Geoscience Reference
In-Depth Information
used to indicate the wide variety of conditions that can
be experienced.
On a hot summer's day it is noticeable that tempera-
tures in a forest are much lower than outside, providing
a respite from the strong glare and baking heat of the
sun. Air movement is weak. It feels humid, and the impres-
sion is quickly gained of an entirely different climate. This
affects plant and animal life as well as people. Quite
different ecosystems develop because of the climatic
environment produced by the forest. Because of the
differences in scale, the microclimates within a forest are
more distinct than those in grassland or low crops.
(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 radia-
tion; 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.
Radiation exchanges in woodland
It is apparent on entering a forest that the forest canopy
cuts out much of the incoming radiation. Most of the
energy is absorbed by the tree canopy. A significant
proportion is reflected - about 5-15 per cent, on average,
although in some cases reflection may reach over 30 per
ground directly, normally in the form of small patches of
light called sunflecks. The remainder penetrates the
vegetation indirectly; it is scattered by the atmosphere and
arrives as diffuse radiation.
Variations over time
These patterns of microclimate are only averages. Con-
siderable 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 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
difference may not be large. Studies in an Oxfordshire
woodland found a mean temperature difference of only
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Spectral changes
During the progress of radiation through the forest
vegetation, considerable changes in its spectral composi-
tion take place, as specific wavelengths are filtered out
or scattered by the canopy. The shorter wavelengths
Table 8.2
Tree albedos (%)
Aleppo pine
17
Monterey pine
10
Loblolly pine
11
C between mature deciduous forest and adjacent
grassland in summer.
Lodgepole pine
9
Scots pine
9
Oak
Summer
15
The effects of woodland type
Spring
12
Eucalyptus
19
The microclimate of woodland depends very much upon
the type of woodland we are dealing with. For example,
deciduous trees show a strong seasonal change compared
with conifers. But considerable variations occur between
different species of deciduous trees. Birch leaves, for
example, are small and have a lower density than beech
or oak, so that, even when they are in full leaf, birch trees
allow more light to reach the ground surface. As a result
Sitka spruce
12
Norway spruce
12
Birch and aspen
Late winter
25
Orange trees
32
Tropical rain forest
13
Cocoa
16
