Reverting homogenous forest landscapes in a Mediterranean site. Transitioning to mixed oak forests

Initial reforestation efforts in the Western Mediterranean basin aimed to convert degraded lands into forests dominated by native Mediterranean oaks following a successional trajectory in which pine pioneer species were extensively used to ameliorate abiotic conditions. However, the transformation into mixed oak forests has not occurred due to changing land use priorities, land abandonment and lack of active forest management leaving a large extent of homogenous pinewood landscapes. Homogenous forests face significant adaptation challenges in the current climate crisis, including susceptibility to pests, diseases, and catastrophic wildfires. This study explores the use of harvest gaps similar to the establishment cuts of the irregular shelter-wood system (ISS), adapted for Mediterranean conditions to transform pure Pinus pinaster reforested stands into mixed-oak forests under the assumption that transforming light-demanding monocultures into mixed forests enhance resilience. The method involves creating small gaps (< 0.02 ha) to avoid natural pine regeneration while allowing for oak establishment and planted oak saplings development. The results indicate that oak seedling survival is higher in gaps compared to control plots, with no significant difference between gap sizes. Pine regeneration density is higher in larger gaps, while oak regeneration density is positively influenced by the presence of harvest residues. Survival of planted oak is high although browsing prevents full height development. Small harvest gaps are effective in controlling pine regeneration while supporting oak survival and growth. However, additional measures like fencing may be required to address browsing. The findings highlight the challenges and potential solutions for transforming homogenous forests into more resilient mixed forests.