The Chihuhuan Desert is the largest hot desert in North America. In fact, according to Medellin-Leal in the reference edited by Bender (1982, p. 321, 323), the Chihuhuanm Desert is "the largest division of the Great North American Desert" and "the largest of the North American deserts". Laity in Omre (2002, p. 389) described the Chihuhuan as "the largest desert in southwestern North America". The Chihuhuan Desert is a "high desert" with an average elevation of 1400 meters (Medellin-Leal in Bender, 1982, p. 323; Brown, 1994, p. 173) and ranging from 600 to greater than 2000 meters (Mares, 2002, p.112). Vankat (1979, 192) described the Chihuhuan Desert as a subtropical desert but with a climate that is more moderate than that of the adjacent Sonoran Desert. Like the Sonoran Desert, the Chihuhuan Desert was named after the Mexican in which most of the desertscrub community occurs. Though the Chihuhun Deert as a general (and vast) vegetational region (ie. Chihuhuan Desert vegetation or Chihuhuan Desertscrub) has been described on par with other vegetational units of equivalent hierarchial rank as, for example, in North American Terrestrial Vegetation (Barbour and Billings, 1988, 2000), there is not a detailed, descriptive ecological study of the Chihuhuan Desert as there is for the sister Sonoran Desert in the life-long work of Forrest Shreve (Shreve and Wiggins, 1964). Good descriptions of the Chihuhuan Desert were included in Jaeger (1957, ps. 33-49), Shelford (1963, ps. 391-394), Bender (1982, ps. 321-381), Brown (1994, ps. 169-179), and Mares (1999, ps. 112-115). Some of the best descriptions of the Chihuhuan Desert were by MacMahon in his chapter, Warm Deserts, in (Barbour and Billings, 1988, 2000).

The Chihuhuan Desert was threated herein as the Eastern Desert Scrub (Larrea-Flourensia Association) of the Larrea-Prosopis Formation of the Desert Scrub Climax as classified and described by Frederic E. Clements (1920, ps. 162-170). The climax (pre-Columbian) Chihuhuan Desert has expanded over course of the past 100-300 years (depending on geographic location) by encroachment (invasion) of the desertscrub vegetation into the semidesert grassland classified and described by Clements (1920, ps. 114-121, 144-149) Desert Plains Grassland (Aristida-Bouteloua Association) of the Stipa-Bouteloua Formation of the Grassland Climax. Relations of these two major units of vegetation were discussed below.

It is generally accepted that Forrest Shreve (1942) was the first to delineate and actually designate this desertscrub vegetation as the Chihuhuan Desert. Shelford (1963, fig. 15-1, p. 374) followed the delineation of Shreve (1942) and predicatably followed the Clementsian scheme of community organization as applied in Shelford and Clements (1939). Dick-Peddie (1993) presented a more restrictive distribution of Chihuhuan Desert Scrub (fig. 8.1D, p. 125), but as was described below this delineation must be taken in conjuction with distribution of Desert Grassland (fig. 7.1C, p. 103). Brown (1994, ps. 169-179) defined and described the Chihuhuan Desert based on the climatic delineation of the "real" Chihuhuan Desert by Schmidt (1979). The Chihuhuan Desertscrub biotic community (# 153.2) within Warm Temperate Desertland climatic zone (# 153) in the description of Brown (1994, ps. 169-179) and classification system of Brown et al. (1998, ps. 24-26) appeared to this author to furnish the most accurate description, classification, and location of the "real" Chihuhuan Desert. While the system of Brown et al. (1998) combined features of numerous classification systems beginning with hierarchial arrangement of organisms devised by Carolus Linnaeus, the biotic community basis and even most of the vegetational units, including disturbance climaxes, are the plant formation-biome concept of the Clementsian school (Brown et al., ps. 11-12). As to be expected the "real" Chihuhuan Desert coincides closely--if not identically--with the Chihuhuan Desert of Clements (1920) and Shreve (1942) who adopted the interpretation of Clements (1920) and Weaver and Clements (1938) minus the elaborate Clementsian monoclimax scheme. (The Clements-Shreve view was discussed below as a historical note.)

MacMahon (in Barbour and Billings, 1988 and 2000) cited previous workers who subdivided the Chihuhuan Desert into three regions: 1) Trans-Pecos region, the northern portion, 2) Mapimian region, the middle portion, and 3) Saladan region, the southern portion. Other workers cited by MacMahon (in Barbour and Billings, 1988 and 2000) had eight primary subdivisions of the Chihuhuan Desert Region including woodlands. In this treatment (accepted by MacMahon and used as basis for his discussion) the Chihuhuan Desert Scrub differed from the other seven subdivisions: Lechuguilla Scrub, Yucca Woodland, Prosopis-Atriplex Scrub, Alkali Scrub, Grpsophilous Scrub, Cactus Scrub, and Riparian Woodland.

In the current treatment, as in that by MacMahon (in Barbour and Billings, 1988 and 2000), all eight of these subdivision units were interpreted as part of the Chihuhuan Desert Region even though woodland was not regarded by the current author as desert (which by definition is shrubland and not woodland or forest). In other words, there was a distinction made between desert in the strict sense of arid shrubland (= arid scrub), or the desert biome, and other parts of the Chihuhuan Desert Region (a climate-defined, large expanse of land) that are not desert as delieated by vegetation. Such non-desert vegetation is nontheless in a general desert climate. For example, riparian woodland is not desert as defined by vegetation or a plant community dominated by the shrub category (it is woodland, a plant community dominated by trees and not shrubs), but riparian woodland as a corridor inside of desert scrub is part of the general or overall desert climatic region. It should be noted that the effective microclimate of riparain vegetation (scrub or woodland or gallery forest; herbaceous or woody) is not desert. It may in effect be largely aquatic (ie. effectively in the humid or the wet climatic--precipitation--zones and not in the arid zone).

Aside from aridity, which is the fundamental and only real basis for all deserts (ie. climatic or "true" desert vs. what some describe variously as "edaphic", 'physiological", etc. "deserts"), basin and range physiogrphy is the major physical-chemical--the abiotic--component that delineates the Chihihuhuan Desert. Indeed all North American Deserts--the major units of Chihuhuan, Sonoran, Mojave, and Great Basin--occur primarily within the Basin and Range physiographic province. The definitive authority for this (and all other) physiographic units in the United States of America and much of adjacent Canada and Mexico is the seminal and comprehensive treatment by Fenneman (1931, 326-395). A more recent, and also highly readable (though abbreviated), description of Basin and Range is Hunt (1974, ps. 480-535). There are a number of detailed treatments of restricted areas within the Basin and Range province ranging from the professional, mostly-for-geologists descriptions such as Gile et al. (1981) for southern New Mexico to less technical, laymen-oriented works like Maxwell (1968) and MacLeod (2002) for the Big Bend of Rio Grande area. Definitive sources for soils of the Chihuhuan Desert are the cooperative county soil surveys, but most counties within this region do not have completed surveys.

In common usage the Chihuhuan Desert is described as being in the Lower Sonoran life zone. In the more precise and strict application of the Life Zone Theory of C. Hart Merriam (1890) the Chihuhuan Desert is in the Arid or Extreme Arid Lower Sonoran Area of the Lower Sonoran Division of the Lower Austral Zone (Schmidly, 2002, ps. 42-44, 73-83). The Chihuhuan Desert falls entirely in the Chihuhuan biotic province; in fact, the Chihuhuan biotic province is restricted to the Chihuhuan Desert (Dice, 1943, ps. 7, 58-60; Schmidly, 2002, ps. 42-44).

"Demarcation of the deserts has been based on multiple criteria, including climate, geology, vegetation physiognomy, and floristic composition, with the result that the defined boundaries of the desert may vary according to the criteria selected" (Laity, in Orme, 2002, p. 380). This succinct statement summarized the inevitably difficult and always controversial delination of any desert. This problem is especially troubling in the case of the Chihuhuan Desert because there are within the overall Chihuhuan Desert Region isolated areas or units of woodland, grassland, and even alpine communities which are, of course, biomes that are distinct from the desert or arid shrubland (= desert scrubland or desertscrub) and, therefor, from the shrubland biome.

In this regard it is helpful to distinguish between the Chihuhuan Desert Region or, better yet, simply Chihuhuan Region and the Chihuhuan Desert or Chihuhuan Desertscrub. In one's "mind's eye" the smaller (in spatial-scale or land area covered) units of non-desert range vegetation within the greater or surrounding Chihuhuan Desert can be seen as "islands in the desert" much like alpine range can be seen as "islands in the sky" or grass-dominated glades as "islands in the forest".

Distinction between Chihuhuan Desert and semiarid grasslands and shrublands in the Chihuhuan Region (eg. climax scrub range communities on sand dunes not in climatic desert) has sometimes been rmade more difficult by systems of vegetation classification, descriptions of range vegetation, and mapping of native vegetation and/or natural regions. For example, the Ecoregions of Texas map (Griffith et al., 2004) included Chihuhuan Desert Grasslands and Chihuhuan Montane Woodlands (level IV ecoregions) within the Chihuhuan Deserts (level III ecoregion). Woodland and grassland biomes were included as part of a shrubland biome which is a vegetation classification-- a hierarchial-- impossibility (analogous to placing species of the reptiles class within the bird class). Furthermore two or more Chihuhuan Deserts (plural usage) were implied. The plural designations of Chihuhuan Deserts may have followed the distinction between the creosotebush-yucca faciation and the succulent desert faciation of Eastern or Chihuhuan Deserts by Shelford (1963, ps. 391-394). Griffith et al. (2004) did not include Shelford (1963) in their Literature Cited. In Ecoregions of Texas "Deserts" at the level III ecoregion did not include other "deserts" as subheadings (ie. not at level IV ecoregions thereunder). To further confuse the designation, delineation, and distinction of the Chihuhuan Desert within Texas, Griffith et al. (2004) placed the map unit, Chihuhuan Desert Slopes (level IV ecoregion), within (under) Arizona/New Mexico Mountains (level III ecoregion) yet on Ecoregions of Texas this ecoregion unit was mapped as being in (where else) Texas!

