How do large wildfires impact sediment redistribution over multiple decades?

Abstract

Wildfires have become an increasing threat for Mediterranean ecosystems, due to increasing climate change-induced wildfire activity and changing land management practices. In addition to the initial risk, wildfires can alter the soil in various waysdepending on fire severity-and cause enhanced post-fire erosion. Usually, post-fire erosion studies focus on a short time window and lack the attention for sediment dynamics at larger spatial scales. Yet, these large spatial and temporal scales are fundamental for a better understanding of long-term destructive effects of multiple recurring wildfires on post-fire erosion processes and catchment sediment dynamics.In this study the landscape evolution model LAPSUS was used to simulate erosion and deposition in the 404 km 2 Águeda catchment in north-central Portugal over a 41-year (1979-2020) timespan, including eight wildfires each burning >1000 ha. To include variation in fire severity and its impact on the soil, four burn severity classes, represented by the difference normalized burn ratio (dNBR), were parameterized.Although model calibration was difficult due to lack of spatial and temporal measured data, the results show that long-term post-fire net erosion rates were significantly higher in the wildfire scenarios (5.95 ton ha À1 yr À1 ) compared to those of a nonwildfire scenario (0.58 ton ha À1 yr À1 ). Furthermore, erosion values increased with burn severity and multiple wildfires increased the overall catchment sediment buildup. Simulated erosion patterns showed great spatial variability, with large deposition and erosion rates inside streams. This variability made it difficult to identify land uses that were most sensitive for post-fire erosion, because some land uses were located in more erosion-sensitive areas (e.g. streams, gullies) or were more affected by high burn severity levels than others. Despite these limitations, LAPSUS performed well on addressing spatial sediment processes and can contribute to pre-fire management strategies, by identifying locations at risk of post-fire erosion.

How Long Is Long? A Bibliographic Review of What Is Meant by the Long-Term Effects of Fire on Soil Properties

“…In the Mediterranean, the most intense erosion is a short-term phenomenon, that is, it occurs immediately after a fire, given that the many plant species that colonize the burned area protect the soil from erosive processes. And yet, Follmi et al (2022) show that although erosion in a Portuguese forest is reduced over time, it remains more important in the long term in areas that have been burned than in those that avoided the effects of fire. To achieve this goal, the authors build a landscape evolution model and find that over a 41-year timespan erosion rates in the burned area were 5.95 ton compared to 0.58 ton ha −1 year −1 in the non-burned area.…”