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Tree pruning: A modern approach

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Tree pruning: a modern approach
In a second article on tree pruning1, HUGH MORRIS focuses on
arboriculture management practices, which will be discussed together
with the physiological processes to which the tree is subjected in
response to such treatments.
We pruned trees historically for purposes other than appearances or
aesthetics. We viewed trees for our gain and survival, rather than for long-
term gains from their inestimable benets. This past attitude is engrained in
our psyche. As Alex Shigo, the father of modern arboriculture, said, ‘many
of the tree care practices that were developed centuries ago have changed
little over the years’. It has been a ‘take’ relationship without any due
consideration for the trees’ health. Shigo further stated that ‘practices are
deep-rooted in attitudes’. From the time of the early settlers, wood has shaped
civilisation: from rewood to shelter, from cups and bowls to tools, from
1 The rst article was published in the IDS Yearbook 2010, pp. 217-225.
Above, The Preston Twins, located in Preston Park, Brighton, are claimed to be the oldest pair
of English elms in the world, both being approximately 400 years old. To increase their longevity
the Brighton and Hove City Council arboricultural team, managed by Bob Greenland, prune the
trees when they feel it is necessary (every three to five years) in order to reduce and thin the sail
area of the crown. According to Bob, this is not retrenchment, as these would not be classified as
veteran trees, owing to their healthy crowns. Reducing the sail area of the crown makes the tree’s
branches less prone to failure from wind, as the branch attachments have weakened with age.
photograph © Hugh Morris
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the wheel to bridges, and from transport to weaponry. It has been the most
important of substances, surpassed by no other. The British Empire felled
oaks (and in many cases topped them for their bows) on a colossal scale for
shipbuilding, an exercise that reaped dividends during the Napoleonic wars
and many other naval conicts. Along with this, the coal mines that depended
on timber pit props for their internal support set the stage for the industrial
revolution. So, our relationship with trees has been one-sided. While such
practices as topping were eminently practical in agriculture and some forestry
applications, they have no place in cultivation for amenity and scientic
ends. However, we have carried past practices through time and across
disciplines. For instance, when forestry on large scale declined, foresters
turned to urban tree management for employment and carried their practices
over to arboriculture. The origins of early arboriculture were deep-rooted in
three treatments: ush cutting (cutting branches back to the trunk), wound
dressing, and the lling of cavities, all now considered maltreatments after
decades of scientic studies, notably in recent times by Dr Alex Shigo. The art
and science of arboriculture has changed the emphasis to tree management for
the benet of the future of mankind, rather than for instant gratication. People
are now concerned for the planet, and trees have the most valuable place in our
ght to save and protect the environment. Correct tree management practices
protects our trees, increasing their appearance, longevity, and resistance to
pests and diseases.
Within the discipline of arboriculture the reasons for pruning trees include:
for people’s safety in high usage areas removal of dead, dying, or diseased
parts; for aesthetics; for clearance of lower limbs along roadsides, streets, and
people’s homes; to create vistas in arboreta, gardens, and parks; to lighten
the load on trees in order to reduce wind damage; to correct a tree’s habit,
especially while young; to remove defects that may result in failure of part of
the tree; to provide more light for understory plants in established gardens
and arboreta; and, of course, for the tree’s health. Trees, for our pleasure
and health, have been taken from their natural environment (among other
trees) so, in compensation for this, must be nurtured, and modied into our
landscape. To achieve this, the way we prune trees is critical.
This paper is a sequel to the one I wrote for the IDS Yearbook in 2010,
entitled, ‘A history of tree pruning’, which focused on the history of the pruning
method, and the general adoption of ‘natural target pruning’ particularly within
the arboricultural industry. The focus will be on arboriculture management
practices, notably six principal techniques: pollarding, crown-reduction,
crown-lifting, crown-thinning, dead-wooding, and formative-pruning. The
practices will be discussed, together with the physiological processes to which
the tree is subjected in response to such treatments.
Pollarding is an ancient agricultural practice in both Europe and the British
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Isles. It involves the removal of the upper scaffold branches to promote a
head of small diameter shoots and foliage. We pollard trees to regulate size
and shape, so it can be a solution when selecting long-lived normally large
species of trees for our streets, such as plane (Platanus spp.) and lime (Tilia
spp.). Pollarding is rst administered when the tree is young, so as to minimise
the impact of decay-causing fungi by allowing the tree to develop what are
termed ‘knuckles’, club-shaped swellings impenetrable to fungi by their
very anatomy. A frame-work is developed during a tree’s formative years,
where each year, all branches are pruned back to the same points, allowing
the knuckle, or pollard head, as it is sometimes called, to develop. Professor
Ed Gilman of the University of Florida, an advocate of correct pruning, has
largely taken over the mantle for this special treatment from the renowned Dr
Alex Shigo. He has recommended, if necessary, leaving one or two inches of
illustration © J R Shepherd www.inkyleaves.com
Topping exposes the wood to decay-causing pathogens, that eventually results in a dysfunctional
column of decay. This affects the long term health of the tree, and its physical stability, especially for
those species with a poor inherited ability to compartmentalise effectively.
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stub to ensure that there will be buds to initiate next year’s sprouts. Welfare of
the newly formed starch-rich knuckle, a mass of buds and callus, then becomes
the principal concern for the arborist when re-pollarding.
Pruning of a similar fashion in mature trees is referred to as ‘topping’, a
malpractice that has its roots in pollarding, a perfectly acceptable method in
built-up environments. Topping is a form of tree mutilation, where after the
treatment, the sealing process around the open wounds is slow. In the majority
of cases, a knuckle is never formed, meaning that an open wound remains,
allowing a large column of dysfunctional wood to develop, as decay moves
in a downward direction. This is the rst wall of the CODIT model (Dr Alex
Shigo’s theory on tree defence as covered in my rst paper for the IDS) that is
One of the primary concerns with topping is the production of epicormic shoots, as the tree tries
to compensate for loss of leaves. These shoots grow rapidly and are much weaker than normal
branch-stem attachments owing to both a weak attachment and the rapid growth. This often
results in failure, a major safety concern.
