Endogenous and exogenous control of tree architectural and hydraulic traits of European beech growing in natural populations

Abstract Key message Genetic components explain a moderate, yet statistically significant proportion of the phenotypic variation in tree architecture and hydraulic traits of beech trees growing in naturally regenerated monospecific stands. Abstract Drought tolerance of European beech has been increasingly questioned in recent years. However, the influence of genetic factors and forest management on growth performance and key hydraulic traits in naturally regenerated beech populations remains insufficiently understood. This study aims to quantify the relative contributions of endogenous and exogenous factors to tree architectural and hydraulic traits of ~ 100-year-old beech trees in monospecific stands growing on a cool-moist northeast (NE) and a warm-dry southwest (SW) aspect in the Swabian Alb, Southwest Germany. In 1999, three treatments (control, strong thinning, and very strong thinning) were applied to both aspects. Kinship analyses based on microsatellite markers assigned 722 trees to 126 half‑sibling families. Terrestrial laser scanning in spring 2023 enabled detailed phenotyping of tree architecture and the calculation of competition indices. Quantitative wood anatomy was used to characterize water conducting cells. Family effects on tree architecture were less pronounced compared to competition and aspect, yet remained the sole significant predictor for secondary branching and crown ratio. Wood anatomical traits were highly plastic and responded strongly to changes in environmental conditions. Contrary to previous assumptions, reduced competition did not decrease vessel size or increase hydraulic safety. Furthermore, vessel diameters were larger on the warm‑dry SW aspect when comparing trees of similar height and competition, challenging the assumption of a safety‑oriented hydraulic strategy under arid conditions. Families significantly influenced hydraulic traits, but most phenotypic variation occurred among individual trees nested within families, providing a potential mechanistic explanation for genotype‑related mortality patterns reported in earlier studies.