The benefits of a nurse plant, shade cloth, and nurse rock on Carnegiea gigantea (saguaro) growth and survival shift along a stress gradient

Abstract

Facilitation tends to be prevalent in arid lands as exemplified by nurse plant relationships. Carnegia gigantea (saguaro), a foundation species in the Sonoran Desert, faces threats of climate change and changing fire regimes. Since it is known to depend on nurse plants, restoring saguaros post-fire or plantings to support regeneration of the species will also require consideration of nurse plants. We sought to understand the specific tradeoffs and mechanisms of planting saguaros with nurse plants or structures mimicking nurse plants. We planted 40 saguaro plants in individual plots with a fully crossed combination of nurse plants, nurse rock, or shade cloth, and controls with five replicates in an outside planter in Scottsdale, Arizona, USA. We watered plots for ten months before turning off the irrigation. Saguaros grew best without nurse plants and nurse rocks in the first year, but only 15 survived the second summer without supplemental water, and of those, 14 were under nurse plants. Photosynthetically active radiation was associated with saguaro survival but not mycorrhizal inoculum potential or temperature. Nurse plants, but not treatments mimicking nurse plants, clearly facilitated saguaro survival under drought conditions, outweighing competition for light and slower growth.