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illustration © J R Shepherd www.inkyleaves.com
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breached in this instance, the weakest of four walls that act as dynamic and/
or static barriers against the movement of fungal hyphae. This dysfunctional
column eventually results in hollowing out, where the core of the limbs and/
or stem disintegrates. It is the new wood, developed after the barrier zone is in
place (wall 4 of the CODIT model) that is the foundation for new growth. This
new growth is often weak, and its failure is a likelihood making the practice of
topping ever the more problematic. In the case of pollarded trees, these issues
do not arise, as one does not breach the swollen knuckles. This situation is
often exacerbated as topped trees are often unattended for years. To rectify
this problem a cyclical pruning regime should be enforced in an attempt to
redeem what might be a recoverable situation, although topping is a practice
Coppicing, a method of woodland management, was practised extensively in Britain for the
valuable new shoots in a variety of applications such as fuel for charcoal iron production, house
building, cart-making, etc. After a gradual wane, we are beginning to see a return of the ancient
practice with biofuels now being used as an alternative to oil.
illustration © J R Shepherd www.inkyleaves.com
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that must be abolished and consigned to the past. Topping large trees is not
pollarding, it is a crime against nature (Shigo, 2008). An interesting legalistic
attitude was expressed in Germany by Otto the Great, the founder of the Holy
Roman Empire (936 to 973), where the law stated, ‘whoever beheads a tree
shall himself likewise be beheaded’. This excessive law was not in place only
to maintain tree health, but to ensure trees were allowed to grow straight for
their valuable timber. Figures for how many people actually were beheaded
are lost in antiquity, though I suspect only a few.
Internal responses by the tree to topping are corrective adjustments to the
root: shoot ratio. Under normal circumstances, when there is a functional
relationship between tree roots and the above ground parts, the former are ve
to six times heavier than the latter. Upsetting this sensitive balance changes
the interactions between the three primary components: leaves, absorbing
roots, and connective tissue (and the transporting xylem and phloem, along
with mechanical tissues). According to Dr Kim Coder of the University of
Georgia, it reects resource allocations while the tree adjusts to maintain a
functional balance between the shoots and roots. By the act of topping, a large
percentage of leaves will also be removed which dramatically reduces a tree’s
ability to photosynthesise. Because of this, sugars become unavailable to the
roots causing their starvation and subsequent death. Of course, no true balance
can ever be met; it is just an ideal state, as the system is always dynamic.
Coppicing is a similar treatment to pollarding, and is a traditional
woodland management technique. The tree or shrub is cut back near ground
level, which in encourages rejuvenation resulting in new stems from dormant
buds at the base. Not all species have a capacity for rejuvenation, though
a number of species show it very strongly, such as: Acer pensylvanicum
‘Erythrocladum’, beech (Fagus spp.), birch (Betula spp.) dogwoods (especially
those for ornamental purposes such as Cornus sanguinea ‘Midwinter Fire’,
gum trees (Eucalyptus gunnii), foxglove tree (Paulownia spp.), hazel (Corylus
spp.), hornbeam (Carpinus spp.), Indian bean tree (Catalpa), Judas tree (Cercis
spp.), sweet chestnut (Castanea sativa), tree of heaven (Ailanthus altissima),
and willow (Salix alba var. vitellia ‘Britzensis’). Conifers tend not to have the
ability, though a few do, such as the coastal redwood (Sequoia sempervirens),
Japanese -cedar (Cryptomeria japonica), Chinese-r (Cunninghamia lanceolata),
Californian nutmeg (Torreya californica) and pitch pine (Pinus rigida) and
P. leiophylla. Although yew (Taxus baccata) can tolerate heavy pruning, and,
in its hedge form, can be coppiced; however, a mature yew when taken back
to ground level will not recover. Coppiced trees are maintained in a state of
juvenility, so, like trees that are pollarded, are the ‘Peter Pans’ of the tree world,
where old age is always in the far distance.
Crown-reduction is an arboricultural operation that results in the overall
reduction in the height and spread of the tree’s crown by means of shortening
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branches to an appropriate nodal point or branch union (where branches
diverge). Pruning at a nodal point should be to reduce a branch to a side
lateral no less than one-third the size of its original girth. This is essential, as
the side branch must be of an appropriate size to take over the role of leader
for the entire limb. Failure to do so would increase opportunities for decay-
causing fungi to enter the increased wound surface areas, and can also result
in dieback owing to the insufciently sized nodal points. The malpractice
of removing branches to an inadequate nodal point or stub is referred to as
‘tipping’. Tipping is a common malpractice, even by professional arborists,
and can cause trees serious harm. Arborists often do this when there are no
A well respected and sound practice among practitioners, crown reduction is a method that can
reduce tree size appreciably when there is a strong argument in favour of it. Of critical importance
is the position of the offensive branches that are reduced. They must be reduced to a nodal point
usually of no less than one third the size of the diameter of the branch being reduced. There are
few exceptions to this.
illustration © J R Shepherd www.inkyleaves.com
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suitable side branches to prune back to. However, in this situation it is better
to remove the branch completely. Tipping encourages sprouts that are weakly
attached, and leads to decay, often spreading into the main trunk. According
to Shigo (2008), tipped branches can live for many years, though usually less,
and can then become a hazard. During routine tree care, another observation
through scientic studies by Gilman (2012) is that the pruning wound of
branches pruned at a junction should not exceed ve centimetres in diameter
in poor compartmentalisers, and 10cm in strong compartmentalisers. At a
junction between a principal and lateral branch, there is no ‘Branch Protection
Zone’, where there lies a specialised mesh of interwoven cells that inhibit
inward progression of fungi. A good crown-reduction will appear natural
and as though the tree has not been pruned. The shape of the tree should be
conserved, and the side branches, which take over the role as leaders around
the periphery of the tree, should hide any otherwise noticeable pruning
wounds. Well performed this is both an art and a science.
