Vegetation of the Peloncillo Region: High-Diversity Crossroads

On first sight the Peloncillo region—particularly the Peloncillo Mountains and adjacent Lordsburg Playa—seem spare and devoid of note to botanical study. But the vegetation of the Peloncillo region is particularly noteworthy for a number of reasons: a high number of species catalogued to date (879 in the central Peloncillo Mountains alone); its placement at the convergence of four major floristic regions1 (Madrean, Chihuahuan, Sonoran, and Southern Rocky Mountain-Mogollon); and its relatively intact and large tracts of native habitats, which offer opportunities for further study and future conservation banking. A major portion of the region’s species richness occurs in the Sierra San Luis, which has not been catalogued; thus the total numbers of species is much higher than now recorded.

Outstanding Features:
  • At least 879 plant species in the Peloncillo Mts. proper, comprising over 24% of all plant species in New Mexico in only 2% of its area.

  • Floral components of four distinct biotic regions.
  • Isolation and difficult access have preserved remarkable number of intact, now-rare habitat types such as Plains grassland.
  • Opportunities to study relationships between desert grasslands, prairie dog towns (the largest left in North America occur here), and grazing.
  • Geographic scope. In describing the vegetation of the Peloncillo region, the boundaries used here are from Steins, New Mexico, and I-10 south to Guadalupe Canyon, including a portion of the Guadalupe Mountains, and the border with Mexico; and from the Animas Mountains in the east to the Peloncillo Mountains in the west, including the Lordsburg Playas. Adjacent to the Peloncillo Mountains in the south, in Mexico, are the Sierra San Luis. This range has not been completely botanically surveyed, and so numbers cited do not include this range, although they are included in discussions in this chapter.

    Compiling the First Peloncillo Mountains Flora

    Prior to 2003, the flora of the Peloncillo Mountains was unknown except for local lists for Guadalupe Canyon2 and the Bioresearch Ranch.3 The flora of the Peloncillo Mountains (also Lordsburg Playas and the Sierra San Luis) was compiled from an electronic search of databased collections in the herbaria at the University of Texas at El Paso (UTEP), New Mexico State University (NMC, NMCR), the University of New Mexico (UNM), and some online searchable databases for some Arizona herbaria (ARIZ, ASU, et al.).

    Composition of the Flora

    Floral diversity is affected by many factors including, area, elevational range, substrate types, amount and nature of surface water, activities of man, and proximity to other floristic regions. The Peloncillo region is large and includes two mountain ranges (Animas and Peloncillo Mountains) and a basin playa (Lordsburg Playa). The elevation range is moderate, from 3,800 – 8,500 feet (1,280 – 2,529 m).

    Available surface water is limited and mostly seasonal. Substrate types include limestone and igneous rock outcrops and derived soils as well as the alkaline playa soils containing a high clay fraction. Disturbances by man, such as grazing and non-native species introductions, are factors. The area is also peripheral to three major floristic regions: Madrean, Chihuahuan, and Sonoran. It also has elements of a fourth: Southern Rocky Mountain-Mogollon. Accordingly, the flora is large and diverse.

    The flora of the Peloncillo Mountains is the best focal point for discussion. Unfortunately the Sierra San Luis is largely botanically unexplored. The flora of Lordsburg Playas is not diverse because it is to a large extent physically controlled by a high clay composition in the soil and is an alkaline environment as well. Nevertheless, it contains important elements worthy of conservation. The Peloncillo Mountains are botanically well- known having been visited on numerous occasions by a number of botanists since the 1930s. Substrates in the Peloncillo Mountains are both igneous and sedimentary. Reasonably good riparian habitats are found in Guadalupe and Cloverdale Canyons, as well as a few other springs. The highest peak in the Peloncillos is 6,625 feet (2,019 m).

    The significance of the large diversity of plant species (879 species) found in the Peloncillo Mountains can only be appreciated after a comparison with the diversity in other ranges. Few mountain ranges in the region have been adequately studied, but one is the Organ Mountains. The Organ Mountains, in Doña Ana County, are located far enough from the Peloncillos to provide a good comparison. The Organ Mountains rise about 5,000 feet (1,524 meters) above the surrounding basins to an elevation of 9,012 feet (2,746 meters), have limestone and igneous substrates, have permanent water, and have a well-developed Douglas-fir and ponderosa pine zone at the highest elevations.5 The Peloncillo Mountains rise only 2,200 feet (670 meters) from the surrounding basins to 6,625 feet (2,019 meters), have igneous and limestone substrates, lack the high-elevation Douglas-fir and ponderosa pine community, but have some wetland habitats. The Organ Mountains flora contains about 850 species while that of the Peloncillos is 879 species. The Peloncillos have perhaps 150 more species than would be expected for a range of its size. The reason for the increased diversity is the location of the range in an area where a number of biotic provinces converge.

