The Importance of Stand Structure in Narrow-Leaved Ash (Fraxinus angustifolia Vahl) Dieback—Insights from an Extensively Managed Stand on a Humogley Soil in Serbia

Ash dieback is a major issue affecting European ash populations, including narrow-leaved ash (Fraxinus angustifolia Vahl). An important factor contributing to the decline of narrow-leaved ash is the fungal disease caused by Hymenoscyphus fraxineus. However, the mortality of trees also depends on stand structure that may influence the disease dynamics. In 2020, we analysed the stand structure of middle-aged, extensively managed, narrow-leaved ash stands growing on Humogley soil (Cariceto remotae-Fraxinetum angustifoliae, Jov., et Tom. 1979). This permanent sample plot is located in Posavina (Serbia), where we observed reduced tree vitality and mortality. The stand originates from natural regeneration after a succession of marsh habitats. At ages 20–25 years (1996) and 30–35 years (2006), selective thinning was carried out. Until the age of 45–50 years (2020), the stand was left unthinned, and the presence of ash dieback fungus was recorded in Serbia. In 2020, we measured the diameter at breast height (DBH) of each tree on the plot and assessed their crown class and degree of isolation. These parameters were evaluated in relation to the crown defoliation of the trees. The results indicate that reduced vitality and mortality of trees manifest in conditions of strongly expressed intraspecific competition in the stand, particularly during the stem exclusion stage. Healthy trees were observed primarily within the predominant/dominant crown class and exhibited the highest mean DBH. Trees classified as dead or dying (81%–100% defoliation) had a lower mean DBH compared to both healthy trees (<25% defoliation) and significantly defoliated trees (26%–80%). This was observed at both the stand level and in predominant/dominant trees, suggesting that tree mortality is primarily linked to their poor growth.