Disturbance, Succession, and Agroecosystem Management - Agroecology: The Ecology of Sustainable Food Systems - page 244

the farmer and farm community and adjusted to fit market
demand, the distance to market, the ability to enter into
the market, and the farmer's ability to purchase and trans-
port inputs. The closer the farm is to inputs, labor, and
markets, the heavier the emphasis can be on the annual
component.
The biggest challenge in managing a successionally
developed system is to learn how to introduce disturbance
in ways that stimulate system productivity on the one
hand, and provide resistance to change and variation
within the ecosystem on the other. This can be done in
many different ways depending on local environmental
conditions, the structure of mature natural ecosystems nor-
mally present, and the feasibility of maintaining modifi-
cations of those conditions over the long term.
For example, in the prairie region of the United States,
where a large percentage of the country's annual grain
production currently takes place, the use of a successional
model for designing a tree-less perennial grain system
(discussed in Chapter 13) might be the focus. Another
example applies to the rice-growing regions of the Yangtze
River valley of China, where the long-term maintenance

of paddy systems is based on knowledge of wetland eco-
systems, periodic flooding, and human alteration of paddy
soil. A successionally developed paddy rice agroeco-
system could incorporate a perennial component by using
trees that tolerate wet, flooded conditions, such as willows,
bald cypress, and other riparian or wetland species, and
by adding an animal component consisting of waterfowl
and fish (Figure 17.5).

AGROFORESTRY SYSTEMS

Although the perennial components of a successionally-
developed agroecosystem do not have to be trees, systems
with trees provide some of the best examples of how suc-
cessional development can be managed. The term agro-
forestry has been given to practices that intentionally retain
or plant trees on land used for crop production or grazing
(Wiersum, 1981; Nair, 1983). Such systems combine ele-
ments of crop or animal agriculture with elements of for-
estry, either at the same time or in sequence, building on
the unique productive and protective value of trees. There
are many variations in practices that fall into the category

FIGURE 17.5(a) Variations in the mixture of annuals and perennials in successionally developed agroecosystems. Corn and
beans grown for the local market are surrounded by persimmon trees in the urban fringe around Beijing, China (above). At a greater
distance from any markets, a rural farm in southern Costa Rica (right) concentrates on perennial shrub and tree crops.