Does parental tree size determine acorn germination in Quercus ilex L. dehesas?

Quercus ilex L. is an evergreen tree species widely spread throughout southwestern Europe. However, its poor natural regeneration poses major challenges since the 1980s, and so assisted regeneration is speculated to be critical for either maintaining existing populations or recovering degraded ones. This method, nonetheless, raises major operational challenges related to fruit harvesting and selection in the field. For instance, the link between parental tree traits, such as age and size, and acorn characteristics has seldom been addressed; yet it could be decisive for streamlining acorn selection. In this study, we employed Structural Equation Models (SEM) to examine the relationship between parental tree Diameter at Breast Height (DBH) and acorn traits determinant for seed germination in Quercus species worldwide. Specifically, we measured acorn dry mass (ADM), seed moisture (SM), and pericarp thickness (PT) in a Q. ilex population in central-western Spain. Our results indicate a significant positive influence of parental tree DBH on ADM, with larger acorns associated with higher germination percentages. Similarly, SM positively influenced germination, though it was influenced by acorn size through two contrasting pathways. PT, on the other hand, negatively impacted germination and was also negatively affected by ADM. These findings suggest that acorn selection should prioritize large/old trees to maximize germination via increased ADM. Further research, nonetheless, is needed to progress in the relationships between parental tree traits and acorn characteristics in this and other similar contexts in which large-scale plans for Q. ilex regeneration would be planned. • Larger (older) holm oaks produce larger acorns. • Seed moisture and pericarp thickness are negatively correlated with acorn size. • (But) pericarp thickness positively influences seed moisture content. • Acorn size and seed moisture enhance germination; pericarp thickness reduces it. • DBH affects germination via direct and indirect effects on acorn traits.