Disturbance, Succession, and Agroecosystem Management - Agroecology: The Ecology of Sustainable Food Systems - page 248

O PTIMIZING P OSITIVE I MPACTS OF T REES

Knowledge of both the positive and negative impacts of
trees on the rest of the agroecosystem is essential to fully
and effectively integrating trees into the system. The posi-
tive impacts discussed above need to be balanced with the
possible negative impacts of trees. These include compet-
itive or allelopathic interference between trees and other
crops, microclimate modification that creates conditions
favoring disease or pest outbreak, and damage to crop
quality caused by branches or fruits falling from mature
trees. These negative effects of trees can usually be
avoided or mitigated by appropriate spatial arrangement
of the trees, choice of tree species, choice of annual spe-
cies, timing of planting, and pruning. Integration of trees
takes extensive knowledge of the full range of ecological
interactions that can occur.

A B C

Boundary planting Shelterbelts Alley cropping

D E F

Trees in fields Trees in fields Rotational fallow

FIGURE 17.8 Models for the arrangement of trees in agrofor-
estry systems (Adapted from Young, A. 1989. In W. S. Reifsnyder
and T. O. Darnhofer (eds.), Meteorology and Agroforestry .
pp. 29-48. International Council for Research in Agroforestry:
Nairobi, Kenya. With permission.)

M ANAGING I NTERDEPENDENCY

As our knowledge of the ecological processes taking place
in complex agroforestry systems becomes more complete,
we can begin to see how the different components of such
systems become interdependent. An annual cropping com-
ponent can become dependent on the trees for habitat
modification, nutrient capture from deeper depths in the
soil, and harboring of beneficial insects. The presence of
the cropping component in the system can displace inva-
sive non-crop plants that might interfere with the growth
of the trees. Animals benefit from the high net primary
productivity of the annual or short-lived crop or forage
part of the system, and return nutrients to the soil in the
form of urine and manures (for further discussion of the
role of animals in agroforestry systems, Chapter 19). Man-
agement of agroforesty systems should focus on maximiz-
ing the benefits of these complex sets of ecological inter-
dependencies.
We must also remember that ecological interdepen-
dencies are only part of the picture. Humans are dependent
on trees in agroecosystems for such items as fire wood,
construction material, browse for animals, fruits and nuts,
spice, and medicinals. Agroforestry systems can be
designed and managed with these needs in mind, so that
the trees serve important roles both ecologically and eco-
nomically. When this occurs, an interdependency can
develop between the farming community and its farms.

If the primary emphasis of the farmer is on silvopas-
toral activities, with trees intended to provide living
fences, wind breaks, occasional forage from prunings,
and harvestable products such as firewood or fruit, then
a boundary planting of trees around areas of pasture (a) may
be the best design. If, in another case, wind is a problem,
but the focus is on crop production, a shelterbelt or
windbreak system (b) may be best. When the tree
component is intended to provide mulch from leaf fall
or prunings to enhance crop production, shelterbelts can
be narrow tree rows between alleys used for agriculture
(c). When the trees also have agricultural value, they
may be dispersed amongst the cropping system or
pasture, either uniformly (d) or more randomly (e).
Finally, if soil conditions are so poor that permanent
cropping or grazing is not feasible, a rotational design
(f) can be employed where the successional period dur-
ing tree development is determined by a range of factors
similar to those used to determine the length of fallow
needed in shifting cultivation. A thorough understanding
of the interaction, integration, and interdependency of
all components of the system will ultimately help in
determining trees' spatial arrangement and how it may
change over time.

TROPICAL HOME GARDENS

S PATIAL A RRANGEMENT OF T REES

An agroforestry system with great complexity and diver-
sity, as well as opportunities for maintaining a mosaic of
stages of succession, is the tropical home garden system.
It is probably one of the most complex and interesting
types of agroecosystems, and one we have much to learn
from regarding resource management for a sustainable
agriculture (Méndez, 2000; Nair, 2001; Kumar and Nair,
2004) (Figure 17.9).

Trees can be arranged in an agroforestry system in a
variety of different ways. The pattern used will depend on
the needs of the farmer, the nature of the agroecosystem,
and the local environmental and economic conditions. As
an example, Figure 17.8 shows six different ways that the
same percentage of ground in an agroecosystem can be
covered by trees.