Interclonal Differentiation in Acorn Morphology of Sessile Oak (Quercus petraea (Matt.) Liebl.) in the Novoselci Clonal Seed Orchard

Abstract

Acorn morphology represents an important component of reproductive biology in oaks and may reflect underlying genetic structure, reproductive dynamics, and environmental influences. In widely distributed species such as sessile oak (Quercus petraea (Matt.) Liebl.), extensive pollen-mediated gene flow may reduce geographic structuring of reproductive traits, while clonal seed orchards provide a suitable framework for assessing interclonal differentiation under relatively uniform environmental conditions. The objective of this study was to quantify interclonal variation in acorn morphology within a clonal seed orchard and to evaluate the relationship between acorn dimensions and the geographic origin of clones.
The study was conducted in the Novoselci clonal seed orchard in Croatia (45.333° N; 17.786° E; Figures 1 and 2). A total of 4800 acorns were collected from 48 clones (one ramet per clone). Acorn length and width were measured, and statistical analyses were performed, including  descriptive statistics, one-way ANOVA, estimation of intraclass correlation coefficients (ICC), linear regression models, and principal component analysis (PCA).
Significant interclonal differences were detected for both acorn length and width (p < 0.001), with ICC values of 0.54 and 0.62, respectively, indicating substantial phenotypic differentiation among clones within the orchard (Table 1). Box‑plot visualisations further highlighted pronounced interclonal variability (Figures 3 and 4). Principal component analysis revealed a continuous morphological gradient primarily driven by acorn size, with PC1 representing overall size and PC2 reflecting shape variation (Figure 5). No significant relationships were found between acorn dimensions and geographic coordinates, elevation, or habitat type (Tables 2 and 3). In a subset of clones represented by two ramets, variance component analysis showed that within‑clone (ramet‑level) variation exceeded clonal variation for both traits (Figure 6), indicating a strong influence of microenvironmental and maternal effects.
Overall, the results suggest that, under the homogeneous environmental conditions of a clonal seed orchard, acorn morphological structure is predominantly shaped by individual (within-population) variability and reproductive dynamics rather than by the geographic or habitat origin of clones. The absence of detectable geographic structuring may reflect high intra-population genetic diversity and extensive historivcal gene flow typical of oak species. Microenvironmental variation, maternal effects, and potential rootstock–scion interactions may additionally contribute to the observed phenotypic differentiation.
These findings improve our understanding of seed trait variability in clonal seed orchards and have implications for interpreting the phenotypic structure and genetic composition of produced forest reproductive material.