Ecological dynamics

This ecological site occurs on channeled, rarely flooded alluvial fans and fan aprons that are within the transitional zone between thermic and hyperthermic soil temperatures. This site has rare sheet flood events. Soils are very deep sands with very gravelly to extremely gravelly modifiers. Flooding, very deep gravelly soils, a channeled topography, and a transitional climate support a diverse shrub mixture, co-dominated by Schott’s dalea, creosote bush and desertsenna.

This site is not considered an ephemeral stream as it does not have a defined drainageway or channel and is primarily a zone of infiltration. This site is on alluvial fans, near the mountain base. This area receives surface flow from mountain runoff. In addition, overflow from adjacent large drainageways may occur during large flood events. Since this site is adjacent to the mountains, there are larger rock fragments and high percentage of gravels on the soil surface and in the soil profile. Because, of the variation of disturbance across the alluvial fan, there is a high degree of temporal and spatial variability in the plant composition.

Lower and more channeled positions tend to have more frequent or intense flooding, and have higher diversity of species, while less flooded topographic positions are dominated by a more homogeneous creosote bush – burrobush (Ambrosia dumosa) shrubland. Creosote bush, Schott’s dalea, and desertsenna are dominant in areas with more flooding. Creosote bush is a dominant species throughout the Mojave and Sonoran Deserts on fan piedmonts with deep soils with thermic to hyperthermic soil temperatures. When disturbance rates are low, extreme longevity and low mortality rates allow creosote bush to be strongly dominant, but with recurrent disturbance, shorter-lived species are able to establish and persist in the plant community. Schott's dalea is a leguminous shrub, which 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. Desertsenna is also a leguminous shrub commonly found in Mojave and Sonoran desert wash habitats (Baldwin et al. 2002), and is associated with creosote bush communities subject to disturbances (Sawyer et al. 2009).

This ecological site is located near the northern distribution ranges of Schott’s dalea, teddy bear cholla (Cylindropuntia bigelovii) and ocotillo (Fouquieria splendens). These species are more common in the Sonoran Desert, which has hotter temperatures and receives a larger proportion of precipitation during summer from monsoons. Ocotillo has low cover, but a distinctive presence in portions of this ecological site. Its tall, thin, spiny branches stand above the shrub canopy, and its red flowers are striking in early spring. Ocotillo is also present in the Chihuahuan Desert in New Mexico and western Texas. Ocotillo is often associated with shallow rocky soils, but exists on very deep soils on this site. It has a shallow root system, quickly sheds its leaves in response to drought, and remains dormant after leaf abscission to avoid water stress. Ocotillo produces leaves immediately after precipitation, and retains them for a few weeks, but multiple leaf flushes can occur in a year. Ocotillo produces multiple winged seeds during winters of ample precipitation, but requires summer rains for seed germination and survival (Matthews 1994). Dense teddy bear cholla patches may occur within this ecological site. Teddy bear cholla is present in the warmer parts of the Mojave Desert, and in the hotter, drier parts of the Sonoran Desert. Teddy bear cholla seldom regenerates from seed, and reproduces primarily by vegetative means from joint segments that detach, develop roots, and new plants. The terminal joints detach easily in the cooler months, by a slight touch or wind. These large cholla gardens may be clones of one plant (Nabhan 2006). Sheet floods associated with this area may help break and disperse cholla pieces.

Other disturbances such as drought, fire, and human hydrologic alterations can affect the community composition and/or hydrologic process of this site. Drought is characteristic of the desert, and can cause mortality or die-back of vegetation. Decreased vegetative cover can lead to increased erosion and change sediment deposition patterns.

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 likelihood of fire. Invasion by non-native annual grasses has increased the flammability of desert vegetation communities ( Brooks , Brooks et al. 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).

Although not truly an ephemeral stream, this site maintains water quality by allowing energy dissipation during high water flow, and store sediments and nutrients in deposition zones. This site also provides 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 in this diverse area, relative to unflooded sites (Levick et al. 2008).

Modifications to hydrology such as surface flow alterations and loss of vegetative cover can have irreversible impacts on hydrologic processes (Nishikawa et al. 2004, Levick et al. 2008). Altered surface flow can alter flow direction, and cause changes in erosion and sedimentation. The site may remain similar, as it has developed with disturbances. However, if water flow is eliminated from an area more stable upland species will dominate, similar to the drought phase described below. An increase in cover of impermeable surfaces (such as pavement, homes, malls, etc.) will reduce the amount of runoff that can infiltrate into the soil creating higher surface runoff and greater peak flows.