Understanding Tree Risk Assessment
A mature oak leans slightly over a car park. A linden in a municipal square shows a cavity at the base. A roadside cherry has a co-dominant stem with included bark at the union. Each of these trees may or may not present an unacceptable risk — the answer depends on a structured assessment, not a glance from the pavement.
Tree risk assessment is the systematic process by which qualified arborists identify defects, estimate the likelihood of failure, evaluate the probable consequences, and arrive at a risk rating that guides management decisions. In the Czech Republic, municipalities, forest managers, and private property owners all have legal obligations regarding trees that could harm people or property. Getting the assessment right — and documenting it properly — matters both ecologically and legally.
The ISA Tree Risk Assessment Framework
The most widely referenced methodology comes from the International Society of Arboriculture (ISA), which publishes the Tree Risk Assessment Manual (2nd edition, 2017). The framework separates assessment into three components:
- Likelihood of failure — how probable is it that the tree, or a specific part of it, will fail under a given load condition?
- Likelihood of impact — if failure occurs, how likely is it to strike a target (person, vehicle, structure)?
- Consequences of failure — what would the severity of harm be if a target were struck?
These three components combine to produce a risk rating on a four-level scale: Low, Moderate, High, and Extreme. The rating then informs whether no action, monitoring, mitigation, or removal is appropriate.
Visual Inspection and Its Limits
The initial stage of any assessment is a ground-level visual inspection — the so-called Level 2 assessment in ISA terminology. The assessor walks around the tree, examining the crown, scaffold branches, trunk, root collar, and soil conditions. A trained eye can identify many high-priority defects: crown dieback, epicormic growth, resin bleeding, fungal brackets, bark cracks, or soil mounding that suggests root failure.
However, visual inspection has clear limits. Decay in the heartwood is often invisible externally until the tree is far advanced in deterioration. A hollow trunk may appear sound on the outside. For trees where structural integrity is genuinely in question, additional diagnostic tools are needed.
Resistograph and Tomography
A resistograph is a fine drilling device that measures resistance as the bit advances through wood. Dense, sound wood resists the drill; decayed or hollow sections show lower resistance. The resulting graph — a resistogram — maps the internal condition along the drill path.
Sonic or electrical tomography takes this further, generating a two-dimensional cross-section of the trunk that distinguishes between sound wood, decayed wood, and hollow zones based on the velocity of impulse transmission through different materials. Czech municipal arborists have used tomography on high-value street trees in Prague, Brno, and Olomouc, particularly on lime and chestnut specimens with deep cavities.
Root Zone Assessment
Structural roots are the anchor; their condition is often neglected in assessments focused on the aerial parts of the tree. Root damage is among the most common causes of whole-tree failure in urban settings, where construction, utility trenching, soil compaction, and paving cut or suffocate roots without any visible change to the crown for several growing seasons.
Assessing the root zone requires knowing the history of the site, examining surface roots at the root collar, and — where conditions warrant — using an air spade to expose structural roots without cutting them. The assessment should note the direction and extent of any compaction, any severed roots within the critical root zone (CRZ), and evidence of fungal decay at the base.
Target Assessment
A dead branch 30 metres from any structure or path is a different proposition from the same branch over a primary school entrance. Target assessment asks: who or what could be struck, how often are they there, and when? A tree over a car park used only during business hours presents a different risk profile from one over a pedestrian path in a public park open at all hours.
In Czech practice, guidance from the Czech Arborist Society (Česká arboristická společnost) and the Technical Standards body (ČSN) frameworks helps arborists assign consistent ratings. The ČSN EN ISO 17420 series covers personal protective equipment for arborists; broader tree assessment methodology draws on both ISA guidance and the German ZTV-Baumpflege standards, which are frequently referenced in Central European arboricultural practice.
Common Defects in Czech Urban Trees
Working across Czech cities, arborists encounter certain defect patterns repeatedly:
- Included bark at co-dominant unions in linden (Tilia spp.) and horse chestnut (Aesculus hippocastanum), particularly in older specimen trees.
- Fungal decay from Ganoderma spp. at the base of mature oaks and limes — the bracket emerges only after substantial internal wood decay has occurred.
- Bark beetle galleries beneath the bark of stressed spruce in peri-urban green belts, weakening stem structure before death.
- Root plate instability in trees planted in tree pits without adequate soil volume, common in post-1990 urban developments.
- Crown asymmetry from historical topping or clearance cuts around power lines, leaving large decay-prone wounds.
Documentation and Legal Context
In the Czech Republic, the removal of a tree with a trunk circumference above 80 cm measured at 130 cm height generally requires a permit from the relevant municipal authority under Act No. 114/1992 Coll. on nature and landscape protection. The permit application typically requires a technical report, and a documented risk assessment strengthens the case where removal is proposed for safety reasons.
Property owners bear responsibility for trees on their land. If a tree fails and causes injury or damage, negligence claims can follow if no assessment was conducted or if a known risk was not acted upon. Documenting assessments — dates, findings, recommended actions, and outcomes — is therefore both good practice and practical protection.
When Assessment Leads to Retention
Risk assessment does not always recommend removal. A tree with a moderate structural defect and low target exposure may be retained with monitoring on an 18-month cycle. A tree with a high failure likelihood but located where failure would cause no harm can also be retained — and its deadwood value to saproxylic invertebrates and hole-nesting birds acknowledged. The goal is a proportionate response to actual risk, not the elimination of every imperfect tree.
For detailed reference on methodology and rating criteria, the ISA Tree Risk Assessment Manual remains the most complete practitioner document available in English. Czech-language guidance is available through the Česká arboristická společnost.
