Every forest fire leaves charcoal and ash residues (Peet 1988). While much of this debris collects in burned forest, some invariably falls or washes into nearby unburned areas. In certain natural catchments, like peat bogs and lakes, these residues tend to collect in distinct sediment layers. Undisturbed, these catchments can record major fire events as signature charcoal layers dating back to the last glacial retreat, about 12,000 years ago.
To search for evidence of past fires, scientists drill out vertical cross-sections - "cores"- of peat bogs and lake beds near forests of interest and note spikes in charcoal abundance along the length of each core. Each spike is assumed to correspond to a large fire event or exceptionally active fire period. Charcoal evenly dispersed throughout a core section is considered evidence of relatively frequent, low-severity fires (Mehringer et al. 1977). Spikes are radiocarbon-dated to determine major fire years and to gauge the typical interval between severe fire events.
Additionally, vegetation present throughout a period of interest can be reconstructed from pollen and plant remains within the cores. Records from different locations are used to glean fire and vegetation patterns over large areas.
Mehringer and others (1977) used this technique to chart fire patterns in one area of the Bitterroot Mountains. They examined six-meter-long cores from Lost Trail Pass Bog near the Montana-Idaho border to determine the fire history of the adjacent lower subalpine forests. Pine pollen was pervasive in layers spanning the past 11,550 years, and frequent fires were evident in the form of continuous charcoal deposition spanning 12,000 years.
Mehringer, P. J., S. F. Arno, and K. L. Petersen. 1977. Post-glacial history of Lost Trail bog, Bitterroot Mountains, Montana. Arctic and Alpine Research 9:345-368.
Peet, R. K. 1988. Forests of the Rocky Mountains. Pp. 63-102 in M.G. Barbour and W.D. Billings, eds., North American Terrestrial Vegetation. Cambridge University Press, New York, New York.