Canopy Gap Dynamics, Disturbances, and Natural Regeneration Patterns in a Beech-Dominated Hyrcanian Old-Growth Forest

Canopy gaps play a crucial role in forest dynamic processes and help preserve biodiversity, influence nutrient cycles, and maintain the complex structure of the forests. This study aimed to quantify the gap dynamics, regeneration establishment, and gap closure in a natural old-growth Hyrcanian forest in the north of Iran. We used a repeated inventory of gap size-frequency and fraction in beech (Fagus orientalis) dominant forest over a 9-year interval (2010–2019). The total gap area documented in 2010, 2016, and 2019 was 2,487, 6,890, and 8,864 m2, respectively. The gap area ranged from the smallest sizes of 139, 83, and 153 m2 to the largest sizes 906, 1,668, and 871 m2 in 2010, 2016, and 2019, respectively. Gap fraction significantly increased from 0.52%, 1.93%, and 3.7% in 2010, 2016, and 2019, respectively. The size distribution of gaps was strongly skewed to the medium class (200–500 m2), with approximately 60% of the gaps. Results revealed that total regenerations are not in correlation with gap size. Small gaps were closed within a few years through rapid horizontal canopy expansion of neighboring beech trees. The gap closure rate decreased by increasing the gap size (70% in 71 m2 to 10% in 1,600 m2). The highest density and greatest regeneration growth occurred mostly along the eastern part of gaps. The spatial distributions of regeneration density demonstrated differences in different gap size classes, which probably resulted from heterogeneity in the microenvironment within the gap and the differences in the regeneration responses to these variations. This investigation provided useful data for managing natural regenerations based on forest sustainability. The changes in gap patterns observed between 2010 and 2019 highlight the high value of repeated gap inventories for better comprehending the disturbance regeneration and dynamics of natural gaps.