Comparative performance of the giant cardon cactus (Pachycereus pringlei) seedlings under two leguminous nurse plant species

Recruitment and survival of cacti in North American deserts are facilitated under the canopy of nurse plants. In the Sonoran desert, the giant cardon cactus (Pachycereus pringlei) is associated with ironwood (Olneya tesota) and mesquite (Prosopis glandulosa) trees. We hypothesized that P. pringlei seedlings would perform better under ironwood than mesquite, on the basis of the mature individuals patterns of association. In Bahia Kino, Sonora, we conducted a field experiment from 1992 to 2000 comparing the performance of P. pringlei seedlings under randomly selected O. tesota, and P. glandulosa trees. Results indicated that P. pringlei seedlings under P. glandulosa had significantly higher survival and were significantly taller than those under O. tesota after 8 years. Micro-environment and soil properties beneath both trees did not differed significantly, while tree physiognomies differed only in height at the base of the canopy and basal area. As a deciduous tree, P. glandulosa provided more litter to the ground than O. tesota. These results therefore did not support our initial hypothesis. We discuss how other biotic factors such as differential seed dispersal might explain why P. pringlei establishment is more strongly spatially associated with O. tesota.