As I mentioned last week, I’ve recently returned from Australia. While I was there, I visited a eucalyptus forest that, in February, was the scene of an appalling wildfire. Perhaps naively, I had expected to find that many trees had been killed. They hadn’t. They had blackened bark, but were otherwise looking rather well, many of them wreathed in new young leaves. This prompted me to consider fire and the role it plays as a force of nature.

Fossil charcoals tell us that wildfires have been part of life on Earth for as long as there have been plants on land. That’s more than 400 million years of fire. Fire was here long before arriviste plants like grasses; it pre-dated the first flowers. And without wanting to get mystical about it, fire is, in many respects, a kind of animal, albeit an ethereal one. Like any animal, it consumes oxygen. Like a sheep or a slug, it eats plants. But unlike a normal animal, it’s a shape-shifter. Sometimes, it merely nibbles a few leaves; sometimes it kills grown trees. Sometimes it is more deadly and destructive than a swarm of locusts.

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The shape-shifting nature of fire makes it hard to study, for it is not a single entity. Some fires are infernally hot; others, relatively cool. Some stay at ground level; others climb trees. Moreover, fire is much more likely to appear in some parts of the world than in others. Satellite images of the Earth show that wildfires are rare in, say, northern Europe, and common in parts of central Africa and Australia. (These days many wildfires are started by humans, either on purpose or by accident. But long before our ancestors began to throw torches or cigarette butts, fires were started by lightning strikes, or by sparks given off when rocks rub together in an avalanche.)

Once a fire gets started, many factors contribute to how it will behave. The weather obviously has a huge effect: winds can fan flames, rains can quench them. The lie of the land matters, too: fire runs uphill more readily than it goes down. But another crucial factor is what type of plants the fire has to eat.

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.

Olivia Judson

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.

Which is suggestive. But without more data from other plants, the evidence remains thin. If a tree were put in the dock and charged with being an accessory to arson, the jury would, for now, have to return the verdict, “Not proven.” Yet as I think of that great eucalyptus forest in Australia, I can’t help wondering. Have the trees actually evolved to make the fires worse?

Notes:

For fossil charcoals and the antiquity of fire, and also for a map of fire on today’s Earth, see Bowman, D. M. J. S. et al. 2009. “Fire in the Earth system.” Science 324: 481-484. For the idea that fire can be likened to an animal, see Bond, W. J. and Keeley, J. E. 2005. “Fire as a global ‘herbivore’: the ecology and evolution of flammable ecosystems.” Trends in Ecology and Evolution 20: 387-394. This paper includes an interesting general discussion of the role of fire in shaping ecosystems and plant diversity.

The idea that plants may have evolved to enhance flammability was first put forward by Mutch, R. W. 1970. “Wildland fires and ecosystems — a hypothesis.” Ecology 51: 1046-1051. His original formulation has been vigorously criticiz ed — an excellent summary of the problems with it can be found on pages 128-130 of Whelan, R. J. 1995. “The Ecology of Fire”. Cambridge University Press. However, the idea has been refined, and made more plausible, by several authors. See, for example, Bond, W. J. and Midgley, J. J. 1995. “Kill thy neighbour — an individualistic argument for the evolution of flammability.” Oikos 73: 79-85 and Schwilk, D. W. and Kerr, B. 2002. “Genetic niche-hiking: an alternative explanation for the evolution of flammability.” Oikos 99: 431-442.

For a general discussion of traits that enhance flammability, as well as a discussion of why it is hard to disentangle selection by fire from selection by herbivores, see page 145 of Bond, W. J. and van Wilgen, B. W. 1995. “Fire and Plants.” Springer. Chapter two of this book also discusses the different kinds of fire, and the conditions that promote them; the text includes a good description of traits normally considered to have been shaped by fires, as well as the disappearance of species associated with the loss of fire. For longleaf pines and the need for fire, see Varner, J. M. et al. 2005. “Restoring fire to long-unburned Pinus palustris ecosystems: Novel fire effects and consequences for long-unburned ecosystems.” Restoration Ecology 13: 536-544.

For a demonstration that dead branches affect flammability, see Schwilk, D. W. 2003. “Flammability is a niche construction trait: canopy architecture affects fire intensity.” American Naturalist 162: 725-733. For the relationship between keeping dead branches and fire-stimulated seed germination in pines, see Schwilk, D. W. and Ackerly, D. D. 2001. “Flammability and serotiny as strategies: correlated evolution in pines.” Oikos 94: 326-336.

Many thanks to Peter Cowan, Jenny Graves, Dan Haydon, Alan Peterson, Dylan Schwilk and Jonathan Swire for help, comments and suggestions.