The upshot of titles for ecoregions and their hierarchial placement in Ecoregions of Texas (Griffith, 2004) is that the Chihuhuan Desert as a desert (ie.an arid shrubland) and a biotic community 1) does not have an autonomous or independent hierarchial standing and 2) it was interpreted so as to include natural plant-animal communities that are not desert at all. Simply put the Chihuhuan Desert was not recognized as a desert. Obviously, ecoregions are not synonymous with units of vegetation at any scale. In some instances there is bound to be overlap especially when both ecoregions and natural vegetation are based primarily on physiographic units (eg. Edwards Plateau).

The key perplexing problem was clearly stated by Laity in Orme (2002, p. 380) as quoted above. How (on what criteria) is the term "desert" to be defined or interpreted (ie. conceptualized)? Is desert "seen" as a climate or climatic regime; general geographic location; or biotic community based on flora and fauna that in turn are functions of climate, physiogonomy, soils, elevation, latitude and longitude, etc.? In regards to size (spatial scale), structure, and function, is a "desert" (in this case, the Chihuhuan Desert) an ecosystem, landscape or ecoregion? What is the relative level of spatial scale for any one or all of these?

In this publication devoted to range cover types, emphasis was-- by definition and necsssity-- laid on biotic community with animal species, including livestock, primarily a reflection of the plant component of habitat or the management unit. Given that rangeland and forest cover types are dominance types these are of inherently based on vegetation. These are recognized and, usually, published units of vegetation. Most of them, especially rangeland cover types, are climax or high seral stage vegetation. Cover or dominance types are based on dominant plant species, physiogonomy, general botanical composition, and plant community structure. Therefore in coverage of the Chihuhuan Desert herein this climatic, topographic, edaphic, geographic, floristic unit was described primarily as range vegetation and secondly as a range biotic community based on climax dominant plants, botanical make-up, and physiogonomy.

In the following coverage Chihuhuan Desert was not treated as an ecosystem, landscape or ecoregion. Published FRES ecosystem designations, Kuchler potential natural vegetation titles and numbers, SRM cover types, SCS/NRCS range sites, and Brown et al. series were given when such existed for the climax or disclimax vegetation shown. Level IV ecoregion titles from Ecoregions of Texas (Griffith et al., 2004) were included with examples of range cover types in the Chihuhuan Desert per se (strictly defined as explained in the succeding paragarph) when photographs of Chihuhuan Desert vegetation and landscapes were from Texas. Texas examples of this desert range vegetation were not arranged according to mapped Texas ecoregions however because ecoregions did not always coincide with long-standing, widely accepted units of vegetation or biotic communities (biomes, associations, or range dominance types).

Range cover types within the greater Chihuhuan Desert Region (= general Chihuhuan Region) that are in biomes other than the shrubland biome were treated as the recognized and described range cover type--Society for Range Management (SRM) rangeland cover type or Society of American Foresters (SAF) forest cover type--under the appropriate biome. For example, the oak-pinyon pine-juniper cover type (SRM 504, SAF 239) was placed under the biome heading of Woodlands and Forests. Cover types of riparian vegetation, as for example Cottonwood-Willow SAF 235, within the surrounding Chihuhuan Desert were likewise covered under the appropriate biome (Woodlands and Forests for SAF 235) and not treated as part of the Chihuhuan Desert. The rationale for this placement was viewed as self-evident. Gallery forests (forest cover types) that developed along streams in the desert are forest (forest vegetation) and not desert (not arid shrubland). Likewise (though perhaps less obvious), shrublands (cover types made up of shrubs; shrub vegetation) along watercourses, including major rivers like the Rio Grande (Rio Bravo), are shrubland but not desert shrubland (not desert, not desert vegetation). Some riparian vegetation may even be aquatic shrubland dominated by hydrophytes like willow (Salix spp.). Scrub plants may be phreatophytes--obligate or facultative--like various woody legumes of such size and/or multi-stemmed habit as to be shrubs rather than trees.Thus mesquite-dominated bosques (SAF 242) were treated logically and to minimize confusion under both the Willow and Riparian and the Miscellaneous chapters of Shrubland.

Distinction between Chihuhuan Desert and semidesert grassland within the Chihuhuan Region (hence, sometimes shown as Chihuhuan Semidesert Grassland) is the most problematic separation of range vegetation (and of biomes) given the conterminous (or closely proximate if not adjacent) occurrence of these two range communities. The "semidesert (Chihuhuan) grassland" (as it is often shown) is second in areal extent only to Chihuhuan Desert per se and vast acreages of these two widespread range communities form a natural vegetational mosaic within the Chihuhuan Region. Frequently there is an ecotone between Chihuhuan Desert and semidesert grassland with this transition zone being a climax desert shrub-grass savanna with creosotebush (Larrea tridentata), the zonal or regional dominant of the Chihuhuan Desert, and other shrubs like ocotillo (Fouquieria splendens) scattered in an understorey "sea" of mostly cespitose grasses, especially gramas (Bouteloua spp.). This could even be viewed as a creosotebush-shrub steppe especially on bajadas (benches) and foothills above the desert plains of basins.

Shreve (1917, ps. 122-123, pl. 3) appears to have been the first to publish designation of this vegetational region as a "desert-grassland transition". Clements (1920, ps. 145, 162, 163, 169) described this same (more-or-less) vegetation zone as "a broad transition region between the desert scrub and the Aristida-Bouteloua grassland" (the latter unit was Clements' major association of the "desert plains grassland climax"), "the broad transition between the desert scrub and the grassland" and a "line of contact with grassland or other scrub communities, and thus forming a broad ecotone of mixed or alternating communities" being thus "... given to forming parks or savannahs with grassland..." The Chihuhuan Desert (Clements' "Larrea-Flourensia association" of the "eastern desert scrub climax") was later interpreted as "a desert-plains savanna" wherein desert shrubs "... largely replaced the grasses in consequence of overgrazing and of fire, also to some extent" (Weaver and Celemnts, 1938, p. 537). Shreve (1942) became the first published (at least in the referred literature) formal recognization of much of this zone of range vegetation as the Chihuhuan Desert.

Dick-Peddie (1993, ps. 106-109, 131-132) devoted much of his discussion of what he denoted as Chihuhuan Desert Scrub and Desert Grassland to the transitional (= ecotonal) nature of these often contiguous natural plant communities. Dick--Peddie (1998, ps.106, 131-132) quoted and concurred with Shreve (1942) that much of what appears as Chihuhuan Desertscrub is "... a desert-grassland transition" and "... part of the Desert Grassland Transition Region rather than part of the desert". Dick-Peddie (1993, p. 131-132) concluded: "It is very difficult in the field to tell whether an area is occupied by an original (pre-1850) or a recently established Chihuhuan Desert Scrub vegetation". Much of current range vegetation that is by all physiogonomic appearances and floristic attributes Chihuhuan Desert is in reality a "... new Chihuhuan Desert Scrub".

Historical note: Students of the history of ecology should find it interesting-- and perhaps revealing if the entire sequence of reference and the designation of "transition" as used by Forrest Shreve and F.E. Clements was known. It is (was at the time) known that these two premier ecologists, in spite of what seemed to be amicable occasional relations, did not care for each other personally and disagreed strongly (and publicly) with some of the conclusions and theories of one another. Shreve rejected Clements' model of a deterministic climax, and kept his personal dislike of Clements "just below the surface", With regards to "desert-grassland transition" and "[a] region intermediate in character..." of Shreve (1917), Clements (1920) in Plant Indicators, his second massive monograph, added the adjective "broad" in his longer descripition of this vegetation. In Plant Indicators Clements (1920) did not cite Shreve (1917). Clements (1920) did cite eight other published works of Shreve including one published in 1917. Did Clements (1920) inadvertently (carelessly but innocently) omit the Shreve (1917) map and brief descriptions of 18 vegetational areas? How could Clements (1920) have overlooked such a recent publication that dealt with the same vegetation and its description (the identical focus of ecological study) by a co-worker? Both ecologists were at the same work location, the Desert Laboratory of the Carnegie Institution in Tucson, Arizona. The essential question is, Did Clements (1920) commit plaigarism? Even if Clements and Shreve had discussed among themselves the transition or ecotone between semiarid grassland and desert, and even if Clements suggested the term prior to publication of either man's work (and there is no evidence for this or even of such discussion), Shreve (1917) beat Clements into scientific press with the term such that it should have been cited by Clements (1920). From the standpoint of range vegetation the importante fact was that two of the leading vegetation scientists of the pioneer era eached identical views regarding the spatial and successional relationship between semidesert grassland and desertscrub in the Chihuhuah Region.

In recent decades (but probably beginning three to four centuries ago) boundaries and pattern of this patchwork of Chihuhuan Desert and a semidesert grassland that extends into the Sonoran Desert, two range communities of climax (potential natural) vegetation, have been greatly altered by action of non-native (invasive) Homo sapiens, such that the area and extent of Chihuhuan Desert has greatly expanded with a commensurate decrease in that of semidesert grassland. Woody species of the Chihuhuan Desertscrub have invaded the virgin Chihuhuan Grassland with the result being an exponential-like expansion of the pre-Columbian Chihuhuan Desert due to various anthropogenic impacts: overgrazing (including that by the freighting and other transportation activities), field crop and orchard agriculture (the aborted remains thereof), mining operations, altered fire regimes, and urbanization which perhaps were coupled and synergistic with climatic shifts or ususually severe shor-term meterological perturbations especially drought).

