Developing Nature-based Solutions in the Alps: an Ex-situ Experiment to Select Willows for Subalpine Soil and Water Bioengineering Structures

Streambank erosion control and management are experiencing a significant paradigm shift, particularly in mountainous regions. There is an increasing demand for Nature-based Solutions such as soil and water bioengineering techniques (SWBE) with living plant material to protect both human assets and biodiversity from streambank erosion. The success of these techniques is highly dependent on vegetation growth and requires solid knowledge of the local species used. However, our knowledge of appropriate subalpine species is still limited, thus hindering the advancement of effective SWBE in these environments. To address this gap, we established an ex-situ experiment to study the biotechnical traits of subalpine species that contribute to streambank protection. Four species were cultivated in a growth chamber for four months: Salix caesia Vill., Salix foetida Schleich. ex DC. and Salix hastata Vill., restricted to the subalpine belt, and Salix purpurea L., a ubiquitous species. We then assessed both aerial and root traits (number, length and biomass) to evaluate the species’ potential for use in erosion control. The survival rate was excellent - up to 96%. S. hastata had high belowground biomass, promising for substrate stabilization, while S. purpurea produced numerous long shoots appropriate for surface protection. The two other willow species provided intermediate benefits, but their presence could promote biodiversity in SWBE structures. Our study reveals promising potential for the use of these species in high-elevation SWBE. Their morphological differences suggest that the four species should be used in accordance with specific streambank contexts.