Mojave Desert Mixed Scrub

This ecological system is an extensive desert scrub dominated by Yucca brevifolia and/or Coleogyne ramosissima. It is found in the transition zone between Larrea tridentata - Ambrosia dumosa desert scrub and lower montane woodlands (700-1800 m elevations) that occur in the eastern and central Mojave Desert, and in southern Great Basin. The vegetation in this ecological system is quite variable. Major communities include Yucca brevifolia and Coleogyne ramosissima scrub. Dominant and diagnostic species include Coleogyne ramosissima, Ericameria parryi, Ericameria teretifolia, Eriogonum fasciculatum, Ephedra nevadensis, Grayia spinosa, Lycium spp., Menodora spinescens, Nolina spp., Cylindropuntia acanthocarpa, Salazaria mexicana, Viguiera parishii, Yucca brevifolia, or Yucca schidigera. Less common are stands with scattered Joshua trees and a saltbush short-shrub layer dominated by Atriplex canescens, Atriplex confertifolia, or Atriplex polycarpa, or occasionally Hymenoclea salsola. In some areas in the western Mojave, Juniperus californica is common with the yuccas. Desert grasses, including Achnatherum hymenoides, Achnatherum speciosum, Muhlenbergia porteri, Pleuraphis jamesii, Pleuraphis rigida, or Poa secunda, may form an herbaceous layer. Scattered Juniperus osteosperma or desert scrub species may also be present.

The vegetation in this ecological system is quite variable. Major alliances include Yucca brevifolia and Coleogyne ramosissima scrub. Dominant and diagnostic species include Coleogyne ramosissima, Ephedra nevadensis, Ericameria parryi, Ericameria teretifolia, Eriogonum fasciculatum, Grayia spinosa, Krameria spp., Lycium spp., Nolina spp., Cylindropuntia acanthocarpa (= Opuntia acanthocarpa), Peucephyllum schottii, Salazaria mexicana, Viguiera parishii, Yucca brevifolia, or Yucca schidigera (Sawyer et al. 2009). Less common are stands with scattered (Yucca brevifolia and a saltbush short-shrub layer dominated by Atriplex canescens, Atriplex confertifolia, Atriplex polycarpa, or occasionally Hymenoclea salsola. In some areas in the western Mojave, Juniperus californica is common with Yucca brevifolia. Desert grasses, including Achnatherum hymenoides, Achnatherum speciosum, Muhlenbergia porteri, Pleuraphis jamesii, Pleuraphis rigida, or Poa secunda, may form an herbaceous layer. Scattered Juniperus osteosperma or desert scrub species may also be present. Stands dominated by Ericameria parryi, Eriogonum fasciculatum, Nolina bigelovii, Nolina parryi, Lycium andersonii, Menodora spinescens, or Viguiera parishii occur on rocky ridges, outcrops, and dry washes and may be too sparse to burn except under extreme conditions (Sawyer et al. 2009). The vegetation description is based on several references, including Beatley (1976), Brown (1982), Turner (1982), MacMahon (1988), Holland and Keil (1995), Reid et al. (1999), Ostler et al. (2000), Anderson (2001c), Gucker (2006a, 2006b), Barbour et al. (2007), Keeler-Wolf (2007), Sawyer et al. (2009), and NatureServe Explorer (2011).

This ecological system is found in the Mojave Desert and in the transition zone into the southern Great Basin. It represents the extensive mid-elevation desert scrub in the transition zone above the lower elevation creosotebush desert scrub and generally below the foothill and lower montane woodlands (700-1850 m elevations (Sawyer et al. 2009). Adjacent ecological systems include Great Basin Pinyon-Juniper Woodland (CES304.773) and Inter-Mountain Basins Big Sagebrush Shrubland (CES304.777) above and Sonora-Mojave Creosotebush-White Bursage Desert Scrub (CES302.756) below.

Climate: Climate is semi-arid with hot summers and cool winters. Annual precipitation is low, averaging between 4 and 25 cm. However, year-to-year precipitation variability can be quite large with drought common and rare wet years producing a bloom of desert annuals.

Physiography/landform: Stands occur on upper bajada and lower piedmont slopes with smaller patches occurring on rocky ridges and outcrops. Slopes are gentle to moderate. Aspect is variable with higher elevation stands found on warmer south- to west-facing slopes.

Soil/substrate/hydrology: Substrates are a mixture of alluvium and colluvium and are variable, ranging from silt to loam to coarse sand, but often shallow, well-drained, sandy and rocky. Many stands occur on alkaline, calcareous substrates and often have biological crusts and a shallow caliche layer (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), Reid et al. (1999), Ostler et al. (2000), Anderson (2001c), Gucker (2006a, 2006b), Barbour et al. (2007a), Keeler-Wolf (2007), and Sawyer et al. (2009).

This system occurs on extremely xeric sites and is well-adapted to prolonged drought and heat stress. Growth slows or stops in winter due to cold and is inhibited at other times by heat. Winter rains are sometimes sufficient to allow ephemeral herbs to flower in the spring. Late summer thunderstorms also contribute moisture.

