Lora Kirova*, Lora Stoeva, and Todor Stoyanov

Forest Research Institute, Bulgarian Academy of Sciences, 132 ‘St. Kl. Ohridski’ Blvd., Sofia, Bulgaria. *E-mail: l.naydenova@gmail.com

This study aimed to evaluate how different thinning intensities influence the accumulation of organic carbon and nitrogen in soils. We analysed key soil parameters under varying thinning regimes in Norway spruce (Picea abies (L.) H.Karst.) stands, compared with unthinned controls, across six state forestry management units in the Central Balkan Mountains, Bulgaria. On the territory of each management unit, soil samples were taken from three types of plots: unmanaged controls (no silvicultural intervention for over 20 years); light thinning (10–15 %, within the past 10 years); and moderate thinning (20–25 %, within the past 10 years). Soils were examined in depth layers of 0–10, 10–20 and 10–30 cm. We measured bulk density, rock content, soil acidity, and soil organic carbon and total nitrogen stocks. Results indicate that low-intensity thinning (10–15 %) significantly alters soil organic carbon. These findings highlight the need for proactive management strategies to guide thinning practices in Bulgarian forests, with particular attention to soil organic carbon and total nitrogen dynamics.

Received: 15 August 2025 / Accepted: 17 October 2025 / Available online: 24 October

Open Access: This work is distributed under the Creative Commons Attribution 4.0 License.

Kiril Vassilev (1)*, Beloslava Genova (1), Stoyan Georgiev (2), Marius Dimitrov (3), Momchil Nazarov (4), Ivaylo Nikolov (4), Gana Gecheva (1,5), Borislav Grigorov (6), Jozef Šibik (7), Maria Šibikova (7), Jan Douda (8,9), Constantin Mardari (10), and Nikolay Velev (1)

1. Department of Plant and Fungal Diversity and Resources, Institute of Biodiversity and
Ecosystem Research, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria.
*E-mail: kiril5914@abv.bg
2. Field Crops Institute – Chirpan, Agricultural Academy, 6200 Chirpan, Bulgaria.
3. Department of Dendrology, Faculty of Forestry, University of Forestry, 10 Kliment Ohridsky
Blvd., 1797 Sofia, Bulgaria.
4. Central Balkan National Park, 3 Bodra smyana Str., 5300 Gabrovo, Bulgaria.
5. Department of Ecology and Environmental Protection, Faculty of Biology, University of Plovdiv
‘Paisii Hilendarski’, 4000 Plovdiv, Bulgaria.
6. Faculty of Geology and Geography, University of Sofia ‘St. Kliment Ohridski’, 1000 Sofia,
Bulgaria.
7. Institute of Botany, Plant Science and Biodiversity Center, Slovak Academy of Sciences,
Bratislava, Slovakia.
8. Faculty of Environmental Sciences, Czech University of Life Sciences Prague, 129 Kamýcká
Str., 16500 Praha, Czech Republic.
9. Institute of Botany, The Czech Academy of Sciences, Průhonice, Czech Republic.
10. Anastasie Fătu Botanic Garden, Alexandru Ioan Cuza University, Iaşi, Romania.

