Key message <p>Forest communities historically dominated by common ash (<i>Fraxinus excelsior</i> L.) were characterized by particularly high species richness in the understorey and included a distinct set of vascular plant species statistically associated with higher ash cover. These findings establish a valuable ecological baseline. The vulnerability index complements this by highlighting species potentially sensitive to environmental changes following ash decline, without aiming to assess current impacts of the ash dieback disease.</p> Abstract:&#xa0;Context <p>Ash (<i>Fraxinus excelsior</i> L.)&#xa0;dieback is causing widespread ecological changes in European forests, yet few baseline data exist to assess its long-term impact on understorey vegetation.</p> Aims <p>The aim of this study was to identify vascular plant species of the understorey associated with common ash prior to ash dieback. Rather than assessing the direct impact of ash dieback, the analysis seeks to characterize pre-disturbance vegetation patterns to inform future developments and contribute to a better understanding of potential vegetation dynamics across similar lowland forest systems in Europe.</p> Methods <p>A vegetation dataset of 1912 ash-forest relevés in the Northern German lowlands was analysed to identify linkages of understorey vegetation to common ash using extended Huisman-Olff-Fresco models. To estimate the potential vulnerability of identified ash-associated species, we applied a multi-attribute point-scoring method based on dispersal distance, habitat preferences and threat status of the understorey species considered.</p> Results <p>Greater crown cover of common ash was linked to greater diversity in the understorey. In addition, 43 understorey species displayed a higher probability of occurrence at ash-dominated sites. Out of these ash-associated species, 35 herbaceous species showed differences in estimated vulnerability scores.</p> Conclusion <p>This study provides a pre-disease reference for understorey vegetation in forest communities historically dominated by <i>F. excelsior</i>. The identification of species associated with higher ash cover establishes an ecological baseline to which post-dieback vegetation can be compared. Functioning as a predictive tool, the applied vulnerability index evaluates species’ vulnerability based on expected environmental changes due to ash decline, rather than on empirical post-decline observations. While not intended to quantify observed impacts, this approach provides a helpful tool for assessing vegetation in relation to historical baselines and may support future considerations in conservation and forest management under potentially changing conditions in ash-rich forest ecosystems.</p>