Fire. Natural Fire Regimes. Agricultural Fire Practices

Fire is a creative force of the living world. Life supplies oxygen and fuel for combustion. The chemistry behind combustion is truly elemental. When it occurs within a cell, it is called "respiration." When it occurs outside, it is called "fire."

Natural Fire Regimes. Fossil records of fire, in the form of charcoal, date to the Devonian period, roughly concurrent with the first land-colonizing plants. Natural fire occurs in patches and pulses, driven by a two-cycle climatic engine of wetting and drying, shaped by the kinds of biomass (living matter) that may or may not thrive under such a regime, waiting on lightning's lottery to kindle.

A place has to be wet enough to grow fuels and dry enough to burn them. The patterns of burning result in what are known as "fire regimes." (Such regimes are a statistical concept. A fire is to a regime as a storm is to a climate. The rhythms are many but orderly.)

That fire simply happened meant that the biosphere (the part of the world in which life can exist) had to accommodate that fact. Fire became a factor in evolutionary selection and an ecological presence. It shaked and baked, a form of creative destruction in nature's economy. Plants and animals adjusted to fire's regime, as fire did to them.

Even so, much of the Earth doesn't naturally burn. Much of the historical Earth so failed to combust that vast quantities of biomass were simply buried (to be exhumed and combusted over the last two centuries). Nature's economy, in brief, lacked a broker that could match flame with fuel.

Anthropogenic Fire. That changed with the later hominids (early humans). It seems that Homo erectus could tend fire, although probably not until Homo sapiens could humans start fire more or less at will. Still, it was easier to keep fire alight than to constantly rekindle it. So, likewise, people kept fire continuously in nature.

The sputtering flame became constant, something that accompanied people wherever they went; and they went every-where. What they didn't burn outright, they relied on catalytic fire to help manipulate.

Since the first tread of Homo sapiens, fire ecology has thus meant human ecology. The biosphere had long exercised some control over fuel and oxygen because life produced both. Now, through humans, it could in principle exert some power over ignition as well. But humans did not set fire by instinct. They set fire for their own ends.

They inscribed lines of fire and fields of fire that sculpted new landscape mosaics; they kindled flame according to new rhythms. Biotas (flora and fauna) would have to adapt to this regime. In reality, anthropogenic (human-caused) fire competed with natural fire. Ever after, fire regimes would have to reconcile culture with climate. Flame, and the landscapes it touched, entered the moral universe of humanity.

This diagram distributed by the National Forest Service points out the benefits of natural fires

Agricultural Fire Practices. What people needed was a means to make fuel on demand as they could spark. From a fire-history perspective this occurs with agriculture. There are systems of agriculture for which fire is irrelevant, that use water instead of flame to disturb, destroy, and fertilize. But they are not many, and they cannot extend far from floodplains or hillside terraces. (To see how much fire matters, one can try this simple thought experiment: Remove fire completely from the web of cultivation and see how long and in what forms farming and herding can endure.)

The reason lies in basic fire ecology. Flame purges and promotes. For a time, fire's catalytic presence drives off the local biota, liberates nutrients, and rearranges microclimates. (Revealingly, almost all of agriculture's hearths occur in places characterized by wet dry seasonality, which are thus intrinsically fire-prone.) Eventually the local biota closes in, and the process must begin anew. The fire ceremonies of agrarian societies testify to the belief that fire destroys the bad and encourages the good.

Farming thus exploits a fire-driven cycle. Either the farm moves through the landscape {as with classic slash-and-burn farming), or the landscape in effect cycles through the farm (as with spatially stable crop rotation). Either way, at some point, the system requires the push-pull of fire to tum its ecological cranks. Fire, however, demands fuel. So the cultivator must either seek out new sites or else bring new fuel to the old site. The agronomic name for that fuel is "fallow." The fallow, in short, was not burned as waste, but rather was grown in order to be burned.

Agriculture expanded enormously the realm of fire. Where fire had not previously existed, it could now thrive. Where it had previously flourished, it could change character. Aboriginal fire regimes could morph into agricultural ones. Particularly if outfitted with livestock—hoof and tooth proved as powerful as ax and harrow—people could return anthropogenic fire to regions like temperate Europe (or at a later time, Amazonia) from which they had been expelled. The flocks and herds replaced the megafauna that had earlier vanished.

The possible combinations of plants, animals, ax, plow, people, and fire are many. What matters is that all demand a controlled disturbance, and the form that best matches human desires with ecological possibilities is burning. What matters, too, is that cultivation placed fire's ecology even more strenuously into human hands.

How fire would behave on Earth became more closely bonded to the will and whim of human life, to a widening gamut of politics, trade, scholarship, war, and legal conceptions of land ownership, none of which had touched natural fire or shaped its regimes. Ideas and institutions would prove as significant as storms and bark thickness.