As I discussed in my previous post, crown fires are fires that burn vertically into the crowns of trees, enabling these fires to spread from treetop to treetop in dense stands. 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 1988 in Yellowstone National Park in areas that hadn’t had significant burns since the 1700s. It’s a good thing that Wyoming is less populous than California, because this would be devastating here!
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, and in the absence of over-growth conditions surrounding these stands, they can light up like the Fourth of July without sending collateral damage deep into stands of different tree species. For this reason, we should not accept crown fires as our new normal. 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 naturally.
The intense heat of a crown fire can cause spontaneous combustion beyond the point of flames itself. They 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 soils that support forest vegetation are similarly decimated 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.
Hydrophobic soils are only one aspect of 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 the beach doesn’t have any plant matter, but if you walk a few hundred feet away from the ocean, bunchgrasses are holding the sands quite nicely? It’s the same concept 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.
Barbour, Michael; Pavlik, Brucel Drysdale, Frank; and Lindstrom, Susan. 1993. California’s Changing Landscapes: Diversity and Conservation of California Vegetation. A publication of the California Native Plant Society.
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. 2016. https://www.nps.gov/yell/learn/nature/fireconsequences.htm
Yellowstone National Park. Fire. 2018. https://www.nps.gov/yell/learn/nature/fire.htm
Yellowstone National Park. Fire Information Continued. 2017. https://www.nps.gov/yell/learn/nature/fireinfo.htm
Yellowstone National Park. Fire History. 2017. https://www.nps.gov/yell/learn/nature/firehistory.htm
Yellowstone National Park. Forests. 2017. https://www.nps.gov/yell/learn/nature/forests.htm
Wikipedia. Carr Fire. 2018. https://en.wikipedia.org/wiki/Carr_Fire#/media/File:2018_Carr_Fire_(42286511740).jpg
Myself. Arcata, California coastline. 2018. Did you actually read the sources? I'm impressed. They are a pain to write, so thank you!