Some recent workers such as Dick-Peddie (1993, ps. 107) followed the traditional view of early range investigtors and pioneer ecologists (including Forrest Shreve, Frederic Clements, and John Weaver) and concluded that overgrazing by livestock was the main factor (albeit only one of several) for this desertification. Overgrazing as the principal cause of desertification resulting in expansion of Chihuhuan Desert into semidesert (Chihuhuan) grassland was proposed by Clements (1920, ps. 145, 148) and was soon thereafter incorporated into the first edition of the famed textbook, Plant Ecology (Weaver and Clements, 1929) and then in the second edition (Weaver and Clements, 1938) as quoted above. Other scientists were more guarded (ambiguous may be more fitting) in their conclusions, but still felt that overgrazing was a pivotal disturbance in de facto conversion of semidesert grassland into Chihuhuan Desert.

One of the locations in which this invasion of desertscrub into semidesert grassland has been most intensively (and consistently) studied has been the Jornada del Muerto (Spanish for Journey of Death) Trough, a system or series of basins through which most of the Rio Grande flows and the famed El Camino Real (see below) once followed. Gibbens et al., (2005) made an analysis of vegetational changes in the Jornada Trough of southern New Mexico based on comparison of surveyors' general descriptive accounts to current vegetation. This was an update of the classic study by Buffington and Herbel (1965). Gibbens et al., (2005, p. 665) cited several investigators (including Buffington and Herbel, 1965) and observed: "Heavy grazing has been almost univesally cited as a cause for reduction of grass cover". Gibbens et al.(2005, ps. 665-666) then discussed other contributing (or potentially contributing) factors including droughts, suppression of fires, climate change, and elevated carbon dioxide concentrations. They concluded that there was no evidence in their data "to support or refute fire, climate change or CO2 as causative factors" whereas "[t]here can be little doubt that heavy grazing in the latter part of the 19th and early 20th centuries helped trigger the encroachment of shrubs" (Gibbens et al. 2005, p. 665-666).

Interestingly, Gibbens et al. (2005, p. 666) then posited the question, What would the vegetation they studied "...look like today if it had been used at moderate grazing intensities or had not been used by domestic livestock"? Their conclusion: "We believe shrubs would have increased under moderate grazing or if not used by domestic livestock". Gibbens et al. (2005, p.666) reached this conclusion because at the time of land survey (subdivision into section lines) in 1858 invading shrubs were already well-established and widely, though sparsely, dispersed on the study area (public domain range that later came under management of the U.S. Forest Service and then Agricultural Research Service as the Jornada Experimental Range and of New Mexico State University as the College Ranch).

This long-term study location (the two experimental units are conterminous) lay along the historic El Camino Real (Spanish for Royal Road or Kings Highway) and, more specifically, the El Camino Real de Tierra Adentro, (Royal Road to the Interior) that connected Santa Fe to Mexico City through El Paso del Norte. The Camino Real is the oldest European road in North America. The entire route of El Camino Real was first used in 1598 and this original "mother road" continued to be a major trade and travel route for more than the next 300 years. Gibbens et al. (2005, p. 665) noted that mesquite (Prosopis glandulosa) became established along El Camino Real "at a very early date" with mesquite seed dissemination by livestock was "undoubted one reason for the wide distribution of mesquite". Gibbens et al. (2005), however, stopped short of the logical extension that the pre-Columbia vegetation had already been impacted and modified by general heavy traffic along El Camino Real (and in the general vicinity of El Paso del Norte and the upstream Rio Grande Valley) for 260 years before the General Land Office survey that described the then-existing vegetation. Palatable (and eventually perhaps not-so-palatable) range grasses, forbs, and shrubs along the Camino Real provided fuel for numerous beasts of burden (horses, asses, mules, oxen) central to the commerce and traffic of this North American major trade route. "Great herds of sheep, goats, cattle, and horses driven along the route" even before establishment of commercial ranching operations (Buffington and Herbel, 1965, p. 145).

In addition to stresses of defoliation exerted on heavily grazed range plants, overgrazing also removed fuel for naturally ignited fires thereby changing fire regimes different from those under which range plants and communities evolved. Furthermore, over time mere human foot traffic as well as camp fire-building, food gathering and toilet activities, and recreational diversions whould like have had major adverse impacts on native flora and fauna and soil in a region receiving eight to ten inches of average annual precipitation. Any who doubt this nongrazing adverse human impact need but to visit extensive portions of the Chihuhuan and Sonoran Deserts being devastated by "wetbacks" crossing the Mexico-United States border even as this was written.

In light of and consistent with conclusions reached by Gibbens et al. (2005) it would seem undeniable that shrubs on both Chihuhuan desert and semidesert grassland range communities had likely already been increasing faster than grasses and other palatable herbaceous plants-- so as to initiate a human-induced woody invasion-- for more than two and a half centuries (a quarter of a millenium) before United States land surveys furnished a by-then biased baseline of former virgin vegetation. The Jornada Experimental Range is to the east of El Camino Real (Buffington and Herbel, 1965, 145), but the Jornada del Muerto stretch of El Camino Real passed directly through a large portion of the New Mexico State University College Ranch that joins the Jornada Experimental Range. It would be safe assumption that some movement of and ranging by livestock along the Jornada del Muerto spanned part of Jornada Experimental Range at least periodically (enough to disperse mesquite seed).

At time of the 1858 land survey, surveyors noted that famers from the Rio Grande Valley were already pasturing cattle on the Jornada (Buffington and Herbel, 1965, p. 145), but such use would have been "only seasonal or intermittent" prior to the 1880's (Buffington and Herbel, 1965, p. 160). The numerous outstanding investigations of changes in range vegetation that were conducted on the Jornada Experimental Range and the College Ranch (most were cited in Gibbens et al., 2005) documented brush invasion on semidesert grassland (probably semidesert grassland -Chihuhuan Desert ecotone) range that could be assumed to have been in some state of retrogression, some departure from virgin conditions (pre-Columbian vegetation), when United States land surveys provided the earliest available records of range conditions in the Jornada del Muerto area. Any such range degradation, however, would have been less than on range nearer the main trail of Camino Real. As such, changes in this range vegetation documented by range reserchers like Gibbens et al. (2005) must be regarded as conservative. If the ecological baseline had been established on pre-Camino Real (ie. climax) vegetation the extent of brush invasion and degree of range depletion would have been, in all likehood, even more drastic (greater departure from potential natural range vegetation).

Research from the Jornada Experimental Range was probably not typical of vegetation changes on semidesert grassland closer to the main route of Camino Real. Most of the great decline in grass and commensurate increase in brush on the Jornada Experimental Range took place after the Genereal Land Office survey in 1858. Survey descriptions of "good grass" cover declined from greater than 90% to less than 25% of the experimental range from 1858 to 1963. (Buffington and Herbel, 1965, p. 162). In this context, it is unfortunate that so much of what is known about range vegetation on semidesert grassland and Chihuhuan Desert-semidesert (Chihuhuan) grassland ecotone was determined from ranges on the Jornada Experimental Range and College Ranch on which heavy livestock use began later than on less remote semidesert grassland range (eg. range closer to El Camino Real, villages, and other areas more typical of earlier Hispanic grazing use).

Long before Spanish-Mexican use of the land American Indians may have contributed to the brush problem, especially of mesquite. Buffington and Herbel (1965, p. 161) felt that some of the mesquite sand dune disclimax range might trace back to farming by Pueblo Indians and, following pueblo abandonment, to transport of mesquite legumes as food by nomadic Indians

With regard to human actions that induced --directly or indirectly-- desertification on the Jornada de Muerto rangeland it should be emphasized that while some of the livestock along El Camino Real were being trailed to markets in Mexico most of the livestock that would have been influential in initiation of range retrogression were draft animals of freighters (mule-skinners and bull-whackers) or travelers and not the range livestock of cattlemen and woolgrowers which came later. The latter appeared on the scene almost 300 years after much of the range extending on either side of a wide corridor trade route had been subjected to what almost assuredly had to have been overgrazing and overbrowsing.

It should be remembered that these trails were not of the extent or size made by a set or two of deeply worn wagon ruts (or even of the width of four-lane interstate highways and their rights-of-way). Instead these priminitive travel corridors were linear overgrazed ranges consisting of many individual trails of different kinds (trails beat by cattle and horse hooves, human foot paths, etc. in addition to ruts cut by carts and wagons). Widths of such trails eventually expanded to natural boundaries like the Rio Grande and San Andreas Range as humans and their animals forrayed ever farther out in search of forage, limited fire wood or dung, game meat, and whatever natural provisions they could find.

Preston and Esquibel, complete with photographs by Christine Preston, (1998) described much of what remained of the route (land, plants, people, and customs) of El Camino Real. After having traveled 13,000 miles by automobile and 350 miles horesback (in addition to distance walked), and over a four year period of time, these authors (Preston and Esquibel (1998, p. 6) described this oldest European road in North America as a "braided trail". Those who have followed trails established by cattle or sheep or who are familar with the geology of braided streams should understand this analogy as applied to a human road. "There were shortcuts and longcuts, rough trails for packtrains and smooth roads for caretas, drier routes and wetter routes, harsher routes and gentler routes. Those in a hurry with light wagons often took different routes from those with more time or heavier cargo..." Preston and Esquibel (1998, p. 6).

Some of the deeper ruts along the more established "braids" of the trail are still visible (Preston and Esquibel (1998, p.25, plates 5, 34, 48) while other portions have been lost in antiquity (or became the route of highways including part of Interstate 25). In essence it is impossible to know which locales-- including specific experimental plots, exclosures, and relict areas--where impacted (and to what degree) by nearly 300 years of travel along El Camino Real.

Presence of some large areas of what was relict vegetation at time of General Land Office surveys (260 years after establishment of a heavily used trade route) furnished a reference for potential natural vegetation on at least some range sites, but the presence of shrubs at some locations-- those at densities and cover induced by human disturbance-- attested to the presence of a brush "foothold" or "beachhead" leading to retrogression marked by prounouced woody plant invasion and, ultimately, a "stranglehold", a threshold of range degradation for which there was no reversal without expensive applications of intensive range improvement practices that cannot be justified on basis of agricultural productivity. Sadly, it was the surveyers' records of shrubs and not of "prairie" , "grama grass", or "very good grass" cover (Gibbens et al. 2005, 655-656) in the 1858 land survey that proved to be of most ecological relvance.

