146
J. D. OVINOTON
and beech stands (Voigt, 1960). The magnitude of the interception-loss
depends upon both weather conditions and the luxuriance and form of
the vegetation cover. Evaporation is more rapid in dry, hot, windy
weather. The percentage loss of water by interception is greater in light
than heavy showers so that less water is lost by interception if the precipitation consists of heavy downpours. Canopies of coniferous trees
with their many needle-like leaves trap more water than those of
deciduous hardwoods as the latter have large, flat leaves from which
water runs off easily. Interception also results in a more patchy distribution of water over the forest floor (Ovington, 1954b) and this is probably of great importance t o the water economy of the trees since
water penetration into the soil is improved and the water flowing down
the tree-stems tends to follow the roots spreading out from the base of
the trunk (Voigt, 1960).
While much of the water failing t o reach ground level is evaporated
directly back into the atmosphere, there is some evidence that significant amounts of water are absorbed by the foliage so that interception
does not represent a complete water loss from the ecosystem. Hartel and
Rudolph (1953) found that when needles of Abies alba are moistened,
between 3 to 10 mg of water are absorbed per g of leaf weight per
hour and in young leaves the rate of absorption rises t o 30 mg. StBlfelt
(1944) calculated that under the most favourable conditions, the absorption of water by spruce leaves is equivalent to 33% of the total transpiration but Rutter (1959) found cuticular absorption of water by Scots
pine needles t o be negligible.
Once precipitation reaches ground level, further loss of water may
occur by evaporation, the amount depending upon the closeness of the
forest cover and the rapidity of infiltration into the soil.
2. Transpiration
Transpiration of the woodland plants is controlled by two main sets
of factors, (i) environmental, e.g. wind, atmospheric humidity and temperature, and (ii) internal plant controls such as leaf structure and root
distribution. Ladefoged (1956) found that during June the daily transpiration of a first quality beech stand with trees 25 to 30 years old,
could be closely related t o weather conditions, thus in wet weather,
under overcast skies and in bright sunshine, 0, 20 and 40 x lo3 kg of
water respectively were transpired per ha. Woodlan.ds of different tree
species growing under identical conditions transpire at different rates.
Polster (1950) gives comparative figures for the transpiration rates of
trees of various species based on measurements of water-loss from twigs
of leaves cut from the tree crowns, if the transpiration rate of spruce is
given a value of 100 the comparative figures for birch, oak, beech,
J. D. OVINOTON
and beech stands (Voigt, 1960). The magnitude of the interception-loss
depends upon both weather conditions and the luxuriance and form of
the vegetation cover. Evaporation is more rapid in dry, hot, windy
weather. The percentage loss of water by interception is greater in light
than heavy showers so that less water is lost by interception if the precipitation consists of heavy downpours. Canopies of coniferous trees
with their many needle-like leaves trap more water than those of
deciduous hardwoods as the latter have large, flat leaves from which
water runs off easily. Interception also results in a more patchy distribution of water over the forest floor (Ovington, 1954b) and this is probably of great importance t o the water economy of the trees since
water penetration into the soil is improved and the water flowing down
the tree-stems tends to follow the roots spreading out from the base of
the trunk (Voigt, 1960).
While much of the water failing t o reach ground level is evaporated
directly back into the atmosphere, there is some evidence that significant amounts of water are absorbed by the foliage so that interception
does not represent a complete water loss from the ecosystem. Hartel and
Rudolph (1953) found that when needles of Abies alba are moistened,
between 3 to 10 mg of water are absorbed per g of leaf weight per
hour and in young leaves the rate of absorption rises t o 30 mg. StBlfelt
(1944) calculated that under the most favourable conditions, the absorption of water by spruce leaves is equivalent to 33% of the total transpiration but Rutter (1959) found cuticular absorption of water by Scots
pine needles t o be negligible.
Once precipitation reaches ground level, further loss of water may
occur by evaporation, the amount depending upon the closeness of the
forest cover and the rapidity of infiltration into the soil.
2. Transpiration
Transpiration of the woodland plants is controlled by two main sets
of factors, (i) environmental, e.g. wind, atmospheric humidity and temperature, and (ii) internal plant controls such as leaf structure and root
distribution. Ladefoged (1956) found that during June the daily transpiration of a first quality beech stand with trees 25 to 30 years old,
could be closely related t o weather conditions, thus in wet weather,
under overcast skies and in bright sunshine, 0, 20 and 40 x lo3 kg of
water respectively were transpired per ha. Woodlan.ds of different tree
species growing under identical conditions transpire at different rates.
Polster (1950) gives comparative figures for the transpiration rates of
trees of various species based on measurements of water-loss from twigs
of leaves cut from the tree crowns, if the transpiration rate of spruce is
given a value of 100 the comparative figures for birch, oak, beech,