In the City of Ljubljana, we are preparing for the maintenance work on the castle slope with various emergency measures, which will significantly contribute to strengthening the protective role of the forest. Together with the representatives of several professional organizations, after many years of coordination and careful consideration, we chose the most appropriate solution with the aim of ensuring the safety of the residents of the buildings on the Town Square and Old Square, walkers and visitors to the forest, and the long-term stability of the forest on Castle Hill.

Based on several initiatives by citizens who feel threatened by falling stones and other materials due to the unstable slope in the buildings at the bottom of the western slope of Castle Hill, after obtaining the necessary expert opinions, guidelines, consents and other documents from the competent institutions, we designed a project that includes felling of trees and installation of protective fences and nets. We will begin with the work after the conclusion of the public procurement procedure for the selection of the contractor, which is expected to be in the middle of November this year.

The expert committee unanimously agreed on the necessary measures

The forest on Castle Hill has several strongly emphasized roles, which are often not complementary, so it is necessary to assess their importance. As we want to conduct the decision-making processes in this matter professionally and transparently, in November 2023 we established the Committee for Planning and Control in Protected Areas of Urban Forests – Castle Hill, which consists of representatives of the Slovenian Forestry Institute (ZGS), the Institute of the Republic of Slovenia for Nature Protection (ZRSVN), the Forestry Institute of Slovenia (GIS), the Biotechnical Faculty of the University of Ljubljana – Department of Forestry and Renewable Forest Resources and the Department of Landscape Architecture (BF, UL), the company Tisa d.o.o., the public company Voka Snaga d.o.o. and the City of Ljubljana. The committee unanimously assessed that in the case of the part of Castle Hill located directly above residential buildings in Old Ljubljana, the first priority is the safety of people and property. This means that it is necessary to implement measures in the forest to increase the ability of the forest to protect itself and the forest floor (protective role of the forest), as well as the residents and property directly below it (protective role of the forest). After an inventory of the forest and an inspection of the trees, which was carried out by experts from diverse fields (forestry, forest technology, nature conservation, arboriculture, landscape architecture, planning of public space and green areas, etc.), we ascertained that it is possible to significantly increase safety with measures listed below.

