PS 28-142 - Ground truthing the relationship between carbon bridgehead fraction of charcoals and fire intensity

Abstract: Wildfires are a fundamental and inherent component of the earth system, heavily influenced by climatic effects on weather, ignition, and fuels. Consequently, wildfire reconstructions are necessary to fully understand changes in past ecosystems. However, fire proxy records often lack key information about fire regimes, i.e., temperature, intensity, frequency, etc., that are essential to quantitative reconstructions. Here, we assess the relationship between charcoal carbon bridgehead fraction, a novel proxy for fire temperature, and modern-day fire conditions to assess the use of the proxy in paleo-wildfire reconstructions. This proxy is based on laboratory pyrolysis experiments, conducted on a variety of vegetation types, that reveal a strong linear relationship between fire temperature and char carbon bridgehead fraction from the temperature range of 250 to 600°C (r² = 0.88). This relationship has an RSME of 46°C, indicating that the bridgehead carbon fraction may be a broad indicator of fire temperature. However, subsequent experiments are showing that biomass moisture and pyrolysis time are also significant controls on carbon bridgehead fraction. To assess the utility of this proxy under natural fire conditions, we analyzed charred surface litter samples from prescribed burns and a wildfire. For one of the prescribed burns, in-situ thermistors were deployed to record fire intensity (temperature × duration), during the burn fire at the Lake Waco Wetlands in central Texas, dominated by dried marsh plant fuels. The charcoal carbon bridgehead fraction for this burn was linearly correlated to fire intensity (r² = 0.7). Charred surface litter samples were also collected after a moderate-severity conifer crown fire (Thompson Ridge) in New Mexico, and low-severity prescription fires in nearby conifer stands. Aromatic carbon percent was higher in moderate-severity fires than low-severity fires, but the bridgehead fraction did not differ. This may indicate that char formation under conditions of intense heating was not substantial, or that high-intensity char carbon was preferentially transported off site during the several months' time between burning and sampling. Additional wildfire research needs to be conducted to determine the reason for the discrepancies between prescribed fire and wildfire. Future research involves comparing burn severity maps calculated from Landsat-9 satellite imagery to carbon bridgehead fraction of charcoals collected post-wildfire to better determine the relationship between the proxy and wildfire effects.