Gap regeneration in two old-growth forest reserves in Slovenia (Rajhenavski Rog) and Croatia (Čorkova Uvala) was analysed in relation to within gap light heterogeneity. Both reserves were located in the Dinaric mountain range in south-central Europe and were dominated by beech (Fagus sylvatica L.) – silver fir (Abies alba Mill.) forest communities with similar growing conditions. In total, the two largest gaps (700–2000 m²) in each reserve were included in the study (n = 4), plus a further four gaps in Rajhenavski Rog and three in Čorkova Uvala (200–500 m²). All the gaps were ∼10 years old and originated from one or two successive events, mostly due to a combination of fungi attack and windthrow. Consequently, all gaps had complex geometry and were covered by a well-developed regeneration layer. Each gap was mapped, followed by establishment of a N–S oriented 5 × 5 m grid within and around the area of the canopy opening. At the grid intersections 773, 1.5 × 1.5 m plots were established. On each plot, the coverage of tree regeneration and ground vegetation, seedling density in different height classes and browsing damage were recorded. In addition, the total stretched length and last growing season increment of the leading stem was measured on selected seedlings in each plot. Relative diffuse and direct radiation were estimated using digital hemispherical photographs. All plots were classified into four microsites according to direct and diffuse radiation levels, and microsites were tested for differences in regeneration height and density. While there was more woody regeneration and an almost fivefold higher (6.2 vs 1.3 m⁻²) average total regeneration density in Rajhenavski Rog, mainly due to a high (5.5 vs 0.6 m⁻²) density of beech, there was more ground vegetation and a higher density of silver fir seedlings in Čorkova Uvala. The within-gap regeneration patterns proved to be similar in both forest reserves and showed that 1-year-old seedlings of beech and silver fir and small beech seedlings preferably recruit on microsites under closed canopy or close to gap edges with lower levels of direct and diffuse radiation. There was no significant difference in density of large-beech seedlings among the microsites, yet height and height increment were higher on microsites receiving the highest levels of direct and diffuse radiation. Within-gap heterogeneity in light conditions appears to significantly influence general patterns of beech and silver fir regeneration, while the differences in total average density, regeneration and ground vegetation abundance and spatial distribution cannot solely be explained by light. Since climate, parent material, soil, exposition, relief and stand conditions were very similar between both reserves, it appears that these differences are a result of different densities of large herbivores, which are reported to be much higher in Rajhenavski Rog. This was also confirmed in our research, as much more browsing damage was found in Rajhenavski Rog compared with Čorkova Uvala. Implications for silvicultural practice are outlined.

Key message: Natural regeneration of beech, maple and fir was investigated in relation to irradiance, ground vegetation and soil features. Seedling establishment may be favoured by creating small and irregular gaps and by successive extension of gaps along the sun-exposed gap edge. Context: To successfully manage natural regeneration, it is important to understand the interactions of forest gap micro-climates and soil features. Aim: The aim of this study was to identify conditions for successful natural regeneration of European beech, sycamore maple and silver fir in mixed forests. Methods: Seedling growth and survival were recorded within and around two artificial gaps, and the relationships to irradiance, ground vegetation and soil features were examined. A simple conceptual model was applied to identify optimal micro-sites for regeneration. Results: Ground vegetation hindered the establishment of natural regeneration in the centre of the gaps. Small seedlings of maple and beech were more abundant within the small gap and along gap edges; beech had the highest density on sun-exposed edges and maple on shaded gap edges. Tall beech and maple seedlings were more abundant along sun-exposed gap edges. Greater variability of irradiance in the larger gap contributed to greater micro-site heterogeneity relative to the small gap. Conclusions: Seedling establishment may be favoured by creating small and irregular gaps and by successive extension of gaps along the sun-exposed gap edge. Circular gaps with diameters greater than stand height contribute to increased ground vegetation coverage and hinder tree regeneration, including semi-shade tolerant maple and spruce.

