Nurse plants were associated with only 4% of O. acanthocarpa approximately doubled and its frequency increased 14-fold. For four elevations ranging from 230 m to 1,050 m, the mean height of O. A common cylindropuntia in the northwestern Sonoran Desert, Opuntia acanthocarpa, was investigated for the following hypotheses: its lower elevational limit is set by high temperatures, so its seedlings require nurse plants its upper elevational limit is set by freezing spine shading is the least at intermediate elevations and changes in plant size and frequency with elevation reflect net CO₂ uptake ability.