Fine-scale interventions can reinforce the forest character of the understory vegetation – The effects of different artificial gaps in an oak-dominated forest

Forest biodiversity is threatened by the use of conventional rotation forestry system, while fine-scale interventions of continuous cover forestry, such as gap-cutting, could protect forest habitats and enhance the stand structural heterogeneity. Consequently, their use could maintain biodiversity during the timber production process. It is unclear which gap sizes and shapes can trigger ample natural regeneration while simultaneously maintaining or improving the near-natural character of the understory. The Pilis Gap Experiment examined the five-year effects of four gap types comparing two gap sizes (150 and 300 m 2 ) and two gap shapes (circular and elongated) on the light and soil moisture conditions and understory vegetation in an oak–hornbeam forest. The investigated understory variables included species richness, total cover, height, shrub cover and cover of five functional groups. Our results indicate an initially increased light in all gap types, but later it decreased in large circular gaps, while remaining more stable in other gap types. Soil moisture increased first, transiently in the circular gaps, and later in the elongated gaps. Species richness temporarily increased in large circular gaps, whereas total cover increased in all gap types. Understory height and shrub cover also increased in large circular gaps. Annual and perennial forb cover remained unchanged in all gap types, although graminoid cover showed transient growth in large elongated gaps. Small gaps had the highest cover of woody seedlings, whereas bramble ( Rubus fruticosus agg.) cover increased the most in large circular gaps. Species composition exhibited the most significant changes in large circular gaps. From a conservation aspect, all gap types can be considered favorable, as they increase the heterogeneity of the openness and understory vegetation in homogeneous closed stands. Vegetation changes are the most prominent in large circular gaps w spread of bramble here multiple vegetation layers developed. However, the dense cover of bramble and shrubs hinders the effective regeneration of sessile oak ( Quercus petraea ). Smaller gaps slightly increase the heterogeneity of the forest understory and provide ample light and soil moisture to initiate regeneration. In larger gaps, oak regeneration may be supported by applying an elongated shape, mitigating the competition from bramble. • The effect of different sized and shaped gaps was studied on oak forest understory. • 150–300 m 2 sized gaps preserved the forest microclimate (light and soil moisture). • All gaps preserved the forest character of the composition, but increased cover. • In large circular gaps, the understory height and the shrub cover increased. • Small gaps with increased light and low competition allow oak regeneration to start.