Soil temperatures during and following low-intensity prescribed burning in a Eucalyptus pauciflora forest

Abstract

Temperatures at the soil surface and at several soil depths were measured during, and at intervals, for 33 months after a low-intensity prescribed burn in a subalpine Eucalyptus pauciflora forest. The high organic matter content, low bulk density, and low moisture content of the surface soil caused steep soil temperature gradients to be generated during the fire. Mean maximum temperatures during the fire were 600 ± 50, 450 ± 52, 54 ± 5 and 42 ± 5°C in the litter and at 0, 2 and 5 cm soil depths respectively. The highest temperatures recorded at 0, 2 and 5 cm depths were 703, 94 and 44°C. Temperatures exceeding 200°C, which result in volatilization of N from soil organic matter, were estimated to have occurred in the upper 3 mm of the soil. Byram fire intensity tended to be negatively correlated with the maximum temperature measured at the soil surface, but was not correlated with the amount of heat absorbed by black cans (thermal integrators) or the increase in the heat content of the soil. After the burn, the mean daily maximum temperatures in the soil were markedly higher on burnt than on unburnt sites. For example, soon after burning increases were 6, 10, 4 and 4°C at 0, 2, 5 and 10 cm depths, respectively, during a 5-day summer period. Mean daily minimum temperatures on recently burnt plots were similar to or slightly lower than those on unburnt areas. Average day-time temperature in recently burnt forest in summer was elevated by up to 8 and 4°C at 0 and 10 cm soil depths.