The Physical and Chemical Characteristics of Wildfire Ash

Following wildfire events a layer of ash is often deposited over the landscape. Much of the existing literature suggests that the ash layer temporarily reduces infiltration, however only two studies have actually documented a reduction in infiltration due to the presence of ash. In fact a greater number of field-based studies have found that, at least in the short term, ash reduces runoff by increasing the initial infiltration rate. Variability in the effect of ash on runoff may reflect differences in the physical and chemical characteristics of the ash layer due to variations in fuel density, fuel moisture content, the type and quantity of fuels present, the fire severity, and permanent structural changes associated with wetting. However, no effort has been made to characterize and quantify the variability in ash characteristics.

The objectives of this study are to: 1) evaluate how the physical and chemical properties of ash vary as a function of combustion temperature and fuel type, 2) determine the effect of wetting on ash properties. Ash was generated in the laboratory using fuels from the three dominant tree species of western Montana (Lodgepole Pine (Pinus contorta), Ponderosa Pine (Pinus Ponderosa) and Douglas Fir (Pseudotoga menziesii)) at 100°C temperature increments from 300 to 900°C. A subsample of each specimen was saturated, left undisturbed for 24 hours and then oven dried at 104°C. Dry and wetted ash samples were characterized in terms of: color, particle size and shape (using scanning electron microscopy), carbon content, mineralogy (using X-ray diffraction), aggregate stability, porosity, water retention properties and hydraulic conductivity. Initial results indicate noticeable structural differences between fuel types at low combustion temperatures, however structural and chemical characteristics of ash are more dependent upon combustion temperature than fuel type. Ash produced at high combustion temperatures also exhibits structural and chemical alterations upon wetting.