The question Gibbens et al. (2005, p. 666) asked regarding range types and composition and structure of the range vegetation if there had been proper livestock grazing or no livestock grazing could be applied only to Jornada del Muerto range after 1858, the earliest time for which they had descriptions of the vegetation. A more important question-- perhaps the most important question-- would have to begin with the first livestock grazing under Hispanic culture rather than being limited mostly to the later (and shorter) Anglo ranching era. This author suspects that the answer to that question would be the opposite. No, the brush invasion would have much less and black grama and tobosa would still stand "stirrup high" over a much larger area of the semidesert grassland if the whiteman had practiced proper grazing management with or without "little ice ages", droughts, increased atmospheric carbon dioxide, etc.

In other words, the process of anthropogenic desertification-- of converting semidesert grassland into desertscrub and expansion of the Chihuhuan Desert-- must be assumed to have been underway for two to three centuries before Anglo races invaded and prevailed over the occupying Hispanic ones (just as the latter summarily destroyed the American Indian civilizations that they encountered while carrying the Cross and the sword en route to the Seven Cities of Cibola). How many droughts occured during the nearly three centuries of intensive range use by pre-Anglo Europeans? How many lightening strikes failed to mantain the natural fire regimen due to absence or, at least, scarcity of fuel that had been burned in equine stomachs, oxen paunches, or as tender for camp fires? What impact did extirpation of native grazers and browsers along Camino Real have on range vegetation? What effects (if any) on the range could be attributed to adoption of the Spanish horse by unconquered Plains and Desert Indian tribes? Did rodents and lagomorph populations change with on-going range retrogression and, if so, did this have an influence on species composition and structure on the range plant community? .

Readers desiring more information on El Camino Real were referred to Moorhead (1958). This classic in American history explained how the Santa Fe Trail developed over time to link up with El Camino Real (a freight wagon prototype of national highways). It is not without historic basis of fact that truckers and cargo handlers (and one of their unions complete with horses-head emblem) are still referred to as "teamsters" (from the original Team Drivers International Union). One interesting agricultural impact of El Camino Real (and its later connecting Santa Fe Trail) was its role in establishing mule breeding in Missouri and adjacent states and territories. Importance of livestock grazing along El Camino Real can be appreciated by reading throughout Moorhead (1998; see Horses, Mules, Oxen, Pack Trains in index).

In other parts of the semidesert grassland-desert ecotone and in range regions as far east as the Gulf coastal prairies and marshes, Rio Grande or South Texas Plains, and Edwards Plateau and as far west as the bunchgrass prairies of the California Central Valley and Coast Ranges the Roman Catholic Church (via Spain) had laid the foundations of the soon-to-be great range livestock industries in their embryonic form at far-flung missions (Lehmann, 1969, ps. 9-22; Jordan, 1993, 65-169 passim). This was at least a century before "gringos" wrest much of the livestock ranges away from Europeans radiating from the Mediterranean Region. The Spanish-Mexican frontier was taking its toll on its ranges at the same time that the predominant Anglo-Saxon frontiers along the Atlantic Ocean and eastern Gulf of Mexico were taking theirs on theirs. Anglo stockraising was as abusive of the range, if not more so, than Spanish-Mexican grazing, but it came much later relative to the history of European use of the grassland and desert ranges of southwestern North America. Analysis of range deterioration under European occupation must include the entire span of such use of range resources.

Regardless of the date and timing and whatever the exact combination(s) of factors brought in by European man, desertification was--according to prevailing tradational and conventional scientific views--primarily human-induced such that much, though probably not most, of what is recognized as the Chiuhuan Desert is not natural, native, climax, or whatever but disclimax (disturbance climax) desert. By definition the Clementsian disclimax is for practical purposes (including management) permanent in human time scale. Consistent with this Clementsian concept Brown et al. (1998, ps.34, 40) recognized this disclimax vegetation as comprising a principal plant-animal series (fifth level) within a biotic community (fourth level). Thus this disturbance climax was interpreted as being the here-and-now (and the for-some-time-to-be) climax. As such proper management practices ahould be based on and applied to this range vegetation as if it were (because it now is) climax.

Brown et al. (1998, p. 40, 41) placed this mostly man-made disclimax (Shrub-Scrub Disclimax Series) within the Chihuhuan (Semidesert) Grassland biotic community and not in the Chihuhuan Desertscrub biotic community within and for which they did not include any disclimax. In a preceding publication Brown (1994, figure 101 and 105 on ps. 175 and 178) included Chihuhuan semidesert grassland disclimaxes in the Chihuhuan Desertscrub noting that designation as to whether grassland or desert "is moot". The more precise or specific classification (the classification system as such ) that incorporated this disclimax was the more recent publication of Brown et al. (1998). Based on Brown et al. (1998) classification the primarily human-induced disclimax Chihuhuan Desert (Shrub-Scrub Disclimax) could logically have been included in this on-line publication under the Semidesert Grassland chapter, but the current author followed the initial format of Brown (1994) and treated the disclimax (man-made) scrubland herein under Chihuhuan Desert because it is desert (= arid shrubland) and no longer grassland.

Of as much, if not more, concern to many range scientists and conservationists--and even more pertenient to ranchmen and other range mangers--than expansion of the Chihuhuan Desert into Chihuhuan Semidesert Grasslands was loss of the herbaceous (primarily grass) layer from natural Chihuhuan Desert vegetation. It seems likely, and has generally been accepted, that the virgin vegetation of all but the most xeric habitats of the Chihuhuan Desert had a well-developed and, in most years, a vigerous understorey dominated by grasses with even more species, though less cover, of forbs and "half-shrubs"(suffrutescent species). Reduction or loss of the herbaceous understorey meant depletion of range forage (the grazing resource of the desert range ecosystem) and this range retrogression was one of the major stages in desertification. The same pattern of loss of plant vigor followed by reduction of cover and density of the more palatable browse plants through overbrowsing accompanied that of palatable grasses and forbs (depending on species and mix of range animals). Ultimately physical and chemical soil loss (erosion) followed losses of soil-protecting plant cover with a consequent reduction in water-holding capacity and dysfuction of hydrologic processes of the soil. The proverbial "downward spiral" was well under way with stresses of normal perturbations (eg. droughts, extreme temperatures) made worse by misuse of the range.

In discussing the photographs below disturbance climax (anthropogenic) Chihuhuan Desert was distinguished from natural (= climax) Chihuhuan Desert to the extent possible based on published criteria, especially indicator species (Dick-Peddie, 1993). The Chihuhuan Desert-semidesert (Chihuhuan) grassland ecotone was interpreted literally (ie. in the restricted rather than broad or generic usage) so as to view transition desertscrub-grassland vegetation strictly as range plant communities the obviously unique assemblage of which was composed of dominant, associate, and indicator species from adjacent desert and grassland range. These composite plant communities were distinct and unique as to physiogonomy, species composition, and structure. They developed as border or boundary vegetation and thus occurred as relatively long and narrow plant communities appearing as if natural "fencerows" along adjoining plains, basins, bajadas, arroyos, foothills, etc.

4. Ocotillo (Foquieria splendens)- Ocotillo or coachwhip is in it's own family, Fouquieriaceae. It is one of the readily distinguishable and characteristic species in southwestern North America. It grows on semidesert grasslands and on the Chihuhuan, Sonoran, Mojave, and Colorado Deserts.

Hudspeth County, Texas. June.

5. Inflorescences of ocotillo- In typical desert fashion ocotillo is one of the generic "resurrection" plants. Ocotillo is one of the ultimate opportunist species that goes in and out of dormancy as atmospheric and edaphic conditions warrent. This shrub may appear to be "dead as a doornail" only to put on new leaves and then burst into brilliant, nearly breath-taking, diffuse bloom. Two examples of the ocotillo inflorescence were shown for instructional and aesthetic purposes.

Drought-deciduous leaves and spines along shoots were also presented.

Hudspeth County, Texas. April.

7. Monahan's Sand Dunes- Coppice shrub from of mesquite. This soil is high in gypsum and various gypsophils are associates. Climax vegetation at edge of salt lakes This is one of the more unique vegetational cover types in the Trans-Pecos section of Basin and Range physiography. Hudspeth County, Texas. June. FRES No. 33 (Southwestern Shrubsteppe Ecosystem). Subunit of K-53 (Trans-Pecos Shrub Savanna) too small to be mapped at Kuchler scale. SRM 729 (Mesquite); as the noted in SRM description this vegetation type "varies widely", and this is a "pure" form of scrub mesquite. This vegetation was interpreted as climax or potential natural vegetation-- not a brush invasion of deteriorated semidesert grassland-- due to local soil conditions (ie. an edaphic climax). Mesquite Series of Brown et al. (1998). Chihuhuan Deserts- Chihuhuan Basins and Playas Ecoregion, 24a (Griffith et al., 2004).

The mesquite scrub disclimax on severely depeted semidesert grassland in the Chihuhuan Region is different from the climax vegetation shown here. Degraded semidesert (Chihuhuan) grassland that became disclimax Chihuhuan Desert largely through human influence was classified by Brown et al. (1998, p. 40) as the Shrub-Scrub Disclimax Series, 143.15. The mesquite scrubland form of anthropogenic (man-made) Chihuhuan Desert develops into mesquite-dominated sand hummocks that can ultimately form coppice sand dunes known as mesquite coppice duneland (Buffington and Herbel, 1965, p. 144). That disclimax mesquite scrub is the most advanced stage of semidesert grassland retrogression It is a brush invasion that in effect is permanent in human time scale, at least without massive financial outlays for intensive range eimprovement practices. That range cover type was treated below under the heading, Disclimax Chihuhuan Desert.

