Abstract
Facilitation by nurse plants likely increases from outside the canopy to the center of the canopy as stresses decrease towards the center of the canopy. These stress gradients may be important in controlling plant distributions, with stress-tolerant species specializing outside the canopy or at the canopy edge, and stress intolerant species specializing at the center of the canopy. We tested if interactions with nurse plants control the distribution of understory species, and if plants species specializing in the understory environment experience higher physiological stress when grown outside the canopy than species specializing in the open environments. We tested these predictions in field sites in the arid environment of Saudi Arabia. We measured the environmental conditions, understory species abundance, and functional and physiological traits of species found under nurse plantAcacia gerrardii. We found thatAcaciatrees have an overall facilitative impact on the understory species. Species found more commonly under nurse tree canopies experience significant physiological stress when growing outside canopies. In contrast, species found more commonly outside canopies do not experience significant physiological stress when growing either under canopies or outside canopies. Our results demonstrate that differences in species ability to tolerate environmental stresses are important in structuring herbaceous plant communities under nurse plants in these extremely stressful environments.