Environmental Engineering Reference
In-Depth Information
large parts of Asia, Africa, and the Americas it not only preceded permanent crop-
ping but has also coexisted with it, often literally side by side. Shifting cultivation
was not only the foundation of ancient Mesoamerican cultures, it left its mark on
relatively large areas of the Amazon.
Those regions are readily identii ed by their soils:
terra preta
(or, more precisely,
terra preta do índio
) can be up to 2 m deep; it contains relatively large amount of
charred phytomass residues as well as elevated levels of organic matter from crop
residues, human waste and bones (often in excess of 13% of total organic matter,
that is, more than 5% C); and it was created by centuries of shifting cultivation,
mostly between 500 BCE and 1000 CE (Glaser 2007; Junqueira, Shepard, and
Clement 2010). Lehmann et al. (2003) estimated that
terra preta
soils cover no more
than 0.3% of the low-lying parts of the Amazon basin, but other estimates are up
to an order of magnitude higher.
In Asia, permanently cropped valleys and shifting cultivation in neighboring hills
and mountains have coexisted for millennia, and in the southeastern part of the
continent the forest clearing for cultivation has survived into the twenty-i rst century
(Allan 1965; Watters 1971; Okigbo 1984). Swidden cultivation was also common
in the temperate and boreal zones of preindustrial North America: Canada's Hurons
were among the northernmost practitioners, cultivating corn and beans for at least
5 and up to 12 years before clearing a new patch of forest and letting the land
regenerate for 35-60 years (Heidenreich 1971). With a typical corn harvest of 1.5
t/ha and a 50-year rotation cycle, they could support between 10 and 20 people/ha.
Shifting cultivation was not uncommon in medieval Europe—even in the Île-de-
France region around Paris it was practiced until the early twelfth century (Duby
1968)—and it survived in parts of Central and northern Europe and Siberia not
only into the nineteenth but even to the middle of the twentieth century. In Ger-
many's Ruhrgebiet, tree fallows (harvested for bark and charcoal) alternated with
rye until the 1860s (Darby 1956); rye was also used in Siberian shifting cultivation
until the 1930s; and in Finland of the late 1940s birch and pines (cut and sold as
logs) were followed by alder, whose burning prepared the soil for turnips. Other
crops used in European shifting cultivation in Scandinavia, Central and Eastern
Europe, and northern Russia (in some cases until the 1950s) included barley, l ax,
millet, oats, and wheat, and fallow periods in the sub-Arctic forests were as long as
40 years.
Net energy returns of shifting cultivation (quotients of harvested edible phyto-
mass and energy consumption above basal metabolism) are as low as six- to tenfold
and as high as 20- to 30-fold in various tropical environments, with rates between
