IELTS Academic Reading Practice Test 29: Passage 1

To most people, a fire sweeping through a forest is a scene of pure destruction. Yet in many parts of the world, fire is as regular a feature of the environment as drought or heavy rain, and plants have had millions of years to adjust to it. In the pine forests of North America, the eucalyptus woodlands of Australia and the shrublands of South Africa, many species have evolved not merely to survive burning but to benefit from it.

The simplest defence is insulation. The giant sequoia of California has soft, fibrous bark that can be more than half a metre thick on old trees. This bark is rich in chemicals called tannins and contains very little of the resin found in many other conifers, so it tends to smoulder rather than burst into flame, protecting the living tissue beneath. The ponderosa pine combines thick bark with a second strategy. As the tree grows taller, it sheds its lower branches, leaving a long, bare trunk. Flames on the forest floor therefore have no 'ladder' to climb, and the crown of the tree stays out of their reach.

Many Australian eucalypts take a different approach: rather than resisting damage, they recover from it. Beneath their bark lie numerous dormant buds which, if the leaves are destroyed, rapidly burst into growth, so that within weeks of a fire, blackened trunks may be covered in fresh green shoots. Some species have an additional insurance policy in the form of a lignotuber, a woody swelling at the base of the stem, largely below ground, which stores both food and buds. Even if everything above the soil is killed, the plant can regrow from this hidden reserve.

Other plants protect not themselves but their seeds. Many lodgepole pines in the Rocky Mountains produce cones that are sealed shut by a resin that melts only at temperatures of roughly 45 to 60°C. These cones can remain closed on the branches for years, building up a store of seeds high above the ground. When a fire passes through, the resin melts, the scales open, and the seeds fall onto ground that has been cleared of competing plants and enriched by ash. After the great fires that swept through Yellowstone National Park in 1988, dense carpets of lodgepole pine seedlings appeared across much of the burned area. Botanists call this habit of holding on to seeds until a particular trigger occurs 'serotiny', and it is also found in many Australian shrubs, including the banksias.

Fire also sends signals to seeds already lying in the soil. The seeds of many acacias and other members of the pea family have coats so hard that water cannot get in until they have been cracked by heat. Others respond to smoke. In 1990, two botanists in South Africa showed that smoke alone could trigger germination in a shrub from the fynbos, a shrubland of exceptional diversity. In 2004, a team in Western Australia identified a chemical in smoke that stimulates germination; compounds of this kind are now known as 'karrikins', from a Noongar word for smoke. Nurseries and conservation workers now soak seeds in 'smoke water' to raise native species that were once extremely difficult to grow.

Fire can also govern the timing of growth. In the south-eastern United States, the longleaf pine evolved in open forests that lightning set alight every few years. Its seedlings spend their early years in a 'grass stage', looking like little more than a thick tuft of long needles. During this period, which may last from two years to more than ten, the needles shield the single growing bud while the young tree develops an extensive root system. Then, drawing on these underground reserves, it shoots rapidly upwards, lifting its bud above the reach of the next ground fire.

For much of the twentieth century, however, the role of fire was poorly understood. In the United States, after enormous fires in 1910 destroyed millions of acres of forest, the Forest Service set out to extinguish every fire as quickly as possible, and from 1935 its official aim was to bring each one under control by ten o'clock on the morning after it was reported. With fire excluded, dead wood and young trees accumulated on the forest floor, so that when fires did break out they had far more fuel and burned hotter. In California's sequoia groves, few young trees were growing, because sequoia seeds need bare soil and plenty of sunlight, conditions that fire had once provided. Since the late 1960s, park managers there have used prescribed burns, lit deliberately under carefully controlled conditions, to recreate them.

Fire-adapted plants are not, however, adapted to any fire at all. Each species is suited to a particular fire regime: a typical pattern of frequency, intensity and season. Where fires are becoming more frequent as the climate warms, young lodgepole pines may burn before they are old enough to produce cones, and the forest may give way to grassland or shrubs. What matters is whether fire arrives in the form to which the plants have become accustomed.