Apache-Chihuahuan Desert Grassland

This ecological system is a broadly defined desert grassland, mixed shrub-succulent steppe, or xeromorphic oak savanna that is typical of the Borderlands of Arizona, New Mexico and northern Mexico (Apacherian region) but extends west to the Sonoran Desert, north into the Mogollon Rim in Arizona and up the Rio Grande Valley into central New Mexico. It also extends east into the Chihuahuan Desert. It is found on gently sloping alluvial erosional fans and piedmonts (bajadas) that lie along mountain fronts of the isolated basin ranges throughout the Sky Island mountain archipelago and on to foothill slopes up to 1670 m elevation in the Chihuahuan Desert. The vegetation in this mixed semi-desert grassland ecosystem is variable. It is characterized by the dominance of a typically diverse layer of warm-season, perennial grasses with scattered stem succulents and shrubs. Frequent species include the grasses Aristida ternipes, Bouteloua barbata, Bouteloua chondrosioides, Bouteloua curtipendula, Bouteloua eriopoda, Bouteloua gracilis, Bouteloua hirsuta, Bouteloua ramosa, Bouteloua repens, Bouteloua rothrockii, Dasyochloa pulchella, Digitaria californica, Eragrostis intermedia, Heteropogon contortus, Hilaria belangeri, Leptochloa dubia, Muhlenbergia porteri, with Muhlenbergia emersleyi, Muhlenbergia setifolia at upper foothill elevation, rosettophyllous, often succulent species of Agave, Dasylirion, Nolina, Opuntia, and Yucca, and short-shrub species of Calliandra, and Parthenium. Tall-shrub/short-tree species of Acacia, Prosopis, Juniperus, Mimosa, and various oaks (e.g., Quercus grisea, Quercus emoryi, Quercus arizonica, Quercus oblongifolia) may be present with low cover (usually <10%). Pleuraphis mutica-dominated semi-desert grasslands often with Bouteloua eriopoda or Bouteloua gracilis occurring on lowlands and loamy plains in the Chihuahuan Desert are classified as Chihuahuan Loamy Plains Desert Grassland (CES302.061). Many of the historical desert grassland and savanna areas have been converted through intensive grazing and other land uses, some to Apacherian-Chihuahuan Mesquite Upland Scrub (CES302.733) (Prosopis spp.-dominated).

The vegetation in this mixed semi-desert grassland ecosystem is variable. It is characterized by the dominance of a typically diverse layer of perennial grasses with scattered stem succulents and shrubs. Frequent species include the grasses Aristida ternipes, Bouteloua curtipendula, Bouteloua eriopoda, Bouteloua gracilis, Bouteloua hirsuta, Bouteloua ramosa, Bouteloua repens, Eragrostis intermedia, Heteropogon contortus, Muhlenbergia porteri, with Muhlenbergia emersleyi, Muhlenbergia setifolia at upper foothill elevation, rosettophyllous, often succulent species of Agave, Dasylirion, Nolina, Opuntia, and Yucca, and short-shrub species of Calliandra, and Parthenium. Tall-shrub/short-tree species of Acacia, Prosopis, Juniperus, Mimosa, and various oaks (e.g., Quercus grisea, Quercus emoryi, Quercus arizonica, Quercus oblongifolia) may be present with low cover.

This system is found on gently sloping alluvial erosional fans and piedmonts (bajadas) that lie along mountain fronts of the isolated ranges throughout the Sky Island mountain archipelago and on to foothill slopes from 1000 m to 1670 m and up to 1800 m elevation in the Chihuahuan Desert and up to 2200 m in lower montane grasslands.

Climate: Climate is semi-arid, warm-temperate with a highly variable, bimodally distributed precipitation. Approximately two-thirds of the 20-40 cm mean annual precipitation occurs in the late summer and early fall, usually as localized high-intensity thunderstorms.

Physiography/landform: Sites are typically gently sloping mesas and piedmonts (LANDFIRE 2007a).

Soil/substrate/hydrology: Substrates are variable, ranging from fine- to coarse-textured soils depending on site. However, most are typically deep, coarser-textured, gravelly soils derived from limestone, sandstone, conglomerate or igneous substrates such as tuff.