So why reduce the crown? In most cases trees are reduced owing to a fear
about their size. People react when they become aware of the danger of a large
tree, though as cited by the protagonist Ishmael in the classic novel, ‘Moby
Dick’, ‘ignorance is the parent of all fear’. It is hard to bring clarity to the
subject when we are overburdened by a litigious society bringing with it an
inated sense to protect both property and people. Trees fail by their very
nature, being living organisms. They can fail for no obvious reason (known
as an ‘Act of God’), though in most cases fail owing to an obvious defect that
TREE PRUNING
The large cavity in the lower bole of this half
of the Preston twins, Brighton, shows why it
is necessary to carry out crown thinning and
reduction. Apart from old trees not being
able to conjure up the energy to transport
nutrients upwards to any great height, a low
stature tree is also more wind resistant. The
bole thickens to an extent unusual among
unpruned trees, as carbohydrate reserves
target a reduced area.
photograph © Hugh Morris
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would be observed by the professional arborist. A crown-reduction is carried
out to reduce the ‘sail area’ of the tree, thereby easing the mechanical stresses
on a tree of weak disposition. For example, a tree with advanced decay within
the main trunk, but too important to remove, is reduced in size to lessen the
likelihood of failure and to increase its longevity. This is especially important
illustration © J R Shepherd www.inkyleaves.com
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if the tree is a specimen of a high amenity value. Among other reasons to carry
out crown-reduction would for instance be in situations where a mature tree
has weak co-dominant stems (the stems may also be braced) or where a tree
is underneath a utility line. Other reductions may be restricted to one side of
the crown, such as when pruning a street tree back from shop windows, or a
house, etc.
There is another situation where crown-reduction is appropriate, and this
is in the case of veteran trees. A veteran tree is one of exceptional merit, both
from a cultural and conservation standpoint, owing to such factors as age,
condition, and/or rarity. Veteran trees or those approaching such status, often
develop twin crowns, with a second developing inside the outer crown. This
is in response to the sheer size and age of the tree, when transport of nutrients
and water to the outermost parts, becomes increasingly difcult. In this
condition the outer crown gradually becomes sparser, while the inner crown
becomes lusher, owing to an increase in bud production. Dr Claus Mattheck, a
professor of biomechanics at Karlsruhe Institute of Technology, Germany, has
observed the nature of trees for decades. He says that in order to prolong the
life of an ancient tree, one must prune the outer crown in order to consolidate
the canopy to compensate for decreasing root function. He maintains that when
the upper crown becomes visibly distinct from the lower this is an indication
to the professional arborist for the crown to be reduced accordingly. This kind
of crown reduction is referred to as ‘retrenchment pruning’, and should be a
gradual exercise (over a period of years) that keeps pace with the decline of
the tree.
The amount of live tissue that can be removed at any one time depends
on the size, condition, age, and the species of tree being pruned. As a general
rule of thumb, the British Standards 3998 recommend pruning no more
than 30% of an individual branch, though this is an arbitrary gure, with no
scientic basis. Species such as birch (Betula spp.), beech (Fagus spp.), honey
locust (Gleditsia spp.), walnut (Juglans spp.), Japanese pagoda (Styphnolobium
japonicum), Laburnum spp., pear (Pyrus spp), and Prunus spp. are poor
compartmentalisers (i.e. with less ability to wall-off decay), so large scale
reductions on more mature specimens should be avoided in order to maintain
both tree health and safety. This is especially the case if there are no suitable
nodes to prune back to. Oak (Quercus spp.), lime (Tilia spp.), plane (Platanus
spp.), poplar (Populus spp.), maple (Acer spp.), willow (Salix spp.), yew (Taxus
spp.), ash (Fraxinus spp.), tree of heaven (Ailanthus altissima), mountain ash
(Sorbus), horse chestnut (Aesculus spp.), and alder (Alnus spp.) can tolerate
heavy pruning, partly because of having sufcient dormant buds in older
wood, and their ability to compartmentalise decay (Cutler and Richardson,
1989). However, both poplar and willow are poor compartmentalisers, so
careful assessment should be made on such genera in the pruning for the
commencing year. A great practical knowledge of trees is required to enable
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The removal of lower branches is a solution to many tree related issues, whether the tree is too
close to a house, blocking a vista in an arboretum, or making contact with vehicles and passer’s by
along a street.
the arborist to estimate what an individual tree can tolerate, and this know-
ledge is only gained after years of experience and schooling.
