Ecological dynamics

Abiotic Factors

The most important abiotic factors driving this ecological site are a dissected, eroded pediment landform, very shallow to shallow soils to a hard bedrock contact, and a hyperthermic soil temperature regime. The site is co-dominated by Mojave indigobush, desertsenna and burrobush, and a high diversity of secondary shrubs and subshrubs is typically present.

Pediments are gently sloping, bedrock erosional surfaces of low relief that form at the base of receding mountain fronts. Pediment surfaces are covered by a discontinuous layer of alluvium that is typically thinner closer to the mountain front and eventually thick enough that the pediment is no longer distinct from the alluvial fan (Dohrenwend and Parsons 2009). Closer to the mountain front, erosion is active, and exposed bedrock is dissected and undulating (Dohrenwend and Parsons 2009). Different topographical positions within these dissected pediment landscapes experience different rates of overland flow and rainsplash erosion (Edinger-Marshall and Lund 1999).

The dissected pediment landscape, with contrasting and localized areas of soil erosion and deposition, and run-off and run-on, supports a diverse plant community. The majority of the landscape is characterized by shallow soils subject to active erosion, which support a sparse but diverse community of shallow-rooted woody shrubs and subshrubs. Species that are capable of sprouting after mechanical damage are more common in this ecological site; these include Mojave indigobush, desertsenna, Fremont’s dalea (Psorothamnus emoryi), California jointfir (Ephedra californica), and range ratany (Krameria erecta). The frequency of disturbance supports the continuous presence of shorter-lived subshrubs, which in more stable landforms are eventually replaced by longer-lived dominants. Annual species are limited in this site by the lack of soil moisture, and by soil erosion, which limits seedling establishment (Guerrero-Campo et al. 2008). Interfluves transport material, receive run-on from higher positions, and are mantled by a deeper layer of alluvium, which supports deeper-rooted species like creosote bush, and species more typical of washes, such as desertsenna.

Disturbance dynamics

The major disturbances affecting this ecological site are drought, invasion by non-native species and erosion.

Drought is an important shaping force in Mojave Desert plant communities (Webb et al. 2003, Hereford et al. 2006). Short-lived perennial shrubs and perennial grasses demonstrate the highest rates of mortality (Webb et al. 2003, Bowers 2005, Hereford et al. 2006, Miriti et al. 2007), and annual species remain dormant in the soil seedbank (Beatley 1969, 1974, 1976). Long-lived shrubs are more likely to exhibit branch-pruning, and or limited recruitment during drought (e.g. Hereford et al. 2006, Miriti et al. 2007), leading to reduced cover and biomass in drought-afflicted communities. Because this ecological site already has sparse cover, further loss of cover due drought-induced mortality increases the susceptibility of this site to increased damages from erosion.

Non-native annual grasses (red brome [Bromus rubens], cheatgrass [Bromus tectorum] and Mediterranean grass [Schismus species]) have become naturalized throughout the Mojave Desert over the past century (Rickard and Beatley 1965, D'Antonio and Vitousek 1992, Brooks 1999, Reid et al. 2006, Norton et al. 2007). Although non-native annuals are present in this ecological site, the site is relatively resistant to invasion. The active erosion that characterizes this site limits seedling establishment annual species (Guerrero-Campo and Montserrat-Marti 2000), and shallow soils and a hot climate reduce available soil moisture, which limits biomass of annuals.

The inability of this site to support a high biomass of annuals, and overall sparse vegetation cover, makes this site resistant to fire, which has increased in other Mojave Desert plant communities due to the continuous fine fuel layer created by non-native annuals in wet years (D'Antonio and Vitousek 1992, Brooks et al. 2004). In the very unlikely instance of ignition in this ecological site, fire is likely to be small and of low intensity so the effects are insignificant at the landscape scale.

Water erosion is the dominant process modifying and maintaining exposed pediment surfaces (Edinger-Marshall and Lund 1999, Dohrenwend and Parsons 2009), and is an important process modifying and maintaining the vegetation community. Without additional disturbance, the affects of erosion are within the natural range of variability of the reference plant community. However, with additional disturbance (anthropogenic or natural), the effects of erosion may be more severe leading to vegetation and soil loss and potentially a new community phase or state. For example, shrub cover protects the gravelly soils of this ecological site from erosion (Edinger-Marshall and Lund 1999). If shrub cover is reduced due to drought, erosion will remove more soil, which reduces the availability of safe sites for plant establishment, which further reduces shrub cover.