A 50-year Record of Soil Erosion Dynamics: Pre- and Post-Wildfire Observations from Sediment Coring Methods in Contrasting Ecosystems (Var, Southern France)

In the context of increasingly frequent and intense wildfires, it is crucial to assess their long-term environmental consequences. While their short-term impacts on soil loss and sediment transport are well documented, their effects on erosion dynamics over broader spatial (e.g., watershed) and temporal (e.g., ≥20 years) scales have not been extensively studied. Most studies are restricted to short post-fire periods (i.e., on timescales comprised from a couple months to a decade), which limits our ability to evaluate the resilience, regeneration, and delayed responses of affected ecosystems.

To understand how ecosystems respond, adapt or degrade after wildfire events, it is essential to capture these dynamics over decadal timescales, both before and after such disturbances. In this context, sedimentary archives represent a powerful although underutilized tool for reconstructing past erosion processes and assessing the long-term (e.g., 20 years) resilience of ecosystems affected by wildfires.

This study focuses on the Peguières area, a complex of three small watersheds (13.5ha, 18ha and 39.8ha) located within the Esterel Massif in the French Mediterranean. These watersheds drain into three small artificial reservoirs, constructed in 1977, offering a unique opportunity to analyze sediment archives from three replicated, ecologically similar watersheds affected by major wildfires in 1987 and 2003. Given the minimal human impact in this Natura 2000 protected area, the recorded sediment signals are expected to be primarily influenced by wildfires, providing a robust means of reconstructing long-term erosion dynamics in Mediterranean fire-prone environments. The deployed analytical methodology, combining radionuclide analyses and clustering of geochemical parameters (Mn, Zr, Rb, Ca, Ti, etc.) with density data, provides a robust and reproducible framework for identifying distinct erosion phases before/during and after the wildfire within each core.

The Peguières site, as it comprises three micro-watersheds disturbed by wildfire, offers a unique opportunity to provide comparisons at the watershed scale. Preliminary findings from one of the watersheds, which burned completely during the 2003 wildfire, revealed that despite a general decline in erosion rates over time, erosion remained 31.15% higher in the recent period compared to pre-fire levels. These results challenge the assumption of full ecosystem resilience achievement within a decade after such a disturbance. These preliminary results demonstrate the value of sedimentary archives in assessing long-term impacts of wildfires. They can also inform the management of fire-prone landscapes and support the development of effective environmental protection strategies to mitigate soil and water resource degradation that may be caused by wildfires.