Geoscience Reference
In-Depth Information
rainforest trees that were especially sensitive to fire, such as the
Auracaria
.Tothisday,
the Aboriginal people of Arnhem Land in seasonally wet tropical northern Australia
burn off the dead grass and undergrowth every year at the end of the wet season
(usually late March or early April) in order to allow the unimpeded growth of new
grass to attract wallabies and similar game (Haynes,
1991
).
So widespread throughout Australia was the Aboriginal use of fire to modify the
plant cover in ways they deemed desirable that the archaeologist Rhys Jones coined
the expression
fire-stick farming
for this activity (Jones,
1968
), although farming in
the strict and narrow sense of cultivating domesticated plants is hardly an accurate
term for this process. One of the great puzzles in the Australian prehistoric record
is the demise of the megafauna roughly 45,000 years ago, at about the time that
prehistoric humans first entered Australia (see
Chapter 17
). Although it is still unclear
whether humans were the sole or main cause of these extinctions, it is possible that
widespread prehistoric burning may have altered the plant cover and habitats of many
of the large browsers and so hastened their demise. A corollary to this argument is
the suggestion put forward by Miller and his co-workers that the change in vegetation
led to a change in run-off and hydrological regime and even weakened the summer
monsoon (Miller et al.,
2005
). In common with Charney's 'biogeophysical' drought
model discussed in
Chapter 23
, this hypothesis is more of a numerical sensitivity
study than a demonstration of an actual synoptic effect.
In the normal course of events, natural fires triggered by lightning strikes are most
common at the start of the summer wet season and frequent low-intensity Aboriginal
fires are most common at the close of the wet season. In either case, the fires are
limited in their impact and seldom spread out of control. This changed with the arrival
more than two hundred years ago of the Anglo-Celtic settlers, who rapidly excluded
the first Australians from their expanding territory and in the process brought to an
end the practice of frequent low-intensity firing of the landscape. As a consequence,
the long-established fire regime was altered quite drastically, so frequent low-intensity
burning became rare except in the far north of the continent, giving way to less frequent
but high-intensity and highly destructive fires (Gammage,
2011
). This pattern is also
evident in other parts of the world, such as North America, where attempts to exclude
fire and promote forest protection allowed a substantial build-up of the fuel load
during wet years, leaving the forests increasingly vulnerable to lightning strikes or
arson.
Fire can be a potent cause of land degradation and loss of biomass and biod-
iversity, even in the wetter parts of the dry subhumid regions of the world.
Table 23.3
(
Chapter 23
) shows that during the 1877-1998 period, ENSO events coincided with
both droughts and fires in Indonesia. About 12.5 per cent of Asia experiences biomass
burning on an annual basis, although it is more severe and widespread during drought
years. Some of the burning results from shifting cultivation, some from the clearing
of forest for oil palms and other plantation products, and some from savanna fires
