Linking sediment trapping efficiency with morphological traits of Salix tiller barriers on marly gully floors under ecological rehabilitation

Soil erosion is considered to be a major threat for soil quality worldwide. Overgrazing and deforestation are the main causes of a recent increase in soil erosion. When devoid of vegetation, marly terrains are particularly prone to severe hydric erosion with gully formation. It is commonly acknowledged that the presence of vegetation interacts with the sediment yield, especially in the gully floor. In the southern Alps, there is a promising ecological engineering approach used to decrease the sediment yield at marly gullies, consisting in implanting bioengineering works to trap sediment in gully floors. Here, vegetation barriers are made of a line of Salix purpurea cuttings (non-native species but widely used in bioengineering), arranged perpendicularly to the flow, on a dead-wood sill, resprouting and forming tillers. Our objective was to better understand how morphological traits of Salix tiller barriers can explain their sediment trapping efficiency. The history of sediment trapping has been recorded on a sample of 77 Salix barriers since their implantation, from 2002 to 2008. Then, focusing on still visible and homogeneous segments of tiller barriers only (49 segments), the basal diameter, basal stem density and basal shoot branching of the tillers, as well as the sediment deposit heights were measured in spring 2010. The results showed that Salix tillers barriers are able to trap sediment in marly gully floors under ecological rehabilitation. The sediment trapping records showed that there is a structural efficient threshold beyond which Salix barriers can trap sediment. We suggest that this threshold is explained by the fact that the basal stem diameter also reaches a threshold value of about 6 mm, usually during the 3rd year after Salix cuttings are implanted. Beyond this threshold, the results suggest that the main tiller morphological trait which explains the efficiency of sediment-trapping barriers is the basal shoot branching. The present study is innovative in the sense that the sediment flow conditions in gully beds are much stronger than those found in previous studies. The results found here provide practitioners with useful data for designing gullies restoration with Salix cuttings.

► We tracked sediment deposit height upstream 77 Salix barriers of up to 8 years-old. ► Beyond 6 mm basal stem diameter, Salix tillers are able to trap sediment. ► Plant traits involved in sediment trapping vary with sediment flow intensity.