United States Department of Agriculture 

A Guide to Identifying, Assessing, and Managing Hazard Trees in 

Developed Recreational Sites of the Northern Rocky Mountains 

and the Intermountain West 

Forest Forest Health Protection Publication October 

Service Northern & Intermountain Regions R1-17-31 2017 
 

Acknowledgements: 

The authors would like to thank the authors of 2 publications from which figures were derived; Figures 1-5 were 

derived from: 

Angwin, P.A., D.R. Cluck, P.J. Zambino, [and others]. 2012. Hazard Tree Guidelines for Forest Service Facilities and 

Roads in the Pacific Southwest Region (Report # RO-12-01): U.S.D.A. Forest Service, Forest Health Protection, Pacific 

Southwest Region. 

Figure 6 was derived from: 

Filip, G.M., J. Biro, K.L. Chadwick, [and others]. 2013. Field guide for hazard tree identification and mitigation in 

developed sites in Oregon and Washington forests. R6-NR-TP-021-2013 USDA Forest Service, Forest Health Protection, 

Pacific Northwest Region, Portland, OR 

Use of these Figures was done with permission from the authors. All photographs are by the authors or from Forest 

Health Protection files unless otherwise noted. 

The authors also express gratitude to several reviewers of drafts of this publication including but not limited to: Pete 

Angwin, Jim Blodgett, Gregg DeNitto, Elise Foster, Jess Jenne, Jim Worrall, and Nick Woychick. 
 

A Guide to Identifying, Assessing, and Managing Hazard Trees in 

Developed Recreational Sites of the Northern Rocky Mountains 

and the Intermountain West 

John Guyon, Plant Pathologist 

Christy Cleaver, Plant Pathologist 

Marcus Jackson, Plant Pathologist 

Angel Saavedra, Plant Pathologist 

Paul Zambino, Plant Pathologist 

Report number R1-17-31 

United States Department of Agriculture 

Forest Service 

State and Private Forestry 

Northern and Intermountain Regions 

https://www.fs.usda.gov/main/r1/forest-grasslandhealth 

This guide is not intended to and may not be relied upon to create any rights, substantive 

or procedural, enforceable at law by any party in any matter civil or criminal. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table of Contents 

Chapter 1. Introduction .....................................................................................................................................................1 

Purpose of Guide and Goal of Hazard Tree Management.............................................................................................1 

Hazard Trees: Identification...........................................................................................................................................1 

What Constitutes a Hazard? What is Risk?....................................................................................................................2 

Chapter 2. Components of Hazard Tree Ratings................................................................................................................3 

Overview ........................................................................................................................................................................3 

Hazard Tree Rating.........................................................................................................................................................3 

What is Failure Potential?..............................................................................................................................................3 

What is Damage Potential?............................................................................................................................................6 

Rating Damage Potential ...............................................................................................................................................6 

What are Potential Impact Zones and Target Value Scores?.........................................................................................6 

Determining Potential Impact Zone for Tree or Tree Part Failure.................................................................................7 

Chapter 3. Planning and Documentation.........................................................................................................................10 

Outline of the Process..................................................................................................................................................10 

Other Factors Useful for Land Managers to Consider: Timing ....................................................................................10 

Type and Intensity of Survey........................................................................................................................................10 

Annual Survey: .........................................................................................................................................................10 

Baseline Survey: .......................................................................................................................................................10 

Ad hoc or “At Need” Surveys: ..................................................................................................................................10 

Where to Survey ..........................................................................................................................................................11 

How to Survey..............................................................................................................................................................11 

Evaluate the site from a distance.............................................................................................................................11 

Establish reference points........................................................................................................................................11 

Examine the area around each tree.........................................................................................................................11 

Begin systematic examination .................................................................................................................................11 

Dead Trees ...........................................................................................................................................................12 

Trees with Insufficient Remaining Wood Strength..............................................................................................12 

Sound Wood.....................................................................................................................................................12 

Sound Wood Shell............................................................................................................................................13 

Measurement of Sound Wood Shell Thickness ...........................................................................................13 

Proper Drilling Technique ............................................................................................................................14 

Example Determination of Sound Wood Shell ............................................................................................14 

Documentation ........................................................................................................................................................15 

Mitigation.................................................................................................................................................................15 

Hazard Tree Rating and Treatment Priority Classes ................................................................................................17 

Chapter 4. Identification of Defects and Diseases...........................................................................................................18 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Dead Trees ................................................................................................................................................................... 18 

Wounded Trees ........................................................................................................................................................... 19 

Aggressive Root Diseases and Root and Butt Rots ...................................................................................................... 20 

Conifers: Heterobasidion (Annosus) Root Disease..................................................................................................22 

Conifers: Armillaria Root Disease ............................................................................................................................ 23 

Conifers: Laminated Root Rot; Cedar Laminated Butt Rot...................................................................................... 24 

Conifers: Schweinitzii Root and Butt Rot................................................................................................................. 25 

Conifers: Tomentosus Root and Butt Rot ................................................................................................................ 26 

