|
|
![](images/tiles/article_left_green.gif) |
Download the article as an Acrobat pdf - get the
article
here, get Acrobat
here - free |
|
![](images/tiles/article_right_green.gif) |
|
|
|
Principles Practice and Specification for Tree Pruning
Andrew Cowan N.D.Arb.
First Published: October 2002
The first decision to be made, when considering pruning, is whether cutting off and removing living branches will actually benefit the tree or shrub. Will the proposed pruning prolong it’s useful life expectancy within the context of the surrounding environment and land use. It is important to remember that pruning may do more harm than good, and in some situations may create more problems than it solves. |
|
|
Every cut made has the potential to change the
growth of the tree or shrub. Removing living foliage,
by pruning, affects the trees physiology and future
growth. The reduction in leaf area, that results
from pruning, will reduce the tree’s overall
photosyntheitc capacity and may reduce overall
growth on the pruned section, or on the entire
tree.
However, the casual observer of the growth that
appears after pruning, could be mistaken for thinking
that cutting off branches and reducing the trees
leaf area, is, beneficial
and encourages new vigour. This may indeed appear to
be the case, but remember that there are often less
branches left for the tree to produce new shoots from
and severely pruned trees have a tendency to initiate
the production and growth of warersprouts, as a response
to the need for new foliage and increased photosynthetic
capacity.
It is important that anyone, considering pruning work
to a tree, has an understanding of the biology of trees
and how they respond to pruning in order to optimize
their health and structure. Plants are reactive, generating
systems, which use basic mineral and organic resources
to build new tissues. They do not have the capacity
to heal or repair damaged areas, so where week points
occur due to injury, all they can do is re-enforce the
site by growing, reactively, additional tissues and
replace the losses. |
![](images/photos/Foliage.jpg) |
|
|
Trees store energy reserves (starch, sugars, and oils)
in branches, stems, trunk and roots. These energy reserves
can be preserved by removing, the fewest number of living
branches necessary to accomplish the desired objective.
Excessive branch removal depletes these reserves and
reduces the ability of the tree to photosynthesize and
store more energy. It is also important to be aware
that if the tree is forced to use vital energy reserves
for growth, they will not be available for defence against
plant pathogens or wood decay organisms.
There should be a good reason to remove more than a
quarter, of a trees leaf area, in a single year. It
is important to consider pruning over the entire life-span
of the tree or trees involved and not as a one-off single
operation. Many trees generate adventitious sprouts,
in response to over-pruning, as they attempt to replace
the stored energy. However live-branch pruning is an
essential part of forming good crown structure, and
is a necessary procedure in the management of specimen
trees within residential parks and gardens.
It is essential when considering pruning a tree, of
any age, that a thorough evaluation is done to determine
the objectives to be achieved, when the work is complete.
The decisions can them, be made as to where, how, when
and how often to prune the specific tree or trees, to
achieve these objectives. This evaluation process is,
an essential, part of planning the management of any
tree or tree population, and should be recorded in a
tree management schedule.
Each time a tree is pruned, according to the timing
regime set out within the management plan, a work specification
will need to be drawn up to provide the practical arborist
with guidance. The arborist will them be able to make
informed decisions, and make appropriate pruning cuts,
based on an understanding of branch attachment and tree
biology, to achieve the pruning objectives.
Removing the correct stems, branches or branchlets
is as important as making the right pruning cuts. Even
with proper pruning cuts, if the wrong branches or too
many branches are removed from a trees crown it can
defeat the object of the pruning regime.
No tree should be pruned without first establishing clearly defined pruning objectives, which may include the following: |
|
|
![](images/photos/vta-hbeam.jpg) |
- Improve crown structure and form
- Reduce risk of failure
- Maintain health
- Prolong useful life expectancy
- Removal of dead, dying or diseased wood
- Influence flowering or fruit production
- Provide clearance
- Reduce shade or wind resistance
- Improve aesthetics or allow a view
- Maintenance and enhancement of wildlife habitat
This list is not exclusive and will vary depending
on the tree itself and the surrounding land use,
but the methods used to achieve them can be selected
from the following pruning types;
- Formative and structural pruning
- Crown thinning
- Crown raising
- Crown reduction
- Crown clearing and dead wooding
- Restoration pruning
- Conservation pruning
- Pollarding
|
|
|
Throughout the entire process, from the initial planning
stage to the completion of pruning work and removal
of arising debris, the following two points should be
considered;
1 All pruning created wounds on trees, and can remove
significant areas of living tissues. The open wound
may allow the entrance of disease organisms, which could
instigate the decay of the exposed woody tissues. Combine
this then with the loss of leaf area when a branch is
removed, leaving that section of the tree with less
photosynthetic, energy producing tissue, and one must
consider any pruning to be potentially detrimental to
the future health and sustainability of the tree.
