Geoscience Reference
In-Depth Information
penetrate when they are leafless. Tree age is also impor-
tant in that this controls both crown cover and height.
Figure 12.11B shows this rather complicated effect for
spruce in the Thuringian Forest, Germany.
Knowledge of the effect of forest barriers on winds
has been used in the construction of windbreaks to
protect crops and soil. Cypress breaks of the southern
Rhône valley and Lombardy poplars (
Populus nigra
)
of the Netherlands form distinctive features of the land-
scape. It has been found that the denser the obstruction
the greater the shelter immediately behind it, although
the downwind extent of its effect is reduced by lee
turbulence set up by the barrier. A windbreak of about
40 per cent penetrability (Figure 12.13) gives the maxi-
mum protection. An obstruction begins to have an effect
about eighteen times its own height upwind, and the
downwind effect can be increased by the
back coupling
of more than one belt (see Figure 12.13).
There are some less obvious microclimatic effects
of forest barriers. One of the most important is that the
reduction of wind speed in forest clearings increases
the frost risk on winter nights. Another is the removal
of dust and fog droplets from the air by the filtering
action of forests. Measurements 1.5 km upwind on the
lee side and 1.5 km downwind of a kilometre-wide
German forest gave dust counts (particles per litre) of
9000, less than 2000 and more than 4000, respectively.
Fog droplets can be filtered from laterally moving air
resulting in a higher precipitation catch within a forest
than outside. The winter rainfall catch outside a euca-
lyptus forest near Melbourne, Australia, was 50 cm,
whereas inside the forest it was 60 cm.
b Modification of airflow
Forests impede both the lateral and the vertical move-
ment of air (Figure 12.12A). Air movement within
forests is slight compared with that in the open, and quite
large variations of outside wind velocity have little
effect inside woods. Measurements in European forests
show that 30 m of penetration reduces wind velocities
to 60 to 80 per cent, 60 m to 50 per cent and 120 m to
only 7 per cent. A wind of 2.2 m s
-1
outside a Brazilian
evergreen forest was reduced to 0.5 m s
-1
at 100 m
within it, and was negligible at 1000 m. In the same
location, external storm winds of 28 m s
-1
were reduced
to 2 m s
-1
some 11 km deep in the forest. Where there
is a complex vertical structuring of the forest, wind
velocities become more complex. Thus in the crowns
(23 m) of a Panama rainforest the wind velocity was 75
per cent of that outside, while it was only 20 per cent
in the undergrowth (2 m). Other influences include the
density of the stand and the season. The effect of season
on wind velocities in deciduous forests is shown in
Figure 12.12B. In a Tennessee mixed-oak forest, forest
wind velocities were 12 per cent of those in the open in
January, but only 2 per cent in August.
Figure 12.12
Influence on wind-velocity profiles exercised by: (A) a dense stand of 20-m high ponderosa pines (
Pinus ponderosa
) in
the Shasta Experimental Forest, California. The dashed lines indicate the corresponding wind profiles over open country for general
wind speeds of about 2.3, 4.6 and 7.0 m s
-1
, respectively. (B) A grove of 25-m high oak trees, both bare and in leaf.
Sources
: (A) After Fons, and Kittredge (1948). (B) After Geiger and Amann, and Geiger (1965).
