Invalid Unit Specified
D039 Acacia greggii - Cylindropuntia leptocaulis - Muhlenbergia porteri North American Warm Desert Scrub & Grassland Division

The U.S. National
Vegetation Classification
Type Concept Sentence: This division contains aridland shrublands and grasslands dominated by xerophytic woody shrubs, succulents and grasses that occur among the lowland intermountain basins and foothills of desert mountain ranges across the southwestern U.S. and northern Mexico.
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Common (Translated Scientific) Name: Catclaw Acacia - Christmas Cholla - Bush Muhly North American Warm Desert Scrub & Grassland Division
Colloquial Name: North American Warm Desert Scrub & Grassland
Hierarchy Level: Division
Type Concept: This division is characterized by the dominance of xerophytic shrubs and grasses (and occasionally trees) that tolerate warm-temperate to subtropical arid conditions of the deserts of the southwestern U.S. and northern Mexico (Viscaino-Baja California and Mojave eastward to the Sonoran, Chihuahuan, and the Tamaulipan mattoral/mezquital of northeastern Mexico). Shrublands have sparse to moderate canopies dominated by drought-tolerant micro-microphyllous or broad-leaved species. Strong diagnostic shrubs include Fouquieria splendens, Larrea tridentata, Acacia greggii, Prosopis glandulosa, and Prosopis velutina. Moderately-diagnostic regional dominants include Flourensia cernua, Jatropha cuneata, Leucophyllum frutescens, and Viguiera stenoloba along with dwarf-shrubs such as Ambrosia dumosa, Eriogonum fasciculatum, and Parthenium incanum. Cacti and rosette stem succulents, and sarcocaulescent trees are also common. Cylindropuntia leptocaulis is a strong diagnostic species; others are characteristic of specific desert regions, e.g., Agave lechuguilla, Bergerocactus emoryi, Bursera microphylla, Carnegiea gigantea, Cylindropuntia bigelovii, Cylindropuntia prolifera, and Yucca treculeana. Desert grasslands are dominated by drought-tolerant, warm-season (C4) bunchgrasses. Moderately strong diagnostic grasses include Bouteloua breviseta, Bouteloua eriopoda, Muhlenbergia porteri, Muhlenbergia setifolia, Pleuraphis mutica, Pleuraphis rigida, Sporobolus flexuosus, and Tridens muticus. Other, often abundant grasses include Bouteloua curtipendula, Bouteloua gracilis, Bouteloua hirsuta, Sporobolus airoides, Sporobolus wrightii, and Panicum obtusum. While xerophytic shrubs can be conspicuous elements of desert grasslands, they typically are not dominants, e.g., Dasylirion leiophyllum, Ephedra torreyana, Nolina microcarpa, Nolina texana, and Yucca elata. Perennial forbs are diverse, but typically low in cover; annual forbs can be locally to regionally abundant in any given year depending on rainfall amounts and timing, or essentially absent (e.g., Eschscholzia californica). There are also grasslands that have been invaded by non-native, often aggressive species such as Eragrostis curvula, Eragrostis lehmanniana, Pennisetum ciliare, and Pennisetum setaceum. Intermixed among the grasslands and shrublands are ephemeral dry washes dominated by shrubs tolerant of high episodic stream flows; characteristic species include Chilopsis linearis, Fallugia paradoxa, Hymenoclea monogyra, and Hymenoclea salsola.

This is a division of warm-temperate to subtropical arid conditions with peak summer temperatures that can exceed 50°C and mean annual precipitation ranges from 30 to 250 (300) mm. In the higher elevation or more continental regions (Mojave and Chihuahuan), temperatures can drop below freezing for extended periods in winter. Desert grasslands are more prevalent in the eastern, summer rainfall-dominated regions (Chihuahuan and Tamaulipan) that favor the more shallow-rooted grasses during the growing season. In contrast, shrublands tend to dominate the Sonoran, Mojave, and Viscaino-Baja California deserts where the predominantly winter-rainfall regime favors shrubs that are able to access deeper stored soil moisture during the growing season. Sites extend from sea level (or below) to about 1600 m for shrublands and 2000 m for desert grasslands. The vegetation types are sorted from low-lying, broad basin bottoms with fine clay alluvial soils (including alkaline ephemeral playa lakes) up adjacent coalesced alluvial fan piedmonts (bajadas) with shallow gravelly soils and desert pavements to the colluvial foothill slopes of bordering desert mountain ranges with their cobbly and rocky soils (including sparsely vegetated cliff faces and boulder slopes). The desert basin can also have extensive sandy plains (sand sheets) and dunelands (including unique gypsum dune communities). Fire plays a role in desert grasslands (return intervals between 10 and 30 years) but has minimal impact on desert scrubs. Excessive burning of desert grasslands can also favor shrubs.
