Promoting oak (Quercus pubescens Willd.) seedling development: finding the balance between canopy opening and grass competition under water stress

To adapt Mediterranean forests to increasingly harsh climatic conditions by promoting genetic diversity, thinning is often considered an effective strategy to enhance sexual regeneration. However, determining an optimal thinning level that both increases light availability and maintains favorable microclimatic conditions for germination, without excessively promoting herbaceous competition, remains challenging. To better understand how abiotic and biotic factors influence oak seedling development and to help identify a balanced thinning level under climate change, we conducted a semi-controlled experiment testing the combined effects of competition with a grass species (Poaceae), two canopy opening levels, and water stress. Our results highlight the crucial role of competition with Poaceae species - in our case Festuca ovina - in oak regeneration. Their presence not only intensifies competition for essential resources, but also modifies soil properties and alters belowground interactions, overall creating conditions less favorable for oak seedling establishment. In addition, our results highlight the significant impact of key abiotic factors that are canopy opening (which influences light availability) and hydric conditions, as well as their interactions with the effects of competition. We observed a consistent need for adequate light to ensure optimal seedling performance, suggesting that successful regeneration depends on balancing sufficient canopy opening to improve light availability with maintaining sufficient cover to mitigate water stress and limit grass competition. Overall, our study contributes to the broader debate on sustainable forest management strategies under changing climatic conditions.