Charcoal morphologies and morphometrics of a Eurasian grass-dominated system for robust interpretation of past fuel and fire type

Abstract. Reconstructing past fire regimes by quantifying charcoal fragments is a commonly used approach, and recent developments in morphological and morphometric analyses of charcoal particles have improved our ability to identify characteristics of burnt plant fuel and interpret fire-type changes. However, burning experiments linking known plants to these metrics are limited, particularly in open ecosystems. This study presents novel analyses of laboratory-produced charcoal of 22 plant species from the steppe regions of Eurasia (Romania and Russia), along with selected Holocene charcoal assemblages from the same areas. We characterize charcoal morphologies and morphometrics in these grass-dominated environments, thereby enabling more robust interpretations of fuel sources and fire types for palaeofire research. Our experiments demonstrate that fire temperature biases the amount of charcoal these plants produce. Grass charcoal production was significantly lower and decreased more strongly with fire temperature than forbs, suggesting an underrepresentation of graminoids in sedimentary charcoal assemblages. While charcoal morphologies enable finer distinctions between fuel types than morphometrics, both approaches are complementary for fuel identification. Morphometric analyses revealed that graminoid charcoal particles were more elongated (length-to-width ratio L / W = 4) and narrower (width-to-length ratio W / L = 0.38) than forbs (L / W=3.1 and W / L=0.42, respectively), in agreement with a global compilation for graminoids ((L / W = 4.3 for grass 5.4 grass and wetland graminoids) and forbs (L / W = 2.9). However, overlapping L / W values present a challenge for establishing absolute cut-off values for fuel type identification in charcoal assemblages with mixed fuel sources. Based on our analyses and compiled datasets from experimental burns, L / W values above 3.0 may indicate predominantly herbaceous morphologies in temperate grassland-dominated ecosystems, though values are likely to be higher for grass than forb-dominated vegetation. Notably, grasses exhibit shorter aspect ratios (4.3) than wetland graminoids (6.4), highlighting that the aspect ratio needs tailoring to the specific environment of its application i.e., dry vs wet open ecosystems. The long forms of graminoid charcoal particles also suggest their potential for longer-distance transport compared to more spherical particles produced from leaves, meaning they likely provide insights into regional fire history. An important finding is that charcoal morphology and morphometrics accurately reflected the dominant herb communities shown by the pollen record, highlighting a solid link between the dominant vegetation type and fuel burnt in grassland-dominated environments. However, the relationship between woody charcoal and pollen may be more complex for trees, as their pollen can travel longer distances compared to woody charcoal. Our results also highlight the complex interplay between local vegetation and charcoal composition with human fire use that needs to be considered when interpreting charcoal morphological records. Overall, these advancements in identifying fuel sources and changes in fire types make charcoal analysis highly relevant to studies of plant evolution and fire management. A critical takeaway from this study is the importance of not assuming the universality of previous research findings and instead employing experimental approaches to characterize charcoal particles in new ecosystems prior to the application of these techniques. For example, experimental charcoal research is needed in tropical grasslands and savannas.