Crop diversification for weed management - Ecological Management of Agricultural Weeds - page 352

labor intensive, highly dependent or largely independent of purchased
inputs, and oriented toward maximizing income or minimizing risk. The
defining characteristic of all of these systems is that they include woody per-
ennials and herbaceous species that interact over periods ranging from
months to years (Young, 1989, p. 11). Agroforestry systems are also character-
ized by a low frequency of tillage.
Woody perennials perform a variety of functions in agroforestry systems.
Trees and shrubs interplanted with annual crops can provide farmers with
fuel and construction materials.They can also be used to add organic matter to
soil, thereby improving water holding and cation exchange capacities, and
increasing microbial activity (Nair, 1984, pp. 31-46; Young, 1989, pp.
93-150). Interplanting deep-rooted tree species, such as Erythrina and Inga
spp., with shallower-rooted crops, such as coffee, fosters nutrient recovery
from lower soil horizons, and recycles nutrients through decomposing leaf
litter and prunings (Aranguren,Escalante & Herrera,1982).Leguminous trees
can also fix atmospheric N and provide it to associated crops through decom-
posing residues (Young, 1989, pp. 130-43).The multistory canopies and deep
roots of some agroforestry systems minimize soil erosion and reduce soil and
leaf temperatures, which is particularly valuable in hot regions (Willey, 1975;
Young, 1989, pp. 53-77). Shade cast by overstory trees can benefit the physio-
logical performance of certain shorter tree and bush crops, such as coffee, tea,
and cacao (Willey, 1975).
Because they are larger and longer-lived than annual crops, trees and
shrubs more strongly affect their environment. Consequently, cropping
systems that include trees offer unique opportunities and challenges for weed
management.
Temporal patterns of weed infestation in agroforestry systems
As in annual crop systems, weed growth and interference in agrofo-
restry systems depend on the presence of weed propagules,conditions for ger-
mination, and resources available for growth, all of which vary over time.
During establishment and early growth phases, trees capture only a small
fraction of available resources.Weeds present in the first months or even years
after tree establishment are a function of the previous use of the site and the
seed rain from surrounding vegetation. Annual crops are often intersown
among trees during early stages of system development to capture as crop
yield some of the resources that would otherwise be captured by weeds. This
period may last from one to three or more years. As trees increase in size and
ability to capture resources, they become less susceptible to interference from
annual and perennial herbaceous weeds.