Inn dikes – Substrate-seed mixture-management combinations for for species-rich and functiona dike grassland
Duration: 2025–2028
Funding: Verbund Innkraftwerke
Contact: Markus Bauer | Josephin Hartmann | Johannes Kollmann
About
Dikes not only fulfill the essential function of flood protection but are also an important component of green infrastructure with ecological and social functions. The species-rich grasslands found on them are often of high conservation value, connect habitats across the landscape, and contribute significantly to local recreation.
To ensure flood protection in the future, dikes must be raised and widened. In the course of these construction measures, biotope types of high conservation value—such as species-rich lowland hay meadows or calcareous nutrient-poor grasslands—are often destroyed. In accordance with the Compensation Ordinance, these habitats must be restored to at least a comparably good condition.
The Inndeiche project aims not only to compensate for the loss of valuable habitats and species following dyke raising but also to optimize the multifunctionality of the dikes.
The project’s goal is to identify measures that ensure flood protection and the technical safety of the dikes, while simultaneously promoting species-rich and resilient grassland vegetation, minimizing maintenance and mowing efforts through reduced biomass growth, and enhancing the value of the dikes for local recreation through high species richness and diverse flowering patterns.
In a controlled experiment conducted along a 3-km stretch on both sides of the Inn River near Rosenheim, we are investigating how the use of different seed mixtures—depending on varying topsoil depths and mowing frequencies—can contribute to achieving this multifunctionality.
To this end, seed mixtures were compiled from native seeds drawn from two different species pools, comprising typical species of lowland hay meadows and calcareous nutrient-poor grasslands, respectively. The seed mixtures and the target plant communities they aim to establish differ in terms of their specific leaf area (community-weighted mean SLA). Since specific leaf area significantly influences photosynthetic performance and thus biomass growth, the study analyzes whether adjusting these functional plant traits can reduce the amount of biomass produced and, consequently, the effort required for mowing.
Furthermore, research is being conducted to determine to what extent the topsoil depth can be adjusted to both ensure the structural integrity of the facility (a continuous grass cover and root penetration that is as homogeneous as possible, as an important contribution to flood protection) and to promote plant communities that are as species-rich and resilient as possible. The thickness of the topsoil layer could be a key factor in achieving the desired multifunctionality: Thinner soil layers can promote reduced productivity, thereby potentially favoring biodiversity while also reducing mowing requirements due to lower biomass growth.
The various combinations of tailored seed mixtures, topsoil depth, and mowing regimes are compared with existing dykes as well as with standard practices in restoration projects (hay transfer). The goal is to identify variants that achieve better functionality than current practices by providing lower biomass, more species-rich stands, and greater recreational value through as diverse a flowering pattern as possible, all while maintaining comparable vegetation cover. The results will be translated into practical recommendations for the restoration of dike grasslands that meet the requirements of flood protection as well as those of nature conservation and local recreation.