9. Fourwing saltbush flat- Alkali sink adjoining the one shown in the immediately preceding photograph of an alkali sacaton-fourwing saltbush-pale wolfberry savanna. In these two photographs an alkali basin (portion of same sink shown above) supported a fourwing saltbush consociation. According to Allaby (1998) consociation was a term used in the Anglo-American tradition of Plant Ecology to mean a plant community composed exclusively (at least, mostly) of a single dominant species . Consociation came into widespread through the influence of F. E. Clements. This alkaline basin was conterminous with the shifting dunes of gypsum (hydrous calcium sulfate) that are the centerpiece of White Sands National Monument. (Gypsum dunes were visible in distant background.)

It was not determined if absence of herbaceous species (eg alkali sacaton) was natural or result of past unnatural (especially anthropogenic) disturbances. This might have been a disturbance climax or, alternatively, it might have the potential natural vegetation (= climax vegetation). It was obvious that this range plant "community" (?) was a "monoculture" in Nature with such absence of any other vascular plant species that would please the most avid advocate of clean-tillage, monocultural cropping systems. Fourwing saltbush is a dioecious species. A female plant heavy laden with fruit was featured in left foreground of second slide. As an inadvertant lesson, a little individual plant of creosotebush (Larrea tridentata) was at the "right hand" of the female saltbush. Creosote- bush is the single most dominant and influencial plant species across both the Chihuhuan and Sonoran Deserts as well as much of the Mojave Desert.

On this gypseous-- and probably saline-- habitat fourwing saltbush was a glysophile (= glysophyte)- and probably also a halophyte (= halophile).

White Sands National Monument, Otero County, New Mexico. June, early estival aspect. FRES No.30 (Desert Shrub Shrubland Ecosystem). Variant of K-34 (Saltbush-Greasewood). Fourwing saltbush variant of SRM 501 (Saltbush-Greasewood). Dick-Peddie (1993) did not offer a Saltbush Series for Chihuhuan Desert Scrub. Saltbush Series 153.26, of Chihuhuan Desertscrub, 153.2 (Brown et al., 1998). Chihuhuan Deserts- Chihuhuan Basins and Playas Ecoregion, 24a in foreground; Chihuhuan Deserts- Gypsiferous Dunes Ecoregion, 24g in background (gypsum dunes) of Omernik and Griffith (2006).

10. Vegetation of a salt playa- A stand (another consociation as presented in the previous two photographs) of alkali sacaton with a few individual plants of pickleweed (Allenrolfea occidentalis) at edge of a playa. Playa or, often, playa lake was defined by Wilson and Moore (1998) as: "A shallow, intermittent lake in an arid or semiarid region, covering or occuppying a playa in the wet season". Jackson (1997) defined playa: "An ephemeral lake that upon evaportation leaves or forms a playa". Playas are common in the Southern Great Plains and, though less common, reasonably plentiful in the Trans-Pecos Basin and Range.

Shown in these two slide was an example of an arid saline playa, part of the Texas Salt Basin. This geologic phenomenon was location of the infamous Texas Salt War. (History buffs may wish to refer to The Handbook of Texas, a version of which is available "online").

Main interest of rangemen is the native vegetation around edges of this saline playa. Second photograph was of extreme outer edge of playa (a salt flat) and "frontier outpost" of range vegetation which was limited to alkali sacaton and pickleweed. The general range plant community was very similar to that the alkaline gypsum sink at White Sands National Monument presented previously. However, the FRES ecosystem designation varied depending on whether native vegetation was grassland or shrubland (ie. Grassland Ecosystems vs. Shrubland Ecosystems[Garrison et al., 1977, ps. III-IV]). Range vegetation shown here could be interpreted as grassland in form of an alkali sacaton flat or, alternatively, a shrubland with presence of the shrub, pickleweed, interpreted as constituting a savanna. Not much feed on this range, but there would be no time wasted putting out stock salt (other minerals?).

Hudspeth County, Texas. June. FRES No. 33 (Southwestern Shrubsteppe Shrubland Ecosystem). No really appropriate Kuchler unit: variant of K-53 (Trans-Pecos Shrub Savanna) was closest. No SRM unit applied either. No biotic community given by Brown et al. (1998), but there should be a Sacaton Series under Chihuhuan Desertscrub,153.2, and/or Chihuhuan (Semidesert) Grassland, 143.1. Chihuhuan Deserts- Chihuhuan Basins and Playas Ecoregion 24a (Griffith et al., 2004).

It was explained in the introduction to the Chihuhuan Desert (named after the Mexican state of Chihuhua) that there were many criteria used in defining "desert" and, as such, various interpretations as to what was (and was not) actual Chihuhuan Desert or the desert per se within the larger Chihuhuan Region. Clearly there are grasslands, woodlands, and forests within the greater or general land area encompassed by a similar climate of the Chihuhuan Region. In fact, any vegetation in this overall area that is not shrubland is not desert (deserts are by definition shrubland, scrubland, scrub; meaning that the dominant plant form is shrub, smaller woody plants typically with multi-stems and shorter-than-tree-height). When defining or distinguishing a unit of vegetation as to range cover type-- by undeniable definition, a grazing land dominance type-- the dominant plant species is the one final criterion. In case of the Chihuhuan Desert (plus the Sonoran and much of the Mojave Deserts) the regional dominant is creosotebush (Larrea tridentata= L. divaricata= L. mexicana= L. glutinosa). With but few exceptions (or possible exceptions) such as big sagebrush (Artemisia tridentata) there is no other woody species that domintes so vast a region as does creosotebush.

Over what has become known as the Chihuhuan Desert tarbush or hojase (Flourensia cernua) is a distant, runner-up co-dominant with creosotebush whereas in the Sonoran Desert there are two co-dominants: the suffrutescent shrubs (halfshrubs), white bursage or burroweed or hierba del burro (Ambrosia dumosa= Franseria dumosa) and triangle or triangleleaf bursage (A. deltoidea= F. deltoidea). Honey mesquite (Prosopis galndulosa of the two varieties, P. glandulosa var. glandulosa and P. glandulosa var. torreyana; P juliflora and P. chiensis were previous and apparently erroneous species names) is the desert shrub that ranks second to creosotebush as a co-dominant over the entire combined Chihuhuan-Sonoran Deserts super-region. Before nomenclature and designation by Shreve (1942) of the Chihuhuan Desert and Sonoran Desert Clements (1920, ps. 162-177), in the heirarchy of his monoclimax theory, designated the Eastern Desert Scrub (Larrea-Flourensia Association) and the Western Desert Scrub (Larrea-Franseria Association) which together comprised the Desert Scrub Climax (Larrea-Prosopis Formation). Subsequent descriptions, including those of Shreve (1942), essentially followed Clements' original outline.

Thus it is that creosotebush, tarbush, and honey mesquite in combination (various combinations) define and deliminate the Chihuhuan Desert. It appears that there is presently a general consensus among students of Chihuhuan Desert vegetation that all of these major shrubs have since arrival of whiteman increased in cover and acreage of land dominated. In one of the longest term studies of vegetation change in the Chihuhuan Region (both semidesert grassland and desertscrub) Gibbons et al. (2005) that increase has been greatest for mesquite and least for tarbush. On some sites such as on bajadas much of the increase in creosotebush came at expense of tarbush (Gibbons et al., 2005, p. 659).

This woody plant invasion in the general Chihuhuan Region (again, this includes both semidesert grassland and Chihuhuan Desertscrub) was discussed in introduction of Chihuhuan Desert. Most of the expansion of Chihuhuan Desert into former areas of semidesert grassland (desertification) was anthropogenic--and, probably, certainly unfortunately, permanent in human time scale-- such that this "man-made" desert is disturbance climax (disclimax) desert. Brown et al. (1998, p. 40) entitled this unit as the Shrub-Scrub Disclimax Series.

In this section of the Chihuhuan Desert emphasis was on desert scrub (= arid range shrubland) in which creosotebush is the dominant or least a major and defining species and, frequently but secondly, mesquite is an important species. Different range cover types and, when known, range sites were presented and described. These units of range plant communities included: 1) climax or potential natural vegetation and 2) disclimax vegetation. For some examples of vegetation it could not be determined which of these two conditions applied, and from a management standpoint such distinction was sometimes irrelevant. Nonetheless, efforts were made to present native vegetation in climax conditon consistent with that same tendency for published rangeland (and many forest) cover types (Eyre, 1980; Shiflet, 1994) and with expressed purpose for other published names and descriptions of natural vegetation such as those by Kuchler (1964) and Garrison et al. (1977). This was explained in this publication in Range Cover Types under Introduction.

11. Chihuhuan Desert landscape at a glance- Physiography of land in the Basin and Range province and physiogonomy of Chihuhuan Desert vegetation as seen from a bajada (bench) across a basin to the next mountain range. Creosotebush was sole dominant, but other shrubs included ocotillo, pricklypear (mostly Texas prickly pear [Opuntia englemannii var. englemannii]), and leatherstem, dragon's blood, or sangre de drago (Jatropha diocia). There was not an herbaceous understorey although grass species included remnant black grama (Bouteloua eriopoda), chino grama (B. ramosa), perennial threeawn (Aristida purpurea complex), and fluffgrass (Tridens pulchellus= Erioneuron pulchellum). Many but not all of these grass plants were near shrubs where presumedly there was some protection from grazing animals.A nice specimen of leatherstem was in extreme left foreground.

This was degraded range, but general appearance of vegetation and topography was instructive. Presence of herbaceous layer in this same kind or type of desert scrub community was included later in this section.

