Charcoal morphologies to discriminate fuel source and fire temperatures

Wildfires can have major impacts on terrestrial ecosystems and climate. The analysis of charcoal fragments in peat and lake sediments is the most widely used approach to reconstruct past biomass burning and fire regimes. This method typically relies on the quantification of the total charcoal content of the sediment. However, there is an increasing effort to use morphologies (finer anatomical features) and morphometrics (length: width ratio) of charcoal particles to advance our understanding of fuel burnt and fire types. We used experimental burnings in the laboratory for plant species from boreal Siberia, which are also commonly found in the Northern Hemisphere, to expand the reference datasets on morphological distinctions between species or fuel types. We also tested the effect of burning temperature (five temperature categories ranging from 250 to 450 °C) on char mass and morphometrics of charred plant material. We found that graminoid charcoal particles are most elongate (6.7-11.5), leaves are the shortest and bulkiest (2.1-3.5) while twigs and wood are intermediate (2.0-5.2). Our findings correspond well to the few existing comparable experimental measurements. Further, the use of fine charcoal features was successful in separating wood, graminoids, and leaves, but it was difficult to further differentiate these fuel types i.e., leaves and wood from trees and shrubs, due to overlapping features. In terms of charred mass, graminoids, Sphagnum, and wood (trunk) lose the most mass at low burn temperatures (<300°C), whereas heathland shrub leaves, brown moss, and ferns at high burn temperatures. Ongoing work applying micro-Fourier Transformed Infrared Spectroscopy (FTIR) on both modern charred particles produced at different temperature ranges and fossil charcoal will help estimate the pyrolysis temperature and fuel type. Similarly, our focus is to expand reference datasets on charcoal morphologies and FTIR to other major biomes, particularly grasslands. We also highlight the further investigations into charcoal experimental studies needed to refine the histories of past wildfires.