Habitat amelioration and associational defence as main facilitative mechanisms in Mediterranean grasslands grazed by domestic livestock

Facilitative interactions alter species dynamics through the mechanisms of habitat amelioration, associational defence against herbivores, and release of competitive interactions. However, the relative importance of each mechanism remains poorly known. This study attempts to fill that gap for systems grazed by domestic livestock, where abiotic stress., grazing, and competition potentially limit plant performance. In southern France, two light-demanding, trees, Pinus sylvestris and Pinus nigra, invade grasslands and often become established near shrubs. We tested the effects of habitat amelioration, associational defence against herbivores and release of herb competition by shrubs on the establishment of pine seedlings by sowing and transplantation experiments. We also assessed the effect of shrubs on seed predation and sapling growth. Shrubs increased seed predation and depressed seedling emergence and sapling growth, but they facilitated the survival of pine seedlings by increasing shade and sod Moisture and by protecting seedlings against sheep grazing. There was no effect of herb competitive release. This Study points out that habitat amelioration and associational defence are both highly Important in dry grasslands grazed by domestic livestock because these mechanisms enhance seedling survival, a major life stage for tree dynamics. This Situation, In which two Facilitative mechanisms are important, is not taken Into account by Current models of facilitation and is probably due to the coupling of domestic herbivore grazing and stressful abiotic conditions.