Geoscience Reference
In-Depth Information
obviously important. In temperate forests the sizes may
be closely similar, whereas in tropical forests there
may be great local variety. Crown coverage determines
the physical obstruction presented by the canopy to
radiation exchange and airflow.
Different vertical structures in tropical rainforests
and temperate forests can have important microclimatic
effects. In tropical forests the average height of the taller
trees is around 46 to 55 m, with individual trees rising
to over 60 m. The dominant height of temperate forest
trees is generally up to 30 m. Tropical forests possess
a great variety of species, seldom less than forty per
hectare (100 hectares = 1 km
2
) and sometimes over 100,
compared with less than twenty-five (occasionally only
one) tree species with a trunk diameter greater than 10
cm in Europe and North America. Some British wood-
lands have almost continuous canopy stratification,
from low shrubs to the tops of 36-m beeches, whereas
tropical forests are strongly stratified with dense under-
growth, simple trunks, and commonly two upper strata
of foliage. This stratification results in more complex
microclimates in tropical forests than temperate ones.
It is convenient to describe the climatic effects of
forest stands in terms of their modification of energy
transfers, airflow, humidity environment and thermal
environment.
of the trees and their density. Coniferous forests have
albedos of about 8 to 14 per cent, while values for
deciduous woods range between 12 and 18 per cent,
increasing as the canopy becomes more open. Values
for semi-arid savanna and scrub woodland are much
higher.
Besides reflecting energy, the forest canopy traps
energy. Measurements made in summer in a thirty-year
old oak stand in the Voronezh district of Russia, indicate
that 5.5 per cent of the net radiation at the top of the
canopy is stored in the soil and the trees. Dense red
beeches (
Fagus sylvatica
) intercept 80 per cent of the
incoming radiation at the treetops and less than 5 per
cent 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 12.11A). Visible light, however, does not give
an altogether accurate picture of total energy penetra-
tion, because more ultraviolet than infra-red radiation is
absorbed into 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 to 75 per cent of the
outside light intensity may penetrate to the floor of a
birch-beech forest, 20 to 40 per cent for pine and 10
to 25 per cent for spruce and fir. However, for tropical
forests in the Congo the figure may be as low as 0.1 per
cent, and 0.01 per cent 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 per cent of the light may
a Modification of energy transfers
Forest canopies change the pattern of incoming and
outgoing radiation significantly. The short-wave reflec-
tivity of forests depends partly on the characteristics
Figure 12.11
The amount of light
beneath the forest canopy as a func-
tion of cloud cover and crown height.
(A) For a thick stand of 120 to 150-
year-old red beeches (
Fagus sylvatica
)
at an elevation of 1000 m 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
30 m.
Source
: After Geiger (1965).
