Plants that burn fastest in a wildfire

Lichen burns cool and quick, while spruce burns slow and hot.

 

This story is part of a package of coverage on wildfire in our recent print edition. See the feature story, 'After one record-setting wildfire, a Washington county prepares for more.'

It’s rare for the rainforest to catch fire; the Pacific Northwest’s old-growth forests are some of the wettest places in North America. But this year, the Olympic Peninsula saw its driest spring in over a century, and in late May, a bolt of lightning ignited the desiccated lichen and timber of the Queets River Drainage in Washington’s Olympic National Park.

Since then, the Paradise Fire has burned more than 1,600 acres, and fire officials expect it to continue until the end of summer. Much of the blaze has been spread by lichen, a composite symbiotic organism generally made up of fungus and algae. Lichen mantles the forest’s centuries-old Sitka spruce, western hemlock and Douglas-fir, forming ladders that flames easily crawl up. When those ladders collapse, fallen logs provide more fuel and intensify the fire. Lichen and mosses are unusual culprits. Typically, fires are driven by one of four vegetation groups — grass, shrub, slash or timber. But climate change and an altered fire cycle mean different fuel types are now carrying fires, and that makes it all the more urgent for scientists to understand each species.

Matt Jolly, at the Missoula Fire Sciences Laboratory in Montana, is one of the only researchers in the world attempting to model fire behavior after individual vegetation types. Instead of looking at a group, like timber, as a whole, Jolly examines the individual needles of a ponderosa pine. How quickly a plant ignites, and gets wet or dries, he explains, is directly related to its surface-area-to-volume ratio. “The fastest way to cook something is to flatten it out,” he says. “By flattening it out, you increase the surface area and therefore the contact with heat.”

That makes lichen, for example, incredibly flammable, far more than any other vegetation group. Moreover, unlike timber, which can take weeks to dry out, lichen can dehydrate in minutes. But just because lichen ignites quickly, doesn’t mean it causes the most intense fires. Very hot fires need a large volume of fuel, or vegetation, to sustain them. That makes timber the most dangerous vegetation group.

Although the Pacific Northwest’s forests are home to both lichen and massive stands of old-growth timber, fires there are typically restrained by other ambient factors, including rainfall. But as the drought persists and climate change alters humidity levels, rainforest fires could become more frequent. “It really depends what happens in the climate over the next 30 years,” Jolly says.

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Sources: Matt Jolly, Missoula Fire Sciences Laboratory; James K. Brown, Ratios of Surface Area to Volume for Common Fuels, 1969.

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