International Journal of Wildland Fire International Journal of Wildland Fire Society
Journal of the International Association of Wildland Fire

Climate, lightning ignitions, and fire severity in Yosemite National Park, California, USA

James A. Lutz A E , Jan W. van Wagtendonk B , Andrea E. Thode C , Jay D. Miller D and Jerry F. Franklin A

A College of the Environment, University of Washington, Box 352100, Seattle, WA 98195, USA.

B US Geological Survey, Western Ecological Research Center, Yosemite Field Station, El Portal, CA 95318, USA.

C School of Forestry, Northern Arizona University, Box 15018, Flagstaff, AZ 86011, USA.

D US Forest Service, 3237 Peacekeeper Way, Suite 101, McClellan, CA 95652, USA.

E Corresponding author. Email:

International Journal of Wildland Fire 18(7) 765-774
Submitted: 8 July 2008  Accepted: 23 December 2008   Published: 27 October 2009


Continental-scale studies of western North America have attributed recent increases in annual area burned and fire size to a warming climate, but these studies have focussed on large fires and have left the issues of fire severity and ignition frequency unaddressed. Lightning ignitions, any of which could burn a large area given appropriate conditions for fire spread, could be the first indication of more frequent fire. We examined the relationship between snowpack and the ignition and size of fires that occurred in Yosemite National Park, California (area 3027 km2), between 1984 and 2005. During this period, 1870 fires burned 77 718 ha. Decreased spring snowpack exponentially increased the number of lightning-ignited fires. Snowpack mediated lightning-ignited fires by decreasing the proportion of lightning strikes that caused lightning-ignited fires and through fewer lightning strikes in years with deep snowpack. We also quantified fire severity for the 103 fires >40 ha with satellite fire-severity indices using 23 years of Landsat Thematic Mapper data. The proportion of the landscape that burned at higher severities and the complexity of higher-severity burn patches increased with the log10 of annual area burned. Using one snowpack forecast, we project that the number of lightning-ignited fires will increase 19.1% by 2020 to 2049 and the annual area burned at high severity will increase 21.9%. Climate-induced decreases in snowpack and the concomitant increase in fire severity suggest that existing assumptions may be understated – fires may become more frequent and more severe.

Additional keywords: burn severity, climate change, climate variability, fire regime attributes, landscape flammability, normalized burn ratio, patch complexity, RdNBR, Sierra Nevada, snowpack, snow water equivalent.


We thank Yosemite National Park, the US Geological Survey Western Ecological Research Center, and the California Department of Water Resources for data and data management. We thank J. K. Agee, A. J. Larson, D. McKenzie, and A. L. Westerling for discussions. Comments from J. A. Freund, A. R. Gillespie, C. B. Halpern, T. M. Hinckley, D. G. Sprugel, and two anonymous reviewers improved previous versions of this manuscript. This research was supported by National Science Foundation IGERT (0333408) and the Seattle ARCS Foundation.


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