Ecological dynamics

Small ephemeral streams are often overlooked from an ecological and management perspective yet these streams may be more important than or at least as important as larger ephemeral streams because of the importance of water in the desert. Small ephemeral streams receive more frequent low flows relative to infrequent large flows in larger ephemeral streams (Griffiths et al. 2006, Levick et al. 2008). 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 and provide important migration corridors for wildlife (Hild et al. 2007, Levick et al. 2008, Vyverberg 2010). 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.

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 are common in ephemeral wash systems (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 a 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 (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) dynamics. 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).

The dynamic nature of ephemeral streams creates a complex of plant communities that do not conform to an equilibrium model and makes temporary channel development and configuration predictions difficult (Vyverberg 2010, Stein et al. 2011). 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. A low diversity of short-lived species will dominate areas shortly after a ‘reset’ event but long-lived species will increase over time to create a mixture of both short-lived and long-lived plant species. 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.

Other disturbances such as drought, climate change, fire, grazing, mining, and land development can affect community composition and/or hydrologic process. Cycles of drought are inherent to the desert, and can cause significant mortality or die-back of vegetation (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 southwest United States 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.

Ephemeral stream vegetation is determined by geographical location, drainage size (which affects water supply and disturbance frequency and intensity), elevation, topographic position (slope, type of watercourse), and soil properties (texture, bed materials, parent material) (Evens 2000, Levick et al. 2008, Stein et al. 2011). Differences in water availability and flooding disturbance distinguish xeroriparian vegetation from the surrounding landform vegetation which is evident by species composition, size, and production (Johnson et al. 1984, Levick et al. 2008). Plants protect soils from erosion and influence water flow by providing bank and channel roughness through channel bar formation and maintenance (Levick et al. 2008, Vyverberg 2010, Stein et al. 2011).

This ecological site describes the dynamics of a moderate sized eastern Mojave Desert ephemeral stream system. It occurs on drainageways (including associated landforms such as channels, terraces, inset fans) that are bound by steep alluvial walls or incised banks that drain fan remnants at elevations of approximately 3,400 to 5,500 ft. Drainageway slopes of 2 to 4 percent are typical. This site has an aridic bordering on ustic soil moisture regime. Soils are very deep, well to excessively drained sands with sandy and sandy skeletal particle size classes. Flood intensity, scour and sediment transport varies both spatially and temporally across the drainageway and along the channel segments, which creates a disturbance dependent complex of xeroriparian plant communities (drought-tolerant vegetation within ephemeral streams) that may include barren active channels, occasionally to frequently flooded channel margins and low bars, and very rarely to rarely flooded terraces and high bars. Community components are used in this ecological site to represent the vegetation response to the various degrees of scour and sediment transport. Soil disturbance from flash flood events is the primary driver of plant community dynamics within this ecological site.

Several different ephemeral stream ecological sites are often connected to each other within the same watershed and given the dynamic nature of ephemeral stream systems community components of one ecological site may be ecologically similar to a community component of another ecological site. Although community components of ephemeral stream ecological sites are often portrayed in a lateral cross-section of the wash, community components also exist on a longitudinal basis as well; creating a very fuzzy boundary between the various ecological sites. Soils for both the Ustic Ephemeral Drainageways Order 2 (R030XY220CA) and the Ustic Ephemeral Drainageway Order 3 (R030XY219CA) ecological sites are very similar and management implications based on soil interpretations will be very similar but due to a growing understanding in the importance of desert washes these ecological sites have been created as separate entities based on the absence/presence of desert willow and Mojave rabbitbrush.

When two order 2 ephemeral streams merge an order 3 ephemeral stream is created. The order 3 ecological site, R030XY219CA, forms at higher elevations and the Mid-Elevation Riparian Complex 4-7" p.z. (R030XY222CA) forms at lower elevations both of which are larger drainageways with significant cover of desert willow. If the moderate sized order 2 drainages do not converge with other streams, surface flow eventually percolates out of the channel into substratum. Below this point the active channel becomes vegetated with stable upland vegetation, such as creosote bush (Larrea tridentata) shrubland.

In the Ustic Ephemeral Drainageways Order 2 system moderate drainage size restricts vegetation to predominately drought-resistant shrubs, with little water input to support phreatophytic trees. Dominant species in occasionally to frequently flooded positions (channel margins, low bars) include disturbance adapted shrubs such as woolly fruit bur ragweed, desert almond, burrobush and purple sage. Rarely flooded higher bars and terraces support a productive community dominated by perennial grasses including black grama, big galleta and bush muhly.

Desert almond is a long-lived deep-rooted species diagnostic of episodic flooding (Stein et al. 2011). In the eastern Mojave Desert it is associated with the cooler temperatures and higher precipitation of upper fan piedmont positions, and gravelly soils (Evens 2000, Sawyer et al. 2009). Purple sage, woolly fruit bur ragweed and burrobush are all shorter-lived, shallow-rooted species also diagnostic of episodic disturbance (Stein et al. 2011). Relatively high summer precipitation supports woolly fruit bur ragweed and black grama. Higher elevations and intermittent disturbance support purple sage. In the eastern Mojave Desert the desert almond – woolly fruit bur ragweed association is associated with confined channels located at mid elevations with coarse to medium sandy soils (Evens 2000). The relative composition of these communities is determined by time since disturbance and the intensity of disturbance events.

Intense lightning storms are common in the middle elevation desert shrubland and grassland. Large fires are likely to have been more common near this ecological site than at lower elevation sites (Mojave National Preserve 2004, Papierski 1993). Many large fire scars are visible in areas surrounding this ecological site in the mid-20th century aerial photos. The dominant species at this ecological site recover rapidly or increase in response to fire however loss of vegetation cover due to fire in the surrounding mountains and hills can contribute to increases in flooding events, sediment deposition, scouring of xeroriparian vegetation, channel avulsion and channel widening.

Livestock grazing has also 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, although wild burros remain at this site. Cattle and burros preferentially use riparian habitat because of access to water, shade, and productive vegetation (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). There is great uncertainty as to the pre-European plant composition of this ecological site but riparian vegetation is resilient and remains distinct from the surrounding uplands, possibly attributable to both manmade and lightning fires in the area and the introduction of foreign ungulates into this ecological system.

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