Temporal evolution of the hydromechanical properties of soil-root systems in a forest fire in China

Plant roots generally enhance soil strength and stabilize slopes through hydro-mechanical effects, especially in forested areas prone to shallow slope failure. Forest fires can severely weaken the hydro-mechanical contribution of roots to slopes, however, the hydro-mechanical characteristics of soil-root systems (SRS) affected by wildfire remain poorly understood. To obtain insight into the post-fire hydro-mechanical characteristics of SRS, a subalpine conifer forested area in Sichuan Province, China that suffered a wildfire on March 30, 2019 was continuously monitored over two consecutive years. Samples from zones with different degrees of burn severity were collected and tests both for roots and SRS were performed. The results revealed a substantial decline in root number, which decreased by 46%-58% two years after the wildfire in the medium- and high-severity areas. The tensile strength tests indicated a reduction of root tensile strength by 36%-47% for roots with diameters less than 2 mm. The shear strength of the SRS determined from saturated direct shear tests strongly and had degraded by 55%-82% two years after the wildfire because of root death and reduced root reinforcement. The results of hydraulic conductivity tests over the same time period indicated an abrupt reduction of SRS hydraulic conductivity within several months after the fire owing to ash clogging and the formation of a hydrophobic layer. After more time had elapsed, however, hydraulic conductivity had increased unexpectedly by a factor of 2.2-3.2 greater than that of unburned soil. We attribute this observation to the formation of macropore flow pathways from decayed roots, which was observed by scanning electron microscopy. The findings presented here provide important insight into the temporal changes of the hydro-mechanical characteristics of SRS in burned areas and their associated mechanisms and could be a useful reference to better evaluate post-wildfire stability of subalpine conifer forest in similar environmental conditions.