Positive interactions under nurse-plants: spatial scale, stress gradients and benefactor size

Positive interactions often play an important role in structuring plant com munities and increasing biological diversity. Using three scales of resolut ion, we examine the importance of a long-lived desert tree, iron-wood (Olne ya tesota), in structuring plant communities and promoting biological diver sity in the Sonoran Desert. We examined the positive effects of Olneya cano pies of different sizes on plant communities in mesic and xeric habitats th roughout the central Gulf Coast subregion of Sonora, Mexico. In xeric sites , Olneya canopies had strong positive effects on plant richness and abundan ce, and small positive effects on the size of plants, underscoring the role of facilitation in extreme environments. In mesic sites, Olneya canopies h ad very little effect on perennials and a negative effect on ephemeral rich ness, suggesting predominantly competitive effects in this less stressful e nvironment. Overall, Olneya canopies increased biological diversity where a biotic stress was high, but did not increase diversity in more mesic areas. Thus Olneya canopies caused consistent shifts in plant-community structure among xeric and mesic sites, but not when these landscapes were combined. Benefactor size also mediated positive interactions, with larger Olneya can opies supporting larger perennials in both xeric and mesic sites. Thus stre ss gradients and benefactor size both influenced the balance of facilitativ e and competitive effects under nurse-plant canopies, and the spatial scale at which facilitative effects shape community structure.