Environmental Engineering Reference
In-Depth Information
tary maritime) foraging societies. But these extensive food production ways shared
yet another important commonality: in areas with low population densities they
proved to be extremely long-lived and resilient to change, while in other regions
they gradually morphed into various combinations of shifting and permanent
farming, or what might be best called seminomadic agropastoralism with some
foraging still present. Rising population densities brought a rather rapid end to these
modes of subsistence, mainly because of shorter soil regeneration cycles in shifting
cultivation and the overgrazing of many pastures.
Shifting cultivation begins by removing natural vegetation (preferably a second-
ary growth, which is usually easier to clear away than a primary forest), often by
burning or (once better tools became available) by felling large trees i rst and then
slashing younger growth before setting woody piles ai re, or simply by slashing a
secondary growth without burning. The resulting patches ranged from fairly level
surfaces that were often turned into densely planted gardenlike plots (interplanting
and multilayered polycultures were common) to roughly surfaced small i elds con-
taining remnants of large burned trees where crops were grown in mounds or
clumps. Regardless of the clearing method, shifting cultivation shared the underlying
strategy of exploiting the nutrients accumulated in plants and soil.
Phytomass burning recycles all mineral nutrients (K, Ca, Mg), and while nearly
all nitrogen in the burned phytomass is lost to the atmosphere as NO
x
, a great deal
of this key macronutrient remains in soil, and additional supply comes from the
mineralization of decaying (slashed or uprooted) vegetation, so that crops can be
grown for a few seasons without any other nutrient inputs. The staples of shifting
cultivation included grains (rice, corn, millet), roots (sweet potatoes, cassava, yams),
and squashes and various legumes (beans, peanuts), and the most diverse plantings
included one or even two scores of edible, i brous, and medicinal plants. Such plots
required a considerable amount of maintenance (weeding, hoeing, pruning, thin-
ning), and they also needed guarding or fencing, while many i elds with staple crops
were left in a semiwild state (subject to considerable damage by pests and animals)
until the harvest.
Relatively large areas of forest were needed in places where just one or two years
of cultivation were followed by long regeneration periods of more than 10 and even
as many as 60 years; more intensive cultivation with a few crop years followed by
only slightly longer (4-6 years) fallow periods were eventually quite common, and
in many locations higher population densities compressed the cycle to the shortest
fallow spans possible, with a crop or two followed by a single year of regeneration.
Australia was the only continent where this practice was never important, while in