Conifers: Yellow Root Rot or Stringy Butt Rot ......................................................................................................... 27 

Ganoderma White Mottled Rot (Artist’s Conk) and Hemlock Varnish-Shelf Conk.................................................. 28 

Other Root Problems...................................................................................................................................................29 

Undermined Roots................................................................................................................................................... 29 

Severed roots........................................................................................................................................................... 29 

Loosened, Cracked, or Broken Roots; Leaning Trees............................................................................................... 30 

Heart Rots .................................................................................................................................................................... 31 

Hardwoods: Aspen Trunk Rot or Aspen Heart Rot .................................................................................................. 32 

Hardwoods: Brown Cubical Rot of Birch (Birch Polypore)....................................................................................... 33 

Hardwoods: White Spongy Trunk Rot (Tinder Conk)............................................................................................... 34 

Hardwoods: Hardwood Trunk Rot (False Tinder Fungus)........................................................................................ 35 

Hardwoods and Conifers: Brown Cubical Rot (Sulfur Shelf or “Chicken of the Woods”)........................................36 

Conifers: Brown Top-rot (“Rose-colored Conk”) ..................................................................................................... 37 

Conifers: Brown Trunk Rot (Quinine Conk or “Chalky Fungus”).............................................................................. 38 

Conifer: Juniper Pocket Rot ..................................................................................................................................... 39 

Conifers: Cedar Brown Pocket Rot or Redcedar Pencil Rot.....................................................................................40 

Conifers: Red Heart Rot (Bleeding Stereum) ........................................................................................................... 41 

Conifers: Red Ring Rot or White Speck (Pini Conk) ................................................................................................. 42 

Conifers: Rust-red Stringy Rot (Indian Paint Fungus) .............................................................................................. 43 

Sap Rots of Conifers.....................................................................................................................................................44 

Conifers: Brown Crumbly Rot (Red-Belt Fungus)..................................................................................................... 45 

Conifers: Gray-brown Sap Rot (Pouch Fungus)........................................................................................................ 46 

Canker Diseases, General Information ........................................................................................................................ 47 

Hardwoods: Sooty Bark Canker ............................................................................................................................... 48 

Hardwoods: Snake (Cryptosphaeria) Canker-rot.....................................................................................................49 

Hardwoods: Hypoxylon Canker ............................................................................................................................... 50 

Hardwoods: Black canker ........................................................................................................................................ 51 

Conifers: Atropellis Canker ...................................................................................................................................... 52 

Conifers: Western Gall Rust.....................................................................................................................................53 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

All Trees, Bole Cracks and Splits...................................................................................................................................54 

Tension and Compression Failure, Limbs.................................................................................................................54 

Lightning Strikes.......................................................................................................................................................54 

Cracks from Wind and Wind Shake..........................................................................................................................54 

Frost Cracks..............................................................................................................................................................55 

Structural Failure Cracks, Main Bole........................................................................................................................56 

Forked Multiple Tops or Trunks (Conifers) ..................................................................................................................57 

Dead Tops (Conifers)....................................................................................................................................................58 

Dead or Defective Branches.........................................................................................................................................59 

Branch Brooms (Conifers)............................................................................................................................................60 

Fire-Caused Damage ....................................................................................................................................................61 

Height to Diameter Ratios............................................................................................................................................62 

Insect-caused Damage .................................................................................................................................................63 

Interacting Indicators (Multiple Defects).....................................................................................................................64 

Glossary............................................................................................................................................................................65 

Appendix. Field Form and Instructions ............................................................................................................................69 

Data Collection Fields...................................................................................................................................................69 

Recommended Tools ...................................................................................................................................................69 

Instructions for Completing Region 1 and 4 Tree Hazard Evaluation Form ................................................................69 

Region 1 and 4 Tree Hazard Evaluation Form..............................................................................................................71 

 

 

 

 

 

 

Chapter 1. Introduction 

Purpose of Guide and Goal of Hazard Tree Management 

Direction on hazard tree management for USDA Forest Service developed recreation sites is found in Forest Service 

Manual section FSM 2332.13 where it states: “Consistent with preserving the recreation resource, remove trees or 

tree limbs identified as hazardous at developed recreation sites. Obtain assistance from timber management, forest 

pest management, and recreation specialists, as necessary”. By developing this guide, Forest Health Protection, 

formerly known as Forest Pest Management, provides managers of developed recreation areas with a process and 

sufficient background information to aid in recognizing, assessing, and making management decisions regarding 

hazard trees. 

The goal of hazard tree management is to minimize, to the extent practical and consistent with overall management 

objectives and constraints, the potential that trees at public sites will cause injury, death, or property damage. 

