Pine Stand Density Influences the Regeneration of Acacia saligna Labill. H.L.Wendl. and Native Woody Species in a Mediterranean Coastal Pine Plantation

Mediterranean plantations are the most suitable areas to assess vegetation dynamics and competitive interactions between native and exotic woody species. Our research was carried out in a coastal pine plantation (Sicily) where renaturalization by native species (Pistacia lentiscus L. and Olea europaea var. sylvestris) and invasion by Acacia saligna (Labill.) H.L.Wendl. simultaneously occur. The regeneration pattern of woody species in the pine understory was evaluated in six experimental plots along a stand density gradient, from 200 to approximately 700 pines per hectare. Both pine stand density and regeneration by native species had a significant negative relationship with Acacia natural regeneration. Olea regeneration was positively correlated with stand density, while Pistacia showed a non-significant relationship. Saplings of both native species were mostly less than 1 m high, whereas approximately 70% of Acacia individuals were higher than 1 m. We found that 400 pines per hectare should be considered a minimum stand density to keep Acacia under control, while favouring the establishment of native species in the understory. The successful control of Acacia requires an integrated management strategy, including different forest interventions according to stand density: thinning, control measures against Acacia, and renaturalization actions