Effect of tree wood density on energy release and charcoal reflectance under constant heat exposure

BackgroundCharcoal increases in reflectance with increased intensity and/or duration of heating, and this has been proposed as a potential quantitative metric for fire severity. Because fuel properties also influence reflectance, relationships between heat exposure and reflectance must currently be considered fuel-specific, limiting the application of the method.AimsWe assessed the effect of wood density on charcoal reflectance, to test whether it could be used as a proxy for overall variation in wood properties.MethodsWood samples from 25 tree species were charred under constant conditions in a microcalorimeter, and reflectance measured by microphotometry.Key resultsA positive linear relationship was found between wood density and charcoal reflectance (r = 0.53). Wood density was highly correlated with total heat release per unit volume (r = 0.94).ConclusionsWood density accounts for a substantial component of the variation in charcoal reflectance under constant heat exposure.ImplicationsThe relationship of density to reflectance shown here is relevant to the assessment of charcoals formed anaerobically, where endogenous heating (combustion of the sample itself) does not occur. In fire-produced chars, an additional increase in reflectance from endogenous heating should produce a stronger correlation, and density might account for a large component of the variation in reflectance under such conditions. The reflectance of charcoal may indicate how much heat it has been exposed to, making it a potential measure of fire severity, but the influences of other factors first need to be understood. Comparing 25 species, we found that denser wood produced charcoal with higher reflectance.