Semi-desert grasslands are complex with many stands having a shrub or stem succulent component (Agave and Yucca spp.) under natural conditions (Burgess 1995). This woody component increases in density over time in the absence of disturbance such as fire (Burgess 1995, Gori and Enquist 2003, Schussman 2006a). Under historic natural conditions (also called natural range of variability or NRV), this ecosystem ranges from open perennial grasslands with low cover of shrubs to grasslands with a moderately dense shrub layer and succulent layer (Burgess 1995, Gori and Enquist 2003). An exception is that some stands with deep argillic horizons appear resistant to shrub and tree invasion without disturbance (McAuliffe 1995).

It is well-documented that frequent stand-replacing fire (fire-return interval (FRI) of 2.5 to 10 years) was a key ecological attribute of this semi-desert grassland ecosystem historically before 1890 (Wright 1980, Bahre 1985, McPherson 1995, Kaib et al. 1996). Other evidence of the importance of fire in maintaining desert grasslands includes the widespread conversion of grasslands to shrublands during the century of fire suppression (McPherson 1995) and the results of prescribed burning on decreasing shrub cover and increasing grass cover (Bock and Bock 1992, Robinett 1994). Additional evidence that frequent fire is a key ecological attribute of this ecosystem is that many common invasive shrubs, subshrubs and cacti are fire-sensitive and individuals are killed when top-burned, at least when they are young (<10 years old) (McPherson 1995), while native perennial grasses generally quickly recover from burning (Wright 1980, Martin 1983, Bock and Bock 1992).

Herbivory by native herbivores in the system is varied and ranges from invertebrates and rodents to pronghorn (Parmenter and Van Devender 1995, Whitford et al. 1995, Finch 2004). Soil-dwelling invertebrates include tiny nematodes and larger termites and ants, are important in nutrient cycling and affect soil properties, such as bulk density (Whitford et al. 1995). Above-ground invertebrates such as grasshoppers can significantly impact herbaceous cover when populations are high. Herbivory by native mammals also impacts these grasslands. Historically, populations of large mammals such as pronghorn (Antilocarpa americana) and mule deer (Odocoileus hemionus) were once abundant in this ecosystem (Parmenter and Van Devender 1995). Populations were greatly reduced and, in the case of pronghorn, extirpated during the 1800s and early 1900s, but effective game management has restored many populations, although habitat changes will limit restoration in other areas (Parmenter and Van Devender 1995). The historic impact of large native ungulates on this ecosystem is not known; however, in the case of wintering elk, it may have been significant locally. The current impact is assumed to be relatively small in this ecosystem.

Herbivory from native small mammals such as rodents is significant as they are the dominant mammals in the semi-desert grassland ecosystem. There is also high diversity of these rodents, especially ground-dwelling ones such as spotted ground squirrels (Xerospermophilus spilosoma), and bannertail and Ord kangaroo rats (Dipodomys spectabilis and Dipodomys ordii). These burrowing rodents have a substantial effect on vegetation composition, soil structure and nutrient cycling (Parmenter and Van Devender 1995, Finch 2004). Historically, black-tail prairie dogs (Cynomys ludovicianus) had extensive colonies in the Great Plains that extended west to southeastern Arizona but were greatly reduced. Although abundant in southeastern Arizona and southwestern New Mexico in the 1800s, the black-tailed prairie dog populations were decimated by 1930 and considered extirpated in Arizona by 1960 (Alexander 1932, Hoffman 1986, Parmenter and Van Devender 1995, Van Pelt 1999, Underwood and Van Pelt 2008). Although there have been several reintroductions of black-tailed prairie dogs, their numbers and impacts are still small in this region. Because of the nature of black-tail prairie dogs (large towns and major impacts to the local ecosystem), they may have historically functioned as a keystone species in lower elevation stands in the northern extent of Apacherian-Chihuahuan Semi-Desert Grassland and Steppe (CES302.735). However, historically black-tailed prairie dogs were likely more abundant in the deeper soiled Chihuahuan Loamy Plains Desert Grassland (CES302.061) that occurs on lower elevation alluvial flats and plains. More research is needed to determine the role of black-tailed prairie dogs in these semi-grassland and steppe systems.

