Admir Avdagić

Mixed mountain forests of Norway spruce (Picea abies (L.) Karst.), silver fir (Abies alba Mill.), and European beech (Fagus sylvatica L.) are ecologically important across Europe, providing ecosystem services, supporting biodiversity, and contributing to the bioeconomy as crucial timber sources. Understanding background mortality in mixed beech-fir-spruce mountain forests is essential to distinguish the effects of this relatively continuous process from those of higher-intensity disturbance-induced mortality, and to inform sustainable management under changing conditions. To assess how stand density, tree-size dominance, species competition and diversity, site geomorphology, and climate influence background mortality, expressed as the annualised basal area loss mortality rate, we applied generalised linear mixed models. We used forest inventory data acquired between 1912 and 2016 from 78 permanent plots located in Bulgaria, Bosnia and Herzegovina, Slovenia, Slovakia, Poland, Germany, and Switzerland. At the stand level, mortality increased with higher stand basal area, fir size dominance, and greater tree species diversity, confirming asymmetric interactions among coexisting species. Spruce mortality increased with stand density, species diversity, and fir dominance, indicating a higher probability of spruce mortality under fir dominance pressure. Fir mortality increased with mean annual temperature, species diversity, and intraspecific tree-size dominance, revealing fir as a species sensitive to rising temperatures. Beech mortality increased with stand density and fir dominance, suggesting that beech suffers more when fir is occupying an upper layer than beech. These findings suggest that sustainable management of mixed mountain forests requires targeted interventions to regulate stand density and control size dominance hierarchies.

The study examined the sources of volume loss in spruce and fir roundwood resulting from the prescribed Huber’s formula, the prescribed method of diameter measurement, the prescribed method of length measurement, and the deduction of double bark thickness. The Riecke’s formula was assumed to provide the most accurate determination of log volume and was therefore used to calculate the gross volume of roundwood, serving as the reference value for quantifying the observed losses. For all analyzed sources of volume loss except when applying Huber’s formula, a dependence on diameter class was observed, with losses decreasing as diameter increased. The highest percentage of volume loss for both spruce and fir logs was found when deducting double bark thickness (ranging from 9.1% to 6.4% for spruce and from 10.7% to 7.7% for fir), followed by the prescribed method of diameter measurement (from 7.3% to 1.7%). Additional losses were associated with the prescribed volume calculation formula (from 5.3% to 1.5% for spruce and from 3.7% to 1.1% for fir) and the prescribed method of length measurement (from 3.1% to 2.1%).

Key message: We found a significant increase in the latewood density of European beech, and a decrease in the latewood and mean wood density of silver fir and Norway spruce in European mountain forests over the period 1901–2016. In the past century, drought did not directly influence the wood density trend of the three studied species. However, for both fir and spruce, drought indirectly affected the mean wood density via changes in the latewood to earlywood ratio, i.e., in the case of extreme drought, trees with high values of latewood to earlywood ratio experienced a slight attenuation in the declining trend of their mean wood density. Context: Century-long wood density measurements can provide novel information on tree response to climate change and the carbon sequestration potential of forest ecosystems. Still, the knowledge about long-term changes in wood density of European beech (Fagus sylvatica L.), silver fir (Abies alba Mill.), and Norway spruce (Picea abies (L.) H.Karst.) in European mountain forests needs to be further explored. Aims: We assessed long-term changes in tree-ring mean wood density, earlywood density, and latewood density in trees of the three species between 1901 and 2016. We investigated the influence of endogenous factors (i.e., tree-ring width, current tree diameter, and latewood to earlywood ratio) and drought events on wood density. Methods: In total, 150 tree cores were sampled from mountain forests in Bulgaria, Bosnia and Herzegovina, Slovenia, Switzerland, and Germany. The mean, early, and latewood density of these samples were measured with the LIGNOSTATION™ system. To address our research aims, we applied a linear mixed-effect modelling approach using the data from 101 correctly cross-dated cores that spanned the entire period of analysis. Results: In the absence of drought, the latewood density of European beech increased by 7.1%, the late and mean wood density of silver fir decreased by 16.8% and 11.0%, respectively, and the late and mean wood density of Norway spruce decreased by 16.1% and 7.2%, respectively, between 1901–2016. In the past century, drought influenced the trends of wood density through an effect mediated by changes in the latewood to earlywood ratio. Specifically, in cases of extreme drought, silver fir and Norway spruce trees with a latewood to earlywood ratio value 50% higher than the median experience a slight attenuation in the declining trend of their mean wood density, making the negative impact of drought marginally less severe. Conclusions: Our findings have significant implications for the accuracy of carbon stock assessments, national greenhouse gas inventories, and the utilization of wood from the three species. Given the fact that changes in wood density follow species-specific patterns and the expectation of more frequent drought events in Europe, in the future, it is essential to build further tree-ring density time series for other species and sites to improve our understanding of how climate change alters wood density and carbon sequestration of forest ecosystems.

The negative influence of timber harvesting on the forest environment is reflected through damage to the residual trees, regeneration, and forest soil. Considering that skidding, a popular extraction method, can cause substantial and severe damage to the remaining stand, the aim of this research was to determine damage to residual trees during skidding by an LKT 81T cable skidder, including oxen bunching. The research was conducted in eastern Bosnia and Herzegovina, in an uneven-aged mixed fir (Abies alba Mill.) and spruce (Picea abies L.) forest with pine (Pinus sylvestris L.) on limestone soils. Tree felling was conducted using a Husqvarna 372 XP chainsaw. Extraction operations caused damage to 6.31% of the residual trees in the stand. The most damage was “removed bark” (65.34%) and occurred on the lower parts of the tree, the butt end (55.11%) and root collar (32.39%). The average size of the damage was 197.08 cm². A statistically significant correlation was found between the damage position and the diameter at the breast height (p < 0.05) and the damage position and damage size (p < 0.01) by Spearman correlation analysis. The conducted analysis by the chi-squared test showed that there is a statistically significant difference in the proportion of damage for trees with different distances to the nearest skid road (p = 0.0487), but the share of damaged trees did not decrease by increasing the distance from the skid road.