Saguaro Cacti Associate with Some Plants More Than Others

(Photograph = Saguaro cactus, Max Ruckman)

Introduction

The saguaro cactus (Carnegiea gigantea) is the northernmost member of the ceroid subtribe of cacti—a group that includes tall columnar species. Its northward spread is thought to be limited primarily by cold temperatures.

The Details

In Organ Pipe Cactus National Monument, Arizona, saguaros were studied for how often they occurred in the open versus beneath perennial shrubs and small trees. Saguaros occurred significantly less often than expected in open areas and more often under the canopies of perennial plants.

Once expectations were calculated from plant cover, saguaros occurred less often than expected with Larrea tridentata (creosote) and more often than expected with Prosopis juliflora and Cercidium microphyllum (now Parkinsonia microphylla). Large, tree-like perennials likely help in two ways: they moderate extremes of heat and cold and they serve as perches for birds that disperse saguaro seeds, concentrating seeds beneath the canopy. In soil samples beneath C. microphyllum, seed density was highest near the trunk and declined toward and beyond the canopy edge—consistent with nonrandom seed deposition.

These associations held across age classes: the composition of nurse associates did not change meaningfully between shorter (<2 m) and taller (>2 m) saguaros. Taller saguaros were more often found with taller, tree-like associates (e.g., C. microphyllum, Prosopis), and less often with smaller shrubs (e.g., Ambrosia).

Why the relative scarcity under Larrea? Conditions for germination may be poorer beneath creosote (including hydrophobic surface soils), and rodent activity and burrowing are common under Larrea, which can increase seed predation and seedling damage.

Possibly, nurse plants such as Prosopis and Parkinsonia help saguaros establish in the northerly parts of their range.

Additional Reading: Distributional Associates of the Saguaro