Riparian shrub and forest vegetation is widespread across Bulgaria, but its syntaxonomic diversity has not been fully studied, which was the main aim of our study. All published relevés were compiled, and new data were collected from different parts of the country, e.g. 905 relevés. Hierarchical clustering was used for classification from class to alliance levels. Diagnostic, constant, and dominant species were identified for all established syntaxa. This study provides the first comprehensive investigation into the distribution, syntaxonomic diversity, ecological characteristics, and threats to riparian forests and shrublands in Bulgaria. Riparian vegetation was classified into two classes, four orders, and seven alliances: 1) Alno glutinosae-Populetea albae P. Fukarek et Fabijanić 1968, Populetalia albae Br.-Bl. ex Tchou 1949, Platanion orientalis I. Kárpáti et V. Kárpáti 1961, Alno-Fraxinetalia excelsioris Passarge 1968, Alnion incanae Pawłowski et al. 1928, Alno-Quercion roboris Horvat 1950; and 2) Salicetea purpureae Moor 1958, Salicetalia purpureae Moor 1958, Salicion albae Soó 1951, Salicion eleagno-daphnoidis (Moor 1958) Grass 1993, Salicion triandrae T. Müller and Görs 1958, Tamaricetalia ramosissimae Borza et Boscaiu ex Doltu et al. 1980, Tamaricion parviflorae I. Kárpáti and V. Kárpáti 1961. The Mediterranean influence in the country is shown in forests of Alno-Quercion roboris, Platanion orientalis, and Tamaricion parviflorae, found in the southern part of Bulgaria. Alliances Alnion incanae and Salicion eleagno-daphnoidis occur in the semi-mountainous and mountainous regions, whereas phytocoenoses of the Salicion albae alliance are widespread in plains along rivers and water bodies. Alliance Salicion triandrae includes shrub communities dominated by Amorpha fruticosa L. and Salix triandra L. On the other hand, the alliance Alno-Quercion roboris is restricted to rivers in northern Bulgaria and the temperate region. The species composition of the studied vegetation was influenced by continentality, which affects climate patterns, such as light and temperature regimes. Additionally, moisture availability and altitude also played a significant role in shaping the studied vegetation. 

Received: 29 July 2025 / Accepted: 17 October 2025 / Available online: 24 October 2025

Open Access: This work is distributed under the Creative Commons Attribution 4.0 License.

Plamen Glogov*, Gergana Zaemdzhikova, and Mariam Bozhilova

Department of Silviculture and Management of Forest Resources, Forest Research Institute –
Bulgarian Academy of Sciences, 132 ‘St. Kliment Ohridski’ Blvd., 1756 Sofia, Bulgaria.
E-mails: plglogov@abv.bg*(PG), zaem.bg@abv.bg (GZ)    

The study aimed to examine the relationship between the occurrence of European mistletoe
(Viscum album L.) and crown defoliation of Canadian poplar (Populus × canadensis Moench) plantations established on coastal dunes along the Central Black Sea coast of Bulgaria. A total of 1,158 trees were assessed for infestation level using a four-class visual scale (no, light, moderate, and heavy infestation) and for crown defoliation following the ICP Forest protocol. Nearly half of the trees (49.8 %) were free of mistletoe, while moderate to heavy infestation occurred in less than 10 %. Defoliation was absent or slight in about two-thirds of the trees, whereas severe defoliation was recorded in 8 %. Statistical analyses revealed a positive correlation between mistletoe abundance and crown defoliation, as well as with stem diameter, indicating that larger and taller trees are more susceptible despite the even-aged structure of the plantations. Although mistletoe did not directly cause tree mortality, it significantly contributed to defoliation and may predispose trees to other biotic and abiotic stressors. A decision-tree model identified threshold values of infestation that predict changes in crown condition, offering a practical tool for assessing host-tree health in dune plantations.

Received: 16 May 2025 / Accepted: 29 October 2025 / Available online: 05 November 2025

Open Access: This work is distributed under the Creative Commons Attribution 4.0 License.

Boyka Malcheva* and Rozalina Koleva

Department of Silviculture, Faculty of Forestry, University of Forestry, 10 Kliment Ohridski Blvd., 1797 Sofia, Bulgaria. *E-mail: b_maltcheva@ltu.bg

