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

Abstract

Background Charcoal 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. Aims We 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. Methods Wood samples from 25 tree species were charred under constant conditions in a microcalorimeter, and reflectance measured by microphotometry. Key results A 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). Conclusions Wood density accounts for a substantial component of the variation in charcoal reflectance under constant heat exposure. Implications The 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.