Summary and Conclusions
Although the first record of man in the Chisos Mountains was in 1747, his influence upon the land was not significant until the cattlemen arrived in the 1880s. From this time until the establishment of Big Bend National Park in 1944, the vegetation which supported man's livestock was visibly changed by overgrazing. In 1934 the Civilian Conservation Corps established the first road which facilitated living in the remote Chisos Basin. These early activities provided the impetus needed to expand the facilities to their present status. During the major National Park Service developmental phase of the 1950s, the basin experienced the most severe drought recorded in the mountains. The earlier effects of grazing were amplified by the drought and the vegetation is only presently exhibiting significant signs of recovery.
The basin is an elevated bowl, carved by erosion from layers of Cenozoic lava and intrusive ridges. The resulting steep, rugged topography presents complex patterns of parent material from which the soils are derived. The soils have no particular pattern of distribution and belong to three classes: sandy loam, sandy clay, and sandy clay loam. Three vegetation formations are supported by the soils: Evergreen Woodland Formation, Chaparral Formation, and the Chihuahuan Desert Formation. The chemical characteristics of the soils supporting the formations exhibit certain general trends. The pH increases with aridity as does calcium carbonate; however, carbon and phosphorus decrease in amount from chaparral to woodland to desert, respectively.
A long-range climate analysis indicates a combined warming and drying trend; however, the character of the basin vegetation presently exhibits a shift from a short, xeric cycle to a more moist and cool period. The adjusted annual rainfall of 18.26 inches occurs primarily in the months of May through October when the daily maximum means are approximately 68-88°F. Only rarely does the low winter moisture come in the form of snow and sleet.
The present basin vegetation, although complex in composition and distribution, can be categorized into three major physiognomic-physiographic types: a tree-dominated, northern exposure, upper elevation, Evergreen Woodland Formation; a shrub-dominated Chaparral; and a southern exposure, lower elevation, semisucculent-dominated Chihuahuan Desert Formation. The major woodland trees in increasing dominance with elevation are; Juniperus pinchoti, Quercus grisea, Pinus cembroides, J. deppeana, Q. emoryi, J. flaccida, and Q. gravesii. Shrubs and grasses can be locally dominant as well as succulents and herbs to complete the three woodland strata, The desert formation, dominated by the semisucculents Agave lecheguilla and Dasylirion leiophyllum and succulent Opuntia engelmannii, is confined to two strata. Acacia constricta, Viguiera stenoloba, and Fraxinus greggi provide the upper stratum, while locally dominant grasses such as Bouteloua curtipendula, B. hirsuta, B. eriopoda, and Aristida glauca control the lower stratum. The latter two formations have many similar features; however, in the chaparral succulence is relatively insignificant.
Vegetation dynamics suggests that the woodland is expanding toward its pre-grazing range status. The process is slowed because the majority of human activity is centered at the formation's lower contact with the man-influenced desert. The woodland species must also compete with the numerous desert and disturbed-area species which responded to the impact conditions and control the region. Many of these species have long lifespans and can occupy the area for considerable time.
Investigations on the present forms of impact indicate that both the vegetation and physical factors of the environment have been altered by the activities of man. The major alterations observed in the vegetation are decreased numbers of natural individuals, decreased numbers of natural species, decreased natural cover, decreased community complexity, and decreased community stability. All of these characteristics are attributes of the disturbed area, unstable due to environmental changes. The severity of the change determines the degree of the exhibited effects. The forms of present impact ranged from totally denuded areas, such as around buildings and along trails, to densely vegetated areas with impaired drainage.
The forms of environmental change most severely affecting the vegetation were the alterations of drainage patterns and disturbances of the soil by upheaval or denuding, which result in the alteration of many soil factors. Since the soil factors and vegetation are reciprocally interrelated, the alteration of the vegetation acts also as an environmental change. The most frequently observed altered soil factors were increased compaction, increased runoff, decreased infiltration, and altered soil strata. These changes influence soil moisture, temperature, gas exchange, microorganisms, pH, nutrients, and stability, all factors which influence the vegetation.
The following actions upon the vegetation by man were observed or sampled in the impacted areas of the basin. They are drawn from the above interrelationships between the vegetation and its environment.
The areas of man's activity have been denuded, initiating a complex natural sequence of factors and plant species. The denuding process favors only a few species in the basin (Table 7) which are adapted for survival in the area. These species are generally herbaceous in the earlier stages and have high physiological tolerances for low moisture, necessitate low nutrients, provide numerous disseminules, and are represented in the later successional stages by long-lived shrubs. Many years are required for the succeeding vegetation to provide adequate conditions for native vegetation to regain dominance. Evidence indicates that from 25 to 50 years are required for reestablishment of the Chihuahuan Desert Formation in the basin, and 75-100 years for the Evergreen Woodland Formation. All too frequently the succession is halted in the initial stages and seldom advances, thus favoring xeric species.
Examples of this impact, roughly in the order of decreasing contributions to basin denuding are: roads and parkings, trails, National Park Concessions, Ranger Station and personnel housing, sewage facilities, pipelines, campgrounds, Chisos Remuda, Campfire Circle, and wells.
