Environmental Engineering Reference
In-Depth Information
Box 8.1
Shifting Cultivation: Phytomass Removal and Harvests
In Central America
milpa
, a plot of land devoted to the shifting cultivation of corn
supplemented by half a dozen (interplanted and planted in sequence) fruit and vege-
table species, is managed by a short cycle of four years of forest fallow and one year
of crops (Carter 1969). An average corn yield of just over 1 t/ha (15 GJ/t) means that
(assuming a 20% preharvest loss to wild animals and pests) the edible yield is about
11 GJ/ha planted. The short rotation entails clearing relatively thin secondary growth;
even if it stores no more than 50 t/ha or (at 16 GJ/t of dry matter and a 90% removal
rate after slashing and burning), the total is about 2.9 TJ of woody phytomass from
the four fallowed hectares. More than 99.5% of the total phytomass removed from
the site on a i ve-year fallow/crop cycle is thus the cleared woody matter, and with an
average annual food requirement of about 10 GJ per capita, this cultivation will need
about 4.5 ha per person, supporting about 20 people/km
2
.
A very different shifting cultivation was studied by Rappaport (1968) in New
Guinea: a long (15-year) fallow and a highly productive two-year gardenlike polycul-
ture of more than 20 root (taro, yam, cassava), vegetable, and fruit species. The planted
area is thus only about 12% of all managed land, and with forest phytomass stores at
150 t/ha and an 80% removal rate, every cycle would destroy 1,800 t of woody matter,
or about 29 TJ. Even if the annual edible harvests were as high as 25 GJ/ha, in two
years they would still amount to less than 0.2% of all the phytomass removed from
the affected land during the 17 years of a fallow/crop cycle. With annual food require-
ments at 10 GJ per capita, this shifting cultivation would need nearly 3.5 ha of fallow
and gardens per person and could support nearly 30 people/km
2
.
extremely unproductive practice once the total phytomass contributions are taken
into account. And the studies encountered a variety of location-specii c practices,
which does not make for any easy quantii cation of typical impacts.
Most recently, a global project on alternatives to slash-and-burn cultivation
quantii ed a wide range of possible carbon stock changes (Palm et al. 2004). While
undisturbed tropical forests averaged 300 t C/ha and managed and logged forests
stored mostly between 150 and 250 t C/ha, short fallow (4-5 years), shifting culti-
vation replaced them with vegetation storing no more than 5-10 t C/ha—but long
fallow (more than 20 years) and more complex alternatives (including agroforestry
and intensive tree crops) could store an order of magnitude more of phytomass,
between 50 and 100 t C/ha. Interestingly, the latest studies of shifting agricultures
in Southeast Asia, the region with the highest remaining concentration of such
farmers, have been reappraising the practice at a time when it is rapidly receding
(Padoch et al. 2008).