Disturbance dynamics in this system are variable because of variation in structure and composition, being dominated by open- to closed-canopy scrub to desert grasslands dominated by Pleuraphis rigida (<1400 m elevation) and Pleuraphis jamesii (>1400 m elevation) sometimes with a Yucca brevifolia overstory (Sawyer et al. 2009). Except for the relatively few stands with an herbaceous layer, fire-return intervals (FRI) also tend to be long because the open stands only burn under extreme conditions. Older Yucca brevifolia individuals can tolerate low-severity fires due to fire-resistant bark, and both Yucca brevifolia and Yucca schidigera can sprout if burned (Gucker 2006a, b).

LANDFIRE developed a VDDT model for this system which has two classes (LANDFIRE 2007a, BpS 1410820):
A) Early Development 1 Open (25% of type in this stage): Shrub cover is 0-50%. Historically, fire was relatively uncommon in this vegetation. The average FRI for replacement fire was 400 years. When burned, the fire-tolerant/crown-sprouting shrubs such as spiny menodora, horsebrush and snakeweed will dominate the site. At higher elevations of mesic blackbrush, a big sagebrush-desert bitterbrush community typically replaces blackbrush for a protracted period. This class can express itself for over a hundred years with varying amounts of blackbrush gradually establishing after decades and eventually succeeding to class B. A few examples of this that have been observed in the field are believed to be over 60+ years. The ground cover varies by elevation and moisture regime with mesic sites being generally 10-35% with some sites only capable of 10% cover. The thermic sites are generally 10-15% ground cover with exception going as high as 35%.

B) Late Development 2 Closed (shrub-dominated - 30% of type in this stage): This community class seems to be stable and occurs after a threshold is crossed. Composition is 50-70% blackbrush-dominated. Other species are perennial grasses of desert needlegrass, Indian ricegrass, galleta grass, fluff grass, and threeawn. Lesser shrub composition includes Nevada ephedra, turbinella oak, desert bitterbrush, fourwing saltbush, and Anderson's wolfberry in mesic sites and Nevada ephedra, creosotebush, Mojave buckwheat, snakeweed, prickly pear, white bursage, and spiny menodora in thermic sites. There are other shrubs also. The FRI for replacement fire is 400 years, which causes a rare transition to class A.

Fire-sensitive shrub species such as the long-lived Coleogyne ramosissima, Menodora spinescens, Nolina bigelovii, or Nolina parryi will convert to early-seral and intermediate shrublands dominated by Hymenoclea salsola, Grayia spinosa, Gutierrezia sarothrae, Ericameria teretifolia, Ephedra nevadensis, Menodora spinescens, Cylindropuntia acanthocarpa, Salazaria mexicana, Tetradymia spp., or Yucca schidigera which have shorter FRIs (Anderson 2001c, Keeler-Wolf 2007, Sawyer et al. 2009). LANDFIRE modelers emphasized that blackbrush is fire-intolerant, may be slow to re-establish following fire, and grasses may dominate immediately following fire. Invasion of non-native annual grasses following fire is likely under current conditions (LANDFIRE 2007a).

Some species such as yucca moths (Tegeticula spp.) and Yucca species have obligate mutualistic relationships (Baker 1986b, Althoff et al. 2006). Yucca sp. are typically dependent on one or sometimes two species of Tegeticula for pollination, which is usually dependent on one to several Yucca host plant species for habitat and food for larvae; for example, Tegeticula mojavella and Tegeticula californica pollinate Yucca schidigera, and Tegeticula antithetica and Tegeticula synthetica pollinate Yucca brevifolia. More study and review are needed to fully understand the many functional roles animals have within this ecosystem.

The primary land uses that alter the natural processes of this system are associated with livestock practices, annual exotic species introduction, fire regime alteration, direct soil surface disturbance, and fragmentation. Excessive grazing stresses the system through soil disturbance (also from ORV use), diminishing or eliminating the biological soil crust, altering the plant species composition by loss of perennial species, and increasing the establishment of native disturbance-increasers and annual grasses, particularly Bromus madritensis and other non-native annual bromes.

Natural fire regimes may have been altered because of grazing by livestock and fire suppression over the last 100 years. This may allow the presence of relatively fire-intolerant species such as Artemisia tridentata, Coleogyne ramosissima, or Larrea tridentata in stands of this system in relatively mesic sites (Keeler-Wolf and Thomas 2000). In sites throughout the range of this system, annual grass invasion has also substantially altered the fire frequency. Fine fuel adjacency from alien annual grasses, such as Bromus madritensis, Bromus tectorum, and Schismus spp., currently represents the most important fuel bed component in desert scrub and can substantially increase the fire frequency. After a year of moderate to high rainfall, the annual vegetation converts into fine fuels that can carry fire through these open scrub stands, killing fire-sensitive species with moderate to long fire-return intervals and converting to exotic annual grasslands (Keeler-Wolf et al. 1998).

Human development has impacted many locations throughout the ecoregion. High- and low-density urban and industrial developments also 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 more 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 is found in the eastern and central Mojave Desert and on lower piedmont slopes in the transition zone into the southern Great Basin.