Title: Community Tree Risk Management: How to Detect Tree Defects and Conduct Tree Risk Assessments
1Community Tree Risk ManagementHow to Detect
Tree Defectsand Conduct Tree Risk Assessments
- Patrick Weicherding, Ph.D.
- Regional Extension Educator
- Natural Resources Environment
- Andover Regional Center
2Community 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.
3Post 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
4Tree Risk Management
- Tree risk management involves the process of
inspecting and assessing trees for their
potential to injure people or damage property.
5Tree 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
6Systematically Inspect Individual Trees
- Tree parts to inspect
- Crown
- Branches
- Branch unions
- Stem or trunk
- Root flare
- Rooting zone
7Systematically Inspect Individual Trees
- Examine all sides
- of the tree
- From a distance
- Close up
- Use diagnostic tools as needed
8Systematically Inspect Individual Trees
- Diagnostic tools
- Using binoculars to assess
9Systematically Inspect Individual Trees
- Diagnostic tools
- Measuring shell thickness with an increment
borer
10Systematically Inspect Individual Trees
- Diagnostic tools
- Measuring shell thickness with a portable drill
and ? inch drill bit
11Systematically Inspect Individual Trees
- Diagnostic tools
- Measuring shell thickness with a Resitograph
12Systematically Inspect Individual Trees
- Diagnostic tools
- Measuring tree decay with a Shigometer
13Systematically Inspect Individual Trees
- Diagnostic tools
- Measuring tree and root health with TreeRadar
(TRU)
14Systematically Inspect Individual Trees
- Diagnostic tools
- Root collar examinations using a metal probe
15Tree 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.
16- Advanced decay is always present when fungal
fruiting bodies are found (mushrooms, shelf
fungi, bracket fungi, and conks).
17- Other easily observed indicators of advanced
decay include cavities, open cracks, and holes in
the wood.
18Tree Risk Categories
- Cracks
-
- Separation of the wood or a deep split through
the bark and into the wood. - Vertical cracks
- Horizontal cracks
19- Cracks are caused by improper wound closure,
splitting of branch unions or flush-cut pruning.
20- Horizontal cracks indicate that the tree is
already failing and the tree is in imminent
danger.
21Tree Risk Categories
- Root Problems
-
- Inadequate anchoring by root systems, damaged
roots or restricted roots including stem girdling
roots.
22- Damaged roots are caused by excavating,
trenching, paving, re-grading, or soil
compaction.
23- Symptoms of root problems include crown decline,
soil/root disturbance, leaning tree or roots
confined.
24Tree Risk Categories
- Weak Branch Unions
- A branch union with included bark or an
epicormic branch.
25- Large epicormic branches on a decaying stem.
26Tree Risk Categories
- Cankers
- Areas where bark and/or cambium are dead.
27- Cankers are caused by insects, diseases, and by
mowers, equipment, vehicles and vandalism.
28Tree Risk Categories
- Poor Architecture
-
- Growth pattern that indicates structural
imbalance and weakness in the stem, branch or
tree.
29- Poor tree architecture includes leaning trees,
harp trees and trees with poor branch form.
30Tree Risk Categories
- Dead Wood
- A dead tree or branches.
31- 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.
32Tree Risk Rating
- USDA Community Tree Risk Evaluation Form
33Tree 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
34Probability 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
35Probability 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
36Probability of Failure
- 1-Inch Rule
- Remember to measure shell thickness and diameter
in the same place.
37Probability of Failure
- 2-Inch Rule
- Remember to measure shell thickness and diameter
in the same place.
38Probability of Failure
- To determine how much damage a root system
sustained, estimate how much of the Critical Root
Radius (CRR) was lost.
39Probability 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
40Probability 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
41Size 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
42Probability of Target Impact
- 1 Occasional Use
- 2 Intermittent Use
- 3 Frequent Use
43Probability 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
44Probability 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
45Subjective 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
46Examples 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
47- 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
48- 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
49- 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
50- 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
51Tree Defect Treatment Options
- 1 - Move the target
- 2 - Treat the tree
- 3 - Convert to wildlife tree
- 4 - Close the site
- 5 - Remove the tree
52Tree Defect Treatment Options
53Tree Defect Treatment Options
54Tree Defect Treatment Options
55Tree Defect Treatment Options
56Tree Defect Treatment Options
57Tree 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