Thorough, systematic surveys of trees that could harm the public or damage property at developed recreation sites, 

conducted by effective evaluators, are an important part of meeting management requirements. An effective 

evaluator can be defined as a person who has received training and has experience in evaluating tree hazards. Types 

of tree defects can vary widely by location and species, so 

familiarity with locally important tree defects may be needed for 

hazard tree evaluators to be effective. This guide provides a means 

to identify tree defects and failure potential using examples of 

hazard trees chosen to represent tree species and defects likely to Guide Objectives: 

occur in the Northern and Intermountain Regions of the USDA 1) Provide a process of evaluating Forest Service. tree hazards in developed recreation 

This guide is primarily written for use on developed sites, such as sites of the Northern Rocky 

areas established for regular use for camping, picnicking, parking, Mountains and the Intermountain 

or other activities. Established, high-use sites are far more likely to West. 

have failures of trees and tree parts that could cause personal 2) Provide an up-to-date field guide injury or property damage than at undeveloped sites where people to aid accurate identification of tree 

and property normally have more infrequent or shorter exposure. defects and Failure Potential. 

Hazard tree management has its basis in science, yet its practice 3) Provide guidance for recording cannot identify and remove all trees that may fail. Under high wind evaluations of developed sites. 

or other storm conditions, trees and forests may experience 

extreme physical stresses that can cause breakage and wind throw 4) Promote development of longin trees lacking detectable defects. Also, as trees and stands age, term vegetation management plans 

they have prolonged and repeated exposure to natural for developed sites. 

disturbances or human-caused injuries that result in increasing 

amounts of both obvious and hidden defects. Such defects, along 

with increasing mass, height, and wind exposure, can make aging trees and groups of trees more prone to failure. 

Hazard Trees: Identification 

Evaluators need to exercise considerable professional judgment, and no guidance can address every possible 

situation. Considerations for evaluators include: the presence of a tree or tree part that has the potential to cause 

harm; the potential that the tree or tree part will fail; the location of the tree or tree part and/or its proximity to a 

developed recreation site; and the overall degree of risk that the tree or tree part might pose. 

Natural resource managers have many, often conflicting, priorities influencing their allocation of resources to 

projects and management functions. Hazard tree management is a major priority, but this activity often occurs within 

the context of all other management activities on developed recreation sites. These are best coordinated within the 

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broader perspective afforded by a vegetation management plan. FSM 2331.4 states to, “Prepare a vegetation 

management prescription for each recreation site. The primary objective of the prescription is to create and maintain 

a natural environment”. Consideration of hazard tree risks can inform broader decisions on long-term vegetation 

management to enable a functioning, healthy, and safe forest environment for future visitors of Forest Service 

recreational sites. 

What Constitutes a Hazard? What is Risk? 

The term “hazard” refers to a tree or tree part of sufficient size, mass, and condition to have the potential to cause 

harm. The term “risk” refers to the relative potential that a hazard may cause harm to someone or something of 

value. The overall risk of a hazard may change due to changes in the condition of the tree and/or the range of 

conditions likely to occur over time. 

Hazard tree rating deals with both evaluating the failure potential of a recognizable hazard and the risk the hazard 

poses to persons, property and/or structures. For example, even a tree identified as a possible hazard may receive a 

low overall hazard tree rating if the evaluator determines that either the potential for damage is low or the distance 

from people or valuable property is too great. 

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Chapter 2. Components of Hazard Tree Ratings 

Overview 

This chapter explains the basis and components of hazard tree ratings and provides a detailed method for assigning 

numerical values for Failure Potential, Damage Potential, Target Value and overall Hazard Tree Rating for individual 

trees. Hazard tree evaluation also involves documentation and record keeping of tree ratings and periodically reevaluating these ratings at some scheduled interval. This chapter also provides guidance to managers and 

responsible officials for deciding acceptable levels of risk and methods of mitigating hazard. 

Hazard Tree Rating 

The Hazard Tree Rating system presented here incorporates three components: 

1. Failure Potential: the potential that a tree or tree part may fail within the time period before reassessment. 

2. Damage Potential: the potential that property damage, injury, or death may result from a tree or tree part 

failure. 

3. Target Value: the value given to a tree based on its proximity to people and property. 

The Hazard Tree Rating for each individual tree is determined by adding numerical values of Failure Potential (FP) 

(scale FP1-4), Damage Potential (scale 1-3), and Target Value (scale 1-3). Possible Hazard Tree Rating ranges from 3 to 

10. Trees with lower ratings have lower risk compared to those with higher ratings. 

What is Failure Potential? 

Failure Potential is the potential for a tree or tree part 

to fail before the next inspection period. Assessing 

Failure Potential requires recognition of weaknesses, 

the conditions that act on those weakness, and the 

frequency at which those conditions occur. Failure is the 

end result of many interacting variables: tree size, age, 

form, species, condition, location; stand structure; site 

and environment conditions; and presence and extent 

of tree defects. 

Tree characteristics commonly contributing to Failure 

Potential include: 

• Declining, dying, or dead trees. 

• Dead, broken, or free-hanging branches. 

• Multiple stems; dead, forked, or multiple tops; 

and if these have recent weakening. 

• Wounds, injuries, exposed or damaged roots 

and associated decay or defect. 

