Environmental Engineering Reference
In-Depth Information
THE EFFECTS OF WOODLAND TYPE
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 more plants grow on the woodland floor. Similarly, pine trees give a less
dense canopy than do spruce; the dark, unvegetated floor of plantations of Sitka spruce
contrast with the much lighter conditions in pine woodland.
In addition, the nature of the understorey is important. An open canopy allows the
development of one or more layers of understorey plants, and these, too, intercept both
incoming and outgoing radiation. The extreme example is shown by the tropical rain
forest. Although radiation inputs are high, the successive layers of trees, bushes and
shrubs intercept so much radiation that only small amounts reach the forest floor.
WINDS IN WOODLAND
Patterns of wind in woodland are similar to those in grassland, although the zone of
modified flow extends to a much greater height. Above the canopy wind speed may
increase slightly, but as the canopy is approached, velocity falls rapidly. Lowest wind
speeds are often found within the leafy canopy, and where the undergrowth is also dense,
velocities may remain low. In most cases, however, the main trunk zone is more open, so
there is less interference with air flow and wind speeds increase again. Near the ground,
friction and the effect of low-growing plants cause velocity to fall once more. Complex
patterns of flow often develop in the forest, with local funnelling and deflection of the
wind. We can often see the results of these flow patterns in the distribution of dead leaves
on the woodland floor. Sheltered areas trap deep layers of leaves, which, by decay, will
add nutrients to the soil, while more exposed zones are swept clear by the wind.
MOISTURE IN WOODLAND
In general, vapour pressure is slightly higher in a forest or in woodland than outside it.
This is mainly due to the large area covered by the leaves in a forest, which transpire
moisture into the atmosphere, from where it is not easily dispersed because of the lighter
winds. In some forests there may be few actively transpiring leaves near the ground so
modifying the pattern. On the other hand, the interception of mois-ture by vegetation
reduces the amount of water available at the forest floor, so the net effect on humidity
levels is small.
As daytime temperatures are cooler than those outside, the relative humidity of the air
should also be greater even if the forest atmosphere contained the same absolute amount
of water vapour. Experiments suggest values about 5 per cent above those outside,
though the precise differences depend upon the type of woodland as well as on the time
of year and the weather conditions (Table 8.3).
