Chihuahuan Desert Mixed Scrub

This ecological system is the widespread desert scrub that occurs on gravelly mid to upper bajadas, foothills and dissected gravelly alluvial fans in the Chihuahuan Desert and has recently expanded at the expense of desert grasslands in the northern portion of its range. It generally occurs on mid to upper piedmonts above the desert plains Chihuahuan Creosotebush Desert Scrub (CES302.731) and extends up to the chaparral zone. Soils are typically well-drained, non-saline, gravelly loams often with a petrocalic layer. Substrates are frequently derived from limestone although igneous rocks are common in some areas. In Texas, this system is best developed over limestone substrates. Vegetation is characterized by the presence of Larrea tridentata, typically mixed with thornscrub or other desert scrub such as Agave lechuguilla, Aloysia wrightii, Baccharis pteronioides, Dasylirion leiophyllum, Flourensia cernua (not bottomland), Fouquieria splendens, Koeberlinia spinosa, Krameria erecta, Leucophyllum minus, Mimosa aculeaticarpa var. biuncifera, Mortonia scabrella, Opuntia engelmannii, Parthenium incanum, Prosopis glandulosa, and Rhus microphylla (in drainages). Grasses are common but generally have lower cover than shrubs. Common species may include Bouteloua curtipendula, Bouteloua eriopoda, Bouteloua gracilis, Bouteloua hirsuta, Bouteloua ramosa, Dasyochloa pulchella, and Muhlenbergia porteri. Stands of Acacia constricta-, Acacia neovernicosa- or Acacia greggii-dominated thornscrub are included in this system, and limestone substrates appear important for at least these species. If present, Prosopis glandulosa has relatively low cover and does not strongly dominate the shrub layer.

This system also includes upper piedmont stands of desert scrub that are strongly dominated by Larrea tridentata, as wells as Larrea tridentata shrublands with a sparse understory that occur on gravelly piedmont slopes that may extend down gravelly upper basins.

In western Texas, this scrub is best developed over limestone substrates. Acacia constricta, Agave lechuguilla, Condalia ericoides, Dasylirion leiophyllum, Larrea tridentata, Leucophyllum spp., Mimosa aculeaticarpa var. biuncifera, Parthenium incanum, Prosopis glandulosa, Viguiera stenoloba, and Yucca torreyi are often present to dominant, but numerous shrub species may be present. The herbaceous cover is generally low with species such as Aristida purpurea, Bouteloua curtipendula, Bouteloua eriopoda, Bouteloua ramosa, Bouteloua trifida, Dasyochloa pulchella, and Muhlenbergia setifolia. Historically, much of this desert scrub was thought to be a more open steppe, characterized by perennial desert grasses (typically Bouteloua eriopoda) and an open creosotebush - mixed desert shrub layer. Remnant stands of this historic composition of Larrea tridentata desert scrub in the Chihuahuan Desert can be seen on remnant early Holocene erosional surfaces that can often have pebbly desert pavement on the soil surface.

This mid to upper piedmont ecological system is characterized by the presence of Larrea tridentata typically mixed with thornscrub or other desertscrub such as Agave lechuguilla, Aloysia wrightii, Baccharis pteronioides, Dasylirion leiophyllum, Flourensia cernua (not bottomland), Fouquieria splendens, Koeberlinia spinosa, Krameria erecta, Leucophyllum minus, Mimosa aculeaticarpa var. biuncifera, Mortonia scabrella (= Mortonia sempervirens ssp. scabrella), Opuntia engelmannii, Parthenium incanum, Prosopis glandulosa, and Rhus microphylla (in drainages). Stands of Acacia constricta-, Acacia neovernicosa- or Acacia greggii-dominated thornscrub are included in this system, and limestone substrates appear important for at least these species. If present, Prosopis glandulosa has lower cover than other shrubs and does not strongly dominate the shrub layer. This system also includes upper piedmont stands of desert scrub that are strongly dominated by Larrea tridentata. In Texas, Acacia constricta, Agave lechuguilla, Condalia ericoides, Dasylirion leiophyllum, Larrea tridentata, Leucophyllum spp., Mimosa aculeaticarpa var. biuncifera, Parthenium incanum, Prosopis glandulosa, Viguiera stenoloba, and Yucca torreyi are often present to dominant, but numerous shrub species may be present. Grasses are common but generally have lower cover than shrubs. Common species may include Bouteloua curtipendula, Bouteloua eriopoda, Bouteloua gracilis, Bouteloua hirsuta, Bouteloua ramosa, Dasyochloa pulchella, and Muhlenbergia porteri. In Texas, herbaceous cover is generally low with species such as Aristida purpurea, Bouteloua curtipendula, Bouteloua eriopoda, Bouteloua ramosa, Bouteloua trifida, Dasyochloa pulchella, and Muhlenbergia setifolia. Also included in this ecological system are shrublands with a sparse understory of Larrea tridentata that occur on gravelly piedmont slopes that may extend down gravelly upper basins. A pebbly desert pavement may be present on the soil surface. This may indicate remnant erosional surfaces from the early Holocene that are thought to be some of the historic distribution of Larrea tridentata desert scrub in the Chihuahuan Desert (Muldavin et al. 2000b). Historically, much of this desert scrub was thought to be a steppe characterized by perennial desert grasses such as Bouteloua eriopoda, Bouteloua ramosa, Muhlenbergia porteri, Bothriochloa barbinodis, or Digitaria californica with an open creosotebush - mixed desert shrub layer.

