Elements for Life

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] 148

OME NATURAL DISASTERS

are strongly linked to meteoro-

logical conditions, and among these forest fires play a very

important role as a major threat to vegetation-covered

areas in many parts of the world.

The impact of fires in a given region is usually evaluated by the

number of fires and the size of the burned area. Unfortunately,

there is no uniformity in these data throughout the world to

render the statistics relevant for comparison.

The relative impor-

tance of the problem in a given area depends both on its

dimension and frequency, and on the perceptions of local people.

Forest fires are a complex mixture of human and natural

factors. Man is quite often the originator of such fires, inter-

venes during their development, and is also impacted by them.

Overall, it is very difficult to assess precisely the relative roles

of man and nature in forest fires. We can only say that there are

aspects of forest fire in which human and social activity are

the most relevant; but there are also aspects in which nature

plays the dominant role.

Fire is part of nature and has shaped vegetation cover and

life throughout millennia. It cannot be eliminated from the

landscape without damage to biodiversity. Conversely, man has

used and continues to use fire for the management of rural and

wild spaces. Therefore it is necessary to distinguish between

controlled fires and wild fires. The latter include those events,

either natural or anthropogenic, in which fire cannot be

controlled before causing undesired damage.

The advent of technology has created the illusion that man can

dominate natural forces and overcome the laws of nature. One of

the consequences of this attitude is the illusory ideal of exclud-

ing fires from the forest altogether. We have to recognize that

even if all anthropogenic fires could be avoided, natural events

such as lightning strikes might originate fires in conditions that

are beyond the control of even the most advanced technologies.

The role of climate and meteorology

It is commonly accepted that physical factors such as topogra-

phy, vegetation cover, climate and meteorology contribute

greatly to the conditions needed for a fire to start and to spread.

Climate and topography determine to a great extent the type

of vegetation cover, its quantity and distribution.

Meteorological factors such as precipitation, air temperature

and humidity affect the growth of fine vegetation and deter-

mine its proliferation and dryness. The moisture content of fine

vegetation, particularly in dead plants, is strongly related to the

risk of ignition – above a certain moisture threshold it is very

difficult to ignite or maintain. Conversely, very dry fuels provide

an ideal environment for fires to start and spread. The presence

of slopes or wind can also contribute to an increase in the rate

of spread of a fire, to a point that may make its control virtually

impossible. In certain conditions including steep slopes and

canyons, the convection induced by the fire modifies the

burning conditions and thus increases its rate of spread.

Climate, man and forest fires

Domingos Xavier Viegas, Department of Mechanical Engineering,

University of Coimbra, Portugal

Forest fire at the borders of River Zezere in Pampilhosa da Serra, Central Portugal, 1 August 2003

Photo: Pedro Palheiro/ADAI