Invertebrate animals are also significant in semi-desert grassland. They are both abundant and extremely diverse, ranging from single-celled protozoans, bacterial and soil nematodes and mites to larger arachnids, millipedes, cockroaches, crickets, grasshoppers, ants, beetles, butterflies, moths, flies, bees, wasps, and true bugs (Whitford et al. 1995). Invertebrates are important for nutrient cycling and pollination, and subterranean species of ants and termites can impact soil properties such as bulk density, infiltration permeability and storage (Whitford et al. 1995). Grasshoppers feed on grasses and forbs and can consume significant amounts of forage when their populations are high. Many species of butterflies, flies, bees, and moths are important for pollination. Some species such as Yucca moths (Tegeticula spp.) and Yucca species have obligate/mutualistic relationships (Whitford et al. 1995, Althoff et al. 2006). In these grasslands, Yucca spp. are typically dependent on a single species of Tegeticula for pollination, which is usually dependent on a single Yucca host plant species for habitat and food for larvae, for example, Tegeticula baccatella and Yucca baccata, Tegeticula carnerosanella and Yucca faxoniana, Tegeticula elatella and Yucca elata, Tegeticula maderae and Yucca x schottii, and Tegeticula yuccasella and Yucca glauca. More study and review are needed to fully understand the many functional roles animals have within the semi-desert grassland ecosystem.

LANDFIRE developed a VDDT model for this system which has three classes (LANDFIRE 2007a, BpS 2711210).

A) Early Development 1 All Structures (20% of type in this stage): Herbaceous cover (0-20%). Grass and herbs, 0-5 years (predicated on moisture regime). Early-succession post-fire grass and herb community. This class encompasses the time period required to recover sufficient fuel loads to carry fire. Perennial bunchgrasses, annual grass, and herb community. Upper layer of shrubs, canopy cover less than 5%.

B) Mid Development 1 All Structures (35% of type in this stage): Perennial grass species dominate with 35-50% canopy cover; <0.5 m height. Shrub cover is 5-10% with shrubs 0-1 m tall. Grass with some low shrubs, 6-50 years old. Perennial bunchgrasses regenerated and young shrubs begin growing. Species are perennial bunchgrasses and shrubs. Canopy cover of shrubs is 5-10%. Maintenance disturbance is drought, occurring approximately every 30 years. Maintenance replacement fire is more frequent with less frequent replacement fire returning to class A. This was modeled to occur every 10 years on average, half the time causing a transition to class A, and half the time maintaining this class.

C) Late Development 1 All Structures (50% of type in this stage): Perennial grass species dominate with 10-35% canopy cover; 1-2 m height. Shrubs continue to increase in size and/or number of individuals. Species are perennial bunchgrasses and shrubs. Canopy cover of shrubs is 10-20%. (Shrub cover will be similar to species composition found in Apacherian-Chihuahuan Mesquite Upland Scrub (CES302.733)). Shrub species diversity increases. FRI=10 years, half are replacement (to class A) and half take class back to class B. The wind/weather stress in this model is drought, occurring approximately every 30 years. It is thought that this is the class that might result with lack of fire and that more would be present in this class currently versus historically.

In the LANDFIRE BpS 2611210 model, mixed-severity fire was modeled for MZ26; however, this was removed for MZ27, as it is thought that only patchy replacement fire would occur in this system (LANDFIRE 2007a). It was noted that the amount of moisture following fire has a significant impact on plant response/recovery. Because historical fire data in this system are lacking, there is uncertainty over the role fire plays in maintaining this system. Some modelers think fire has a major impact on control of woody species, whereas others think fire is less important in control of woody species than maintenance of perennial grass cover in this system (LANDFIRE 2007a).

