Acorn removal and seedling age determine oak ('Quercus ilex' L. and 'Q. suber' L.) restoration outcome in ungulate-dominated Mediterranean environments

Key message: We test the potential benefits of planting 2-year vs. 1-year-old seedlings to restore Mediterranean oak-dominated systems. Planting 2-year-old 'Quercus suber' L. seedlings is recommended for improved survival and resilience against wild boar ('Sus scrofa' L.) and drought in dry sandy soils. The removal of acorns in seedlings did not apparently influence leaf biochemical traits and could reduce wild boar damage, particularly in 1-year-old seedlings. Context: In the face of anthropogenic global change, Mediterranean oak-dominated ecosystems confront increased biotic (ungulate herbivory) and abiotic (drought) stressors, compromising forest regeneration. Restoration measures are imperative to address this scenario. Aims: This study assesses the impact of different mitigation measures on the survival and biochemical traits of two oak species. Methods: We planted 'Quercus ilex' L. and 'Q. suber' L. seedlings in Cabañeros and Doñana National Parks (Spain), subjecting them to three treatments: cotyledon/acorn removal, seedling age (1- vs. 2-year-old), and herbivore protection (fenced vs. non-fenced). Results: Wild boar ('Sus scrofa' L.) damage peaked in winter and early spring, while drought prevailed from late spring to fall. In sandy soils, wild boar uprooted 1-year-old more often than 2-years-old seedlings (40% vs. 18%). One-year-old seedlings without acorns showed higher survival rates against wild boar only in sandy soils. The removal of acorns in seedlings did not influence plant biochemical traits. Conclusions: Planting 2-year-old seedlings in sandy soils may mitigate wild boar damage and improve drought resilience. Seedling age seems more important than acorn removal against biotic and abiotic stressors although acorn removal could reduce wild boar damage in 1-year-old seedlings. Our results underscore the importance of considering multiple stressors in oak restoration strategies.