    Plant Communities

    The plant communities in the region are pine-oak woodlands, oak savanna, chaparral, short-grass prairie (including desert grasslands), and Chihuahuan desert-scrub. The Lordsburg Playas is largely a saltbush and dropseed community. Riparian systems in some canyons such as Guadalupe Canyon and Cloverdale Canyon are limited but floristically diverse where they occur. In the area surrounding Cloverdale at an elevation of 5,262 feet (1,603 meters) is the highest valley bottom in the Apachean or “sky island” region. This intact grassland is probably the most important community in need of conservation. Grasslands of this type are quickly vanishing in Mexico and elsewhere through abuse.

    Biogeographic Considerations

    The Peloncillo Mountains, along with their southern extension, the Sierra San Luis, are part of the Sierra Madre Occidental Phytogeographic Province, where Apachean and Madrean biotic provinces blend. The area contains species from the Madrean province, which ends about 90 miles (150 km) south of the U.S. border but is mostly Apachean. The area’s species mix is complex because the mountains or “sky islands” are surrounded by basins that contain floristic elements characteristic of other vegetative assemblages. For example, the Deming Plain, largely an arid grassland basin, passes east-to-west through the area and constitutes a “filter barrier” for species characteristic of the Chihuahuan and Sonoran Deserts. The Continental Divide is at its lowest elevation in the area of the Deming Plains, allowing further species interchange. A discussion of each biotic province’s and floristic region’s characteristics follows.

    Apachean. The Peloncillo and San Luis Mountains are within what is recognized as the Apachean Biotic Province. It consists of high-elevation grassland and emerging mountain masses (“sky islands”) in southeastern Arizona, southwestern New Mexico, and adjacent area of northeastern Sonora and northwestern Chihuahua. The following plants have distributions that are Apachean:

    Berberis wilcoxii Hackelia ursina
    Brickellia simplex Hedyotis greenei
    Bouteloua eleudens Mammillaria wrightii v. wilcoxii
    Carex chihuahuensis Muhlenbergia arizonica
    Desmodium batocaulon Penstemon supurbus
    Eysenhardtia polystachya Platanus superbus
    Fraxinus papillosa Verbesina longifolia

    Madrean. The Sierra Madre Occidental is within the Madrean Biotic Province. It extends to within 90 miles (150 km) of the U.S.-Mexico border. Some argue that the “sky islands” within the Apachean province are actually isolated assemblages of the Madrean province. Many plant species have distributions primarily within Mexico but reach the United States in the Peloncillo and nearby mountains. These plants reach the northern limits of their distribution in and around the Peloncillo Mountains. Representative plants showing this pattern of distribution (Madrean and/or more widely distributed to the south in Mexico) are the following:

    Abutilon malacum Machaeranthera riparius
    Acacia milifolia Mecardonia vandelloides
    Amoreuxia palmatifida Mimosa dysocarpa
    Anoda pentaschista Mimosa grahamii
    Aspicilia hirtella Porophylum ruderale
    Bouchea prismatica Rivina humilis
    Castilleja ornate Senecio salignus
    Castilleja wrightii Sideroxylon lanuginosum
    Cuphea wrightii Silene thurberi
    Dalea filifolia Tephrosia tenella
    Dalea grayi Tripsacum lanceolatum
    Dalea greggii Yucca madrensis
    Jatropha macrorhiza
    Krameria grayi

    Chihuahuan. Some elements of the Chihuahuan Desert flora extend northeast or east to reach their western limit of distribution in the area of the Peloncillo Mountains. Some representative examples are the following:

    Escobaria orcuttii
    Philadelphus mearnsii
    Vaquilenia californica ssp. pauciflora

    Sonoran. Some plants of the Sonoran Desert extend east into the Peloncillo Mountains region and even beyond. Representative examples are the following:

    Cylindropuntia spinosior
    Echinocereus rigidissimus
    Ferocactus wislizeni

    Western species reaching the eastern limit of distribution in the region. A number of plant species range from California and Baja California east to the area of the Peloncillo Mountains. Representative examples are the following:

    Castilleja exserta
    Cirsium arizonicum
    Eriophyllum lanosum
    Mirabilis pumila
    Phoradendron californicum
    Prosopis velutina
    Sophora arizonica
    Yaba microcarpa

    Northern—including Rocky Mountain—species that reach the southern limit of distribution in the region. Some species have distributions centered to the north of the region but extend south into the area of the Peloncillo Mountains. Representative examples are the following:

    Aquilegia desertorum
    Heuchera novomexicana
    Oenothera elata ssp. hirsutissima
    Purshia stansburiana
    Solidago missouriensis var tenuissima
    Endemics. Endemism in plants is uncommon in the region. Two species qualify as endemics.
    Hymenoxys ambigens var. neomexicana [Animas and Peloncillo Mountains]
    Limosella pubiflora [Chiricahua and Peloncillo Mountains]

    Flora of the Sierra San Luis

    The Sierra San Luis (San Luis Mountains) are a small north-south trending range of about 25 miles (15 km) in length, extending from just across the U.S. border at San Luis Pass where it is contiguous with the Animas Mountains, south into Mexico along the borders of Chihuahua and Sonora to the vicinity of Mexico Highway 2 at Puerto San Luis. The elevation range is 4,300 - 8,300 feet (1,310 – 2,530 meters). Substrates include volcanic ash, basalt, andesite, felsite, quartz latite tuff, rhyolite tuff and conglomerate. Wetland habitats are limited. In some places a Douglas-fir community is present.

