Environmental Engineering Reference
In-Depth Information
15 and 20 being perhaps the most common and with maxima above 50 for some
crops (Smil 2008). Average per capita requirements for shifting cultivation range
from as much as 10 to less than 2 ha of land in fallow and under the crops, and
the area actually cultivated is most commonly between 5% and 20% of those totals.
This translates to population densities as low as 10/km
2
and mostly between 20 and
40/km
2
, or an order of magnitude above the rates of 1-2/km
2
typical of pastoral
societies.
Specii c claims made by shifting cultivation on the biosphere's phytomass vary
widely, with the difference due mainly to the kind of cleared vegetation and to the
length of the fallow period. Short fallows of less than i ve years, removal of relatively
thin secondary (or later) growth, and a single year of cropping may claim only a
small fraction of phytomass that is destroyed by burning primary growth, followed
by a longer (more than 10 years) fallow period. Detailed monitoring of a Javanese
short-rotation system—one year of mixed vegetables followed by a year of cassava
followed by four years of bamboo fallow—showed the importance of the regenera-
tive period: at its end, organic matter in the top 25 cm of soil had increased by
about 7 t/ha, an outcome well justifying a traditional saying that “without bamboo,
the land dies” (Christanty, Mailly, and Kimmins 1996). Two specii c examples based
on data obtained by i eld studies illustrate the overall impact of short- and long-
rotation practices (box 8.1).
Even if the population densities supportable by shifting cultivation were to cluster
rather tightly around 20-30 people/km
2
and even if we knew approximate totals of
people engaged in these practices in the past, we still could not calculate the overall
claims on the biosphere because the standing phytomass destroyed by felling or
slashing and burning can vary by a factor of four. Shifting cultivation still survives
in some tropical and subtropical regions, and some of its practitioners—including
the Hanunoo of the Philippines (Conklin 1957), the Kekchi Maya of Guatemala
(Carter 1969), the Tsembaga of New Guinea (Rappaport 1968), the Yukpa of
Colombia (Ruddle1974), the Iban of Sarawak (Freeman 1980), the Kantu' of
Kalimantan (Dove 1985), and the swidden cultivators of India's Manipur state
(Shah 2005) and China's Yunnan province (Yin 2001)—were studied in considerable
detail by modern anthropologists.
Perhaps the most revealing conclusion is that they found fairly similar and rela-
tively high net energy returns (the quotient of the food energy in harvested crops
and the energy spent in their cultivation) ranging between 15 and 25. But McGrath
(1987) correctly pointed out that what has traditionally been seen as a highly pro-
ductive system when assessed in terms of energy outputs and inputs is really an
