High-severity fire reduces soil erosion resistance: A study based on 4-year field monitoring data in dry-warm valley in Southwest China

Forest fire has occurred frequently in the dry-warm valley of Southwest China in recent years. Fire changes physicochemical properties, which may further affect soil erosion resistance (rill erodibility (K_r) and critical shear stress(τ_c)). However, few studies have examined the effects of high-severity fire on K_r and τ_c and the temporal variation characteristics. To evaluate the temporal variation in K_r and τ_c and the influencing factors after high-severity fire, soil samples were collected from burned areas 0.5, 1, 2 and 3 years after the fire, and from unburned areas. Then, K_r and τ_c were determined via a flume experiment at three slopes (17.6, 36.3 and 57.7 %) and five flow discharges (0.8, 1.6, 2.4, 3.2 and 4.0 L s⁻¹). The results revealed that K_r and τ_c varied in the ranges of 0.0006–0.0013 s m⁻¹ and 0.8144–2.2584 Pa, respectively. Compared with the unburned areas, the burned areas presented an increase in K_r of 20 %–160 % and a decrease in τ_c of 37.29 %–77.39 %. With the prolonging of restoration years, K_r gradually decreased and τ_c gradually increased, but it was still lower than the level in unburned areas. Correlation analysis and structural equation modeling showed that median soil particle size (total effects of 0.49 and −0.27, respectively, p < 0.05) and soil organic matter (total effects of −0.71 and 0.90, respectively, p < 0.05) were the most significant factors affecting K_r and τ_c during the 0.5–3 years experiment post-fire. K_r (R² = 0.613, NSE = 0.582) and τ_c (R² = 0.974, NSE = 0.974) could be well estimated by bulk density, median soil particle size, root mass density, soil shear strength, soil organic matter and recovery time. These results provide new insights into the mechanism of the influence of post-fire on soil erosion resistance in the dry-warm valley.