The Blue Mountains are a fire prone environment. The occurrence of bushfire depends on four conditions (Tasker and Hammill 2011):
1. A sufficient quantity of fuel (leaf litter, tree bark, etc.),
2. Fuel that is dry enough to burn,
3. Weather conditions suitable for fires to spread – this is determined by the weather on any given day, and
4. A source of ignition.
Further breakdown of these conditions will highlight how these conditions come together in the Blue Mountains.
1. Enough fuel to carry a fire:
The sclerophyll forests and woodlands that dominate the Blue Mountains provide an almost continuous supply of fuel. These woodlands contain a great diversity of eucalypts recognised by the United Nations as of outstanding universal value when defining the Greater Blue Mountains World Heritage Area. Flora of the genus Eucalyptus, as well as other natives such as Angophora, have evolved with fire such that they are not only fire tolerant, but fire promoting.
These trees and the understorey species that usually come to dominate lose their leaves and shed bark both of which add to fire fuel loads (Gill 1981). Combine these habits with a capacity to readily regenerate from shoots left in tree trunks and underground, and it becomes clear that fire is no enemy to Blue Mountains vegetation.
In fact, certain species require fire in order for seeds to flourish or at other points in the life cycle.
Of the four conditions above, the quantity of fuel is rarely the limiting factor. Models of fuel accumulation over time follow a negative exponential curve, where fuel builds up rapidly for several years after a fire and then reaches a relatively steady state level (OEH 2012). The rapid accumulation of dead leaf litter and living vegetation results in capacity for another fire typically within four to ten years of being burnt (van Loon 1977; Birk and Bridges 1989).
2. Fuel that is dry enough to burn:
When fuel is dry enough to burn, it is said to be “available” to the fire. While the amount of fuel is rarely the limiting factor, its availability often is. Only when long, dry spells occur will the fuel dry to the point which it can readily burn. The influence of the El Niño Southern Oscillation (ENSO) on the climate of south‐eastern Australia, however, creates these long, dry spells from time to time.
In fact, the oscillation between wet (La Niña) and dry (El Niño) conditions could be said to be fire promoting. La Niña conditions bring wetter than average years, and plant growth flourishes. With an increase in plant growth comes an increase in fuel loads, which builds up during the wet period. Should El Niño conditions follow and bring hot, dry years shortly thereafter, the scene is set for a fire. Cunningham (1984) suggests: “A very wet season or succession of wet seasons should lead to increasing alertness to danger especially should they be followed by a dry spring. October to February and particularly November and December are months of most critical hazard.”
At the same time, this dependence of major bushfire on fuel availability, which in turn is dependent on ENSO cycles, means that severe bushfire seasons in south‐east Australia generally occur once or twice per decade and correspond with strong El Niño events (Tasker and Hammill 2011). OEH (2012) also noted a positive relationship between major bushfires and El Niño episodes and the tendency for the most significant fire seasons to occur following major La Niña episodes (wetter years that allow fuel buildup).
3. Weather that promotes rapid fire spread:
While fuel and terrain do influence the behaviour and spread of fires in local areas, weather exhibits the strongest influence at the landscape level (Bradstock et al. 2009; Bradstock et al. 2010).
Not all fires turn into major bushfires. When major fires do occur, however, they are aided by hot, dry, and often windy conditions on the day. While Blue Mountains summers are generally mild, several times in a given spring and summer the weather may become hazardous. Every once in a while, the Blue Mountains experiences temperatures in the mid‐to‐high 30s as a result of a common occurrence where strong winds from the northwest bring dry heat into the area.
While conditions rarely last for more than two days and are often alleviated by a welcome southerly gust, Cunningham (1984) suggests that all major crisis days of bushfires in the Blue Mountains, including the 1957 fires, have been triggered by this characteristic weather sequence. He suggests the sequence is quite common and can occur between six and ten times in a normal spring‐summer period. The Forest Fire Danger Index describes fire weather by combining variables of temperature, rainfall, relative humidity, and wind speed into a single metric (Noble et al. 1980).
