The effect of gap size on the establishment and initial development of natural regeneration of Scots pine (Pinus sylvestris L.) in oligotrophic sites

Abstract Artificial canopy gaps represent an effective tool for promoting natural regeneration, increasing structural diversity in forest ecosystems. However, the influence of specific gap characteristics on regeneration success remains insufficiently understood. This is especially important for the transformation of even-aged, single-layered Scots pine ( Pinus sylvestris L.) stands on oligotrophic sites, where there is a clear need for evidence-based silvicultural recommendations. This study investigated the effects of gap size (0.05 ha vs. 0.30 ha), soil cover type, microtopography, and within-gap position on the establishment and early development of naturally regenerated Scots pine. To assess these factors, generalised additive models were applied to data from a regeneration experiment established in a 98-year-old pine stand during the winter of 2013/2014. Regeneration density was recorded after the 2nd and 8th growing seasons, and in the latter inventory, seedling height and proportions of soil cover types were also measured. Initial findings showed that seedling density in large gaps was over three times higher than in small gaps (3.42 vs. 1.04 seedlings·m −2 ). However, after 8 years, this difference had disappeared, and the gap size no longer significantly affected either seedling density or height. Dwarf shrubs and grasses negatively influenced regeneration density, while lichens and litter had a positive effect on seedling density but were negatively associated with seedling height. The results suggest that both gap sizes may be suitable for regeneration cuttings under similar site conditions.