In the nurse plant syndrome, or nurse association, seedlings (beneficiaries) are associated with adult shrubs/trees (benefactors). This phenomenon has been documented in several regions of the planet. Abiotic stress amelioration (one mechanism of facilitation) is one of the causes of this association. Most of the studies addressing the nurse syndrome have been conducted on spatial scales of a few hectares and have focused on only one or a few species. Moreover, there is an almost complete lack of studies addressing the incidence and characteristics of the nurse phenomenon in the arid Andes of South America. We undertook a first approximation to the study of facilitation in these ecosystems. The study was conducted at local and regional scales and involved the assessment of the spatial distribution of juveniles (seedlings and saplings) of 51 populations of 16 shrub and 12 cactus species in relation to shrub cover at 20 localities of the Prepuna (subtropical Andes of Bolivia and Argentina, 20-26°S). In terms of spatial distribution, the juveniles of most of the populations of shrubs studied were distributed both under the shrubs and in open spaces, thereby showing an apparent indifference to microhabitat. Globose and opuntioid cacti were preferentially distributed below the canopies of shrubs and were usually more associated with the dominant shrub species, which stood out as better potential nurses. The pattern was consistent throughout the region, including the more mesic and arid localities. The fact that Prepuna woody species are capable of establishing in open spaces would confer this region a greater resilience. Our findings further suggest that community dynamics in arid and semi-arid environments are more variable than previously thought.
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Acknowledgments We wish to thank Gonzalo Subieta for field assistance; Daniel Larrea-Alcázar, Andrea Loayza, Kazuya Naoki, Rodrigo Ríos, and three anonymous reviewers for important comments provided to the manuscript; Roberto Kiesling and Lázaro Novara for their assistance in plant identification; Parques Nacionales (Salta, Argentina) and Los Cardones National Park for permission to work there; the Los Cardones park rangers (especially ranger Marco) for their assistance. This study was supported by the Rufford Small Grants Program (RSG, grant 07.02.06) and by the Endangered Species Initiative, Werner Hanagarth grants, provided by PUMA Foundation and Conservation International-Bolivia (FPUMA-DIMEP, grant 132/06). This study complies with the current laws of the countries in which it was performed.