Crown-lifting operations involve the removal of the principal lower lateral
branches of a tree, thus achieving a desired vertical clearance. This commonly
undertaken operation is carried out most usually on street and roadside trees to
provide the minimum satisfactory clearance for vehicles and/or passers-by. It
is also undertaken in arboreta and woodland gardens to create vistas and to
provide more light to plants underneath the canopy layer. In arboreta it would
be referred to as stem clearance rather than height clearance, the latter being
more specic to urban trees. In nearly all cases stem clearance is undertaken
illustration © J R Shepherd www.inkyleaves.com
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on mature trees, where major scaffold limbs (those arising from the trunk) are
only removed as a last resort. However, in mature trees that have grown tall in
response to light, principal lower limbs are often not of great diameter, so some
can be removed with due care and attention. The preferred time to crown-
lift a tree is at the tree nursery, while a tree is young. This is referred to as a
‘standard’ tree, where the lower branches are removed to a height of 1.8m. This
formative pruning is cheaper and easier (and carried out with a secateurs),
with the tree responding favourably to such mild treatment. As advocated by
Shigo (2008) and Gilman (2011), it also stimulates a stronger central leader on
a young tree by removing auxin coming from lateral branches. This hormone
TREE PRUNING
The Jersey Elm (Ulmus minor subsp. sarniensis was once commonly planted on streets throughout
the UK for its strong apical dominance (ability to retain a central leader). This splendid attribute
meant it could grow tall, with branches being kept duly subordinated. Such tall trees can be crown-
lifted above vehicles and passers-by without any compromise to tree health. These trees are still
evident throughout Brighton and Hove, where Dutch elm disease has been kept under control.
photograph © Hugh Morris
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produced in branch tips stimulates apical but suppresses lateral growth.
In nature lower branches are often shed owing to lack of light, as seen in
conifers, a particularly good example being the monkey-puzzle tree (Araucaria
araucana). However, in most deciduous trees lower branches are retained, often
drooping downwards until contact is made with the ground. Lower branches
are the tree’s best insurance in its old age (Mattheck, pers.com., 2010). Lower
branches, especially in mature specimens in open spaces, should be removed
only for valid reasons. In many cases crown lifting encourages mowers and
strimmers to cut too close to the trunk and cause damage to the bark (both outer
and inner bark, with the latter including the phloem, a key component of the
tree’s circulatory system). This is among the major causes of death of young
trees. In one event I know of, the offender had ploughed into a mature birch
tree, causing large sections of bark to detach. I inspected the tree upon noticing
dieback in the crown, where I observed to my astonishment bark secured
back in its rightful place by numerous small nails! One should take care not
to remove too many branches: a clearance along one third of the main trunk
should be the maximum permitted from the overall crown, and preferably
carried out over a number of seasons. Removing lower branches changes the
balance of stress throughout the tree, and this is particularly harmful when
pruning is excessive. Carbohydrate distribution alters radically, changing the
taper of the stem as the lower trunk becomes depleted of energy. This also
hinders the pathway of carbohydrates and auxin to the roots, both essential for
root function. In response to this alteration in structural biomechanics, a tree
will often grow stress-induced epicormic shoots associated with increased light
levels and auxin production. Sun-scorch is also often a possibility in exposed
thinner barks of some species, such as beech (Fagus spp.); however recent
information may cast doubt over a beech tree’s vulnerability to exposure, with
some workers suggesting anecdotally that oak may be more susceptible.
In some species, notably conifers such as the bald cypress (Taxodium
distichum) and the dawn redwood (Metasequoia glyptostroboides), crown lifting
should be avoided purely for aesthetic reasons. Their beauty, particularly in
autumn, should be admired from the ground upwards.
Crown-thinning is the selective removal of branches from within the crown
to increase air movement and light penetration. Generally it involves the
removal of live tissue (including diseased material), although it can also be an
opportunity to remove deadwood. Collectively the process is termed a ‘crown
clean’. The removal of crossing branches, diseased and defective wood should
be considered before any other branches are removed. By allowing light to
penetrate the crown through selective removal of branches, one attempts to help
the remaining branches survive and increase in diameter. This prerequisite
also allows one to stand back and consider the next move. Like a chess player,
one doesn’t want to move too soon, as a better selection may lie ahead. One
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must also know when enough is enough. Like painting a masterpiece, one
must know when to walk away.
Crown-thinning is the selective pruning of small diameter branches back
to suitable nodes, with the majority of the work being carried out in the
outer third of the crown. Adjustments to the amount of live tissue removed
depend on the species and age of the tree being worked on, and this should
be estimated only by an experienced arborist. For instance, it may state in
guidelines that no more than one quarter of foliage should be removed during
a single treatment, and there is a danger that inexperienced and/or unschooled
TREE PRUNING
A highly debated pruning method, crown thinning is certainly of benefit to a small number of
species with dense unruly crowns, such as Prunus cerasifera ‘Pissardii’ (along with other cultivars of
purple plum), Prunus serrula, and Tilia species. while young. A crown clean, similar to a crown thin,
includes the removal of deadwood.
illustration © J R Shepherd www.inkyleaves.com
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arborists may look at this as being the percentage to target rather than the
uppermost limit. Over-thinning is a common event that often results in ‘lion
tailing’. This malpractice all too commonly involves the excessive removal of
lateral branches along the lower principal (or scaffold) limb. It shifts the load-
bearing capacity of the limb to the upper regions, placing the entire limb under
great stress; just as excessive crown-lifting does to the trunk. Retaining lower
branches maintains branch taper from the base of the limb towards its tip, so
the removal of branches reduces this necessary taper, thus producing greater
overall stress. This is due to the re-allocation of sugars to the uppermost
This is a practice that is frowned upon in the industry, though does not appear harmful to the
amateur. The removal of side branches along the inner scaffold limb leaves the branch exposed
to potential failure when its mechanics are manipulated. It may also be an entrance point for
pathogens. Epicormic growth (water sprouts from adventitious buds) is an unsightly response by
the tree to such malpractice.
illustration © J R Shepherd www.inkyleaves.com
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parts. Decay is also a likely consequence, and many pruning wounds in close
proximity may lead to large decay pockets. These factors could easily result
in failure because of weak lateral branches. Excessive epicormic growth and
sun scorch, due to too much reduction of shade, are among the many concerns
caused by this practice.
