(Photograph = Saguaro cactus, Max Ruckman)
Introduction
The saguaro cactus (Carnegiea gigantea) is the northernmost member of the ceroid subtribe of cacti, a group that contains tall columnar cacti. Its northward spread is thought to be limited primarily by cold temperatures.
The Details
Saguaro cacti were studied in the Organ Pipe National Monument of Arizona. The cacti were studied with respect to their occurrence in open space or space covered by perennial plant species (shrubs and small trees).
The cacti occurred significantly less often than predicted by chance in open areas, and significantly more often under the canopies of perennial plants. In other words, saguaro cacti occurred more often in association with shrubs and trees than chance would predict.
Interestingly, once the expectations had been calculated, the cacti were found less often than expected growing with Larrea tridentata (creosote) and more often than expected with Prosopis juliflora and Cercidium microphyllum. It is possible that the latter two small trees serve to limit cold air from reaching seedlings during extreme periods and are preferred nurse plants. The seeds may fall under such plants when birds eat the fruits and then fly to the shrubs/trees to roost.
Possibly, nurse plants such as P. juliflora and C. microphyllum help saguaro plants grow in northerly areas of their range.
Additional Reading: Distributional Associates of the Saguaro
Parkinsonia microphylla (syn. Cercidium) belong to the nitrogen fixing legume family, Larrea is a Zygophyllaceae. That seems a more likely reason.