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Trans-Pecos Texas is the region of the Pecos River in the extreme western part of the state (Fig. 1). New Mexico borders to the north and the Rio Grande separates Texas from Mexico to the south. There are nine relatively large counties (El Paso, Hudspeth, Culberson, Reeves, Jeff Davis, Presidio, Brewster, Pecos, and Terrell) completely within the Trans-Pecos. Brewster County is larger than the state of Connecticut. Val Verde County to the southeast is partially in the region. Counties bordering the Pecos River to the east (Loving, Winkler, Ward, Crane, Upton, and Crockett), adjacent southern New Mexico, and adjacent Mexico are considered to be associated with the Trans-Pecos in this floristic account of woody plants. There are about 20,500,000 acres, about 32,000 square miles, in the Trans-Pecos, an area roughly equal to the state of Maine (Schmidly, 1977).
Most of the Trans-Pecos is under private ownership in the form of relatively large ranches. Cattle ranching is emphasized in most areas, particularly where there are abundant desert grasslands. Sheep and goats are still favored on a few ranches, located mostly on the Stockton Plateau, but in most areas predators and poisonous weeds have forced ranchers to stop raising these animals. There are few cultivated areas, these being centered around El Paso, Valentine, Van Horn, Pecos, Balmorhea, Presidio, and Fort Stockton. These all depend upon irrigation water either from wells or a diminishing river flow. In 1979 a vast new farming area was plowed in the grassland near Valentine, where irrigation water is supplied from deep wells. There are two national parks, Guadalupe Mountains National Park in northern Culberson County, and Big Bend National Park in southern Brewster County.
Physiography And Climate
Most of the Trans-Pecos is physiographically diverse and different from the rest of Texas mainly because of the numerous mountain systems and low, arid basins (Fig. 1). The lowest elevation in the Trans-Pecos is about 1,000 ft. at the mouth of the Pecos River. Elevations increase steadily up the Rio Grande to 1,360 ft. at the mouth of San Francisco Canyon on the Brewster-Terrell county line, 1,650 ft. at Reagan Canyon, and about 1,800 ft. at the head of Boquillas Canyon. The highest elevation in the Trans-Pecos is 8,749 ft. at Guadalupe Peak. Elevations of other peaks in the Guadalupe Mountains include the following: Bush (8,631 ft.), Shumard (8,615 ft.), Bartlett (8,508 ft.), Hunter (8,368 ft.), and El Capitan (8,085 ft.). Other major peaks in the Trans-Pecos include Mt. Livermore (8,382 ft.) in the Davis Mountains, Emory Peak (7,835 ft.) in the Chisos Mountains, and Chinati Peak (7,730 ft.) in the Chinati Mountains. In all there are 90 peaks that are a mile or more above sea level. The elevations of some major towns are: El Paso, 3,762 ft.; Sierra Blanca, 4,512 ft.; Van Horn, 4,010 ft.; Pecos, 2,580 ft.; Fort Davis, 5,000 ft.; Marfa, 4,688 ft.; Alpine, 4,481 ft.; Presidio, 2,594 ft.; Fort Stockton, 3,050 ft.; Sanderson, 2,775 ft.
The mountains are composed of either igneous or sedimentary (mostly limestones) substrates and in some formations both substrates are present. In El Paso County the Franklin Mountains are predominantly limestone. In Hudspeth County the Hueco and southern Quitman Mountains and Sierra Diablo are limestone, while the Northern Quitman and Eagle mountains are composed of igneous rocks. In Culberson County the Guadalupe, Delaware, and Apache mountains are limestone. The Van Horn Mountains are composed of igneous rocks as are the Davis Mountains that dominate Jeff Davis County and northern Brewster County. Igneous mountains, the Sierra Vieja, Chinati, and Bofecillos, also dominate Presidio County. In Brewster County, the Glass, Del Norte, Santiago, and Dead Horse mountains are limestone, while the magnificent Chisos Mountains are of igneous origin. The Edwards Plateau of central Texas extends west across the Pecos River in Terrell County and parts of Pecos, Brewster, and Val Verde counties where it is also known as the Stockton Plateau. The Stockton Plateau is a highly dissected array of limestonecapped mesas and deep canyons at about 2,000-3,200 ft. that extend north to Ft. Stockton and west to the Glass Mountains and Marathon Basin. Erosional outwash materials have in some areas accumulated to form high basins between the mountains. Alluvial fans or bajadas (alluvial slopes formed by rushing water) are deposited characteristically at the bases of desert mountains. In overall profile the Trans-Pecos can be pictured as extensive lower elevation flats, slopes, dunes, basins, hills, and ridges surrounding the higher, islandlike mountains, plateaus, and basins.
