Which willows for Soil and Water Bioengineering structures on high-elevation streambank? Ex situ study of cutting capacity of three shrub species

Soil and Water Bioengineering techniques are a sustainable alternative to civil engineering to prevent erosion processes that threaten streambank stability. These techniques are still poorly developed and documented in subalpine streams, where climatic and hydrological conditions are particularly challenging. It is well known that the success and integration of a SWBE technique is best achieved when it is possible to use indigenous plants and plant material. At the subalpine belt, shrub and tree willows are among the dominant woody species on streambanks. Even if a few past studies claimed that they could play a full role in stabilising the banks of high-elevation streams, their biotechnical characteristics are nearly unknown. The unique information available comes from empirical and not detailed results showing a low resprouting rate of cuttings. Still, no data or information on these capacities are known in the subalpine environment. We conducted an in situ experimental study to assess the cutting capacity of willow species at high elevations to improve SWBE on these streambanks.

Three species of subalpine shrub willow were chosen: S.caesia, S.foetida and S.hastata. These willows have no protected status in the French Alps, making them ideal for soil and water bioengineering. S.purpurea, known for its high resprouting ability in foothill streambanks, was used as a control species. The cuttings were placed in culture chambers for 4 months under controlled conditions of light, humidity, and temperature. We tested the effect of a growth hormone (indole-3-butyric acid), with the assumption that it would stimulate willow growth. 25 cuttings from each species were treated with the hormone while another 25 were not treated. At the end, we measured root diameter, primary root number, root biomass, cumulated stem length, stem number and, stem and leaves biomass. 

The recovery rate was high for all four species (>95%). No significant differences were found between the hormonal treatments. For each trait measured, there were significant differences for at least one of the species, reflecting significant differences in root and aerial morphology between species. S.hastata was distinguished by its very high biomass and S. purpurea by its very long structure. The results revealed significant differences between these species, in terms of morphology, resource allocation and therefore properties for soil and water bioengineering structures. All of the four species appeared to be suitable in SWBE structures. These findings have important implications for the effective installation of cuttings in SWBE structures in subalpine environments.