FAILURE POTENTIAL (FP) SCORES 

FP4 = HIGH 

Dead trees; live trees with major defects; decayed or 

burned trees with less than acceptable remaining 

sound wood. 

FP3 = MEDIUM 

Live trees with moderate defects; decayed trees where 

sound wood shell thickness is at or near the minimum 

preferred amounts. 

FP2 = LOW 

Live trees with only minor defects such as wounds that 

don’t impair tree structure; trees with slight decay. 

FP1 = VERY LOW 

Live trees without visible defects. 

See Table 1 for examples. 

• Lean in a tree, its amount and type, and factors that contributed to the lean. 

• Recent lean, so tree has not had time to add strength to prevent failure. 

• Compromised anchoring of lateral roots (tree is “root-sprung”). 

• Disease, decay, or insect infestation and whether these will be lethal or weaken roots, stem, or branches. 

• Exposure to wind and other site-specific influences. 

• Soil structure and shallow water table that may make a tree prone to root failure. 

• Proximity to pockets of root disease. 

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Table 1. Failure Potential Rating in Whole Trees. 

Indicator Failure Potential (FP) 

High; FP4 Medium; FP3 Low; FP2 Very Low; FP1 

Bole 

Cankers 

>50% bole 

circumference 

affected 

25-50% bole 

circumference affected 

<25% bole circumference 

affected None 

Bole wounds 

(includes fire 

damage >5 years) 

>50% bole 

circumference 

affected OR <20% 

sound wood shell 

See Table 2 

<50% bole 

circumference affected 

AND 

20-30% sound wood 

shell 

<50% bole circumference 

affected AND >30% sound 

wood shell 

Well-calloused 

or superficial 

wound 

Cracks (see 

Chapter 4) 

Structural failure 

cracks OR if <20% 

sound wood shell 

Significant cracks 

without movement AND 

20-30% sound wood 

shell 

Small shallow cracks OR 

partially callus sealed cracks 

(e.g., frost cracks) AND 

>30% sound wood shell 

None 

Height : diameter 

ratio 

Ht (ft)/dbh (ft) 

H:D ratio >100 H:D ratio 80 to 100 H:D ratio 60 to 80 H:D ratio <60 

Fungal decays 

(Guidelines for 

some specific 

conks / fungi in 

Chapter 4) 

Conks visible 

OR <15% sound 

wood shell (Table 2) 

if conifers without 

openings or wounds. 

<20% if openings, 

wounds or cracks are 

present 

Significant decay 

present, but >15% 

sound shell without 

openings, wounds or 

cracks OR >20% with an 

opening, crack or 

wound. 

See Table 2 

Some decay present, sound 

wood quantity over 

thresholds. 

See Table 2 

None 

Butt and Roots 

Fire damage (<5 

years) 

Bole cross-section 

<50% sound wood 

OR 

>1 quadrant of 

damaged structural 

roots 

If “other species” at 

right, bole cross-section 

50-75% sound wood 

(see Fig. 6), OR any 

species with 1 quadrant 

of damaged structural 

roots 

Cedar, ponderosa pine, 

larch: bole cross-section 

>50% sound wood 

Bole crosssection 100% 

sound wood, 

and NO 

damaged 

structural roots 

Other species: bole crosssection >75% sound wood 

Leaning and/or 

root-sprung trees 

Soil cracked or lifted 

or root breakage 

near tree 

Uncorrected lean ≥15°,

soil looks intact 

Corrected lean, soil looks 

intact 

Lower bole 

vertical with 

sweep in upper 

Undermined or 

severed roots 

<50% of the 

structural roots 

remaining in the 

ground 

50-75% of the structural 

roots remaining in the 

ground 

>75% of the structural roots 

remaining in the ground 

Few roots 

exposed and lack 

decay 

Aggressive Root 

Disease or Root 

and Butt Rot 

Tree with aggressive 

root disease: has 

thinning or fading 

crown or basal resin 

or stain OR butt rot 

extensive with <15% 

sound wood shell OR 

decay in >50% of the 

structural roots 

Aggressive root disease 

suspected from diseased 

neighbor but not 

confirmed in tree OR 

moderate butt rot with 

15-25% sound wood 

shell OR decay in <50% 

of the structural roots 

Little or no butt rot with 

>25% sound wood shell and 

no decay in structural roots; 

no evidence of root disease 

in neighbors 

None 

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Table 1 continued. Failure Potential Rating in Tree Parts and All Parts (including whole trees). 