1. Felling of trees that pose a direct safety risk (endangering lives and property)
The felling of trees will start at the castle, in the lower risk zone (zone C, slope incline of about 35°), continue in the middle, in the higher risk zone (zone B, slope incline of about 45°) and end in the critical zone (zone A, slope incline of about 65°), where we are dealing with vertical and overhanging walls up to a height of 25 meters. It will be implemented in accordance with the issued decision of the Slovenia Forest Service, which took into account the guidelines of the Expert opinion on the treatment of protective forests at the Ljubljana Castle (Biotechnical Faculty, University of Ljubljana, Department of Forestry and Renewable Forest Resources, August 2023) and the Committee for Planning and Control in Protected Areas of Urban Forests – Castle Hill. In the zones of higher and lower risk, 51 larger trees with trunk diameters over 25 cm will be removed. On the steepest and overhanging parts of the slope, 154 smaller trees are planned for felling, as they are slanted, asymmetrical and damaged. According to the decision of the Slovenian Forestry Institute, 205 examined trees will be removed (out of a total of 908 trees), which represents a good 22% of all registered trees, or a total of approximately 105 m3 of trees. The felling of mostly damaged, weakened and unstable trees will also enable better growing conditions for lower-growing, younger, vital trees, which will further strengthen the protective role of the forest.
The committee considered several felling and harvesting technologies based on the mentioned expert opinion of the Biotechnical Faculty at the University of Ljubljana, among which are manual removal, collection with a portable winch, collection with a tractor winch (for all three, removal with a narrower machine is taken into account), cable harvesting (removal with tractor trailer), crane harvesting (removal with a tractor-trailer or truck) and helicopter harvesting (removal with a truck). The discussion and comparison of felling and harvesting technologies showed that the most suitable technology is the simultaneous execution of sectional felling with helicopter harvesting, as it enables phased execution and the shortest execution of the work, while at the same time causing the least damage to the rest of the trees, herbaceous and shrub vegetation, soil, buildings and paths. Helicopter harvesting does not cause new erosion hotspots, thus preserving the soil for the future emergence of young trees.
2. Preservation and arboricultural treatment of old trees of larger dimensions that do not pose a direct risk to people and buildings
In order to maintain the ecological roles of the forest, which also includes the preservation of biotic and habitat diversity, we will preserve trees of larger dimensions that do not pose a direct risk, provide them with appropriate arboricultural treatment and monitor their condition in the future.
3. Installation of protective fences above residential buildings
As part of the project for the implementation of the protection of buildings below the western slope of the Ljubljana Castle (APUS d.o.o., May 2022; Geologija d.o.o. Idrija, August 2024), a multi-phase installation of protective fences is planned. To secure the area under the hill, three types of protection will be installed: a protective fence, a flexible rockfall barrier and a system for active slope stabilization. Flexible rockfall barriers will be erected only in areas where it is absolutely necessary to improve the safety of people and property in the area below Castle Hill. The decision on the form, quantity, micro locations and duration of protection will be made by the committee and the client in accordance with the current agreement, taking into account the progress of the work and the current situation on site.
4. Regular monitoring and maintenance of the forest on Castle Hill with measures of lower intensity at least every 5 years, which will reduce the risk of erosion events and improve the condition of the urban forest below the castle and its management
The castle slope maintenance project is primarily intended for the greater safety of people and buildings on the Town Square and Old Square, but also for the long-term protection of the forest since its stability or successful resistance to erosion processes is a basic condition for performing all its other roles. A system for active slope stabilization will be installed on the steepest and overhanging parts of the slope, which will be regularly monitored and maintained. After the completion of the project, we will continue with regular maintenance work, arboriculture, and, if necessary, additional planting of suitable tree and shrub species with the aim of strengthening the highlighted functions of the forests of Castle Hill.

All necessary consents and authorizations from private forest owners have been obtained

All the necessary consents were obtained for the felling of trees and land development, namely Consent for felling from the Institute for the Protection of Cultural Heritage of Slovenia (2021), Consent for land development from the Institute for the Protection of Cultural Heritage of Slovenia (2022), Expert opinion regarding felling from the Institute of the Republic of Slovenia for Nature Conservation (2021), Consent for securing buildings from the Slovenia Forest Service (2022), Expert opinion for securing buildings from the Institute of the Republic of Slovenia for Nature Conservation (2022), Nature conservation consent of the Ministry of Natural Resources and Spatial Planning (2022), Decision of the Ministry of Agriculture and Forestry on the assessment of the impact of interventions on the protection forest (2023) and Decision on approval of felling of selected trees (2024). The City of Ljubljana also obtained the authorizations of private forest owners in the area in question for the implementation of the project.

The committee took the common view that, on the basis of the presented state of the forest and tree volume on Castle Hill and the expert discussions at the four committee meetings held between November 2023 and May 2024, including several site inspections, the proposed course of action is both appropriate and necessary to ensure the safety of the occupants of the buildings on the Town Square and the Old Square, of the walkers and visitors to the forest, and to ensure the long-term stability of the forest on Castle Hill.

In the City of Ljubljana, we support the above-mentioned, expertly substantiated measures, as we are convinced that with this we are pursuing the goals of the strategies for the further development of the city's green areas and the quality of life in the city.

Closures in the work zone

We intend to publish a public procurement for the execution of the work within the framework of the project, which is planned to be carried out from mid-November 2024 to the beginning of March 2025. Due to the work and ensuring safety on and around the work site, temporary closures of selected areas and paths will be in place in the area of Castle Hill. Access to the Ljubljana Castle and areas where maintenance work will not be performed will remain open.

We will inform the public about the details of the project, the method of implementation, the exact timeline, the closures of the area, the selected contractor and the value, etc., after the completion of the public procurement procedure and before the start of work on site. We will inform the public about the implementation of the project and key milestones in real time, depending on the progress of the on-site work.