Specificities of Population Structure and Genetic Diversity of European Sweet Chestnut (<em>Castanea sativa</em> Mill.) of Three Chestnut Populations from the Largest Continuous Chestnut Area in the Republic of Croatia

The European sweet chestnut, Castanea sativa Mill., is an ecologically and culturally significant Croatian forest tree. However, its genetic diversity and population structure re-main insufficiently understood. This research examined three chestnut populations (PET, HRK, and BAC) from Zrin Mountain, Croatia’s largest continuous chestnut area, utilizing seven nuclear SSR markers. In order to assess the genetic diversity, and population structure, 153 individuals were genotyped. The study revealed that all populations exhibited a moderate level of genetic variety (mean He = 0.571), with the BAC population exhibiting the highest allelic richness and number of private alleles. AMOVA revealed that 67% of the total genetic variance resided within individuals and only 3% among populations. Population differentiation was low to moderate (FST = 0.064; PhiPT = 0.146), with high inferred gene flow (Nm = 7.48). The STRUCTURE and PCoA studies showed that the HRK population was the most genetically different. In contrast, PET and BAC had in-creased genetic similarity and integration. These findings emphasize the relevance of gene flow among chestnut stands and local genetic resources. The findings establish a founda-tion for the sustainable management and conservation planning of Croatian chestnut populations in the context of a broader European environment.