Have some plants evolved to promote fire?

Olivia Judson, an evolutionary biologist, recently traveled to Australia where she learned a little about fires in eucalyptus forests. In an article she wrote for the New York Times, she wonders if some plants have specific characteristics that make it more likely that fires will burn intensely. Here is an excerpt from her article.

….It’s common knowledge that plants regularly exposed to fire tend to have features that help them cope with it — such as thick bark, or seeds that only grow after being exposed to intense heat or smoke. But what is less often remarked on is that the plants themselves affect the nature and severity of fire.

For example, dead branches burn more readily than living branches, so a tree that keeps dead branches (rather than letting them fall) makes it easier for a fire to climb into a forest canopy: the dead branches provide a ladder for the fire. Deadwood also allows fires to get hotter. Leaves that are high in cellulose, or that contain oils, also stoke the flames. Resins and gums are highly flammable. And as any girl scout knows, twigs catch light more readily than branches, so a twiggy sort of plant can catch fire more readily than its non-twiggy sister.

But here’s the odd thing. Many plants that live in places prone to fire are highly flammable — more flammable than plants that live elsewhere. This has led some to speculate that these plants have actually evolved to cause fires: that they “want” fire, and have evolved features that make it more likely that a spark will become a flame, and a flame will become a fire. I call this the torch-me hypothesis.

The argument goes like this. Many plants depend on fire for their propagation. Indeed, without fire, these plants disappear. If, for example, longleaf pine forests do not burn regularly, the pines will be replaced by water oaks and other species. So — runs the argument — fires are desirable because they kill the competition. Plants that enhance fires may thus have an evolutionary advantage: they murder the competition while creating the right circumstances for their own seeds to sprout.

This idea has sparked a heated debate. The problem is, showing that a trait has evolved because it enhances fire is difficult. Yes, oily leaves are more flammable; but perhaps the real advantage of oily leaves is that insects don’t enjoy eating them. Then, their flammability may be a by-product of tasting terrible.

The best evidence that some plants may have evolved to promote fire comes from pines. Some species of pine keep their dead branches; others tend to self-prune. As you would expect under the torch-me hypothesis, the more flammable species — the ones with the dead wood — also tend to have seeds that are released by fire. In short, the two traits go together….

The author seems to think she is the first person to consider this concept.

Lodgepole pine, photo: N. C. Heywood

One example of this is the lodgepole pine. It has shaggy bark and does not self-prune its limbs readily, so a fire at the base of the tree can, under dry conditions, run up the trunk of the tree and become a crown fire. It has a fire return interval of about 300 years, and fires tend to be of the stand replacement type, leaving nothing but snags. The serotinous cones open and disperse the seeds after the fire, promoting the resurgence of another lodgepole forest.

Lodgepole pine infested with dwarf mistletoe. Photo: D. Johnson

UPDATE: Chuck Bushey wrote to us about this.

Bob Mutch actually wrote his MS thesis on this topic at the University of Montana in the late 70’s. It was later published in the Journal of Ecology and I think he was the first to formally express the concept in the scientific literature.

Chuck later said the actual citation is:

Mutch, Robert W., 1970. Wildland Fires and Ecosystems – A Hypothesis.

Ecology, Volume 51(6):1046-1051.

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Author: Bill Gabbert

After working full time in wildland fire for 33 years, he continues to learn, and strives to be a Student of Fire.