Modeled dispersal patterns for wood and grass charcoal are different: Implications for paleofire reconstruction

Abstract

Sedimentary charcoal records provide useful perspectives on the long-term controls and behavior of fire in the Earth System. However, a comprehensive understanding of the nuances, biases, and limitations of charcoal as a fire proxy is necessary for reliable paleofire interpretations. Here, we use a charcoal dispersal model to answer the following questions: (1) How does the dispersal of wood and grass charcoal particles differ? (2) Do traditional conceptual models of charcoal dispersal reliably characterize grass charcoal dispersal? We find that small differences in shape (length:width (L:W)) and density of grass and wood charcoal can cause substantial differences in particle dispersal and source area. Whereas the modeled dispersal of wood charcoal shows a localized deposition signal which decays with distance, grass charcoal shows more diffuse deposition lacking a localized center (for both >125 µm and >60 µm). Although paleofire research has typically not distinguished between fuel types when interpreting source area, we show that the dispersal of charcoal derived from different fuels is unlikely to be uniform. Because differences in localization, production, and preservation could bias aggregate charcoal accumulation, caution should be taken when interpreting wood and grass-derived charcoal particles preserved in the same record. Additionally, we propose an alternative, dual background conceptual model of grass charcoal dispersal, as the traditional, two-component (peak and background) conceptual model does not accurately characterize the modeled dispersal of grass charcoal. Lastly, this mismatch of conceptualizations of dispersal mechanics implies that grass charcoal may not fit the criteria necessary for peak analysis techniques.

Long‐and short‐term vegetation change and inferred climate dynamics and anthropogenic activity in the central Cerrado during the Holocene

“…An increase in pollen influx is usually associated with an increase in fire intensity and a shorter distance from its source (Tinner et al, 2006;Duffin et al, 2008). Experimental analysis of charcoal particles has shown that the charcoal area increases with local fire (Leys et al, 2015(Leys et al, , 2017, especially when charred wood particles are more abundant (Vachula and Rehn, 2023). From 810 to 730 cal a BP, a mid-magnitude fire event (CHARarea <4 mm 2 cm −2 a −1 ) with gradual increase in charred grass particles (Figs 4 and S4) was observed after a short dry episode, and did not prevent the expansion of woody cerrado and gallery forest taxa, with a recovery of about 12 years.…”

Fire-human-climate interactions in the Bolivian Amazon rainforest ecotone from the Last Glacial Maximum to late Holocene

“…Charcoal morphometrics based on aspect ratio can successfully differentiate woody and grass fuels (Vachula et al, 2021), however, previous studies document the overrepresentation of woody charcoal in grass dominated systems (Leys et al, 2015). To complicate this issue, recent charcoal dispersal models indicate that the dispersal of wood versus grass charcoal is not uniform, with fine grass charcoal particles being transported further than woody charcoal particles of the same size class (i.e., 125 µm) (Vachula and Rehn, 2023). Thus, the grass-charcoal component present in charcoal records from grass-dominated and mixed wood-grass ecosystems, such as NKMNP, likely represent broader regional fire histories than woody charcoal records from wood-dominated ecosystems.…”

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

“…Thirdly, dispersal models accounting for shapes, sizes and densities of charcoal show that small differences in shape and density of charcoal can cause substantial differences in particle source area (Vachula and Richter, 2018;Vachula and Rehn, 2023). Elongated, lighter particles such as those from graminoids tend to have higher residence time in the atmosphere and can travel longer distances than spherical or heavier particles, such as those from leaves or wood (Clark and Hussey, 1996;Pisaric, 2002;Vachula and Richter, 2018;Courtney Mustaphi et al, 2022;Vachula and Rehn, 2023). Charcoal particles produced by grassland fires tend to be smaller than those produced by forest fires (Belcher et al, 2005;Leys et al, 2017), and our results show that the size of charcoal particles decreased with the increased fire temperature.…”

“…Another uncertainty in charcoal-based fire reconstruction is how charcoal production is affected by the biomass quantity, density, moisture content and fire temperature (Simpson et al, 2016;Pereboom et al, 2020;Feurdean, 2021;Hudspith et al, 2018). Finally, the size, shape and density of the charcoal fragments can cause substantial differences in particle dispersal and incorporation into sediments (Clark and Hussey, 1996;Vachula and Richter, 2018;Vachula and Rehn, 2023). Microscopic particles may be transported considerable distances and deposited into various sedimentary environments such as peats, lakes and rivers, in contrast to larger particles that tend to be deposited nearer to the source of the fire (Scott et al, 2000;Scott, 2010).…”

“…Microscopic particles may be transported considerable distances and deposited into various sedimentary environments such as peats, lakes and rivers, in contrast to larger particles that tend to be deposited nearer to the source of the fire (Scott et al, 2000;Scott, 2010). Additionally, the morphology of charcoal characterised by longer and thinner shapes, typical of grass, allows for longer-distance atmospheric transport compared to spherical or denser particles, such as those from leaves and wood (Clark and Hussey, 1996;Pisaric, 2002;Scott et al, 2000;Vachula and Richter, 2018;Courtney Mustaphi et al, 2022;Vachula and Rehn, 2023). Conversely, water favours the transport of wood charcoal (Scott et al, 2000).…”