Environmental Engineering Reference
In-Depth Information
SPECTRAL CHANGES
During the progress of radiation through the forest vegetation, considerable changes in its
spectral composition take place, as specific wavelengths are filtered out or scattered by
the canopy. The shorter wavelengths (i.e. blue light) are removed preferentially by the
leaves, while the amounts of longer-wave red and infra-red radiation increase. This
change in the composition of the light is responsible for the characteristic colours that we
encounter in woodland. It also makes the light less suitable for plant growth. As a result,
the range of plants that can survive on the forest floor is limited.
The woodland affects not only the inputs of radiation; it similarly affects outputs. The
manner of this modification is far more complex, for outgoing long-wave radiation comes
from a wide range of sources - from the atmosphere, the top of the canopy, the leaves
and branches of the trees, the undergrowth, and the soil surface. There is inevitably a
great deal of interception, absorption and re-emission of the long-wave radiation, so that
little escapes direct to space.
VARIATIONS OVER TIME
These patterns of microclimate are only averages. Considerable variations occur over
time, owing to changes in the inputs of solar radiation and to changes in the woodland
itself. If we measured short-wave inputs of radiation throughout the day we would find
that levels remained low
Table 8.3 Difference of temperature and relative
humidity (%) between the inside and outside of a
forest
Forest
January
April
July
October
Year
Deciduous broad-leaf
0·1
3·4
0·0
3·2
0·8
-0·8
0·5
1·1
0·3
2·2
Needle tree (conifer)
0·7
4·8
0·7
4·8
0·8
6·5
1·0
9·5
0·9
6·8
Japanese cedar
0·2
1·6
0·1
-1·1
0·4
1·5
0·2
0·5
0·2
0·8
Source
: After Yoshino (1975).
Note
: Positive values indicate that inside the forest is more humid.
with the exception of brief periods associated with the passage of sunflecks. During the
night, the vegetation traps and returns much of the outgoing long-wave radiation, so
cooling is slow.
This pattern also changes seasonally. In winter the inputs of radiation are low and the
effect of the forest on the microclimate diminishes. Moreover, in deciduous woodland,
the trees lose their leaves, so that there is much less interception and absorption. If we
compare woodland temperatures with those on open land, therefore, we find a much
smaller difference in winter. The effect of the woodland is at a maximum when the trees
are in full leaf and radiation inputs are high (Table 8.3). Even then the differences may
not be large. Studies in an Oxfordshire woodland found a mean temperature difference of
only 0·9° C between mature deciduous forest and adjacent grassland in summer.
