Recognizing and Managing Tree Risk in the Urban Forest

Presentation given to 2026 Master Gardeners Tree Stewards Class on November 2, 2026.

Transcript

1. Recognizing and Managing Tree Risk in the Urban Forest P. Eric Wiseman, PhD Associate Professor of Urban and Community Forestry Dept. of Forest Resources and Environmental Conservation Virginia Tech vtuf@vt.edu linktr.ee/EricWiseman 2026 Master Gardeners Tree Stewards Class • November 02, 2026 Source: www.facebook.com/DianePathieuABC7
2. Lecture Overview Purpose Provide insights on tree risk that will enable you to provide sound advice about selecting and managing trees Outline • Defining and contextualizing tree risk • How and why trees fail • Recognizing tree failure risk • Managing tree failure risk Source: E. Wiseman 2026 Master Gardeners Tree Stewards Class • November 02, 2026 02 • 25
3. No Trees – No Problems…Right? No Social Benefits No Economic Benefits No Environmental Benefits TREES PAY US BACK Source: blog.gouldevans.com/the-wealth-of-street-trees 2026 Master Gardeners Tree Stewards Class • November 02, 2026 03 • 25
4. Basic Premises • The majority of tree failures are inconsequential to people – more so an inconvenience than cause of harm • Trees are at higher risk of failure when they have abnormalities and defects • Not all abnormalities and defects are inherently hazardous • Tree risk can be successfully managed through proper tree selection, maintenance, and protection Source: www.channel3000.com/news/local-news 2026 Master Gardeners Tree Stewards Class • November 02, 2026 04 • 25
5. Key Concepts and Principles • Tree owners and managers have a legal duty to identify and mitigate unreasonable risks posed by trees • Failure to mitigate such risks may be judged as negligence and result in legal liability for harm caused by trees • Trees posing the greatest risk of harm are those with defects and site conditions that reduce tree stability We have a legal, ethical, and moral duty to protect people from harm by trees Source: www.mdjonline.com/neighbor_newspapers 2026 Master Gardeners Tree Stewards Class • November 02, 2026 05 • 25
6. What is Tree Risk? Likelihood of an event involving a tree that has negative consequences RISK = (likelihood of event) × (likelihood of negative consequences) Types of Tree Risk COMBINED LIKELIHOOD Threshold of Unacceptable Risk LOW MODERATE HIGH OVERALL TREE RISK EXTREME Conflict Risk Potential harm due to a tree interfering with people, property, or activities Failure Risk Potential harm due to falling tree debris impacting people, property, or activities • There are no zero-risk trees in the urban forest – all trees pose some level of risk • It is impractical to eliminate all risks – management is about limiting risk to a reasonable level 2026 Master Gardeners Tree Stewards Class • November 02, 2026 06 • 25
7. A Closer Look at Conflict Risk • Conflict risks often occur when we violate the “right tree, right place” tenet • Consequences range from minor nuisance to serious harm Leaves, flowers, Allergies, stinging insects, injuries, fruits, sap, slips/trips, wildfires, storm drain clogs thorns, debris Branches Contact with powerlines, obstruct signs, block views, hinder vehicles and pedestrians, obstruct lighting Trunks Road-edge hazard, lightning strike, obstruct signs, block views, animal nests Roots Hardscape damage, pipe damage, septic intrusion, tripping hazard Source: Smiley, E.T. et al. (2025). Best Management Practices - Tree Risk Assessment. 2026 Master Gardeners Tree Stewards Class • November 02, 2026 07 • 25
8. A Closer Look at Failure Risk? Tree failure occurs when load (force) exceeds load-bearing capacity of tree parts Most failures involve the combination of a loading event (like a storm) and a tree defect Failure Impact Harm Likelihood of failure depends on severity of defects and site factors Likelihood of impact depends on target zone and target occupancy Likelihood of harm depends on target vulnerability , tree part size, and fall distance Harm Impact Failure A tree that poses an unreasonable risk of failure, impact, and harm is called a hazardous tree 2026 Master Gardeners Tree Stewards Class • November 02, 2026 08 • 25
9. How Do Trees Fail? Failure is loss of structural integrity in tree parts that compromises tree stability and jeopardizes people and property STEM FAILURE UPROOTING Source: theloopny.com Source: www.gfc.state.ga.us 2026 Master Gardeners Tree Stewards Class • November 02, 2026 BRANCH FAILURE Source: www.purdue.edu 09 • 25
10. Why Do Trees Fail? • Trees fail when mechanical stress (referred to as load) exceeds root anchorage or wood fiber strength • Gravity acting on the mass of the tree exerts internal load on tree parts • Ice, snow, and rain add mass and exert external load on tree parts • Wind intensifies loading stress on tree parts • Moisture decreases soil strength and root adhesion to soil particles 2026 Master Gardeners Tree Stewards Class • November 02, 2026 10 • 25
11. What Defects Predispose Trees to Failure? Trees with defects fail with higher frequency and under milder loading conditions than sound trees COMPROMISED ROOTS Source: www.theday.com WEAK BRANCH UNIONS Source: www.rynolawncare.com 2026 Master Gardeners Tree Stewards Class • November 02, 2026 DECAYED/CRACKED WOOD Source: www.bartlett.com 11 • 25
12. Tree Risk Prevention Mitigating hazardous trees is complicated and expensive An ounce of prevention is worth a pound of cure Best Practices for Tree Risk Prevention • Species selection for growth habit, wood strength, branch unions, and decay compartmentalization • High-quality nursery stock with good structure • Proper tree placement, planting, and aftercare • Structural pruning when trees are young • Preserve suitable trees during construction • Protect trees from physical injuries Source: www.arborvalleynursery.com/plants/swamp-white-oak 2026 Master Gardeners Tree Stewards Class • November 02, 2026 12 • 25