The soils are variable in texture and profile. In general the soils are basic. Salt lakes and alkali flats in poorly drained areas are located throughout the region, most conspicuously in northern Hudspeth and Culberson counties and near the Pecos River in Reeves and Pecos counties. Extensive or localized gypsum substrates are scattered throughout the region, perhaps best exemplified by gypsum plains of the Castile Formation south of the Guadalupe Mountains in northern Culberson and Reeves counties. Deep clay, sand, and gravel deposits predominate in certain areas.
The general climate of the Trans-Pecos may be characterized as arid. It is cool and dry during the winter and hot and dry during the summer. The average annual precipitation is about 12 in. with most of the rainfall coming in the form of thundershowers in July, August, and early September. Annual precipitation generally decreases from east to west and increases with elevation. El Paso annually records less than 8 in. of precipitation while Presidio averages 8.5 in., Alpine 15.5 in., and up to about 20 in. are usual in the higher elevations. In Big Bend National Park annual records furnished by Jim Liles show less than 10 in. at Rio Grande Village (1,850 ft.), about 13 in. at Panther Junction (3,800 ft.), and almost 19 in. in the Chisos Basin (5,400 ft.). Average midsummer high temperatures range from about 85 to 95ºF, depending largely upon elevation, to 100ºF in Presidio, where the highest official temperature is 122ºF (125ºF reported). Average mid-winter low temperatures are from 27º to 32ºF, with a record low temperature of -10º F in Ft. Davis.
Ten Vegetational Areas are recognized for Texas (Gould, 1962; Correll and Johnston, 1970), one of them being the Trans-Pecos, Mountains and Basins. The Trans-Pecos Vegetational Area is more a geographic distinction than a vegetative one because there is extreme diversity of habitats and vegetation in the region. The Edwards (Stockton) Plateau Vegetational Area extends across the Pecos River into the southeastern Trans-Pecos according to Gould (1962), thus adding to the plant diversity of the Trans-Pecos as a geographic region.
Perhaps the unique vegetative aspect of the Trans-Pecos is the Chihuahuan Desert, the largest desert in North America. About one-third of the Chihuahuan Desert, and its only representation in the United States, is in the Trans-Pecos and parts of southern New Mexico and southeastern Arizona. Two-thirds of the Chihuahuan Desert is in Mexico. In the Trans-Pecos essentially all of the lower elevational vegetation is more or less part of the northern Chihuahuan Desert flora. The Desert vegetation is most characteristic in southern Brewster County surrounding the Chisos Mountains, and it extends in northerly directions intermixing with other vegetation types where physiographic (mostly erosional) and climatic factors allow encroachment of desert plants. The author often has been asked to define, in his opinion, what are the precise boundaries of the Chihuahuan Desert in Trans-Pecos Texas. There are of course no precise topographic features that contain the Desert but its boundaries can be defined generally on the basis of characteristic vegetation or indicator plant species such as Lechuguilla (Agave lechuguilla), Creosotebush (Larrea tridentata), Tarbush (Flourensia cernua), Ocotillo (Fouquieria splendens), sotof (Dasylirion spp.), yucca (Yucca spp.), and certain other shrubs (Parthenium spp.; Acacia spp.; Mimosa spp.). The mountains of the Trans-Pecos "rise up out of the desert," so to speak, and certain desert plants extend up the slopes and canyons as far as growth factors will permit, perhaps to 6500 ft. or more on some southern slopes. The elevational boundary of the Desert flora as a whole, depending upon slope exposures and other factors, is approximately 4500 ft. There is no clear boundary of the Chihuahuan Desert in the southeastern Trans-Pecos where desert plants intermix with Edwards Plateau vegetation on eroded rangelands to the vicinity of Langtry in Val Verde County. Desert plants have slowly encroached upon grasslands of the Trans-Pecos as over-grazed ranges permitted soil erosion and the establishment of new habitats suitable for desert species. Van Devender and Spaulding (1979) have presented paleoecological data, plant fossils from ancient packrat middens, that the Chihuahuan Desert achieved much of its present northerly distribution less than 8000 years ago when the climate started to become arid. Precise boundaries of the northern Chihuahuan Desert are difficult to define partly because the Desert is still growing. Schmidt (1979; 1985) provides a rather precise delineation of the whole Chihuahuan Desert based upon climatic data. An excellent synopsis of the Chihuahuan Desert is available in Key (1980).