Forest managers are often required to restore forest stands following natural disturbances, a situation that may become more common and more challenging under global change. In parts of Central Europe, particularly in mountain regions dominated by mixed temperate forests, the use of relatively low intensity, uneven-aged silviculture is a common management approach. Because this type of management is based on mimicking less intense disturbances, the restoration of more severe disturbance patches within forested landscapes has received little attention. The goal of this paper is to synthesize research on the restoration of forests damaged by disturbances in temperate forests of Slovenia and neighbouring regions of Central Europe, where uneven-aged silviculture is practiced. Research indicates that active management aimed at favouring mixed uneven-aged forest reduces the risk of disturbance and improves the resilience of stands. Salvage logging may have positive or negative effects on regeneration, much of which is due to the method applied and the quality of work. The most prominent factors that negatively affect restoration are: lack of advanced regeneration and decomposed woody debris, high altitude, steep slopes, dense ground vegetation, and overbrowsing. Planting or sowing should be applied in post-disturbance forests where many negative factors interact and where a high demand for sustainability of forest ecosystem services is present.

Since the mid-19th century, Pinus nigra plantations have played a key role in the restoration of degraded European landscapes. Nowadays, these plantations are aging and prone to natural disturbances, insect infestations, and diseases. For their successful gradual conversion, knowledge of optimal gap spatiotemporal dynamics is crucial. We studied herb and natural regeneration patterns along with site factors on 477 subplots within 44 plots distributed over four stand types: closed stand (14% diffuse light), open stand (21%), gap edge (23%), and gap (57%). Despite the abundant Quercus petraea, Q. cerris, and Q. pubescens mast year, no one-year seedlings were recorded, which is likely due to the summer drought. Short seedlings (h < 20 cm) of Quercus sp., Fraxinus ornus and Ostrya carpinifolia were more successful within closed stands. Short Quercus seedlings were positively associated with soil depth and negatively associated with soil nutrients, distance to seed trees, and Sesleria autumnalis coverage. Taller Quercus seedlings required more light than both of its strongest competitors and were positively related to humid soils and less rocky sites. Ungulate overbrowsing significantly impeded natural regeneration. The results indicate a satisfactory Quercus density for conversion and the importance of advanced regeneration, which should be gradually, but persistently, released by progressively widening gaps.

Ice storms cause widespread damage to forests in many temperate regions, leaving behind many live trees with severe crown damage. Following a severe ice storm in 2014 that damaged forests across Slovenia, we examined how tree-level attributes influenced survival and crown rebuilding three growing seasons after the storm. Field sampling was carried out in four mature stands dominated by native broadleaf species. Of the 763 sampled trees, the annual mortality rate following the storm was 2.2%, and nearly all trees that died experienced >75% crown removal. Oak (Quercus petraea (Matt.) Liebl.) and chestnut (Castanea sativa Mill.) had higher rates of mortality than beech (Fagus sylvatica L.) and maple (Acer pseudoplatanus L.). Mixed models revealed that survival significantly increased with tree diameter and decreased with increasing crown damage. Although we observed sprouting across all the dominant species, maple, oak, and chestnut showed a more vigorous response than beech, and maple had the fastest sprout growth. Model results showed that sprout density and length increased with level of crown damage. The results indicate that these broadleaf forests are resilient to severe ice damage. Consequently, hasty salvage cutting of trees with canopy damage should be avoided, as many individuals with >75% crown damage are likely to survive and recover.

Forest managers are often required to restore forest stands following natural disturbances, a situation that may become more common and more challenging under global change. In parts of Central Europe, particularly in mountain regions dominated by mixed temperate forests, the use of relatively low intensity, uneven-aged silviculture is a common management approach. Because this type of management is based on mimicking less intense disturbances, the restoration of more severe disturbance patches within forested landscapes has received little attention within the context of uneven-aged silviculture in the region. The goal of this paper is to synthesize research on the restoration of forests damaged by disturbances in temperate forests of Slovenia and neighbouring regions of Central Europe, where uneven-aged silviculture is practiced. We place particular emphasis on the most important biotic and abiotic drivers of post-disturbance regeneration, and use this information to inform silvicultural decisions about applying natural or artificial regeneration in disturbed areas. We conclude with guidelines for restoration silviculture in uneven-aged forest landscapes.