Columnar cacti-shrub relationships in an Andean semiarid valley in western Venezuela

Abstract

Spatial associations between columnar cacti and mimosoid shrubs were evaluated in an Andean semidesert enclave. An analysis on the capacity of other deciduous shrub species and medium-sized ephemeral plants to modify the spatial distribution of cacti was also included. The number of Stenocereus griseus (Haw.) F. Buxb., Cereus repandus (L.) Backeb. and Pilosocereus tillianus Gruber & Schaftzl that grow below the canopies of perennial plants were assessed and compared with open areas. Comparison of observed and expected number of cactus individuals shows a positive spatial association between S. griseus and Prosopis juliflora DC. Moreover, abundance of C. repandus and P. tillianus under the canopies of mimosoid shrubs were not statistically different from what was expected by chance. Positive spatial association between S. griseus and Capparis odoratissima Jacq., Cassia emarginata L., Cordia curassavica (Jacq.) Roem. & Schult., Croton rhamnifolius H.B.K. and Jatropha gossypifolia L. was also found. Evidence suggests that positive columnar cacti-shrub spatial association may occur either in the cactus thickets or in the thornscrubs. Due to the clumped distributions of S. griseus beneath P. juliflora and other perennial plants, we here postulate that S. griseus is the primary nursed columnar cactus of the enclave. The presence of cacti in open areas suggests that facilitation may be less important for establishment of columnar cacti species in this Andean semidesert landscape than in other temperate and tropical semiarid zones. We here discuss the importance of the nurse syndrome phenomenon for recruitment of S. griseus and for the conservation of this Andean semiarid environment.