NOVA Online | Fire Wars | How Plants Use Fire

	Fire from the heavens, lightning has kindled wildfire for millions of years, causing plants over the eons to adapt or die out.

How Plants Use Fire (And Are Used By It)
by Stephen J. Pyne

The Earth has known fire for over 400 million years. The reason is simple: Life made it possible. Marine life pumped the atmosphere full of oxygen; terrestrial life lathered the crust with fuels. When oxygen and fuel meet a spark under the right circumstances, a fire kindles. (Lightning is an ancient and ample ignitor.) The fundamental chemistry of combustion lies at the core of the living world. When it happens within a cell, it's called respiration. When it happens outside organisms, it's called fire. It's that basic.

Rhythm of the heat

So fire happens. Because it does, living systems adjust to its presence, just as they would to sunlight or frost or flooding. This process of accommodation, however, is complex. Organisms do not adapt to fire in the abstract but to particular patterns of fire, what we call fire regimes. A regime is a statistical concept that assimilates many rhythms of burning and often many kinds of fires. An individual fire is to a fire regime as an individual storm is to climate.

As if possessed of personality, fires in conifer stands might burn along the ground or shoot into the canopy, seemingly dependent on whim.

Unsurprisingly, there exists a climatic basis for fire's regimes, and this relies on rhythms of wetting and drying. A place must be wet enough to grow combustibles and dry enough to ready them for burning. Places that are chronically wet or relentlessly dry do not experience fires or have them only rarely, after, say, an exceptional drought drains a jungle or a rainy spell floods deserts, thereby encouraging combustibles to grow. Some regions undergo annual cycles that manifest regular fire seasons. Others experience fires on multi-year or decadal rhythms, such as those associated with the oscillations of El Niño. For some places fires occur on the order of a century or even longer.

These rhythms mean that fires thrive in a kind of habitat. Fires in grasslands burn one way; fires in rainforest another; fires in temperate conifers in several ways, sometimes skipping along the surface, sometimes soaring through dense crowns. Indeed, varieties of each kind of fire exist. Even grassfires may burn with the wind or against it; they may creep and smolder or rage at the pace of a galloping horse. But rough patterns do emerge, and biota adapt to these patterns, much as they would to patterns of rainfall.

Opportunist extraordinaire, a polar bear backlit by fire patrols the edge of a blaze.

Fauna and Fire
Animals, too, adapt to fire. Some seek to avoid it. If they burrow, they go underground until the flames pass. If they can fly, they soar away. If they are otherwise mobile, they skirt the fire. The common perception of terrified animals as depicted in Bambi is a gross exaggeration. Only very rarely do they become trapped.

In fact, as with plants, some take advantage of the flames. They actively hunt along their perimeter searching for snakes, insects, or larger creatures moving ahead of the flaming front. Some beetles possess infrared-seeking organs that help direct them to smoldering stumps and logs, where they will feed and nest.

In places for which fire is a major force, animals accommodate themselves to the kind of landscape that fire creates, and they may suffer if that habitat changes from fire's exclusion. Here, too, they can become active agents themselves in shaping those landscapes by their grazing, browsing, and hunting habits, each of which determines what kind of fuel is available for burning.

Coping with fire

Most fire adaptations among plants involve suites of traits that adapt to a variety of stresses—to drought and to browsing and grazing as well as to burning. Yet many traits seem more specific to fire. Some plants survive, for example, by protecting their vital organs from heat. Larch bear dense bark capable of withstanding surface fires. Ponderosa pine shed lower branches as they mature, shielding sensitive needles from scorching fires below. African aloes and Brazilian proteas surround buds with moist tissues capable of absorbing heat.

Some plants use the heat to their advantage. Scrub oak and chamise, an evergreen shrub, resprout from roots or branches after fire has incinerated their outer limbs. If fire destroys their branches, many eucalypts will throw out new branches by sprouting from their trunks. After flames destroy their trunks, mallee, low-growing, shrubby eucalypts, send out vigorous shoots from special nutrient-storage organs found in their roots. Australian grass trees push out new leaves and even bloom—and may only bloom—after a scouring fire. (Animals, too, may adjust to and even take advantage of fire—see Fauna and Fire.)

Other plants opportunistically seize a site temporarily purged by fire to promote their own reproduction. Cape lilies lie dormant until flames brush away the covering, then blossom almost overnight. Lodgepole pine and jack pine rely on the pulse of flame through their crowns to melt away the waxy bond that holds their cones closed; their seeds then fall to fresh ash below, where they can take root without much competition. Sequoia seedlings flourish best on deep-burned sites free of grasses and other rivals.

To protect the bulk of its precious cargo of needles from ground fire, ponderosa pine trees slough off low-hanging branches as they mature.

Still other plants depend on fire in different ways. Longleaf pine, in its young "grassy" phase, withstands a fire then grows quickly, now housed in tough bark, immunized against brown spot fungus, and ready to take on the next flames. Mountain ash, while burned only every few centuries, depends on those devouring conflagrations to prepare the seedbed for its next generation. Fireweed seed lies in litter and soil until liberated by a deep burn. Such plants are true pyrophytes—fire-lovers whose survival depends on the predictable return of flame.

A provocative question concerns whether such species may actually encourage fire, whether they have acquired traits that make them more likely to burn. As it ages, for example, chamise increases the proportion of dead to living limbs, the moisture content of the living leaves drops, the ratio of combustible chemicals to water rises, and deadwood piles up on the ground. After a fire passes through, chamise resprouts quickly and enjoys a nutrient-fluffed advantage over its rivals. Is chamise a fire-promoting, not merely a fire-favored, plant? It's hard to say.

What we can say is that reciprocity exists between fire regimes and the biota that sustain them. After all, wildfire does not burn blocks of carbon bullion; it combusts the biomass of living systems shaped by evolutionary and ecological stresses, of which fire is one. In this sense, organisms do exercise some "control" over fire behavior, because fire will follow its fuels. Only that part of the landscape that is available to burn can burn. Plants thus do shape the kinds of fires they may experience.

	Of all the millions of species of life on Earth, only one has learned to control fire: Homo sapiens.

Taming flame

The other indispensable insight into fire ecology is the role of humans. We are, uniquely, a fire creature: We hold a species monopoly over the manipulation of fire. We can start and, within limits, stop fires. We can shape fuels and do so to make them more or less combustible. We can even reach into the geologic past to gather fossil biomass and burn that.

Our capacity as fire creatures defines our ecological imprint as no other trait can. Other animals knock over trees, dig holes, eat plants, and hunt, but only human beings apply and withhold fire. The ecology of fire on Earth reflects our own existence; that is, its dynamics embody our knowledge (and ignorance) as well as our will and our capacity to choose. The living world claimed ignition from lightning's lottery but lost sovereignty to something called culture.

We cannot ignore fire, because no neutral position is possible. Fire can be as ecologically effective withheld as applied. Places that have known one regime will suffer if that regime changes, just as they would by a change in rainfall regimes. Nor can we ignore ourselves as fire agents. Our ancient ancestors made a Faustian pact. We gained fire, which brought power; in return, we agreed to manage fire. We can't put the torch aside just because we find its ecology complicated or its smoke annoying or its legacy vexing. In the end, fire is more than an ecological process or an environmental problem. It is a relationship. We have made each other what we are in the world today.

Stephen J. Pyne is professor in the Biology and Society Program at Arizona State University. He is the author of the "Cycle of Fire," a series of books on how humans and fire have interacted to shape the Earth. Among the titles in this series are Fire in America: A Cultural History of Wildland and Rural Fire and, most recently, Fire: A Brief History, both published by the University of Washington Press.

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