Agriculture Reference
In-Depth Information
) values of relative irradiance
across a row of hedgerow orchards at 2.9
Figure 7.5
Comparison of observed (
0.9 m spacing under
diffuse light conditions with those predicted using a 45
◦
leaf angle
(solid line) and a spherical leaf angle distribution (broken line). From
Palmer (1977b). Reproduced with permission.
×
orchards consist of trees that are spatially separated within as well as between
rows. In this situation light may be received on the lower, but outer, portions
of the tree canopy without passing through foliage vertically above. It may,
however, have been attenuated by passage, at an angle, through the upper part
of the foliage of an adjacent tree or row of trees. It may also be intercepted by
stems or fruits.
The light transmission to the ground, or to any particular point in the
canopy, can be calculated as a process in which beams of light pass through the
canopy in the manner of a point quadrat. Penetration depends on the altitude
and azimuth of the source of light during the day, the areas and arrangement
of leaves, stems and fruits per unit canopy volume and separate light extinc-
tion coefficients for each of these shading structures. Computer models have
been developed, both for direct light penetration and interception, and for
that of diffuse light. The models of Palmer (
b)
give quite close correspondence with measured values of light transmission
to the orchard floor (Figure
b) and Wagenmakers (
). Wagenmakers' simulations showed that cor-
rect assessment of crown size and leaf area were very important to accurate
.
