132
J. D. O V I N Q T O N
cerned, woodlands normally have a relatively large mass of green photosynthetic tissue. This is primarily tree leaves, the maximum weight of
tree leaves recorded in Table I being 19-8 x lo3 kg per ha, but photosynthesis also occurs in the tree twigs and fruiting bodies and in the
understorey vegetation. The leaves of the woodland flora have a large
surface area, Tadaki and Shidei (1960) give a range of tree leaf areas
from 2.2 t o 7.9 ha per ha of land surface for stands of deciduous tree
species. Much of the incident light is intercepted and there is a progressive decrease in light intensity through the successive plant layers
until the light a t ground level may only amount t o about 1% of that
falling on the ecosystem. Passing downwards from the upper plant
surface of the ecosystem there is a gradation in photosynthetic efficiency from sun t o shade-leaves and t o the shade-tolerant plants of the
ground flora which presumably benefit from the increased carbon
dioxide concentration at ground level. Large scale absorption of carbon
dioxide from the atmosphere is necessary for high productivity and
would be facilitated by having the large area of chlorophyll-bearing
tissue dispersed through a considerable depth of air. Evidently, the
relatively high rate of organic matter production attained by woodlands can be partly attributed to the large and well-distributed mass of
photosynthetic tissue. Ultimately the upward extension of the photosynthetic zone might be expected t o lower the efficiency of organic
matter production since with increasing height more energy will be
expended in translocation and in transporting water from the roots t o
the leaves. It is significant that the greatest annual production of
organic matter is achieved at about the pole stage when the trees are
about 7 m tall and the weight of tree leaves present per unit area, of
land is at a maximum and the living canopy is just being raised clear
of the ground (Ovington, 1957a).
Any evaluation of the potential capacity of an ecosystem t o manufacture organic matter also needs t o take into account the annual
duration of photosynthesis. This can be illustrated by comparing forest
and agricultural crops. Agricultural crops such as spring cereals may
attain high daily rates of net photosynthesis but the annual production
of plant matter may still be low compared t o that of woodland ecosystems because of the short annual life history of the cereals. Frequently agricultural crops are only present for about five months of
the year and only during part of this time do they make their fullest
use of the site, since time is required for the leaf and root systems t o
develop froni the planted seed and, photosynthesis of the crop is greatly
reduced during the ripening period. In contrast, evergreen woodlands
have a large weight of leaves and roots present throughout the year so
that positive values for net photosynthesis may be recorded even in the
J. D. O V I N Q T O N
cerned, woodlands normally have a relatively large mass of green photosynthetic tissue. This is primarily tree leaves, the maximum weight of
tree leaves recorded in Table I being 19-8 x lo3 kg per ha, but photosynthesis also occurs in the tree twigs and fruiting bodies and in the
understorey vegetation. The leaves of the woodland flora have a large
surface area, Tadaki and Shidei (1960) give a range of tree leaf areas
from 2.2 t o 7.9 ha per ha of land surface for stands of deciduous tree
species. Much of the incident light is intercepted and there is a progressive decrease in light intensity through the successive plant layers
until the light a t ground level may only amount t o about 1% of that
falling on the ecosystem. Passing downwards from the upper plant
surface of the ecosystem there is a gradation in photosynthetic efficiency from sun t o shade-leaves and t o the shade-tolerant plants of the
ground flora which presumably benefit from the increased carbon
dioxide concentration at ground level. Large scale absorption of carbon
dioxide from the atmosphere is necessary for high productivity and
would be facilitated by having the large area of chlorophyll-bearing
tissue dispersed through a considerable depth of air. Evidently, the
relatively high rate of organic matter production attained by woodlands can be partly attributed to the large and well-distributed mass of
photosynthetic tissue. Ultimately the upward extension of the photosynthetic zone might be expected t o lower the efficiency of organic
matter production since with increasing height more energy will be
expended in translocation and in transporting water from the roots t o
the leaves. It is significant that the greatest annual production of
organic matter is achieved at about the pole stage when the trees are
about 7 m tall and the weight of tree leaves present per unit area, of
land is at a maximum and the living canopy is just being raised clear
of the ground (Ovington, 1957a).
Any evaluation of the potential capacity of an ecosystem t o manufacture organic matter also needs t o take into account the annual
duration of photosynthesis. This can be illustrated by comparing forest
and agricultural crops. Agricultural crops such as spring cereals may
attain high daily rates of net photosynthesis but the annual production
of plant matter may still be low compared t o that of woodland ecosystems because of the short annual life history of the cereals. Frequently agricultural crops are only present for about five months of
the year and only during part of this time do they make their fullest
use of the site, since time is required for the leaf and root systems t o
develop froni the planted seed and, photosynthesis of the crop is greatly
reduced during the ripening period. In contrast, evergreen woodlands
have a large weight of leaves and roots present throughout the year so
that positive values for net photosynthesis may be recorded even in the