Disturbance, Succession, and Agroecosystem Management - Agroecology: The Ecology of Sustainable Food Systems - page 243

development. The trade-offs and constraints change. In the
early stages of development, for example, constant removal
of NPP restricts the accumulation of biomass, whereas
restricted harvest of NPP forces a farmer to wait several
years for harvest. Grazing animals can help accelerate bio-
mass turnover as long as their manures are kept in the
system. At the intermediate stages of development, NPP is
high enough for part of it to be harvested as fruit or nuts
and part allowed to accumulate as biomass. By the later
stages (e.g., stage 7 in Figure 17.3), NPP declines to a low
enough level that a workable strategy is to allow all new
NPP to accumulate as biomass, and to harvest the biomass
selectively for fuel, timber, forage, paper pulp, or even food.

1. Bare soil 2. Annual monoculture

4. Polyculture of mixed annuals and
short-lived perennials

3. Annual polyculture

5. Annual / perennial polyculture with
tree seedlings

6. Agroforestry

M ANAGING S UCCESSIONALLY D EVELOPED
A GROECOSYSTEMS

Once a successionally developed agroecosystem has been
created, the problem becomes one of how to manage it.
The farmer has three basic options:

7. Tree crop agroecosystem

FIGURE 17.3 Steps in the successional development of an
agroecosystem. At any stage in the process, disturbance can be
introduced to bring all or part of the system back to an earlier
stage of development.

• Return the entire system to the initial stages of

succession by introducing a major disturbance,
such as clear-cutting the trees in the perennial
system. Many of the ecological advantages that
have been achieved will be lost and the process
must begin anew.

2

NPP

NPP
(tons/ha/
year)

100

Biomass
(tons/ha)

1

• Maintain the system as a perennial or tree crop

agroecosystem, with or without livestock.

Biomass

50

• Reintroduce disturbance into the agroecosys-

tem in a controlled and localized manner, taking
advantage of the intermediate disturbance
hypothesis and the dynamics that such patchi-
ness introduces into an ecosystem. Small areas
in the system can be cleared, returning those
areas to earlier stages in succession, and allow-
ing a return to the planting of annual or short-
lived crops. If care is taken in the disturbance
process, the below-ground ecosystem can be
kept at a later stage of development, whereas
the above-ground system can be made up of
highly productive species that are available for
harvest removal. Such a mixture of early and
later stages of development leads to the forma-
tion of a successional mosaic. This mosaic can
be adjusted and managed according to the eco-
logical conditions of the area, as well as
the needs of the farmer and changes in market
conditions. It can also incorporate livestock.

0 0

t 1 t 2 t 2 t 4

Time

FIGURE 17.4 Change over time in the relationship between
annual net primary productivity (NPP) and accumulated
living and dead biomass in a representative successionally
developing ecosystem. A time interval (e.g., one season) in the
early stages of succession (such as t 2 -t 1 ) will witness a rapid
increase in NPP, whereas NPP will decline slightly during a time
interval of similar length (such as t 4 -t 3 ) during the latter stages
of succession. (Modified from Whittaker, R. H. 1975. Commu-
nities and Ecosystems . 2nd ed. MacMillan: New York.; Odum,
E. P. 1993. Ecology and Our Endangered Life-Support Systems .
Sinauer Associates Incorporated: Sunderland, Massachusetts.
With permission.)

developmental process, and provide the greatest amount of
harvestable material in the shortest time. This could also be
the point at which the most biomass is available for grazing
animals. In the later stages of development (e.g., Stage 7 in
Figure 17.3), when the rate of NPP begins to decrease, stand-
ing biomass (in the form of accumulated perennial biomass)
is relatively high, but the actual amount of new harvestable
material produced in each time interval begins to drop.
The changing relationship between NPP and biomass
over time determines what management and production
strategies can be used at each stage of agroecosystem

The latter option provides the most advantages and
offers the greatest flexibility to the farmer. Within the
constraints imposed by the ecological limits of the crop-
ping region, the final mixture of annual and perennial
plants and grazing animals can be tailored to the needs of