Fig 1.
Description of the study site.
A. Map of the 51 site locations, the grassland formations in the Great Plains. B, C and D. 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 5th and 95th quantile of the data distribution.
Fig 2.
Shapes and sizes of charcoal in the Great Plains.
A Typical shape of a woody fuel type charcoal piece; B Typical shape of an herbaceous fuel type charcoal piece; C 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. D Boxplots of the length and width of the charcoal particles. The dashed line is drawn at 120μm. E 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.
Fig 3.
Random forest analyses of 13 explanatory factors at a 1060m buffer.
Random forest results expressed in percentage of mean standard error (%MSE) of A the charcoal count (Char Count), and B the sum of the particles’ area per sample (Char Area). C For each random forest analysis, the partial plot of the factor explaining more than 5% of the variance.
Fig 4.
Relationships among the width to length ratio (W:L ratio) of charcoal particles and the environmental parameters.
A 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 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 Ambrosia to Artemisia 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%. C 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.