Presidio County, Texas June, estival aspect. FRES No. 30 (Desert Shrub Shrubland Ecosystem). K-35 (Creosotebush). SRM 508 (Creosotebush-Tarbush). Creosotebush-Tarbush Series, 153.21, of Chihuhuan Desertscrub 153.2 (Brown et al., 2004, p. 41). Chihuhuan Deserts- Low Mountains and Bajadas Ecoregion 24c (Griffith et al. 2004). Predominately a Gravelly range site in Desert Shrub vegetation zone.

12. Sample of Chihuhuan Desert vegetation- A "photo-quadrant" of the plant community on the range shown in the preceding photograph. Creostebush was dominant, but ocotillo, leatherstem, Texas pricklypear, tasajillo or pencil cholla (Opuntia leptocaulis), whitethorn acacia or largancillo (Acacia constricta), honey mesquite, fluffgrass, perennial threeawn, and chino grama were also present. Desert pavement on soil surface was pronounced.

Presidio County, Texas. June, estival aspect. Ecosystem and range vegetation units were presented in the preceding slide.

13. What a big difference a little grass makes- Paired photographs of vegetation in creosotebush plains form of Chihuhuan Desert. Shrub layers of range vegetation was the same in both potographs: creosotebush was dominant; other common woody plants included Texas pricklypear, ocotillo, tasajillo, leatherstem, and honey mesquite. There were both "regular" and succulent and/or semi-succulent shrub species present. Tarbush was so rare as to be lacking. In the lower photograph an herbaceous layer of perennial native grasses consisted of chino grama (most common), bush muhly (Muhlerbergia porteri), black grama, and red or dogtown threeawn (Aristida longiseta). Forbs were very limited. Range vegetation in the lower photograph matched general descriptions of virgin vegetation from a species compostion standpoint. Herbaceous cover might have been greater in pre-whiteman desert vegetation, but there was a remnant of grasses present, including decreaer species. Range vegetation in the upper photograph was typical of much of the Chihuhuan Desert with the herbaceous layer being absent for all practical purposes.

Desert pavement was obvious.

These photographs presented the sterotypic or arch-typical view of the Chihuhuan Desert. This was the creosotebush plains form with the regional dominant forming a shrubland physiogonomy across one basin to the next mountain range. Desert range across Basin and Range.

Presidio County, Texas. June, early estival aspect (prior to onset of summer rainy season). FRES No 30 (Desert Shrub Shrubland Ecosystem). K-35 (Creosotebush). Creosotebush variant of SRM 508 (Creosotebush-Tarbush). Creosotebush-Tarbush Series, 153.21, of Chihuhuan Desertscrub, 153.2 (Brown et al., 1998, p. 41). Chihuhuan Deserts- Low Mountains and Bajadas Ecoregion 24c (Griffith et al., 2004). Predominate range site was Gravelly in Desert Shrub vegetation zone.

14. Bajada Chihuhuan Desertscrub- Range plant communities co-dominated by creosotebush and tasajillo (known also as Christmas cactus and pencil cholla) on benches at foot of Bofecillos Mountains. Range vegetation in first of these slides had a patchy grassy understory consisting of red threeawn, fluffgrass, chino grama, and black grama. There were very few plants of annual grasses and these were limited to sixweeks grama (Bouteloua barbata). Forbs were absent (essentially so anyway). Other plants included mesquite, pricklypear (mostly O. englemannii var. englemannii), and leatherstem. Vegetation in the second photograph was the same as that in the first except that herbaceous species were absent.

Landform or topographic feature at local scale was bajada (Spanish for bench, and a standardized term in Geology). Formation of an alluvial fan though not pronounced was readily distinguished in the second photograph.

Presidio County, Texas. June, early estival aspect. FRES No. 30 (Desert Shrub Shrubland Ecosystem). K-30 (Creosotebush). Creosotebush-tasajillo variant of SRM 508 (Creosotebush-Tarbush). Creosotebush-Tarbush Series, 153.21, of Chihuhuan Desertscrub, 153.2 (Brown et al. 1998, p. 41). Chihuhuan Deserts- Low Mountains and Bajadas Ecoregion 24c (Griffith et al., 2004). Gravelly Outwash range site in Desert Shrub vegetation zone.

15. Bajada vegetation- Creosotebush, regional or zonal dominant plant species of Chihuhuan Desert (and Sonoran Desert), shared the desert spotlight with tasajillo or pencil cholla on a bajada. These two range plant species were co-dominants of a desertscrub community on a xeric mountain side. In the section devoted to cactus species presented below the successional relationship between tasajillo and creosotebush and the concept of "cyclic succession" was described and discussed. At this point, attention of the range management student was directed to co-existence and co-dominance of these two species in two families of range plants.

This was a closer-in view of the Chihuhuan Desert vegetation shown in the immediately preceding slide (note rock ledge or small cliff at far right in both photographs). Photograph was taken from a location farther up on the footslope of Bofecillos Mountains. Alluvial deposit of geologically eroded earth and subsequent soil forrmation led to development of this range vegetation with climax dominant plant species. View of a bajada and, in background, alluvial fan or small mountain delta. The landform pattern was repeated many times in this one range of mountains.

Presidio County, Texas. June, early estival aspect. FRES No. 30 (Desert Shrub Shrubland Ecosystem). K-30 (Creosotebush). Creosotebush-tasajillo variant of SRM 508 (Creosotebush-Tarbush). Creosotebush-Tarbush Series, 153.21, of Chihuhuan Desertscrub, 153.2 (Brown et al., 1998, p. 41). Chihuhuan Deserts- Low Mountains and Bajadas Ecoregion 24c (Griffith et al., 2004). Gravelly Outwash range site in Desert Shrub vegetation zone.

16. Cheek-by-jowl- Photographic evidence of side-by-side survival of creosotebush and tasajillo. These two range shrubs were co-dominate species on a bajada (= bench) that was formed by orogenic (= mountain-forming) processes. These range plants were part of the Chihuhuan Desert scrubland in the Bofecillos Mountains shown in the three immediately preceding slides.

Creosotebush has generally been viewed as one of the least xerophytic of desert plants, but it is the single most widespread dominant species in the hot deserts of North America. Cresotebush lacks many of the anatomical and morphological features of many other desert plant species. Cresostebush does not have leaf pubescence or irregularily distributed, recessed stomates (Barbour et al.,1999, p. 550), but this dominant desert species with its almost mesophytic-like morphology does have the evergreen freature that is characteristic of xerophytes (Barbour et al., 1999, p. 628). Tasajillo or pencil cholla is obviously more xerophytic and xeromorphic than creosotebush. Yet the two species have survived as co-equals for generations over scale of evolutionary time. The ecological relations, especially in regards plant succession, was described below under discussion of tasajillo as a member of Cactaceae. These examples were prelude to the lesson on cyclic succession (or dynamic equilibrium). Should be more than enough to whet the ecological appetite of any real rangeman.

Presidio County, Texas. June.

17. Epiphytic layer in the desert- Not exactly a liana in a tropical rainforest. Nor will Tarzan be swinging from it, but shown here was an ephiphyte about as large as the dominant woody species of the Chihuhuan Desert. Climbing or vine milkweed (Sarcostemma cynanchoides) added a layer to the structure of range vegetation. This is not a parasitic plant and its presence had no obvious effect on creosotebush so the interrelation appeared to be that of commensalism, the interrealtionship that is beneficial or positive to one organism and neutral to the other species. The nurse plant syndrome is the best-known form of commensalism among desert plants. There did not appear to be a nurse plant factor involved in the milkweed's use of creosotebush as a "trellis". Vine milkweed also grows over rocks and, in junk and salvage yards, on old refigerators, farm implements, and wrecked vehicles.

Although S. cynanchoides does not extend its grow far beyond its "living trellis" it nonetheless was viewed by this author as adding another dimension or contributing to another layer of this range shrubland vegetation. This commensalism took place in the same range plant community presented in the immediately preceding four photographs (scrubland vegetation on a bajada in the Bofecillos Mountains in which creosotebush and tasajillo were co-dominants).

Presidio County, Texas. June.

18. Climbing or vine milkweed (Sarcostemma cyanchoides)- Basal portion of climbing milkweed (first photograph) and details of its shoot, including tiny leaves, (second photograph). Last growing season's (year's) dead shoots provided amble evidence that this unusual and interesting desert range plant was a perennial. Climbing milkweed is in Ascelpiadaceae and does have the characteristic milky latex. Nifty little fellow.

Presidio County, Texas. June, pre-bloom phenological stage.

19. Draw in the Bofecillos Mountains- Not big enough for a canyon and yet not an arroyo (and too far south for a coulee) so this topographic feature was a draw. Due to both increased flow of water following rains (rarely snows) and shading the draw is a more mesic and generally favorable habitat than that of surrounding envirnoments. Honey mesquite was dominant with creosotebush and Texas pricklypear were associate species. Tasajillo was also abundant. There was some black grama (perhaps a remnant of "happier times", but most the common grass was fluffgrass.

Big Bend Ranch State Park, Presidio County, Texas. June. FRES No. 30 (Desert Shrub Shrubland Ecosystem). . K-30 (Creosotebush). No SRM rangeland cover type was specific to this range vegetation, but mesquite variant of SRM 508 (Creosotebush-Tarbush) might cover it. One form of Mesquite Series, 153.24, of Chihuhuan Desertscrub, 153.2 (Brown et al., 1998, p.41). Chihuhuan Deserts- Low Mountains and Bajadas Ecoregion 24c (Griffith et al., 2004). Draw range site in Desert Shrub vegetation zone.

20. Desert draw vegetation- The extremely limited (restricted) land feature known as a draw produces a distinctive range vegetation. Plant life in desert draws does not necessarily develop into a unique range plant community, but it is an assembly of plant forms and plant species that to the astute observer differs from that on adjacent bajadas and basins. This was a different draw from the one shown in the preceding photograph, but range vegetation was more-or-less the same. Honey mesquite was dominant with creosotebush and Texas pricklypear (and also, perhaps, tasajillo) the associates. Fluffgrass was the most common herbaceous species. No forbs for all practical purposes.

