Ecological dynamics

This ecological site is associated with relatively large third order or larger ephemeral streams, situated close to mountain slopes where high intensity water inputs occur. Large, frequent flash-flood events create a wide active channel that is mostly barren with scattered, patchy smoketree. The active channel has a shallow braided pattern across a broad, relatively level drainageway. Desert ironwood and blue paloverde are present across this area on slightly raised positions, or along channel margins. The associated occasionally flooded bars have higher shrub and forb diversity, with burrobush (Hymenoclea salsola), desert lavender (Hyptis emoryi), and sweetbush (Bebbia juncea) common. Stable islands may occur within the drainageway, and are dominated by brittlebush (Encelia farinosa) and creosote bush (Larrea tridentata). There is a high range of variability within this ecological site due to variation in flood intensity, stream volume, and flow disruptions due to road crossings.

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, Levick et al. 2008, Shaw and Cooper 2008). Physical disturbance of soils as a result of flash flooding makes predictability of temporary channel development and configuration very low except when considered at a very coarse scale. 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. Channel avulsion of the main channel would cause high mortality of vegetation in the old channel due to water stress, and initiate development of xeroriparian communities in the newly created channel

Channel avulsion is “the diversion of the majority of the surface flow to a different channel, with total or partial abandonment of the original channel” (Field 2001). Because flow in these ephemeral streams is inconsistent in force, volume, and frequency, deposition and scour of sediment varies with each flood event. Changes in sedimentation and erosion alter flow dynamics, potentially redirecting flow to an alternate channel. Cycles of channel avulsion on fan piedmonts is an ongoing and long-term process in the development of alluvial fans and associated landforms, and can occur after any substantial overland flow event when existing channel capacity is very rapidly and dramatically exceeded. If channel avulsion occurs at the apex of the alluvial fan, it is more likely to capture the majority of the stream flow. Upper fans extend into the base of mountains, which provide a direct sediment source that is transported over time, by larger flood events, to distal reaches of the drainage. This ecological site generally occurs on the upper to medial positions of the fan piedmont. At the distal position, the stream loses velocity, and becomes more braided while the large active channel dwindles. This is the transition to another ephemeral ecological site, the Gravelly, Braided Ephemeral Stream (R031XY034CA). Eventually, even the braided channels dissipate, and become vegetated with upland creosote bush plant communities.

The frequency and impact from flood events varies across the drainageway and channel segments, developing a complex of xeroriparian plant communities. Xeroriparian vegetion refers to the drought-tolerant vegetation that exists on ephemeral streams and drainageways. 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.

Blue paloverde, smoketree, desert lavender, and catclaw acacia are present along active channel margins. These species are phreatophytes, that is, they have deep roots and primarily rely on a deep water source. A deep water source typically refers to a water table or a zone of saturated soils. However, these ephemeral desert streams do not generally have water tables within reach of plant roots, and here plants are accessing deep ground water in unsaturated soils (Nilsen et al. 1984). This ephemeral stream receives surface run-off from mountain slopes and mountain drainages. When this flow reaches the broad deep sandy channels on the fan piedmont, flow is able to spread out and infiltrate deep into the soils. A high volume of water infiltrates into these coarse soils, which supports the phreotophyte species as well as shallower rooted species.

During larger flood events, flow will sheet flood across the higher areas of the drainageway. These occasionally flooded areas are dominated by shorter-lived shrubs with shallower root systems (burrobush and sweetbush). These species are absent in the active channel because they will uproot during high velocity floods. Shorter lived shrubs are adapted to lower intensity flood disturbance because they have high seed production, high seed viability, and seeds that are light and easily dispersed by wind from nearby sources. The new generation matures quickly, producing more seed.

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 Deserts. These species are restricted to the warmer Sonoran desert and its biomodal precipitation pattern and lack 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, about 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 are not included in the state and transition model (STM), because the incidence of frost is uncommon, and since this site exist because of the relatively frost free conditions of this area.

Another climatic driver for this ecological site is the reliance of several of these species on 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, desert lavender and smoketree. 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 northern portion of the Sonoran desert is limited, with about 2 to 4 inches falling in a given year. The amount of total precipitation, and the timing and frequency of precipitation is highly variable from year to year. In addition, the spatial distribution of precipitation is patchy, as squalls may downpour on one area, but completely miss adjacent mountains or valleys. Hereford et al. (2006) describes shifts from dry to wet periods which may last several decades. During wetter cycles plant species show increases in cover, size, and regeneration; while during drier periods and droughts, regeneration is low and shorter-lived shrubs have high mortality (Hereford et al. 2006, Miriti et al. 2007). Severe droughts have even more pronounced mortality. In the STM below, the drought phase is referring to the more 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 or above average 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).

Fire is historically rare in desert ephemeral drainageway communities. An increase in the abundance of invasive annual grasses and annual forb cover in associated upland communities has led to an increase in fire frequency (Brown and Minnich 1986, Brooks et al. 2004, Brooks and Matchett 2006, Rao et al. 2010, Steers and Allen 2011) in upland communities as well as ephemeral drainageways.

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 hydrology of the 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, 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 generally scour and incise. 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 phreatophytes, such as blue paloverde, desert lavender, smoketree, and catclaw acacia, potentially eliminating these species from certain areas.