The aesthetics of the tree can be enhanced by emphasising individual
limbs, rather than leaving a great mass of unkempt foliage and interwoven
branches. Species that would benet from thinning include purple plum
(Prunus cerasifera ‘Pissardii’), Tibetan Cherry (Prunus serrula), olive (Olea
europaea), dogwood (Cornus spp.), and Ginkgo biloba, all of which tend to
develop overcrowded crown structures. Limes (Tilia spp.) can also benet
from this practice, especially in young trees.
Recent research carried out by Gilman (2008) showed that thinning in
mature trees was largely a pointless exercise, as new growth, stimulated by
the greater penetration of light, quickly grows back. However, during the
period between the thinning operation and subsequent regrowth, the trees
were shown to be more wind tolerant (Gilman, 2011). Where crown-thinning
is required in mature trees it shows that little or no ‘structural pruning’ was
carried out during the tree’s formative years. Young trees respond much more
favourably to selective branch removal, though structural pruning would
be a welcome alternative to crown-thinning in mature trees, as advocated
by Gilman. Structural-pruning is the removal of live branches and stems to
inuence the orientation, spacing, growth rate, straight of branch attachment,
and ultimate size of branches and stems (Gilman, 2011). Shigo, on his online
website, assigns crown-thinning as a practice of the past by theorising that the
inner crown has a function in throwing wind off course, as do the fronds on a
palm tree. Though, with considerable respect for the work of Shigo, eld trials
by Gilman (2008) demonstrated that this seems not to be the case. However,
crown-thinning might become a practice of times gone if the advent of
structural pruning takes hold in modern arboriculture.
Deadwooding is the removal of dead and diseased wood from within the tree’s
crown. It is removed by ‘Natural Target Pruning’, the prescribed pruning
method as advocated by Shigo that keeps the branch collar intact, thus limiting
the entry of decay. This swollen area of live tissue is more pronounced around
deadwood, as the stem wood continues to grow engulng the basal region of
the deadwood in the process. Although not strictly dened as a branch collar,
it should be treated as such. The principal reason deadwood is removed is
due to the fact that it can pose a safety threat. This is a concern when dead
branches overhang a footpath, highway, buildings, or amenity areas However,
as expressed by Mattheck (pers. Com., 2010), deadwood is not necessarily
weak, in fact, as a result of its low moisture state, it is stronger than live-wood
when decay is not advanced. Also deadwood does not have the additional
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weight of foliage to carry. The species of tree is of course another factor, as for
instance oak (Quercus spp.) deadwood can remain on the tree for long lengths
of time, whereas that of poplar (Populus spp.) and willow (Salix spp.) may
break away quite rapidly, owing to the wood type and its susceptibility to
fungal rot. Whether it is a hardwood or softwood must also be considered, as
the former, with some exceptions tends to decay more rapidly and therefore
break away sooner.
Aesthetics must also be considered in specimen trees, those that provide
high amenity value, whether in a park, arboretum or along a street. Removal
of deadwood in these instances should be recommended, as it can be
unsightly. Also, as Shigo has acknowledged, the removal of deadwood as trees
age, is a ‘health precaution’, because the deadwood is an energy source for
fungi. His analogy likened the tree to a business stating that, ‘as your business
grows and ages; you must take care to recognise deadwood among employees
and non-living things. The faster you discard deadwood, the healthier your
business will be’. Fungi can invade living tissue of a tree beyond the branch
collar via deadwood. If the deadwood is removed by ‘Natural Target Pruning’,
then the spread of decay-causing fungi will be restricted.
Where trees are at distance from built-up environments, arboreta,
gardens etc., they should, where possible, be left to their own devices, since
deadwood forms a very valuable component of the natural ecosystem. Where
aesthetics and tree health would be of concern in trees of high amenity value
in the landscape, such clinically treated trees would be out of place, e.g.
in the countryside, woodlands, or in conservation areas such as national
parks. Design of public access routes around trees with retained deadwood
may be necessary to reduce risk. Neville Fay, a British arboricultural
consultant, is an advocate of ‘Coronet Pruning’, a natural fracture technique
carried out to mimic the effects of natural breakage in storms. This technique
combines safety concerns with wildlife objectives, and is a popular practice
in large countryside parks, such as Knepp Park, Sussex. Unfortunately, this
skilled chainsaw-honed sculpting does not have a place where amenity and
tree health are priorities, as it quite simply encourages microbial rot.
Formative-pruning is used on young trees. It is a form of structural-pruning,
though the latter term includes trees of all ages. As a mild treatment in
young trees, it allows the development of a good and sound crown structure
that prepares the tree for its future. This pre-emptive approach prevents
development of weaknesses by removing suspect branches. One of the
principal weaknesses that can occur in young trees is the development of
‘co-dominant stems’, which are, in many cases, mechanically weak unions
resulting from bark-to-bark contact. This can be as a result of the genetic
constitution of the tree, or, as is most often the case, the way it develops when
growing in a human-made environment. It occurs when there is a scramble for
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dominance between competing branches. Formative pruning is carried out in
the tree nursery as a matter of course, though it is dangerous to assume that
this has been done. Also, if trees are purchased as seedlings, or raised from
seed, the task lies with the grower. A leader must be selected by removing or
subordinating (reducing in size at least by one quarter) potential ‘takeover’
branches, together with the removal of branches with poor unions. Gilman
(2011) also recommends the subordination (reducing by pruning back) of
lower branches with a high aspect ratio (diameter of branch to trunk ratio) by
as much as 30 to 60%, as anything less apparently makes little difference. The
auxin is then redistributed resulting in greater terminal growth (i.e. at the top
of the tree), as once-competing stems are now restricted.