    Botanically the San Luis Mountains are largely unexplored. The range is in a sparsely settled region of Mexico where the plant communities should be intact. Accessibility is difficult, hence its biota is poorly known except for incidental collecting in the San Luis Pass area at the north and Puerto San Luis area to the south. Plant communities would be expected to be similar to those known from the Peloncillo and Animas Mountains. Because of their higher elevation (than the Peloncillo Mountains), a stronger Madrean component of the flora is expected. It is known that the Peloncillo Mountains rise to 6,625 ft. (2,019 meters) and support a flora of 879 species and the nearby Chiricahua Mountains reach 9,726 feet and support a flora of 1,200 species. It can be safely concluded that the San Luis Mountains flora would consist of at least 900 species, although this remains to be documented.

    The first explorations into the Sierra San Luis were those of Mearns12 conducted in 1892 and 1893. Mearns explored several canyon systems in the Sierra San Luis and climbed to the tops of the tallest peaks. He collected plant and animal specimens and recorded observations on the elevational distribution of trees and shrubs. He provides a short plant list for the range but the taxonomy has changed so much that some of the observations cannot be accurately recorded in the following list of species without reviewing his vouchers (if vouchered) that are at the U. S. National Herbarium.13 Mearns published some photographs of the area.

    A more recent survey conducted in the Sierra San Luis is that of Marshall for birds inhabiting pine-oak woodlands in the mountains of southern Arizona and adjacent Mexico. Marshall worked in the Turkey Canyon area, a site also explored by Mearns. Marshall writes: “In Turkey Canyon there was a patch of a few acres consisting of this improbable mixture of trees: Douglas- fir, piñon, Chihuahua pine, Apache pine, Arizona cypress, alligator juniper, net-leaf oak, silver- leaf oak, and Arizona madrone.” He reports that Turkey Spring is still (in 1953) like the photograph in Mearns. He reports for that time in 1953 no logging and no grazing in the Sierra San Luis was occurring.

    The plant list has come from the literature such as it exists and a survey of holdings at four herbaria for collections made at San Luis Pass, where the San Luis Mountains are contiguous with the Animas Mountains. The only significant collections from the south end of the range are from Puerto San Luis along Mexico Highway 2 and a microwave tower road that were made by Van Devender and are at ARIZ. The observations Van Devender made on the flora about his campsite have been provided.

    Flora of Lordsburg Playa

    A conspicuous feature of the landscape when driving between Lordsburg and Road Forks on Interstate 10 are the alkali flats of Lordsburg Playas. These landscapes are barren, generally dry, flat, and undrained. The playas soils are strongly alkaline and contain considerable amounts of clay, reducing permeability. This area would seem unworthy of any conservation effort except that an important aquatic arthropod assemblage survives there and flourishes when the playa holds water.

    Surrounding the alkali flats is a zone of vegetation on Horndale soils of medium grasses, mesquite, saltbush, and althorn. The surface of the soil is silty loam with abundant clay below that. This soil is also strongly alkaline and has slow permeability. The plant community is not diverse but contains some rare plant species worthy of conservation efforts. The greatest threat to this community is grazing, since the vegetation that grows there can tap water resources throughout the year.

    Rare and Endangered Plants

    A number of resources document rare and endangered plant species.18 Within New Mexico plants are assigned a “R-E-D Code.” More information on the meaning of agency codes can be found on the New Mexico Rare Plants Technical Council (1999) website [] and in the Arizona Rare Plant Field Guide.

    Knowledge Gaps

    A full botanical survey of the Sierra San Luis would be an important addition to the knowledge of the Peloncillo region, elevating its stature as an important botanical area even further.

    While the Peloncillo region as a whole is fairly intact in its ecosystem health, a few areas of note should be targeted for specific, targeted conservation: Grasslands at Cloverdale are in the highest valley bottom in the Apachean or “sky island” region. Native grasslands of all types are quickly vanishing on both sides of the international border, but these plains-type grassland associations dominated by blue - and black- grama are especially rare. Lordsburg Playas contains rare plant species, which are threatened by grazing in this unusual environment.

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    Section Authors:

    VEGETATION - Richard D. Worthington, Ph.D. Associate Professor of Biological Sciences, University of Texas at El Paso. Richard is curator of the UTEP herbarium, which is part of the Centennial Museum. His research specialty is floristics of mountain masses in the Southwest.