Principles of terrestrial ecosystem ecology.pdf

108 5. Carbon Input to Terrestrial Ecosystems
carbon fixation by the canopy as a whole. This
is one of many examples in ecosystem ecology
in which selection for optimal performance at
the level of individuals gives rise to predictable
patterns at the level of ecosystems.
Leaf area is the major factor governing the
light environment experienced by individual
leaves within the canopy. There is a maximum
leaf area that plants in an ecosystem can attain,
because light is a directional resource that is
greatest at the top of the canopy and decreases
exponentially within the canopy, according to
the following equation:
I = I0e -kL (5.1)
where I is the irradiance (the quantity of
radiant energy received at a surface per unit
time) at any point in the canopy, Io is the irradiance
at the top of the canopy; k is the extinction
coefficient, and L is the projected leaf area
index (LAI; the leaf area per unit of ground
area) above the point of measurement.
LAI is a key parameter governing ecosystem
processes because it determines the light attenuation
through a canopy and strongly influences
the capacity of vegetation to gain carbon
and transfer water and energy to the atmosphere
(see Chapter 4). LAI has been defined in
two ways: (1) Projected LAI is the leaf area
projected onto a horizontal plane. (2) Total
LAI is the total surface area of leaves, including
the upper and lower surface of flat leaves
and the cylindrical surface of conifer needles.
Total LAI is approximately twice the value of
projected LAI, except in the case of conifer
needles, for which the projected leaf area is
multiplied by p (3.14) to get total leaf area.
Total LAI is particularly useful in describing
the effective leaf area of conifer forests, in
which leaves are more cylindrical than flat. A
flat leaf cannot absorb more photons than
move through a horizontal plane, because light
is a directional resource. Conifer needles can,
however, absorb considerably more light per
unit of projected leaf area than flat leaves.
Conifer needles are particularly effective in
absorbing diffuse light, which provides a more
uniform illumination of the overall canopy.
Diffuse light makes up a larger proportion of
total irradiance at low sun angles and under
cloudy conditions. This may explain the predominance
of conifers in high-latitude forests,
where sun angles are low and in temperate
rain forests, where conditions are usually
cloudy. Unfortunately, there is no consistent
agreement on whether data should be
expressed as projected LAI or total LAI, and
many review papers do not specify clearly
which definition of LAI is being used. Micrometeorologists
measuring radiation transfer and
ecologists working with broad-leaved forests
tend to use projected LAI, whereas ecophysiologists
interested in carbon exchange and
ecologists working in conifer forests tend to use
total LAI. Each definition of LAI has advantages
for addressing particular questions. The
important thing is to specify which definition is
being used.
Projected LAI varies widely among ecosystems
but typically has values of 1 to 8m 2 leaf
m -2 ground for ecosystems with a closed
canopy. The extinction coefficient is a constant
that describes the exponential decrease in irradiance
through a canopy. It is low for vertically
inclined or small leaves (e.g., 0.3 to 0.5 for
grasses), allowing substantial light penetration
into the canopy, but high for near-horizontal
leaves (0.7 to 0.8). Clumping of leaves around
stems, as in conifers, and variable leaf angles
result in intermediate values for k. Equation 5.1
indicates that light is distributed unevenly in an
ecosystem and that the leaves near the top of
the canopy capture most of the available light.
Irradiance at the ground surface of a forest, for
example, is often only 1 to 2% of that at the top
of the canopy. At very low irradiance, leaf
respiration completely offsets photosynthetic
carbon gain, resulting in zero net photosynthesis,
the light compensation point of the leaf (Fig.
5.8). A mature shaded leaf typically does not
import carbon from the rest of the plant, so the
leaf senesces and dies if it falls below the light
compensation point for extended periods of
time. This puts an upper limit on the leaf area
that an ecosystem can support, regardless of
how favorable the climate and the supply of soil
resources may be.
Do differences in light availability explain
the differences among ecosystems in carbon
gain? In midsummer, when plants of most