Indicator Failure Potential (FP) 

Tops and Branches 

High; FP4 Medium; FP3 Low; FP2 Very Low; FP1 

Multiple 

interacting 

Indicators (if not 

interacting, use 

highest single 

score) 

Two or more interacting 

medium-FP indicators 

Two or more 

interacting low-FP 

indicators 

Two or more 

interacting very low-FP 

indicators 

None 

Forked or 

multiple tops 

V-shaped forks with 

embedded bark AND 

open cracks, decay, or 

conks 

V-shaped forks with 

embedded bark, and 

no other FP2 defects 

V-shaped forks without 

embedded bark and 

no cracks, decay, or 

conks OR unbalanced 

multiple tops 

None 

Decay of Upper 

Bole 

Any species with 

indicators of significant 

decay (open cracks, 

multiple or large conks, 

or exposed rot) 

True fir, hemlock, 

spruce, or hardwoods 

with little decay (few 

small conks) 

Douglas fir or pine with 

little decay None 

Detached or 

partially 

detached tops, 

branches, (≥3”

diameter) 

All detached parts 

Live and attached tops 

or limbs but cracked 

or split 

Cedar or larch; pine 

tops killed slowly by 

rust fungi and showing 

resin impregnation 

Small hanging 

branches or tops 

<3” diam. 

Dead tops 

Dead tops ≥3” diam. with

signs of decay or other 

defects, or if already 

detached 

Dead tops in aspen, 

cottonwoods and 

large birches or nonresinous conifers 

≥3“diam. without

decay or other defects 

Non-decayed dead 

tops <3” diam., OR 

dead non-decayed tops 

of any size in resinous 

species 

None 

Dwarf mistletoe 

brooms 

Douglas-fir with large 

(>10’ diam.) dead 

brooms OR any species 

brooms with cracked 

branches 

Douglas-fir with small, 

dead brooms OR 

other tree species 

with large, dead 

brooms 

Douglas-fir with large, 

live brooms OR other 

tree species with small, 

dead brooms 

Live brooms with 

no other sign of 

defect 

Cottonwoods and 

large birches with 

large damaged or 

dead limbs 

Large (>3” diam.) live or 

dead branches with 

evidence of decay and 

past breakage 

Large (>3” diam.), live 

branches with 

evidence of past 

breakage but no 

decay 

Large (>3” diam.), live 

branches without 

decay or breakage 

None 

All parts 

Multiple 

interacting 

Indicators (if not 

interacting, use 

highest single 

score) 

Two or more interacting 

medium-FP indicators 

Two or more 

interacting low-FP 

indicators 

Two or more 

interacting very low-FP 

indicators 

None 

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What is Damage Potential? 

Damage Potential is an estimation of extent or severity 

of damage or injury that could result from tree failure. 

Damage Potential takes account of tree or tree part size 

and the height at which a defect occurs. The Damage 

Potential rating is most useful for evaluating tree parts, 

which are more likely than whole trees to vary in the 

severity of damage they could cause. Obviously, even a 

small, defective tree part could cause greater damage if 

it fell from high in a tree than if it was lower in a tree. 

Rating Damage Potential 

Damage Potential is rated on a scale of 1 to 3, with 3 

indicating greatest severity. Damage Potential scores 

given in the text box at right reflect these assumptions: 

• Small branches and tops (less than 3 in. in 

diameter) typically cause minor damage. 

• Medium size tree parts can cause serious injury 

DAMAGE POTENTIAL SCORES 

3 = EXTENSIVE 

Tree or tree part’s size and falling distance indicate its 

force of impact could cause serious personal injury or 

death or severe damage to property. 

2 = MEDIUM 

Tree or tree part’s size and falling distance indicate its 

force of impact could cause at least moderate injury, 

or moderate but not major structural damage. 

1 = MINOR 

Tree or tree part’s small size (<3 inch diameter) and 

short falling distance to target indicate its force of 

impact could cause minor injury or light damage to 

property. 

to people, but rarely cause death and can cause some property damage but not loss of an entire structure. 

• Larger tree parts can cause extensive damage resulting in death or property/structure loss. 

• Tree parts might have less Damage Potential if deflected off or falling through tree canopies, because 

interactions with branches and trees can slow their fall. Damage Potential could be assigned a lower value for 

some of these cases. 

• Large, whole trees with an unobstructed path to a person or structure should always be given a Damage 

Potential value of 3. 

What are Potential Impact Zones and Target Value Scores? 

A Potential Impact Zone (PIZ) is the area that a tree or 

its parts could normally reach if they fail. Target Value 

is a score assigned to a tree based on how long and 

how often the PIZ will be occupied by persons or 

structures or other valued property. Note that 

although a Target Value score is determined relative to 

people and property a hazard tree might impact, this 

score is assigned to a tree. It is not a value assigned to 

a particular object or to people. Target Value score is 

rated on a scale of 1 to 3, with 3 indicating greatest 

Target Value score. Some important points to 

consider: 

• Length of use and types of activity are critical. 

For example, sleeping campers are less likely 

to move during windy or stormy conditions 

than hikers or day use visitors who may leave 

an area. 

• Target Value scores will always be high when 

people are frequently in the Potential Impact Zone. 

TARGET VALUE SCORES 

3 = HIGH 

Tree’s potential impact zone often has people or 

valuable property present for long periods. This score 

is used for trees around developed tent sites, toilets, 

parking spurs, and other high-occupancy sites. 

