Often, trees are damaged but not killed by a forest fire. Nonlethal fires are typically those that keep close to the ground, leaving the trees' foliage largely unscathed (Brown 1995).
Wounds inflicted by these fires are usually on the uphill side and at the base of trees, where accumulated fuels like pine needles, leaves, twigs, branches, and other debris concentrate heat as they burn. Fire in these hotspots can heat through tree bark and kill living tissue. A tree damaged in this way bears a burn mark shaped like an upside-down "V" once the fire has passed. Over time, the charred bark sloughs away, leaving a smooth, light-colored scar.
Fire-killed tissue cannot produce new wood. Yet a scarred tree will still grow in diameter as it produces wood from the surrounding live tissue. This new growth will ever so slowly spill into the scarred area and may eventually conceal the wound. In the meantime, the dry and resin-covered wound will remain especially flammable.
If the tree experiences another fire during the time it takes new wood to cover the original wound, it will likely catch fire in the same spot once again. As the scar reburns, surrounding tissue is apt to be killed, leaving a new, peripheral wound as a telltale sign of the second fire. In this way, a tree can record the passage of several fires in the same spot on its trunk.
If you cut a tree in cross section, you'll notice a series of concentric rings. Each ring represents one season's growth. The light-colored wood in each ring was produced during the first part of each growing season, while the dark-colored wood was produced as each growing season came to an end. By counting back from a dated ring (e.g., the ring formed in the year that the tree was cut), you can age a tree.
If you cross-section a tree at its fire scar, you often can associate the original wound with one particular annual ring. If the tree has witnessed repeated fires, you're likely to find multiple wounds, each associated with its own ring. By linking the rings to calendar years, you can date these fires (Dieterich and Swetnam 1984). What's more, by noting whether the scars tend to fall in the light-colored earlywood versus the dark-colored latewood, you can tell whether the tree usually saw fire at the beginning or end of its growing season (Dieterich and Swetnam 1984).
Information gleaned from the fire scars on a single tree provides a glimpse into the history at a single point (Maruoka and Agee 1994). Information from many, many trees must be compiled to reveal patterns of fire occurrence over a large area of interest (e.g., Arno 1976, Davis et al. 1980). Fortunately, you don't have to cut down trees for tree-ring analysis. A special auger, known as an increment borer, can extract slender cores from live trees (Barrett and Arno 1988). Otherwise, a partial cross-section from the fire-scarred area will suffice (Arno and Sneck 1977). Full cross-sections usually come from dead trees, logs, or the stumps of logged trees.
The science of tree-ring analysis, or dendrochronology, can shed light on centuries-old patterns of largely nonlethal fires and can be used to date relatively recent stand-replacing events. Other methods must be used to characterize fire patterns predating the oldest fire-scarred trees and forest stands within a given area. For most of the region's forests, reliable fire-scar data span only the past four centuries or so (Arno and Sneck 1977).
For more on dendrochronology, check out Henri Grissino-Meyer's Ultimate Tree-Ring Web Pages, the website of The University of Arizona's Laboratory of Tree-Ring Research, the PBS timber-dating webpage, or the Introduction to Dendrochronology developed by Brian McCarthy and Darrin Rubino.
Arno, S. F. 1976. The historical role of fire in the Bitterroot National Forest. USDA Forest Service, Intermountain Forest and Range Experiment Station, Research Paper INT-187.
Arno, S. F., and K. M. Sneck. 1977. A method for determining fire history in coniferous forests of the mountain west. USDA Forest Service Intermountain Forest and Range Experiment Station, General Technical Report, INT-42.
Barrett, S. and S.F. Arno. 1988. Increment-borer methods for determining fire history in coniferous forests. USDA Forest Service, Intermountain Research Station, General Technical Report, INT-244.
Brown, J.K. 1995. Fire regimes and their relevance to ecosystem management. Pp. 171-178 in, Proceedings of the Society of American Forester's 1994 National Convention.
Davis, K. M., B. D. Clayton, and W. C. Fischer. 1980. Fire ecology of Lolo National Forest habitat types. USDA Forest Service, Intermountain Forest and Range Experiment Station, General Technical Report, INT-79.
Dieterich, J. H., and T. W. Swetnam. 1984. Dendrochronology of a fire scarred ponderosa pine. Forest Science 30:238-247.
Maruoka, K. R., and J. K. Agee. 1994. Fire histories: overview of methods and applications. Blue Mountains Natural Resources Institute, Technical Note, BMNRI-TN-2. Available online at http://www.fs.fed.us/pnw/bmnri/pubs/tn2.html#intro.
ADDITIONAL AVAILABLE LITERATURE:
Agee, J. K. 1990. The historic role of fire in Pacific Northwest forests. Pp. 25-38 in J. D. Walstad, S. R. Radosevich, and D. V. Sandberg, Editors, Natural and prescribed fire in Pacific Northwest forests. Oregon State University Press, Corvallis.
Arno, S. F. 1980. Forest fire history in the Northern Rockies. Journal of Forestry 78:460-465.
Arno, S. F., and S. Allison-Bunnell. 2002. Flames in our forests: Disaster or renewal? Island Press, Washington DC.
Baker, W. L. and D. Ehle. 2001. Uncertainty in surface-fire history: the case of ponderosa pine forests in the western United States. Canadian Journal of Forest Research. 31:1205-1226.
Barnes, B. V, D. R. Zak, S. R. Denton, and S. H. Spurr. 1998. Forest Ecology, 4th ed. John Wiley and Sons, New York, New York.
Gutsell, S. L, and E. A. Johnson. 1996. How fire scars are formed: coupling a disturbance process to its ecological effect. Canadian Journal of Forest Research 26:166-174.
Pyne, S. J. 1982. Fire in America: a cultural history of wildland and rural fire. Princeton University Press, Princeton, New Jersey, USA.
Shigo, A. L. 1984. Compartmentalization: a conceptual framework for understanding how trees grow and defend themselves. Annual Review of Phytopathology 22:189-214.
Shigo, A. L., and H. G. Marx. 1977. Compartmentalization of decay in trees. USDA Forest Service, Northeastern Forest Experiment Station, Information Bulletin, INF-NE-405. Available online at http://www.srs.fs.usda.gov/pubs/viewpub.jsp?index=5292.
Weisberg, P. J. and F. J. Swanson. 2001. Fire dating from tree rings in western Cascades Douglas-fir forests: An error analysis. Northwest Science 75:145-156.
Whelan, R. J. 1995. The ecology of fire. Cambridge University Press, New York, New York, USA.