Chihuahuan Creosote Desert

This matrix ecological system is the common lower elevation Larrea tridentata-dominated desert scrub that occurs throughout much of the Chihuahuan Desert and has recently expanded into former lower elevation desert grasslands in the northern portion of its range. Stands typically occur in flat to gently sloping desert basins and on alluvial plains, extending up into lower to mid positions of piedmont slopes (bajada). Substrates range from coarse-textured loams on gravelly plains to finer-textured silty and clayey soils in basins. Soils are alluvial, typically loamy and non-saline, and frequently calcareous as they are often derived from limestone, and to a lesser degree igneous rocks. The vegetation is characterized by a moderate to sparse shrub layer (<10% cover on extremely xeric sites) that is typically strongly dominated by Larrea tridentata with Flourensia cernua often present to codominant. A few additional shrubs or succulents may also be present, such as Agave lechuguilla, Parthenium incanum, Jatropha dioica, Koeberlinia spinosa, Lycium spp., and Yucca spp. Additionally, Flourensia cernua can often be abundant and the sole dominant in silty basins which are included in this ecological system. In general, shrub diversity is low as this ecological system lacks thornscrub and other mixed desert scrub species that are common on the gravelly mid to upper piedmont slopes. However, on deeper soils and along minor drainages, shrub diversity and cover may increase with occasional Atriplex canescens, Gutierrezia sarothrae, or Prosopis glandulosa. Herbaceous cover is usually low and composed of grasses. Common species may include Bouteloua eriopoda, Dasyochloa pulchella, Muhlenbergia porteri, Pleuraphis mutica, Scleropogon brevifolius, and Sporobolus airoides. Included in this ecological system are Larrea tridentata-dominated shrublands with a sparse understory that occur on gravelly to silty, upper basin floors and alluvial plains. A pebbly desert pavement may be present on the soil surface.

This desert scrub is common on alluvial plains and is characterized by a moderate to sparse shrub layer (<10% cover on extremely xeric sites) that is typically strongly dominated by Larrea tridentata with Flourensia cernua often present to codominant (Brown 1982, MacMahon and Wagner 1985, Henrickson and Johnston 1986, MacMahon 1988, Dick-Peddie 1993). A few scattered shrubs or succulents may also be present such as Agave lechuguilla, Parthenium incanum, Jatropha dioica, Koeberlinia spinosa, Lycium spp., and Yucca torreyi. Additionally, Flourensia cernua will often strongly dominate in silty basins that are included in this ecological system. In general, shrub diversity is low as this ecological system lacks codominant thornscrub and other mixed desert scrub species that are common on the gravelly mid to upper piedmont slopes. However, shrub diversity and cover may increase locally where soils are deeper and along minor drainages with occasional Atriplex canescens, Gutierrezia sarothrae, Parthenium incanum, Acacia constricta, or Prosopis glandulosa. In Texas, succulents such as Fouquieria splendens, Agave lechuguilla, Yucca torreyi, Opuntia spp., and Echinocereus spp. may be conspicuous on particularly hot desert sites at low elevations. In the southern Chihuahuan Desert, stands are dominated by Larrea tridentata with Agave parryi (= Agave scabra), Cylindropuntia kleiniae (= Opuntia kleiniae), Cylindropuntia imbricata (= Opuntia imbricata), and Yucca filifera (Huerta-Martinez et al. 2004). Herbaceous cover is usually low and composed of grasses. Common species may include Bouteloua eriopoda, Dasyochloa pulchella (= Erioneuron pulchellum), Muhlenbergia porteri, Pleuraphis mutica, Scleropogon brevifolius, and Sporobolus airoides. Included in this ecological system are Larrea tridentata-dominated shrublands with a sparse understory that occur on gravelly to silty, upper basin floors and alluvial plains. A pebbly desert pavement may be present on the soil surface.

Climate: Climate is semi-arid to arid with annual precipitation ranging from 200-250 mm that falls mostly in the summer. Summers are hot and winters can be cold with freezing temperature occurring in the northern extent.

Physiography/landform: This ecological system is the common lower elevation desert scrub that occurs throughout much of the Chihuahuan Desert and has recently expanded into former desert grasslands in the northern portion of its range. Elevation ranges from 1000-2000 m. Stands typically occur in flat to gently sloping, desert basins and on alluvial plains, extending up into the lower to mid positions of piedmont slopes (bajada), sometimes on colluvium.