2 = MODERATE 

Tree’s potential impact zone includes, at most, 

features with moderate use and short use length such 

as water sources or waste disposal stations. 

1 = LOW 

Tree’s potential impact zone includes, at most, 

features with limited use and short use length such as 

garbage cans, campground signs, or fences; may also 

be used for trees with moderate-use features just 

outside estimated impact zones. 

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Determining Potential Impact Zone for Tree or Tree Part Failure 

Potential Impact Zone (PIZ) is the area on the ground that could be reached by ANY portion of a tree or a tree part 

that is considered a hazard and is determined by tree height, lean, and slope. Guidelines are as follows: 

• Flat ground, whole straight tree as hazard: The PIZ for a straight, non-leaning tree on flat ground is a circle 

around the tree with a radius equal to the total tree height (Fig. 1). For instance, if a tree is 100 feet tall, then 

its PIZ has a radius of 100 ft. 

• Sloping ground, whole straight tree as hazard: On sloping ground, the PIZ may be extended downhill from 

the tree to a distance (the “slide zone”) the evaluator deems necessary to protect people or property from a 

tree sliding, rolling or bouncing if it fails (Fig. 2). The evaluator needs to use judgement and account for 

steepness of slope, topographic features, and neighboring trees, but the PIZ is often extended to 1.5 times 

tree height of the downhill side for trees on moderately steep slopes. 

• Flat ground, whole leaning tree as hazard: For trees leaning more than 15 degrees, failure from the base 

most often causes the tree to strike anywhere from the direction of the lean to 90 degrees to either side. The 

PIZ in this area has the same radius as the tree’s height. Backlash away from the lean could occur in storms 

with high winds and would be mostly branches, so the “backlash zone” portion of the PIZ away from the lean 

is considered to be narrower (Fig. 3). 

• Flat ground, bole top on a straight tree as hazard: The PIZ for a dead top, or one or more stems of a split or 

multiple top of a straight, non-leaning tree on flat ground is a circle around the tree with a radius equal to the 

total tree height, similar to Figure 1. The calculation in this guide accounts for high variability in where short 

upper bole failures may land if they fail in high wind, because of the distances that they can be cast when 

trees are swaying. A PIZ of 1.0 times tree height is also less likely to overinflate the PIZ of failures in the lower 

bole and is easy to calculate. 

• Flat ground, bole top on a leaning tree as hazard: Like the PIZ for a dead top on a straight tree, the PIZ for a 

dead top on a leaning tree is a circle with a radius equal to the total tree height, but it is centered below the 

likely break point of the top instead of at the tree’s base (Fig. 4). 

• Flat ground, large lower branch as hazard: The PIZ for a hazardous lower branch is calculated as a circle 1.5 

times the branch length, centered below the likely place it will fail (Fig. 5). 

7 
Figure 2. The Potential Impact Zone (PIZ) for a whole tree on a slope 

is calculated as a circle with a radius equal to the height of the tree 

plus a “slide zone” to account for a tree’s sliding, rolling, or bouncing. 

Figure 1. The Potential Impact Zone (PIZ) for a whole tree on flat Here, maximum PIZ with slide zone is shown as 1.5 times tree height, 

ground is a circle around the tree with a radius equal to the tree but larger or smaller slide zones may be appropriate (see text). 

height. 

Figure 3. The Potential Impact Zone (PIZ) for a tree with a significant lean (>15°) on flat ground is a 

half circle with a radius equal to the height of the tree in the direction of the lean, plus a narrower 

backlash zone away from the lean (see text). 

8 
 

Figure 4. The Potential impact zone (PIZ) for a hazardous bole top for a tree on flat ground and a 

slight lean is a circle with a radius equal to the height of the tree, centered below the base of the 

dead top or likely break point. 

Figure 5. The Potential Impact Zone (PIZ) for a large defective branch is calculated as a 

circle with a radius 1.5 times the length of the defective branch, centered below the 

presumed break point. 

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Chapter 3. Planning and Documentation 

Outline of the Process 

It is not necessary to mitigate all levels of hazard from trees, only those hazards that have been deemed to create an 

unacceptable risk to persons, property and/or structures in the judgment of the appropriate managing authority. 

Recommended steps for evaluating potential hazard trees at a developed recreation site include: 

1) Identify staff and managers responsible for all evaluations, mitigation activities and other subsequent 

activities. 

2) Identify and gather the necessary equipment (see Appendix), forms (see Region 1 and 4 Tree Hazard 

Evaluation Form in Appendix), maps, and previous hazard tree evaluation data if available. 

3) Determine area to be surveyed within each recreation site. 

4) Determine a Hazard Tree Rating for individual trees; 

• Identify tree defects. Determine a numerical Failure Potential score using tables and data sheets. 

• Estimate Potential Impact Zone (zone a tree or its parts could reach), and numerical Damage 

Potential and Target Value scores. 

• Add numerical scores of Failure Potential, Damage Potential, and Target Value to determine Hazard 

Tree Rating. 

5) Document the date the site is being evaluated, individual Hazard Tree Ratings and treatment options if 

practicable. 

