Crop diversification for weed management - Ecological Management of Agricultural Weeds - page 353

Once orchards and plantations are established and productive, the poten-
tial for weed infestation depends on the proportion of available resources cap-
tured by the trees. When trees achieve nearly complete resource use, as in
dense plantations, weed problems are often minimal, though vines, parasitic
canopy weeds, and undesirable trees can invade. In other cases, practical man-
agement factors such as access for annual harvest or wide spacing for fruit
quality and pest control preclude maximal resource capture.This fosters weed
persistence.
Following a multiyear period of peak production,orchards and plantations
enter a period of replanting or renovation. Weed pressure and weed manage-
ment strategies during renovation depend on resident weed vegetation, the
soil seed bank, and inputs to the seed bank. Since many orchard and planta-
tion systems have a productive life of 10 to 30 years or more,weedy vegetation
in the last years before renovation represents the accumulated effectiveness of
weed management over an extended period.
Other agroforestry systems, such as forest fallow and alley cropping, are
cyclic, with regular, periodic shifts in the relative proportions of woody and
non-woody components. When annual crops are produced in cyclic systems,
trees are temporarily suppressed, which favors weed growth. Later, however,
when tree growth occurs during fallow periods, weed growth and reproduc-
tion are reduced by competition from trees. This managed fluctuation is
essential to maintaining both soil productivity and annual crop production.
From an ecological perspective, weed management in agroforestry systems
differs from that in annual cropping systems because it often exploits succes-
sional phenomenon, i.e., long-term, directional changes in species composi-
tion and environmental conditions driven by species interactions.
Additionally, because distances between woody perennials in agroforestry
systems generally are farther than those between crop plants in annual
systems, agroforestry provides more opportunities for spatially manipulating
growth resources to the advantage of crops and detriment of weeds.Trees can
also produce large quantities of leaves and branches that can be used as mulch
for weed suppression. Integrating these processes to achieve the desired
balance between tree growth, annual crop production, weed suppression, and
soil improvement is the major management challenge that must be addressed
in agroforestry systems.
Forest fallow systems and weed dynamics
Historically, in many tropical and temperate agricultural systems,
crop fields were surrounded by forests,and trees re-established in fields after a
few years of crop production. After several decades of growth, trees were cut