National Tree Safety Group
5 key principles:
- Trees provide a wide variety of benefits to society.
- Trees are living organisms that naturally lose branches or fall.
- The overall risk to human safety is extremely low.
- Tree owners have a legal duty of care.
- Tree owners should take a balanced and proportionate approach to tree safety management.

• Health and Safety Executive
  SIM 01/2007/05: Management of the risk from falling trees or branches

• NatureScot: A brief guide to occupiers’ legal liabilities in Scotland in relation to public outdoor access

• National Tree Safety Council
  Common sense risk management of trees

• National Tree Safety Group
  1) Informal observation
  2) Formal observation
  3) Detailed Observations

• International Society of Arboriculture
  1) Limited visual
  2) Basic assessment
  3) Advanced assessment

• Lonsdale, D. (2021) Principles of Tree Hazard Assessment and Management. Ninth edition. UK. Arboriculture Association. Research for Amenity Trees No. 7. ISBN 9780900978579
  i) Tree Assessment Strategy, and;
  ii) Strategy for detailed assessment of decay (first, second and third stage)

• Breloer, H. & Mattheck C. (1995) The body language of trees. A handbook for failure analysis. UK. Stationery Office Limited. Research for Amenity Trees No. 4. ISBN 9780117530676
  Visual Tree Assessment (VTA)
  1) Visual assessment of biological and mechanical health
  2) Sounding and /or drilling to evaluate defect further
  3) Drilling with an increment borer and testing with a Fractometer

Many examples of what constitutes a tree 'hazard' are included in the resources under 'Processes'. Other sources you may wish to consult include:

• Forest Research
  Hazards from trees: a general guide

• US Forest Service, Department of Agriculture (USDA)
  Urban Tree Risk Management (Chapter 3)

• Strouts, R. G & Winter, T. G. (2019) Diagnosis of ill-health in trees. Third edition. UK. Arboriculture Association. Research for Amenity Trees No. 2. ISBN 9780900978586

• Institute of Wood Science & Technology, Bangalore
  Urban Tree Risk Assessment -A pictorial guide

• Sorbus International
  An introduction to wood inspection

• Arboriculture Association
  James Chambers
  Decay Detection - When it works, when it doesn't, and what to look for [from 39 minutes]

• Urban Forestry Today
  Nicholas Brazee
  Detection of Internal Decay in Trees Using Tomography

Further background information:

• Urban Forestry Today
  Karl J. Niklas
  Tree Biomechanics 1, and;
  Tree Biomechanics 2

In tree management, risk is a combination of likelihood (of a tree or part of a tree failing) and severity (how serious the harm could be to, for example, a person, property or activity).

Calculating likelihood (or probability), qualitatively or quantitatively, requires recognition of what constitutes a significant hazard. However, perceptions of what constitutes a hazard can vary among arborists.

Calculating severity requires identifying potential targets (people, property or activities that could be struck and damaged) and how severely targets might be harmed. Where mobile targets exist (e.g people), frequency and duration of occupancy also requires consideration.

Methods for calculating risk may involve qualitative and/or quantitative information.

• International Society of Arboriculture
  Basic risk assessment
  1) Liklihood matrix
  2) Risk rating matrix

• Quantified Tree Risk Assessment
  QTRA

• Forbes-Laird Arboricultural Consultancy
  Tree Hazard: Risk Evaluation And Treatment System THREATS

• VALID Tree Risk-Benefit Management and Assessment
  Vitality Anatomy Load Identity Defect

• USDA Forest Service Southwestern Region
  Tree Risk Assessment Form

• Urban Tree Risk Management: A Community Guide to Program Design and Implementation
  USDA Community Tree Risk Evaluation Form 3.3

• USDA Forest Service Pacific Southwest Region
  Hazard Tree Identification and Mitigation R5 Hazard Tree Evaluation Form

• International Society of Arboriculture
  A Photographic Guide to the Evaluation of Hazard Trees in Urban Areas Tree Hazard Evaluation Form 2nd Edition ('Hazard rating')
  

Access

• Stopping or diverting traffic

• Deterring informal parking upon roots

• Relocating infrastructure (seats, information boards)

• Creating mown paths away from high-risk zones

• Planting thorny shrubs within drip line

Surgery

• Felling

• Pruning

• Monolith creation

• Place deadwood in high-risk area

Inspection

• Further (advanced) inspection

• Follow up inspection/monitoring

Other

• British Standards
  BS3998:2010 Tree Work - Recommendations.

• Barrell Tree Consultancy
  Trees and the law - Useful documents

• Quantified Tree Risk Assessment
  Case Law

• British and Irish Legal Information Institute |
  Case Law Search