The geographic area in which desert grasslands occur is quite large (500,000 km2, 193,000 square miles), ranging in the north from west-central Arizona through New Mexico to west Texas, and extending south through thirteen Mexican states (see map; McClaran 1995). The desert grassland has been described as a transitional vegetation type that separates the true deserts of the lowlands from higher elevation shrublands and woodlands. The spatial distribution of desert grasslands is strongly influenced by the interaction of climate, soils, and topography.
Desert grasslands generally occur in valleys and basins at the southern end of North America’s Basin and Range Province. Due to the variable topography of the Basin and Range Province, desert grasslands are distributed in discontinuous patches, forming transitional areas with desert vegetation at lower elevations and woodlands or chaparral at higher elevations (Burgess 1995). This mosaic of desert grasslands exists at minimum elevations of 1100 m (3600 ft) at the northern extreme to maximum elevations of 2500 m (8200 ft) at the southern extreme of the range in Mexico; the maximum elevation in the United States is 1800 m (5900 ft) (Brown 1982, McClaran 1995).
Desert grasslands are the hottest, driest, and sunniest of North American grasslands. The total amount and seasonal distribution of precipitation varies regionally within the desert grasslands. Desert grasslands in the United States receive, on average, 230 to 460 mm (9 to 18 in) of precipitation annually, yet desert grasslands in central Mexico receive up to 600 mm (23.5 in) mean annual precipitation. Summer rains are important in all desert grasslands, and a bi-seasonal precipitation pattern is typical of all but the extreme southern parts of the region. However, the relative proportions of summer moisture follow a gradient from the northwest to southeast. With the exception of the extreme northwest portion of the desert grassland, 50% or more of the average annual precipitation occurs between May and October; desert grasslands in eastern New Mexico and Texas receive approximately 75% of the annual precipitation during this period, and this value increases to nearly 90% at the southeastern limit of the grasslands in south-central Mexico. However, it is important to note that both seasonal and annual precipitation in this region are extremely variable, and wide year-to-year fluctuations in total precipitation are considered normal. Summer rains are primarily generated by convective thunderstorms, often capable of delivering localized heavy precipitation over brief time periods, whereas in winter, frontal storms affect larger regional areas and typically deliver light rains that may fall intermittently for several days. Overall, desert grasslands are characterized by mild winters and warm to very hot summers. The average annual temperatures range from 13′ to 16′ C (55′ to 61′ F), and the frost-free period generally ranges between 200-240 days. During summer, maximum temperatures frequently exceed 38′ C (100′ F) and minimum temperatures of -18′ C (0′ F) in winter are not uncommon. Diurnal temperature differences (the difference between minimum and maximum temperatures during a 24-hour period), average 14′ to 19′ C (57′ to 66′ F).
Topography and Soils
Desert grasslands primarily occur in valleys and basins of the Basin and Range physiographic province. The distribution of these grasslands is discontinuous, constrained locally by soils and topographic position within the landscape. Semidesert grasslands are primarily located on alluvium-filled basins separating block-faulted mountain ranges. The long, narrow mountain ranges characteristic of this province result from block uplifting, and are mainly composed of granite, quartz monzonite, granodiorite, limestone, or metamorphic rocks. Volcanic rock, such as rhyolite and andesite, may be locally abundant. These block-faulted mountains are the primary source of sediments that create extensive piedmont slopes, or bajadas, that characteristically skirt mountain ranges in this region, and ultimately form the deep deposits of alluvium typical of the surrounding basins. Depending on the age and juxtaposition of the piedmont slopes (foothills) in the landscape, these surfaces are incised by washes and arroyos that flow intermittently, draining either into closed basins known as bolsóns, or into river valleys.
Soils of the region are also diverse, strongly affected by local geology, physiography, and age. Soils are generally classified as Aridisols, Entisols, or Mollisols. Soils range from coarse-textured sandy soils to finer-textured silty clays and clay loams. Recently formed soils may be shallow or deep, with little horizon development. Older soils tend to have well-developed horizons and, depending on the parent material, may include clay-rich argillic horizons or calcium-rich petrocalcic (caliche) layers (Fig. 2). Soil texture, depth, landscape position and climate are important determinants of soil moisture relations. The interaction of soil texture, subsurface horizons, slope and aspect have important implications for soil water availability and likely affect plant distribution in these communities.
