Combustion Behavior and Pyrogenic Carbon Formation in Beetle-Impacted Lodgepole Pine Forest Fires

Abstract:

Changes in wildfire behavior due to climate change and ecosystem disturbance may significantly impact the distribution of carbon after wildfire events. Pyrogenic carbon (PyC), produced during wildfires and broadly defined as a continuum of carbonaceous species that are highly stable and recalcitrant to environmental degradation, has been identified as a potential carbon sequestration mechanism. The lengthening of wildfire seasons due to increased global temperatures and snow-free land area, in addition to disturbances such as insect-induced mortality in forest ecosystems, can considerably modify the fuel complex in a wildland setting, thereby altering the ignition and fire behavior. However, predicting wildfire propagation, intensity and products is complex due to dependence on the burning environment and fuel type and composition. The formation of combustion products such as PyC is highly dependent on parameters such as temperature, oxygen availability, heating rate and duration in addition to fuel physiochemical properties. This work assessed the impacts of the mountain bark beetle, prevalent in Northern Hemisphere lodgepole pine forests, on pyrolysis kinetics, net heat output, and PyC production. Healthy (HP) and beetle-killed (BK) lodgepole pine boles burned in a 2018 Medicine Bow-Routt National Forest fire were characterized and compared with their unburnt counterparts. Charring was found to be limited to the bark and cambium of the tree boles and significant differences in charring extent between HP and BK were observed. Thermogravimetric analysis revealed dissimilarities in bark/cambium composition between unburned HP and BK and, thus, differences in devolatilization behavior, while fuel pyrolysis kinetic analysis showed higher activation energies for HP, indicating a greater resistance to thermal degradation. A hydrogen pyrolysis characterization method was utilized to isolate and compare PyC present in burned HP and BK boles and to estimate total amounts of PyC produced during the wildfire event. The results of this work present significant advancements in the understanding of combustion behavior and PyC production in beetle-impacted lodgepole pine forest fires and provide important data for carbon modelling and accounting.