Tree roots are an important source of mechanical strength within the soil. When roots are destroyed by wildfire, slopes may become unstable. In addition to land stability, trees act as barriers to rock falls by deflecting or stopping falling rocks and providing stability for stationary rocks. The danger from falling rocks can loosely correlate to the severity of the burn area. Higher severity burn areas see almost no vegetation remaining on the slopes whereas lower severity burns only affect the understory. Additionally, heat from the wildfire can fracture rocks and send large pieces tumbling downslope.
Rock falls occur where a source of rock exists above slopes steep enough to allow rapid downslope movement of rocks by falling, rolling, and bouncing. Early recognition and avoidance of rock-fall-prone areas is the most effective way to reduce rock-fall hazard and risk. Once a rock-fall hazard has been identified and characterized, a geotechnical consultant experienced in rock-fall-hazard mitigation should provide design or site-preparation recommendations to reduce the hazard.
According to the New Mexico Bureau of Geology, slopes over 17° (30%) have a high susceptibility to rock falls. Slopes between 8° and 17° (14% - 30%) have moderate susceptibility and gentler slopes are classified as a low susceptibility.
Keep in mind that recent fires have impacted slope stability and events like heavy rain or earthquakes will exacerbate rock-fall potential.