Diagnostic Characteristics: Aridland shrublands and grasslands are dominated by a combination of xerophytic woody shrubs and grasses, while succulents and grasses occur from basin bottoms to desert mountain foothills. Shrubs include tall and dwarf multi-stemmed woody shrubs that are microphyllous or broad-leaved, evergreen or drought-deciduous species. Acacia greggii, Fouquieria splendens, Larrea tridentata, Prosopis glandulosa, and Prosopis velutina are strong diagnostic species with Ambrosia dumosa, Eriogonum fasciculatum, Flourensia cernua, and Leucophyllum frutescens as regionally important dominants. In addition, there are cacti and rosette stem succulents, and, on occasion, sarcocaulescent trees and shrubs. Cylindropuntia leptocaulis and Cylindropuntia bigelovii are strong and moderately diagnostic species, respectively; others are regionally diagnostic: Encelia farinosa (Mojave), Bursera microphylla (Baja California), Carnegiea gigantea (Sonoran), Agave lechuguilla (Chihuahuan), and Yucca treculeana (Tamaulipan). The division also includes communities of ephemeral desert washes with Chilopsis linearis, Fallugia paradoxa, and Hymenoclea monogyra as the strong diagnostic dominants, along with ruderal communities dominated by invasive grasses such as Eragrostis curvula, Eragrostis lehmanniana, Pennisetum ciliare, and Pennisetum setaceum.
Rationale for Nominal Species or Physiognomic Features: Acacia greggii and Cylindropuntia leptocaulis range across the entire division distribution and are near-endemics that are representative of the woody and succulent shrub components, respectively (although Cylindropuntia leptocaulis is never a dominant). Muhlenbergia porteri is also a near-endemic that ranges across the division except in the Tamaulipan zone. It represents the grass component, but it too is seldom a dominant. Larrea tridentata may also be a good candidate species to include in the name.
Classification Comments: Species linkages to the Tamaulipan component remain weak, and this element may need to be considered under a subtropical division in the future (e.g., 3.A.1 Tropical Thorn Woodland Formation (F039)). Brown (1982a) also considers the Sonoran subtropical, but the clear linkages to the cooler Mojave and Chihuahuan deserts preclude removing it from this division.
Similar NVC Types:
D061 Western North American Interior Chaparral, note: intersperses with D039 at higher elevations.
D040 Western North American Cool Semi-Desert Scrub & Grassland, note: "shares many species with D039, particularly at the southern edge of it distribution."
D010 Western North American Pinyon - Juniper Woodland & Scrub, note: intersperses with D039 at higher elevations.
D022 Western North American Grassland & Shrubland, note: intersperses with D039 along its northeastern edge.
D327 Californian Scrub & Grassland, note:
Physiognomy and Structure: This division is characterized by the dominance of xerophytic shrubs and grasses (and occasionally trees). Shrub growth forms are diverse and include tall and dwarf multi-stemmed, woody shrubs that are microphyllous or broad-leaved which can be evergreen or drought-deciduous (and some cases, cold-deciduous), and cacti and rosette stem succulents (and on occasion, sarcocaulescent trees). Thorns and spines are common, lending the term thorn scrub. Shrub-dominated desert communities are typically low in production and may form very open (10% cover) to moderately (50-66%) closed canopies, and may or may not have a significant herbaceous layer of grasses and forbs. Desert grasslands, in contrast, are dominated by drought-tolerant, often robust warm-season (C4) bunchgrasses that can range from 10% to nearly 100% in cover.
