Structural Dynamics and Disturbance Regime in an Old-Growth Oak–Beech Forest: Integrating Long-Term Observations, Dendroecology and Canopy Gap Analysis

The Muški bunar old-growth forest on Mount Psunj represents one of the rare preserved mixed ecosystems of sessile oak (Quercus petraea) and European beech (Fagus sylvatica) in Southeastern Europe, providing an important reference for understanding natural forest dynamics. This study aimed to analyse stand structure, age distribution, growth dynamics, and disturbance regime based on repeated field surveys conducted in 1979 and 2021. The results revealed pronounced structural heterogeneity, a wide range of tree sizes and ages, and clear interspecific differences. European beech dominates smaller and medium diameter and age classes, whereas sessile oak is primarily present in older and larger diameter classes. A very high growing stock (1155.81 m³ ha⁻¹) indicates exceptional stand productivity, with maximum cambial ages of 295 years for oak and 253 years for beech. Basal area increment analysis showed that even old trees maintain substantial growth. Although both species exhibit positive long-term growth trends, recent decades show divergence, with increasing growth in beech and stagnation or decline in oak. Stand dynamics are mainly driven by low-intensity disturbances, while recent windthrows have further increased structural heterogeneity and regeneration. These findings highlight the importance of old-growth forests as reference systems for close-to-nature forest management.