Q U A N T I T A T I V E ECOLOQY A N D WOODLAND E C O S Y S T E M 147
Douglas fir, pine and larch become 740, 460, 372, 94, 139 and 212. By
combining these data with foliage biomass figures such as those of
Burger, Polster obtained the following mean daily transpiration rates
for the trees expressed as lo3 kg of water per ha: birch 47, beech 38,
larch 47, Douglas fir 53, spruce 43 and pine 24. The results of Ivanov et
al. (1951) indicate that the amount of water transpired by trees varies
as a woodland matures, in 10, 33, 65, 80 and 150-year-old pine stands,
the daily traiispiratioii from May to September (the transpiration period
according to Ivanov et al.) amounts to 17, 23, 18, 17 and 13 x lo3 kg of
water per ha compared with 22, 22 and 19 x lo3 for birch stands 25, 60
and 70 years old. Apparently maximum transpiration is achieved about
the peak period of organic matter production.
Little attention has been paid t o the water loss attributable t o interception and transpiration b j the woodland understorey vegetation but
in some circumstances this may be considerable. The removal of the
dense shrub understorey of rhododendron and laurel from a wooded
catchment in North Carolina increased the annual water yield by 50 x
1 O4 kg per ha (Johnson and Kovner, 1956).
3. Evaporation and Transpiration as Related Processes
The relative magnitudes of evaporation and transpiration depend
largely upon the type of vegetation cover present but it has been suggested that the combined total water-loss of these two processes is not
greatly influenced by the vegetation. Evaporation and transpiration are
alike in needing a supply of energy to provide heat for the vaporization
of water and in both cases incoming solar radiation is the main source
of energy, although water vaporization may also be increased by drying
winds. Once energy is used in either of the two processes it is no longer
available, so that energy supply places an upper limit t o the water loss
from woodland ecosystems by evaporation and transpiration together.
In Britain only about 40% of the incident radiation is used for the
vaporization of water, energy being dissipated in various other ways,
e.g. by reflection, re-radiation, heating the air, etc. Penman (1956) has
stressed that the amount of energy available can be determined from
meteorological data so that the potential loss of water from the
ecosystem attributable t o evaporation and transpiration can be
calculated. He suggests, on theoretical grounds, that when there is adequate soil moisture and the vegetation forms a continuous cover, the
total water loss due t o evaporation and transpiration depends mainly
on weather conditions and is not greatly influenced by differences in the
type of vegetation cover. Considerable evidence has accumulated
verifying the validity of Penman’s conclusions (Zahner, 1955) ; a t the
same time it is recognized that Penman’s qualifications (e.g. adequate
Douglas fir, pine and larch become 740, 460, 372, 94, 139 and 212. By
combining these data with foliage biomass figures such as those of
Burger, Polster obtained the following mean daily transpiration rates
for the trees expressed as lo3 kg of water per ha: birch 47, beech 38,
larch 47, Douglas fir 53, spruce 43 and pine 24. The results of Ivanov et
al. (1951) indicate that the amount of water transpired by trees varies
as a woodland matures, in 10, 33, 65, 80 and 150-year-old pine stands,
the daily traiispiratioii from May to September (the transpiration period
according to Ivanov et al.) amounts to 17, 23, 18, 17 and 13 x lo3 kg of
water per ha compared with 22, 22 and 19 x lo3 for birch stands 25, 60
and 70 years old. Apparently maximum transpiration is achieved about
the peak period of organic matter production.
Little attention has been paid t o the water loss attributable t o interception and transpiration b j the woodland understorey vegetation but
in some circumstances this may be considerable. The removal of the
dense shrub understorey of rhododendron and laurel from a wooded
catchment in North Carolina increased the annual water yield by 50 x
1 O4 kg per ha (Johnson and Kovner, 1956).
3. Evaporation and Transpiration as Related Processes
The relative magnitudes of evaporation and transpiration depend
largely upon the type of vegetation cover present but it has been suggested that the combined total water-loss of these two processes is not
greatly influenced by the vegetation. Evaporation and transpiration are
alike in needing a supply of energy to provide heat for the vaporization
of water and in both cases incoming solar radiation is the main source
of energy, although water vaporization may also be increased by drying
winds. Once energy is used in either of the two processes it is no longer
available, so that energy supply places an upper limit t o the water loss
from woodland ecosystems by evaporation and transpiration together.
In Britain only about 40% of the incident radiation is used for the
vaporization of water, energy being dissipated in various other ways,
e.g. by reflection, re-radiation, heating the air, etc. Penman (1956) has
stressed that the amount of energy available can be determined from
meteorological data so that the potential loss of water from the
ecosystem attributable t o evaporation and transpiration can be
calculated. He suggests, on theoretical grounds, that when there is adequate soil moisture and the vegetation forms a continuous cover, the
total water loss due t o evaporation and transpiration depends mainly
on weather conditions and is not greatly influenced by differences in the
type of vegetation cover. Considerable evidence has accumulated
verifying the validity of Penman’s conclusions (Zahner, 1955) ; a t the
same time it is recognized that Penman’s qualifications (e.g. adequate