Canopy gap structure across diverse old-growth forest types in the Western Carpathians

• The disturbance regime in the Carpathians is primarily driven by small-scale dynamics. • Although rare, intermediate and large-scale gaps account for 32% of the total gap area. • Tree species composition significantly influences the disturbance regime. • Silvicultural practices should mimic natural disturbance variability of gap sizes. Canopy gaps are a fundamental component of forest stand and landscape heterogeneity, playing a crucial role in forest dynamics. This study aimed to characterize the influence of species composition and stand structure on the characteristics of canopy gaps in eight National Nature Reserves in Slovakia, representing diverse vegetation types and altitudinal gradients. The forest compositions ranged from oak-dominated to beech-dominated and mixed spruce-fir-beech stands. Stem density varied from 260 to 543 n.ha -1 , while growing stock ranged from 376 to 829 m 3 .ha -1 . In total, 310 canopy gaps were recorded, with an average abundance of 12 gaps per hectare (ranging from 6 to 17 at different sites). On average, canopy gaps occupied 17% of the permanent research plot area (9–25%), while expanded gaps covered 43% (24–56%). The disturbance regime was primarily driven by small-scale dynamics with gaps smaller than 200 m 2 representing 78% of the total number (ranging from 59 to 85% across sites). We found that species composition significantly affects gap characteristics: a higher proportion of beech correlated with a decrease in gap frequency, total gap area and the canopy-to-expanded gap ratio, likely due to beech’s rapid lateral crown expansion. Conversely, a higher proportion of conifers increased the number of gapmakers per gap, reflecting their higher susceptibility to group mortality. Our findings indicate that while small-scale gap dynamics predominate, intermediate and large-scale gaps account for a disproportionate 32% of the total gap area and thus are critical for maintaining structural heterogeneity and provide regeneration niches for less shade-tolerant species. To mimic such natural disturbance variability and to enhance forest resilience in the face of climate change, silvicultural practices that integrate various cutting forms with gaps of diverse sizes should be preferred.