Agriculture Reference
In-Depth Information
Canopy light interception and
distribution
The light intensity
I
L
at any given depth in a continuous leafy canopy, like
a field of grass, is related to the vertically-summed leaf area index
L
(LAI
measured as m
leaf m
−
ground) above that plane according to equation
in which
I
is light intensity above the canopy and
K
the light extinction
coefficient.
.
I
L
I
=
e
−
KL
(
.
)
The total percentage interception can be calculated from this with LAI
summed down to ground level.
The light extinction coefficient depends on the average spectral properties
of the leaves and their orientation in relation to the spatial distribution of solar
radiation. In theory, stands with randomly spaced opaque horizontal leaves
have
K
=
; in practice
K
ranges from about
.
for planophile canopies to
for erectophile canopies. Clumping of leaves decreases the interception per
unit leaf area and so reduces the value of
K
. In practice
K
is usually calculated
using equation
.
I
and the intervening leaf area
index
L
. This derivation ignores the fact that some of the reduction in light
intensity from
I
to
I
L
may be a result of intervening stems, etc. but has the
practical advantage of relating the photosynthesizing leaf surface to irradiance
levels. For apple,
K
measured in this way ranges from
.
after measurement of
I
L
,
.
to
.
for PAR,
with a mean value of
).
In terms of light climate a continuous canopy can be considered as a series
of strata separated by horizontal contours of irradiance as measured on a
horizontal plane. The leaf area, in LAI terms, above the irradiance contour
at which the irradiance is
I
L
can be defined as
L
I
and is determined by
K
(equation
.
( Jackson,
,
.
).
L
I
=
(ln
I
L
)
/
(
−
K
)
(
.
)
or
L
I
is the total LAI if this is less.
Some apple orchards, e.g. mature orchards of 'open centre' bush trees,
horizontally trellised systems and very closely planted multirow bed systems
(Wagenmakers,
) may give virtually continuous canopies. In these cases,
and assuming an extinction coefficient of
%of
those above the canopy can only be achieved in the strata above a vertically-
summed LAI,
L
,of
.
, light levels of more than
.
, the
% irradiance contour would correspond with
an
L
of
.
Much more commonly, however, orchards consist of individual hedgerows
or trellised systems separated by alleyways and, in their early years, almost all
and the
% irradiance contour with an
L
of
.
