BIG BEND The Impact of Human Use Upon the Chisos Basin and Adjacent Lands NPS Scientific Monograph No. 4

This topic is a most difficult one to assess, but necessary to confront if one is to assess the role of human impact. The major problem is to determine whether man's influence is a single factor which must be included in the sum of ecosystemic dynamics, or whether it has an additive or compounding effect upon one or several factors. Because there are no documented natural factors of extreme effect such as fire, fungal diseases, pestiferous infestations, unusually high native animal populations, faulting to effect hydrology, etc., since European man's influence, only climate needs to be superimposed upon man's impact.

In considering climate there is little information available. Droughts of short duration are recorded, but the majority were of little consequence to the vegetation as a whole. Droughts which lasted for a year reduced forage grasses, but had little effect on sheep or goat browse. The drought of the fifties, following the cessation of grazing, created a major problem because the effects of the drought were accentuated by the impoverished state of the vegetation at the time. The drought began only 6 years after grazing ceased, making assessment of vegetation recovery from grazing difficult. McDougall (1953) reports that vegetation recovery by 1953 for the Chisos Mountains was rapid; for instance, the grasses on the South Rim were probably in as good a condition as before grazing. Unfortunately, nothing was stated concerning the Chisos Basin but, because of its elevation, the condition of the basin vegetation was not as improved as that higher in the mountains. This is implied from conditions described by McDougall for lower areas in the park.

I can see continued improvement in the Boot Spring to South Rim region from 1964 to the present, indicating that the condition on South Rim today is superior to that of 1953. This then makes McDougall's report of rapid vegetation recovery quite difficult to accept, as either the vegetation was very poor during his first visit (1943-45) or the condition deteriorated greatly between 1953 and 1964. The latter could certainly have occurred, since 3-5 years of drought ensued. The question then arises: Were the conditions worse after a 6-year recovery from 14 or more years of grazing-browsing or after a 13-year recovery from a 5- to 7-year drought? I tend to accept the former explanation, because even after the added effects of good moisture and no grazing since the drought, one still finds many areas poor in natural vegetation composition. This seems to negate the concept that vegetation can recover rapidly in a 5-7 year period.

If a similar reasoning can be applied to basin conditions, but at a slower pace of improvement due to more human activity, vegetation conditions improved little prior to the drought and thus compounded the effect of the drought. Such a hypothesis cannot be proved or substantiated because of the lack of extant data. The case is ecologically sound for 13 years after the drought or 24 years since grazing ceased, one finds areas of the basin still in very poor condition. The area in point is a triangle, one side being a line drawn from the junction of Pulliam Ridge-Ward Mountain drainage to a ravine south of National Park Concessions prefabs; a second drawn from the prefabs to the Panther Pass parking lot; and the final side following the drainage along the base of Pulliam Ridge to Ward Mountain junction. All transects sampled in this area (sites 1, 5, 7, 8, and 11) had vegetation of a hybrid or disturbed nature except for site 7. On the basis of results of this study, this condition is due to man and his livestock.

Predrought grazing in the basin had its greatest effect by reducing grass cover and density, thus permitting the first major invasion of desert herbs, shrubs, and succulents and the beginning of the soil moisture-nutrient depletion, erosion cycles, and the loss of the natural vegetation's competitive edge. Since all of these factors could not be corrected in 6 years, the drought supported the trend initiated by the overgrazing. Unfortunately, the invasion plants were of xeric origin, possessed long life-spans, and were maintained by small disturbances which permitted them to control the area at the expense of natural grasses and woody plants. The presence of the xeric plants delayed vegetation recovery and development and excluded grasses which promote soil nutrition, moisture, stabilization, structure, and reduced runoff. Without these improvements the drought supported establishment of the Chihuahuan Desert Formation plants at the expense of Evergreen Woodland Formation components.

The construction of Fig. 4, which presents the hypothetical distribution of vegetation, essentially involved the addition to the Evergreen Woodland Formation to those areas which are hybrid or disturbed in nature. Evidence that the area would be woodland were it not impacted is the occasional large trees and small seedling-saplings which occur in the region. In small microclimates on some northern exposures along ravines, very highly developed woodland vegetation is present, including grasses and shrubs in some instances.

The species composition of the Evergreen Woodland Formation would not include in any great number such invaders or disturbed area species as Xanthocephalum spp., Chrysactinia mexicana, Opuntia engelmannii, Prosopis glandulosa, and Acacia constricta. Species occurring in the woodland such as Viguiera stenoloba, Artemesia ludoviciana, Bothriochloa barbinodis, Bouteloua curtipendula, B. gracilis, and Juniperus pinchoti would also be of lower importance. Quercus grisea and Q. emoryi would be of greater importance as trees in the lower basin area. Accompanying the oaks and various woodland grasses would also be woodland shrubs, especially along the ravines of the lower basin. now dominated by desert shrubs.

Fig. 4. Map of hypothetical vegetation in the basin.

The Chihuahuan Desert Formation in the basin would be of the sotol-grass association type with some scattered oaks and Agave scabra, rather than the lechuguilla-grass association which presently dominates large areas. On isolated areas of the more xeric exposures lechuguilla-grass would still occur, but the grasses would predominate. Low elevation invader species such as Larrea tridentata, Erioneuron pulchellum (fluffgrass), Aristida glauca, and Heteropogon contortus would not be of their present importance. The following lowland species would not be expected to occur in the basin at all: Atriplex canescens (fourwing saltbush), Opuntia kleiniae (candle cholla), O. leptocaulis (pencilcholla), Parthenium incanum (mariola parthenium), Baileya multiradiata (desert baileya), Machaeranthera pinnatifida, Hilaria mutica, and Erioneuron grandiflorum (large flowered tridens). These species are currently present in the campground and adjacent areas.

The effect of grazing upon the Chaparral Formation is more difficult to analyze. Quercus grisea and Q. emoryi would be, at present. of much greater importance on the upper slopes of Casa Grande, while Xanthocephalum spp. would be much lower in importance in this area. Agave lecheguilla, Larrea tridentata, and Aristida glauca would not be expected to occur on the limestone substrate, while Quercus intricata and Fraxinus greggi would be of greater importance on the southern exposures than they are presently.

The area to the northeast of the campground would resemble vegetation at sites 7 and 16 which are presently dominated by pinyons, junipers oaks, and grass (Table 4). Below the campground and Chisos Remuda would be a vegetation dominated by grass, with scattered gray oaks and pinyons especially along the ravines. The lower slope of Pulliam Ridge would again be dominated by grass, gray oaks, and scattered sotol and pinyons.