LONG-TERM VEGETATION CHANGE AT A FULLY PROTECTED SONORAN DESERT SITE

At MacDougal Crater in the Sierra del Pinacate Reserve, Sonora, Mexico, 3 sources of data were used: a series of exactly matched photographs, begun in 1907; detailed permanent-plot maps, dating from 1959-1960; and an age-distribution analysis of a 170-yr old population of Carnegiea gigantea. The crater vegetation is dominated by the woody perennials Cercidium microphyllum, Encelia farinosa, Prosopis sp. and Larrea tridentata, and the columnar cactus Carnegiea gigantea. Various populations of L. tridentata declined 50-90%, and Cercidium declined 60% during the first half of this century with little or no recruitment since. Carnegiea numbers increased 4-fold over the same period. A 200-fold increase in Prosopis in the playa-like crater center occurred in the early 1970s. Elsewhere on the crater floor, Encelia density increased markedly during the same period from insignificant levels in the early 1960s. Age distribution analysis for the Carnegiea population reveals three major establishment peaks during the 1790-1960 period. The high mortality for some of the species was probably the result of the prolonged drought during 1936-1964. Establishment surges for some appear related to periods of unusually heavy precipitation during certain seasons. -from Author