Crop diversification for weed management - Ecological Management of Agricultural Weeds - page 354

and burned, and annual crops were planted again. In recent years, numerous
factors, including human population increase and land conversion to export
crops and pastures, have led to shorter fallow periods and a reduction in tree
cover relative to cropped area.
Saxena & Ramakrishnan (1984) investigated weed responses to shortened
periods of forest fallow in northeastern India,where the interval between suc-
cessive crops has declined from 20 to 30 years to four to five years. In their
study, weed seed density, weed plant density, and weed biomass were more
than twice as high in fields cropped after four and six years of fallow in native
vegetation than in fields cropped after 10 and 20 years of fallow. As a result,
fields in the area generally were hand-hoed twice after long fallow periods,but
required three or four hoeings after shorter fallow periods. Saxena &
Ramakrishnan (1984) noted that repeated crop cycles with only four to six
years of fallow resulted in an “arrested succession”in which herbaceous weedy
species with high reproductive potential persisted through fallow periods. In
contrast, with longer fallow periods, weedy species were replaced by bamboo,
trees, and shade-tolerant herbs, and weed seeds and perennating vegetative
parts were largely destroyed by decay, physiological exhaustion, and
predation.
On river bottom lands in Amazonian Peru, four- to six-year periods of tree
fallow are rotated with a sequence of short and medium cycle crops that are
progressively more tolerant of weeds (Staver, 1989 a ). Indigenous farmers cut
and burn trees and shrubs, and then plant maize and cassava. Plantain is
added during the first six months of crop production and cassava may be
planted a second time after the first crop is harvested. During the second and
third years of cropping, which are dominated by plantain, weed biomass
increases, insect pests of plantain multiply, and crop nutrient deficiencies
become more prevalent (Staver, 1989 a ). Weeding frequency declines during
the same period. Shrubs and trees begin to re-emerge in the field, although
the harvest of plantain continues. As trees form a complete overhead canopy,
herbaceous weeds begin to disappear. Staver (1991) found that herbaceous
weed biomass in this system was eliminated by two to five years of fallow, but
that readily germinable weed seeds in the soil continued to decline through
10 years of tree cover (Figure 7.9). Typically, fields are cleared again after fal-
lowing for three to five years,even though viable weed seeds are still present in
the soil. As a result, early weed control in the maize-cassava-plantain relay
sequence is necessary to minimize crop yield reductions.
In this system, trees and shrubs act both as competitors with crops and as
agents of soil restoration and weed control. In the early stages of crop produc-
tion, trees and shrubs sprout from stumps and roots or germinate from the