Floristics: Shrub growth forms are diverse. There are tall and dwarf multi-stemmed woody shrubs that are microphyllous or broad-leaved and they can be evergreen or drought-deciduous (and some cases, cold-deciduous). Among these, Acacia greggii (= Senegalia greggii), Fouquieria splendens, Larrea tridentata, Prosopis glandulosa, and Prosopis velutina are strong diagnostic species with Ambrosia dumosa, Eriogonum fasciculatum, Flourensia cernua, Jatropha cuneata, Leucophyllum frutescens, and Viguiera stenoloba as regionally important dominants. In addition, there are cacti and rosette stem succulents, and on occasion, sarcocaulescent trees. Among these, Cylindropuntia leptocaulis and Cylindropuntia bigelovii are strong and moderately diagnostic species, respectively. Others, while restricted to the division, are common and often conspicuous elements of specific desert regions, e.g., Bursera microphylla (Baja California), Carnegiea gigantea (Sonoran), Agave lechuguilla (Chihuahuan), and Yucca treculeana (Tamaulipan). Shrub-dominated desert communities are typically low in production and may form very open canopies (10% cover) to moderately closed (50-66%) ones at best, and with or without a significant herbaceous layer of grasses and forbs.

Desert grasslands, in contrast, are dominated by drought-tolerant, warm-season (C4) bunchgrasses and can be more productive with grass cover that can range from 10% to nearly 100%. Among desert grasslands of the division, moderately strong diagnostics include Bouteloua eriopoda, Muhlenbergia porteri, Pleuraphis mutica, Pleuraphis rigida, and Sporobolus flexuosus. Xerophytic shrubs can be conspicuous and moderately diagnostic elements of the grasslands, but not the dominants, e.g., Dasylirion leiophylla, Ephedra torreyana, Nolina texana, and Yucca elata. Perennial forbs are diverse, but typically low in cover; annual forbs can be locally to regionally abundant in any given year depending on rainfall amounts and timing, or essentially absent. Desert grasslands are more prevalent in the eastern, summer rainfall-dominated regions (Chihuahuan and Tamaulipan) that favor the more shallow-rooted grasses during the growing season. In contrast, shrublands tend to dominate the Sonoran, Mojave, and Viscaino-Baja California deserts where the predominantly winter-rainfall regime favors shrubs that can access deeper stored soil moisture from the winter during the growing season.

In desert washes, strong diagnostic species include Chilopsis linearis, Fallugia paradoxa, and Hymenoclea monogyra. In addition, Baccharis sarothroides, Brickellia laciniata, Juglans microcarpa, Prosopis velutina, Prosopis glandulosa, Rhus microphylla, Olneya tesota, and Parkinsonia florida are moderately diagnostic regional species (the latter two are small trees). Lastly, where sites have been heavily disturbed or near the epicenter of the introduction of aggressive non-native noxious weeds, de-novo ruderal (weedy) communities dominated by drought-tolerant species can form. Of particular concern is the invasion of desert grasslands by perennial graminoids such as Eragrostis curvula, Eragrostis lehmanniana, Pennisetum ciliare, and Pennisetum setaceum. Annuals such as Brassica tournefortii, Bromus rubens, Schismus arabicus, and Schismus barbatus can also be problematic, particularly in winter-rainfall regions where they germinate early and can alter fire regimes later in the summer dry season.
Dynamics: Fire plays a role in desert grasslands (return intervals between 10 and 30 years) but has minimal impact on desert scrubs. Excessive burning of desert grasslands can also favor shrubs. However, in recent years exotic perennial and annual grasses have introduced a fire regime into the desert scrub which lacked a fire regime prior to this. These exotic grasses carry fires which burn the non-fire-adapted shrubs and small trees.