Current Plant Communities
The term desert grassland is somewhat debatable because these are the least “grassy” of the North American grasslands. Furthermore, the relative contribution of co-dominating life forms, such as subshrubs, shrubby trees, succulents, and forbs, varies widely in desert grasslands throughout the region (Fig. 3). Desert grassland vegetation is highly responsive to subtle differences in soils and topography, resulting in localized patches of distinct vegetation in areas that share a common climate and elevation. Superimposed over this natural patchwork, the composition of vegetation may be transformed as a result of disturbances, be they large or small, short-term or long-term, or natural versus human-caused. Although classified as grasslands, some areas support only sparse amounts of grass, and vegetation is quite patchy compared to other North American grasslands. In general it often supports patchy vegetation that may or may not include grasses.
Historic Plant Communities
The desert grasslands of the southwestern United States are believed to have developed after the last Little Ice Age. The grasslands were originally dominated by perennial, C4 grasses, many of which reproduce by stolons and tillering. Primary grass species include black grama (Bouteloua eriopoda (Torr.) Torr.), blue grama (Bouteloua gracilis (Willd. ex Kunth) Lag. ex Griffiths), bush muhly (Muhlenbergiaporteri Scribn. ex Beal), dropseeds (Sporobolus spp.) and three-awns (Aristida spp.) on sandy upland sites, and tubosa (Pleuraphis mutica Buckley) and burrow grass (Scleropogon brevifolius Phil.) on playa and lowland sites. Records and photos from the mid-1800s to early 1900s demonstrate that grasses were dominant life-forms on the landscape. However, shrub species have developed a dominant presence in the landscape through anthropogenic and climatic stressors. Some people argue the heterogeneous plant composition resulting from such transitions has changed what was grassland into a mixed shrub-savanna composition.
In the time period between the last Little Ice Age and colonial settlement, shrubs existed in low densities in the desert grassland. Native species primarily associated with increased production, and encroachment, include mesquite (Prosopis spp.) (Fig. 4), creosote bush (Larrea tridentata (DC.) Coville), and tarbush (Flourensia cernua (DC.). Questions and controversies exist as to the triggers and mechanisms driving the transition from grassland to shrub-dominant state, but the effects of these transitions are commonly regarded as loss of overall productivity and diversity.
Additional shrub, subshrub and succulent species that coexist in the grasslands include four-wing salt brush (Atriplex canescens (Pursh) Nutt.), snake weed (Gutierrezia sarothrae (Pursh) Britton & Rusby), winterfat (Krascheninnikovia lanata (Pursh) A. Meeuse & Smit), prickly pears (Opuntia spp.), barrel cactus (Ferocactuswislizeni (Engelm.) Britton & Rose), agave (Agave spp. L.), yucca (Yucca spp.), cholla (Cylindropuntia imbricata (Haw.) F.M. Knuth) and ocotillo (Fouquieriasplendens Engelm.)
Both venomous and non-venomous snakes exist in the desert grasslands, including prairie rattlesnake (Crotalus viridis), western diamondback (C. atrox), desert grassland massasauga (Sistrurus catenatus edwardsi), gopher snake (Pituophis melanoleucus affinis, and P. m. sayi), and coachwhips (Masticophis flagellum cingulum). Lizards species include lesser earless (Holbrookia maculate), side-blotched (Uta stansburiana), horned lizards (Phrynosoma spp.), southern prairie lizards (Sceloporus undulatus consobrinus), and whiptails (Cnemidophorus spp.). An additional reptile is the desert grassland box turtle (Terrapene ornate luteola). Ants and termites also play crucial roles in the desert grassland systems.
