How Does an Arborist Decide Whether to Remove or Save a Tree?

A qualified arborist doesn't make the call to remove or save a tree lightly. The decision involves a structured assessment of the tree's health, structural integrity, location, and any legal protections that apply — and in many cases, saving the tree is the preferred outcome. Here's what goes into that decision.

The Initial Visual Assessment

The first thing an arborist does is walk around the tree and take in the full picture. They're looking at the overall form, canopy density, bark condition, and any obvious signs of stress or damage. This visual tree assessment (VTA) is a recognised method used by arborists worldwide and forms the foundation of any recommendation.

Key things they look for include:

• Dead or dying branches (known as deadwood) in the canopy
• Cracks, splits, or cavities in the trunk or major limbs
• Fungal fruiting bodies (mushrooms or conks) at the base or on the bark, which can indicate internal decay
• Leaning that has developed recently or is combined with root disturbance
• Root zone damage from construction, compaction, or soil changes
• Pest and disease signs such as unusual leaf drop, discolouration, or insect activity

Assessing Structural Integrity

A tree can look healthy on the outside but be structurally compromised internally. Arborists use tools like a mallet tap test to listen for hollow sections, and in more complex cases may use a resistograph (a fine drill that measures wood density) or sonic tomography to map internal decay without causing significant damage to the tree.

Structural defects that raise serious concern include:

• Co-dominant stems (two trunks of equal size) with included bark, which can split under load
• Cracks running through the trunk or major unions
• Significant root damage or girdling roots
• A compromised root plate, especially after nearby earthworks

Weighing Up the Risk

Once the tree's condition is understood, the arborist considers risk — specifically, what is the likelihood of failure, and what would be the consequences if it did fail? This is often called a target assessment. A large tree leaning over an empty paddock poses very different risk to one leaning over a children's playground or a house.

Arborists in NZ often reference the TRAQ (Tree Risk Assessment Qualification) framework or similar structured risk rating systems to make this assessment consistent and defensible. BRANZ and MBIE Building Performance both provide guidance on managing risks associated with trees near buildings.

Can the Tree Be Saved?

Removal is generally a last resort. Before recommending it, a good arborist will consider whether the tree can be retained safely through:

• Crown reduction or thinning — removing weight and wind resistance from the canopy
• Deadwooding — removing hazardous dead branches
• Cabling or bracing — installing support systems to reduce the risk of structural failure
• Root zone management — aerating compacted soil, applying mulch, or improving drainage
• Pest and disease treatment — fungicide or insecticide applications where appropriate

If the tree can be made safe and is worth retaining (for shade, amenity, habitat, or sentimental value), most arborists will recommend a management plan rather than removal.

Legal Protections and Council Rules

This is a critical part of the process that homeowners often overlook. In New Zealand, many trees are protected under District Plan rules administered by your local council. A tree may be listed as a Notable Tree, sit within a scheduled tree protection overlay, or be subject to rules around vegetation clearance in sensitive zones.

Removing a protected tree without consent can result in significant fines. Your arborist should check — and so should you — before any work begins. Contact your local council's planning or resource consent team to confirm the tree's status. In some cases, a resource consent is required even for pruning.

For trees near boundaries, power lines, or public land, there may be additional considerations involving your network utility operator or local authority.

Species and Ecological Value

Native species such as tōtara, kauri, pōhutukawa, and kahikatea often carry greater ecological and cultural value, and councils may apply stricter rules to their removal. An arborist familiar with NZ native species will factor this into their recommendation. Kauri in particular requires careful biosecurity protocols to prevent the spread of kauri dieback (Phytophthora agathidicida).

When to Call a Professional

Tree assessment and removal is skilled, physically dangerous work. You should always engage a qualified arborist — ideally one with an NZ Certificate in Arboriculture or membership with the New Zealand Arboricultural Association (NZAA). WorkSafe NZ sets out strict requirements for working at height and near power lines, and tree work regularly features in serious harm statistics.

DIY tree removal, especially for large trees or those near structures and power lines, carries serious risk of injury, property damage, and legal liability. A professional arborist carries appropriate insurance and will obtain any necessary permits.

You can find qualified local arborists on tradedirectory.co.nz.

Sources & Further Reading

• MBIE Building Performance – trees and building consent
• WorkSafe NZ – working safely at height
• BRANZ – building and construction research
• Tenancy Services – Healthy Homes and property maintenance
• Standards NZ – relevant NZ standards