Shigo (2008) refers to a young tree as a tree of 100% dynamic mass (no
heartwood or ripewood). This means that a young tree is composed of living
cells throughout its cross-section. A young tree can therefore have living
tissue removed without serious detriment, as there is enough remaining
energy to maintain order in the system. Such energy can only be stored in
living parenchyma cells in the form of starch, which is readily converted into
glucose when required for growth processes. As a tree matures, the ratio of
living to dead tissue changes, dead tissue being ‘static’ and living tissue being
‘dynamic’. Shigo calls this the ‘core-skin hypothesis’, where a skin of dynamic
TREE PRUNING
Formative pruning, a type of structural pruning, reduces the necessity to prune in later years.
Defects develop in trees from a young age and it is at this time that they should be removed.
Defects include co-dominant stems with included bark, where a lateral branch competes with
the leader for dominance often resulting in compression forks.
illustration © J R Shepherd www.inkyleaves.com
227
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STUDY DAYS
mass (annual growth) is formed over the aging core (later to become heartwood
or ripewood). This transition inuences our approach to tree pruning. As the
tree ages we must remove less live tissue, as it becomes increasingly required
for the continued functioning of the tree. What I mean here is that as the tree
ages and gets larger, more energy is required by the tree, so more needs to
be produced a reserve of potential energy (stored energy) is necessary to
meet the demands for kinetic energy (working energy). This is quite difcult
to grasp, but its elegantly simplied theory as expressed by the late Alex
Shigo has changed the way we view and work with trees. The message is
simple: prune trees while young to avoid having to do it when older.
When to prune trees
There is no stipulated time to prune, as pruning can be carried out all year
around, though whether or not certain times of the year are more acceptable
depends on the severity of the pruning and the species of tree concerned. When
pruning, it is important to consider phenology, i.e. the timing of occurrences,
such as bud-break, owering, fruiting and leaf-fall, and how these affect
carbohydrate (energy) availability. The level of parasites such as fungi,
insects, and micro-organisms should also be taken into account. So when
we prune we must take these relationships into consideration. For instance,
fungal spores are most abundant in autumn, when trees happen to be at their
weakest. Also, because of the advent of dormancy in trees (particularly in
deciduous trees), active glucose is transferred to starch for storage over the
winter. So the tree lacks an active defence, and this is especially serious when
pruned. CODIT (Compartmentilisation of Decay in Trees) is an active process
and does not work in dormant trees. Another concern is that wood moisture
levels are at their lowest point in the dormant part of the phenological calendar,
a position that is most favourable for the germination of hyphae from fungal
spores. My advice would be to avoid pruning at this time, unless it is very
mild pruning on younger trees. Winter is a better time to prune, as although
the tree is inactive micro-organisms and insects tend to also be inactive owing
to the lower temperatures. However, this is changing with warmer winter
temperatures brought about by Climate Change. As a result of this climatic
factor, micro-organisms and insects are becoming more active during winter,
which is a worrying trend. Also, extreme low temperatures can induce cracks
and dieback from pruning wounds during the winter months. The approach
is to be cautious at this time by not making large pruning wounds, thus leaving
the tree more susceptible. The branching network of deciduous trees is more
visible at this time, so it is certainly more enticing for the arborist or amateur
to prune. Apple growers carry out pruning during mid-to-late winter by way
of tradition, as it ts into their fruit-harvesting schedule. So the approach one
takes to pruning involves critical judgement and consideration of a number of
variable factors.
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INTERNATIONAL DENDROLOGY SOCIETY
To conclude: the management of trees when you consider the complex
nature of their growth and the plethora of physiological differences between
species, should be carried out by an experienced climbing arborist, and often
only after consultation with an academic or consulting arborist. Trees and
their management is a specialist subject in the realm of arboriculture, the
discipline devoted to the management and cultivation of trees, shrubs, and
woody climbers. Much poor pruning practice is evident throughout arboreta,
parks, and estates owing to the lack of arboricultural expertise among those
who carry out the work. We have taken a backward step over recent decades,
where the arborist has been epitomised as one who only cuts trees down, or
prunes trees, often badly. This is, of course, true, though we must recognise
the difference between a ‘tree surgeon’, and an arborist, where the latter term
should only be used for those with excellent tree pruning skills and a sound
knowledge of how the tree functions. An academic arborist (or arboriculturist),
one who is educated to at least degree level in the science of arboriculture, is
highly knowledgeable in matters related to woody plants. Through science,
they can make sound judgements with regards to their pruning whereby the
practitioner of arboriculture will be informed of any advancement. Charles
Sprague Sargeant, the rst Director of Arnold Arboretum, in Jamaica Plain,
Massachusetts, referred to himself in the preface of Des Cars famous text,
A treatise on Pruning Forest and Ornamental Trees, as Professor of Arboriculture,
Harvard College, which was his ofcial title at Harvard from 1879. Sargeant
was not only an academic arborist, but also an excellent botanist.
Trees are hugely important in our landscape, and with scientic
management of their correct pruning especially in their formative years – can
make marked differences to their inuence on society. Trees become safer with
fewer opportunities to fail; they become healthier having fewer entrances for
decay-causing fungi to inhabit; trees become more beautiful as less deleterious
pruning is required in later life; and they live longer. Trees cannot adapt to
our mistreatment of them. As Shigo stated, ‘the danger is that our insults and
assaults are coming faster than the system can adjust or adapt’. Trees are long-
lived organisms with fewer opportunities for rapid evolutionary adaptation,
so we must consider this and treat them with dignity.
I will end this on a quote by the famous dramatist Anton Pavlovich Chekhov
written in 1888, ‘A tree is beautiful, but what’s more, it has a right to life; like
water, the sun and the stars, it is essential. Life on earth is inconceivable
without trees. Forests create climate, climate inuences peoples’ character,
and so on and so forth. There can be neither civilization nor happiness if
forests crash down under the axe, if the climate is harsh and severe, if people
are also harsh and severe. ... What a terrible future!’.