Bird species are the most diverse of the vertebrate groups in the desert grasslands. Species include horned lark (Eremophila alpestris), lark bunting (Calamospiza melanocorys), meadowlarks (Sturnella magna and S. neglecta), scaled quail (Callipepla squamata), multiple sparrows (Emberizidae, Passeridae), falcons (Falconidae), doves (Columbidae), roadrunner (Geococcyx californianus), and burrowing owl (Athene cunicularia).
The diversity of mammals in the desert grasslands is largely attributed to rodent populations. Such species include mice (Cricetidae, Heteromyidae, Muidae), gophers (Geomyidae), ground squirrels (Sciuridae), kangaroo rats (Dipodomys spp.), black tailed jackrabbits (Lepus californicus), desert cottontail rabbits (Sylvilagus audubonii), and woodrats (Neotoma spp.).Javelina (Pecari tajacu), and foxes are additional small mammal species. Large herbivores were extirpated throughout most of the desert grasslands during the 1800s and early 1900s; however, rigorous game management policies have restored a multitude of populations. These species include desert mule deer (Odocoileus hemionus crooki), pronghorn antelope (Antilocapra americana), and big horn sheep (Ovis canadensis).
Several predatory mammal species exist in the desert grassland, though their territories are subjected to threat by increasing urbanization and human presence in the landscape. Three primary mammal predators are mountain lion (Felis concolor), bobcat (F. rufus), and coyote (Canis latrans).
The Chihuahuan desert is believed to have formed approximately 8,000 years before the present in the Mid-Holocene when continental ice sheets melted. The region has potentially endured three transitions between grassland and shrubland in the last 3,000 years. Many plant species currently found in the desert grassland are believed to be relicts of a more mesic climatic condition that presumably occurred in the Mid-Holocene. Controversy exists over geobiotic change and landscape formation in the Nearctic deserts of North America. However, it has been determined that woodlands predated the current grass and shrub species, there have been varying levels of coexistence between dominant grass and shrub species for a few thousand years, and minimal to no large ungulate herbivory has occurred in the desert grasslands in the 10,000 years prior to European settlement.
Yearly and Seasonal Variation
Vegetation changes and state transitions in desert grasslands are generally assumed to be the result of environmental stressors (e.g. prolonged drought), human impacts (e.g. heavy grazing pressure) and climatic change. Variation in annual aboveground net primary production depends upon factors such as amount and timing of precipitation, seasonal temperatures, ecological sites, and plant types.
Conditions that make fire spread possible, such as ignition source and adequate fine fuel that is dry enough to burn, occur frequently in desert grasslands. Accounts from the early-1900s state that fire was historically used by Native American tribes to keep woody plant and cactus encroachment minimized (Humphrey 1953). Conversely, some believe fire was of minimal importance in the maintenance of un-encroached desert grassland and that fire occurrence was historically infrequent.
A historic assumption is that above-ground biomass is severely affected by fire in desert grasslands. However, recent studies show that, while cover is altered by fire, survivorship is similar between burned and unburned black grama- (Bouteloua eriopoda) dominated areas and belowground biomass does not show any effect in change due to fire.
The desert grasslands typically receive between 200-500 mm (8″-20″) of mean annual precipitation. Precipitation is highly variable throughout the desert grasslands. The variability is primarily driven by mountain ranges that create a rain shadow effect as Pacific storms move eastward (Western Regional Climate Center, DRI). Plant species therefore have developed mechanisms for dealing with harsh environmental conditions, such as long viability of annual seeds and drought deciduous tendencies in perennials. In the last century two extreme meteorological drought events took place: first in the 1930s, then in the 1950s, both with a Palmer drought index of -4.0 or lower (Western Regional Climate Center, DRI). Periodic, moderate drought events have occurred every 10 to 20 years in the last century; however, more extensive dry periods are predicted for the future.
