Multiple Recruitment Limitation Causes Arrested Succession in Mediterranean Cork Oak Systems

Lack of tree regeneration and persistency of species-poor shrublands represent a growing problem across Mediterranean evergreen oak forests. What constrains forest regeneration is poorly understood, and restoration attempts have been largely unsuccessful. We assessed the contribution of four different mechanisms of tree recruitment limitation (that is, source, dispersal, germination, and establishment) in a cork oak (Quercus suber) system in southern Portugal. Using a combination of field studies and experiments, we quantified seed production, seed removal and dispersal, seed survival and germination, seedling establishment and survival, as well as cork oak natural regeneration for the three dominant vegetation types in this system (Cistus ladanifer shrubland, oak forest, and oak savanna). We found that all four forms of cork oak recruitment limitation were significantly more severe in shrublands than in oak forests and savannas, so that oak seedling recruitment in shrubland was impeded in multiple ways. Our results explain why transitions from shrublands to oak savannas and forests are extremely difficult, and that the release from arrested succession in this system requires the simultaneous relief of multiple constraints on recruitment limitation in the early life history of oaks. These results have important implications for the restoration and conservation of Mediterranean oak systems.