6) Determine and record possible mitigation options, and whether any immediate action is recommended. 

7) Submit hazard tree evaluations to the site’s manager or a supervisor as appropriate. 

8) Be sure to record dates when recommended actions are completed. 

Other Factors Useful for Land Managers to Consider: Timing 

• Essentially all trees pose some hazard, it is not possible to eliminate all hazards, and trees and forests are 

constantly changing, so it is advised to repeat assessments of trees over time at reasonable intervals or after 

major disturbance events. 

• Preserving the safety of visitors and employees is crucial, so evaluators should allow sufficient time to make 

the decisions on the soundness of individual trees, especially if the evaluator has limited experience. 

• Being aware of differences in visitor use and recurrent risk levels at different sites enables managers of 

multiple sites to prioritize hazard tree evaluation efforts and implement treatments that respond to ongoing 

patterns of hazard and risk. 

• The most intensively used sites may benefit from more frequent or intensive assessments. 

Type and Intensity of Survey 

Annual Survey: Timing and frequency of examinations may vary, but all developed sites could be examined annually 

for trees with new or increased hazard. Winter storms can cause failure or weakening of trees or limbs, especially in 

portions of stands with root disease or stem or branch decay. Annual pre-season site reviews could be a systematic 

walk-through examination and ideally, will refer to a previous baseline survey. 

Baseline Survey: A survey that is more thorough and formal than an annual walk-through is recommended at least 

every five years, where each tree in all areas of the developed site is examined for new evidence of hazard or defect. 

Baseline surveys could be more frequent for recreation areas with special site factors such as bark beetle outbreaks, 

root disease, and soils that become saturated in spring floods. Some forest types, such as white fir or cottonwood, 

often have many defective trees and if practicable, more frequent assessments may be useful. 

Ad hoc or “At Need” Surveys: It is also advisable to examine sites after special circumstances, such as after severe 

weather events or fires that occur during the usage season. Storm damage often weakens trees, and can be followed 

within weeks by additional tree and branch failures. Fire can rapidly decrease wood strength and can result in failures 

in a relatively short time. 

10 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Where to Survey 

Obtain a detailed map of the developed site and review any past survey information. Places to survey include: 

• Roads: Roads within the site and immediately entering or exiting the site. 

• Structures: Buildings, parking areas, restrooms, waste disposal stations, water pumps, picnic tables, or ski 

lifts. 

• Gathering places of people: Around tent pads, picnic tables, parking areas, ski-lifts, fire rings, barbecue pits, 

and all other recognized gathering places or focal points of human activity. Ask site hosts or maintenance 

workers for additional features. 

How to Survey 

Evaluate the site from a distance 

Examine trees from different angles and vantage points. While approaching the site, look for severe defects that 

would be a dangerous to examiners. Evaluation by two people is more effective and efficient than with one person, 

and increases safety. With a 2-person crew, one examiner can work near the tree to look at defects in detail while 

the other examines the crown and measures height which helps determine Damage Potential, PIZ, and Target Value. 

Compare the vigor and overall appearance of trees relative to their nearest neighbors. This allows detection of dead 

trees or tops and live-crown symptoms of root disease. Indicators could include reduced lateral branch and terminal 

growth, thinning crowns, chlorosis, distress cone crops, and dead tops and branches. Other indicators of Failure 

Potential such as defoliator activity, dwarf mistletoe infection, stem conks, bark beetle attack, and recent leans are 

often initially detected from a distance. 

Establish reference points 

Locate a large object that is a permanent fixture within the site for a reference point. Examples of useful fixtures 

include a fire pit in a numbered campground site, an outhouse along a road, and a lift tower at a ski area. Within the 

survey area, one reference point can often be used per every one to two campsites or other site subunit. These are 

essential for making maps and for documenting and relocating individual trees. Reference points can be indicated on 

a site map and/or by GPS. Note distance and bearing of each tree from the reference point. Ideally, reference points 

will be available from a prior hazard tree evaluation. Evaluations of trees are best made in a systematic fashion, 

starting at the first reference point. 

Examine the area around each tree 

Look for evidence of past and current pathogen and insect attack, or other damage. Nearby stumps and old roots can 

be examined for evidence of decay or root and butt pathogens. Broken-out tops that are lying on the ground, and 

wind-thrown or wind-broken trees can be examined for causes of failure. Examine conks, mushrooms, and other 

fruiting bodies on and around trees to aid identification of causes and extent of decay. 

Begin systematic examination 

• Look at the tree top, limbs, bole, butt, and soil mounds surrounding the roots. All sides of each tree can be 

examined. Binoculars can be useful for tall trees. 

• Look for dead trees, dead tops and branches, whether attached or free-hanging, splits and other crown 

defects, crown symptoms of root disease, crown symptoms and bole bark beetle attack, and conks and 

mushrooms that indicate decay and root and butt diseases. Dead tops can be examined for decay and 

instability indicated by conks, crumbling sapwood, woodpecker feeding activity, and wildlife nesting cavities. 

