Geoscience Reference
In-Depth Information
may intercept up to 94 percent of low-intensity
precipitation but as little as 15 percent of high
intensities, the average for temperate pines being
about 30 percent. In tropical rainforest, about 13
percent of annual rainfall is intercepted. The
intercepted precipitation either evaporates on the
canopy, runs down the trunk or drips to the
ground. Assessment of the total precipitation
reaching the ground (the through-fall) requires
careful measurements of the stem flow and the
contribution of drips from the canopy. Canopy
interception contributes 15-25 percent of total
evaporation in tropical rainforests. It is not a total
loss of moisture from the forest, since the solar
energy used in the evaporating process is not
available to remove soil moisture or transpiration
water. However, the vegetation does not derive
the benefit of water cycling through it via the
soil. Canopy evaporation depends on net radia-
tion receipts and the type of species. Some
Mediterranean oak forests intercept 35 percent of
rainfall and almost all evaporates from the canopy.
Water balance studies indicate that evergreen
forests allow 10-50 percent more evapotranspira-
tion than grass in the same climatic conditions.
Grass normally reflects 10-15 percent more solar
radiation than coniferous tree species and hence
less energy is available for evaporation. In addi-
tion, trees have a greater surface roughness,
which increases turbulent air motion and, there-
fore, the evaporation efficiency. Evergreens allow
transpiration to occur year-round. Nevertheless,
research to verify these results and test various
hypotheses is needed.
minima are higher (
Figure 12.16
). This is
particularly apparent during periods of high
summer evapotranspiration, which depress daily
maximum temperatures and cause mean monthly
temperatures in tropical and temperate forests to
fall well below that outside. In temperate forests
at sea-level, the mean annual temperature may be
about 0.6°C lower than that in surrounding open
country, the mean monthly differences may reach
2.2°C in summer but not exceed 0.1°C in winter.
On hot summer days the difference can be more
than 2.8°C. Mean monthly temperatures and
diurnal ranges for temperate beech, spruce and
pine forests are given in
Figure 12.17
. This also
shows that when trees transpire little in the
summer (e.g., the
forteto
oak maquis of the
Mediterranean), the high daytime temperatures
reached in the sheltered woods may cause the
pattern of mean monthly values to be the reverse
of temperate forests. Even within individual
climatic regions it is difficult to generalize,
however. At elevations of 1000m the lowering of
temperate forest mean temperatures below those
in the open may be double that at sea-level.
The vertical structure of forest stands gives
rise to a complex temperature structure, even
in relatively simple stands. For example, in a
ponderosa pine forest (
Pinus ponderosa
) in
Arizona the recorded mean June to July
80
70
20
Maximum
60
10
50
40
Modification of the thermal
environment
Forest vegetation has an important effect on
microscale temperature conditions. Shelter from
the sun, blanketing at night, heat loss by evapo-
transpiration, reduction of wind speed and the
impeding of vertical air movement all influence
the temperature environment. The most obvious
effect of canopy cover is that, inside the forest,
daily maximum temperatures are lower and
0
30
Minimum
20
-10
10
0
J
FM
A
M
J
Month
J
A
S
O
N
D
Figure 12.16
Seasonal regimes of mean daily
maximum and minimum temperatures inside and
outside a birch-beech-maple forest in Michigan.
Source: After US Department of Agriculture Yearbook (1941).
