Ecological dynamics

This ecological site occurs on gently sloping, narrow, first and occasionally second order ephemeral drainageways in the eastern Mojave Desert. These drainageways receive frequent flash flooding from runoff from adjacent fan remnants and hills. Soils are very deep sands with little soil development. Ephemeral drainageways, even small ones such as this ecological site, are ecologically important components of arid ecosystems where water is the principal limiting resource (e.g. Schlesinger and Jones 1984, Levick et al. 2008, Schwinning et al. 2010). Small drainage systems direct the flow of surface water, thereby temporarily increasing water availability during flooding (Schwinning et al. 2010). Very deep coarse-textured soils also act to store water in arid ecosystems (Noy-Meir 1973), so the temporal availability of water is higher in ephemeral drainageways than surrounding uplands (Schwinning et al. 2010). This increase in the amount and availability of water increases the productivity and cover of vegetation in these systems relative to adjacent uplands, and allows for a xeroriparian plant community to occur (Levick et al. 2008). The xeroriparian community is comprised of drought-tolerant species that also may occur on upland sites, but is more productive, has higher cover, and is typically more diverse than adjacent upland sites.

Soil disturbance from flash flood events is the primary driver of plant community dynamics within this ecological site. Ephemeral streams lack permanent flow except in response to rainfall events (Balling and Wells 1990, 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). Physical disturbance of soils as a result of flash flooding makes predictability of the plant community low except when considered at a coarse scale. Typical runoff events may result in an apparently stable mosaic of plant species distribution, 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.

The vegetation of this ecological site consists of a unique xeroriparian assemblage. All species occurring on the site may occur in upland habitats. The rare eastern Mojave Desert endemic (CNPS Inventory of Rare and Endangered Plants on list 1B.2) Mojave desert plum is a characteristic species of this ecological site. Mojave desert plum was described as a new species in 2002 (Prigge 2002), and is known only from the Vontrigger Hills area of the Mojave National Preserve (Consortium of California Herbaria (ucjeps.berkeley.edu/consortium/). Mojave desert plum habitat is typically small ephemeral drainageways, but it may also occur on adjacent hills. This species is thought to be a neo-endemic, recently evolved from the more common desert almond (Prunus fasciculata), which occurs in similar habitats, but little is known about the ecology of P. eremophila. Further data collection on the abiotic factors characteristic of Mojave desert plum habitat versus desert almond habitat is required, as is research into how these two species differ ecologically.

Other characteristic species include catclaw acacia (Acacia gregii), purple sage (Salvia dorrii), and big galleta (Pleuraphis rigida). Catclaw acacia usually reaches dominance only where there is regular flooding, or where surface fragments channel water (Sawyer et al. 2009). Purple sage and big galleta frequently occur on upland habitats, but are often abundant on disturbed soils, or soils receiving additional run-on.

Disturbances such as drought, fire, and human hydrologic alterations can affect the community composition and/or hydrologic process of this ecological site. Episodic drought is characteristic of the desert (Hereford et al. 2006), and can cause mortality or die-back of vegetation. Decreased vegetative cover can lead to increase in erosion and change sediment deposition patterns, possibly increasing the chance of channel migration. Historically fire was very 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 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), and loss of vegetation cover in these hills 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 causing temporary decreases in vegetative cover; widening of channels, which would increase the complexity of plant communities in the ecological site (i.e. areas receiving different flooding intensity or frequency would be dominated by different suites of species); sediment deposition could cause channel avulsion; abandoned channels would be less suitable to support the vegetation of this ecological site.