Climate: Climate is semi-arid with annual precipitation ranging from 200-250 mm that falls mostly in the summer.

Physiography/landform: This ecological system is the widespread desert scrub that occurs on gravelly mid to upper bajadas, foothills and dissected gravelly alluvial fans in the Chihuahuan Desert and has recently expanded into former desert grasslands in the northern portion of its range. It generally occurs on mid to upper piedmonts above the xeric basins and plains dominated by Chihuahuan Creosotebush Desert Scrub (CES302.731) and extends up to the chaparral zone.

Soil/substrate/hydrology: Soils are typically well-drained, non-saline, gravelly loams often with a petrocalic layer. Substrates are frequently derived from limestone, although igneous rocks are common in some areas (Brown 1982a, MacMahon and Wagner 1985, Henrickson and Johnston 1986, MacMahon 1988, Dick-Peddie 1993).

In the U.S., much of this scrub is thought to be a result of recent expansion of Larrea tridentata into former desert grasslands and steppe in the last 150 years as a result of drought, overgrazing by livestock, and/or decreases in fire over the last 70-250 years (Buffington and Herbel 1965, Ahlstrand 1979, Donart 1984, Dick-Peddie 1993, Gibbens et al. 2005). This expansion has created challenges in determining ecologically historic stands from more recent ones. 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 desert scrub, but more research is needed. Conversely, sparse understory Larrea tridentata shrublands on remnant early Holocene erosional surfaces (often with shallow calcareous soils and a pebbly desert pavement) may indicate historic distributions of Larrea tridentata desert scrub in the Chihuahuan Desert (Stein and Ludwig 1979, Muldavin et al. 2000b).

Larrea tridentata, a dominant and diagnostic species, is very long-lived (some clones have been estimated to be over 10,000 years). 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). Larrea tridentata is poorly adapted to fire because of its highly flammable, resinous leaves and limited sprouting ability after burning although it may survive lower-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. Other dominant shrubs such as Acacia constricta, Acacia greggii, Acacia neovernicosa, Fouquieria splendens, Flourensia cernua, Mimosa aculeaticarpa var. biuncifera, Mortonia scabrella, and Parthenium incanum are generally top-killed by low- to moderate-severity fires, while severe fires may kill them. The nitrogen-fixing ability of Acacia neovernicosa and other leguminous shrubs in this system allow it to colonize harsh environments well (Muldavin et al. 1998a).

This system also includes invasive Flourensia cernua shrublands that occur in former (degraded) tobosa (Pleuraphis mutica) flats and loamy plains (Muldavin et al. 1998a). Presence of Scleropogon brevifolius is common in these invasive stands. Flourensia cernua is relatively shallow-rooted and therefore competes strongly with grasses for soil moisture (Muldavin et al. 1998a). Buffington and Herbel (1965) report that Larrea tridentata has displaced many stands of Flourensia cernua and cite that it may be because Larrea tridentata only competes with grasses during the shrub's seedling stage. Muldavin et al. (1998a) state that stands with no graminoid layer are unlikely to develop one; but stands with a graminoid layer are likely to maintain it if not overgrazed. Impermeable caliche and argillic horizons are not uncommon on these sites. These layers restrict deep percolation of soil-water and may favor the shallower root grasses and shrubs such as Flourensia cernua over more deeply rooted shrubs such as Larrea tridentata and Prosopis spp. (McAuliffe 1995).

Drought is a relatively common occurrence in this desert scrub, generally occurring every 10-15 years and lasting 2-3 years with occasional long-term drought periods (10-15 years duration). Larrea tridentata 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).

LANDFIRE (2007a) developed three VDDT model for this system, all have two classes. (1) Chihuahuan Mixed Desert and Thorn Scrub (BpS 2511001) occurs in basins, plains and into foothills in the Chihuahuan Desert. Substrates are generally fine-textured, saline soils. Does not do well in poorly aerated soils. Stands of Acacia constricta-, Acacia neovernicosa- or Acacia greggii-dominated thornscrub are included in this creosotebush system, and limestone substrates appear important for at least these species.

A) Early Development 1 All Structures (15% of type in this stage): Characterized by low shrub cover (typically 5-10%). Little disturbance was considered in Class A, except for replacement fire every 300 years on average. In the historic 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 (85% of type in this stage): Typically, >10% shrub cover and <10% grass and forb cover; associated with more productive soils. Larrea tridentata characteristically dominates shrub layer. Acacia species may dominate locally in patches. Few fine fuels are associated with this community, therefore the MFRIs 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.

(2) Chihuahuan Mixed Desert Shrubland (BpS 2511002) a minor desert scrub that occurs on gravelly mid to upper bajadas, foothills and dissected gravelly alluvial fans in the Chihuahuan Desert and has recently expanded into former desert grasslands in the northern portion of its range. It generally occurs on mid to upper piedmonts above the desert plains Chihuahuan Creosotebush Desert Scrub (CES302.731) and extends up to the chaparral zone (LANDFIRE 2007a, BpS 2511002).