I would like to extend particular thanks to Dr Jim Ratter for reading the article and making
useful suggestions. A special thanks to Jessica Rosemary Shepherd for her valuable artistic
contribution to this work. Her illustratory work can be found at: http://inkyleaves.com
TREE PRUNING
229
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STUDY DAYS
References
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Derirtas, N. M., Bolat, I., Ercisli, S., Ikinci, A., Olmez, H., Sahin, M., Altingag, M., and
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Des Cars, A. (1884). A treatise on pruning forest and ornamental trees. Massachusetts Society for
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Dujesiefken, D., Liese, W., Shortle, W., and Minocha, R. (2005). Response of beech and oaks to
wounds made at different times of the year. European Journal of Forest Research 124: 113-117
Fay, N. (2002). Environmental arboriculture, tree ecology and veteran tree management.
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Fay, N. (2004). Natural fracture pruning techniques and coronet cuts. www.treeworks.co.uk
Fay, N. (2004). Survey methods and development of innovative arboricultural techniques in key
UK (United Kingdom) veteran tree sites. In: The trees of history: Protection and exploitation of
monumental trees. Proc. International Congress 64–75. Torino, Italy
Ferrari, F. (2004). Summer pruning. Acer 3:53–55.
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Gilman, E. F., and Grabosky, J. (2006). Branch union morphology affects decay following pruning.
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Gilman, E. F., and Lilly, S. (2002). Best Management Practices Tree Pruning. International Society
of Arboriculture. Champaign, IL.
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Gilman, E. F., Master, F., and Grabosky, S. (2008). Pruning affects tree movement in hurricane
force wind. Arboriculture and Urban Forestry, 34(1): 20-28
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Haddad, Y., Clair-Maczulajtys, D. and Bory, G. (1995). Effects of curtain-like pruning on
distribution and seasonal patterns of carbohydrate reserves in plane (Platanus × hispanica
Wild) trees. Tree Physiology. 15, 135-140.
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American Edition, Boston. 65p
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Shigo, A. L. (1984b). Tree decay and pruning. Arboricult.l J. 8:1–12.
Shigo, A. L. (1985b). How tree branches are attached to trunks. Can. J. Bot. 63:1391–1401.
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Shigo, A. L. (1986). A new tree biology: Facts, photos, and philosophies on trees and their problems
and proper care. Shigo and Trees, Associates, Durham, NH. 595 p.
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... 98 que presentan suficientes yemas latentes en madera, y a su mayor capacidad para compartimentar la descomposición (Morris, 2013). ...
Book
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Este Manual propone un nuevo enfoque para una antigua disciplina. Cada capítulo ofrece fundamentos que desde la arboricultura moderna valoran el entendimiento de la biología del árbol como un sistema vivo, complejo y altamente eficiente. Esta nueva perspectiva requiere la formación de profesionales idóneos, calificados y diversificados, que posibiliten una gestión equilibrada, inteligente y sensible del árbol como individuo y del bosque urbano como universo. Por otro lado demanda una permanente actualización y colaboración, a fin de promover la creación de equipos interdisciplinarios capaces de diseñar y gestionar planes estratégicos para satisfacer las metas exigibles para la sostenibilidad del arbolado como componente esencial de la infraestructura verde urbana. Este Manual surge como resultado del trabajo en áreas de gestión pública de arbolado urbano, del desarrollo docente en el ámbito universitario y de la interacción en distintas asociaciones profesionales. Su contenido está ideado para cubrir los saberes del técnico que debe gestionar el arbolado en el ámbito público o privado, así como también al estudiante de alguna de las carreras de Ciencias Agropecuarias que se oriente en la formación en Silvicultura Urbana.
... Pogosti razlogi za obžagovanje dreves so odstranitev odmrlih ali odmirajočih vej, izboljšanje strukture, estetika in zmanjšanje tveganja. Drevesa obžagujemo tudi, kadar želimo povečati prodor svetlobe in zraka v notranjost ali v spodnji del krošnje (Argent, 2000;Morris, 2013;Purcell, 2015; ISA, 2020). ...
Research
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Izvleček: Sever Brglez, B., Brglez Sever, M.: Nega dreves na višini v urbanem okolju; Gozdarski vestnik, 78/2020, št. 5-6. V slovenščini z izvlečkom in povzetkom v angleščini, cit. lit. 42. Prevod Breda Misja, jezikovni pregled slovenskega besedila Marjetka Šivic. Arboristika je že uveljavljena veja urbanega gozdarstva, ki omogoča ohranjanje zdravih, estetskih ter varnih dreves. Arboristične metode in negovalni ukrepi lahko pripomorejo k ohranitvi dreves, ki prispevajo k biolo-ško-ekološki, družbeni, gospodarski in kulturni vrednosti v mestnem okolju. Članek vključuje kratek pregled objav, ki definirajo pojem arboristike in različne profile arboristov. Razprava se osredotoči na pojmovanje ar-borista-negovalca dreves na višini in izvedbo nege ter sanacije dreves na višini. Zbrane informacije opisujejo vzroke in vrste poškodb dreves v urbanem okolju ter predstavijo priporočila pri obžagovanju dreves ter primeren čas rezi glede na starostno obdobje dreves in koledarski čas. Vključene so tudi vsebine raziskav glede ocene vitalnosti in nevarnosti dreves, izvedbe obžagovalnih rezi ter različnih tehnik obžagovanja in odziv dreves na mehanske poškodbe. Abstract: Sever Brglez, B., Brglez Sever, M.: Tree Care on Height in Urban environment; gozdarski vestnik (Professional Journal of Forestry), 78/2020, vol 5-6. In Slovenian, abstract in english, lit. quot. 42. Translated by Breda Misja, proofreading of the Slovenian text Marjetka Šivic. Arboristics is an already established urban forestry branch enabling the trees to stay healthy, esthetical, and safe. Arboristical methods and treatment measures can aid in the preservation of the trees that contribute to biological-ecological, social, economic, and cultural value in the city environment. This article incorporates a short review of publications defining the concept of arboristics and diverse profiles of arborists. The discussion focuses on the concept of an arborist-tree care worker on height and his performing of care and rehabilitation of trees on height. The gathered information describes the causes and types of tree damages in the urban environment and presents suggestions for pruning as well as an appropriate time for pruning with regard to the age of the trees and calendar season. Also, the contents of the researches regarding the vitality and dangerousness of the trees, execution of the pruning as well as diverse pruning techniques, and the reaction of the trees to the mechanical injuries are included.