Another unique aspect of the Trans-Pecos vegetation is the higher mountain flora. The Davis Mountains in the central Trans-Pecos are reported to be the southern end of the Rocky Mountains, while the Glass Mountains in the central Trans-Pecos may be the end of the Appalachian range. Present floristic surveys suggest phytogeographic connections between certain mountains of the Trans-Pecos and the southern Rocky Mountains of New Mexico, the mountains of southeastern Arizona, the Sierra Madre Occidental of Mexico, and the Del Carmen-Sierra Madre Oriental of Mexico.
Although the flora of the Trans-Pecos is diverse and complex, at least five general vegetation types may be recognized:
Chihuahuan Desert Scrub
. At lower elevations (Fig. 2) where annual precipitation ranges from seven to nearly 12 in. about half of the Trans-Pecos is dominated by various shrub species and semi-succulents such as Lechuguilla, sotol (Dasylirion spp.), and yucca (Yucca spp.). Paleoecological and contemporary ecological data suggest that the desert scrub has gained its present distribution to a large extent through invasion of eroded grassland. Creosotebush (Larrea tridentata) is a prominent element of the desert scrub (Fig. 3), occurring with mixed shrubs or in some areas forming extensive, almost pure stands. Present evidence suggests that Creosotebush dispersed from South America (Argentina) to the Chihuahuan Desert region, probably in Mexico, from whence it spread to the western Sonoran and Mohave deserts. Other common shrub species include Catclaw Mimosa (Mimosa aculeaticarpa var. biunicifera), acacias (Acacia constricta, A. neovernicosa, and A. greggii), Mariola (Parthenium incanum), Honey Mesquite (Prosopis glandulosa), Fourwing Saltbush (Atriplex canescens), Tarbush (Flourensia cernua), Javelinabush (Condalia ericoides), Skeletonleaf Goldeneye (Viguiera stenoloba), Allthorn (Koeberlinia spinosa), and Ocotillo (Fouquieria splendens). Among the common grasses are gramas (Bouteloua ramosa, B. gracilis, B. curtipendula, and B. eriopoda), threeawns (Atristida spp.), tridens (Tridens spp.), and Fluffgrass (Dasyochloa pulchella).
Within the Chihuahuan Desert scrub there are several kinds of plant associations that may be recognized (even as distinct vegetation types), these usually being associated with specific soil types or substrates. Saline habitats exhibit characteristic halophytic vegetation (Henrickson, 1977), including Fourwing Saltbush, Alkali Sacaton (Sporobolus airoides), Winged Sesuvium (Sesuvium verrucosum), Frankenia (Frankenia jamesii), Pickleweed (Allenrolfea occidentalis), Seepweed (Suaeda spp.), and Saltgrass (Distichlis spicata). Fourwing Saltbush is also a widespread species in various habitats. Crusty gypsum exposures are scattered throughout the Trans-Pecos (Fig. 4), often near the saline habitats, and characteristic assemblages of plants are restricted to these areas, for example Tiquilia hispidissima, Gaillardia multiceps, Gypgrass (Sporobolus nealleyi), Selinocarpus spp., and Anulocaulis spp. (Powell and Turner, 1977). Gypsum endemic species are also found in the gypseous clays which occur near Terlingua in southern Brewster County and from southern Hudspeth County southeast near the Rio Grande to Presidio County. Deep sandy soils are found in parts of El Paso and Hudspeth counties where Honey Mesquite, Broom Psorothamnus (Psorothamnus scoparius), and Sand Sagebrush (Artemisia filifolia) are common. Extensive deep sand and dunes are also found adjacent to the Trans-Pecos in Ward, Winkler, Crane, and other counties where Havard Oak (Quercus havardii), Honey Mesquite, Plains Yucca (Yucca campestris), and Sand Sagebrush are prominent plants.
Limestone and igneous-rock habitats are widespread throughout the Chihuahuan Desert scrub and these support several characteristic plant associations including Sotol-Lechuguilla on rocky limestone hills, and Sotol-grassland on igneous-rock slopes. The Stockton Plateau is a limestone area that might be categorized as a vegetative entity because of its somewhat distinct plant associations (Schmidly, 1977). The highly eroded area is characterized by rolling tablelands, caprocked mesas, steep canyons, low rocky hills, and shallow valleys. The vegetation is a mixture of Chihuahuan Desert scrub and grassland with Creosotebush, Lotebush (Ziziphus obtusifolia), Texas Persimmon (Diospyros texana), Guayacan (Guaiacum angustifolium), Lechuguilla, and Texas Sotol (Dasylirion texanum) constituting some of the important scrub plants. Important grasses include several gramas, Chino Grama (Bouteloua ramosa), Red Grama (B. trifida), Black Grama, and Sideoats Grama, several threeawns (Aristida spp.), and tridens (Tridens spp.). Either Red Berry Juniper (Juniperus pinchotii) or Ash Juniper (J. ashei) may be found on the mesas with Mohr Shin Oak (Q. mohriana) and Vasey Shin Oak (Q. pungens var. vaseyana).
