Researchers measure dead wood in Rohrach, Vorarlberg

Natura 2000 protected areas extend around the Rickenbach Gorge in the north of Vorarlberg on both sides of the Austrian-Bavaria border. On the Austrian side, in the municipalities of Hohenweiler and Möggers (district of Bregenz), the 50-hectare Rohrach natural forest reserve was established in 1992 and all forestry use was prohibited. Since then, the undisturbed forest development there has been scientifically observed. The data collected is particularly relevant with regard to climate change adaptation, recording the condition of the forests and their sustainable use.

"island" for rare animal and plant species

An initial survey of flora and fauna in the 1990s under the direction of the now deceased ecologist Georg Grabherr was repeated in Rohrach in 2022, this time adapted to the present thanks to the further development of surveying methods. According to Miriam Simma from the regional management, a collective publication on this is still pending, but the individual scientific results are already inspiring the experts: after 30 years of undisturbed conditions, many rare birds, insects, fungi and plants can be found in Rohrach. 26 large fungi were detected for the first time in Vorarlberg, 2 of them for the first time in Austria. 34 red-list species of beetles were found, including primeval forest relic species such as the bark beetle (Ceruchus chrysomelinus) and the shovel beetle (Prostomis mandibularis).

Deadwood is important for healthy forests

This is because the area constantly has something available that is still too rigorously removed in traditional commercial forests: dead wood. The dead trees and branches provide habitat, support the nutrient cycle, reduce erosion and temporarily store carbon. Intensive forest management has led to a decline in this important building block in the ecosystem and thus in the biodiversity of the forests. There is now a rethink: a certain proportion of dead wood, usually between five and ten percent, is recommended.

However, the important monitoring of dead wood is particularly difficult in natural forests with trunks lying on top of each other and dense vegetation. So how do you measure such a "jungle"? The geodesist Jakob Galle dealt with this very issue in his master's thesis in the field of photogrammetry at the Department of Geodesy and Geoinformation at the Vienna University of Technology. The Salzburg native made his way through the impassable and damp Rohrach with his heavy laser in his luggage.

New algorithm for laser detection of dead wood

Galle investigated options for the automated detection of deadwood and compared the time-consuming field measurement with the proven line sampling method ("Line Intersect"), the already developed drone-based laser scanning method (ULS) and a new algorithm for terrestrial laser scanning (TLS) developed by Galle. The TLS approach uses high-resolution point clouds to detect lying deadwood based on geometric features. The deadwood biomass of the area can then be calculated from this.

Galle's conclusion: The ULS method was efficient for large-scale mapping, but less so for sample areas with smaller or hidden trunks. The ground laser provided more detailed measurements, but there were problems in parts of the forest with dense vegetation and trunks lying on top of each other. The line samples offered a quick and easy overview, but were less accurate and, like manual field recording, were not suitable for large areas.

For Galle, the solution lies in a coordinated mix of methods: "A combination of ULS and TLS data could offer a more comprehensive solution by using ULS for large-scale surveys and TLS for detailed analyses," says the scientist. This would be interesting not only for monitoring in Austria's natural forests, but generally for large forest areas such as those managed by the Austrian Federal Forests.