Chihuahuan Desert Cactus Scrub

This ecological system is found in the Chihuahuan Desert on colluvial slopes, upper bajadas, sideslopes, ridges, canyons, hills and mesas. Sites are hot and dry, typically with southerly aspects. Gravel and rock are often abundant on the ground surface. In Texas, this system is typically associated limestone. The vegetation is characterized by the relatively high cover of succulent species such as Agave lechuguilla, Dasylirion leiophyllum, Dasylirion texanum, Euphorbia antisyphilitica, Fouquieria splendens, Ferocactus spp., Opuntia engelmannii, Cylindropuntia imbricata, Cylindropuntia spinosior, Yucca baccata, Yucca torreyi, and many others. Perennial grass cover is generally low. The abundance of succulents is diagnostic of this desert scrub system, but desert shrubs are usually present. In Texas, shrub species such as Larrea tridentata, Parthenium incanum, Viguiera stenoloba, and Forestiera angustifolia may be present. Herbaceous cover is low with grasses such as Bouteloua eriopoda, Bouteloua ramosa, and Bouteloua curtipendula sometimes present. Fern and fern allies such as Astrolepis spp., Cheilanthes spp., and Selaginella lepidophylla are often common. Stands in rolling topography may form a mosaic with more mesic desert scrub or desert grassland ecological systems that would occur on less xeric northerly slopes. Agave lechuguilla is more abundant in stands in the southern part of the mapzone. This system does not include loamy plains desert grasslands or shrub-steppe with a strong cacti component such as cholla grasslands.

The vegetation is characterized by the relatively high cover of succulent species such as Agave lechuguilla, Dasylirion leiophyllum, Dasylirion texanum, Euphorbia antisyphilitica, Fouquieria splendens, Ferocactus spp., Opuntia engelmannii, Cylindropuntia imbricata (= Opuntia imbricata), Cylindropuntia spinosior (= Opuntia spinosior), Yucca baccata, Yucca torreyi, and many others. Perennial grass cover is generally low. The abundance of succulents is diagnostic of this desert scrub system, but desert shrubs are usually present. In Texas, shrub species such as Larrea tridentata, Parthenium incanum, Viguiera stenoloba, and Forestiera angustifolia may be present. Herbaceous cover is low with grasses such as Bouteloua eriopoda, Bouteloua ramosa, and Bouteloua curtipendula sometimes present. Fern and fern allies such as Astrolepis spp., Cheilanthes spp., and Selaginella lepidophylla are often common. Stands in rolling topography may form a mosaic with more mesic desert scrub or desert grassland ecological systems that would occur on less xeric northerly slopes. Agave lechuguilla is more abundant in stands in the southern part of the mapzone.

Occurrences are found on a variety of hot, dry sites, typically rocky or gravelly slopes with southerly aspects. Gravel and rock are often abundant on the ground surface. In Texas, it is typically associated with limestones, but can also be found on calcareous gravels, igneous and sandstone substrates on rocky or gravelly slopes associated with Igneous Hill and Mountain, Limestone Hill and Mountain, Sandstone Hill, Limestone Hill, Gravelly, and similar ecoclasses.

[from M086] In the U.S., much of this desert scrubland is thought to be a result of recent expansion of Larrea tridentata and Prosopis glandulosa into former desert grasslands and steppe in the last 150 years as a result of a combination of drought, overgrazing by livestock, wind and water erosion, and/or decreases in fire over the last 70-250 years from fire suppression and fine-fuel removal by livestock, and changes in the seasonal distribution of precipitation (Buffington and Herbel 1965, Herbel et al. 1972, Humphrey 1974, Ahlstrand 1979, McLaughlin and Bowers 1982, Gibbens et al. 1983, Hennessy et al. 1983, Donart 1984, Brown and Archer 1987, Schlesinger et al. 1990, Dick-Peddie 1993, McPherson 1995, Gibbens et al. 2005). Seed dispersion by livestock is an additional factor in the increase of Prosopis glandulosa (Brown and Archer 1987). It is believed that Prosopis glandulosa stands formerly occurred in relatively minor amounts and were largely confined to drainages until cattle distributed seed upland from the bosques into desert grasslands (Brown and Archer 1987, 1989). This macrogroup also includes invasive Flourensia cernua shrublands that occur in former (degraded) tobosa (Pleuraphis mutica) flats and loamy plains. Presence of Scleropogon brevifolius is common in these invasive stands. Dick-Peddie (1993) suggested that absence of Flourensia cernua as codominant and presence of Dasyochloa pulchella, Acourtia nana, and Yucca elata may be indicators of recent conversion of desert grasslands into desertscrub, but more research is needed. Conversely, sparse understory Larrea tridentata shrublands on remnant early Holocene erosional surfaces often with shallow calcareous soils and desert pavement may indicate pre-historic distributions of Larrea tridentata desertscrub in the Chihuahuan Desert (Stein and Ludwig 1979, Muldavin et al. 2000b).

Historical natural-ignition fires were relatively small, probably 10-15 acres in size. Repeated fire is thought to help maintain a general mosaic pattern between open grassland and shrub-dominated areas (Johnston 1963). Wright et al. (1976) found that Prosopis glandulosa is very fire-tolerant when only 3 years old. Most plants resprout after being top-killed by fire. Thus, prior to livestock grazing reducing fire frequency, repeated grassland fires probably maintained lower stature of shrubs and prevented new establishment by killing seedlings.

Drought is a relatively common occurrence in this desertscrub, generally occurring every 10-15 years and lasting 2-3 years with occasional long-term drought periods (10-15 years duration). Prosopis spp. and other shrubs have extensive root systems that allow them to exploit deep-soil water that is unavailable to shallower rooted grasses and cacti (Burgess 1995). This strategy works well, especially during drought. However, on sites that have well-developed argillic or calcic soil horizons that limit infiltration and storage of winter moisture in the deeper soil layers, Prosopis spp. invasion can be limited to a few, small individuals (McAuliffe 1995). This has implications in plant geography and desert grassland restoration work in the southwestern United States.

On sandsheet and dune sites, Prosopis glandulosa is more common on warmer, drier sites on sandsheets with subsoils composed of clays or carbonate substrates, whereas Artemisia filifolia is more common on relatively cooler/moisture sites with coarse, deep sand (S. Yanoff pers. comm. 2007). These sites are also more susceptible to grazing pressure.

This Chihuahuan Desert ecological system occurs on colluvial slopes, upper bajadas, sideslopes and mesas. It extends east to the Devils River in Texas.