Fire - Agroecology: The Ecology of Sustainable Food Systems - page 137

But many of these systems have recently begun to move
in an unsustainable direction, because an array of social,
economic, and cultural factors create pressures that
shorten the fallow period, remove fallen timber for fire-
wood, introduce inappropriate crops, or overgraze ani-
mals, eventually promoting the invasion of noxious
weedy species or leading to a breakdown of the processes
that enhance the recovery of native species ground cover.
Overuse of fire is often one cause of the breakdown in
sustainability (Figure 10.5).

TA B L E 1 0 . 1
Slash Loads Available for Burning as a Part of Land
Clearing in a Range of Ecosystems

Slash Load
(kg/m 2 ) System Location Ref.

Napier grassland Tabasco, Mexico 1.63 Gliessman

(1982)

2 years second
growth

Tabasco, Mexico 1.18 Gliessman

(1982)

8 years second
growth

Turrialba, Costa
Rica

3.85 Ewel, et al.

(1981) M ODERN A GRICULTURAL S YSTEMS

Mature tropical dry
forest

Jalisco, Mexico 1.18-1.35 Ellingson,

et al. (2000)

In modern agricultural systems, fire plays many diverse
roles. The examples presented below represent different
levels of technology and have different levels of use
depending on the agroecosystem type, part of the world,
and cultures involved. They can be used at any time during
the cropping cycle, from preplant to harvest, depending
on the system and the purpose. The biggest challenge in
the use of fire overall is to understand how to take advan-
tage of the beneficial effects of fire while avoiding or
minimizing the negative ones. Skill, experience, and
knowledge are all required.

Upland rice and
barley

Central Japan 0.34 Koizumi,

et al. (1992)

Upland rice Tabasco, Mexico 0.51 Gliessman

(1982)

Paddy rice Central Valley,

California

0.7-0.9 Blank, et al.

(1993)

Douglas fir with red
alder (9 years old)

Oregon, U.S. 0.986 Cromack,

el al. (1999)

Conifer forest Pacific

Northwest, U.S.

0.5-3.0 Dell and Ward

(1971)

Annual pasture Central Coast,

California

0.2-0.3 Gliessman

(1992b)

Land Clearing

In many parts of the world today, fire continues to be the
most accessible and affordable tool for clearing vegetation
and plant biomass from the soil surface prior to preparing
the land for planting, especially in present-day versions
of shifting cultivation. The use of fire for land clearing is
particularly important in many forestry systems, where the
large slash load left after logging is burned to make
replanting easier, as well as to reduce the chance of a
wildfire moving through the dry slash and suppressing the
establishment of seeded or transplanted tree seedlings.
The amount of dry matter that needs to be cleared will
obviously have a great impact on the type and intensity
of the fire. As shown in Table 10.1, these amounts, called
slash loads, vary considerably depending on the system.
Slash left on the soil in tropical shifting cultivation sys-
tems can easily exceed 4 kg/m 2 , and if adequately dried
and burned at an appropriate time, will carry a hot, uni-
form fire that will consume most all of the plant material
except large diameter branches and trunks (Ewel et al.,
1981). Even young second-growth produces 1 to 2 kg/m 2
of dry matter and can easily carry a fire (Gliessman, 1982).
Logging of older forest systems invariably leaves the
forest littered with logs, tops, and branches, which can
become a fire hazard as they dry out. Such slash can also
harbor pests and be detrimental to the recovery of tree
seedlings. On the other hand, as the debris decomposes,
it improves soil structure and nutrient status while pro-
tecting the soil against erosion. All of these factors need
to be taken into account in deciding if slash should be

burned uniformly over the surface, piled so that impacts
of burning can be localized, or left unburned as a mulch.
In some traditional systems, when slash is limited in sup-
ply (usually less that 0.5 kg/m 2 ), it is piled, burned, and
the ash scattered uniformly over the cleared fields as a
fertilizer (Figure 10.6).
A unique example of the use of fire for land clearing
is a system for renovating old cacao plantations in
Tabasco, Mexico that are no longer profitable (Figure 10.7).
First, bananas are planted in the understory. The next
year, all overstory shade trees and old cacao trees are
cut, leaving a heavy slash load of more than 5 kg/m 2
that covers the corms of the bananas. Once adequately
dried, the slash is burned. Immediately after the fire, a
traditional corn/bean/squash intercrop is planted in the
same way as in local shifting cultivation systems, allow-
ing for a harvest within 6 months after cutting of the
trees. While the annual crops are being planted and
cared for, sprouting bananas and new shoots from the
trunks of the leguminous shade trees are protected and
allowed to develop. After the annual crop has com-
pleted its cycle, short-lived perennial crops such as yuca
(cassava) or papaya are planted. By the time these crops
are harvested, the bananas have formed a fairly continu-
ous canopy, producing bananas (or plantain) for local
use or sale. By the third year, the resprouted shade trees
have also begun to become part of the shade-producing
canopy. At this point, shade conditions at the soil sur-
face have returned to the reduced levels appropriate for