Microbial biomass carbon (MBC) and the enzymatic activities of catalase and cellulase were investigated in soils within Sofia Municipality in an agricultural ecosystem (Vrazhdebna district), forest ecosystems (Vitosha Nature Park, Zheleznitsa, and Dolni Pasarel areas), and urban soil (green area, Dolni Pasarel village), in a seasonal context, with and without the influence of local water sources. MBC and the activities of catalase and cellulase were higher in the agricultural soils than in the forest soils, which is also attributable to the fact that they are tilled and irrigated, with trends of higher microbiological activity in spring compared to summer. In most soils of Vitosha, the inflow of water from local sources creates anaerobic conditions that reduce the accumulation of microbially derived carbon and the enzymatic activity of microorganisms. Soils in Vitosha, Zheleznitsa and Dolni Pasarel are acidic, while those in Vrazhdebna are neutral, suggesting mainly bacterial biomass in the latter and more mould fungi in acidic soils; anaerobic conditions in waterlogged sites further limit mycelial microorganisms. Correlation analysis showed a strong negative relationship between MBC and soil moisture (r ≈ -0.73; -0.69), a weak positive relationship with pH (r ≈ 0.08–0.15), a moderate positive relationship with temperature (r ≈ 0.30–0.38) and catalase (r ≈ 0.50–0.60) and a strong positive relationship with cellulase (r ≈ 0.61–0.66). Catalase activity was moderately negatively correlated with moisture (r ≈ -0.53; -0.45) and moderately positively with pH (r ≈ 0.49–0.52); cellulase showed a weak to moderate negative association with moisture (r ≈ -0.33; -0.27), a weak to moderate positive association with temperature (r ≈ 0.28–0.31) and a moderate negative association with pH (r ≈ -0.21; -0.26). Regression analysis confirmed the role of pH as a stable predictor – positive for catalase and negative for cellulase in both seasons, and showed that in spring catalase was the strongest positive predictor of cellulase, whereas in summer cellulase was significantly influenced by temperature, MBC, depth and catalase.

Received: 30 June 2025 / Accepted: 12 November 2025 / Available online: 19 November 2025

Open Access: This work is distributed under the Creative Commons Attribution 4.0 License.

Silvena Boteva (1)*, Boyanka Angelova (1), Kameliya Petrova (2), and Dimitar Dimitrov (3)

1. Department of Ecology and Environmental Protection, Faculty of Biology, Sofia University ‘St.
Kliment Ohridski’, 8 Dragan Tsankov Blvd., 1164 Sofia, Bulgaria. E-mails: sbboteva@biofac.unisofia.
bg*, b_angelova@biofac.uni-sofia.bg
2. Department of Silviculture, Faculty of Forestry, University of Forestry, 10 Kliment Ohridski Blvd.,
1797 Sofia, Bulgaria. E-mail: kpetrova@ltu.bg
3. Department of Forest Ecology, Forest Research Institute, Bulgarian Academy of Sciences, 132
St. Kliment Ohridski Blvd., 1756 Sofia, Bulgaria. E-mail: dimitrov_117@abv.bg

Forests are highly productive ecosystems that act as carbon sinks, where soil organic matter is formed from the residues of biomass decomposition as well as carbon deposited by the rhizosphere. Soil microorganisms play a crucial role in these processes by decomposing plant litter, thereby causing changes in the physical and chemical properties of the soil. This study aimed to determine whether soil bacterial activity can serve as an indicator of soil health and forest condition. In this study, Biolog EcoPlate™ assay was used to analyse the community-level physiological profiles (CLPPs) from spruce forest soils. Samples were taken from two plots in coniferous forests with soils classified as Dystric Cambisols (Sample plot 1, SP1) and Eutric Cambisols (Sample plot 2, SP2) in three depths (0–10, 10–20 and 20–30 cm). Soils were sandy clay loam and clay loam, respectively. Average microbial activity (expressed as the area under the curve, AUC) was nearly twice as high in SP2 as in SP1. In SP1 highest activity was measured at the 10–20 cm depth, while in SP2 it was measured at 20–30 cm. CLPP showed higher bacterial affinity for polymers and lower for carbohydrates. There was a significant difference in the metabolic activity among carbon guilds utilisation (p < 0.05) but not among different depths (p > 0.1). Tween 40, Tween 80 and γ-aminobutyric acid were preferentially utilised, while 2-hydroxybenzoic acid, α-ketobutyric acid and glucose-1-phosphate had the lowest utilisation rates. The strong linear correlation between microbial Shannon diversity and Evenness indices showed that depth influenced these rates, but not the number of utilised carbon sources. Results indicate that microbial activity can serve as a quick and reliable indicator of forest ecosystem functioning.

Received: 24 June 2025 / Accepted: 19 November 2025 / Available online: 21 November 2025

Open Access: This work is distributed under the Creative Commons Attribution 4.0 License.