The Recognition of Hazardous Trees
Some Common factors
Windblow can occur when a tree is subjected to wind which it
has not experienced previously and is usually encountered where other trees have
been removed in the recent past and the existing tree or trees are now more exposed
than previously. Removal of buildings or the erection of new buildings can also
cause changes in wind directions potentially adding to this hazard. Windblow can,
however, also occur when a tree has a damaged root system.
Summer Branch Drop is difficult to predict and usually involves
a mature or over-mature tree losing a large limb on a calm summer day. In my experience,
it is often the case that the tree has not been inspected for many years and was
in need of some maintenance.
Previously pollarded trees and previously topped trees have
usually been badly treated and evidence of inter-nodal pruning, cavities, slime
flux, major deadwood, included bark and many other symptoms should be viewed with
suspicion. After pollarding the rapidly growing branches can be founded upon decaying
wood or on the outer cambial layer of the tree causing potentially unstable limbs.
Pruning wounds such as flush cutting in the past often allows
pockets of decay to develop beyond what may appear to be a clean wound.
Broken and hanging branches and storm injury often involve portions
of the tree, which may have already failed but may be lodged within the upper
crown, yet to fall.
Weak forks & cracks or splits in the stem are usually indicative
of a potential problem and the need for action. Loose bark is often indicative
of previous injury to the tree and requires close examination to assess the seriousness
of the damage.
Basal cavities may be a sign that more serious decay is present
in the tree's stem or root system.
Damaged roots are often hard to predict, but on new housing sites
evidence of digging or raised levels around trees should be carefully investigated
and commented upon.
Leaves or needles if small, light in colour and sparse may be
a sign that the tree is stressed in some way.
Dead twigs and branches can fall out of a tree causing personal
injury or damage to property.
Cankers or other malformation in the tree's canopy or on its
stem are worthy of further inspection.
Root rot is not always evident but should be closely watched
for.
Abrupt bends in branches or stems can be indicators of internal
faults or physical weaknesses.
Fungal fruiting bodies should always be identified and their
significance reported upon. The species of fungi and the type of decay caused
along with the location on the tree and extent of decay are all important factors.
Cracks in the soil around a tree's base may be
simply dry soil but it could indicate that the tree is insecurely rooted.
Leaning trees may have leaned all their lives but may also have
partially failed. Look for the angle of the new extension growth in the tree's
top.
Species Characteristics
Different species have different characteristics. Willow and Poplar for example
are liable to lose limbs or fail because of the cell structure in their wood whilst
Ash is renowned for losing limbs when mature. Anyone inspecting trees should have
a thorough knowledge of what defects are inherent in the various species present
in the area in which they operate and what weaknesses these can lead to.
Location of the tree
When considering what the risk is of a tree failing and what action is appropriate,
attention should be given to the following points:
Targets - if the tree fails in part or in whole, what will be
damaged? The ‘target’ is what the tree will fall upon or is likely to damage.
The frequency and pattern of use should be considered. You should ask, ‘Is the
target static? If so can it be moved? If not what other options are available?’
Houses - if a house is within falling distance of a tree, the
situation is deserving of very careful consideration.
Greenhouses or sheds - these can often be moved if a tree is
only a minor risk and they are situated beneath.
People - this involves trees over footpaths and roads or trees
in places where people are lawful visitors. Can people be kept away? If a path
can be diverted to keep people away could this lessen the risk?
Roads and traffic - The Highways Act 1980 S154, requires trees
over roads and footpaths to be maintained to allow for the passage of vehicles
and pedestrians.
Power lines - consider the potential for the tree to cause damage
to power lines or other services.
Reducing the risk from dangerous trees
Removing the target is not always possible especially in the urban situation
when the target is a house or other building. Below are the considerations to
be borne in mind when deciding how one can reduce risk.
- Remove target
- Prune tree
- Strop brace tree
- Remove tree
It is evident from the above that hazard tree assessment involves a thorough
and methodical, if not formalised approach and the ability to recognise a number
of subtle phenomena and relationships. This requires expert arboricultural knowledge
and experience. JCA can offer Tree Hazard Analysis and, if required, a decay detection
service using latest technology.
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