Natural regeneration of sessile oak forests is a complex process whose final outcome is influenced by numerous factors. The initial phase of development of sessile oak seedlings stands out as the most critical period in the process of natural regeneration of sessile oak forests. As the available light and competition from the accompanying woody species and ground vegetation are some of the main predictors of the success of sessile oak forest regeneration, this paper presents the results of studying the influence of these factors on the regeneration dynamics and development of sessile oak seedlings. The research was conducted in sessile oak forests in northeastern Serbia in the period from 2015 to 2020. At the end of the growing season each year, the following data were collected on 180 smaller sampling plots of 1 m² in different conditions of canopy openness: the number, height, and root collar diameter of sessile oak seedlings. Also, the percent cover of competing woody species and ground vegetation was estimated on these sample plots. The obtained results indicated that the success of sessile oak forest regeneration largely depends on the initial number of sessile oak seedlings and silvicultural treatments during the rejuvenation period, which determine the microclimatic conditions in the stand and control the competing vegetation. They also indicate that with an increase in the available light, the impact of the competing vegetation on the dynamics of regeneration and development of sessile oak seedlings is less pronounced. Accordingly, as young sessile oak seedlings need a large amount of light for survival and development, it is necessary to increase the available amount of light intensively in a short period of time (six–eight years) by reducing canopy openness and thus providing optimal conditions in which sessile oak seedlings can gain an advantage over the competing vegetation.

Old-growth forests are spontaneously developed forest ecosystems without direct human influence in which only natural processes take place. In this study we analyzed the structural sustainability of beech-fir-spruce old-growth forests on dolomite and limestone in the Bosnian Dinaric Mountains. The field work was carried out on permanent experimental plots of 1.0 hectare in size. Thereby, the diameters (d_1.30) and the height (h) of all trees within the plots were measured. Based on the available literature, we hypothesized that the structure of old-growth forests provides sustainability through tree-size demographic equilibrium. Thus, the data collected were used to test possible differences between the actual and the theoretically balanced structure in the studied old-growth forests. Statistically significant difference in the actual structure between the two old-growth forests on limestone and dolomite was determined. However, both of them exhibited sustainable diameter distributions. These results point to the importance of preserving old-growth forests for future research as they exemplify the tree-size demographic sustainability and can thus serve as an appropriate reference to managed forests. Concretely, certain structural attributes from old-growth forests could be embedded into the management objectives for increased resilience of managed forests.

This paper presents research results on the effects of the extremely high temperatures during the heat wave of August 2017 on the growth and development of injury symptoms in sessile oak (Quercus petraea /Matt./Liebl.) seedlings of different age in the area of the Majdanpek municipality in northeastern Serbia. The starting hypotheses of this study is that the resistance of sessile oak seedlings to extremely high temperatures changes with age and that the stand canopy has a significant protective role in situations where sessile oak seedlings are endangered by extremely high temperatures. The extreme weather conditions at the beginning of August manifested themselves in extremely high temperatures and prolonged absence of precipitation. The average temperature at the beginning of August 2017 was 5.0 to 5.1oС (depending on the altitude) higher compared to the period of seedling growth (2010-2016). During the heat wave, the recorded precipitation was in the range from 0 to 1 mm. These climate conditions significantly affected the development of young seedlings, causing wilting of smaller or larger parts of the leaf surface and sometimes leading to plant death. Using analysis of variance, differences in the intensity of seedling damage were found to depend on age, height, and the protection provided by the mature stand canopy. The obtained results point to the very important role of mature trees in the protection of seedlings from the dangers of extremely high temperatures.

Understanding the processes occurring in old-growth forests and identifying their key aspects can significantly enrich modern forestry practices with innovative ideas and concepts. The natural regeneration process in beech old-growth forests exhibits distinct spatial heterogeneity and temporal variability. To define the key drivers that influence the spatial heterogeneity of regeneration processes and their effects, research was conducted in three beech old-growth forests situated in Serbia, Southeastern Europe: Felješana, Vinatovača, and Kukavica. In each old-growth forest, a network of circular sample plots with an area of 0.1 ha (totaling 45 plots) was established to gather data on structural characteristics and ecological conditions. Within each circular sample plot, data on the regeneration layer were collected on four square sample plots of 1 m² (180 in total). Using linear mixed models, the key drivers of spatial heterogeneity of regeneration processes in beech old-growth forests were analyzed. Based on the results, several key factors contribute to the highly heterogeneous distribution of seedlings, including the canopy, the presence of a middle layer comprising young trees, ground vegetation, and soil stoniness, while a significant influence of the combined effect of the canopy and the presence of a middle layer of young trees is also defined. The spatial heterogeneity of the regeneration process is also represented through the assessment of the ratio between the abundance of one-year-old and older seedlings. The dominance of one-year-old seedlings intensifies with increased canopy density (in instances of very dense canopy (1.0), the ratio of one-year-old and older seedlings is 70:30%). Seedling growth characteristics are shaped by multiple factors, including the influence of the canopy, the presence of the middle layer of young trees, slope, and soil stoniness, with a substantial combined influence of the canopy and the middle layer of young trees. This indicates that the spatial variability of the regeneration process in beech old-growth forests is primarily driven by factors with a substantial individual influence, which may also act combined. It is of paramount importance to understand these factors and determine their influence on the regeneration process in managed beech forests.

Key message: We provide data on seedlot germination potential—a key trait related to regeneration—of 12 oak species. Germination was tested at the University of Granada following international protocols with 8985 acorns from 93 batches and 16 countries across Europe. Data on germination probability, acorn origin, mass, and moisture content measured on another 4544 acorns are available at https://doi.org/10.30827/Digibug.87318. Associated metadata are available at https://metadata-afs.nancy.inra.fr/geonetwork/srv/fre/catalog.search#/metadata/a742c6d8-bc37-4ca2-8b81-2447c5a8858d.