Partial forest harvesting effects on erosion flux in a headwater catchment (Strengbach catchment, France)

Due to the increasing global need for wood, forest management and especially tree harvesting have become increasingly challenging for the sustainability of forest ecosystems. Indeed, the natural dynamics of solid exports in rivers can be strongly disturbed by anthropogenic activities including forestry. The impact of forest management on erosion flux can be due to tree logging but also to forest roads, skid trails, stream crossings required for silvicultural operations.

The impact of forestry on solid exports in mountainous environment has been studied in a small granitic watershed (0.8 Km²) located in the Vosges massif. Between July and August 2014, the Strengbach catchment (Observatoire Hydro-Géochimique de l’Environnement) was concerned by clear-cutting on some plots located near the main stream. This small extended forestry operation (2.3% of the catchment) involved the logging of trees and the implementation of skid trail network including poorly designed stream crossings. The bedload flux was estimated since April 2009. The suspended sediment (SS) flux was evaluated on the basis of stream water samples collected every 16 hours and during high-flow events since December 2012.

Before the forestry operation, the mean bedload flux was 2.5 T/yr±8% for a mean outlet runoff of 730 mm/yr, although the SS flux was 7.7 T/yr±10% for an outlet runoff of 950 mm/yr.

The forestry operation occurring in 2014 has involved a significant and quasi-immediate impact on the SS concentration and flux. As an illustration, the mean SS concentration of the stream was 129 mg/L (outside high-flow periods) the fortnight after the forestry operation beginning, whereas it was only 6.2 mg/L just before. In addition, the forestry operation led to approximately 5 to 6 times larger SS flux than that expected for the July-August 2014 period. The impact on annual SS flux was significant during two hydrological years, with an increase of +100% and +50% for 2014 and 2015, respectively.  This relatively high disturbance is mainly due to the implementation of non-improved stream crossings and skid trails, responsible for the introduction of a huge amount of fine soil particles into the stream. At the opposite, no clear influence of the forestry operation on the bedload export could be observed in 2014 whereas it was 2 times higher than that expected the following year. This delay of the tree harvesting impact on coarse sediment export can be explained by the trapping of bedload upstream of the logs constituting stream crossings during the forestry operation. After the logs removal, the trapped sediments needed several flood events to reach outlet, explaining the delay. Overall, a post-logging recovery time of approximately 10 months can be assumed for the solid exports following the forestry operation.