ASSESSING WILDFIRE INTENSITY ON THE KAIBAB PLATEAU, ARIZONA, USA

Fire intensity (maximum temperature and duration of burning) is an important metric to characterize the apparent modern fire-regime shift to larger and hotter fires. This fire-regime shift may be due to forest management practices that have suppressed fires and allowed fuel accumulation over the past century and may be enhanced by recent climate change. To test if fire intensity has increased in recent decades, this study develops and assesses fire-intensity methods to compare pre-twentieth century and modern fire intensities. Methods include documenting the color of heat-altered rock, characterizing charcoal reflectance, and measuring luminescence characteristics of burned rock and soils.

Samples of rock, charcoal and soils were collected from three sites on the Kaibab Plateau that remained unburned in the past century, and eight sites from the adjacent 2020 Mangum Fire, which ignited 71k acres of Ponderosa pine forest on the north rim of the Grand Canyon. The study employs and tests four intensity metrics to determine if the Mangum Fire burned at greater intensities than nearby historic fires (pre-1900 AD).

Fire intensity methods are largely in development. This study uses and tests the application of: 1. Thermally-altered rock color; temperature-color relationships will be calibrated using native rocks collected outside the burn area, heated in a muffle furnace, and assessed with the Munsell color scheme. 2. The optical reflectance of charcoal is proportional to the temperature and duration of combustion, measured as proportion of microscopic reflective grains. 3. Optically Simulated Luminescence (OSL) of quartz sand from 21 cm soil cores and surface rocks will be tested for thermal resetting; quartz heated >500°C is expected to produce anomalously young OSL ages. 4. Soil samples will also be tested for changes in thermoluminescence (TL) sensitivity following exposure to intense heat. Sensitivity of the 110°C TL signal is expected to increase at the temperature of previous heat exposure for temperatures >200-500°C.

Preliminary results provide fire-intensity metrics for comparison between historic burns and the 2020 Mangum fire. This work also provides a proof-of-concept for the novel application of luminescence properties and thermally-altered rock color to estimate paleo-wildfire intensity.