Desert grassland areas, some of them extensive, are widespread throughout the Trans-Pecos, and they are best developed at 3500-5200 ft. on plateaus, rolling hills, and basins where the soils are relatively deep and fertile and annual precipitation is 10 to 18 in. Much of the original grassland in the Trans-Pecos is now covered by desert scrub vegetation, or is severely invaded by a mixture of desert scrub species. The invasion of desert scrub into grassland began or was enhanced when livestock were introduced to the Trans-Pecos and when subsequent over-grazing, soil erosion, and periodic droughts allowed the opportunistic desert plants to become established. Encroachment of desert scrub has proceeded most successfully in marginal grassland areas at lower elevations, but many scrub species, e.g., Catclaw Mimosa, Cane Cholla (Opuntia imbricata), prickly pear (Opuntia spp.), Honey Mesquite, Soaptree Yucca (Yucca elata), Spanish Dagger (Y. torreyi), sacahuiste (Nolina spp.), sotol, and broomweeds (Gutierrezia spp.) can be found scattered throughout most grassland areas at any elevation.
Perhaps the best remaining grassland area in the Trans-Pecos is the "highland grassland" (Fig. 5) of the Davis Mountains near Valentine, Marfa, Alpine, and Fort Davis. Blue Grama (Bouteloua gracilis) is the dominant species but several other gramas and many other grasses are interspersed. Other grassland areas with relatively good cover or grassland remnants are associated with most every mountain range or large plateau in the Trans-Pecos. Blue Grama still dominates in most areas but soil conditions and other factors promote various dominant species such as other gramas, Tobosa Grass (Hilaria mutica), threeawns, needlegrass (Stipa spp.), bluestems (Bothriochloa spp.; Schizachyrium spp.), Burrograss (Scleropogon brevifolius), and tridens. Extensive tobosa flats occur particularly in lower elevation grassland basins where run-off water tends to accumulate. About 268 species of grasses are known to occur in the Trans-Pecos.
In the major mountain systems of the Trans-Pecos, where annual precipitation exceeds 15 in., a woodland association (Fig. 6) begins characteristically at middle elevation slopes and valleys, intergrading with upper grassland, and extending to upper mountain slopes and valleys. About 8000 years ago woodland extended across much of the Trans-Pecos (Wells, 1966; Van Devender and Spaulding, 1979). At present dense woodland usually is found on north slopes, east slopes, in valleys, and on some mesas while south and west slopes typically support rather open woodland with scattered trees. The lowest woodland association in the Davis Mountains is usually oak-juniper at about 4400-5500 ft. The dominant oaks are Gray Oak (Quercus grisea) and Emory Oak (Q. emoryi). The dominant juniper species are Rose-fruited Juniper (Juniperus coahuilensis) and Red Berry Juniper. A pinyon-juniper association occurs potentially at an even lower elevation, 3200-5600 ft., in the limestone southeast portion of the Trans-Pecos from Altuda Peak, the Glass Mountains, and the Del Norte Mountains southeastward along the western edge of the Stockton Plateau. Here the dominant juniper is Red Berry Juniper and the pine is Papershell Pinyon (Pinus remota).
A pinyon-juniper-oak association occurs on upper mountain slopes and valleys, about 5500-7500 ft. Alligator Juniper (Juniperus deppeana) replaces the other junipers at higher elevations. At higher elevations in the Davis Mountains the pine species is Mexican Pinyon Pine (Pinus cembroides), while in the Guadalupe Mountains and Sierra Diablo the pine is Colorado Pinyon (P. edulis). The dominant oaks are Silverleaf Oak (Quercus hypoleucoides), Gray Oak, and Emory Oak. Various grasses, including needlegrass (Stipa spp.), Pinyon Rice Grass (Piptochaetium fimbriatum), Bulb Panicum (Panicum bulbosum), and muhlys (Muhlenbergia spp.) are found beneath the trees and in open areas. Chisos Red Oak (Q. gravesii), Texas Madrone (Arbutus xalapensis), and Bigtooth Maple (Acer grandidentatum) are locally common especially in moist canyons.
