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

Random forest analyses of 13 explanatory factors at a 1060m buffer.

<p>Random forest results expressed in percentage of mean standard error (%MSE) of <b>A</b> the charcoal count (Char Count), and <b>B</b> the sum of the particles’ area per sample (Char Area). <b>C</b> For each random forest analysis, the partial plot of the factor explaining more than 5% of the variance.</p

Description of the study site.

<p><b>A</b>. Map of the 51 site locations, the grassland formations in the Great Plains. <b>B</b>, <b>C</b> and <b>D</b>. boxplots of the annual precipitation (in mm), the percentage of area burned, and the fire frequency (# of fires in ten years), respectively, of the three grassland types of the Great Plains (tallgrass, mixed grass and shortgrass prairies). Boxplots represent the mean (solid black line), first and third quartile (box limits), and 5^th and 95^th quantile of the data distribution.</p

Relationships among the width to length ratio (W:L ratio) of charcoal particles and the environmental parameters.

<p><b>A</b> Scatter plot between W:L ratio averaged by site and the proportion of Grassland and Shrubland on the landscape, within a 5000m buffer from the depositional environment. The line corresponds to a W:L ratio of 0.5. <b>B</b> Principal component analysis of the 15 most abundant pollen taxa present in the 51 surface sediment samples, the mean value of W:L ratio for each site (meanW:L), and the sum of area of particles for each site (sumCharArea). Amb.Art is the ratio of <i>Ambrosia</i> to <i>Artemisia</i> pollen, Chenop_Amaranth for Chenopodiaceae/Amaranthaceae pollen, and Aster_Undiff for undifferentiated species of Asteraceae pollen. Axis 1 explains 27% and axis 2 explains 13%. <b>C</b> the random forest analysis of the 13 explanatory factors of the W:L ratio within a 5000m buffer from the depositional environment, and the partial dependence plots for the factors explaining more than 5% of the variance. The y axes of the partial dependence plots correspond to the W:L ratio (unitless). AP/NAP for the ratio of arboreal pollen to non-arboreal pollen.</p

A comparison of charcoal measurements for reconstruction of Mediterranean paleo-fire frequency in the mountains of Corsica

Fire-history reconstructions inferred from sedimentary charcoal records are based on measuring sieved charcoal fragment area, estimating fragment volume, or counting fragments. Similar fire histories are reconstructed from these three approaches for boreal lake sediment cores, using locally defined thresholds. Here, we test the same approach for a montane Mediterranean lake in which taphonomical processes might differ from boreal lakes through fragmentation of charcoal particles. The Mediterranean charcoal series are characterized by highly variable charcoal accumulation rates. Results there indicate that the three proxies do not provide comparable fire histories. The differences are attributable to charcoal fragmentation. This could be linked to fire type (crown or surface fires) or taphonomical processes, including charcoal transportation in the catchment area or in the sediment. The lack of correlation between the concentration of charcoal and of mineral matter suggests that fragmentation is not linked to erosion. Reconstructions based on charcoal area are more robust and stable than those based on fragment counts. Area-based reconstructions should therefore be used instead of the particle-counting method when fragmentation may influence the fragment abundance

Shapes and sizes of charcoal in the Great Plains.

<p><b>A</b> Typical shape of a woody fuel type charcoal piece; <b>B</b> Typical shape of an herbaceous fuel type charcoal piece; <b>C</b> Boxplot of the range size of area of particles from the 51 surface sediment samples. The dashed line at 0.014 mm² corresponds to the area of a particle of 120x120μm. <b>D</b> Boxplots of the length and width of the charcoal particles. The dashed line is drawn at 120μm. <b>E</b> Linear regressions of charcoal count between (i) 60μm to 1 mm (x-axis) and between 120μm to 1mm (y-axis) (60–120μm); (ii) 60μm to 1mm (x-axis) and 180μm to 1mm (y-axis) (60–180μm); and (iii) 120μm to 1mm (y-axis) and 180μm to 1mm (x-axis) (120–180μm). The r-square and the p-values are indicated in the graph.</p