Ecological dynamics

Abiotic Factors

Extreme temperatures and aridity, coarse soils with high rock content by volume and a heterogeneously distributed surface rock fragment cover that increases run-on, drives the vegetation community of this ecological site. This site occurs on stable, broad fan remnants and fan aprons in an extremely arid portion of the Lower Colorado Desert. Mean annual precipitation is 2 to 4 inches, mean maximum temperatures are over 90 F for five months of the year, and strong winds are typical during the summer months. On landforms where additional water is not available, this extremely water-limiting environment can support only a sparse, low production plant community.

In arid regions, coarse textured soils have greater water availability because water quickly infiltrates the soil to depths where it is not lost to evaporation (Noy-Meir 1973, Hamerlynk et al. 2002). Thus, greater amounts of water are held for a longer duration in deep, coarse soils where the development of restrictive soil horizons (such as argillic or petrocalcic horizons) has not occurred (Hamerlynk et al. 2002, Hamerlynk and McAuliffe 2008). However, little water is available at shallow depths, which favors deep-rooted species, or species that can retain large amounts of water when it is available.

The distribution of rock fragments on the soil surface and within the soil horizon has a significant effect on the distribution and availability of soil water (Poesen and Lavee 1994, Cerda 2001, Martre et al. 2002, Li et al. 2007). Soils with greater rock content by volume have less water availability, which limits productivity relative to sandier soils (Martre et al. 2002). Small surface rock fragments can increase infiltration rates (Poesen and Lavee 1994, Cerda 2001), while large surface rock fragments increase run-on. Thus, surfaces where gravel and large fragments are heterogeneously distributed, such as this ecological site, have localized patches of relatively high water availability (i.e. patches of gravel adjacent to patches of large surface fragments). Further, large rock fragments on the soil surface and within soil horizons locally reduce evaporation rates, increasing the temporal availability of water (Martre et al. 2002, Li et al. 2007, Hamerlynk and McAuliffe 2008), and buffering soil temperatures (Cerda 2001). These provide microsites for plant establishment.

The reference plant community of this ecological site is dominated by creosote bush, and Schott’s dalea is a characteristic secondary species. This site has relatively high cactus diversity, and annual species comprise 37 to 52 percent of production during years of average to above average precipitation.

Creosote bush is a long-lived, deep-rooted evergreen shrub dominant across vast areas of the North American warm deserts. Creosote bush maintains its evergreen status by using water held in deep soil layers. Creosote bush is typically the dominant shrub on deep coarse soils with little development (Hamerlynk et al. 2002, Hamerlynk and McAuliffe 2008), such as on this ecological site. Schott's dalea is a deep-rooted shrub that occurs from the southern Mojave Desert, the Colorado Desert and into the Sonoran Desert in northern Mexico. It occurs on hyperthermic soils in habitats receiving additional run-on such as rocky hill-slopes, washes, and alluvial fans with regular sheet-flow. It is an indicator of gravelly soils in the Colorado Desert region (Marks 1950). Data is lacking on the ecology of this species, but it is suggested that it requires water for dispersal and seed germination (Vogl and McHargue 1966); although this ecological site does not receive regular flooding, run-on from large rock fragments provides enough to support Schott’s dalea. Cacti are adapted to the most arid of environments with their ability to store large amounts of water with stem succulence; plasticity to expand and contract with water gain or loss; rapid root growth in response to even small amounts of moisture to capture water when it is available; spines, which are modified leaves that shade the stem and do not lose water like typical leaves; and Crassulacean acid metabolism (CAM) whereby photosynthesis occurs at night so that stomata are open during the cool of the night and water losses are minimized. Cactus can store so much water in their succulent stems that they can withstand water losses of 70 to 90 percent (Ingram 2008).

Disturbance dynamics

The primary disturbances influencing this ecological site are drought and invasion by non-native annual plants.

Desert regions are characterized by low mean annual precipitation and extreme variability in the amount of precipitation received in any year or decade (Hereford et al. 2006). Thus, episodic mortality in response to periods of drought is important in shaping desert community dynamics (Hereford et al. 2006, Miriti et al. 2007). This ecological site is already shaped by extremely arid conditions; nevertheless, extreme drought can still have an impact. Drought can cause reductions in cover and production due to mortality of short-lived species, branch-pruning and mortality of longer-lived species, lack of recruitment, and lack of emergence of annual species (e.g. Webb et al. 2003, Bowers 2005, Hereford et al. 2006, Miriti et al. 2007). Creosote bush exhibits branch-pruning, but low mortality in response to drought in the Mojave Desert (Webb et al. 2003, Hereford et al. 2006, Miriti et al. 2007). In the Sonoran desert, mortality of creosote bush due to severe drought may be more pronounced, but still less than 5% (Bowers 2005).

Non-native annual species such as red brome (Bromus rubens), Mediterranean grass (Schismus barbatus), redstem stork’s bill (Erodium cicutarium) and Asian mustard (Brassica tournefortii) have become naturalized in the Colorado Desert over the past century (Brown and Minnich 1986, D'Antonio and Vitousek 1992, Brooks and Berry 2006). In lower elevations, where soil temperature regimes are hyperthermic and soil moisture is more limiting, Mediterranean grass is the dominant non-native grass (Brooks and Berry 2006). Like native annuals, nonnative annual cover and production is directly related to winter precipitation (Beatley 1969, Brooks and Berry 2006, Barrows et al. 2009). When undisturbed, the high surface rock fragment cover and low water availability at shallow depths of this ecological site restrict establishment and biomass of non-native species, but when the soil surface is disturbed, Mediterranean grass may become more abundant.

Soil disturbance, such as from construction or off-road vehicle use not only increases the susceptibility of this site to invasion by non-native species, but increases establishment opportunities for native species as well. This can lead to higher diversity, productivity and cover in this ecological site.