During the last century, the area occupied by this desert grassland and steppe decreased through conversion of desert grasslands as a result of drought, overgrazing and Prosopis glandulosa seed dispersion by livestock, and/or decreases in fire frequency (Buffington and Herbel 1965, Brown and Archer 1987). Conversion of this type has also commonly come from urban and exurban development near cities such as Sierra Vista, Arizona, altered hydrological regimes (water developments/reservoirs) (Cooke and Reeves 1976), and irrigated agriculture, especially hay meadows dominated by non-native forage grasses. Fire suppression has allowed succession and conversion to shrublands, desert scrub and woodlands, especially from oak, pinyon or juniper tree invasion (Gori and Enquist 2003). This grassland has also been converted to invasive non-native, perennial forage grasses Eragrostis lehmanniana and Eragrostis curvula (Cable 1971, Anable et al. 1992, Gori and Enquist 2003).

It is believed that mesquite formerly occurred in relatively minor amounts and was largely confined to drainages until cattle distributed seed upland into desert grasslands (Brown and Archer 1987, 1989). Shrublands dominated by Prosopis spp. have replaced large areas of desert grasslands, especially those formerly dominated by Bouteloua eriopoda, in Trans-Pecos Texas, southern New Mexico and southeastern Arizona (York and Dick-Peddie 1969, Hennessy et al. 1983). Studies on the Jornada Experimental Range suggest that combinations of drought, overgrazing by livestock, wind and water erosion, seed dispersal by livestock, fire suppression, shifting dunes, and changes in the seasonal distribution of precipitation have caused this recent, dramatic shift in vegetation physiognomy (Buffington and Herbel 1965, Herbel et al. 1972, Humphrey 1974, McLaughlin and Bowers 1982, Gibbens et al. 1983, Hennessy et al. 1983, Schlesinger et al. 1990, McPherson 1995).

These native mixed semi-desert grasslands are the dominant grassland type and range from open grasslands with low shrub canopy cover (less than 10% cover) to denser grassland with higher shrub and succulent cover. Over time without fire or other disturbance, stands become dominated by woody vegetation and convert to shrublands or woodlands (Gori and Enquist 2003). Conversion to juniper woodlands or mesquite shrublands is common when trees or mesquite exceed 15% cover (Gori and Enquist 2003). These grasslands were historically maintained as open grasslands with low shrub cover by fire-return intervals of 2.5 to 10 years (Wright 1980, Robinett 1994, McPherson 1995, Brown and Archer 1999). Both drought and livestock grazing interact with grass cover and fire-return intervals can affect the rate of shrub increase (Wright 1980, Robinett 1994, McPherson 1995, Brown and Archer 1999). Gori and Enquist (2003) found that after grassland conversion to shrubland there is a loss of perennial grasses and increases of bare ground. If not protected by surface rock, topsoil erosion can occur changing the site to be less suitable for grass recolonization (McAuliffe 1995).

Hydrological alterations also occurred in many semi-desert grasslands during early Anglo-American settlement time with a period of arroyo formation from 1865 to 1915 (Cooke and Reeves 1976). During this time many broad valley bottom drainages were incised, lowering water tables. This resulted in changes to more xeric vegetation because of decreased water availability, as well as increased sediment movement, altered hydrologic relationships, and loss of productive land (Cooke and Reeves 1976). Although there is debate of causes of these hydrologic changes (arroyo formation), Cooke and Reeves (1976) found strong evidence that arroyo formation was initiated by building ditches, canals, roads and embankments along channels that altered valley floor hydrology.

The introduction of the invasive non-native, perennial grasses Eragrostis lehmanniana and Eragrostis curvula has greatly impacted many semi-desert grasslands in this ecoregion (Cable 1971, Anable et al. 1992, Gori and Enquist 2003). Anable et al. (1992) and Cable (1971) found Eragrostis lehmanniana is a particularly aggressive invader and alters ecosystem processes, vegetation composition, and species diversity.

Common stressors and threats include fragmentation from housing and water developments, altered fire regime from fire suppression and indirect fire suppression from livestock grazing and fragmentation, introduction of invasive non-native species, and overgrazing by livestock which can lead to severe soil compaction and reduce vegetation cover exposing soils to erosion of topsoil, especially if soil surface does not significant rock cover.

This system is found in the Borderlands of Arizona, New Mexico and northern Mexico (Apacherian region), extending to the Sonoran Desert and throughout much of the northern Chihuahuan Desert.