The Forest Fire Danger Index describes fire weather by combining variables of temperature, rainfall, relative humidity, and wind speed into a single metric (Noble et al. 1980). The index is used for fire prevention, awareness, and management through assessing fire risk, planning prescribed burns, and declaring Total Fire Ban days. Forest Fire Danger Index categories are:
- Low-moderate (0-11)
- High (12-24)
- Very high (25-49)
- Severe (50-74)
- Extreme (75-99)
- Catastrophic (100+)
4. An ignition
Despite the three conditions above, you still can’t start a fire without a spark. Natural causes of ignition include dry lightning strikes, which start around one‐third of all fires in the Blue Mountains (Cunningham 1998). Sources of fire ignition other than from lightning are from human causes close to areas of rapid urban expansion or access roads, and are more likely to occur on weekends during early spring to late summer (Bryant 2008).
Deliberate and accidental human‐caused fires often occur on or preceding days of very high or extreme fire danger and can quickly reach proportions beyond the limits of suppression (OEH 2012). The Blue Mountains is unique in having a vast urban‐bushland interface and claiming the status of a City within a World Heritage Area. At the same time, expanding human settlement within a fire prone landscape requires caution.
How fire travels
While fuel availability and weather affect how a fire travels, topography is also important. Here, we are talking about the slope of the land. Unlike humans, fires usually travel uphill much faster than downhill. The steeper the slope, the faster the fire travels. This is because the ambient wind usually travels uphill and the fire is able to preheat the fuel uphill because of rising smoke and heat. Fires tend to burn out once they reach the tops of hills, and struggle to move downhill because of an inability to preheat the downhill fuel compared to uphill fuel (Bonsor 2013).
Managing to reduce the chance of major bushfires
One way managers attempt to reduce the likelihood of major bushfires that threaten life and property is through initiating smaller, controlled burns to reduce fuel loads and create a mosaic of vegetation at different life stages across the landscape. As fuel loads are bushfire hazards, these controlled burns are often referred to as hazard reduction burns. Hazard reduction may also be completed through mechanical (hand) removal of fuel. Attempting to keep the entire landscape entirely free of fuel, however, is beyond management capacity.
Furthermore, fire is a vital ecosystem process, and Blue Mountains flora and fauna have adapted to a particular fire regime. While individual fire events are rarely of concern when it comes to ecosystem conservation, the cumulative effect of different types of fires, that is, the fire regime, risks generating potentially undesirable long‐term change (DSE 2011; OEH 2012)
The importance of appropriate fire frequency:
- Fires that are too frequent (too short of an interval) may kill juvenile plants before they have had the chance to seed, may reduce availability of nutrients, and may cause erosion.
- If fire is excluded from an area for too long of an interval can affect ecosystem health by failing to provide the disturbance necessary for landscape renewal. Species composition may change, which, in turn, may change the fire risk of an area.
The importance of appropriate fire intensity:
- Intense fires can sometimes cause a decline or loss in species sensitive to fire intensity. Repeated high-intensity fires, therefore, may change to species composition of an area.
- Intense fires generally result in slower rates of recovery.
The importance of appropriate fire season:
- Natural fires occur in the warmer months, meaning species are more likely to have adapted to survive fire during this period.
- Striped Legless Lizards, for example, are vulnerable to fire in the winter and autumn. They escape summer fire by using cracks in the soil. If fire occurs during wetter conditions when these cracks are unavaiable, as is more common in winter and autumn, the lizards are much more likely to die.
The importance of fire extent:
- Large, uniform fires generally have greater impacts on plant and animal species than smaller patchy fires. This is because habitat resources are affected across wide areas.
In the end, a variety of fire regimes, including occasional intense crown fires and low intensity frequent fires, facilitates a diverse and rich mix of plants and animals, structural diversity in vegetation and habitat and promotion of more robust ecosystem structures and functions (Bradstock et al. 1995; Bradstock and Kenny 2003; OEH 2012). When planning fire management activities, land managers use biodiversity thresholds based on plant species functional types and life history to identify the domain of acceptable fire intervals within broad vegetation types (Kenny et al. 2004). In general, fires of a frequency shorter than the acceptable fire interval and of repeatedly high intensity are problematic. High frequency fire is listed as a key threatening process under the Threatened Species Conservation Act 1995 (NSW).
While appropriate fires are generally beneficial for Australian ecosystems, management also needs to consider areas where fire is generally inappropriate (DSE 2011).
- While peat‐based vegetation, such as upland swamps, can resprout following fire, should the fire be intense enough to also ignite the peaty soil, the peat may smoulder. These peat fires result in the loss of peat habitat that takes millennia to accumulate.
- Many types of rainforest are sensitive to fire.
- If an area is known to be critical habitat for a threatened species, burning the area may result in species extinction.