Facilitation of underplanted broadleaf trees by pine canopy under Mediterranean conditions depends on thinning intensity and type, not on species drought tolerance

Planted forests in dry Mediterranean regions are increasingly threatened by extreme climatic events, making adaptive management strategies such as thinning and underplanting essential. To apply these restoration practices at large scales, it is necessary to understand how different tree species respond to varying thinning intensities. This study evaluated the performance of seedlings from four late-successional, resprouting Mediterranean broadleaf species — Acer monspessulanum L., Quercus pyrenaica Willd Sorbus torminalis (L.) Crantz, and the evergreen Quercus ilex L. subsp. ballota (Desf.) Samp— underplanted along a canopy openness gradient created by thinning treatments in a 50-year-old Pinus pinaster plantation. Four thinning treatments were compared: no thinning (control), moderate thinning from below (20% basal area reduction), heavy thinning from below (35%), and heavy thinning from above (35%), leaving 28.7, 24.9 and 23.2 m 2 ·ha −1 residual basal area, respectively. Thinning significantly increased light availability, throughfall, and soil moisture, with most improvements occurring under moderate thinning. Soil water availability increased at deeper soil layers in all thinned stands, while shallow soil moisture improved mainly under moderate thinning. After five years, seedling survival was high (>80%) across all species and generally increased with greater canopy openness. Quercus pyrenaica and Q. ilex showed the strongest positive response in survival to increased canopy opening, whereas A. monspessulanum and S. torminalis were less affected, likely due to differences in functional traits. Seedling growth was also enhanced by thinning, reaching maximum rates under moderate thinning, with S. torminalis exhibiting the highest growth and A. monspessulanum the lowest. Improved seedling performance under thinning was associated with reduced summer water stress and higher photochemical efficiency during dry periods. Overall, the study demonstrates that moderate thinning creates optimal environmental conditions for the facilitation of underplanted seedlings and provides guidelines for promoting structural and species diversification in Mediterranean pine plantations. • Thinning increases resources and the performance of underplanted seedling. • Seedling water and photosynthetic stress is alleviated under moderate thinning. • Response of underplanted seedlings to canopy openness is species-specific. • Thinning and species mixing speeds the conversion of monospecific pine plantations.