144
J. D. OVINOTON
culation. The two most significant features of the woodland water
cycle are, (i) the relatively rapid passage of large amounts of water
through the ecosystem and, (ii) the small amount of water retained
within the ecosystein, particularly in the organic matter. This is shown
by Table VII where the distribution of water in late summer is given
for three softwood and three hardwood plantations in Britain, all growing on very similar sites and in close proximity t o one another. After 47
years of afforestation, the water present in the forest ecosystems only
averages about 15% of the annual precipitation and less than O.lyo of
the annual precipitation is contained within the organic matter accreted
each year. Seasonal changes in the water content of the woodland ecosystems occur, e.g. the water content of the mineral soil increases during
the wetter, winter months, but if this trebled, the total water in the ecosystem would still be small compared with precipitation. Another
interesting feature shown by Table VII is the greater weight of water in
the mineral soils of the hardwood plantations. Conversely, the organic
matter of the softwood plantations contains more water than that of the
less productive hardwood stands. Differences in the water content of
the organic matter of softwood and hardwood plots are insufficient t o
account for the larger differences in mineral soil moisture, which must,
be attributed mainly t o a greater annual loss of water from those ecosystems having coniferous trees present. Water is known t o be removed
from woodland ecosystems in several ways and considerable scientific
effort has been devoted t o the measurement of different forms of water
loss, mainly from the point of view of slowing the speed of run-off t o
reduce soil erosion and of increasing the amount of water directly available for use by man, i.e. the water yield.
A. WATER LOSS FROM WOODLAND ECOSYSTEMS
Since the amount of water likely t o be removed from woodland ecosystems by cropping is relatively insignificant, water loss occurs mainly
through evaporation, transpiration and run-off.
1. Evaporation
Much of the precipitation is intercepted by the woodland vegetation :
of this, part is evaporated directly back into the atmosphere, some is
absorbed by the vegetation and the rest reaches the ground as water
flowing down the plant stems or dripping from the foliage.
Numerous data are available giving the proportion of the annual precipitation failing t o reach ground level because of interception and the
following are typical; Loblolly pine 14% (Hoover, 1953), 10% t o 55%
for a range of British plantations (Ovington, 1954b), 350/6 in mixed
Uganda tropical forest (Hopkins, 1960) and 19% t o 33% for Red pine
J. D. OVINOTON
culation. The two most significant features of the woodland water
cycle are, (i) the relatively rapid passage of large amounts of water
through the ecosystem and, (ii) the small amount of water retained
within the ecosystein, particularly in the organic matter. This is shown
by Table VII where the distribution of water in late summer is given
for three softwood and three hardwood plantations in Britain, all growing on very similar sites and in close proximity t o one another. After 47
years of afforestation, the water present in the forest ecosystems only
averages about 15% of the annual precipitation and less than O.lyo of
the annual precipitation is contained within the organic matter accreted
each year. Seasonal changes in the water content of the woodland ecosystems occur, e.g. the water content of the mineral soil increases during
the wetter, winter months, but if this trebled, the total water in the ecosystem would still be small compared with precipitation. Another
interesting feature shown by Table VII is the greater weight of water in
the mineral soils of the hardwood plantations. Conversely, the organic
matter of the softwood plantations contains more water than that of the
less productive hardwood stands. Differences in the water content of
the organic matter of softwood and hardwood plots are insufficient t o
account for the larger differences in mineral soil moisture, which must,
be attributed mainly t o a greater annual loss of water from those ecosystems having coniferous trees present. Water is known t o be removed
from woodland ecosystems in several ways and considerable scientific
effort has been devoted t o the measurement of different forms of water
loss, mainly from the point of view of slowing the speed of run-off t o
reduce soil erosion and of increasing the amount of water directly available for use by man, i.e. the water yield.
A. WATER LOSS FROM WOODLAND ECOSYSTEMS
Since the amount of water likely t o be removed from woodland ecosystems by cropping is relatively insignificant, water loss occurs mainly
through evaporation, transpiration and run-off.
1. Evaporation
Much of the precipitation is intercepted by the woodland vegetation :
of this, part is evaporated directly back into the atmosphere, some is
absorbed by the vegetation and the rest reaches the ground as water
flowing down the plant stems or dripping from the foliage.
Numerous data are available giving the proportion of the annual precipitation failing t o reach ground level because of interception and the
following are typical; Loblolly pine 14% (Hoover, 1953), 10% t o 55%
for a range of British plantations (Ovington, 1954b), 350/6 in mixed
Uganda tropical forest (Hopkins, 1960) and 19% t o 33% for Red pine