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Lightning vs. Human-caused Forest Fires

"The agent by which fire was first brought down to earth and made available to mortal man was lightning. To this source every hearth owes its flames."

— Lucretius, De Rerum Natura, 50 B.C.

Click for larger image
Annual number of lightning fires reported in forested areas. Source: Pyne, Stephen J. 2001. Fire: A Brief History. University of Washington Press. p. 5. Click for larger image.

Nationwide, humans cause almost 80% of wildfires, but in the American Southwest, 60% to 70% of forest fires are ignited by lightning. The region leads the nation in the average number of forest fires due to lightning strikes and the number of acres burned by these fires each year (Pyne 2001, p. 6).

Even so, by some estimates, 10% to 25% of all wildfires might be deliberately set. And the number of human-caused fires in the West, intentional or not, is rising because more people are moving into the wildland-urban interface, where private homes border on forests and other public lands.

The fire season usually begins in May with isolated lightning strikes. The maximum number of acres burned most often occurs in the dry and windy month of June, while the maximum number of fires occurs in July with the onset of the summer monsoon and its frequent thunderstorms.

A mature ponderosa pine tree is supremely adapted to survive fire. Its thick bark can withstand fire-excavated cavities in its bole. Lower branches prune away as it matures, exposing its needles not only to sunlight but but away from the flames of surface burns. Ponderosa forests of the American Southwest not only endure frequent fire—they thrive on it.

References

Dettinger, M.D., Cayan, D.R., and Brown, T.J. 2000. Summertime intraseasonal and interannual lightning variations in the western United States. Proceedings, 24th Annual NOAA Climate Diagostics and Prediction Workshop, Tucson, AZ, 1-5 November 1999.

Huffines, G. and R. E. Orville, 1999. Lightning ground flash density and thunderstorm duration in the continental United States: 1989-96. Journal of Applied Meteorology 38(7) 1013-1019.

Knapp, P.A. 1997. Spatial characteristics of regional wildfire frequencies in grass-dominated communities. Professional Geographer 49:39–51.

Pyne, Stephen J. 2001. Fire: A Brief History. University of Washington Press. 204 p.

Pyne, S.J., Andrews, P.L., and Laven, R.D., 1996. Introduction to wildland fire. John Wiley and Sons, 769 pp.

Swetnam, Thomas, W. "Fire and Climate History in the Western Americas From Tree Rings." Laboratory of Tree-Ring Research, The University of Arizona, Tucson, AZ. http://www.ltrr.arizona.edu/~tswetnam/graphics/PAGES/PAGES3.pdf 4/10/03.

Last edited June 6, 2003

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