Geoscience Reference
In-Depth Information
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 1.5km upwind, on the lee
side and 1.5km downwind of a kilometer-wide
German forest gave dust counts (particles per
liter) 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 eucalyptus
forest near Melbourne, Australia was 500mm,
whereas inside the forest it was 600mm.
800
R
n
600
LE
400
H
200
Δ
S
0
-100
00
04
08
12
Hour
16
20
24
Figure 12.14
A computer simulation of energy
flows involved in the diurnal energy balance of a
primary tropical broad-leaved forest in the Amazon
during a high-sun period on the second dry day
following 22mm daily rainfall.
Source: After a Biosphere Atmosphere Transfer Scheme
(BATS) model from Dickinson and Henderson-Sellers (1988).
By permission of the Royal Meteorological Society, redrawn.
Modification of the humidity
environment
The humidity conditions within forest stands
contrast strikingly with those in the open.
Evaporation from the forest floor is usually much
less owing to the decreased direct sunlight, lower
wind velocity, lower maximum temperature, and
generally higher forest air humidity. Evaporation
from the bare floor of pine forests is 70 percent of
that in the open for Arizona in summer and only
42 percent for the Mediterranean region.
Unlike many cultivated crops, forest trees
exhibit a wide range of physiological resistance to
transpiration processes and, hence, the propor-
tions of forest energy flows involved in evapo-
transpiration (
LE
) and sensible heat exchange (
H
)
vary. In the Amazonian tropical broad-leaved
forest, estimates suggest that after rain up to 80
percent of the net solar radiation (
Rn
) is involved
in evapotranspiration (
LE
) (
Figure 12.14
).
Figure
12.15
compares diurnal energy flows during July
for a pine forest in eastern England and a fir forest
in British Columbia. In the former case, only
0.33
Rn
is used for
LE
due to the high resistance of
the pines to transpiration, whereas 0.66
Rn
is
similarly employed in the British Columbia fir
forest, especially during the afternoon. Like short
green crops, only a very small proportion of
Rn
is
ultimately used for tree growth, an average figure
being about 1.3W m
-2
, some 60 percent of which
produces wood tissue and 40 percent forest litter.
During daylight, leaves transpire water through
open pores or
stomata
. This loss is controlled by
the length of day, the leaf temperature (modified
by evaporational cooling), surface area, the tree
species and its age, as well as by the meteorological
factors of available radiant energy, atmospheric
vapor pressure and wind speed. Total evaporation
figures are therefore extremely varied. The
evaporation of water intercepted by the vegetation
surfaces also enters into the totals, in addition
to direct transpiration. Calculations made for a
catchment covered with Norway spruce (
Picea
abies
) in the Harz Mountains of Germany showed
an annual evapotranspiration of 340mm and
additional interception losses of 240mm.
The humidity of forest stands is closely linked
to the amount of evapotranspiration and increases
with the density of vegetation present The increase
in forest relative humidity over that outside
averages 3-10 percent and is especially marked in
summer. Vapor pressures were higher within an
oak stand in Tennessee than outside for every
