森林火灾对浅层含根土力学特性的影响

Effects of forest fire on the mechanical properties of roots-soil composites

根系对土体的力学特性有显著影响,可以对土壤侵蚀和浅层滑坡起到作用。森林火灾发生后,对表层土壤产生影响,植物根系的固坡作用明显减弱,然而对灾后根土复合体力学特性的影响及根系腐败研究尚不充分。为揭示火灾对根系固土能力的弱化机理,对2022年8月21日发生山火的重庆市缙云山国家级自然保护区开展调查,从不同火烧程度的区域采集土样,采用电子万能试验机测得根系抗拉强度、ZJ型应变控制式直剪仪测定含根土体抗剪强度。结果表明:1)轻度火烧区与未被火烧区的细根数量差异较小,而中等火烧区与重度火烧区明显减少,对直径<2 mm的根系,针阔混交林在轻度、中度与重度火烧地区分别比未被火烧地区减少16%、28%和44%,竹林分别减少17%、25%和50%;2)根系抗拉强度随直径整体呈幂函数下降,对比火灾前后根系的抗拉强度,其值随火烧程度的增加而降低;当法向应力相同时,针阔混交林和竹林的未火烧区根土复合体的抗剪强度均明显大于火烧区,中度火烧区与重度火烧区下降较为明显;3)火烧等级对土壤黏聚力显著影响,针阔混交林重度火烧区0~10 cm土壤黏聚力较未火烧区域降低86%,竹林重度火烧区20~30 cm下降72%。结果可为研究森林火灾对根土复合体力学特性的影响及其相关机制提供依据,为灾后浅层滑坡等地质灾害易发性评价和防灾减灾提供数据支撑。

[Background]The root system has significant influence on the mechanical properties of soil and can play a role in soil erosion and shallow landslide.After the occurrence of forest fires,the surface soil was affected,and the slope-fixing effect of plant roots was significantly weakened.However,the effects of forest fires on the mechanical properties of the root-soil system and root corruption have not been fully studied.[Methods]To reveal the weakening mechanism of fire on root soil fixation ability,on August 21,2022,Chongqing Jinyun mountain national nature reserve of fire occurring has carried out investigation.According to the fire intensity classification criteria,the burned severity was divided into three grades:low,moderate and high.In this study,plant root investigation and soil sample collection were carried out on two kinds of forest types and three burn severity of fire areas within one month after the disaster.Collecting soil samples from different degree of fire area,electronic universal testing machine was used to measure root tensile strength,ZJ direct shear device to measure shear strength of soil-root systems.[Results]1)There was a small difference in the number of fine roots between the lowly burned area and the unburned area,while the number of fine roots in the moderately and the highly burned area decreased significantly,on the roots of the diameter of less than 2 mm,theropencedrymion in lowly,moderately and highly burned area,respectively,than those unburned area decreased by 16%,28%and 44%,bamboo forest decreased by 17%,25%and 50%respectively.2)The root tensile strength decreased as a power function with the diameter.Compared with the root tensile strength before and after fire,the root tensile strength decreased with the increase of fire intensity.3)When the normal stress was the same,the shear strength of the root-soil complex in the unburned area of theropencedrymion and bamboo forest was significantly greater than that in the burned area,and the shear strength decreased significantly in the moderately burned area and the highly burned area.4)The soil cohesion was significantly affected by the burned severity.The soil cohesion of 0-10 cm in the highly burned area of coniferous and theropencedrymion was 86%lower than that in the unburned area,and the soil cohesion of 20-30 cm in the highly burned area of bamboo forest was 72%lower.[Conclusions]This study revealed the change of mechanical properties of root-soil complex and root failure mechanism of different burned severity and forest types after forest fire.The result may provide a basis for the study of the effects of forest fires on the mechanical properties of root-soil complex and its related mechanisms,and provide data support for the assessment of the vulnerability of geological disasters such as shallow landslides after disasters,as well as the prevention and reduction of post-disasters.