Interplay Between Genetic Diversity and Tree Vitality in Fraxinus excelsior Populations Affected by Ash Dieback

<b>Background:</b> Ash dieback, driven by the invasive fungal pathogen <i>Hymenoscyphus fraxineus</i>, has precipitated severe declines in <i>Fraxinus excelsior</i> L. populations across Europe, threatening genetic diversity and ecosystem stability. <b>Methods:</b> This study investigates the interplay between phenotypic vitality and genetic variation in five Polish ash stands using nuclear simple sequence repeat (nSSR) and chloroplast DNA (cpDNA) markers. Vitality assessments of 186 trees across three reserves (from Białowieża Primeval Forest and Wolica Reserve) were conducted. <b>Results:</b> Vitality assessments revealed a slight predominance of dying individuals (36%, 3rd degree of Roloff classification). Nuclear analyses indicated moderate to high diversity (mean <i>H</i>_E = 0.826), significant inbreeding (<i>F</i>_IS = 0.178, <i>p</i> < 0.001), and low inter-population differentiation (<i>F</i>_ST = 0.044) among all five stands. Chloroplast markers showed elevated differentiation (Φ_ST = 0.228, <i>p</i> < 0.0001), reflecting phylogeographic structure. Vitality degrees assessed in three chosen populations (Browsk FD, Hajnówka FD, and Chojnów FD) exhibited negligible genetic differentiation (nSSR <i>F</i>_ST = 0.009; cpDNA Φ_ST = 0.003), suggesting gene flow mitigates pathogen-induced selection. Bayesian clustering (STRUCTURE, K = 3) revealed admixture with distinct genotypes in dying trees, potentially linked to susceptibility. <b>Conclusions:</b> These findings underscore the resilience of ash genetic pools and advocate for selective breeding in nurseries to prevent the spread of dieback, prioritizing resistant genotypes for conservation.