Geoscience Reference
In-Depth Information
radiation at the top of the canopy is stored in the
soil and the trees. Dense red beeches (
Fagus
sylvatica
) intercept 80 percent of the incoming
radiation at the treetops and less than 5 percent
reaches the forest floor. The greatest trapping
occurs in sunny conditions, because when the sky
is overcast the diffuse incoming radiation has
greater possibility of penetration laterally to the
trunk space (
Figure 12.11
). Visible light, however,
does not give an altogether accurate picture of
total energy penetration, because more ultraviolet
than infrared radiation is absorbed in the crowns.
As far as light penetration is concerned, there are
great variations depending on type of tree, tree
spacing, time of year, age, crown density and
height. About 50-75 percent of the outside light
intensity may penetrate to the floor of a
birch-beech forest, 20-40 percent for pine and
10-25 percent for spruce and fir, but for tropical
forests in the Congo the figure may be as low as
0.1 percent, and 0.01 percent has been recorded
for a dense elm stand in Germany. One of the
most important effects of this is to reduce the
length of daylight. For deciduous trees, more than
70 percent of the light may penetrate when they
are leafless. Tree age is also important in that this
controls both crown cover and height.
Figure
12.11
shows this rather complicated effect for
spruce in the Thuringian Forest, Germany.
Modification of airflow
Forests impede both the lateral and the vertical
movement of air. In general, 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 30m of penetration
reduces wind velocities to 60-80 percent, 60m to
50 percent and 120m to only 7 percent. A wind of
2.2m s
-1
outside a Brazilian evergreen forest was
reduced to 0.5m s
-1
at 100m within it, and was
negligible at 1000m. In the same location, external
storm winds of 28m s
-1
were reduced to 2m s
-1
some 11km deep in the forest. Where there is a
complex vertical structuring of the forest, wind
velocities become more complex. Thus, in the
crowns (23m) of a Panama rainforest the wind
velocity was 75 percent of that outside, while it was
only 20 percent in the undergrowth (2m). 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 12.12
. In a
Tennessee mixed oak forest, forest wind velocities
were 12 percent of those in the open in January,
but only 2 percent in August.
Knowledge of the effect of forest barriers
on winds has been utilized 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 landscape. 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 percent penetrability (
Figure 12.13
) gives
the maximum protection. An obstruction begins
to have an effect about 18 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
).
30
30
120
(A)
(B)
Crown
tops
100
Canopy
80
20
20
60
Trunk
space
Growing
spruce
forest,
Thuringia
40
10
10
Mature beech
forest, Austria
20
Crowns develop
0
0
0
0
20
40
0 10 20
% of brightness in open country
60
80
100
30 40 50
Figure 12.11
The amount of light beneath the
forest canopy as a function of cloud cover and crown
height: (A) For a thick stand of 120-150-year-old red
beeches (Fagus sylvatica) at an elevation of 1000m
on a 20° southeast-facing slope near Lunz, Austria;
(B) For a Thuringian spruce forest in Germany over
more than 100 years of growth, during which the
crown height increased to almost 30m.
Source: After Geiger (1965). By permission of Rowman and
Littlefield.