Livestock, including sheep, goats and cattle, were first introduced to the desert grasslands in the 1500s by Spanish expeditions sent by royal decree to find and mine mineral riches, colonize the New World, and convert native peoples to Catholicism. Desert grasslands were colonized by expeditions starting from Mexico City in the early 1500s, after the Spanish conquest of the Aztecs, with expansion moving north into present-day New Mexico. Establishment of colonial settlements and pastoralism did not begin in the northern desert grasslands until the late 1500s with expeditions by Juan de Onate. However, settlements were typically small, with only a few thousand head of livestock, because of scarcity of water sources and frequency of Apache raids. Extensive settlement did not occur in the northern desert grasslands until the 1870s-1880s when the Southern Pacific Railroad was extended west, the Apaches were subjugated, mining began to boom in southeastern Arizona, and cattle ranching became a major enterprise.
Grazing, specifically overgrazing, in conjunction with historic drought events is commonly perceived as the primary reason for vegetation change in the desert grasslands. Scientists using survey notes from 1858 as historic context for mapping vegetation changes concluded that heavy grazing (or overgrazing) was the causative reason for shrub expansion. Restoration of desert grasslands is predicted to be impossible without an extreme shift in climate (i.e. greater and more frequent precipitation events) given the extensive loss of soil A horizons from overgrazed areas. However, researchers determined in an 18-year study that shrub expansion occurs in all areas irrespective of grazing.
Stressors, such as severe wind, may play a role in determining the conditions under which fragmentation of vegetation communities can occur. Geomorphologic agents, such as wind, can also influence plant growth and reproduction through sandblasting, which can result in physical damage to plants, and restrict vegetative reproduction and seedling establishment. Plants can be negatively affected by wind through burial from aeolian deposits, which can result in plant mortality. Conversely, the redistribution of nutrients by wind may also have a positive effect on plant growth through the island effect. Erosive winds in the northern desert grasslands come primarily from a southwesterly direction between March and May. Frontal winds passing through the desert grasslands may reach 50 km/h (30 mph) with maximum speeds reaching more than 80 km/h (50 mph). Researchers did not detect an effect of fetch length on grass reproduction; however, wind erosion may have important effects on the growth and survival of perennial plants in this system.
Tornadoes, though uncommon and often occurring in sparsely populated areas, can and do occur. Recent periods of extensive below average lows (NOAA.gov) also seem to have a significant impact on vegetation growth and mortality, most noticeably affecting cactus species.
Controversy exists over the historical presence and use of fire in the desert grasslands. Regardless of historical fire frequency, discontinuity in fuel loads due to shrub invasion currently leads to minimal natural occurrences of fire in desert grasslands. Variability also exists in grass recovery time after fires primarily because of the sporadic nature of precipitation in the region.
Heavy grazing is a critical issue even though many studies have found conservative grazing has greater financial and biological rewards. Conservative grazing in the desert grassland has been defined as 25-35% use of key forage species even during droughts. Distance from watering points in grazed areas has a large effect on relative abundance of perennial grasses and it is suggested that a downward stocking rate adjustment be made for the 1,600 m (1 mile) radial area from a water point.
On the Jornada Experimental Range, scientists observed that a key grassland species, B. eriopoda, was resilient to a four-year pulse of intense defoliation followed by nine years of grazing rest, even when foliar cover had been reduced to very low levels (< 5%) and plant patches were highly fragmented. It is suggested that historical extinctions of B. eriopoda populations associated with state transitions were likely caused by multiple drivers.
Woody Plant Encroachment
Desert grasslands have undergone widespread transitions to shrubland states over the past two centuries. Transitions to shrubland states are believed to be initiated by the extinction of dominant perennial grass (Bouteloua eriopoda Torr., Sporobolus spp., Pleuraphis mutica Buckley) in association with colonization of shrubs (Prosopis glandulosa Torr., Larrea tridentata (DC.) Coville, Flourensia cernua DC.), and increased soil erosion rates (Branson 1985; Van Devender 1995). The expansion of bare ground areas associated with grass loss is believed to govern feedbacks between grass loss, shrub proliferation, and soil degradation. Change in climate, livestock grazing and alteration of fire regimes are causal factors often attributed to shrub proliferation; however, an alternate hypothesis that has been proposed is the effect of post-industrial CO2 on C3 woody plants versus C4 desert grasses.
