Geoscience Reference
In-Depth Information
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).
d Modification of the thermal environment
Forest vegetation has an important effect on micro-scale
temperature conditions. Shelter from the sun, blanketing
at night, heat loss by evapotranspiration, 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 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
1000 m the lowering of temperate forest mean temper-
atures below those in the open may be double that at
sea-level.
Figure 12.15 Energy components on a July day in two forest
stands. (A) Scots and Corsican pine at Thetford, England (52°N),
on 7 July 1971. Cloud cover was present during the period 00:00
to 05:00 hours. (B) Douglas fir stand at Haney, British Columbia
(49°N), on 10 July 1970. Cloud cover was present during the
period 11:00 to 20:00 hours.
Sources : (A) Data from Gay and Stewart (1974), after Oke (1978).
(B) Data from McNaughton and Black (1973), after Oke (1978).
cycling through it via the soil. Canopy evaporation
depends on net radiation receipts, and the type of
species. Some Mediterranean oak forests intercept 35
per cent of rainfall and almost all evaporates from the
canopy. Water balance studies indicate that evergreen
forests allow 10 to 50 per cent more evapotranspiration
than grass in the same climatic conditions. Grass
normally reflects 10 to 15 per cent more solar radiation
than coniferous tree species and hence less energy is
available for evaporation. In addition, trees have a
greater surface roughness, which increases turbulent
air motion and, therefore, the evaporation efficiency.
Evergreens allow transpiration to occur year-round.
Nevertheless, research to verify these results and test
various hypotheses is needed.
Search WWH ::




Custom Search