How Rock Fragments and Moisture Affect Soil Temperatures during Fire

Soil heating during forest fires can considerably impact the soil system, with effects ranging from seed and microbe mortality to nutrient losses and structural degradation. Because soil heating is related to soil moisture and composition, the impact of fire may also depend on the presence of rock fragments in and on the soil. In laboratory burning experiments, the effect of rock fragments on soil heating was evaluated using factorial combinations of soil moisture, rock fragment cover, and rock fragment content. Soil moisture significantly reduced maximum temperatures as well as the depth and duration of sustained temperatures (duration of heating) above 60 and 175 degrees C. Effects decreased with depth. A rock fragment cover similarly protected the soil from high maximum temperatures, especially in dry soil. While it decreased the depth of lethal heating from 3 to 2 cm, it increased the duration of heating at the soil surface. Incorporated rock fragments had no significant effect on maximum temperature or the depth of lethal heating, and effects on heating duration were limited to dry or bare soil. The data suggest that by changing the degree of soil heating, rock fragments may reduce the risk of fire-induced biological, chemical, and physical degradation but increase the biological impact of fire at the soil surface. These findings have implications for controlled fire decision making in rocky areas where soil heating is desired or should be avoided.