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Originally published in Science Express on 6 July 2006
Science 18 August 2006:
Vol. 313. no. 5789, pp. 940 - 943
DOI: 10.1126/science.1128834
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Research Articles

Warming and Earlier Spring Increase Western U.S. Forest Wildfire Activity

A. L. Westerling1,2*, H. G. Hidalgo1, D. R. Cayan1,3 and T. W. Swetnam4

1 Scripps Institution of Oceanography, La Jolla, CA 92093, USA.
2 University of California, Merced, CA 95344, USA.
3 U.S. Geological Survey, La Jolla, CA 92093, USA.
4 Laboratory of Tree-Ring Research, University of Arizona, Tucson, AZ 85721, USA.


Figure 1 Fig. 1. (A) Annual frequency of large (>400 ha) western U.S. forest wildfires (bars) and mean March through August temperature for the western United States (line) (26, 30). Spearman's rank correlation between the two series is 0.76 (P < 0.001). Wilcoxon test for change in mean large–forest fire frequency after 1987 was significant (W = 42; P < 0.001). (B) First principle component of center timing of stream-flow in snowmelt dominated streams (line). Low (pink shading), middle (no shading), and high (light blue shading) tercile values indicate early, mid-, and late timing of spring snowmelt, respectively. (C) Annual time between first and last large-fire ignition, and last large-fire control. [View Larger Version of this Image (52K GIF file)]
 

Figure 2 Fig. 2. (A) Pearson's rank correlation between annual western U.S. large (>400 ha) forest wildfire frequency and streamflow center timing. x axis, longitude; y axis, latitude. (B) Average frequency of western U.S. forest wildfire by elevation and early, mid-, and late snowmelt years from 1970 to 2002. See Fig. 1B for a definition of early, mid-, and late snowmelt years. [View Larger Version of this Image (28K GIF file)]
 

Figure 3 Fig. 3. Average difference between early and late snowmelt years in average precipitation from October through May (A) and average temperature from March through August (B). Contours enclose regions in which a t test for the difference in mean between 11 early and 11 late years was significant (P < 0.05). The null hypothesis that precipitation from October through May is normally distributed could not be rejected using the Shapiro-Wilk test for normality (P > 0.05 for more than 95% of 24,170 grid cells, n = 49 for precipitation; P > 0.05 for more than 95% of 24,170 grid cells, n = 50 for temperature). See Fig. 1B for a definition of early, mid-, and late snowmelt years. [View Larger Version of this Image (76K GIF file)]
 

Figure 4 Fig. 4. Index of forest vulnerability to changes in the timing of spring: the percentage difference in cumulative moisture deficit from October to August at each grid point in early versus late snowmelt years, scaled by the forest-type vegetation fraction at each grid point, for 1970 to 1999 (26). See fig. S3 for a map of forest vulnerability for 1970 to 2003 over a smaller spatial domain. See Fig. 1B for a definition of early, mid-, and late snowmelt years. [View Larger Version of this Image (45K GIF file)]
 

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