Vegetation of the Guadalupe Escarpment, New Mexico‐Texas

Abstract

The Guadalupe Escarpment, a Persian limestone reef, is the eastern face of a semiard—mesothermal mountain mass that rises 1,000—4,000 ft above the southwestern edge of the Great Plains. Phytocenoses range from xerophytic (Larrea, Flourensia, Acacia dominance types) on the Silty to gravelly plains, through less xerophytic (Agave, Dasylirion, Juniperus dominance types) on gravelly to bouldery lower slopes, to comparatively mesophytic (Juniperus, Quercus, Pinus dominance types) on rocky upper slopes and the escarpment peneplain. This general gradient is a vegetational continuum in which species' ecologic amplitudes are distinct but form overlapping assemblages of similar structure. Special environmental gradients are produced by topographic discontinuity. On low elevation canyon slopes, south—facing exposures support more xerophytic dominance types than north—facing exposures, although floristic differences are minimal. At highest base levels and slope elevations, south—facing exposures support the most xerophytic of all canyonside vegetation, and floristic differences between slopes are maximal. Dominance types with a tree stratum are well developed inside canyons; they form a continuous streambed—stream terrace—canyonhead sequence of increasing mesophytism. Canyon depth and water are important in supporting relict woodland dominated by Acer, Quercus, Juniperus, and Juglans. Height and coverage as stratal features plus growth contribute to a synusial description of the vegetation. Based on relative concordance of synusiae, shrub desert, succulent desert, evergreen woodland, and deciduous woodland plant formations are present. Microphyllous vs. succulent—semisucculent shrubs and half—shrubs distinguish the two desert formations. Narrow and broadleaf evergreen trees vs. broadleaf deciduous trees distinguish the two woodland formations. A mid—gradient predominance of structural diversity characterizes the ecosystem. Comparisons with other southwestern desert regions reveal that dominance types vary considerably; but the same formations recur, although in differing patterns based largely on regional edaphic differences. Formations are shifted toward the xerophytic end of an environmental gradient in the igneous—based Chisos Mountains, Texas; and succulent desert is not well developed on the north side of these mountains. Succulent desert of the Chihuahuan Desert region may not be classable with that of the Sonoran Desert region, because a tree stratum is added in the latter region. Succulent desert and evergreen woodland of the Guadalupe Escarpment could have had a common Madro—Tertiary origin based on evergreen—succulent dominance and paucity of deciduous growth forms.