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Thursday, Dec. 6, 2001
Nature allows for few winners in the face of fire
By MARK BRAZIL
First of two parts Fire! It's a word that strikes terror. Images of leaping, roaring, scorching flames spring to mind; images of billowing smoke and suffocating fumes; of people and other creatures great and small fleeing for their lives.
Within seconds, the awful, fearful unreality of a major fire sears the imagination of all who see one, indelibly imprinting itself on the mind's eye in orange against black.
Few things can cause such emotive panic so rapidly. Why is this? Is it because this elemental force, harnessed so long ago, and whose control distinguishes the human race from its nearest brethren, is so unforgiving once that control lapses? Is it because the natural events associated with fire -- volcanic eruptions and lightning -- are themselves so powerful, so very much beyond man's ability to control?
Just a few years ago, wildfires swept across parts of tropical Borneo, and the devastation was astonishing. There, forest clearance has changed a once perennially damp tropical forest growing on moist, peaty soil to a region where droughts have now become an annual threat. Drought in a tropical forest region! The thought of it beggars belief, but the out-of-control peat fires brought dense air pollution to major cities in the region, threatening human health and causing the deaths of countless forest-dwelling creatures.
On a very much smaller scale, I clearly recall a series of fire-raising attacks on some of the few small remnants of heathland in the eastern English county of Norfolk where I used to live. Then, visiting Salthouse Heath a few days after an attack, I saw for myself the awful and eerie aftereffects on that popular landscape. Previously, on many a balmy summer evening there, I had listened as nocturnal nightingales sang their powerful songs and watched the hawk-winged shapes of nightjars flitting back and forth in search of insect prey or perching on the tops of bushes churring endlessly.
Then, instead of those evocative bird songs of summer, there had been only the dry, ashy crackle of blackened stumps of gorse bushes. Between them, the ground was now devoid of living vegetation. Slow-moving snakes and lizards, the nestlings of ground-nesting birds, a host of specialized insects and plants had been seared, scorched and withered to death by the flames or the heat.
The effects of fire on wildlife are indeed complex. Some species flee, others burrow deep, some are scorched and die, others are merely triggered to set seed, while still other species may actually be drawn to the flames. I recall vividly a massive pall of smoke stretching like a curtain across an enormous sugarcane plantation in Brazil. There the fields were purposefully blazed before harvest to reduce the leaf trash. On the hot updrafts rising far above the flames, numerous birds of prey were gathering; they were drawn not so specifically by the fire, but by the presence of fleeing reptiles and small mammals. There were winners and losers at that fire.
Nevertheless, fire surely is an enemy, although in those instances in Norfolk, Borneo and Brazil, human activities were to blame for unleashing the flames, not nature. While the immediate and the most visible damage is caused by the flames, in many cases -- and particularly in Indonesian Borneo -- more insidious damage was caused by the heat and the far-drifting smoke.
Fires are not all the same; they vary in their heat, their intensity and in the speed at which they move through the habitat, or even the speed at which they climb up into it. Once started, sustained fire is dependent on a trio of factors: the availability of suitable fuel, existing heat and plentiful oxygen. Starve a fire of one of these three, and it will falter and die; increase them, and it will spread.
The actual behavior of a fire, its rapidity of movement, whether it burns at ground level or up into the canopy, and whether it scorches the whole of an area, or merely patches, is dependent on another trio of factors: weather, topography and fuel. Wind-fanned flames can leap streams, rivers and roads, but a wind trap, a gully, gorge or stream may in other situations prove a barrier to the progress of fire. The fuel that is available will feed the fire, shaping the character of the fire itself.
Yet fire, as a force, doesn't merely burn vegetation. Fire also heats and dries soils, affecting the invertebrate life there. Although one fire may leave certain plants only marginally affected, it also leaves them drier in a drier environment, making them more prone to future fires.
More significant environmentally than the impact of a single fire is any change in a region's fire regime. The arrival of fire in an area not previously fire-prone can be devastating, utterly destroying natural habitats and the suites of species that occurred there. While in some environments fire is quite obviously entirely detrimental, such as in areas once covered with lush tropical forest, in others it is entirely natural. Some environments, such as in Australia and in the southwestern parts of North America, seem to have evolved with fire as an important vital force. Fire has in fact helped shape some landscapes, some ecosystems and even some species. In such environments, many species are not merely fire-adapted, some are actually fire-dependent.
Meanwhile, fire has been used as a tool over nearly 2 million years of hominid history in Africa. As a tool, it was carried across Europe and Asia and very much more recently, about 12,000 years ago, controlled fire was carried to America by the human settlers who crossed the land bridge from Siberia to Alaska. Fire was used not merely for cooking and warmth, but as a tool to drive game and to easily modify large segments of the landscape, encouraging light-favoring plants and the herbivores that grazed them -- and in turn provided food for people. Fire is still used today as the single most powerful tool for converting rich, three-dimensional tropical forests to impoverished two-dimensional agricultural land.
Understanding the factors leading to fires, and the consequences of them, has spawned a field of science known as fire ecology, that has in turn generated its own vocabulary. In this case, it is a fascinating one, sprinkled with words such as "avoiders" (plants poorly adapted to fire), "endurers" (plants that resprout after a fire or endure its effects) and "invaders" (plants that invade a site after a fire).
As well, there are "surface fires" that burn along the ground, consuming the ground-cover plants but hardly moving up even into the understory, let alone the high canopy. Then there are "understory fires." These are more intense than surface fires; they burn hotter and blaze up into the understory. However, the most dramatic ones are "crown fires." These reach high into the canopy of trees, burning hotly and with huge destructive power. Finally, of course, there are the "wildfires," which run unchecked, and it is these that humans seem to have grown up fearing most.
You can e-mail Mark Brazil at email@example.com