I sampled spine length in the keystone Carnegiea gigantea (Engelm.) Britton & Rose in two northern Sonoran Desert populations where this species is ultimately limited by cold temperatures. I compared spine length (1) between two sites, (2) on north versus south facing sides of plants, (3) by level of shading, (4) by proximity to other potential conspecific competitors, and (5) by presence near sources of surface runoff from which all their water is obtained. Spine length increases, providing thermal protection and reducing photosynthesis, where temperature is more extreme (winter and summer) and where conditions are more xeric. The difference in spine length by direction on plant was much smaller at the more extreme site. Clumped plants that necessarily had increased competition for water but also greater shading were split; at the more extreme site, greater spine shading occurred where water competition was fiercest, while at the less extreme site, greater shading occurred in association with sunlight rather than water.