Density dependent responses of Cacti, Cylindropuntia bigelovii and C. echinocarpa (Cactaceae), in the desert of Southern California, USA

Densities of vegetative propagules (joints) of two cactus species (cylindropuntias) were manipulated in an experiment in the Lower Colorado Desert (Sonoran) of southern California and observations were made from 1981 to 1999. Stem joints of Cylindropuntia bigelovii break off easily and are the sole means of reproduction. Cylindropuntia echinocarpa reproduces primarily by sexual means but has been reported to have a low rate of asexual reproduction by detached joints. The experiment tested 3 predictions: (1) because of its usual asexual mode of reproduction, C. bigelovii would have better establishment success than the vegetative parts of C. echinocarpa; (2) because areas occupied by C. bigelovii often are densely carpeted with joints, this species would be more density-tolerant than would C. echinocarpa; (3) the more tolerant species should show the slowest growth. Overall, results were contrary to these predictions: survival of C. bigelovii showed a clear density effect but C. echinocarpa did not and the more tolerant species, C. echinocarpa, also showed better growth. At the end of 16 years, however, both species had similar numbers of survivors.