landscape matters volume 5 - Flipbook - Page 21
4.
Spatial thoughts about
countryside tree planting
Hal Moggridge OBE PPLI VMH RIBA
EXTENSIVE WOODLAND PLANTING is proposed in England
and Wales to help counteract global warming. Success with
this sensible policy needs several years subsequent management to ensure establishment, and avoidance of ploughing
land beforehand, which releases carbon sequestrated in the
soil. However, the potential contribution of free standing trees
and hedgerows is being overlooked by government policy, and
needs to be grasped as well. Much of the British lowlands are
already well treed, a bosky rural character as if wooded when
trees and hedges are in fact scattered. Perhaps this mass of
incidental trees and hedgerows has been overlooked, together
with the potential for its volume to be significantly increased,
because any area of trees less than 0.5ha (1.25 acres) is not
defined as woodland by Forestry Research.
There are currently some 450,000 kilometres of hedgerow in
the farmed countryside of England and Wales. Many hedges
are unduly ferociously cut, but an average surface leaf area
of 3.5m2 per metre run is a fair estimate, so that these hedges
provide 1,575 million m2 of leaf surface to help oxygenate
the environment. This is the same as 1,575 km2 of continuous
woodland canopy which, at 1000 trees per hectare, is 157.5 million trees. Here is great potential for increasing oxygenation by
vegetation. The average bulk of hedges could be increased
to 5m2 per metre run. This would provide an extra 675 million
m2 of leaf surface, equivalent to 675 km2 of continuous woodland or 67.5 million woodland trees. Grants for such hedgerow
bulking would be an effective help in the battle against global
warming. Additionally in 1939 there was twice the length of
farmland hedgerow as today. If another 100,000 kilometres of
bulky hedgerow were now replanted, then there would be an
additional 500 million m2 of leaf surface, the same as 50 million
woodland trees. From these benefits must be subtracted the
carbon produced trimming hedges. But farmland hedgerows
S-W view from the edge of Bagley
Wood, near Oxford, across bosky
countryside, full of scattered trees in
also provide invaluable benefits to wildlife. Furthermore additional hedgerow trees could provide added benefit like free
standing wood pasture trees as discussed below.
a mixture of field hedgerows
and gardens.
Little past consideration seems to have been given to the
comparative oxygen productivity of free standing as against
woodland trees. Woodland by necessity raises a leafy
canopy in a generally horizontal plane nurtured by light
above.There is little leaf activity in shaded woodland interiors. Thus a square of woodland 20m x 20m has 400m2 of productive leaf surface. This may be compared with the oxygen
producing leaf surface of a typical free-standing mature
deciduous tree within the same space. The leaf surface of
such a tree with a canopy radius of 8m is some 1005 m2*,
which is two and a half times the leaf surface of woodland
occupying similar space. Therefore spaced out free standing trees in wood pasture seem a more efficient producer of
oxygen than continuous woodland.
The bare trunks of old trees in Bagley
Wood with a top canopy of leaves.
However, as it takes time after planting before wood pasture
is oxygen productive, unlike new woodland which soon has a
continuous horizontal canopy, the creation of wood pasture
needs a special design method. Permanent trees should be
planted irregularly about 20m apart. The interstices of open
space can then be filled with fast growing species, but keeping space around the permanent trees so that they develop
vigorous side branches. As these long-term trees grow the
trees in the interstices will gradually be cut away, until a wood
pasture is established. Thus short term oxygen production can
be combined with the long term maximum productivity.
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