Methods for quantifying fire severity in shrubland-fires

Methods are presented to relate temperature-residence-time at the soil surface, i.e., time above 150 ºC as an estimation of the severity of a fire, with measurements made during an experimental fire or on two post hoc measurements. The experiment was carried out in a shrubland dominated by the woody-legume Cytisus striatus subsp. eriocarpus, in Central Spain. Temperature-sensitive paints, and steam-releasing open-calorimeters were used as fire-meters during the burn. Post hoc measurements used were estimations of heat output per unit area, and measurements of the minimum diameter of branches of Cytisus remaining after the fire. Time above 150 ºC was obtained from measurements made with thermocouples placed at the soil surface in 20 contiguous 1×1 m squares of the burn plot. All other measurements were made at each 1×1 m in the 22×3 m rectangle surrounding, and including, the thermocouple squares. Various simple and multiple regression models were constructed to predict time above 150 ºC from each of the four measurements made during or after the fire. Maximum coefficients of determination obtained for regressions were 0.61 and 0.62 for water mass loss from open-calorimeters and branch diameter, respectively. Using all the variables in a multiple regression model, time above 150 ºC was related to water mass loss from open-calorimeters and heat output per unit area with a coefficient of determination of 0.77. It is concluded that estimations of time above 150 ºC at the soil surface during the passage of fire may be possible based on simple devices, such as open-calorimeters, or on biological indicators, such as minimum branch diameters. Additionally, combining two methods (open-calorimeters, estimations of heat output per unit area) may allow the reconstruction of the time above 150 ºC during the fire at a scale of 1 ², an important characteristic of a burn to understand ecosystem response to fire.