Abstract
AbstractData from recent assessments indicate that the annual area of wildfires burning at high severity (where most trees are killed) has increased since 1984 across much of the southwestern United States. Increasing areas of high-severity fire can occur when greater area is burned at constant proportion of high-severity fire, or when the proportion of high-severity fire within fire perimeters increases, or some combination of both. For the Sierra Nevada Forest Plan Amendment (SNFPA) area, which includes forestlands in eastern California and western Nevada, Miller et al. (2009a) concluded that the proportion of area burning at high severity in mixed-conifer forests had risen over the 1984 to 2004 period. However, no statistical assessment was made of the temporal trend in high-severity fire area because the analyzed dataset was incomplete in the early years of the study period. In this update, we use satellite-derived estimates of fire severity from the three most widely distributed SNFPA forest types to examine the trend in percent high severity and high-severity fire area for all wildfires ≥80 ha that occurred during the 1984 to 2010 period. Time-series regression modeling indicates that the percentage of total high severity per year for a combination of yellow pine (ponderosa pine [Pinus ponderosa Lawson & C. Lawson] or Jeffrey pine [P. jeffreyi Balf.]) and mixed-conifer forests increased significantly over the 27-year period. The annual area of high-severity fire also increased significantly in yellow pine-mixed-conifer forests. The percentage of high severity in fires ≥400 ha burning in yellow pine-mixed-conifer forests was significantly higher than in fires ≥400 ha. Additionally, the number of fires ≥400 ha significantly increased over the 1950 to 2010 period. There were no significant trends in red fir (Abies magnifica A. Murray bis) forests. These results confirm and expand our earlier published results for a shorter 21-year period.
Highlights
Recent research has concluded that, over the last four decades, wildfires have become larger, and large fires have become more frequent across the western United States (Calkin et al 2005, Westerling et al 2006, Miller et al 2009a, Miller et al 2012a)
A dataset analyzed by Miller and colleagues (Miller and Safford 2008, Miller et al 2009a) from the Sierra Nevada Forest Plan Amendment (SNFPA) area of eastern California and western Nevada during the same time period showed that the proportion of fire area burning at high severity was rising over time
Because of the large number of wildfires that occur every year, the Monitoring Trends in Burn Severity (MTBS) program restricts its severity assessments to large fires, defined as ≥200 ha in the eastern US, and ≥400 ha in the western US. It excludes more than 95 % of all wildfires, the 400 ha restriction in MTBS western US fire severity assessments captures about 95 % of the total area burned across the western US area during 1984 to 2010
Summary
Recent research has concluded that, over the last four decades, wildfires have become larger, and large fires have become more frequent across the western United States (Calkin et al 2005, Westerling et al 2006, Miller et al 2009a, Miller et al 2012a). The area of high-severity fire increased during the study period, but the data were not statistically analyzed due to an incomplete dataset from the early years of the study These contemporary assessments of fire severity were made using remotely sensed Landsat Thematic Mapper (TM) and Enhanced Thematic Mapper (ETM) images, which first became available in 1984. Because of the large number of wildfires that occur every year, the MTBS program restricts its severity assessments to large fires, defined as ≥200 ha in the eastern US, and ≥400 ha in the western US It excludes more than 95 % of all wildfires, the 400 ha restriction in MTBS western US fire severity assessments captures about 95 % of the total area burned across the western US area during 1984 to 2010 (http://www.mtbs.gov/ faqs.html)
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