Nurse Rocks as a Minimum-Input Restoration Technique for the Cactus Opuntia basilaris

Much plant recruitment in deserts occurs in shaded microsites below canopies of mature perennial plants. Absence of "nurse plants" from disturbed sites often hinders ecological recovery. Given uncertainty and expense of directly restoring live plants in deserts, we explored using abiotic structures-nurse rocks- as a restoration option for reestablishing Opuntia basilaris (beavertail pricklypear) on a disturbed site in the Sonoran Desert, of the U.S. Wild populations at this site were strictly associated with large varnished surface rocks. To examine whether rocks functioned similarly as nurses for O. basilaris recruitment in disturbed and undisturbed reference habitats, we transplanted 30 rooted individuals each into habitat in which large varnished surface rocks were removed and into nearby undisturbed habitat. Within habitats, half the individuals were transplanted into open (no rocks) or rock (rocks surrounding transplants) microsites. In the first 15 months after planting, which had average precipitation, transplant survival did not differ between microsite types in either habitat but functional measures were influenced positively by nurse rocks. Nurse rocks sharply increased vegetative growth in 91.7% of transplants in undisturbed habitat and in flowering occurrence in 28.6% of transplants in disturbed habitat. Nurse rocks became even more important during extreme drought as the experiment progressed. By 27 months after planting, 2x (disturbed habitat) and 8x (undisturbed habitat) more transplants survived with nurse rocks compared to without. As a low-cost technique utilizing on-site material, nurse rocks show promise for enhancing revegetation success on disturbed desert sites, especially during drought.