Forest Fires
Forest Fires
Forest Fires
Forest Fires
Origins and Ecological Paradoxes
K Narendran
Fire occurring naturally have been known ever since the evolu-
tion of land plants some 350 to 400 million years ago. The first
evidence of a wild fire in the world dates back to the early
Carboniferous era (approximately 390 MY ago). Wildland
fires, as they are commonly known today, occur at varying Natural causes of
frequencies, in different parts of the globe. Natural causes of forest fires are
forest fires are usually lightning strikes or burning coal seams. usually lightning
Lightning strikes when the lower portion of clouds becomes strikes or burning
negatively charged and sufficient electrical potential has built coal seams.
up compared to the positively charged ground. During light-
ning an arc of electrically charged atomic particles, called elec-
trons, passes from the cloud towards the ground, carrying a
current of as much as 60,000-100,000 Amperes. When lightning
comes in contact with dry litter, dead and fallen logs or dry
canopy it could ignite a natural fire. In Australia, in the interior
of the state of Queensland, the drought prone grasslands of the
Mitchell Plains experience repeated natural fires from lightning
strikes, these fires are not accompanied by rain and hence fire in
these ecosystems has greater bearing on their ecology.
Fires may occur naturally also, from burning coal seams below
forested areas. Natural heat production within the core of the
earth is only partly understood. However it is clear that some
areas on the Earth have a higher flow of heat from below the
surface. Most such locations are associated with the movement
of tectonic plate boundaries where mountains are being uplifted
and volcanic island arcs forming. It is likely that, in such areas,
heat from within the Earth could be transferred to the charcoal
layers above, from there onto the peat and other vegetation in
the soil and finally when it comes in contact with forest litter it
would develop into a natural forest fire. Ironically, in regions
usually thought of as cool and wet, forest fires do occur naturally
from time to time originating from charcoal layers found to-
wards the base of peat as in the Western Highlands of Scotland.
The first ever forest fire in India occurred about 200 million
years ago coinciding with the time of evolution of early mam-
mals on the Earth. This is based on an analysis, of charred fossil
log remains of a late Triassic era forest fire in the South Rewa
Gondwana basin of Madhya Pradesh, by the Geological Survey
of India. Fires occur on a regular basis in many parts of India.
Fires are frequent along the Himalayan foothills and also in the
deciduous forests elsewhere. The pine forests of the Himalayas
are highly susceptible to fire. The debris of pine accumulates as
it takes a long time to decompose; the intervening period is one
of high fire risk. In the deciduous forests of southern India, leaf
shedding begins early in the year and by April the ground below
is covered with large amounts of litter. This pattern in leaf
shedding is common in the Mudumalai, Bandipur and Wyanad
wildlife sanctuaries, which have substantial areas under the
deciduous forests. The biomass generated by many grasses such
as Themeda cymbaria and T. triandra also contributes signifi-
cantly to the combustible material of these forests. Humans are
responsible for most of the fires here. Often the forest tribes
light fires, during the collection of non-timber forest products,
to make way within the densely grown patches of grasses and
shrubs. Fires also do occur due to human negligence. Fires set
Forest fires are classified as crown fires, surface fires and ground
fires on the basis of their intensity. The occurrence and intensity
of fire in a forest depends on the topography, weather (precipi-
tation and humidity, temperature and winds) and the forest
type. These factors are popularly known as the fire environ-
ment in the USA. A long dry season will provide a significant
amount of flammable material. This coupled with gusty winds at
the time of a fire could greatly enhance the intensity and extent
of a fire.
The vegetation type is a decisive factor in forest fires. In the
Mudumalai Sanctuary, the tropical dry deciduous forests are
more susceptible to fire (see Figure 1) than the moist deciduous
and dry thorn forests. While the moist deciduous forests receive
Figure 1. A forest fire in the
Mudumalai Wildlife Sanc- higher rainfall, the dry thorn forest has very low rainfall and
tuary hence an insignificant amount of combustible matter. The dry
deciduous forests have a longer dry season of about 5 months
and relatively large amounts of burnable
matter in the form of litter and grass biom-
ass. The species composition of a forest can
also be decisive in forest fires. In the
Mudumalai Sanctuary, the northeastern por-
tions having a high density of Shorea roxburgii
have the highest fire incidence than the rest
of the forest 7 times during the 11 years of
recorded fire data available. We need to un-
derstand further whether such domination
Not all impacts of fire in ecosystems are direct; there are some
indirect impacts too. Post-fire outbreaks of insect and fungal
attacks also do happen. In New Zealand the Sirex wasp had done
immense damage to plantations; while it attacked forests that
had not been under fire, a recent burn greatly enhanced its
establishment and development. Sal heartwood borer
(Hoplocerambyex spinicornis), a beetle, is associated with the sal
forests (S. robusta) of Madhya Pradesh. In 1997, over one-sixth
of the total Sal forests in Madhya Pradesh had been infested. In
one of the most affected districts, about 800,000 trees had been
attacked. It is interesting to note that the insect attack began in
1995 and assumed epidemic proportions in 1997. There are
contradictory viewpoints about sal infestation by this beetle.
Whereas one view attributes the condition to the lack of fire the
other considers disturbances by humans, grazing or fire to be
responsible for the outbreak of the epidemic. There also oc-
curred large-scale mortality of adult trees of S. roxburghii at
Mudumalai during the early 1990s. The cause of the mortality
has been attributed to an unidentified stem boring cerambycid
beetle. Incidentally this part of the forest has the highest fire
frequencies in the whole sanctuary.
Beneficial Impacts
Suggested Reading
[1] Robert J Whelan, The Ecology of Fire, Cambridge University Press, Cambridge, 1995.
[2] Stephen J Pyne, Americas Fire Management on Wildlands and Forests, Forest History Society, North
Carolina, 1997.
[3] Stephen J Pyne, World Fire: The Culture of Fire on Earth, Henry Holt and Company, New York, 1995.