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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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