Frugivorous birds mediate sex-biased facilitation in a dioecious nurse plant

8 páginas, 1 figura, 2 tablas.Facilitation by dispersal occurs if the nurse plant acts as a focus which is actively selected by seed dispersers and enhances the fitness of the facilitated plant. Sex-biased facilitation may be produced if seed dispersers tend to concentrate the seeds under female, fruit-bearing plants of dioecious species more often than under conspecific males. Juniperus sabina is a dioecious shrub with a prostrate growth form from Mediterranean high mountains that modifies many microhabitat characteristics related to seedling establishment and survival. Soil water availability, maximum soil temperature in summer, organic matter and total nitrogen content, were different on open ground as compared with beneath J. sabina shrubs, irrespective of its sex. Other studied characteristics such as soil bulk density and soil compaction after rain did not differ between the microhabitats considered. Some species, such as Juniperus communis, Pinus nigra, Helleborus foetidus and Euphorbia nicaeensis, are spatially associated to J. sabina shrubs, strongly suggesting a facilitative role. The anemochorous P. nigra and myrmechorous H. foetidus and E. nicaeensis did not associate preferentially to any sex of J. sabina. Only J. communis, an endozoochorous species sharing the same bird dispersers as J. sabina, presented a female-biased spatial association with the nurse plant. Seed dispersal mediated by birds attracted by the fruit-rewarding females of J. sabina explains the sexbiased spatial pattern of Juniperus communis.We thank the people of Puebla de San Miguel, especially Major L. Alcusa, the foresters J. Monedero and V. Tortajada, L. Azcutia, V. Tortajada jr., and Vicente "el pastor" for their hospitality and help during our research. Gori helped with field work and in the laboratory. Special thanks are due to D. Montesinos for his work on seed dispersal and predation. Chemical and soil analyses were done by technicians at the Centro de Investigaciones sobre Desertificación (Valencia). M. Debussche, D. García, C.M. Herrera, C. Körner, J. Lepš, F. Pugnaire and L. Soldaat provided valuable comments on the manuscript. This research was supported by projects FEDER 1FD97-0551 and AGL2001-1061 from the Ministerio de Ciencia y Tecnología. MV was granted contracts of the Reincorporación de Doctores y Tecnólogos and Programa Ramón y Cajal del Ministerio de Ciencia y Tecnología during this study.Peer reviewe