13. Tree Characteristics and Sturdiness Tree species are quite variable in their wood strength, physical conformation, and vulnerability to defect development PHYSICAL CONFORMATION DEFECT VULNERABILITY Density Modulus of Rupture/Elasticity Decay Resistance Root Architecture Growth Habit Branch Angle, Aspect, & Taper Girdling Roots Codominant Leaders Included Bark 2026 Master Gardeners Tree Stewards Class • November 02, 2026 logs.scientificamerican.c r ce : b Sou Source: doi.org/10.1039/D0NJ06234K om WOOD STRENGTH 13 • 25
14. Selecting Sturdy Tree Species There is no perfect tree species! Every species has assets and liabilities – species selection is about tradeoffs! SPECIES THAT SHOULD BE IN YOUR URBAN FOREST SPECIES YOU SHOULD PONDER BEFORE PLANTING Japanese cedar (Cryptomeria japonica) Silver maple (Acer saccharinum) Ginkgo (Ginkgo biloba) Yellowwood (Cladrastis kentukea) Ky. Coffeetree (Gymnocladus dioicus) Leyland cypress (Hesperocyparis leylandii ) East. Redcedar (Juniperus virginiana) Tulip-poplar (Liriodendron tulipifera) Blackgum (Nyssa sylvatica) White pine (Pinus strobus) Hophornbeam (Ostrya virginiana) Poplar (Populus spp.) Swamp white oak (Quercus bicolor) Callery pear (Pyrus calleryana) Baldcypress (Taxodium distichum) Basswood (Tilia americana) 2026 Master Gardeners Tree Stewards Class • November 02, 2026 14 • 25
15. Tree Risk Mitigation Options Mitigation: Process of reducing tree risk to a reasonable level so that the residual risk is acceptable by the tree owner or manager COMBINED LIKELIHOOD Threshold of Unacceptable Risk LOW MODERATE HIGH OVERALL TREE RISK RETAIN & MONITOR EXTREME Target Based Options Tree Based Options PERFORM MITIGATION 2026 Master Gardeners Tree Stewards Class • November 02, 2026 15 • 25
16. Tree Risk Mitigation Options Target-Based Mitigation aims to reduce target exposure to impact or reduce severity of harm if there is target impact • Exclude the target • Move the target • Protect the target 2026 Master Gardeners Tree Stewards Class • November 02, 2026 16 • 25
17. Tree Risk Mitigation Options Target-Based Mitigation aims to reduce target exposure to impact or reduce severity of harm if there is target impact • Exclude the target • Move the target • Protect the target Source: twitter.com/PortlandGov/status/1649508336285495297 2026 Master Gardeners Tree Stewards Class • November 02, 2026 17 • 25
18. Tree Risk Mitigation Options Target-Based Mitigation aims to reduce target exposure to impact or reduce severity of harm if there is target impact • Exclude the target • Move the target • Protect the target 2026 Master Gardeners Tree Stewards Class • November 02, 2026 18 • 25
19. Tree Risk Mitigation Options Tree-Based Mitigation aims to reduce likelihood of tree failure, likelihood of target impact, or severity of harm if there is a target impact • Remove the tree • Create a wildlife snag • Prune the tree • Support the tree • Treat the tree Source: www.raneytreecare.com/services-tree-removal.html 2026 Master Gardeners Tree Stewards Class • November 02, 2026 19 • 25
20. Tree Risk Mitigation Options Tree-Based Mitigation aims to reduce likelihood of tree failure, likelihood of target impact, or severity of harm if there is a target impact • Remove the tree • Create a wildlife snag • Prune the tree • Support the tree • Treat the tree 2026 Master Gardeners Tree Stewards Class • November 02, 2026 20 • 25
21. Tree Risk Mitigation Options Tree-Based Mitigation aims to reduce likelihood of tree failure, likelihood of target impact, or severity of harm if there is a target impact • Remove the tree • Create a wildlife snag • Prune the tree • Support the tree • Treat the tree Source: www.urbanforestdweller.com/pruning-how-to-understand-an-arborist 2026 Master Gardeners Tree Stewards Class • November 02, 2026 21 • 25
22. Tree Risk Mitigation Options Tree-Based Mitigation aims to reduce likelihood of tree failure, likelihood of target impact, or severity of harm if there is a target impact Cabling • Remove the tree • Create a wildlife snag • Prune the tree • Support the tree • Treat the tree 2026 Master Gardeners Tree Stewards Class • November 02, 2026 Bracing 22 • 25
23. Tree Risk Mitigation Options Tree-Based Mitigation aims to reduce likelihood of tree failure, likelihood of target impact, or severity of harm if there is a target impact Pest Mgt. Soil Mgt. Source: www.hharborists.com/plant-health-care Source: sorenson-companies.com/tree-services • Remove the tree • Create a wildlife snag • Prune the tree • Support the tree • Treat the tree 2026 Master Gardeners Tree Stewards Class • November 02, 2026 23 • 25
24. Take Home Messages • We accept reasonable risks of trees causing harm as a tradeoff for their benefits • We have a legal and moral duty to recognize and mitigate potentially hazardous trees • Preventing defect development through proper tree selection, placement, and management is critical • There are a range of options for mitigating tree risk without removing trees Source: Larry Harwell Photography, Charlotte 2026 Master Gardeners Tree Stewards Class • November 02, 2026 24 • 25
25. Recognizing and Managing Tree Risk in the Urban Forest P. Eric Wiseman, PhD Associate Professor of Urban and Community Forestry Dept. of Forest Resources and Environmental Conservation Virginia Tech vtuf@vt.edu linktr.ee/EricWiseman 2026 Master Gardeners Tree Stewards Class • November 02, 2026 25 • 25