SHADE AS A CAUSE OF THE ASSOCIATION BETWEEN THE CACTUS NEOBUXBAUMIA TETETZO AND THE NURSE PLANT MIMOSA LUISANA IN THE TEHUACAN VALLEY, MEXICO

SUMMARY (1) The establishment phase of Neobuxbumia tetetzo, a giant columnar cactus dominant in the Tehuacan Valley in central-southern Mexico, occurs mostly beneath the canopies of trees and shrubs which act as nurse plants. This pattern cannot be attributed to preferential seed dispersion, as Neobuxbaumia fruits open while still on the plant, dropping c. 1000 seeds fruit-1 randomly around the parent plant. (2) An experimental field analysis of germination and survivorship of cohorts under different environmental conditions was performed in order to determine the underlying mechanisms associated to the establishment of N. tetetzo beneath the canopies of Mimosa luisana, a legume which is the most abundant shrub in the community. The experimental treatments followed a factorial design, with three levels of radiation (open space, artificial homogeneous shade and the shade produced by Mimosa luisana), two levels of predation (exclusion and non-exclusion of predators), and four topographic aspects (north, east, south and west). (3) Seed germination was lowest in open spaces. In all treatments, exclusion from predators significantly increased seedling survival. Only the shaded treatments had live individuals at the end of the experiment, 2 years later. The results suggest that the nurse-plant effect between Neobuxbaumia tetetzo and Mimosa luisana is chiefly the result of differential survival in shaded microsites with less direct solar radiation, and consequently with lower daytime temperatures and lower evaporative demand. (4) In order to determine the generality of these results, field samplings were conducted in two Mexican deserts located outside the tropical belt: the Vizcaino Desert in Baja California and the Gran Desierto de Altar in Sonora. In these deserts direct solar radiation has a southern azimuth all year round. Five of six succulent species analysed showed a significant pattern of greater establishment on the shaded north sides of nurse plants. (5) The amelioration of the physical environment produced by the nurse plants is important in the patch-structurated population dynamics of many communities of desert plants.

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