Big Bend Ranch State Park, Presidio County, Texas. June.FRES No. 30 (Desert Shrub Shrubland Ecosystem). K-35 (Creosotebush). No really specific SRM, but mesquite variant of SRM 508 (Creosotebush-Tarbush) might be close. One form of mesquite Series, 153.24, of Chihuhuan Desertscrub, 153.3 (Brown et al., 1998, p. 41). Chihuhuan Deserts- Low Mountains and Bajadas Ecoregion 24c (Griffith et al., 2004). Draw range site in Desert Shrub vegetation zone.

21. Chihuhuan Desertscrub landscape- Basin and Range physiography and cresotebush-ocotillo desertscrub physiogonomy were shown at this landscape-scale view of the Chihuhuan Desert. An extremely narrow basin with bajadas running along either side provided the desertic environment to which creosotebush and ocotillo are so well adapted. Ocotillo is not as common as honey mesquite and tarbush on much of the Chihuhuan Desert shrubland, but ocotillo does frequently occur in such relative proportions as to form a conspicuous (a self-evident) creosotebush-ocotillo rangeland cover type. This range type is very prominent on much of the Trans-Pecos Basin and Range country, especially in the Big Bend (of the Rio Grande) vicinity. Absence of an herbaceous layer might have been the result of human-induced deterioration (say, by way of argument or for illustration purposes, overgrazing). However, this bajada and basin range vegetation was adjacent to higher hill slopes on which sotol-lechuguilla-chino grama grassland had developed and between which and this creosotebush-ocotillo desertscrub community there as an ecotone or transition zone that was a chinograss-creosotebush-succulent shrub savanna. Further upslope range vegetation was shown in the very next slide.

MacMahon (in Barbour and Billings, 1988, p. 249-250) described the creosotebush-ocotillo-lechuguilla-cactus community as "the Mixed Desert Scrub phase" of the Chihuhuan Desert.

Attention was drawn to the somewhat uniform or regular dispersion pattern of shrubs that is characteristic of desert plant communities. The uniform or regular dispersion was at level of individual plants for creosotebush whereas for ocotillo it was groups of plants that had uniform dispersion. This was attributed to dispersal of ocotillo seeds close to parent plants (note the ocotillo "family" in far left foreground) such that it was these reproductive groups that exhibed uniforml dispersion. The groups of ocotillo could by strict use of terminology be regarded as aggregated dispersion (aggregated distributional pattern) so that ocotillo had aggregated or contagious or underdispersed dispersion. These ocotillo groups, however, exhibited uniform or regular dispersion. More frequently the regular or uniform dispersion pattern is of individual ocotillo plants. This was shown in several photographs. Just stay in the saddle.

Individual creosotebush plants secrete allelochemicals that directly reduce or redard establishment and growth of other plants in the space around these secreting plants. Allelopathy is one form of competition or, more precisely, one phenomenon that prevents or reduces competition with established plants. There have been various studies suggesting that creosotebush is auto-alleopathic meaning that individuals of this species secrete allelochemicals that prevent or reduce establishment of their own kind (even theri own progeny). This at least a partial explanation for the uniform or regular dispersion (distribution) pattern often seen within populations of creosotebush or in range plant communities dominated by creosotebush..

Students should know the difference between dispersion and dispersal. Dispersion is the distribution of organisms in space or, more descriptively, the pattern or arrangement of individual (or groups) of organisms in space (ie. spatial arrangement of plants realtive to one another as, for instance, random, uniform, etc.). Dispersal is the movement or transport of plant propagules or, the synonyms, diaspores or germules (seeds, fruits, spores, bulbils, stolons, tillers, rhizomes, etc.) from parent plants to another place.

Big Bend Ranch State Park, Presidio County, Texas. June. FRES No. 30 (Desert Shrub Shrubland Ecosystem). K-35 (Creosotebush). Variant of SRM 508 (Creosotebush-Tarbush). Creosotebush-Tarbush Series, 153.21, of Chihuhuan Desertscrub, 153.2 (Brown et al., 1998). Chihuhuan Deserts- Low Mountains and Bajadas Ecoregion 24c (Griffith et al., 2004). Primarily a Gravelly range site in Desert Shrub vegetation zone. Gravelly range site in Desert Shrub vegetation zone.

22. East side of a mountain side- Transect view up a low mountain tht was adjacent (conterminous with) the bajada and basin creosotebush-ocotillo desertscrub shown in the immediately preceding photograph. This range vegetation "joined up with" that of the preceding pictured range plant community to form a continuum of vegetation from basin creosotebush to sotol-lechuguilla-chino grama grassland higher up on this east slope. East slopes are more mesic than south and west slopes because the latter two receive more solar radiation and thus have drier soil environments. North slopes are usually the most mesic. Likewise, higher parts of mountains and, sometimes, even hills support more mesic vegetation due to several factors including cooler ambient and soil temperatures (hence more soil water content) and sometimes more precipitation.

The range vegetation from foreground and including most of this small mountain was an ecotone of creosotebush, pricklypear, and leatherstem from upper elevational parts of creosotebush-dominated Chihuhuan Desertscrub and sotol, lechuguilla, chino grama, black grama, sideoats grama, red threeawn, and more widely scattered dropseed and muhly species. This transition zone formed a grass-shrub savanna.

Big Bend Ranch State Park, Presidio, County, Texas.

23. Desert boulder field- Featured here was a desertscrub community dominated by leatherweed, known also as dragon's blood or sangre de drago within (as part of) the overall creosotebush-dominated Chihuhuan Desertscrub. Local patches (some of them at intermediate spatial scale) in the Trans-Pecos area are distinctive from predominate range cover types. In context of Landscape Ecology each of these is a patch, an ecosystem or landscape element such as an area of vegetation "that is relatively homogenous internally and differs from surroounding elements " (Hlems, 1998) within the larger surrounding cover type that functions as the matrix, "the most eextensive and connected landscape element that plays the dominant role in landscape functioning ... ande"surrounding a patch" (Helms, 1998). Both patch and matrix can be interpreted as a separate cover type. Such was the case for the leatherstem comunity which could be seen as a more restricted range cover type of the greater Chihuhuan Region.

The first photograph provided a landscape-scale view of leatherstem-dominated range vegetation within the Bofecillos Mountains. The edge of a typical creosotebush-dominated community was in the foreground behind which was a leatherstem-dominated community in which creosotebush was an associate species. The second photograph showed details of species composition and plant community structure of the leatherstem range type. Several species of cactus (primarily Texas pricklypear) and ocotillo joined creosotebush as major shrubs on the leatherstem-dominated range. Grasses were present, but widely scattered. The most common grass was black grama followed by chino grama and red threeawn. Annual grasses was n no present. Forbs were scarce.

This boulder field range was a tilted ridge outcrop consisting of conglomerate, mostly sandstone, at foothills of and eroded from the volcanic Bofecillos Mountains (MacLeod, 2002, ps. 99, 179, 190, 229). This was a bajada or bench in the basic landform of the Basin and Range physiographic province.

FRES No. 30 (Desert Shrub Shrubland Ecosystem). A form of K-35 (Creosotebush). Leatherstem variant of SRM 508 (Creosotebush-Tarbush). A form or variant of Mixed Scrub-Succulent Series, 153. 25, of Chihuhuan Desertscrub, 153.2 (Brown et al, 1998, p. 41). Chihuhuan Deserts- Low Mountains and Bajadas Ecoregion 24c (Griffith et al., 2004). Igneous Hill an Mountain range site in the Desert Shrub vegetation zone.

24. Synopsis view of Chihuhuan Desertscrub- An "all-in-one" shot of what the potential natural (= climax) vegetation of the Chihuhuan Desert "should be". Range vegetation shown here was what this author-photographer interpreted as a relict area that served as a photogrphic reference of the virgin Chihuhuan Desert vegetation, including a composite of species composition and plant community structure. It was a range vegetation scientist's picture postcard of: FRES No. 33 (Southwestern Shrubsteppe Shrubland Ecosystem), K- 53 (Trans-Pecos Shrub Savanna).

In this range vegetation there was a well-established herbaceous understorey, and it was of the climax decreaser grasses, chino grama and black grama, as well as increasers including red threeawn and invaders or early seral perennials like fluffgrass. Both Texas or Lindheeimer pricklypear (Opuntia englemannii var. lindheimeri), left foreground,. and Englemann pricklypear (O. englemannii var. englemannii), right foreground and left midground, were present. Creosotebush and ocotillo were dominant and associate shrub species, respectively, and visible throughout.

Vegetation purists feast your eyes on this scene. It portrayed the quitesence of pristine Chihuhuan desertscrub savanna. If there remains a sample of vegetation that is representative of the general or regional range cover type of the Chihuhuan Desert this is "in the running" for the honor.

Big Bend National Park, Brewster County, Texas. June, early summer estival aspect.. Leatherstem variant of SRM 508 (Creosotebush-Tarbush). A form or variant of Mixed Scrub-Succulent Series, 153.23, of Chihunuan Desertscrub, 153.2 (Brown et al., 1998, p. 41). Chihuhuan Deserrts- Low Mountains and Bajadas Ecoregion 24c (Griffith et al., 2004).

25. A slope situation- The term slope has different meanings and connotations depending on context and/or situation. Most precisely, slope refers to change in elevation over a specified distance expressed in different units, most commonly as degrees or percentage (Helms, 1998). Slope is also used to refer to or as more-or-less synonymous with aspect defined as "the predominant direction of slope of the land" (Jacoby, 1989) as in a north slope, east slope, south slope, northwest slope, etc.Aspect in this context is sometimes confused with aspect that means "the seasonal changes in the appearance of vegetation" (Jacoby, 1989). Further confusion can arise with the term exposure meaning "direction of slope with respect to points of a compass" (Jacoby, 1989). Jacoby (1998) defined slope: "A slant or incline of the land surface, measured in degrees from the hoizontal, or in the percent (defined as the number of feet or meters change in elevation per 100 of the same units of horizontal distance); may be further characterized by direction (exposture)". In other words slope has a literally, restricted meaning and some generic meanings that are, again, situation- or context-dependent.

