Structural diversity as an indicator of forest degradation in Quercus robur stands along the Tundzha river, Southeast Bulgaria

The structural integrity and compositional stability of Quercus robur dominated riparian forests in southeastern Bulgaria but have been increasingly compromised by long-term anthropogenic pressures, particularly hydrological modifications. This study investigates the structural diversity and regeneration dynamics of four representative stands along the Tundzha River, situated within protected areas and a strict reserve. Data were obtained through systematic plot-based sampling and analysed to evaluate diameter distribution, regeneration status, and radial growth patterns. Diameter distributions conformed predominantly to gamma and lognormal functions, indicative of uneven-aged stand structure in three of the four sites. Gini coefficients ranged from 0.39 to 0.58, reflecting varying degrees of structural heterogeneity. Regeneration of Q. robur was notably scarce across all stands, with understory layers largely dominated by shade-tolerant and competitive broadleaved species. This compositional trend is characterised by a consistent absence of oak in the smaller diameter classes and suggests a trajectory toward long-term replacement of oak by other species. Tree-ring chronologies further revealed a marked decline in radial growth beginning in the mid-20 th century, coinciding with the onset of intensive flood control and drainage infrastructure. The results indicate that, despite retaining elements of structural complexity, these forests are undergoing gradual ecological degradation. The persistence of Q. robur in these systems is unlikely without targeted silvicultural measures aimed at promoting regeneration and mitigating the effects of altered hydrological regimes.