2 The tree supports a divers ecosystem of organisms
which life around it’s crown, on it’s roots
and in the soil around it. These organisms may use the
living or dead tissues of the tree for food, while others
will form symbiotic relationships with the tree itself.
So it is important to consider that any removal of dead
wood from the crown or the clearance of pruning debris
or leaves from the area around the tree could potentially
be detrimental to the continued viability of the ecosystem
in which the tree lives.
Pruning types that could form part
of a work specification to achieve a particular objective.
Several types of pruning may be used
to achieve a particular management objective and work
on one occasion could be just part of a pruning regime
over several years, and ultimately the entire life-span
of the tree. Not all objectives can be successfully achieved
after just one prune and others may need to be repeated
in order that a desired outcome is maintained. The following
pruning types can be used in isolation or in combination,
depending on the management requirements and pruning objectives.
Formative and structural pruning
Structural pruning is most often completed at the establishment
stage of tree development, when it is known as formative
pruning. The main objective of this type of pruning
is to encourage the formation of good stem and branch
structure, by improving the orientation, spacing, growth
rate, strength of attachment and ultimately size of
branches. Well planned, formative pruning during the
establishment of a young tree can prolong its useful
life expectancy within the context of a particular land
use.
This type of pruning can reduce the need for large
branch removal, and the creation of oversized wounds,
when the tree is older. Structural pruning can be completed
on semi-mature trees, but should be avoided on mature
specimens. The main management objectives of this type
of pruning are to help engineer a crown form which need
less pruning when mature, and where ever possible limit
the development of weak structural features which may
fail in latter life.
Crown thinning
Crown thinning, is the selective removal of small, live
branches throughout the entire crown, with the aim of
reducing the density of the tree leaf area. There is
no external alteration to the trees size or shape because
just internal branches are removed. The important aspect
to remember here is that the majority of branches should
be removed from the outside third of the tree crown.
The maintenance of an inner crown leaf area is essential
to sustain good branch, stem taper.
The excessive removal of branches from the lower two
thirds of a branch or stem can lead to ‘lions
tailing’ which may have adverse effects on their
long term structural integrity, resulting in early failure.
It is also important to limit the amount of foliage
removed, each time a tree is thinned, to no more that
25% (a quarter) of the leaf area, and ideally between
10-20% where possible within the management objectives.
The size of branches to be removed, during thinning
operations, should also be limited, and wherever possible
within the specification a recommended maximum diameter
should be given. In most cases this can be limited to
a size between 3cm to 4cm.
Crown raising
Crown raising, is the selective reduction and removal
of branches to create some vertical lift of the tree
canopy, allowing space under the tree for light, people,
vehicles or buildings. When specifying this pruning
type it is essential to consider the importance of maintaining
as many low branches as possible to sustain good trunk
growth and the formation of even trunk taper.
It is important to remember that excessive removal
of low branches can lead to the development of poor
trunk crown balance, where a tree may become top heavy.
It is also essential to be aware that all wounds around
the main trunk of a tree could potentially allow the
development of decay which may reduce the long term
integrity of the trees main supporting structure. Where
every possible the number and size of would should be
limited and well spaced, so there is less chance of
decay pockets combining to form larger cavities.
Some of the problematic issues described above can
be addressed by the reduction of branches rather than
their complete removal. In such cases the size and age
range of branches to be remove should be specified,
while it may also bepossible
to reduce the end weight of some drooping branches
to bring about some lift, in the overall canopy. |
|
|
Crown reduction
Crown reduction or shaping, involves the removal
and reduction of branches and stems to decrease
the height or spread of a trees crown area. This
type of pruning can be completed for a number
of reasons to achieve a range of management objectives,
from purely aesthetic when used to shape an entire
canopy, to the reduction of one limb where excessive
end weight may threaten failure.
