Charcoal occurrence is extensively used as a tool for understanding wildfires over geological timescales. Yet, the fossil charcoal literature to date rarely considers that fire alone is capable of creating a bias in the abundance and nature of charcoal it creates, before it even becomes incorporated into the fossil record. In this study we have used state-of-the-art calorimetry to experimentally produce charcoal from twenty species that represent a range of surface fuels and growth habits, as a preliminary step towards assessing whether different fuel types (and plant organs) are equally likely to remain as charcoal post-fire. We observe that charcoal production appears to be species specific, and is related to the intrinsic physical and chemical properties of a given fuel. Our observations therefore suggest that some taxa are likely to be overrepresented in fossil charcoal assemblages(i.e. needle-shed conifers, tree ferns) and others poorly represented, or not preserved at all (i.e. broad shoot-shed conifers, weedy angiosperms, shrub angiosperms, some ferns). Our study highlights the complexity of charcoal production in modern fuels and we consider what a bias in charcoal production may mean for our understanding of palaeowildfires.

Hudspith et al_Table1Table 1. Summary of all taxa tested under ambient atmospheric oxygen conditions (21 vol. % pO2). Calorimetry test information along with an image analysis determined %charcoal left in the remaining ash/char residue post-burn are presented. Species are grouped according to growth habit.Hudspith et al Table 2Table 2. Summary of the fifteen species tested under superambient atmospheric oxygen conditions (26 vol. % pO2). Calorimetry test information along with an image analysis determined %charcoal left in the remaining ash/char residue post-burn are presented. Species are grouped according to growth habit.Supplemental Fig1Figure 1. Podocarpus salignus dead foliage ash/char (Fig. 2E) illustrating the stages of image processing used to determine charcoal percentages. (A) Histogram values obtained for all pixel values in the 8 bit greyscale photograph. (B) Image manually thresholded to a greyscale value of 50 to highlight the charcoal in the image. (C) Default threshold applied to the whole image to separate the background from the ash/char sample and to derive a maximum ash greyscale value.