Calibration of charcoal measurements for fire history reconstruction in grassland ecosystem
Fire is one of the most important disturbances acting on vegetation structure, composition and dynamics, especially in grassland ecosystems where management must consider historic fire regimes. Past fire regimes are reconstructed using charcoal particles preserved in depositional environments. Although it is well known that fires produce charcoal particles, the amount of charcoal pieces is difficult to interpret and the relationship between amount of charcoal pieces and fire parameters (area burned, fire intensity, fire frequency) remains poorly understood. In this study, we used a unique four-year dataset of fire history from a native tallgrass prairie in mid-North America to test which environmental factors led to certain charcoal measurements at three spatial scales. We investigated charcoal morphotypes that reflect fuel source, quantities of small and large charcoal particles reflecting fires at different scales, climate, vegetation and landscape variables, and fire parameters such as burned area, fire frequency, and time since last fire.
We found that small and large particles of charcoal are well inter-correlated (R2= 0.67) and likely reflect the same spatial scale of fire activity in a system with both herbaceous and woody fuels. There was no significant relationship linking charcoal pieces and fire parameters at a scale <500m. However, total charcoal amount was predicted well by the area burned <1km, and regional burning explained the ratio of non-arboreal to total charcoal pieces (NA/T ratio). Charcoal variables, including total charcoal count and NA/T ratio, were not correlated with other fire parameters as well as vegetation cover, landscape or climate variables. Thus, in long-term studies, total amount of charcoal particles, even of a small size (>125µm), could be used as an indicator of local area burned. Further studies are needed to determine the relationship between amount of charcoal recorded and vegetation cover or climatic parameters.