Effects of shrub species dominance and diversity on oak seedling survival in Mediterranean woodlands: The interplay of abiotic conditions and plant traits

Understanding plant species interactions, including facilitation and competition, is fundamental to improve the success of woody seedling establishment. This is particularly important for the restoration of sclerophyllous oak forests, one of the main forest types in Mediterranean Europe currently facing a severe lack of oak natural regeneration. However, the effects of shrub communities dominated by different species and diverse shrub communities on the establishment of Mediterranean oaks remains poorly understood. We conducted a 3-year experimental study in cork oak (Quercus suber L.) woodlands in Portugal to assess the effects of species dominance and diversity of the shrub community and associated abiotic resources on cork oak seedling emergence and survival. We hypothesized that the emergence and survival vary with the identity of the dominant shrub, due to distinct shrub functional traits affecting understory abiotic conditions. Additionally, we predict that the survival of seedlings increases with shrub diversity, driven by net facilitation processes. We sowed 960 cork oak acorns across five replicated microhabitats: four with distinct shrub species dominance and diversity and one without shrub cover. We used Bayesian statistics to model cork oak seedling emergence and survival as a function of microhabitat type and abiotic variables (soil water and temperature, soil characteristics and leaf area index). Seedling emergence and survival varied with species dominance and diversity of the shrub microhabitat. The highest survival was observed in microhabitats characterized by higher shrub diversity, and the lowest survival in microhabitats dominated by Cistus ladanifer. Maximum soil temperature was associated with higher emergence but lower survival in open conditions or areas with sparser shrub cover. Water logging constrained emergence and survival, but higher soil potassium was associated with higher survival, for all microhabitats. Our study shows that interactions between cork oak seedlings and their shrub neighbors are species and community dependent, and that a more diverse shrub composition improves cork oak seedling survival, likely due to distinct shrub traits that ameliorate the physical environment and promote facilitation. Management and restoration of Mediterranean sclerophyllous oak forests need to consider the specificity and diversity of the understory plant community, as well as the different stages of oak early life-cycle, to identify the most favorable conditions for oak establishment. This becomes particularly important under more frequent climatic extremes predicted for Mediterranean Europe.