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Wildfires Growing in Size, Not Number

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Since 1984, the size of wildfires seems to have grown, though the number of fires does not.


Source: Flathead National Forest.

Figure 1: Source: Flathead National Forest.

In previous posts I have looked at wildfire in the national parks and the role it has in promoting a healthy forest ecosystem. How is wildfire changing over time? Is it increasing? I can address that question two ways: with maps and with statistics. Maps first.

Figure 1 is the top half of a map that shows the history of wildfires in Glacier National Park and the surrounding area from 1984-2015. On the map, the park is shown in pale yellow. Flathead National Forest is shown in pale green. The line at the top represents the border with Canada. The fires are mapped, and the year they occurred is color coded. Bright red represents fires that occurred in 2015, orange represents fires that occurred in 2003, and dark green those that occurred in 2001. In the previous post, I gave you photos of forest recovery in 3 of these fire areas: the Reynolds Creek Fire, the Red Eagle Fire, and the Moose Fire.

Looking at the map, you can see that over the last 22 years, a significant fraction of the area has burned in one fire or another. Orange is the color most represented on the map, and 2003 was, indeed, a very active fire year in this region.

(Click on graphic for larger view.)

Source: Yellowstone National Park.

Figure 2: Source: Yellowstone National Park.

Figure 2 shows the fire history of Yellowstone National Park from 1988-2013. The park is outlined with a black line. As in the previous figure, fires are colored according to the year in which they occurred. If you recall, 1988 was the year of the terrible fires in Yellowstone. Those fires are shown in pale yellow. Approximately 1/3 of the park burned that year. (The map shows the final fire boundaries – not every acre within the boundary burned. Wildfire is very fickle regarding what it burns and doesn’t burn.) Since 1988, fires have been much smaller, and have consumed much less acreage.

What I want you to take away from these maps is that fire is anything but rare in these national parks. It is a yearly occurrence. Over time, significant portions of the park burn – in Yellowstone most of the park has burned in the last 28 years. This is a natural pattern, and evidence from burn scars and layers of ash in the soil suggest it has been this way for thousands of years.

Fire experts estimate that healthy lodgepole pine forests burn about every 90 years on average. Now, that’s an average, so it means that some areas burn more frequently, and some less. About 2/3 of western fires are started by lightening, and nobody knows where lightning will strike.

Total number of reported wildfires in USA. Source: National Interagency Fire Center.

Figure 3: Data source: National Interagency Fire Center.

Tens of thousands of wildfires are reported every year. Figure 3 shows the number of wildfires reported in the USA from 1960-2015. These statistics are compiled by the National Interagency Fire Center from situation reports on individual fires that come in from many sources. Situation reports have been in use for several decades. Prior to 1983 the source of the data is not known, and the data for 1983 and 1984 seem to have been affected by the phasing in of situation reports. Thus, data up to 1984 should not be compared to data after 1984.

Because of variability in the data, I have dropped a 5-year moving average on it for the period after 1984. The trend in the data is not strong, however it may be toward a slightly decreasing number of fires. The years from 2010-2015 all saw a below average number of fires, although in some cases barely.

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Source: National Interagency Fire Center.

Figure 4: Data source: National Interagency Fire Center.

Figure 4 shows the total number of acres burned by wildfire each year. As in Figure 3, data before and after 1984 should not be compared. That said, this data is less ambiguous: there has been an increase in the number of acres burned in both active and inactive fire years.

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Data source: National Interagency Fire Center.

Figure 5: Data source: National Interagency Fire Center.

Figure 5 shows the annual number of acres burned per fire – it is just a graph of the number of acres divided by the number of fires. Again, don’t compare data from before and after 1984 . The first thing I notice is how small the average number of acres is – somewhere between 15 and 150. Given the results of the two previous charts, it is inevitable that the number of acres burned per fire has increased in recent years, and the chart confirms the change. The size of the change is astounding, however. The number of acres per fire in 2012 and 2015 is about double the highest number in the 10 years after 1984.

The data seem clear: the number of fires each year has not increased, but the number of acres burned per fire has, leading to an increase in total acres burned. This goes along with the statement from the U.S. Forest Service that I quoted in the lead post of this series: the size and ferocity of fires has increased in recent years.

I will try to bring the data home to Missouri in the next post.

Sources:

Flathead National Forest. 2015. Current Fires and Fire History 1984-2015: Flathead National Forest & Clacier National Park. August 21, 2015. Downloaded 10/3/16 from http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprd3851454.pdf.

National Interagency Fire Center. 2016. Total Wildland Fires and Acres. Data downloaded 10/3/2016 from https://www.nifc.gov/fireInfo/fireInfo_stats_totalFires.html.

Yellowstone National Park. 2014. Yellowstone Fires: 1988-2013. Downloaded 10/3/16 from https://www.nps.gov/yell/learn/nature/upload/Fires_88_2013.pdf.

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