Wild Fire Behaviour

The Mt Lujar fire nearly out in 2000

Australia is the source of some of the most compelling information about wildfires.   Now, wild fires are not just fires that happen naturally and spontaneously, they include fires that are started by humans in the natural environment.   In fact, in the Alpujarras, southern Spain, all wild fires are caused by humans.   The impact can be devasting, as witnessed by ourselves at Semilla Besada   in 2006.   Two walkers were the cause of a fire that burnt out 2,500 hectares of Natural Park, which resulted in the death of a fire fighter and the loss of a number of homes and livelihoods.

For those who live in temperate environments, it is difficult to imagine what a wild fire is like.   The following are some of the statistics collected by the Australians, which highlight the danger of fire.

• The flames of a grassfire can rise to five times the height of the grass.
• Burning undergrowth can ignite the crown of trees which then develps into a ‘running crown fire’ and is the most dangerous stage of a forest fire.
• Burning debris can be carried ahead of the fire.   Eucalypt trees encourage this behaviour and can start a spot fire 4km ahead of the fire.   Pine trees similarly can create spot fires 2.5km and grasslands up to 100m ahead of the fire front.
• Fast forest fires can move at 15km an hour at the head.   Grassfires are faster and can move at 20km an hour.
• The speed at which a wild fire spreads increases as it goes uphill.  For every 10% increse in slope, up to 30 degrees, the speed of the wild fire doubles.
• Fire traveling upa 20 degree slope goes four times faster than on flat land.   Conversely, fire travels slower downhill.
• Fire intensity at the head of the fire can be 60,000 kW/m:  that’s the equivalent to the same heat generated by 60,000 single bar electric heaters for every metre of the fire front.
• Fire fighters can only just control a fire of an intensity of 2,000 kW& m.

Sobering statistics, but hopefully will make all those new to fire risk areas, think twice before they act carelessly in natural areas.

Aspen signing off in a sombre mood in the Alpujarras

Fire in the Mediterranean

Fire opposite Semilla Besada 2000

Surprisingly, the Mediterranean is ranked third in the world as a fire-risk area.   Why is this?   Unfortunately, as rural areas are abandoned or grazing animals are no longer used, dead vegetation simply builds up and becomes fire fuel.   There is the popular belief that if things are ‘left to nature’ then that plant material will break down providing fertility for the ultimate establishment of trees.   This may be the tendency in temperate and tropical environments, but in dryland areas this is not the case.   In fact, leaving dead vegetation to build up on the soil surface without any animal impact simply provides a suffocating mat of material through which little can grow.   As wind and rain eventually break this debris down, all that will be beneath will be bare soil.   Bare soil is dead soil.   It will continue to be eroded by the elements, providing an inhospitable environment for plants to establish above ground, or soil microbes to survive below ground.   The net result is a desert.

Using grazing animals in a holistically planned way can ensure that this does not happen.   At Semilla Besada, a flock of sheep prevent the build up of dead vegetation, help build soil fertility and provide both meat and fleece.  

Aspen signing off in anticipation of rain and in the hope of the last of this year’s fire risk.