A) Early Development 1 Open (25% of type in this stage): Under natural conditions shrub cover represents <20% canopy cover and is likely not affected by disturbance. The grass community may be as low as 10% canopy cover after a combination of drought/fire. Little disturbance was considered in Class A, modeled drought every 50 years on average, resetting the age to zero (Option 2). In the historic 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. After 100 years, class A transitions to class B. However, if the upper soil horizon and/or microbes are lost, then a longer recovery time is required or complete recovery is not possible.

B) Late Development 1 Open (75% of type in this stage): Typically, <40% shrub canopy cover and as much as 25% grass and forb canopy cover; associated with more productive soils. Shrubs characteristically dominate the upper layer. Replacement fire followed by prolonged drought every 500 years (min-max: 300-1000 years) on average (Option 1). Wind/weather stress also affected this community on average every 80 years, but did not cause a transition to class A.

(3) Chihuahuan Grama Grass-Steppe (BpS 2511003) a minor desert scrub steppe that occurs on the bajadas and into foothills in the Chihuahuan Desert. Substrates are generally coarse-textured, gravelly soils and may have a petrocalic layer. This site exhibits a high degree of topographic diversity, including limy uplands (LANDFIRE 2007a, BpS 2511003).

A) Early Development 1 Open (20% of type in this stage): Under natural conditions shrub cover represents <10% canopy cover and is likely not affected by disturbance. The grass community may be as low as 10% canopy cover after a combination of drought/fire. Little disturbance was considered in class A. Modeled drought every 50 years on average, resetting the age to zero (Option 2). In the historic 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. After 100 years, class A transitions to class B. However, if the upper soil horizon and/or microbes are lost, then a longer recovery time is required. Or complete recovery is not possible.

B) Late Development 1 Open (80% of type in this stage): Typically, <10% shrub canopy cover and as much as 40% grass and forb canopy cover; associated with more productive soils. Grasses characteristically dominate shrub layer. Replacement fire followed by prolonged drought every 500 years (min-max: 300-1000 years) on average (Option 1). Wind/weather stress also affected this community on average every 80 years, but did not cause a transition to class A.

In the northern Chihuahuan Desert, this creosotebush mixed desert and thornscrub shrubland ecological system is thought to occur in presettlement conditions largely as mixed desert shrub-steppe on upper bajada gravelly soils and dissected gravelly alluvial fans (S. Yanoff pers. comm. 2006). This grama grass steppe with an open canopy of desert scrub species is a mostly historical grama grass steppe BpS that was described during LANDFIRE MZ25 BpS modeling workshops as Chihuahuan Grama Grass Creosote Steppe (LANDFIRE 2007a, BpS 2511003). It is distinct from creosotebush mixed shrublands on similar sites because it has an open shrub layer characterized by dense perennial grasses (typically black grama).

Chihuahuan Mixed Desert and Thornscrub is a widespread, long-lived ecosystem that occurs above Chihuahuan Creosotebush Desert Scrub (CES302.731) in the xeric desert basin. Although thornscrub occurring on limestone rock outcrops is stable, other stands may be sensitive to altered fire regimes caused by invasive species, as well as anthropogenic disturbance such as mechanical/chemical shrub removal. Altered (uncharacteristic) fire regimes greatly influence ecosystem processes.

The historical desert scrub has a very long fire-return interval (FRI) ranging from 300-1000 years (500 years on average) (from LANDFIRE BpS Model 2510740). Larrea tridentata and other desert scrub plant species did not evolve with fire and are sensitive to burning; most of them do not resprout after burning and are slow to recover, and therefore fires should be rare events to be avoided. Invasion of non-native grasses provides fine fuels that can increased fire frequency, intensity and severity. Fires in desert scrub are becoming more common, especially after a series of wet years when fine fuels from non-native herbaceous species build up enough to carry fire.

The impact of livestock grazing to the historical stands of desert scrub is expected to be relatively small because there is little forage available for them in this type, but where livestock grazing or other anthropomorphic disturbance occurs there may be increased soil erosion (Milchunas 2006).

Human development has impacted many locations throughout the ecoregion. These sites represent a poor-condition/non-functioning ecosystem that is highly fragmented, or much reduced in size from its historical extent; the surrounding landscape is in poor condition either with highly eroding soils, many non-native species or a large percentage of the surrounding landscape has been converted to pavement or disturbed by off-road vehicles; the biotic condition is at the limit or beyond natural range of variation, e.g., vegetation composition is altered and is not dominated by native shrubs such as Larrea tridentata and Flourensia cernua. Characteristic birds, mammals, reptiles, and insect species are not present at expected abundances or the ratio of species shows an imbalance of predator-to-prey populations; abiotic condition is poor with evidence of high soil erosion, rill and gullies present or exposed soil sub horizons. Non-native grass invasion provides fine fuels that may increase fire frequency, intensity and severity.

This system occurs in the Chihuahuan Desert (LANDFIRE 2007a).