These barrel cacti are most common on the drier places on the south side of Tempe Butte.  You will find most of them on the driest upper slopes.  However, in the earlier stages of life some cacti need protection to get started.  If you look carefully in the middle of Creosote bush (Larrea tridentata), you will sometimes see small cacti being "nursed" or protected from desiccating sunlight.