As I discussed in my previous post, crown fires are fires that burn vertically into the crowns (leaved, upper portions) of trees. Crown fires then spread from treetop to treetop, quickly setting dense stands ablaze. Rather than gently burning an area, crown fires cause wide swaths of uninterrupted devastation. For this reason, the term crown fire is often synonymous with stand-replacing wildfire.
Crown fires aren’t always unnatural. In fact, in Yellowstone’s Lodgepole Pine (Pinus contorta) forests, crown fires are actually built into the ecology of the area. While surface fires do play a smaller role in Lodgepole Pine forests, dense, even-aged stands of Lodgepole Pines are known to burn quite spectacularly as crown fires. As with California’s forests, if tree density is high, fire frequency is low, and ladder fuels are present, an unusually dry and hot summer could be the catalyst for stand-replacing crown fires. We saw this in Yellowstone National Park's 1988 fires, where areas that hadn't burned since the 1700s went up like a matchbox. It’s a good thing that Wyoming is less populous than California; as we've seen recently, such fires would be absolutely devastating in a populated area.
While Lodgepole Pines do exist as one of the mixed conifers in certain California forests, they are not as dominant as they are in the Rocky Mountains. Lodgepole Pines naturally grow in dense clusters; still, in the absence of over-growth conditions surrounding these stands, Lodgepoles can burn catastrophically in their region without generating a runaway crown fire. That is, fire behavior should change as a fire approaches a forest patch with lower tree density, fewer ladder fuels, and more fire-resilient trees. A crown fire should reduce intensity and morph to a surface fire as it moves out of Lodgepole territory.
For this reason, we should not accept crown fires as our new normal. Put simply, the effects of these fires are devastating in areas not designed to burn this way.
Temperature, Destruction, and Soils
Crown fires burn much hotter than surface fires. Whereas a ground fire is more like lighting matches one by one in a sequence, a crown fire is like igniting the whole matchbox. The density of fuels on the ground plus access to the tree canopy results in a fire orders of magnitude more intense than what would have existed historically.
The intense heat of a crown fire can lead to combustion beyond the point of flames itself. Crown fires can also create their own micro-weather, creating wind events. If heat rises, so does the air surrounding it. As a tragic example, the Carr Fire famously created a “fire tornado.” I have chosen not to depict it here due to its role in taking the lives of multiple first-responders.
The complex soil ecosystem that supports forest vegetation is similarly annihilated by unnatural crown fires. Whereas ground fires reintroduce nutrients to the soils via ash, crown fires alter the physical and chemical structure of the soils they torch. If you’ve ever heard of lightning striking a beach and creating a glass-like structure from the melted sand, then you understand the concept of hydrophobic soils. In the presence of a crown fire, soil components can melt together and take a glass-like composition, which makes them repel water. Just like glass and water don’t ever mix together, water will not penetrate the soil; it will just run off the surface. With enough rain, this is the perfect recipe for widespread sheet erosion, where entire layers of soil slide away in the rains.
The lack of vegetation post-fire also contributes to this devastating erosion. The intense fires leave behind little in the way of plant matter, often burning plants down into their roots. Roots normally function as a soil stabilizer. You can think of each root as one in a series of chains helping hold the soil together. Notice how a shifting, sandy beach contains little plant matter; however, if you walk a few hundred feet away from the ocean, bunchgrasses and other plants hold the sand quite nicely. The same concept applies in the forest.
With little plant matter to slow the rain as it falls to the soil, no roots to help absorb the rainwater after it falls, and hydrophobic soils where runoff is all but guaranteed, things are about to get ugly. Stay tuned for Part 4, where we’ll discuss the after-fire effects on humans.
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Forestry Institute for Teachers. Personal Correspondence. July 2016.
Lotan, James; Brown, James; and Neuenschwander, Leon. 1985. Role of Fire in Lodgepole Pine Forests. Utah State University. https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1122&context=barkbeetles
University of California, Division of Agriculture and Natural Resources. Forest Research and Outreach. Lodgepole pine (Pinus contorta). 2018.
Yellowstone National Park. Ecological Consequences of Fire. 2020. https://www.nps.gov/yell/learn/nature/ecological-consequences-of-fire.htm
Yellowstone National Park. Fire. 2018. https://www.nps.gov/yell/learn/nature/fire.htm
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Wikipedia. Carr Fire. 2018. https://en.wikipedia.org/wiki/Carr_Fire#/media/File:2018_Carr_Fire_(42286511740).jpg
Jacob Ewald. Arcata, California coastline. 2018.
Yellowstone National Park. Ecological Consequences of Fire. 2016. https://www.nps.gov/yell/learn/nature/fireconsequences.htm