Reconstructing grassland fire history using sedimentary charcoal: Considering count, size and shape

Citation: Leys, B. A., Commerford, J. L., & McLauchlan, K. K. (2017). Reconstructing grassland fire history using sedimentary charcoal: Considering count, size and shape. Plos One, 12(4), 15. doi:10.1371/journal.pone.0176445Fire is a key Earth system process, with 80% of annual fire activity taking place in grassland areas. However, past fire regimes in grassland systems have been difficult to quantify due to challenges in interpreting the charcoal signal in depositional environments. To improve reconstructions of grassland fire regimes, it is essential to assess two key traits: (1) charcoal count, and (2) charcoal shape. In this study, we quantified the number of charcoal pieces in 51 sediment samples of ponds in the Great Plains and tested its relevance as a proxy for the fire regime by examining 13 potential factors influencing charcoal count, including various fire regime components (e.g. the fire frequency, the area burned, and the fire season), vegetation cover and pollen assemblages, and climate variables. We also quantified the width to length (W: L) ratio of charcoal particles, to assess its utility as a proxy of fuel types in grassland environments by direct comparison with vegetation cover and pollen assemblages. Our first conclusion is that charcoal particles produced by grassland fires are smaller than those produced by forest fires. Thus, a mesh size of 120 mu m as used in forested environments is too large for grassland ecosystems. We recommend counting all charcoal particles over 60 mu m in grasslands and mixed grass-forest environments to increase the number of samples with useful data. Second, a W: L ratio of 0.5 or smaller appears to be an indicator for fuel types, when vegetation surrounding the site is before composed of at least 40% grassland vegetation. Third, the area burned within 1060m of the depositional environments explained both the count and the area of charcoal particles. Therefore, changes in charcoal count or charcoal area through time indicate a change in area burned. The fire regimes of grassland systems, including both human and climatic influences on fire behavior, can be characterized by long-term charcoal records

Novel and Lost Forests in the Upper Midwestern United States, from New Estimates of Settlement-Era Composition, Stem Density, and Biomass

<div><p>Background</p><p>EuroAmerican land-use and its legacies have transformed forest structure and composition across the United States (US). More accurate reconstructions of historical states are critical to understanding the processes governing past, current, and future forest dynamics. Here we present new gridded (8x8km) reconstructions of pre-settlement (1800s) forest composition and structure from the upper Midwestern US (Minnesota, Wisconsin, and most of Michigan), using 19th Century Public Land Survey System (PLSS), with estimates of relative composition, above-ground biomass, stem density, and basal area for 28 tree types. This mapping is more robust than past efforts, using spatially varying correction factors to accommodate sampling design, azimuthal censoring, and biases in tree selection.</p><p>Changes in Forest Structure</p><p>We compare pre-settlement to modern forests using US Forest Service Forest Inventory and Analysis (FIA) data to show the prevalence of lost forests (pre-settlement forests with no current analog), and novel forests (modern forests with no past analogs). Differences between pre-settlement and modern forests are spatially structured owing to differences in land-use impacts and accompanying ecological responses. Modern forests are more homogeneous, and ecotonal gradients are more diffuse today than in the past. Novel forest assemblages represent 28% of all FIA cells, and 28% of pre-settlement forests no longer exist in a modern context. Lost forests include tamarack forests in northeastern Minnesota, hemlock and cedar dominated forests in north-central Wisconsin and along the Upper Peninsula of Michigan, and elm, oak, basswood and ironwood forests along the forest-prairie boundary in south central Minnesota and eastern Wisconsin. Novel FIA forest assemblages are distributed evenly across the region, but novelty shows a strong relationship to spatial distance from remnant forests in the upper Midwest, with novelty predicted at between 20 to 60km from remnants, depending on historical forest type. The spatial relationships between remnant and novel forests, shifts in ecotone structure and the loss of historic forest types point to significant challenges for land managers if landscape restoration is a priority. The spatial signals of novelty and ecological change also point to potential challenges in using modern spatial distributions of species and communities and their relationship to underlying geophysical and climatic attributes in understanding potential responses to changing climate. The signal of human settlement on modern forests is broad, spatially varying and acts to homogenize modern forests relative to their historic counterparts, with significant implications for future management.</p></div

Microscopic Charcoal Signal in Archaeological Contexts

The recovery of archaeological wood charcoals from combustion features provides insights into the exploitation and use of wood fuel resources and past landscapes. The quality of our interpretation based on wood charcoals, however, depends on reliable information about the charcoal assemblages resulting from taphonomy. Charcoal is very fragile in comparison to other combustion residues such as burnt bones. In archaeological contexts, charcoal can easily be fragmented into small pieces (<0.25 mm) due to their fragile property. The investigation of small fragments and particles is particularly important for the interpretation of combustion residues when large pieces of charcoal are rare or apparently absent in archaeological sites, which is mainly true for many European Palaeolithic sites. Here, archaeologists get incomplete information when only the largest pieces and fragments are considered. In this chapter, we present a method for extracting and quantifying charcoal pieces, fragments, and particles. This approach can be considered as a strategy to minimize the impact of sample incompleteness and biases related to combustion residues in archaeological contexts. We further provide (1) a definition of what the charcoal signal means in an archaeological context; (2) an overview of taphonomy that causes charcoal fragmentation; (3) a review of charcoal sampling, extraction, observation and quantification protocols; (4) a manual (pictures and descriptions) for the observation of charcoal, from large pieces to the smallest particles; and (5) a discussion about why the charcoal signal is useful for archaeologists. By taking into account the consequences of taphonomy, the microscopic charcoal analysis in archaeological contexts provides a reliable assessment of firewood and fuel management practices and the related resilience of societies through time. The microscopic charcoal analysis can further offer additional information about the intensity of taphonomical processes and dating