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Fire and the Regeneration of Aspen Trees

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Figure 1. Regeneration after the Red Eagle Fire in Glacier National Park. Photo: John May.

After returning from a trip to several national parks in 2016, I wrote a series of posts on wildfire, and the role wildfire has in keeping forests healthy. (See here.) In those posts, I reported that wildfire was essential for regenerating species of conifer that have serotenous cones. The cones of these species are coated with a waxy resin that prevents them from opening and releasing their seeds. Fire must melt the resin, and only then are the seeds released – millions of them. Thus, after a fire, the forest regenerates with thousands-upon-thousands of saplings, all the same age. Figure 1 shows the forest regenerating after the Red Eagle Fire near Glacier National Park. These are lodgepole pine, the dominant species in the forests of that area.

I also wrote that aspen trees require fire to regenerate. After a few decades, stands of aspen are invaded by conifers. Aspens are not shade tolerant, and they are not long-lived. Because the conifers create too much shade, the aspens cannot regenerate, and the stand dies out. Fire clears away the shade, and the aspen rhizomes, which remain beneath the ground, send up new shoots, and the aspen stand can be regenerated.

Figure 1. Effects of the Warm Fire (2006) in Kaibab National Forest. Photo by John May.

I just returned from the North Rim of the Grand Canyon. In 2006, the Warm Fire (what a name for a wildfire!) burned across Arizona Hwy. 67, the route to the North Rim. Figures 2, 3, and 4 show the scene. The Red Eagle Fire and the Warm Fire both occurred in 2006, but what has happened since is very different. The scene of the Red Eagle Fire is covered in thousands of small lodgepole pines, all the same age. The scene of the Warm Fire has nary a conifer to be seen. These are all aspens. They haven’t leafed-out yet, so they are a little difficult to see. Aspens turn brilliant colors in the fall – imagine what this area will look like when these trees are mature.

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Figure 2. Effects of the Warm Fire (2006) in Kaibab National Forest. Photo by John May.

To my eye, the area burned by the Warm Fire looks blasted in a way that the area burned by the Red Eagle Fire does not. The reasons might include higher altitude, a more arid climate, and a hotter fire that sterilized the ground. But in addition, this is usually a mixed conifer forest. These species are less tolerant of full sunlight than are the aspens. Thus, the aspens recolonize the burned areas more quickly.

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Figure 3. Effects of the Warm Fire (2006) in Kaibab National Forest. Photo by John May.

Eventually, an interesting thing will occur: the aspens will provide the light shade that the conifers need, and they will be able to start growing. In time, they will begin to shade out the aspens, which will die out, and there will be no more aspens until once again the area burns in a fire. Nature has her ways.

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Figure 5. Warm Fire Progression Map. Source: United States Forest Service, 2006.

The Warm Fire was started in the Kaibab National Forest by lightning on 6/6/2006. At first, it was judged to be a small fire of low intensity that could be allowed to burn and would help renew the forest. In its first 10 days, it burned 1,049 acres.

After 2-1/2 weeks, however, suddenly it blew up into a very hot, rapidly-spreading fire. Between 6/23 and 7/4 it burned about 43,000 acres. Figure 5 shows the fire map through 6/27, but the fire wasn’t contained until 7/4.

Sources:

United States Forest Service. Warm Fire Recovery Project. Viewed online 5/27/2019 at https://www.fs.usda.gov/detail/kaibab/home/?cid=fsm91_050264.

United States Forest Service. Warm Fire Progression Map. Downloaded 5/27/2019 from https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/fsm91_050152.pdf.


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