Light - Agroecology: The Ecology of Sustainable Food Systems - page 49

FIGURE 4.5 Smog in the Valley of Mexico. The high level of air pollution in this mountain-ringed valley impacts light quality at
ground level. One of the peaks of Volcán Ixtacihuatl extends above the smog.

The relative rate of light transmission of a canopy is
expressed as the average amount of light that is able to
penetrate the canopy as a percentage of the total incident
light available at the top of the canopy or on the surface
of an adjacent area free of vegetation. Since we also know
that the change in average light penetration depends on
the density of the foliage and arrangement of leaves,
another way of determining the potential for light absorp-
tion of a particular canopy is to measure leaf-area index
(LAI). This is done by calculating the total surface area
of leaves above a certain area of ground; since the units
for both are identical (m 2 ), LAI becomes a unitless mea-
sure of the amount of cover. If the LAI is determined to
be 3.5, for example, the given area is covered by the
equivalent of 3.5 layers of leaves in the canopy, implying
that light will have to travel through that many layers
before reaching the ground. The height of each layer,
however, is an important determinant of the sequential
reduction of light as it travels through the canopy.
Not only is the more obvious measure of total light
intensity reduced as we enter deeper into the vegetative
cover, but also the quality of that light changes as well.
The “light of shade” usually has a very low amount of red
and blue light, and a relatively high amount of green and

IR light. This effect is particularly pronounced under
broad-leaved evergreen canopies. Conifer forests, on the
other hand, have much more red and blue light at the forest
floor because of the structure of the leaves (needles) and
the fact that they are much more reflective rather than
absorbing and transmitting of visible light.
Given the extreme variations in canopy structure
among natural vegetations and cropping systems, light lev-
els inside canopies are highly variable as well. They can
range from only a few percent of full sunlight at soil level
in a dense forest to nearly 100% of full sunlight in a
cropping system in the early stages of crop development.
The light intensity in a fully mature cotton crop is reduced
to 30% of full sunlight at a point halfway between canopy
top and soil surface, and is less than 5% of full sunlight at
the soil surface. The ways in which a squash crop, a corn
crop, and a corn/squash intercrop modify the light envi-
ronment under their canopies is illustrated in Figure 4.6.

PHOTOSYNTHETIC RATE

Once light is absorbed by the leaf and activates the pro-
cesses in the chloroplast that eventually lead to the pro-
duction of energy-rich sugars, differences in the actual