The use of mixed forests and non-native tree species have the potential to mitigate climate change impacts and enhance biodiversity. However, little is known about how forest composition and environmental factors affect each step of natural regeneration in mixed forests, especially in mixtures with non-native trees. Here we investigated how forest composition affected European beech seed survival (through seed tracking), seed sprouting (via field germination experiments), and seedling survival (monthly seedling monitoring) in pure beech forests and in admixtures with Norway spruce and the introduced Douglas-fir in a mast and non-mast year of beech. We also assessed how biotic and abiotic factors (small mammal abundance, ungulate access, seed production, seed burial, canopy cover, distance to nearest adult tree, seedling aggregation, understory density, browsing damage, and soil properties) affected these regeneration dynamics. Seed survival was negatively affected by the presence of conifers and mouse abundance. Seed germination was influenced by whether seeds were buried or not. Seedling survival increased in Douglas-fir admixtures, and in forests with higher soil quality. Browsing damage and ungulate access diminished seedling survival. Seed production had the greatest influence on absolute number of seedlings. Forest composition and environmental factors had distinct impacts on regeneration of beech depending on its ontogenetic stage. Here, we provide evidence supporting the notion that Douglas-fir is not impairing the regeneration of native trees in mixed forests. In fact, mixtures with Douglas-fir benefited the survival of beech seedlings, likely due to better soil properties and less damage from herbivores on these stands.

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 Caba & ntilde;eros and Do & ntilde;ana 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.

1. The relationship between biodiversity and multitrophic ecosystem functions (BEF) remains poorly studied in forests. There have been inconsistent reports regarding the significance of tree diversity effects on ecosystem functions, which may be better understood by considering critical biotic interactions of trees. 2. This study investigates the role of tree-mycorrhizal associations that may shape forest BEF relationships across multiple ecosystem functions. We used a field experiment (MyDiv) that comprises 10 deciduous tree species associated with either arbuscular mycorrhizal (AM) or ectomycorrhizal (EcM) fungi to create gradients in species richness (1, 2, 4 species) and different mycorrhizal communities (only AM-species [AM fungi associated tree species] or EcM-species [EcM fungi associated tree species], or a combination of both). We investigated the effects of tree species richness and mycorrhizal types on crucial multitrophic ecosystem functions (foliage damage, predation [using artificial caterpillars] and soil fauna feeding activity [similar to 0-10 cm]) and assessed how these effects were mediated by stand characteristics. 3. Overall, we found that tree species richness and mycorrhizal types strongly affected multitrophic ecosystem functions. Compared to monocultures, 4-species mixtures with both mycorrhizal types experienced significantly lower foliage damage. The mixtures of EcM-species supported significantly higher predation (i.e. a greater proportion of artificial caterpillars being attacked), and this effect strengthened with tree species richness. The effects of tree species richness on soil fauna feeding activity were negative across all mycorrhizal types in the lower soil layer. Moreover, we showed that tree diversity effects were mediated by above-ground tree biomass, vertical structural complexity and leaf quality, with the dominating mechanisms largely depending on the mycorrhizal types. 4. Synthesis. Tree species richness affected multitrophic ecosystem functioning by (1) directly decreasing the proportion of foliage damage in the communities with both mycorrhizal types, where AM-species benefited from mixing with EcM-species, and (2) increasing predation rates via changes in the vertical structural complexity in mixtures of EcM-species. Our results highlight the importance of considering mycorrhizal types for managing well-functioning mixed-species forests and contribute to broadening the mechanistic understanding of the context-dependent BEF relationships in forests.