Agriculture Reference
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estimation of light interception and she concluded that fruit contributed little.
Charles-Edwards and Thorpe (
) found that a model using only leaf
areas and tree dimensions gave simulated transmission values very close to
experimental measurements.
Solar altitude and azimuth vary throughout the day, throughout the year,
and with latitude. Light interception and penetration vary not only with tree
or hedgerow dimensions but also with canopy density and hedgerow orien-
tation. The range of potential orchard designs made possible by the use of
rootstocks giving different degrees of vigour control and of training and prun-
ing systems to control tree shape, together with the change of tree dimensions
over the years, limits relevant field investigations. The computer modelling
approach enables light interception and light levels within canopies to be cal-
culated for orchards with different configurations and leaf area densities at any
latitude over the chosen period (Figures
). Results are conventionally
expressed as percentage interception and percentage of above-canopy irradi-
ance but can readily be converted to absolute values if irradiance at the site is
known.
An alternative modelling approach is to consider light transmission to the
orchard floor ( T ) to be made up of that which misses the trees altogether ( T f )
and that which penetrates through the tree canopy ( T c ) including sunflecks on
the ground within the cast shadow area, i.e.
.
,
.
T
=
T f +
T c
(
.
)
In this T f depends on tree form (size and shape) and between-tree spacing,
and T c on leaf density within the tree outline expressed as leaf area per unit of
shaded area, i.e. the cast-shadow area including sunflecks ( Jackson
). The
fractional interception of light ( F ) can then be calculated from the fractional
interception which would occur if the trees were opaque ( F max which equals
T f ) minus the transmission through the trees. This transmission, from
equation
, will be F max e KL where F max is the ground area shaded and L
is the projected leaf area on the cast shadow area, i.e. LAI divided by F max .
Thus, from Jackson and Palmer (
.
,
),
F max e KL
F
=
F max
(
.
)
F max can be determined using relatively simple computer models assuming
opaque 'trees' and calculating their interception of direct and diffuse light
throughout the day and season at different latitudes ( Jackson and Palmer,
). It can also be
measured using opaque scale models of orchards placed on surfaces such
as solar panels or arrays of cosine-corrected sensors giving electrical output
proportional to incoming radiation ( Jackson,
), or by computer graphics ( Johnson and Lakso,
; Middleton and Jackson,
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