The Saguaro: A Population in Relation to Environment

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Abstract
The saguaro (Cereus giganteus, Carnegiea gigantea) is a major plant of the Sonoran Desert, occurring in a number of types of desert and extending into some desert grassland. The center of maximum population density in the Tucson area is on the driest slopes of mountains, at low elevations; the finest stands of large individuals occur on some of the upper parts of the valley plains or bajadas. Toward higher elevations in the mountains the populations is limited by low winter temperatures, which periodically kill large proportions of the population by freezing. Down the bajada slopes the population is limited by the occurrence of finer soils and by other factors. The population is reproducing well on rocky slopes and in some bajada communities but is failing to reproduce on the finer soils of bajadas affected by grazing. The kill by freezing is a temporary catastrophe, for many younger individuals survive the freeze. Grazing subjects the population to a gradual disaster, with slow decline to disappearance resulting from failure of the saguaro to reproduce. When the effects of grazing are far advanced and rodent populations are high, as in parts of Saguaro National Monument, these effects are largely irrversible.