Soil/substrate/hydrology: Substrates range from coarse-textured loams on gravelly plains to finer-textured silty and clayey soils in basins. Soils are alluvial, typically loamy and non-saline, and frequently calcareous as they are often derived from limestone, and to a lesser degree igneous rocks (Brown 1982a, MacMahon and Wagner 1985, Henrickson and Johnston 1986, MacMahon 1988, Dick-Peddie 1993). In Texas, this system typically occurs on flat and gently rolling landforms, often on gravelly alluvial plains, outwash plains and intermountain basins. A pebbly desert pavement may be present on the soil surface.

This is a stable ecosystem that is well suited to the hot, very dry basins and low hills where it occurs. The dominant and diagnostic species, Larrea tridentata, is a very long-lived species (some clones have been estimated to be over 10,000 years old). 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.

Historic fire regimes for Chihuahuan Creosotebush Desert Scrub are difficult to quantify but fires were rare with a fire-return interval (FRI) ranging from 300-1000 years and 500 years on average (from LANDFIRE BpS Model 2510740). The fire characteristics range from low- to moderate- to high-intensity, moderate-severity, stand-replacing crown fires that occur during spring, summer and fall seasons. Fires tend to be small or medium in size and need unusual conditions (e.g., a drought following an unusually wet year so there are adequate fine fuels that are available to carry a fire) (Brown and Minnich 1986, Paysen et al. 2000).

Weather stress such as drought also affects this community by reducing vegetation cover (especially grasses) every 80 years or so but does cause significant shrub mortality although shrubs may die back some (from LANDFIRE BpS Model 2510740) (Humphrey 1974). 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).

Biotic pollination by bees is important for creosotebush (Cane et al. 2000). Seed dispersal is primarily by wind and gravity as fruits are adapted for tumbling (Maddox and Carlquist 1985). However, seed burial by rodents may improve germination and survival of creosotebush (Chew and Chew 1970) so biotic dispersal may enhance regeneration especially in undisturbed, smooth desert pavement areas where seed burial is unlikely. Most seed germination requires between 80-150 mm (3-6 inches) of summer precipitation (Marshall 1995).

Herbivory by native herbivores in Chihuahuan Creosotebush Desert Scrub includes small mammals, reptiles and invertebrates. Larrea leaves are not edible to most animals; however, seeds are eaten by many small mammals (Paysen et al. 2000).

LANDFIRE developed a VDDT model for this system which has three classes (LANDFIRE 2007a, BpS 2510740):
A) Early Development 1 All Structures (15% of type in this stage): Under natural conditions shrub cover generally 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 and fire. Little disturbance was considered in class A, modeled drought every 50 years on average keeping the class in A (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 (85% of type in this stage): Typically <30% shrub canopy cover. 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. Class B is likely over-represented on the landscape today.

Although Chihuahuan Creosotebush Desert Scrub is a widespread ecosystem that has increased in abundance at the expense of native desert grasslands, it is sensitive to altered fire regimes caused by invasive species, as well as anthropogenic disturbance such as mechanical/chemical shrub removal. Currently much of the extent in the U.S. of this desert scrub is the result of recent expansion of Larrea tridentata into former desert grasslands in the last 150 years from the combined effects of drought, overgrazing by livestock, and/or decreases in fire frequency over the last 70-250 years (Buffington and Herbel 1965, Ahlstrand 1979, Donart 1984, Dick-Peddie 1993, Gibbens et al. 2005). This system now includes vast areas of loamy plains that have been converted from Pleuraphis mutica and Bouteloua eriopoda desert grasslands to Larrea tridentata scrub. This system also includes expanding Flourensia cernua shrublands that occur in former (now degraded) tobosa (Pleuraphis mutica) flats and loamy plains. Presence of Scleropogon brevifolius is common on these degraded sites. Dick-Peddie (1993) suggested that absence of Flourensia cernua as codominant and presence of Acourtia nana (= Perezia nana), Dasyochloa pulchella, and Yucca elata may be indicators of recent conversion of desert grasslands into desert scrub, but more research is needed. Conversely, Larrea tridentata shrublands with a sparse understory on remnant early Holocene erosional surfaces (often with desert pavement) may indicate historical distributions of Larrea tridentata desert scrub in the Chihuahuan Desert (Stein and Ludwig 1979, Muldavin et al. 2000b).

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 increase 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 subhorizons. Non-native grass invasion provides fine fuels that may increase fire frequency, intensity and severity.

This extensive, lower elevation desert scrub ecological system occurs in the Chihuahuan Desert in broad desert basins and alluvial plains extending up into the lower bajada.