Hardwood seedling establishment below Aleppo pine depends on thinning intensity in two Mediterranean sites

Context: Developing silvicultural methods to help Mediterranean forests adapt to climate change is of high importance. Introducing resprouting hardwood species below pine stands is expected to promote diversity and resilience of these stands, particularly to forest fires. Aims: To examine how the intensity of pine thinning influences understory micro environment and the establishment of various hardwood seedlings in two Mediterranean sites. Methods: Aleppo pine stands were thinned down to three levels of basal area (uncut: 30 m²/ha, moderate thinning: 13-20 m²/ha, heavy thinning: 7-10 m²/ha) at two Mediterranean sites (South-East France and South-East Spain). Seedlings of six hardwood species were introduced in the understory and their survival and growth were monitored and related to changes in micro-environment induced by thinning. Results: At both sites, thinning improved light availability and seedling diameter increment of all target species. Thinning increased extreme temperature and evaporative demand. Heavy thinning increased summer soil moisture in SE Spain but not in SE France. The worst conditions for seedling survival were reached under uncut stands in SE France and low-density stands in SE Spain. Conclusion: Thinning in pine stands accelerated seedling growth but excessive thinning worsened summer drought and affected seedling survival. Moderate thinning (15-20 m²/ha) seems to be the best option in support of the introduction of hardwoods in the understory, which can improve forest diversity and resilience in the future.