In the melange of pagan midwinter mythology and religion that we now call "Christmas," trees, particularly evergreens, have come to occupy a prominent place in the festivities. In times gone by, mistletoe and holly were most common, but since the 17th century the Christmas tree has typically been some form of spruce, with its pleasant, resinous scent.

Pick up a cone from any relative of the Christmas tree, and you are likely to have to spend a long time scrubbing it to remove the sticky sap that oozes out. In nature, winds dessicate the cones and help them to open and shed their seeds.

However, those of some species, such as the North American lodgepole pine, are so heavily sealed with resin that drying is not enough to open them and allow their seeds to disperse. For such species, survival depends on fire. A fire of the appropriate intensity will melt the dried resin, allowing the glued-shut cones to pop open, thus facilitating seed-dispersal and germination. As well, the flames will have created favorable conditions for seedling growth by burning away forest-floor litter, so ensuring plenty of sunlight is available for them.

Fire temperature and frequency are crucial. A fire that is too hot will burn the seeds within cones; one that is insufficiently hot will not open the cones. Then, if there are successive, frequent fires, new seedlings will be too small to withstand the flames.

In the northern Rockies, frequent afternoon thunderstorms often fail to deliver a deluge -- instead they produce dry lightning. In a typical season, there can be thousands of dry strikes powerful enough to blast the bark off trees in a shower of sparks. Fortunately, most of these don't trigger wildfires because there is not enough combustible matter.

However, where there is an accumulation of fuel -- dry grass, pine needles, wood, twigs, branches and even whole stormblown trees -- this, along with the understory layer of vegetation, constitutes a blaze just waiting to happen. In 1988, for example, fire raged through Yellowstone National Park, which straddles Wyoming, Montana and Idaho, and more than 400,000 hectares were incinerated due to the prolonged practice of fire suppression, which had led to an enormous accumulation of fuel within the forests.

In the past, certain regions of North America were very familiar with fires; they occurred frequently enough to prevent the spread of trees into meadowlands and to maintain open grasslands. Natural fires blazed different types of forest at different intervals, so high-mountain spruce-fir forests might experience fire only once in 150 years, whereas lower pinyon-juniper woodlands maybe burned every 10 to 30 years, and ponderosa pinewoods as frequently as every two to 10 years.

With the arrival of European settlers, the frequency of fires fell sharply as the newcomers did their best to suppress them. Inadvertently, this practice led to the accumulation of massive amounts of fire fuel in some habitats -- and even to alterations in patterns of local flora. In the dry Southwest, the region's vegetation has changed dramatically as a result of settler fire-suppression practices.

Areas that were naturally adapted to frequent, low-intensity fires, such as pinyon-juniper and ponderosa woodlands, were now exposed to long periods without fire, punctuated with rare fires of such high intensity that they completely devastated whole areas of trees. The interrupted cycle of fire no longer checked the spread of scrub and woodland into meadows, and over time the tree-density of forests became much higher than before.

This combination of more trees closer together, and huge accumulations of debris, meant that when fires did break out they burned with such fierce intensity that they climbed up smaller trees and leapt into the crowns, where the intense flames destroyed the photosynthesizing canopy.

Plants respond to fire in a variety of ways. Some are stoically tolerant, thanks to a thick skin. The mighty Douglas fir, for example, has bark so thick that it insulates the tree's vital cambium against natural blazes. Other species are thin-barked and less fire-tolerant, though some may take advantage of it in different ways -- their seeds may require smoke to promote germination, or flames to burst open their cones. Others are resistant in less visible ways: the thin-barked aspens may have poor fire-tolerance, and may be killed off above ground, but in subsequent years they readily regenerate by sprouting from their root networks insulated underground.

In the long term, if fire frequency falls, seed-spreading species benefit; whereas if fire frequency increases, species propagating by sprouting do better -- though too many blazes may prevent some species reaching maturity, and so they are wiped out.

Fire is a crucial natural part of certain landscapes. Amazingly, while some species have adaptations allowing them to survive its ravages, others seem hell-bent on promoting burning. These species include the eucalypts, which produce volatile oils, have highly inflammable leaves and drop carpets of leaves along with drapes of dangling fibrous bark. In so many ways, they seem geared up to promote the spread of fire and contribute to rapidly spreading, highly intensive blazes. The evolutionary origins of the Australian eucalypts are hotly debated, but the presence of their charcoal fossil deposits from 50-34 million years ago shows they were no strangers to fire even then.

On the one hand, wildfires consume an average of 12 to 13 million hectares a year worldwide, destroying both natural and human environments in the process -- yet on the other hand, they also serve to promote a mosaic of species in forests.

As a result, fire ecology is now a hot topic for conservation managers. However, it is hard to convince the public that the "Fire!" of their fears is also the "fire" in "fire ecology" -- and a necessary aspect of environmental education.

Recognizing habitats and species that are fire-adapted or fire-dependent is the first step in helping to restore ecosystem health, since North American research has shown that without active management -- including prescribed burning -- diverse, healthy forests cannot be sustained. Other benefits of prescribed burning may include the control of arboreal diseases (often exacerbated by an unnatural density of trees), enhancing wildlife habitats and reducing dangerous accumulations of fuel.

But it's still not easy to persuade a pyrophobic public to tolerate fires burning their natural course through protected national parks.