The Vegetation, Floristics and Phytogeography of the Summit of Cerro Potosi, Mexico

Abstract

In a quantitative ecological study of the vegetation of Cerro Potosi above 3,500 m, four community types were recognized. 1) The structurally homogeneous alpine meadow is dominated by shade-intolerant, prostrate dicotyledons. Three dominant species comprised 25.7% of the total frequency and 49.3% of total cover. Predominant life-forms are hemicryptophytes and chamaephytes. Typical alpine species tend to be positively associated among themselves and negatively associated with subalpine species which have invaded the alpine meadow. 2) Subalpine meadows on north slopes are dominatedbyerectforbsand cespitose grasses. They are interspersed mostly within the P. hartwegii forest. 3) On east, south, and west slopes a dense, krummholz-like P. culminicola scrub extends from about 3,450 m to the periphery of the summit. 4) Immediately below the scrub is a forest dominated by P. hartwegii. On the northeast slope this species extends to the summit. Principal understory species in both the scrub and the forest are forbs and cespitose grasses, very few of which are common to the alpine meadow. Distribution of the summit communities corresponds closely to topographic features. High wind velocities and limited water availability apparently are critical factors in preventing woody species from attaining dominance on the alpine summit. The four communities are climax vegetation types. The summit flora includes 81 vascular species in 65 genera and 30 families. Species endemic to the Sierra Madre Oriental comprise 42% of the flora; 13 species are known only from Cerro Potosi; 16 species range northward and are generally centered in the southern Rocky Mountains, 15 are distributed principally to the south; Cerro Potosi is the northern limit of two Andean species; four species have semicosmopolitan, arctic-alpine ranges. The flora is probably of Pleistocene age and mostly derived from neighboring populations of lower elevations. The numerous endemics may be the result of catastrophic selection or other factors which have facilitated rapid evolution in isolated, small populations. The unique alpine meadow and Pinus culminicola scrub communities may have been more widely distributed during cooler or more arid periods of the Pleistocene.