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TREE PROBLEMS AND HOW TO AVOID/AMELIORATE THEM
1) Tree roots and their effects on structures
Subsidence damage to buildings is often a result of hot summers and dry winters but trees may contribute to damage:
Direct damage: this can occur to light structures by the physical expansion of woody roots. Root cutting should be avoided if possible as it may destabilise the tree or introduce disease. It may be possible to modify the structure to allow for future root expansion.
Roots only grow into drains that are leaking water. The solution here necessitates the sealing of the leak.
Indirect damage: this results from extraction of soil moisture by fibrous tree roots but tends to cause a problem only in shrinkable soil types such as clays. Irrespective of root action such soils shrink and expand depending on weather conditions and this alone may cause damage to structures. However if it is felt that a tree is responsible pruning may reduce the tree's potential to absorb moisture and allow closure of cracks. Subsequent pruning may often achieve long-term stability.
The presence of a tree near a building does not mean that damage is inevitable.
2) Risks associated with falling wood
The hazard from falling limbs or trunks can be lessened by the recognition of potential danger points. These are:
- Abrupt bends in branches. Often a new shoot will replace a broken or dead limb and grow at 90º or so to the original branch. A combination of stress and decay can sometimes lead to failure.
- A dangerous hazard is that of brittle decay where fungi associated with the breakdown of the cellulose content of wood cause the possibility of sudden snapping with little warning. Fruiting bodies of the fungi are the only sign. The commonest fungal species causing this problem are:
Laetiporus sulphurous (on some broadleaves and yew)
Sparassis crispa (on conifers)
Phaeolus schweinitzii (on conifers)
Ustulina deusta (on many broadleaves)
- "Bottle-butt". Here decay at the base of the trunk causes extra wood to be laid down making the base of the tree appear swollen. Other signs are cracking in the stem, cracking around the root plate as roots die and wind-rock occurs. Fungal bodies may also be visible.
- Sinking of branches. Bark plates buckling on the underside of a branch where it joins the trunk and stretched out on the upper side indicate that a branch is sinking and may snap. A gap in the crown tends to be left above the branch.
- Exposure of previously sheltered trees. Trees that have grown in a closed canopy are not adapted to strong winds and may snap or blow over. Such trees often have a narrow crown and few lower branches due to shading when growing.
- Forks between co-dominant stems. This is the most common cause of above ground failure. The risk is particularly high where the angle between the stems is very acute and if an area of bark-to-bark contact is obvious.
- Instability due to restricted rooting. In waterlogged conditions root penetration is restricted due to anaerobic soil. This tends to result in a wide shallow root system that results in the tree being susceptible to wind throw.
- Neglected pollards. Trees that are pollarded when young but then not cut again have top-heavy branches that are susceptible to snapping.
3) Prevention and management of hazards
Hazards can be prevented or managed by:
- Branch reduction or removal
- Cable or rod bracing of a branch
- Reducing crown to reduce mechanical stress on main trunk
- Formative pruning of young trees to remove co-dominant leaders and low branches over paths
- Re-routing of paths away from veteran trees
- Re-cutting above old pollard joints for pollarded trees
- Felling if other options not adequate or feasible.
Nick Benge
Tree Warden, Cramond Association
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