Environmental Description: Climate: This is a division of warm-temperate to subtropical arid conditions with peak summer temperatures that can exceed 50°C and mean annual precipitation ranges from 30 to 250 (300) mm. In the higher elevation or more continental regions (Mojave and Chihuahuan), temperatures can drop below freezing for extended periods in winter. Desert grasslands are more prevalent in the eastern, summer rainfall-dominated regions (Chihuahuan and Tamaulipan) that favor the more shallow-rooted grasses during the growing season. In contrast, shrublands tend to dominate the Sonoran, Mojave, and Viscaino-Baja California deserts where the predominantly winter-rainfall regime favors shrubs that are able access deeper stored soil moisture during the growing season. Sites extend from sea level (or below) to about 1600 m for shrublands and 2000 m for desert grasslands.

Soils/substrate: From a landscape perspective, a basin-and-range physiography forms the primary physical template for the expression of communities of the division. That is, vegetation communities are assorted from low-lying, broad basin bottoms with fine clay alluvial soils (including alkaline ephemeral playa lakes) up adjacent coalesced alluvial fan piedmonts (bajadas) with shallow gravelly soils and desert pavements to the colluvial foothill slopes of bordering desert mountain ranges with their cobbly and rocky soils (including sparsely vegetated cliff faces and boulder slopes). The desert basin can also have extensive sandy plains (sand sheets) and dunelands (including unique gypsum dune communities). The mountain ranges are composed of either fault-block uplifted sedimentary rocks (limestone and sandstone) with underlying basement granitic rocks that are sometimes exposed, or extrusive volcanics such as rhyolite. Associated with the volcanic regions are extrusive, sometimes large basaltic lava flows in the basins that also support vegetation different from those in the surrounding landscapes. Flowing through these landscape elements are ephemeral dry washes (arroyos) that support unique desert vegetation that is able to tolerate high episodic stream flows driven primarily by summer thunderstorms.
Geographic Range: This division extends from warmer deserts of the southwestern U.S. and northern Mexico from the Pacific to Atlantic oceans (Viscaino-Baja California up to the Mojave and eastward through the Sonoran and Chihuahuan deserts to the Tamaulipan mattoral/mezquital).
Nations: MX, US
States/Provinces: AZ, CA, MXBC, MXBS, MXCH, MXCO, MXDU, MXNU, MXSO, MXTM, NM, NV, TX, UT
US Forest Service Ecoregions (2007)
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Plot Analysis Summary:
Confidence Level: High
Confidence Level Comments:
Grank: GNR
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Concept Lineage:
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Obsolete Names:
Acacia greggii - Opuntia leptocaulis - Muhlenbergia porteri North American Warm Desert Scrub & Grassland Division
Obsolete Parents:
Synonomy: < Matorral Xerofilo (Rzedowski 1978)
> Tropical-Subtropical Desertlands (Brown 1982a)
> Tropical-Subtropical Desertlands (Brown et al. 1998)
> Warm Deserts (MacMahon 1988)
> Warm Temperate Desertlands (Brown 1982a)
> Warm Temperate Desertlands (Brown et al. 1998)
Concept Author(s): J.A. MacMahon (1988)
Author of Description: E. Muldavin
Acknowledgements: Todd Keeler-Wolf, Marion Reid
Version Date: 28Oct2015
References:
  • Brown, D. E., editor. 1982a. Biotic communities of the American Southwest-United States and Mexico. Desert Plants Special Issue 4(1-4):1-342.
  • Brown, D. E., F. Reichenbacher, and S. E. Franson. 1998. A classification of North American biotic communities. The University of Utah Press, Salt Lake City. 141 pp.
  • Faber-Langendoen, D., J. Drake, S. Gawler, M. Hall, C. Josse, G. Kittel, S. Menard, C. Nordman, M. Pyne, M. Reid, L. Sneddon, K. Schulz, J. Teague, M. Russo, K. Snow, and P. Comer, editors. 2010-2019a. Divisions, Macrogroups and Groups for the Revised U.S. National Vegetation Classification. NatureServe, Arlington, VA. plus appendices. [in preparation]
  • MacMahon, J. A. 1988. Warm deserts. Pages 232-264 in: M. G. Barbour and W. D. Billings, editors. North American terrestrial vegetation. Cambridge University Press, New York.
  • Rzedowski, J. 1978. Vegetación de México. Editorial Limusa, México D. F. 432 pp.