Community Tree Risk Management: How to Detect Tree Defects and Conduct Tree Risk Assessments

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Community Tree Risk ManagementHow to Detect
Tree Defectsand Conduct Tree Risk Assessments

• Patrick Weicherding, Ph.D.
• Regional Extension Educator
• Natural Resources Environment
• Andover Regional Center

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Community Tree Risk Management

• Each year, city foresters and municipal managers
  face billions of dollars in property damage,
  personal injury, and even death resulting from a
  falling branch or tree. This public safety risk
  is predictable and preventable but requires a
  systematic approach to assess and treat defects
  in trees.

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Post Storm Surveys in Minnesota

• 84 of the damaged trees had pre-existing defects
  that were preventable and predictable.
• Most common pre-existing defects include
• - Decay
• - Weak branch unions
• - Codominant leaders
• - Stem girdling roots

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Tree Risk Management

• Tree risk management involves the process of
  inspecting and assessing trees for their
  potential to injure people or damage property.

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Tree Risk Management Inspections

• Tree risk management inspections include
• A systematic inspection of individual trees
• An assessment of tree defects
• Estimating the potential for failure based on
  accepted signs and symptoms of tree failure

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Systematically Inspect Individual Trees

• Tree parts to inspect
• Crown
• Branches
• Branch unions
• Stem or trunk
• Root flare
• Rooting zone

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Systematically Inspect Individual Trees

• Examine all sides
• of the tree
• From a distance
• Close up
• Use diagnostic tools as needed

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Systematically Inspect Individual Trees

• Diagnostic tools
• Using binoculars to assess

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Systematically Inspect Individual Trees

• Diagnostic tools
• Measuring shell thickness with an increment
  borer

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Systematically Inspect Individual Trees

• Diagnostic tools
• Measuring shell thickness with a portable drill
  and ? inch drill bit

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Systematically Inspect Individual Trees

• Diagnostic tools
• Measuring shell thickness with a Resitograph

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Systematically Inspect Individual Trees

• Diagnostic tools
• Measuring tree decay with a Shigometer

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Systematically Inspect Individual Trees

• Diagnostic tools
• Measuring tree and root health with TreeRadar
  (TRU)

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Systematically Inspect Individual Trees

• Diagnostic tools
• Root collar examinations using a metal probe

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Tree Risk Categories

• Decay
• Wood that has rotted or is missing. Indicators
  of advanced decay are rotten wood, fungal
  fruiting bodies, cavities, holes, open cracks or
  bulges in the wood.

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• Advanced decay is always present when fungal
  fruiting bodies are found (mushrooms, shelf
  fungi, bracket fungi, and conks).

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• Other easily observed indicators of advanced
  decay include cavities, open cracks, and holes in
  the wood.

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Tree Risk Categories

• Cracks
• Separation of the wood or a deep split through
  the bark and into the wood.
• Vertical cracks
• Horizontal cracks

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• Cracks are caused by improper wound closure,
  splitting of branch unions or flush-cut pruning.

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• Horizontal cracks indicate that the tree is
  already failing and the tree is in imminent
  danger.

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Tree Risk Categories

• Root Problems
• Inadequate anchoring by root systems, damaged
  roots or restricted roots including stem girdling
  roots.

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• Damaged roots are caused by excavating,
  trenching, paving, re-grading, or soil
  compaction.

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• Symptoms of root problems include crown decline,
  soil/root disturbance, leaning tree or roots
  confined.

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Tree Risk Categories

• Weak Branch Unions
• A branch union with included bark or an
  epicormic branch.

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• Large epicormic branches on a decaying stem.

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Tree Risk Categories

• Cankers
• Areas where bark and/or cambium are dead.

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• Cankers are caused by insects, diseases, and by
  mowers, equipment, vehicles and vandalism.

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Tree Risk Categories

• Poor Architecture
• Growth pattern that indicates structural
  imbalance and weakness in the stem, branch or
  tree.

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• Poor tree architecture includes leaning trees,
  harp trees and trees with poor branch form.

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Tree Risk Categories

• Dead Wood
• A dead tree or branches.

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• Lodged branches are particularly problematic.
  They have already failed and only wait to be
  dislodged by the wind or the failure of the
  supporting branch.

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Tree Risk Rating

• USDA Community Tree Risk Evaluation Form

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Tree Risk Rating

• 1-4 1-3 1-3 3-10
• ________ ________ ________
  ________
• Probability Size of
  Probability Risk
• of Failure Parts of Target
  Rating
• Impact
• 1 low 1 - lt 4 inches 1
  occasional use
• 2 medium 2 4-20 inches 2
  intermittent use
• 3 high 3 - gt 20 inches 3
  frequent use
• 4 x-high

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Probability of Failure

1 - Low some minor defects 2 - Moderate one to
several moderate defects 3 - High defects
multiple or significant 4 - Extremely High any
combination of high defects
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Probability of Failure

• 1 Low some minor defects present
• Minor branch dieback
• Minor wounds or defects
• 2 Moderate one to several defects present
• Stem decay or cavity within safe shell limits
• Crack(s) without decay
• Major branch with weak union and included bark
• Defect(s) affecting gt 40 of trees circumference
  or roots within CRR

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Probability of Failure

• 1-Inch Rule
• Remember to measure shell thickness and diameter
  in the same place.

