The greatest benefit of trees is derived from healthy, structurally sound trees growing in locations that support their development. Damage to trees during construction activities is common and can lead to tree decline and death.
If a tree is worth retaining then it needs to be protected throughout the construction phase of a development and ensure that there is sufficient space for the tree to continue to grow in a safe and aesthetically pleasing manner.
Tree protection guidelines
When evaluating existing trees on a property, careful consideration must be given to tree survival and growth, balanced with the intended site usage and building requirements. An arborist should evaluate the health and viability of the trees and ascertain which trees are suitable for retention prior to either design or construction.
Tree roots are generally shallow in the soil and can travel well beyond the dripline of the trees crown. Tree root growth is opportunistic and occurs where the essentials for growth are present and is not always symmetrical in form and depth. Fine absorbing roots that collect water and nutrients are located primarily within the top 150-300mm of the soil. The roots and the soil in this surface layer must be protected from injury.
Diagram 1. Tree root growth.
It is often assumed that tree roots extend deep underground and out to the crown dripline.
In the majority of cases, tree roots are actually shallower in the soil profile and can extend well beyond the crown dripline. As a result, tree roots are vulnerable to surface disturbances during development projects.
A balance between roots and shoots maintains sufficient foliage and roots to support each other. If shoots or roots are suddenly cut, it places stress upon the trees energy system and roots or branches may be shed to compensate. If tree energy reserves are depleted over a number of years, the tree finds it increasingly difficult to get over any new stress placed upon it, and it may ultimately decline.
Tree protection distances
The most important activity, after determining which trees will be retained is the implementation of a Tree Protection Zone (TPZ). The intention of the TPZ is to:
- provide a safe tree resource
- provide adequate root space to sustain tree health, aesthetics and stability
- minimise changes to the tree’s growing environment
- minimise physical damage and loss to the trees root system, crown and trunk.
The most common activities associated with root damage include soil compaction from pedestrian and machinery passage, open trench excavations, and site cuts or fills to achieve level changes. All can theoretically harm the health and stability of trees and consequently, a TPZ should be established around each tree within the construction/development area.
To calculate the TPZ, the diameter of the tree is measured in centimetres (cm) at 1.5 metres (m) above grade. Where a tree branches below 1.5 m, measure the smallest trunk diameter below the lowest branch. To establish the radial TPZ distance, multiply the diameter by 10. This distance should then be measured from the edge of the trunk of the tree to provide the circular protection area in all directions.
The TPZ distances calculated using this method is intended to be a guide for the planning process and not an absolute rule. There will obviously be times when it is not possible to retain the optimum TPZ around each tree to be preserved.
Diagram 2. Determining the tree protection zone.
1. Measure the trees trunk diameter (cm) at 1.5m above grade.
2. Calculate the TPZ by multiplying the trunk diameter by 10. (This is measured from the edge of the trunk).
A mature Brush Box (Lophostemon confertus) street tree with a trunk diameter of 30cm.
30cm x 10 = 300 cm or, 3.0m radius TPZ
Key requirements to protect trees
- Establish the TPZ and install fixed fencing to protect the TPZ eg. Chain-mesh.
- No persons, vehicles or machinery to enter the TPZ.
- No stockpiling of building materials, debris or soil within the allocated TPZ.
- No fuel, oil dumps or chemicals shall be allowed in or stored within the allocated TPZ.
- Soil levels must not be altered within the allocated TPZ.
- No open trenching is to occur within the TPZ.
- All pruning shall be done in accordance with AS 4373 – 1996 Pruning of Amenity Trees.
- A tree shall not be used to attach temporary service wires, nails, screws or any other fixing device or as a winch support or anchorage.
- Care is to be taken to ensure that no damage is caused to tree trunks, roots and structural branches.
- Supplementary watering should be provided to all trees through any dry periods during and after the construction process.
If required, an arborist can suggest recommendations for sympathetic construction techniques specific to the tree and site conditions.
Some information within this fact sheet has been obtained from published sources. For further reading, refer to the Trees – Further reading Fact Sheet.