A review of the development of Mediterranean pine-oak ecosystems after land abandonment and afforestation: are they novel ecosystems?

Mediterranean landscapes are composed of different interacting vegetation patches. Pine and oak ecosystems form contiguous patches within these landscapes, in pure stands, or as mixed pine-oak ecosystems. During the nineteenth century, pine forest distribution in the Mediterranean Basin increased dramatically as a result of large-scale re-forestation and spontaneous forest regeneration. At the same time, secondary succession of abandoned agricultural land allowed development of pine and oak ecosystems. Consequently, a pine-oak mosaic has developed, which created opportunities for cross-colonization, i.e. species colonization from one ecosystem in the reciprocal system. Pines shed their wind-dispersed seeds and colonize Mediterranean oak vegetation. Oaks regenerate in different ecosystems, including pine forest understories. This paper reviews fire-free landscape-scale dynamics of pine-oak Mediterranean mosaics and analyze how landscape-scale interactions are leading to pine-oak ecosystems by different processes. Published information from the Mediterranean Basin illustrates pathways of pine-oak ecosystems formation. Using Mediterranean literature, I try to elucidate the factors that (1) control colonization potential and (2) modulate the resistance to colonization, in different habitats, land uses, and landscape settings. Management implications for these mixed pine-oak ecosystems are suggested. The question of whether they are novel ecosystems is discussed.