USDA Forest Service

Pacific Southwest Research Station

Pacific Southwest
Research Station

800 Buchanan Street
Albany, CA 94710-0011
(510) 883-8830
United States Department of Agriculture Forest Service. USDA logo which links to the department's national site. Forest Service logo which links to the agency's national site.

Research Topics Fire Science

Masticated Fuels:
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Masticated Fuels Research

Below-ground fire effects: soil heating

: Soil heating experiment at the PSW Research Station Laboratory in Redding, CA.

Temperatures as high as 212-572°F have been measured in upper soil horizons under burning masticated fuel beds (Busse et al 2005), when only 140°F are enough to kill plant roots. In order to develop a planning tool for managers interested in reducing masticated fuels using prescribed fire without causing adverse soil impacts, we conducted a study of heat penetration at varying soil moisture levels and in different soil types.

Twelve inch wide and 6 inch deep soil cores were collected by pushing a PVC mould into the ground. This enabled us to collect the intact soil, without cracking and other changes to soil structure. Water was added to these soil cores, and then allowed to dry so that the soils were at: field capacity, 2/3 field capacity, 1/3 field capacity, and summer dry (approximately 40%, 30%, 20%, and 10% volumetric soil moisture). Once at the appropriate moisture level, the soil core was wrapped in plastic to prevent additional drying. When all moisture levels were ready, cores were placed in pits in an open field. Thermocouples were installed at the soil surface, and at 1, 2, 4, and 6 inch depths in the soil.

The pit was filled with soil around the core, and a masticated fuel bed, approximating some of the highest fuel loadings witnessed in the field, was created over the top. A final thermocouple was placed on top of the fuel bed. Fuels were ignited in backing fires and thermocouples removed approximately 24 hours later. Four replicate burns were done for each fuel moisture and soil combination. Three different soil types were investigated: a clay, a loam, and a pumice-sand.

Results of the controlled laboratory burning experiment were validated in the field by burying thermocouples at the same depths in the soil immediately prior to the prescribed burns at the Challenge and Whitmore sites. A total of 62 locations were instrumented.

: installing thermocouples to measure temperature at the center of soil cores.
Key findings:
  • Heat penetration into the soil is highly dependent on soil moisture, with the greatest heat penetration when soils were driest. Heat penetration was greatly reduced with soil moisture at or exceeding 20%.
  • There was no significant effect of soil type. Heat penetration was very similar for all three soil types.
  • Results suggest that most roots and soil organisms will be little affected by prescribed burning of masticated fuels when soils are moist. Soil heating may be considerable with summer wildfires or prescribed burns conducted when soil moisture is low, especially if the loading of masticated residues is high.
Last Modified: Oct 26, 2020 03:23:06 PM