Tree that have defects can cause harm to people and damage property.
Recognising the features of hazardous trees and taking proper corrective
actions can prevent loss of life and property. A tree with hazardous
defect can cause failure (toppling) of all or part of the tree, which
can hit a "target".
Evaluating trees for hazardous defect can be complex because of the
variability of trees, the severity of their defects, and site
differences. This article highlights guidelines, not absolute rules for
recognising and correcting hazardous defect. Corrective actions should
be undertaken by certified arborists.
What to Look For
1. Dead wood
Dead trees and branches must be removed as they can fall at any moment.
Dead branches that have already broken off and hanging are all the more
dangerous.
2. Cracks
A crack is split deep into the wood, sometimes completely through the
stem/trunk. Cracks are dangerous as they are an indication of potential
tree failure.
3. Weak branch unions
Areas where branches are not strongly attached to a tree are called weak branch unions.
When two or more branches grow very closely together, the area where
they join swells with ingrown bark, which does not have the strength of
wood, and thus the union is weaker than one that does not have included
bark.
Trees with a tendency to form upright branches, such as Alstonia, often
produce weak branch unions. They are also formed after a tree is topped
(a cut at a right angle to the direction of growth).
4. Decay
Decay leads to wood that crumbles or is soft, or a cavity where wood is missing, creating a hazard.
Signs of fungal growth such as mushrooms, conks, and brackets growing on
the base of the tree, stems or branches can be indicators of advanced
decay.
5. Cankers
Cankers are caused by mechanical injury or disease, occuring at a
localised area where the bark is sunken or missing. A canker that covers
half or more of the circumference of the tree may be hazardous even if
exposed wood appears sound.
6. Root problems
Root problems may be due to the raising or lowering of soil grade near
the tree, severing of roots, installing of pavement over roots, driving
vehicles over roots, or advanced decay in root and root flares.
As defective roots are underground and not readily seen, aboveground
symptoms such as dieback and off-colour leaves serves as an indication
of root problems.
7. Poor tree form
Poor tree form could be caused by damage from storms, improper pruning
or unfavourable environment. These trees are structurally unsafe. Trees
that lean excessively are also a hazard.
