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

The vegetation of the Chisos Basin in the future will show a shift in dominance in several areas, provided prevailing conditions continue to support the present trees and seedlings. Present evidence suggests that, above the Upper Basin complex, conditions are favoring the more mesic species, whereas in the impacted areas the more xeric species are favored.

A total census taken along the trail from Juniper Flat to the Laguna Meadow Trail is presented in Table 16. The census was taken in the manner described previously under trail impact. The Quercus grisea-Pinus cembroides tree-dominated trailside is shifting to a possible Q. emoryi-Juniperus deppeana vegetation type. This infrequently used trail is on a western exposure; the lower end is more southern and the upper portion more northern. The numerous Q. gravesii seedlings censused are significant as they are the most mesically adapted species listed.

The census of the circle at Boulder Meadow, at a higher elevation, presents a different dynamic pattern (Table 17). The importance of this census is the significant number of Juniperus flaccida and J. deppeana. This site gives the highest reproductive ratio, 1 tree: 13 seedlings, of any area censused in the basin. A similar ratio for the previous trail sector was 1 tree: 7 seedlings, a rather high value. A similar value for the impacted areas of the basin is less than 1:2.

The results of a series of censuses taken in the Upper Basin region are presented in Table 18. A raw comparison between the areas is not valid, but a comparison with percentages and within an area is significant. The total census presents the trend for the area from the stone cottages to the Campfire Circle, with emphasis upon the mesic northern exposure. The large number of Prosopis glandulosa seedlings is of interest for this species is a "weedy" introduction. All sites favor the xeric species, Juniperus pinchoti and Quercus grisea, as their tree-to-seedling reproductive ratios are greater. Every area had less Pinus cembroides seedlings than trees.

The outlook for tree reproduction is good for those areas not presently under severe impact. This impact, as demonstrated by Table 18. need not be more than a trail dissecting the slope or area. The flat area (3), below the Ranger Station, no longer has heavy impact and shows the greatest tree-to-seedling ratio, 1:3. Included in those areas under impact would be the vegetation directly below the miles of basin trails, which show a decreased reproductive rate. This, I believe, results from falling rock and soil along with disrupted natural drainage patterns by the trails. If the trails continue to deteriorate, the condition will become more severe with time. These areas are now more open, thus more xeric, during a time when the major vegetation conditions are favoring the more mesic species.

Table 16. Tree and seedling census along trail from Juniper Flat to Laguna Meadow Trail.

Table 17. Tree and seedling census along trail in circle at Boulder Meadow.

Table 18. Tree and seedling census of several areas in the Chisos Basin.