Crown reduction work can be specified to cover
every branch within a trees crown or it can be
limited to just one. However, one principle should
be applied at all times; the desired effect should
be accomplished with reduction or removal cuts
and not heading cuts. Because biologically, the
tree has more effective wound decay barriers where
branches are removed from their point of origin,
close to the branch collar. The use of heading
cuts, also spoils good tree architecture and can
significantly increase maintenance requirements.
An important
point to consider in branch reduction is the amount
of foliage, or photosynthetic material, which is left
to sustain the remaining branch or stem tissues. When
a branch of a mature tree is reduced, no more that a
quarter (25%) of its foliage should be removed, while
more can be removed in younger trees to achieve particular
management objectives. If insufficient foliage is left,
large areas of the main branch wood may become dysfunctional
and open to decay fungi, which could lead to branch
die-back. |
![](images/photos/split.jpg) |
|
|
A common rule of thumb is that the remaining lateral
branch should be at least one-third to one-half the
diameter of the removed portion. At such a size, the
lateral branch should be able to produce enough energy
to keep the parent branch alive, and there should be
enough growth regulators present to suppress excessive
sprouts. This rule varies with tree species, age and
condition, while localised variations in climate will
also have an impact. Old, stressed or mature trees could
decline or become more stressed if too much foliage
is removed.
Crown clearing and dead wooding
This type of pruning is used where a tree is being maintained
as a specimen within the context of an ornamental garden.
Here the removal of dead, dying, diseased, detached
or broken branches is specified to improve crown appearance
and the overall tree aesthetics. The removal of such
branches may also be considered desirable where they
represent a risk to persons or property. However, it
is also important to remember that dead wood is an essential
habitat for a large number of organisms in the ecosystem
in which the tree lives. The formation of dead wood
within the crown of a tree is part of the natural system
of tree life and should not be considered to be a negative
thing that has to be removed to maintain healthy tree
growth.
Restoration pruning
Restoration pruning, is necessary where a tree has been
damaged, poorly pruned or where a once regular management
regime has lapsed, resulting in the formation of poor
structural features. The principles behind this type
of pruning are similar to those used in structural or
formative pruning on establishing trees, but more care
is required due to the maturity of the specimens involved.
This type of pruning has to be planned over a much longer
time frame and only a limited percentage (perhaps only
10%) of a trees leaf area should be removed at any one
time.
Restoration pruning may involve the training of young
epicormic, or watersprout, shoots to form new branches
and allow the reestablishment of new area of crown.
It is therefore important to provide a more detailed
pruning specification, which may involved the identification
of a specific area of the trees crown or even a particular
branch.
Conservation pruning
Not all, pruning work is completed to improve tree health
or structural form, some management objectives allocate
more weight to the creation and management of wildlife
habitat. In such cases it may be considered advantageous
to create large wounds with the aim of increasing the
area of dead and decaying wood in a tree, or to top
a limb to encourage a crown of dense re-growth which
could provide good nesting opportunities for birds.
The retention of dead wood around the crown of a tree
may be considered an important aspect of trees managed
for their wildlife value. However, there are often conflicts
that arise when health and safety issues have to be
addressed. In these situations a specification that
allows for the sympathetic reduction of dead branches
or stems, using cutting techniques like ‘coronet
cuts’ to maintain a more natural appearance to
the cut ends, may be considered desirable.
Pollarding
Pollarding is beheading a maiden tree by removing the
main leader or stem and then subsequently cutting on
a regular basis (Re-pollarding) back to the same point
(Pollard Head). Pollarding is a management system used
to control the growth of a tree throughout its life,
and is initiated at a young age. When older mature trees
are treated in this way as a crude form of size control
this can be described as topping, and can seriously
damage or even kill the tree.
Where pollarding is specified it should be part of
a long term management plan for a tree or tree population.
If a pollarded tree has lapsed out of regular management,
re-pollarding should not be specifed. In such cases
restoration pruning could be considered to either train
the branches to form a normal crown area, or using staged
reduction work re-establish a pollard management regime.
back to top |
For
more info see:
Articles
Archive
Resources |
|
|
|
|