Ecological dynamics

This ecological site occurs on relatively confined, occasionally flooded, stony, inset fans among cobbly fan remnants. Soil surfaces are stony to cobbly. Blue paloverde is dominant, and desert lavender and Schott’s dalea are important species. The additional run-on provided by the rocky surfaces of both the inset fans and fan remnants supports high cover of blue paloverde and is favored habitat for desert lavender and Schott’s dalea.

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). 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. Channel avulsion of the main channel is rare, but it occurred it 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 a 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. This ecological site generally occurs on the upper to medial positions of a fan piedmont. At the distal reaches, flow dissipates and infiltrates and in the absence of flood disturbance or additional moisture, the inset fans become vegetated with upland creosote bush plant communities.

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 (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, desert lavender, and Schott’s dalea are present along the stony sediment bars. 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).

The dominant species present in this ecological site (Blue paloverde, desert lavender, and Schott’s dalea) 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. Blue paloverde and desert lavender 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 is not included in the state and transition model (STM), because the incidence of frost is uncommon, and since this site exists 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 about 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 blue paloverde and desert lavender. Winter precipitation is important for seed production, and summer rains with warmer temperatures initiate germination of desert lavender and blue paloverde seeds. Without summer rains and warmer temperatures some of these species would not regenerate well.

Precipitation in this portion of the Sonoran desert is limited, with approximately 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) describe 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 more severe droughts, which cause high mortality, lack of regeneration and affect plant community composition. Although it is somewhat ambiguous to define, the Reference phase implies that the area is receiving average to 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).

This ecological site is unlikely to burn, due to sparse vegetation within the drainageways and adjacent fans. Non-native grasses are not abundant, and are not likely to develop high fuel loads due to the very rocky surfaces.

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 that affect this ecological site are generally surface flow alterations for roads or aqueducts. The diversion of flow can either divert flow away from or to an area. Diversion away from an area will cause a decline in blue paloverde and desert lavender over time, due to the absence of flooding and additional moisture. Addition of flow to an area may cause erosion and channel incision or widening in the flood zone. 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 and desert lavender, potentially eliminating these species from certain areas.