sediment flux, and channel migrations result in a dynamic complex of hydrologically and disturbance determined plant communities that do not conform to an equilibrium model of vegetation community dynamics (Vyverberg 2010, Stein et al. 2011). Physical disturbance of soils and vegetation as a result of flash flooding makes predictability of temporary channel development and configuration very low except when considered at a coarse scale. Typical runoff events may result in an apparently stable mosaic of plant species distribution and channel configuration, while more extreme events may completely reconfigure the mosaic and establish the foundation of a new or modified plant community mosaic until the next extreme runoff event occurs. Vegetation communities reflect the time in the recurrence interval, or time between large magnitude ‘reset’ events. Early on in the recurrence interval there is lower diversity in the communities present, with dominance by short-lived species. For the majority of the interval, a mixture of long-lived and short-lived species is present. The late phase of the cycle is characterized by abundant vegetation with narrowing of the channel, making it more susceptible to resetting by a large flood as flow capacity diminishes.

Other disturbances such as drought, climate change, fire, grazing, mining, and land development can affect community composition and/or hydrologic processes. Cycles of drought are inherent to the desert, and can cause significant mortality or die-back of vegetation (e.g. Hereford et al. 2006). Decreased vegetative cover can lead to an increase in erosion and change sediment deposition patterns, possibly increasing the chance of channel migration. Global climate change models for the desert southwest predict increased drought intensity, increased warming and drying, and greater variability in precipitation (Levick et al. 2008). These changes could lead to a decline in xeroriparian vegetation with greater intensity floods and erosion.

Historically fire was uncommon in these ephemeral drainages, and since the dominant species of this site recover rapidly or increase in response to fire, fire is not considered a direct threat to this ecological site. However, native annual vegetation may fuel fire in adjacent communities, especially after high precipitation years. Further, the presence of continuous and flashy fuels from non-native grasses in adjacent upland sites has increased the frequency of fire in adjacent communities (Brooks 1999, 2005, Brooks et al. 2007, DeFalco et al. 2010, Brooks 2011, Engel and Abella 2011). The loss of vegetation cover in adjacent communities can contribute to increased flooding and sediment deposition in this ecological site. This could have a number of effects, including increased scouring of xeroriparian vegetation within the drainage channels; widening of channels, which would increase the complexity of plant communities in the ecological site (areas receiving different flooding intensity or frequency would be dominated by different suites of species); and sediment deposition and channel avulsion.

Livestock grazing has impacted this ecological site. Ranching was established in the eastern Mojave desert in approximately 1875 (Nystrom 2003). Grazing occurred unregulated in the area until the passage of the Taylor Grazing Act in 1934, which divided public land into allotments that were regulated by the Bureau of Land Management (BLM), and among other things, called for fenced ranges and multiple developed water sources (http://www.blm.gov/wy/st/en/field_offices/Casper/range/taylor.1.html). The Federal Land Policy and Management Policy Act of 1976 (FLPMA) brought further regulations, including 10-year grazing permits. In 1994 the California Desert Protection Act created the Mojave National Preserve, and the National Park Service took over management of grazing allotments in much of the eastern Mojave.

Most of the area occupied by this ecological site within the Mojave National Preserve was retired from grazing in 2000 (Kim 2004), and ecological communities are still recovering. Cattle and burros preferentially use riparian habitat because of access to water, shade, and productive vegetation (e.g. Kauffman and Kruegger 1984, Kie and Boroski 1996, Belsky et al. 1999). Livestock grazing can alter riparian vegetation species composition by selective grazing, plant cover removal, trampling stream banks, and compacting soil (Kauffman and Kruegger 1984, Trimble and Mendel 1995, Belsky et al. 1999 Increased runoff resulting from compacted soil and/or loss of vegetation may have led to channel incision, more intense flooding erosion, loss of sheet flow, and declining xeroriparian communities. Grazing in adjacent upland communities may have further increased runoff, erosion, and incision (Trimble and Mendel 1995, Belsky et al. 1999). Since ungrazed examples or detailed historical data of this system do not exist, it is not possible to quantify these impacts here.

Altered hydrological processes such as surface flow diversions, ground water depletion, and loss of the xeroriparian vegetation can have irreversible impacts such as headward erosion, increased flooding and sediment deposition, and/or channel abandonment (Nishikawa et al. 2004, Levick et al. 2008, and Stein et al. 2011). Impermeable surfaces (such as pavement, homes, malls, etc.) reduces soil water infiltration, creates higher runoff, greater peak flows, and more frequent high intensity flooding events (Levick et al. 2008). Stream channelization also increases flood intensity and sediment transport within some reaches, while reducing flow to other reaches. Dams and improperly constructed roads and railroads can cause aggradation and flooding upstream, channel incision and channel abandonment downstream (Levick et al. 2008). Channel abandonment, incision and/or significant reductions in flow can convert xeroriparian vegetation communities to upland communities by altering traditional flow patterns. Channel incision may also scour channel features and lead to more frequent high intensity floods, reduce channel vegetation diversity and create a community dominated by short-lived species that can withstand the new flooding regime.

When disturbances such as those describe above affect the hydrologic function of this ephemeral stream system, this ecological site has the potential to transition to hydrologically altered states (States 2 and 3). Data are not available to describe these states, and they are described in general terms as provisional states in the state-and-transition model.

All tabular data listed for a specific community phase within this ecological site description represent a summary of one or more field data collection plots taken in communities within the community phase. Although such data are valuable in understanding the phase (kinds and amounts of ground and surface materials, canopy characteristics, community phase overstory and understory species, production and composition, and growth), it typically does not represent the absolute range of characteristics nor an exhaustive listing of species for all the dynamic communities within each specific community phase.