Here is another person confusing the reality of trees adapting to
environmental facts (fire) with the erroneous idea that trees want to do
something and so make it happen. To those looking for "why" instead of "how"
the fact that trees in fire-prone regions have survived, is once again
misconstrued to mean that the plants wanted to survive: grew thicker bark
and then tossed some dry branches in to assure that other competitors would
be destroyed while the trees with the thick bark would survive. Also
reiterating the old resin and pitch line, with half a grasp that these
compounds are useful to the trees, but missing the critical one: in places
with long dry seasons, wax is very convenient to hold moisture in leaves,
but inconveniently wax also burns very hot when moisture level in leaf drops
below a threshold. The fact that pine or eucalyptus forests that are moist
don't burn didn't even get into the article. oh well. Kay

From Olivia Judson Blog, NY Times: July 7, 2009, 10:00 pm

On Fire

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.

David McNew/Getty Images Wildfire in the San Jacinto Mountains, California,
in October 2006.

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 A eucalyptus puts out new shoots.

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?

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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.