Ecological dynamics

This ecological site occurs on upper to medial portions of the fan piedmont, below order 3 or larger drainages, where flow from flash flood events dissipates and sediments are deposited on the fans. This site develops multiple braided channels because low stream velocity and sediment deposition causes channel migrations. The majority of this site is occasionally flooded, but it includes a small main channel with more frequent flooding frequency, and raised islands that rarely flood

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 (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, Shaw and Cooper 2008, Levick et al. 2008).

This site often experiences 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]). As sediment deposits in the active drainageway the likelihood of channel avulsion increases because of decreased drainageway volume. Cycles of channel avulsion on alluvial fans are an ongoing and long-term process in the development of alluvial fans, and can occur after any substantial overland flow event when existing channel capacity is rapidly and dramatically exceeded. Channel avulsion of the main channel would cause high mortality of vegetation in the old channel due to water stress and lack of regeneration, but would initiate development of xeroriparian communities in the newly created channel.

The majority of this area is a broad, extensively braided drainageway. There are barren active channels dominated by fine sands and gravels, and low relief sediment bars with higher vegetative cover and larger rock fragments, primarily cobbles. The drought-tolerant vegetation that exists on ephemeral streams and drainageways is referred to as xeroriparian vegetation. It is distinct from the surrounding landforms due to a difference in species composition, size, and production (Levick et al. 2008, Johnson et al. 1984). Xeroriparian vegetation is present because the increased availability of water and flood disturbances in these drainageways. The braided area is dominated by burrobush, desert lavender, and brittlebush but a variety of shrubs and forbs are present. The main active channel has low cover of smoketree. Two notable larger trees, blue paloverde and desert ironwood, are scattered across the drainageway. Blue paloverde is a relatively fast growing, shorter lived tree which is confined to washes in the drier parts of the Sonoran Desert. It needs greater moisture than the average precipitation that this ecological site provides, and its hard shelled seeds need physical scarification from floods or a long time of weathering in the soils to enable germination. Desert ironwood is long lived, slow growing tree with very dense wood, which is also largely confined to washes in this area of the Sonoran Desert.

Many of the species present in this ecological site (Blue paloverde, desert ironwood, Schott’s dalea, and desert lavender) are near their northern limit of distribution along the border of the Mojave and Sonoran Desert. These species are restricted to the warmer Sonoran Desert, and its biomodal precipitation pattern and absence of frost. The dominant species are frost-intolerant and rely on summer precipitation for seed germination. Climate data from the climate stations listed above indicate that temperatures below 28 degrees F may occur between late Dec to mid February, but do not occur every year. In the cooler areas (Hayfield Reservoir) temperatures may fall to the low 20s, approximately every 3 years. The Eagle Mountain station indicates closer to a 10-year period between frosts. Both stations indicate a prolonged period of freezing temperatures in January, 1950, with temperatures as low as 14 degrees F. at Hayfield Reservoir. The prolonged low temperatures in 1950 would likely have caused some mortality throughout this area. Less severe freezes of shorter durations or less severe temperatures will cause die back of frost-sensitive younger stems and branches, but may not kill the mature or hardened plants. The dynamics of frost is not included in the state and transition model (STM), because the incidence of frost is uncommon, and this site exists because of the relatively frost free conditions of this area.

Another climatic driver of this ecological site is summer precipitation. This area receives approximately 30 percent of its precipitation from July to October, from monsoons coming up from Mexico. Summer precipitation is important for the germination and survival of desert ironwood, blue paloverde, and desert lavender. Winter precipitation is important for seed production for desert ironwood and other species. Summer rains and warmer temperatures are needed to initiate germination for desert ironwood, desert lavender, and blue paloverde. Without summer rains and warmer temperatures some of these species would not regenerate well if at all.

Precipitation in this ecological site is very limited, with approximately 2 to 4 inches falling in a given year. The amount of total precipitation, and the timing and frequency of precipitation, highly is variable from year to year. In addition, the spatial distribution of precipitation is variable, as squalls may downpour on one area, but completely miss adjacent mountains or valleys. Hereford et al. (2004) describes shifts from dry to wet periods which may last several decades. During wetter cycles plant species show increased cover, size, and regeneration, while during drier periods and droughts, regeneration is low and shorter-lived shrubs have high mortality (Miriti et al. 2007). In the State-and-Transition model below, the drought phase is referring to severe droughts which cause high mortality, lack of regeneration and affect plant community composition. Although it is somewhat ambiguous to define, the Reference plant community phase implies that the area is receiving average (2 to 4 inches) or higher precipitation, and vegetation is not in decline due to drought.

When precipitation events occur, these ephemeral streams provide important hydrologic functions, such as maintaining 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. These streams are important ecologically because they provide water and habitat for a variety of plant and animal species. 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 also provide important migration corridors for wildlife (Levick et al. 2008).

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 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, 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 of creosote scrub may burn (Brown and Minnich 1986).

When modifications affect the hydrologic function of this ephemeral stream system, this ecological site has the potential to transition to a hydrologically altered state (State 3). Once this threshold is crossed, it is extremely difficult to repair the hydrologic system.

Modifications to hydrology such as surface flow alterations, ground water depletion, and loss of the xeroriparian vegetation can have irreversible impacts on hydrologic processes (Nishikawa et al. 2004, and Levick et al. 2008). An increase in cover of impermeable surfaces (such as pavement, homes, malls, etc.) reduces the amount of runoff that can infiltrate into the soil, creating higher surface runoff and greater peak flows. The runoff is collected in ditches, culverts, and drainage networks, and diverted to the nearest ephemeral stream. In some areas, retaining walls are built along ephemeral streams to reduce damage to property from flood events. These confined channels reduce the ability for the stream to spread out and decrease flow velocity to allow sediment deposition. As a result, the channels will generally incise, with a higher volume of concentrated flows. These processes eventually cause higher peak flows due to increased runoff and concentrated flows. Higher flow velocities may cause uprooting, stem breakage or scour under the roots of xeroriparian vegetation. This loss of root structure along the stream increases scour potential, and the loss of above ground vegetation will increase flow velocity. When the xeroriparian community is lost, important animal species dependent upon this community may be lost from the area as well. Ground water drawdown from household wells (Nishikawa et al. 2004) can deplete the water source for phreotophytes, such as desert lavender, blue paloverde, ironwood, and smoketree and may eliminate these species in affected areas.