Mojave Creosote Desert

This ecological system forms a desert scrub matrix blanketing broad valleys, lower bajadas, plains and low hills in the Mojave and lower Sonoran deserts. This desert scrub is characterized by a sparse to moderately dense layer (2-50% cover) of xeromorphic microphyllous and broad-leaved shrubs. Larrea tridentata and Ambrosia dumosa are typically dominants, but many different shrubs, dwarf-shrubs, and cacti may codominate or form typically sparse understories. Associated species may include Atriplex canescens, Atriplex hymenelytra, Encelia farinosa, Ephedra nevadensis, Fouquieria splendens, Lycium andersonii, and Opuntia basilaris. The herbaceous layer is typically sparse but may have abundant seasonal ephemerals. Herbaceous species such as Chamaesyce spp., Eriogonum inflatum, Dasyochloa pulchella, Aristida spp., Cryptantha spp., Nama spp., and Phacelia spp. are common. This system can often appear as very open sparse vegetation, with the mostly barren ground surface being the predominant feature.

This desert scrub system occurs as open to intermittent vegetation cover, with the mostly barren ground surface being the predominant feature (Sawyer et al. 2009). It is characterized by a sparse to moderately dense layer (2-50% cover) of xeromorphic microphyllous and broad-leaved shrubs that is typically dominated or codominated by Larrea tridentata usually with Ambrosia dumosa. However, several other shrubs may dominate or codominate this system, including Atriplex spp., Ephedra viridis, Ephedra spp., Grayia spinosa, or Lycium spp. Low-elevation stands typically have low cover and diversity, whereas in higher-elevation stands, many different shrubs, dwarf-shrubs, and cacti may be present to codominant or form sparse understories. Associated species may include Atriplex canescens, Atriplex hymenelytra, Atriplex polycarpa, Croton californicus, Dalea spp., Echinocactus polycephalus, Encelia spp., Ephedra funerea, Ephedra nevadensis, Lycium andersonii, Opuntia basilaris, Krameria grayi, Krameria erecta, Psorothamnus arborescens, Psorothamnus fremontii, Salazaria mexicana, Senna armata, and Viguiera parishii. Some common disturbance-related species include Acamptopappus sphaerocephalus, Bebbia juncea, Cylindropuntia acanthocarpa (= Opuntia acanthocarpa), Ericameria teretifolia, Grayia spinosa, or Hymenoclea salsola (Sawyer et al. 2009). If Encelia farinosa or Yucca schidigera is present, cover is generally low (< 1-2% cover). Occasional emergent Fouquieria splendens or Yucca brevifolia may be present with low cover. The herbaceous layer is typically sparse and intermittent, but may be seasonally abundant with ephemerals. Herbaceous species, such as Chamaesyce spp., Eriogonum inflatum, Dasyochloa pulchella, Aristida spp., Cryptantha spp., Nama spp., and Phacelia spp., are common. The vegetation description is based on several references, including Beatley (1976), Brown (1982a), Turner (1982), MacMahon (1988), Holland and Keil (1995), Marshall (1995), Reid et al. (1999), Barbour et al. (2007), Keeler-Wolf (2007), Schoenherr and Burk (2007), and Sawyer et al. (2009).

Climate: Climate is semi-arid to arid with hot summers and warm to cool winters depending on latitude and elevation.

Physiography/landform: This ecological system forms the vegetation matrix in broad valleys, lower bajadas, plains, flats and low hills in the lower Sonoran (Colorado) and Mojave deserts extending into the southeastern Great Basin where it forms the vegetation matrix. Other habitats include minor washes and rills, alluvial fans, and upland slopes. Elevation ranges from -75 to 1200 m. Adjacent ecological systems include Mojave Mid-Elevation Mixed Desert Scrub (CES302.742) above and Inter-Mountain Basins Playa (CES304.786) below.

Soil/substrate/hydrology: Substrates are typically well-drained, sandy soils derived from colluvium or alluvium, and are often calcareous with a caliche hardpan and/or a pavement surface that is derived from limestone and dolomite (Turner 1982b, Sawyer et al. 2009).

The environmental description is based on several references, including Beatley (1976), Brown (1982a), Turner (1982b), MacMahon (1988), Holland and Keil (1995), Marshall (1995), Reid et al. (1999), Barbour et al. (2007a), Keeler-Wolf (2007), Schoenherr and Burk (2007), and Sawyer et al. (2009).

