The use of shrub cover to preserve Mediterranean oak dehesas: a comparison between sheep, cattle and wild ungulate management

Questions: Oak savanna-like systems (dehesas) are highly valuable and protected systems that are strongly compromised by a continuous failure of oak recruitment. Our question is whether different long-term management types involving cattle, sheep and wild ungulates affect the abundance and diversity of safe microsites (shrubs) for oak recruitment. We also assess microsite location and species-specific differences among the nurse shrubs in order to provide management guidance. Location: Dehesas of holmoak (Quercus ilex) in central Spain. Methods: We examined oak regeneration in four different microsites (tree canopy, shrub cover, open and tree-shrub) and across three distinctive and representative management types for a period of, at least, 30 yr: (1) traditional management (i.e. extensive sheep rearing); (2) high commercially competitive management (extensive cattle); and (3) wildlife management (wild ungulate grazing, mostly deer). Results: Wildlife management showed higher abundance and diversity of shrubs. Predicted seedling density under wildlife management was three-fold higher than sheep, and almost nine-fold higher than under cattle management. Highest oak density was found under shrubs that grew beneath trees, twice as high than under trees, ten-fold higher than under shrubs outside the tree canopy, and almost 40-fold higher than in open microsites. However, the proportional facilitative effect of shrubs was higher in open areas than under trees. Three shrub species (Erica australis, Genista hirsuta and Rosmarinus officinalis) showed significantly higher probability of facilitating oak seedlings in comparison to other shrub species, indicating species-specific differences in plant facilitation. Most young oak trees (saplings and juveniles) exceeded the average shrub height, suggesting that facilitation occurs mostly at the seedling stage. Conclusions: Only dehesas under reasonable wildlife management showed a clear three-layered structure with a significant representation of the shrub layer, which increased seedling density. Current cattle management was the least sustainable in terms of oak regeneration. Even though shrub cover, in general, facilitated oak seedlings, not all species were equivalent facilitators. Only three low-palatable and large-sized shrub species were keystone facilitators that should be especially considered in restoration and management practices. Further agriculture policies should promote reduction of cattle densities, rotational grazing and mixed species management to enhance the regeneration of these systems.