How aluminum wrap protects sequoias from wildfire
The material, developed from fire shelters used by wildland firefighters, is often wrapped around at-risk buildings in national parks. Now, it’s protecting some of the biggest trees on Earth.
The pictures seemed to be everywhere last week: towering giant sequoias in California, wrapped in silver blankets to protect them from encroaching flames. Even the 3,000-year-old General Sherman tree, taller than the Statue of Liberty, got the gift-wrapping treatment as two huge wildfires burned to the north and south.
The images brought national attention to the blazes and prompted many people to wonder: Can wrapping a tree like that really protect it? Are these extraordinary organisms even really in danger? Didn’t they evolve to withstand—and even need—fire?
Yes, sequoias do need fire. In fact, one of the reasons they’re currently in danger is because they lacked fire for so long. According to the National Park Service, tree-ring records show evidence of frequent fires for 2,000 years—until European settlers began overgrazing the groves and violently displacing the Native American communities that had previously set maintenance blazes. Without fire, sequoias’ small, pebble-like cones can’t open and spread their seeds, leading to what the park service calls a “massive failure” in sequoia reproduction.
That long buildup without fire has created much more fuel ready to burn, meaning fires there are likely to be hotter and more intense, CNN reports. These kinds of conflagrations are more likely to severely affect sequoias than smaller fires; that’s one reason last year’s Castle Fire killed some 10,000 sequoias in California’s Sierra Nevada, the tree’s main habitat.
Eager to avoid a repeat of last year’s loss, the park service and the U.S. Forest Service opted to use the aluminum wrap seen ‘round the world. Wildland firefighters have long carried aluminum-based personal fire shelters for emergency protection. National parks began adapting the technology into “cabin wrap” to protect buildings in the 1980s, during a fire at Yellowstone National Park, when crews stapled old fire shelters to the outside of at-risk structures. The technique quickly became standard practice. "We basically told the fire crews to treat all our special sequoias like they were buildings and wrap them all up,” Christy M. Brigham, chief of resources management and science at the adjoining Sequoia and Kings Canyon National Parks, told CNN.
Though fire shelters used by wildland firefighters and in cabin wrap are formulated with slightly different materials, they have a few ingredients in common. Fire shelters are made of fire-resistant material like silica or fiberglass joined with a layer of aluminum foil, often with a slight air gap between the layers, according to the National Wildfire Coordinating Group. (Cabin wraps may combine those elements with materials that provide structure and strength.) The aluminum reflects radiant heat; the fiberglass and silica are non-combustible, slowing heat transfer to the inside and limiting oxygen; and the air gap provides further insulation. The result is remarkably effective.
A 2019 study published in “Frontiers in Mechanical Engineering” testing the effectiveness of different formulations of fire blankets found that this combination blocked 92% of heat transfer and reflected 96% of thermal radiation. According to Firezat, a company that makes commercially available cabin wrap, wood reaches combustion temperatures of 575 to 600 degrees within 34 seconds of fire exposure. But lab research by Firezat indicates that wood structures wrapped in its material rise to only 248 degrees after 20 minutes of exposure.
The key cabin wrapping technique, parks service workers told The Missoulian in 2013, is wrapping tightly and leaving no holes that might allow embers to get inside. In the case of the sequoias, that means focusing in particular on covering fire scars from past fires, where the trees are still tender and vulnerable, Brigham added in an interview with ABC station KGO-TV. So far, the effort seems to be working: Initial reports indicate both the General Sherman and the nearby Four Guardsmen trees in Sequoia National Park are safe. But at least one named tree, the Bench tree, has burned, and trees in other groves throughout the park have been reported as “surrounded by fire”—far from out of the proverbial woods.