Fire

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Fire is a naturally occurring ecological force (and interestingly one of the few major forces that humans have any control over, at least in terms of starting fires) found in all continents except Antarctica. Plants and animals in fire-prone forests have often been co-existing with fire for millions of years and have come not just to tolerate fire but need it to ensure survival and reproduction. Most natural fires are caused by the 8 million lightning strikes that hit the Earth each day, although volcanism and earthquakes (causing sparks) can be locally important. Now, of course, humans play their part and in densely populated areas, such as much of Mediterranean Europe, the majority of fires are human-caused.

Fires started by lightning strike to a single tree in damp conditions will produce small gaps similar to a single tree death. However, the main role of fire is in producing large stand-replacing events that may kill trees over hundreds of square kilometres. Most large forest fires are in evergreen, usually coniferous, forests because of the high content of flammable resin in the foliage and the accumulation of masses of small, quick-drying pieces of litter on the forest floor which readily carry fire when dry.

Fire is often treated as a uniform type of disturbance. However, the effect of a fire very much depends on what sort of fire took place. A slow and gentle surface fire that trickles through the dry surface litter may kill thin barked trees (such as birch) and herbaceous plants, at least above ground, but will leave thick-barked trees unharmed. Most top-killed plants will resprout from stored

Figure 9.13 Canopy or crown fire in dead and dying balsam fir Abies balsamea in eastern Canada. Fire is burning through the whole canopy, producing flames twice the height of the trees. (Photograph courtesy of the Canadian Forest Service.)

buds. Few vertebrates will be killed and the survivors will benefit from the young, nutritious regrowth of vegetation. As surface fires become more intense, they may leap into the tree canopy to add a canopy or crown fire above the surface fire. Crown fires can be extremely intense (Fig. 9.13), completely consuming the foliage and small branches, usually killing the tree, and leaving a blackened skeleton. Regeneration is from seed rain from undamaged trees but especially from seed stored in the canopy in fire-proof serotinous cones that are stimulated to open by the heat of the fire and shed their seeds a few days after the fire. Serotiny is common in many pines, such as the lodge-pole pine Pinus contorta and jack pine P. banksiana of North America and the bottle-brush trees (Banksia spp.) and eucalypts of Australia. Again, relatively few animals, apart from invertebrates, are likely to be killed, and most fire sites attract grazing animals which may have their own effect on the regenerating forest. A third type of fire is the ground fire that burns into the organic matter overlying mineral soils, including the living roots of plants. Trees otherwise unmarked can thus be killed when their roots are consumed and the whole tree falls over. Post-fire colonization tends to be from seed rain since stored buds and the soil seed bank are increasingly destroyed as the severity of the ground fire increases.

When an intense surface fire damages the cambium of a tree it leaves a scar. The fire scar record of an individual trunk thus indicates actual fire frequencies at its location. In North America native Indians formerly set light to surface litter and a ponderosa pine in the Bitterroot Mountains of western Montana records 21 fires between 1659 and 1915, an average interval of almost 13 years. Such periodic fires created the park-like stand of giant sequoias in the Yosemite National Park shown in a photograph of 1890. Eighty years later thickets of white fir Abies concolor occupied much of the ground. If fire were to occur under these conditions the firs would provide a fuel ladder carrying fire up into the canopy. Unlike coastal redwood Sequoia sempervirens, the stumps of giant sequoia (or big tree) Sequoiadendron giganteum lack the ability to sprout again so the trees would die (Spurr and Barnes, 1980).

A long-term experiment on the influence of fire began in 1929 when three plots were established in an area of the Olokomeji Forest Reserve, Nigeria. Though each was initially burnt, coppiced and cleared of trees, they were all treated differently. Their vegetation was analysed in 1957 (Hopkins, 1965). Grasses were almost absent from the fire-protected plot in which forest vegetation was rapidly re-establishing and a well-developed canopy of trees and shrubs was present. The other two plots were burnt annually, but whereas that burnt at the end of the dry season had become a tree savanna in which fire-scarred trees were widely spaced, that burnt at the start of the dry season, when fires were less intense, contained a fairly mature savanna woodland with some fire-tender forest trees protected inside fire-resistant clumps of closed woodland.

Fires are rarely completely uniform across a landscape, usually forming a mosaic of differently burnt patches and often unburnt islands (Fig. 9.14). This further complicates patterns of seed rain, regrowth from buds or seeds and grazing pressure. It is perhaps not surprising then, that forest fires will often, at least temporarily, increase biodiversity by creating a wide range of niches.

Even a casual interest in the media will have shown that fires have become more common with increasing drought and high temperatures around the world. Conditions previous to the fire are crucial to its subsequent development, as in the case of the lightning strikes that hit drought-stricken forests and created huge fires in Victoria and Canberra, south-east Australia in January 2003. The ability of gum trees Eucalyptus spp. to survive the fires which ravage them so rapidly and intensely is described in Section 1.3.1.

The European summer of2003 was exceptionally hot; the highest temperature ever recorded in England (38.1 °C at Gravesend, Kent) until then occurred in August. Four months of continuous drought preceded the vast forest fires in southern France when arson was involved in the virtually simultaneous initiation of some 30 outbreaks in July. Huge areas of forest were destroyed,

Figure 9.14 Part of the Muleshoe burn in Banff National Park, Canadian Rocky Mountains in 1993. The fire has left some areas untouched, some with scorched foliage (e.g. the large tree on the right) which may recover but will cast less shade for a number of years, and other areas where canopy fire (see Fig. 9.13) has killed the trees (e.g. mid foreground and the circles in the distance), leaving an opening in the forest. (Photograph by Peter A. Thomas.)

Figure 9.14 Part of the Muleshoe burn in Banff National Park, Canadian Rocky Mountains in 1993. The fire has left some areas untouched, some with scorched foliage (e.g. the large tree on the right) which may recover but will cast less shade for a number of years, and other areas where canopy fire (see Fig. 9.13) has killed the trees (e.g. mid foreground and the circles in the distance), leaving an opening in the forest. (Photograph by Peter A. Thomas.)

much damage done and four people killed in conflagrations which jumped from place to place as the winds changed direction. With so much tinder-dry forest even the thousands of fire officers from all over France, together with some from Italy, took a long time to put the blazes out, despite the most modern methods, including water bombing from aircraft, being employed. Huge fires later that year also caused deaths and loss of timber in Mexico, California, Spain, Portugal, Alberta and British Columbia.

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