Man, by constructing service facilities in the basin, has altered the terrain and the natural drainage patterns. This impaired flow causes excess moisture and eroded soil to occupy the area, resulting in dense vegetation which occurs below the Ranger Station. The structures may also divert the flow so that little runoff reaches the site, as occurs in the desertoid area on the east side of the Ranger Station.
The major promoters of impairment action are roads and parkings, trails, and National Park Concessions buildings and their access walks. Not only do the roads and parking areas in their construction alter the drainage, but thereafter their hard, undulating surfaces influence runoff. The numerous eroding road-cuts demonstrate the undesirable consequences that can result. By dissecting the steep slopes and crossing the numerous ravines, the trail system also functions as an unnatural runoff diversion. The heavily silted drainbars, which increase in size with each downpour, demonstrate trail interception of runoff. Each of these alterations significantly influences the vegetation.
The activities of man have created many new, disturbed habitats in the basin which exclude the natural vegetation and promote the creation of new vegetation types. An outstanding example is the disturbed vegetation type to the northeast of the main campground, created by man's livestock. At least seven new habitats were created in the main campground that support the exotic species (Table 14), along with the many "weedy" species habitats. All newly created areas and their vegetation detract from the integrity of the regional landscape. Few of man's activities have created suitable habitats for the native woodland trees and shrubs, since most sites are highly compacted or in a state of erosion.
Man has played an important role in the introduction of new plant species into the basin, as is evident in his impacted activity areas. In this study alone nine exotic species were collected in the Upper and Lower Basin complexes, even though no special attempt was made to include them in sampling. Introduced "weedy" native species are also evidence of man's activities. The establishment of new species has been promoted by the introduction and frequent use of desert soils in basin construction and maintenance. This practice not only introduces numerous species but also provides them with the native soil most conducive to their growth. Frequently. the species are able to invade other habitats and extend their influence.
The ability of these species to succeed is evident, and methods to control them in the basin should be developed and employed. The special man-provided habitats which promote them must be alleviated and suspected methods of introduction eradicated or reduced. The major avenues of introduction in the basin are the Chisos Remuda, visitors in the main campground and National Park Concessions facilities, and the National Park Service in its transport of soils for road maintenance and construction programs. The source of Sorghum halpense and Solanum elaeagnifolium can probably be traced to the feed or fecal matter of the horses fed outside the basin prior to their entrance. The frequency and density of both species increase greatly as one approaches the operation from all directions, especially along the road leading to the Remuda and around the stables. The seeds are further disseminated as the animals traverse the mountains.
The campgrounds and National Park Concessions harbor the greatest variety of exotic species, reflecting the numerous distant origins of the visitors and the conditions conducive to growth offered by the many diverse habitats in the areas these visitors frequent in the basin. A considerable stand of Sorghum halpense is present in the moist ravine to the north of National Park Concessions warehouse.
Man's activities have polluted the Chisos Basin and have affected the vegetation. The effect of added organic material in the form of raw sewage and fecal matter of animals can be observed. The effects of raw sewage are presently localized to the Upper Basin near the Trailhead and infrequent seepage from the lower lagoon. The action of this pollutant has caused death to some plants, while increasing growth of others. The unnatural, large populations of animals at the Chisos Remuda have significantly altered the vegetation by their pollution. The total effect of pollution by horses cannot be readily determined, but cognizance of the condition may eventually lead to the measurement of more effects. A similar condition exists in the attempt to assess the pollution due to service station spillage, automobile effects, and the gallons of cooking grease and waste water poured into the campsite vegetation in the campgrounds. Since the latter are imparting a distinctive odor to the campgrounds, they have residual effects.
All the above actions are taking place in the basin, and without altered programs the activities of man will continue to perpetuate the impact. Unfortunately, man's demands upon this unique basin are increasing exponentially as are his effects upon the landscape. As the major factors which influence the vegetation are known, programs to curtail the actions can and should be initiated to reverse the effects of impact.
If the basin vegetation is to continue its struggle to recover from the grazing and drought years and to regain any semblance of integrity from present impact, every effort must be made to curtail activities in the basin. All present activities should be evaluated to determine if they are absolutely necessary or desirable. Many of the observed activities could be performed equally as well at lower elevations, thereby reducing impact upon the unique, less stable landscape of the basin. The order of priority for immediate evaluation based upon present vegetation impact is: National Park Concessions, Chisos Remuda Concessions, National Park Service personnel housing and Ranger Station, and trails, roads, sewage, and campgrounds.
To assist the revegetation cycle in impacted areas and those closed to use, a vigorous program of rehabilitation should immediately be initiated. The revegetation program should at all times be biologically sound and in accord with the natural revegetation cycle of the basin. To aid rapid recovery, all species used in the program should promote the mesic woodland species, as the native desert vegetation and exotic species will prevail by default of the woodland vegetation's inability to become established initially.
Last Updated: 1-Apr-2005