| The association of columnar cacti with nurse plants and the mechanisms that account for this association were examined in the cacti Pachycereus pringlei (cardon), Carnegiea gigantea (saguaro), Stenocereus thurberi (organ pipe), and Lophocereus schottii (senita), in the Sonoran Desert of northwestern Mexico.; Except for organ pipe, all columnar cacti were associated with nurse plants in greater proportion than expected by available plant cover. The most important nurse plants were Olneya tesota, Bursera mycrophylla, Larrea tridentata, Hymenoclea monogyra, Atriplex spp., Jatropha cuneata and J. cinerea.; Seed dispersal by animals to nurse plants initiated the cactus-nurse plant association. Six species of birds (Auriparus flaviceps, Myiarchus cinerarscens, Toxostoma curvirostri, Campylorynchus brunneicapillus, Cardinalis cardinalis, and Psaltriparus minimus) and one reptile (Dipsosaurus dorsalis) were the most effective dispersers. During fruiting, cactus seed density under mesquite canopies was 4% (saguaro) and 31% (cardon) of seed density around parent cactus plants. After fruiting, the soil seed bank was denser below mesquite canopies than around cacti or in open areas.; Contrary to expectation, shadiness vs. openness was not a significant factor in explaining seed predation. Cryptic substrates under nurse plants conferred an advantage to seeds and was the significant factor. Thus, despite rodent seed predators being more frequent under nurse plants, seed predation was lower under nurse plants than in the open.; Seedling establishment was experimentally compared among open and shaded microhabitats, and under exposure to or protection from predators. Seedling survival was higher in the shade and when protected from predators than in the open but predators consumed almost all seedlings when not excluded. Furthermore, significantly more seedlings (58%) survived when growing under thorny twigs than seedlings growing completely exposed to predators (3%). Air temperature was more favorable and soil nitrogen levels were higher in the shade than in the open. Photosynthetically active radiation (PAR) was lower in shaded than in open microhabitats.; I conclude that columnar cactus-nurse plant association is the result of three successive mechanisms: animal dispersal of seeds to bushes, escape from predators of seeds/seedlings concealed by the litter that accumulates under bush canopies, and buffering of environmental stress by shade. |