Lehmann lovegrass (Eragrostis lehmanniana Nees) was originally introduced to the desert grasslands from South Africa as a surrogate species for restoration purposes in the 1930s. However, the plant quickly spread, reducing native species composition through competitive reproduction (seeds establish under dry and adverse conditions) and high tolerance to disturbance, such as continuous heavy grazing. Spread of Lehmann lovegrass does not necessitate the presence of grazing, but it is seen to increase in areas with higher grazing, which is likely due to reduced spatial cover of native plants.
Saltcedar (Tamarix spp.) is a highly invasive woody plant species from Eurasia that was introduced in the 1800s as a horticultural species for windbreaks and streambank erosion. Once planted, saltcedar spread through a large majority of riparian corridors in the western US and continues to out-compete riparian vegetation for habitats with moist soils or shallow water.
Additional exotic invasive species include King Ranch bluestem (Bothriochloa ischaemum (L.) Keng), African bermudagrass (Cynodon dactylon L.), buffel grass (Pennisetum ciliare (L.) Link), and Johnsongrass (Sorghum halepense (L.) Pers.).
Urban encroachment into surrounding environs has an increasing impact on local and global ecosystems. In particular, the spread of housing and industrial development is reducing habitat for native species, severely fragmenting landscapes and blocking corridors for species movement.
Grazing Management Practices
Stocking rate is the most important factor to consider when managing a grazing operation, especially in the desert grasslands, where periodic drought presents major challenges. A general grazing intensity of 25-35% use of key forage species is considered best for maintaining forage production and ecosystem resources.
Several poisonous plant species exist in the desert grasslands. The most prevalent are the species loco weed (Astragalus and Oxytropis spp.), redroot pigweed (Amaranthus retroflexus L.), silverleaf nightshade (Solanum elaeagnifolium Cav.), and white margin euphorbia (Euphorbia marginata or E. albomarginata Pursh). Loco weed species present a problem for ranchers in that certain cattle will develop an “addiction” to the plant and consume it until toxicity levels are exceeded. Often ranchers must separate loco weed-consuming cattle and put them to pasture in areas without loco weed.
Restoration of native grass species in the desert grasslands has been of minimal success since the initiation of efforts to restore degraded areas in the early 1900s. Exotic species with greater establishment capacities, such as the African lovegrasses, were introduced because of failure to restore native species, and consequently became invasive, outcompeting remnant native species. Many current efforts are focusing on controlling the spread of African lovegrasses and restoring native species (Fig. 8). Researchers found that a few native grass species were able to germinate with shorter moisture periods than Lehmann lovegrass (Eragrostis lehmanniana Nees); however, once germinated, these natives require a consistent pattern of soil moisture in order to establish. It is suggested that native plant seeding is more effective in the middle of a rainy season, when available water may occur in longer initial periods.
Agricultural practices are typically limited to valleys and areas with easily accessible water sources or irrigation because of the hot and arid growing environment produced by the desert grassland climate. Major crops in the region include onions, cotton, corn, sorghum, alfalfa, chiles, and pecans (http://www.nass.usda.gov).
Commercial Hunting and Recreation
Hunting is a popular hobby in the desert grasslands. Mule deer, elk, Pronghorn antelope, ibex, oryx, javelina, Barbary sheep, cougar and furbearers are all species available for tag draws. Many native and migratory bird species, such as quail, dove, pheasant, Dusky grouse, Band-tailed pigeon, duck, goose, Sandhill crane and September teal are also popular species to hunt (US Fish and Wildlife).
Some species have been introduced with the sole intention of providing hunting opportunities. Oryx (Oryx gazella) are an antelope brought to the Tularosa Basin from South Africa in 1969 whose populations are maintained through depredation hunts. Barbary sheep from the North African range have been introduced to many areas as well as being raised on game ranches (Gray and Simpson 1982). Feral hogs (Sus scrofa) are the most devastating and problematic of any introduced game species. They transmit diseases such as pseudorabies and trichinosis; eat native reptiles, amphibians, birds and bird eggs; and, damage the environment (www.wildlife.state.nm.us, volume 56, no. 4 winter 2011-2012).