Colourimetric variations in burnt granitic forest soils in relation to fire severity

Forest fires constitute a major environmental problem. Information about fire severity would enable the application of measures to offset the effects of forest fires on the ecosystems. In this study, we evaluated the usefulness of soil colour measurement as a rapid inexpensive method of determining fire severity. For this purpose, we subjected samples of granitic soils (Leptic Umbrisol, Umbric Regosol and Humic Cambisol) from different forest sites to laboratory controlled thermal shock. The thermal shock was performed in topsoil by the action of infrared lamps, which allowed reproducing similar temperatures to those reached in low, moderate and high severity fires. The maximum temperature and the degree hours (DH) reached during the heating were measured as indicators of fire severity. Changes in the colour of the soil and in the contents of organic matter and iron compounds were also analysed. Colour measurements were applied in three different processed burnt samples and results were compared in order to determine the best for the purpose of the study. The results obtained in the present study for granitic soils show that the values of the CIELAB parameters defining the colour of the surface of burnt granitic forest soils increased with the temperature and DH reached. Although the amount of organic matter was significantly correlated with all colour parameters and with temperature and DH reached, the Fe compounds were not significantly correlated with either CIELAB parameters, temperature or DH reached. Determination of variations in the organic matter content appears to be a reliable method of estimating the temperature and degree hours reached in granitic soils during burning.