Light - Agroecology: The Ecology of Sustainable Food Systems - page 54

due to either successional development or intentional
human intervention make for a highly diverse light
environment that promotes one of the correspondingly
highest plant species diversities known for an agroeco-
system. Much needs to be known about the specific light
requirements and tolerances of each component of such
a system.
A study of the light environments of the nine different
agroecosystems in Mexico and Costa Rica provides some
impression of the possible variation in the structure and
characteristics of light environments. The data from this
study are presented in Table 4.1.
In general, the polycultures in the study were more
effective at intercepting light than the monocultures,
although the sweet potato monoculture, with its broad
leaves, intercepted light as effectively as the home garden
and the shaded coffee system. These mixed results point
out the difficulty of determining a system's efficiency of
light use. Simply measuring vegetative cover, LAI, and
the transmission of light to the surface does not by itself
elucidate how light is used by the components of the
system. Nor does it show how a well-designed system

can create a light environment that meets the needs of a
diversity of different plants at the same time.

T EMPORAL M ANAGEMENT

Over time, the light environment in an agroecosystem
changes. One type of change results from the growth of
the plants in the system, and another from seasonal
changes. Both kinds of changes can be taken advantage of,
modified, or used as cues for initiating specific techniques.
One kind of temporal management that takes advan-
tage of the changes in the light environment that occur as
a crop matures is the “over-sowing” of one crop into
another. This is done, for example, to produce an
oat/legume hay crop: instead of sowing the oats, harvesting
the oats, and then planting the legume cover crop (such as
clover or vetch), the seed of the legume can be sown when
the oats reach a particular stage of development and the
light environment is most conducive to the establishment
of the legume. Specifically, the legume is planted just
before the heads of oats begin to form, when light levels
at 3 in. above the soil are about 40% of full sunlight. Clover
seems to establish best around 50% of full sunlight, so
over-seeding that occurs just before heads start to form gets
the legume off to a good start. After the oats are harvested,
the light levels reaching the established clover plants
approach once again those of full sunlight, promoting the
rapid growth of this species as a nitrogen-fixing cover crop
(Figure 4.7).
Management of seasonal variations in light is common
in perennial and agroforestry systems. Coffee systems in
Costa Rica — the subject of considerable applied shade
management research — offer a good example of this form
of temporal light management (Lagemann and Heuveldop,
1982; Bellow and Nair, 2003). As discussed previously,
coffee is typically grown under the shade of trees, often
species of the leguminous genus Erythrina . Although
coffee is a very shade-tolerant plant, it suffers when shade
becomes too dense. This is especially true during the
wet-season time of the year, when relative humidity inside
the coffee cropping system stays close to 100% most of
the time, promoting fungal diseases that can cause coffee
defoliation and fruit drop. Therefore, a common practice
is to heavily prune the shade trees at the beginning of the
wet season (during June) in order to allow more light into
the interior, promoting drier conditions and hence a
reduced chance of disease. The greater cloud cover during
the wet season lessens the need for shade over the coffee.
Close to the end of the wet season (usually November or
December) another less intensive pruning occurs that opens
up the canopy of the plantation again, possibly promoting
not only the development of flower buds that open later in
the dry season, but also stimulating the turnover of nitrogen-
rich biomass that aids the more rapid growth of the coffee
plants during this period (Figure 4.8).

TABLE 4.1
Measures of the Light Environment in a Range of
Agroecosystems and Natural Ecosystems in Costa Rica
and Mexico

Trans-
mission
(%)

Cover
(%) Species LAI

2-month-old corn monoculture,
conventionally managed

7 1.0 56 35

3.5-month-old corn monoculture,
traditionally managed

20 2.6 88 12

Sweet potato, weeded and treated
with insecticide

8 2.9 100 11

2.5-year-old intercrop of cacao,
plantain, and the native timber
tree Cordia alliodora

4 3.4 84 13

Old wooded home garden
containing a diverse mixture of
useful plants

18 3.9 100 10

Coffee plantation with an overstory
of erythrina trees

7 4.0 96 4

Plots planted with useful plants
to mimic natural succession, 11
months after clearing

27 4.2 98 7

Gmelina plantation (trees grown for
timber and pulp intercropped with
beans and corn)

8 5.1 98 2

Plots undergoing natural
succession, 11 months after
clearing

35 5.1 96 <1

Source: Ewel et al., 1982. Agro-Ecosystems 7: 305-326.