Which willows for Soil and Water Bioengineering structures on high-elevation streambank? In situ study of cutting capacity of six 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.

Six willow species were selected: three subalpine shrub species (Salix caesia, S.foetida, S.hastata) and three tree species (S.daphnoides, S.myrsinifolia, S.purpurea) presents in both foothill and subalpine belt. The cuttings were planted in three different experimental sites with varied conditions: in Val Thorens at 1 800 m on subalpine grassland habitat and in Lautaret garden at 2 100 m on subalpine tall herb communities and on sand. For each site, 52 cuttings per species were planted in October 2022. In September 2023, at the end of the vegetative period, the recovery rate and the cuttings' aerial growth were assessed. Aerial growth was estimated by biomass dry weight and cumulative stem length.

All recovery rates were above 70%. In Val Thorens, S.purpurea had a recovery rate of 100%, S.hastata 80% and the other species between 88 and 96%. At the Lautaret garden on grassland, the recovery rates, ranged for all species from 71 (S.caesia) to 98% (S.purpurea). The recovery rates in the Lautaret garden on the sand were higher with 100% for S.caesia, S.foetida and S.purpurea and over 88% for the other three species. Despite differences in in situ conditions, all six species had excellent recovery rates for their use in SWBE structures. The willows growing on the sandy substrate at the Lautaret garden showed a higher growth rate. Within sites, there was no significant difference in growth between species. After the first year of growth, these six species seemed suitable for SWBE structures. Recovery rate and aerial growth will be re-estimated in autumn 2024, after the second growing season.