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We built a machine with a, propeller capable of generating 33.5 m/s (75 mph) winds to determine the influence of pruning dose and American National Standards Institute A300 pruning type on trunk movement of Quercus virginiana 'QVTIA' PP #11219, Highrise® at various wind speeds. Trunk movement was regressed against wind speeds and pruning doses for each tree tested. Increasing wind speed increased trunk movement, and the magnitude of the increase depended on pruning dose and pruning type. Increasing pruning dose reduced trunk movement and the magnitude of the reduction was greater at higher wind speeds. The predicted trunk movement of thinned trees was statistically greater than movement of structurally pruned, raised, and lion's tailed trees at wind speeds of 20.1 m/s (45 mph) and was greater than all pruning types at 26.8 m/s (60 mph). There was no difference in movement among reduced, raised, structurally pruned, and lion's tailed trees; and there were no statistical differences in trunk movement among pruning types at the lower wind speeds. We found that thinning the outer edge of the crown was one of the least effective pruning types for reducing trunk movement in wind.
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Two hundred one research publications including 152 journal articles were compiled. Forty-four journals were represent-ed with the Journal of Arboriculture, Arboricultural & Urban Forestry, and Arboricultural Journal as the most frequently cited. Com-partmentalization, wounding, wound response, decay development, and wound treatment were the most frequently noted topic areas. The bibliography was organized in Zotero, an application using the Firefox web browser. Keywords were identi-fied for each publication. Where either the article or its abstract was available, an annotation was created. This pa-per describes the major topic areas identified in the review and discusses the future directions for pruning research. Pruning is at the heart of arboriculture, one of the most impor-tant services arborists provide. To paraphrase Alex Shigo (1989), pruning can be one of the best things an arborist can do for a tree and one of the worse things an arborist can do to a tree. Pruning impacts both tree health and structure. It is practiced worldwide. In 2007, the International Society of Arboriculture (ISA) contracted HortScience, Inc. to prepare a literature review on the topic of pruning. The focus of the review was the research literature. The emphasis was on arboriculture but the review could reference forestry and pomology literature as appropriate.
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The wind load, bending moment, height, and weight were determined for 81 red maples (Acer rubrum) before and after pruning. Trees were thinned, reduced, lion tailed, or stripped of foliage. All three pruning treatments reduced wind load signifi- cantly compared to unpruned trees at all tested velocities (11, 16, and 20 m/sec (25, 35, and 45 mph)). Reduction in wind load increased with increasing velocity. Differences in wind-load reduction between reduction pruning and thinning were not significant at any velocity. The reduction in wind load was linearly related to the amount of weight removed by pruning treatments. Compared to the same trees prior to pruning, the center of pressure height was significantly lowered on thinned and reduced trees, while the center of pressure height did not change on lion-tailed trees.
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Summary The quality and condition of Britain's old tree heritage is reflected in the great number of ancient tree sites found in the British Isles, reputed to be among the finest in Northern Europe. Environmental arboriculture and veteran tree conservation methods have been influenced by a multidisciplinary approach that considers the tree in co-evolutionary terms, inherently linked to its ecological context. This approach considers the importance of the ageing process and the use of terminology relating to the developmental stages a tree passes through. The interactions between trees, fungi and other dependent organisms are considered, particularly in the light of the need for arboriculture to take account of how tree management can work to maintain or re-establish conditions suitable for a functioning tree-ecosystem. Understanding the rates of decline in old tree populations has been improved considerably in the UK since the development and use of the Specialist Survey Method. Studies of important veteran tree sites indicate that the rate of tree loss may be unsustainable. This in turn threatens the continuity of dependent saproxylic communities. Innovation in arboricultural management techniques has been influenced by observations of natural processes. The recent emergence of environmental arboriculture provides a framework for considering such issues and for developing appropriate practices to manage trees to enhance longevity and biodiversity. Principal among these is the development of Individual Tree Management Plans for veteran trees, which set planned treatments programmes for 30 to 100 years. Environmental arboriculture, while responding to all current conventional arboricultural considerations and practices (including tree hazard management and amenity tree care), offers scope for an interdisciplinary synthesis of perspectives between all those involved in the appreciation of veteran and ancient trees.
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Trees have a spectacular survival record. Over a period of more than 400 million years they have evolved as the tallest, most massive and longest-lived organisms ever to inhabit the earth. Yet trees lack a means of defense that almost every animal has: trees cannot move away from destructive forces. Because they cannot move, all types of living and nonliving enemies?fire, storms, microorganisms, insects, other animals and later man?have wounded them throughout their history. Trees have survived because their evolution has made them into highly compartmented organisms; that is, they wall off injured and infected wood.
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Trees respond to injuries and infections by setting boundaries to resist the spread of microorganisms. The boundaries also resist the spread of microorganisms from dying branches into the joining stem. Pruning cuts should not be made behind the branch bark ridge. Such cuts remove the protective boundaries, allowing microorganisms to spread rapidly into the stem. When branches are pruned properly, there is no need for wound dressings. When branches are pruned improperly, no amount or type of wound dressing will help.