150
J. D. OVINOTON
tion slightly exceeds transpiration. At all eight locations, the removal
of water by the combined effects of evaporation and transpiration is
greatest and run-off is least where woodland conditions exist. That
forested areas give lower yields of water for use by man, compared with
other types of vegetation such as grassland, is now generally accepted.
The greater water use of forests can be attributed t o several factors, e.g.
the large amount of interception and the extensive tree root system
which in dry weather permits trees to draw upon water reserves deep
in the soil. In countries such as Britain, where the provision of an adequate water supply is a serious problem, the policy of the afforestation
of water catchments has been questioned in view of the possible reduction of water yield (Law, 1956) but it is important to take into account
the effect of woodlands in regulating run-off and preventing erosion.
The greatest recorded annual loss of water due to evaporation and
transpiration occurs at Coweeta and would involve the expenditure of
over 6 x 10l2 cal per ha, over 150 times the energy released yearly by
organic matter breakdown. At Coweeta and Castricum, the differences
in run-off between tree-covered and treeless areas is such that the trees
must be utilizing an additional 2 x 10l2 cal per ha of radiation for
evaporation and transpiration in an average year.
VI. CIRCULATION OF CHEMICAL ELEMENTS
The interchange of chemicals between living organisms and between
the physical and biological components of woodland ecosystems forms
an extremely intricate system, essentially of a cyclic nature and broadly
following the organic system. The magnitude and detailed pattern of
circulation are also characteristic of the individual chemical element
concerned, since elements vary in their availability to plants, and are
absorbed selectively by woodland organisms and distributed unevenly
through the bodies of plants and animals. The circulation does not form
a closed system, the ecosystem capital changes as chemical elements
are added to or removed from it in various natural and artificial ways,
some of which are listed in Table IX.
The passage of chemical elements through woodland ecosystems is
usually expressed on an annual basis, a legacy of the pioneer work done
in temperate woodlands, where the regular autumn leaf fall dominates
the system. In some ways this is unrealistic, since a particular molecule
may take less or more than a year to pass through the circuit. For
instance, it is possible that, nutrients present in the bud scales or the
spring inflorescences are released by decomposition and re-absorbed by
the trees during the summer months, t o be shed in the same year as
part of the autumn leaf fall. The rapidity with which elements can be
moved in woodland ecosystems has been demonstrated by the use of
J. D. OVINOTON
tion slightly exceeds transpiration. At all eight locations, the removal
of water by the combined effects of evaporation and transpiration is
greatest and run-off is least where woodland conditions exist. That
forested areas give lower yields of water for use by man, compared with
other types of vegetation such as grassland, is now generally accepted.
The greater water use of forests can be attributed t o several factors, e.g.
the large amount of interception and the extensive tree root system
which in dry weather permits trees to draw upon water reserves deep
in the soil. In countries such as Britain, where the provision of an adequate water supply is a serious problem, the policy of the afforestation
of water catchments has been questioned in view of the possible reduction of water yield (Law, 1956) but it is important to take into account
the effect of woodlands in regulating run-off and preventing erosion.
The greatest recorded annual loss of water due to evaporation and
transpiration occurs at Coweeta and would involve the expenditure of
over 6 x 10l2 cal per ha, over 150 times the energy released yearly by
organic matter breakdown. At Coweeta and Castricum, the differences
in run-off between tree-covered and treeless areas is such that the trees
must be utilizing an additional 2 x 10l2 cal per ha of radiation for
evaporation and transpiration in an average year.
VI. CIRCULATION OF CHEMICAL ELEMENTS
The interchange of chemicals between living organisms and between
the physical and biological components of woodland ecosystems forms
an extremely intricate system, essentially of a cyclic nature and broadly
following the organic system. The magnitude and detailed pattern of
circulation are also characteristic of the individual chemical element
concerned, since elements vary in their availability to plants, and are
absorbed selectively by woodland organisms and distributed unevenly
through the bodies of plants and animals. The circulation does not form
a closed system, the ecosystem capital changes as chemical elements
are added to or removed from it in various natural and artificial ways,
some of which are listed in Table IX.
The passage of chemical elements through woodland ecosystems is
usually expressed on an annual basis, a legacy of the pioneer work done
in temperate woodlands, where the regular autumn leaf fall dominates
the system. In some ways this is unrealistic, since a particular molecule
may take less or more than a year to pass through the circuit. For
instance, it is possible that, nutrients present in the bud scales or the
spring inflorescences are released by decomposition and re-absorbed by
the trees during the summer months, t o be shed in the same year as
part of the autumn leaf fall. The rapidity with which elements can be
moved in woodland ecosystems has been demonstrated by the use of