This system covers vast areas of sandy and gravelly alluvial fans and bajadas and rocky slopes in the northwestern Sonoran, Mojave and Colorado deserts (Keeler-Wolf 2007, Sawyer et al. 2009). The dominant shrub, Larrea tridentata, is very long-lived, with clones living >10,000 years (Keeler-Wolf 2007) and is very tolerant of drought and high temperatures. It is highly adapted to minimized evapotranspiration both daily and seasonally using stomatal regulation, resinous leaves, and a leaf structure and habit to minimize self-shading and maximize photosynthesis during favorable growing periods (Hamerlynck et al. 2002, Ogle and Reynolds 2002). It may die back during extreme drought but can sprout from the base (Meinzer et al. 1990). It has low recruitment and is slow to re-establish from seed (Keeler-Wolf 2007). Larrea tridentata is poorly adapted to fire because of its highly flammable, resinous leaves that burn hot such that fires usually kill the shrub. If the shrub is not killed, it has limited sprouting ability after low-intensity fires (Humphrey 1974, Brown and Minnich 1986, Marshall 1995, Paysen et al. 2000). McLaughlin and Bowers (1982) reported that burned individuals surviving a fire regained their former size in five years.

The main codominant shrub, Ambrosia dumosa, is short-lived with a relatively shallow root system, and tends to dominate sandy and rocky sites. It can quickly establish after disturbance or drought (Vasek 1980). Post fire, it also has a limited ability to sprout, but can readily re-establish from seed (Sawyer et al. 2009).

Fire-return interval is long for this open-canopied shrub system with typically discontinuous fuels (Sawyer et al. 2009). Fire occurs under extreme conditions often following a wet year when more fine fuels are available. When it burns, fires are usually of high intensity and moderate severity (Sawyer et al. 2009). Fires in historic creosote-bursage stands were thought to be infrequent except along the margins of the ecological system where it mixed with shrub-steppe containing greater grass fuel loading. Although bunchgrass species can fill in some of the interspaces between shrubs and provide fine fuels, their distribution is generally patchy and rarely provides fuel continuity sufficient to carry fire (Brooks et al. 2007). Periodic drought is occasionally sufficient to thin grass and shrub cover.

LANDFIRE developed a VDDT model for this system which has two classes (LANDFIRE 2007a, BpS 1310870):
A) Early Development 1 Open (15% of type in this stage): Dominant cover is herbaceous, 5-10% canopy cover. Creosotebush scrub is characterized by low cover 5-10%. Little disturbance was considered in class A, except for replacement fire every 300 years on average. Historical condition where invasive annual grasses are absent, the fire-return interval is virtually nonexistent except for areas near the base of mountains experiencing locally higher rainfall and fine fuel buildup from native annuals. After 100 years, class A transitions to class B.

B) Late Development 1 Closed (shrub-dominated - 85% of type in this stage): Greater than 15% shrub cover and 20-40% grass and forb cover; associated with more productive soils. Less fine fuel is associated with this community, therefore the FRIs for replacement fire and mixed-severity fire is 650 years (min-max: 300-1000 years). Wind/weather stress also affected this community on average every 80 years, but did not cause a transition to class A.

LANDFIRE modelers emphasized that pre-settlement fire conditions in warm desert plant communities are not known. However, it is thought that fires in creosotebush scrub were absent to rare events in pre-settlement desert habitats, because fine fuels from winter annual plants were probably sparse, only occurring in large amounts during the spring following exceptionally wet winters (LANDFIRE 2007a).

Primary land uses that alter natural processes of this system directly affect vegetation and soil surface with disturbance and fragmentation, and annual non-native species invasion. Excessive stress to the system occurs through soil disturbance from off-road vehicle (ORV) use, and heavy grazing that alters the species composition by reduction of perennial species and increases native disturbance-driven increaser species as well as non-native annual grasses. Fine fuels from non-native annual grasses, such as Bromus madritensis, Bromus tectorum, and Schismus spp., currently represents the most important fuel bed component in creosotebush scrub and can substantially increase the fire frequency. In years of good moisture, non-native annual grasses can comprise 66-97% of the total annual biomass in this system (LANDFIRE 2007a, BpS model 1310870). In contrast to native annuals, non-native annual plants produce fine fuel beds that persist throughout the summer and greatly increase the continuity of fuels for much of the fire season (Brooks et al. 2007). Historic year-round livestock grazing has contributed to the deterioration of this system.

Human development has impacted many locations throughout the distribution of this system. High- and low-density urban and industrial developments have large impacts. For example, residential development has significantly impacted locations within commuting distance to urban areas. Impacts may be direct as vegetation is removed for building sites or indirectly through natural fire regime alteration, and/or the introduction of invasive species. Mining operations can drastically impact natural vegetation. Road building and power transmission lines continue to fragment vegetation and provide vectors for invasive species.

This system occupies broad valleys, lower bajadas, plains and low hills in the Mojave and lower Sonoran deserts.