Michal Bosela

Climate change leads to less water in forest ecosystems and higher evapotranspiration during the growing season, increasing the risk of drought. This study evaluates microclimate and soil hydrology at two different sites in the Dobroč Primeval Forest (National Nature Reserve, NATURA 2000): a near-natural fir–beech buffer zone and a managed Norway spruce monoculture. Measurements cover two hydrological years with very different climatic conditions. The Climatic Water Balance (CWB) was used to assess precipitation deficit, and soil moisture dynamics were simulated with the GLOBAL mathematical model. In 2021, precipitation was 223.7 mm below the long-term average, and the cumulative CWB deficit from March to September was 224 mm. Drought risk peaked in summer 2021. The spruce stand’s A/B horizon was 197 days below the point of decreased availability (PDA), compared to 179 days in the beech buffer zone. Drought moved through the soil profile with a 3–4-day lag between horizons at both sites. Results confirm that Norway spruce monocultures are more drought-vulnerable than near-natural beech stands under identical conditions, supporting active forest conversion in Central European mountain regions.

Spruce monocultures have been intensively planted across a wide area of Europe to increase timber production and meet the demand from society. However, evidence suggests that species monocultures may not be as resilient to drought spells and heat waves compared to mixtures of two or more species. The advantage of mixed forests over monocultures is particularly evident when the mixed species occupy different niches, reducing inter-specific competition and enabling better growth and increased carbon sequestration. However, it remains unclear how drought events and heat waves affect carbon sequestration in the soil and how this differs between mixed forests and species monocultures. In this study, we conducted two years of intensive monitoring of soil CO2 and CH4 fluxes, measured soil microbial diversity, and assessed long-term (tree ring) and seasonal tree growth to quantify carbon sequestration in a mixed forest and a spruce monoculture. Results showed that severe drought in 2022 significantly reduced the growth of Norway spruce stand and its' forest floor and soil CO2 fluxes but at lesser intensity impacted C fluxes of European beech and silver fir stand. The bark beetle outbreak in 2023 caused rapid tree infestation and die-back only in the spruce stand (followed by salvage clear-cut harvesting) which subsequently increased soil CO2 emissions via a sudden increase in litter input from dead trees, soil temperature and water content from reduction of shade and evapotranspiration.

Gap dynamics are driving many important processes in the development of temperate forest ecosystems. What remains largely unknown is how often the regeneration processes initialized by endogenous mortality of dominant and co-dominant canopy trees take place. We conducted a study in the high mountain forests of the Central Western Carpathians, naturally dominated by the Norway spruce. Based on the repeated forest inventories in two localities, we quantified the structure and amount of deadwood, as well as the associated mortality of standing dead canopy trees. We determined the basic specific gravity of wood and anatomical changes in the initial phase of wood decomposition. The approach for estimating the rate of gap formation and the number of canopy trees per unit area needed for intentional gap formation was formulated based on residence time analysis of three localities. The initial phase of gap formation (standing dead tree in the first decay class) had a narrow range of residence values, with a 90–95% probability that gap age was less than 10 or 13 years. Correspondingly, a relatively constant absolute number of 12 and 13 canopy spruce trees per hectare died standing in 10 years, with a mean diameter reaching 50–58 cm. Maximum diameters trees (70–80 cm) were represented by 1–4 stems per hectare. The values of the wood-specific gravity of standing trees were around 0.370–0.380 g.cm⁻³, and varied from 0.302 to 0.523 g.cm⁻³. Microscopically, our results point out that gap formation is a continuous long-lasting process, starting while canopy trees are living. We observed early signs of wood degradation and bacteria, possibly associated with bark beetles, that induce a strong effect when attacking living trees with vigorous defenses. New information about the initial phase of gap formation has provided a basis for the objective proposal of intervals and intensities of interventions, designed to promote a diversified structure and the long-term ecological stability of the mountain spruce stands in changing climate conditions.