Fires in Nature: Causes and Distribution (AQA A-Level Geography): Revision Notes
Fires in Nature: Causes and Distribution
The nature of wildfires
Wildfire represents a natural phenomenon within many ecosystems and can serve important ecological functions in certain environments. However, fires can also cause severe destruction in natural areas, resulting from both natural processes and human activities.
Large-scale fire events can cause widespread devastation and kill significant amounts of wildlife. Surface fires move rapidly across the ground, consuming plant debris, grasses, and herbs whilst scorching trees. These blazes can heat the ground to temperatures exceeding 1,000°C. At higher elevations, crown fires spread through tree canopies, causing even more extensive damage.
The distinction between surface and crown fires is important for understanding fire behavior. Surface fires, while destructive, often allow tree survival. Crown fires are far more devastating as they consume entire trees and spread more rapidly through the canopy.
The characteristics of any fire depend on several interacting factors:
- The types of plants present in the affected area
- Wind strength and direction
- Local topography
- The condition and dryness of vegetation
Once plant material has dried out, wind becomes a crucial factor in determining how the blaze develops. The most extensive fires occur during dry, windy conditions with low humidity levels. Wind propels flames forward and spreads burning embers, which can ignite additional vegetation and spread more easily in windy conditions.
Pyrophytic (vegetation)
Pyrophytes are plant species that have evolved to tolerate and survive fire. Their survival adaptations include thick protective bark, tissues with high moisture content, and underground storage structures that protect vital organs.
Retardants
Retardants are chemical compounds applied to fires to slow their progression. These substances consist of nitrates, ammonia, phosphates, sulphates, and thickening agents.
Causes of wildfires and their spread
For a natural fire to start and spread, two essential conditions must be met:
Ignition source
In natural fires, lightning represents the primary ignition source by a considerable margin. Climate influences how frequently electrical storms occur, particularly those with minimal rainfall.
Increasingly, human activity causes fires, especially those occurring in and around populated areas. Common human causes include:
- Power lines falling or sparking
- Carelessly discarded cigarettes
- Children playing with matches
- Camp fires and barbecues
- Agricultural burning that escapes control
Evidence suggests that some devastating fires have been deliberately started by arsonists. Greater tourist access to wilderness areas in countries such as the USA and Australia has increased fire risk through some of the activities mentioned above.
Fuel
The fuel source must be sufficiently abundant and dry enough to burn effectively. Climate determines the frequency and length of drought periods during which vegetation and plant debris have the opportunity to accumulate and become thoroughly dry. Climate also influences which types of vegetation will grow in particular areas and the rate at which plant litter can be produced.
Distribution
Wildfires are essentially a rural hazard and can occur in most environments. However, with continued expansion of human habitation, fires now occur within the boundaries of even substantial settlements in California, Australia, and southern European countries.
Areas most vulnerable to wildfire are those with Mediterranean climates and tropical wet season/dry season (savanna) climates.
Mediterranean climates
In Mediterranean regions, winter rainfall encourages plant growth, but hot dry summers with occasional thunder storms create fire conditions. Key locations include:
- Australia: Southwest region, southeast (Victoria and southern New South Wales), and parts of coastal southern Australia
- California: Throughout the state
- Southern Europe: Including southern France, Italy, Greece, Turkey, and Mediterranean islands
Savanna climates
Savanna regions follow a similar pattern, though wet summers and dry winters characterize these areas. Such regions include:
- Northern Australia
- Tropical regions of Africa and South America
- Northeast India
- Northern Florida
The pattern in both climate types is similar: a period of growth (winter rainfall in Mediterranean, summer rainfall in savanna) followed by a period of drying (summer drought in Mediterranean, winter drought in savanna). This cycle creates ideal conditions for fire - abundant fuel that becomes thoroughly dried.
Fire and natural ecosystems
Fire and natural ecosystems are closely connected, particularly in the regions identified above. Fire performs several important ecological functions:
- Clearing vegetation and promoting new seed germination
- Stimulating the growth of certain plant species
- Removing an area of insect pests and parasites
Some species are pyrophytic, meaning they can survive fire through various mechanisms. For example, the baobab tree has resistant bark. In Australia, plants such as banksia only release their seeds after a fire, ensuring that seeds fall on well-cleaned, brightly-lit ground recently fertilised with ash.
Examples of Pyrophytic Adaptations
Baobab tree: Has evolved thick, fire-resistant bark that protects the living tissue beneath from heat damage.
Banksia (Australia): Uses fire as a seed release trigger. The plant's seed pods remain sealed until exposed to fire temperatures, after which they open and release seeds onto freshly cleared, ash-fertilized ground with optimal light conditions for germination.
Fire has not traditionally been a major hazard in tropical rainforest areas due to the humid climate. However, recent burning for forest clearance has frequently escaped control, resulting in widespread fires that burned uncontrolled for extended periods. Both the Amazon Basin (Brazil) and southeast Asia (Indonesia) have experienced such fires in recent years.
Case study: Los Angeles Basin, California
Case Study: Los Angeles Basin Wildfire Risk
The Los Angeles Basin provides an excellent example of an area where wildfires represent a continuing problem. Wildfires occurring here often receive extensive media coverage because it is likely that fire events could threaten the homes of famous film and television personalities. Wildfires are common in this region and pose a serious threat for several reasons:
Vegetation characteristics: Much of the area is covered by drought-resistant chaparral vegetation (a type of scrub), as the region is too dry for much tree growth. This vegetation, after the summer drought, can become extremely dry and acts as tinder.
Wind patterns: The dry Santa Ana wind descends from the local mountains, increasing the dryness of vegetation to dangerous levels. A spark from lightning or a carelessly discarded cigarette can trigger a major fire. This wind also facilitates the easy spread of fire and makes control very difficult.
Development patterns: Much of the area outside the centre of Los Angeles consists of low-density building developments where natural vegetation has been allowed to remain between properties, exposing many homes to fire risk.
The impacts of wildfires
Primary effects
Loss of crops, timber, and livestock
Forest fires can have enormous impacts in timber-producing areas, with the loss of trees requiring many years to replace. The long growth period for mature timber means that economic recovery can take decades.
Remember!
Key Points to Remember:
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Wildfires require two essential conditions: an ignition source (mainly lightning, increasingly human activity) and sufficient dry fuel
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Wind is crucial in determining fire spread, with the largest fires occurring in dry, windy conditions with low humidity
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Wildfires mainly affect areas with Mediterranean climates (hot dry summers) and savanna climates (distinct wet/dry seasons)
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Some plant species (pyrophytes) have evolved adaptations to survive fire, such as thick bark or seeds that only release after burning
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Fire plays important ecological roles including clearing vegetation, promoting seed germination, and reducing pests