Ecological dynamics

This ephemeral stream ecological site occurs in headwaters and first order drainageways on channels and stream terraces on upper fan piedmonts. There is a rarely flooded (1 to 5 times in 100 years) narrow active channel with an associated very rarely flooded (surface flooding may occur 1 in 100 years) broad stream terrace or inset fans. This site has developed in response to infrequent flash flood events, which form sheet flow across the broad flat areas. Headwater streams are hydrologically connected with lower stream reaches, which may be the mid-sized Thermic to Hyperthemic Ephemeral Stream (R030XY186CA) or the Large, Sandy, Thermic Ephemeral Stream (R030XY167CA). The headwater and upper watershed first order drainageways may accumulate nutrients and organic matter during moderate storm events. During dry periods these areas may accumulate organic matter. The organic matter and nutrients are mobilized during large storm events to lower reaches of the drainageway. These nutrients can influence downstream productivity (Levick et al. 2008). The soils are very deep and sandy with very little cohesive structure. They are easily redistributed with flood events. The soil moisture regime is aridic or aridic bordering on xeric, with a thermic soil temperature regime. This ecological site has cooler temperatures and more moisture than the other ephemeral stream ecological sites within Joshua Tree National Park. Drought and fire are also important drivers of plant community succession and hydrological dynamics.

This site is in broad, low lying, headwater positions which accumulate surface and groundwater flow. The drought-tolerant vegetation that exists on 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 site does not provide a deep water source for phreatophytes that rely primarily on deep water source, so species like desert willow (Chilopsis linearis) are absent from this site. The active channel is generally bare of vegetation. Along the channel margins, California jointfir, peach thorn (Lycium cooperi), and desert almond (Prunus fasciculata) can be relatively productive. California jointfir, Mojave indigo bush, burrobrush, peachthorn, desert almond, Joshua tree (Yucca brevifolia), and California juniper (California juniper) form an open mosaic on broad terraces. Collectively, all of these plant communities are part of the xeroriparian vegetation, and provide xeroriparian habitat.

California jointfir is a long lived shrub, often associated with ephemeral drainages in arid and semi-arid regions. It is a gymnosperm, reproducing by wind dispersed pollen and bearing seed in cones (Anderson 2004). It only produces abundant seeds during years of ample precipitation. The seeds have a short dormancy period, and will germinate only after the dormancy period and sufficient moisture. Once established, California jointfir is fairly drought tolerant. Desert almond and peachthorn are also long lived shrubs. They produce seeds that are dispersed by animals, by gravity, or by water. Sheet flow disperses the heavier seeds of these long lived shrubs, which germinate in moist soil after floods. Burrobrush is a shorter lived shrub that will establish with any disturbance and brief moisture availability. Mojave indigobush is most abundant in habitats where additional run-on is available.

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.

Historically fire was very uncommon in these ephemeral drainages; however the presence of continuous and flashy fuels from non-native grasses can increase the possibility of fire. Invasion by non-native annual grasses has increased the flammability of desert vegetation communities (Brooks 1999, 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). The abundance and biomass of these grasses is highest on sandy soils where nitrogen deposition from air pollution from adjacent urban areas is high (Rao et al. 2010). This ecological site is susceptible to high biomass loads of non-native annual grasses due to its sandy soils and proximity to the greater Los Angeles area. Very wet (El Nino) years followed by severe drought produce conditions where large areas of desert scrub may burn (Brown and Minnich 1986).