EFFECTS OF PRESCRIBED BURNING ON SELECTED SOIL PROPERTIES IN STEEP CHAPARRAL, SOUTHERN CALIFORNIA

In chaparral wildlands, land managers use prescribed burning to reduce fuel loads especially at the urban/wildland interface. Because wildland soils of chaparral are often shallow with low water holding capacities, and are situated on slope gradients prone to erosion, any disturbance such as prescribed fire can adversely affect the physical and hydrological properties of the soil. A prescribed burn in the San Dimas Experimental Forest provided an opportunity to quantify pre- and post-fire effects of fire on soil properties in chaparral-covered steeplands. The 1.28 ha study watershed was located in the San Gabriel Mountains about 50 km northeast of Los Angeles. Soil and weathered bedrock were described using pits excavated by hand. Pre- and post-fire litter and duff depth, bulk density, particle size distribution, and soil water content were determined. Potential dry ravel, held in place within the unburned litter and duff layers, was measured using metal plot frames. Soils were shallow, coarse loamy sands overlying highly weathered bedrock. Average litter and duff thickness for the watershed was reduced from 39.4 mm to 18.8 mm. Average bulk density increased by 27% to 1.16 g cm –3. Average percent clay decreased from 5.4 to 3.4%. Moisture content was reduced by 69% from 0.13 to 0.04. Combined potential surface dry ravel (all inorganic matter >2 mm) was 50,646 kg/ha-1, and 31,066 kg/ha-1 for only material >8 mm. Our research will provide land resource managers with needed information to protect soil and watershed health during fuel reduction treatments that will in turn provide safer conditions to those living at the urban/wildland interface.