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Probability of Failure

• 2-Inch Rule
• Remember to measure shell thickness and diameter
  in the same place.

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Probability of Failure

• To determine how much damage a root system
  sustained, estimate how much of the Critical Root
  Radius (CRR) was lost.

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Probability of Failure

• 3 High - multiple or significant defects
  present
• Stem decay or cavity at or approaching shell
  safety limits
• Cracks, particularly if in contact with the soil
• Weak union with crack or decay
• Defect(s) affecting gt 40 of trees circumference
  or roots in the CRR
• Leaning tree with recent root lifting or soil
  mounding, crack or decay
• Dead tree without other significant defects

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Probability of Failure

• 4 - Extremely high any combination of high
  defects
• stem decay or cavity exceeding shell safety
  limits and severe crack
• multiple deep cracks or a single crack which goes
  completely through stem
• weak union with crack and decay
• defect(s) affecting gt 40 of trees circumference
  or roots within CRR and decay present
• leaning tree with recent root lifting or soil
  mounding and other significant defects
• dead branches broken (hangers) or cracked
  branches
• dead trees with other significant defects

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Size of Defective Part

• 1 Parts less than 4 inches in diameter
• 2 Parts from 4 to 20 inches in diameter
• 3 Parts greater than 20 inches in diameter

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Probability of Target Impact

• 1 Occasional Use
• 2 Intermittent Use
• 3 Frequent Use

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Probability of Target Impact

• 1 Occasional Use
• Low use roads and park trails, natural areas,
  riparian zones, industrial parks
• 2 Intermittent Use
• Moderate to low-use parks
• Dispersed campgrounds and picnic areas
• Secondary roads with congested intersections
• Bus stops along moderate-use roads

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Probability of Target Impact

• 3 Frequent Use
• Emergency access routes
• Medical and emergency facilities
• High-use public buildings
• High-use school playgrounds and parks
• Primary roads congested intersections and bus
  stops
• Drive-in campsites in high-use parks

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Subjective Hazard Rating (0-2 Points)

• No numeric rating system can accommodate all
  situations
• Subjective hazard rating builds flexibility into
  a numeric system and allows for professional
  judgment

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Examples of Subjective Hazard Rating

• Can be used if professional judgment suggests the
  need to increase the risk rating and invoke
  immediate corrective action
• Especially helpful to use when tree species
  characteristics become a factor in determining
  risk, for example, some tree species are
• Poor decay compartmentalizers
• Shed branches readily
• Total risk rating typically should not exceed 10
  points

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• Probability of Failure 3
• Stem with cavity and crack
• 2-inch rule just meets shell thickness
• Size of Defective Part 2
• Part is from 4 to 20 inches in diameter
• Probability of Target 2
• Impact
• Open area without fixed picnic tables
• Risk Rating 7

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• Probability of Failure 3
• Weak union with included bark
• Narrow crack is present
• Size of Defective Part 2
• Part is 4 to 20 inches diameter
• Probability of Target 2
• Impact
• Parking lot adjacent to moderate-use park area
• Risk Rating 7

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• Probability of Failure 4
• Crack with extensive decay and cavity
• Outer shell width below shell safety limits
  (minimum ratio 26 stem diameter
• Size of Defective Part 3
• Greater than 20 in diameter
• Probability of Target 3
• Impact
• High-use area
• Risk Rating 10

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• Probability of Failure 4
• Silver maple with multiple deep cracks
• Decay and woodpecker damage
• Size of Defective Part 3
• From 4 to 20 in diameter
• Probability of Target 3
• Impact
• High-use area
• Risk Rating 10

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Tree Defect Treatment Options

• 1 - Move the target
• 2 - Treat the tree
• 3 - Convert to wildlife tree
• 4 - Close the site
• 5 - Remove the tree

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Tree Defect Treatment Options

• Move the target

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Tree Defect Treatment Options

• Treat the tree

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Tree Defect Treatment Options

• Convert to wildlife tree

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Tree Defect Treatment Options

• Close the site

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Tree Defect Treatment Options

• Remove the tree

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Tree Risk Assessment Review

• Lucky Seven Principles (according to Gary
  Johnson)
• Risks (not hazards) occur at different levels
• Most risks are predictable and preventable
• Careful planning and design will eliminate most
  of the risks associated with the urban forest
• Healthy trees are more likely to be sound trees
• Sometimes, the risk can be eliminated or reduced
  by moving the target
• Some risks are simply worth it
• Some trees are not worth the risk