The phenomenon of slope portrayed in these two photographs encompasses each of the three meanings (at least partially). The upper (top) photograph was a typical "creosotebush plains" form of the Chihuhuan Desert: creosotebush-dominated arid shrubland with succulent shrubs including pricklypear species and tasajillo, succulent-like shrubs like Spanish bayonet or Faxon (Yucca faxoniana= Y. carnerosana), and ocotillo which in this general area (Rio Grande Big Bend) is often an associate species to creosotebush. There was a poorly developed herbaceous layer consisting mostly of scattered large plants of chino grama. Slope was only 1 to 2% (ie. essentially no slope).

The lower (bottom) photograph was a small area of chino grama semidesert grassland typical of foothill habitats, and a climax range plant community. Creosotebush and even a few pricklypear plants were present, but the vegetation was grassland not shrubland; in fact, not even a savanna.Percent or degree slope was not determined on this parcel of land, but it was obviously substantially greater than on the level plain dominated by creosotebush..

These two examples of Chihuhuan range vegetation (two range cover types) were located almost adjacent to each other. The unmistakable landmark of Cerro Castellan (and cumulus clouds) proved closeness of these two range plant communities. Slope made the difference in range vegetation. "Creosotebush plains" shrubland (typical Chihuhuan Desert) had little slope and no exposure or aspect (as defined above) features. Chino grama grassland was on a predominately north-facing slope (a north aspect) where there was enough shade to maintain higher soil water content for chino grama that is more mesic (or less xeric), less adapted to aridity than creosotebush, the regional dominant species.

Aspect was probably not the only factor. Soil series may have been different between these two habitats; certainly the two range cover types were on different positions on a catena, " a sequence of soils of about the same age, derived from similar parent mterial and occurring under similar climatic conditions, but having different charcteristics because of variantion in relief and in drainage" (Helms, 1998). Nonetheless, aspect was a (probably the) major variable. Water runoff was greater on the slope dominated by chino grama so that water infiltration would be less on the slope. Northern exposure was obviously more important than degree or percent slope or, perhaps, water infiltration properties did not vary that much between the two environments due to similar soil properties (eg. similar soil texture). Either way, the slight differences in shading of soil due to sloping land was more critical to development of different range types.

Big Bend National Park (near Cerro Castellan), Brewster County, Texas. June, early estival aspect. Left photograph: example of FRES No 33 (Southwestern Shrubsteppe Shrubland Ecosystem). K-53 (Trans-Pecos Shrub Savanna). Variant of SRN 508 (Creosotebush-Tarbush). Creosotebush-Tarbush Series, 153.21, of Chihuhuan Desertscrub, 153.2 (Brown et al., 1998, p. 41). Right photograph: example of FRES No. 40 (Desert Grasslands Ecosystem). Chino grama variant of K-48 (Grama-Tobosa Prairie). Chino grama variant of SRM 505 (Grama-Tobosa Shrub). Grama Grass-Scrub Series, 143.11, of Chihuhuan (Semidesert) Grassland, 143.1 (Brown et al., 1998, p. 40). For those who doubt the validity of distinguishing between grassland and shrubland (and the various units of vegetation as provided by numerous authors) at close proximity within a desert climate, it was pointed out that the same degree of differences in vegetation are found at the same (or even shorter) distances between alpine and subalpine forest, aspen parkland and grassland, deciduous forest and grassland balds, and so forth. Both range cover types were in Chihuhuan Deserts- Low Mountains and Bajadas Ecoregion 24c (Griffith et al., 2004).

26. Succulent form of Chihuhuan Desert- Even though the Chihuhuan Desert supposedly has more cactus species than the Sonoran Desert it is the latter that is regarded as the "succulent desert". This is due to dominance of what is generally interpreted as the succulent life or growth form (eg. species of giant arborescent cactus, abundance of various barrel cactus, numerous Agave species, etc.) in the Sonoran Desert. The general situation is that the Chihuhuan Desert is not a succulent desert, but there are infrequent exceptions to this overall pehnomenon. This photograph presented such a local exceptional range plant community. The dominant species was the succulent, lechuguilla (Agave lechuguilla). Smooth sotol (Dasylirion leiophyllum) was also present (eg. the plant with tall flower stalk) as were cactus species, especially Englemann pricklypear. Likewise ocotillo, a frequent local associate (to creosotebush) was "alive and well". Non-succulent shrubs also common in this range vegetation included creosotebush, the regional dominant of the Chihuhuan and Sonroan Deserts, and the woody composite mariola (Parthenium incanum). The most common of the uncommon grasses were chino grama, black grama, and red threeawn (in that order). Forbs were essentially absent with the Compositae being represented by mariola. Woody legumes were absent from this local community. Hurray, no mesquite!

This was an example of "the Mixed Desert Scrub Phase" consisting of creosotebush, cactus, ocotillo, and lechuguilla identified by MacMahon (in Barbour and Billings, 1988, ps. 249-250).

Big Bend National Park, Brewster, County, Texas. June, early estival aspect. FRES No. 33 (Southwestern Shrubsteppe Shrubland Ecosystem). K-53 (Trans-Pecos Shrub Savanna). No SRM applied; succulent vriant of SRM 508 (Creosotebush-Tarbush) was a real "stretch" (too much so, but it might help some viewers). One form or expression of Mixed Scrub-Succulent Series, 153.25, Chihuhuan Desertscrub, 153.2 (Brown et al., 1998, p. 41). Chihuhuan Deserts- Low Mountains and Bajadas Ecoregion 24 c (Griffith et al., 2004).

27. Creosotebush-Ocotillo-Grama Savana range cover type- These three photographs were examples of a rangeland cover type that has not been published, at least not per se as for example by the Society for Range Management (Shiflet, 1994) or as a specific biotic community in Brown et al. (19198). There are large acreages in the Trans-Pecos Basin and Range where ocotillo rather than tarbush is the associate or even co-dominant with creosotebush (ie. a creosotebush-ocotillo rather than the more common creosotebush-tarbush cover type, association, series, or whatever unit designation was used by the publishing authority). In the observation and interpretation of this author there should be a creosotebush-ocotillo savanna range type, and this presented photographic evidence of the existence of such a rang plant community.

There was a well-defined and prominent herbaceous layer among the two major shrub species. This layer was comprised primarily by chino grama (dominant) and black grama (associate or second co-dominant). These were the major range plant species on foothill grama semidesert grassland that developed as zones of range vegetation at higher elevations on hill and mountain slopes above the more common "creosote-bush plains" or "creosotebush flats" form (range type) on basins and bajadas. Red threeawn and bush muly were other grass species that were present, but neither species approached general abundance of either of the two Bouteloua speceies. The semidesert grassland cover types were dealt with in that chapter under the Grasslands biome herein.

The range vegetation displayed and described here was clearly a creosotebush-ocotillo shrub steppe or creosotebush-ocotillo-gramagrass savanna. Additional woody plant species included pricklypear cactus, especially Englemann pricklypear (Opuntia englemannii var. englemannii), lechuguilla, smooth sotol, Faxon yucca or Spanish bayonet, whitethorn acacia (Acacia constricta), and honey mesquite. This was an example what MacMahon (in Barbour and Billings, 1988, ps. 249-250) labeled "the Mixed Desert Scrub phase" of the Chihuhuan Desert.

According to local authorities of the National Park Service livestock had been effectively removed from this rangeland for a period of about 55 years (at time photographs were taken) and the grass understorey (especially chino grama) had made tremendous recovery. At least this is the official conclusion of the National Park Service (1983, p. 81). "That's my story and I'm sticking to it."

Big Bend National Park, Brewster County, Texas. June, early estival aspect (before beginning of summer rainy period). FRES No 33 (Southwestern Shrubsteppe Shrubland Ecosystem). K-53 (Trans-Pecos Shurb Savanna). Ocotillo variant of SRM 508 (Creosotebush-Tarbush). One form of Creosotebush-Tarbush Series, 153.21, of Chihuhuan Desertscrub, 153.2 (Brown et al., 1998, p. 41). Chihuhuan Deserts- Low Mountains and Bajadas Ecoregion 24c (Griffith et al., 2004).

28. Landscape of Creosotebush-Ocotillo-Grama Savanna range cover type- View at landscape-scale of one variant, form, or expression of creosotebush-dominated Chihuhuan Desertscrub. This proposed rangeland cover type was displayed at more restricted spatial scale and described immediately above. This slide was presented to convey physiography of the Trans-Pecos Basin and Range and the physiogonomy of this Chihuhuan Desertscrub savanna or creosotebush-ocotillo-grama shrubsteppe. Dominant grass was chino grama with black grama the associate species. Red threeawn and bush muhly were sparse, but other perennial native grasses. In addition to creosotebush and associate species, ocotillo, there were such succulent shrubs as pricklypear species, lechuguilla, and smooth sotol. Other shrub species included the woody legumes honey mesquite and whitethorn acacia.

Big Bend National Park, Brewster County, Texas. June, early estival aspect. FRES No. 33 (Southwestern Shrubsteppe Shrubland Ecosystem). K-53 (Trans-Pecos Shrub Savanna). Ocotillo variant of SRM 508 (Creosotebush-Tarbush). A form of Creosotebush-Tarbush Series, 153.21, of Chihuhuan Desertscrub, 153.2 (Brown et al., 1998, p. 41). Chihuhuan Deserts- Low Mountains and Bajadas Ecoregion 24c (Griffith et al., 2004).

29. Species make-up of creosotebush-ocotillo-grama savanna- Detail of range vegetation shown in the immediately preceding three photographs. Creosotebush, ocotillo, various species of cacti, and widely; scattered mesquite made up the shrub component. Grasses present in this &