At the highest elevations of the Guadalupe, Davis (Mt. Livermore), and Chisos mountains there are remnants of a coniferous forest mixed with oaks (Fig. 7). The pine-oak forest also has been referred to as a montane Woodland (Schmidly, 1977). The trees are thought to be relictual of the widespread coniferous forest in western North America, although there are some phytogeographical differences between the three mountain ranges. Characteristic forest trees are Ponderosa Pine (Pinus ponderosa), Pinus arizonica, Southwestern White Pine (P. strobiformis), Douglas-Fir (Pseudotsuga menziesii), Quaking Aspen (Populus tremuloides), and Chinkapin Oak (Quercus muehlenbergii). In the Chisos Mountains, Arizona Cypress (Cupressus arizonica) is also present. Douglas-Fir is not known to occur in the Davis Mountains, but it is present in both the Guadalupe and Chisos mountains. In the Chisos Mountains Pinus arizonica var. stormiae is found in place of Ponderosa Pine. Quaking Aspen is more common in the Guadalupe Mountains than in the Davis and Chisos mountains where the plant is known only from one or a few small populations in each range. Other characteristic plants of the conifer forest are Birchleaf Buckthorn (Rhamnus betulifolia), snowberry (Symphoricarpos spp.), and needlegrass (Stipa spp.). Ponderosa Pine and P. arizonica are not restricted to the highest elevations but may also occur on north and east slopes and in protected canyons at intermediate elevations.
Riparian vegetation (Fig. 8) occurs along the only two major rivers of the Trans-Pecos, the Pecos and Rio Grande, along ephemeral streams such as Limpia Creek and Alamito Creek, and along the many watercourses (canyons, arroyos, draws) that carry runoff from the mountains. The banks of some stock tanks and small lakes also support riparian vegetation. The introduced salt cedar (Tamarix spp.) is thick along the Pecos River and is choking out the upper half of the River. Honey Mesquite and Desertwillow (Chilopsis linearis) are also common trees along the Pecos River. Parts of the Rio Grande, particularly between El Paso and Presidio, are also choked with salt cedar. Screwbean (Prosopis pubescens), Honey Mesquite, Rio Grande Cottonwood (Populus deltoides ssp. wislizeni), willows (Salix gooddingii and S. exigua), Desertwillow, and two cane grasses, Common Reed (Phragmites australis) and Giantreed (Arundo doxax) also prevail along the Rio Grande. The introduced small tree or shrub species Elaeagnus angustifolia L., Russian Olive (Elaeagnaceae), is reported to have become established in recent years along the Rio Grande in El Paso County. This potentially troublesome species has either moved down the Rio Grande from New Mexico, where already it is reported to be a problem along the river, or it has escaped from cultivation in El Paso. In the mountain areas riparian vegetation may be comprised of many different trees including oaks, cottonwoods (Populus spp.), willows (Salix spp.), Little Walnut (Juglans microcarpa), Texas Madrone, Bigtooth Maple, Velvet Ash (Fraxinus velutina), Netleaf Hackberry (Celtis laevigata var. reticulata), and Seepwillow Baccharis (Baccharis salicifolia).
In the Stockton Plateau area well-defined riparian vegetarian marks even dry arroyos, where Little Walnut is a prominent species. Along Independence Creek in Terrell County, where water is usually present, Netleaf Hackberry and Plateau Oak (Quercus fusiformis) are common.
Number of Woody Species in the Trans-Pecos
The present work includes keys and descriptions for 447 species of woody plants in 203 genera and 70 families. At least 29 of these species are endemic to the Trans-Pecos and about 210 woody species are found in Texas only in the Trans-Pecos. Additional species, either woody or herbaceous, are briefly mentioned or discussed in connection with the trees and shrubs native to the Trans-Pecos.
The family with the most woody species represented in the Trans-Pecos is the Asteraceae with 70 species in 44 genera. Fabaceae is next in representation with 43 woody species in 21 genera, while Rosaceae has 20 species in 11 genera, Fagaceae has 21 species in one genus, Agavaceae has 21 species in five genera, and Grossulariaceae and Hydgrangeaceae together have 15 species in four genera.
Most of the illustrations include a branch of the tree or shrub and one or more enlargements of diagnostic characters (e.g., leaf, pubescence, flower, fruit). Sizes of the drawings are not included on the figures or in the legends, but they may be inferred from measurements given in keys and descriptions.