• Examine the bole above the upper butt visually, and look for signs of past injury or fungal fruiting bodies. 

Fruiting bodies often develop at the site of old branch stubs or wounds. By the time trees exhibit fruiting 

bodies of stem decay fungi, decay levels often are substantial. Absence of conks, however, does not 

necessarily mean that a tree is free from decay. Record the presence of all signs of potential defect to aid 

examiners in the future. 

11 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

• If decay columns, hollows, and dead sapwood are present low 

on the bole, physically examine the Sound Wood Shell 

thickness as described below, or for irregular cavities, visually 

determine amounts of wood remaining as illustrated in Fig. 6. 

If it won’t damage bark, tap suspect trees with an axe or 

rubber mallet and listen for hollows. If necessary to increase 

accuracy, determine the extent of decay in suspect trees 

(trees with open wounds, plates of dead- attached bark, 

conks, or mushrooms) with a cordless drill or increment 

borer. 

• Look for evidence of root disease at the base of the bole and 

root crown such as resin flow, dead bark, and depressions. If a 

root disease problem is strongly suspected, consider 

examining the most symptomatic roots with a drill, axe, or 

Pulaski. 

• If root disease symptoms are evident or the tree is close to 

another tree where root disease has been confirmed, the root 

collar, butt, and major lateral roots can be inspected. Look for 

characteristic signs of fungal pathogens or other evidence of 

decay agents. If root disease is suspected, soil can be carefully 

removed from roots, out to a distance of one meter to look 

for evidence of root disease at the root surface. At least two 

major roots can be checked for root disease if preliminary 

evidence suggests its presence. The roots most likely infected 

will be those near other infected trees, hollow stumps, windthrown trees with decayed roots, or flat or sunken faces of 

the tree’s bole. For some root diseases, chopping into 

Decay in Trees 

Decay in trees is a complex process 

involving fungus colonization and 

host tree defenses. As the two 

organisms interact, decay weakens 

the structural integrity of wood. 

Several methods visually estimate or 

calculate amounts of remaining 

sound wood and whether remaining 

strength is sufficient for safety. Each 

method has its own uses and 

limitations, but all are based on a 

finding that trees do not tend to fail 

where they retain at least one third of 

their sound wood. 

Hazard tree evaluators should keep in 

mind that trees are dynamic, living 

things that adapt to their world, and 

can change where they deposit wood 

and add strength in response to 

damage and defect. 

symptomatic roots or coring with an increment borer or cordless drill can show internal infection. 

Dead Trees 

All dead trees are rated as high hazard (FP4) if their PIZs affect high-use areas and tree failure could result in 

moderate to extensive damage. Attempt to determine causes of tree mortality, as some causes will affect additional 

trees at the site. Tree mortality in Intermountain and Northern Rocky Mountain forests often has complex causes. 

Severe or recurrent drought, competition with other trees, root diseases, bark beetles at endemic or outbreak levels, 

and prior damage by fire can all contribute. If present, some of these factors will also be affecting nearby trees, 

predisposing them to mortality or damage. 

Trees with Insufficient Remaining Wood Strength 

Sound wood or sound wood shell can be estimated to determine Failure Potential values for trees with visible or 

presumed decay, conks, wounds, open cavities, cankers or wood loss by fire. Sound wood is the remaining wood not 

affected by decay or other deterioration. Sound wood shell is the remaining wood unaffected by any decay or 

deterioration that contributes to a tree’s strength. Proportions of remaining sound wood and sound wood shell 

representing high, medium, and low Failure Potentials are given in the Failure Potential Value Table (Table 1). 

Sound Wood 

Sound wood is estimated visually as the percentage of non-decayed wood remaining in a cross-sectional area of the 

bole. Figure 6 provides representations of highly irregular wood loss in bole cross-sections that can be used for 

estimating percentage of sound wood remaining after fire, decay, or both causes. These examples can help assess 

trees against sound wood values in Table 2 to identify those with obviously high Failure Potential. 

12 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Using a minimum sound wood value to estimate Failure Potential is most applicable for conifers. Using a minimum 

sound wood value for some hardwood species can overestimate Failure Potential for the following reasons: 1) Trunk 

failures well above the ground line are relatively rare in hardwoods with high mechanical strength such as oaks and 

maples. This is not true for species with weaker wood and rapid decay, such as cottonwood, alder, and aspen; 2) 

Many hardwoods have spreading crowns that are more influenced by leverage from wind and gravity than in 

conifers, so branch and fork conditions cause failures more frequently than trunk conditions. 

Figure 6. Diagrams of bole cross-sections of remaining sound wood representing percentage ranges used in Table 1 to assign Failure Potential 

values. 

Sound Wood Shell 

Sound wood shell is the sound wood completely surrounding internal decay and is estimated as a bole’s average 

thickness of wood with no decay, excluding bark. Calculating the ratio between sound wood shell and diameter can 

indicate how much of a tree’s original strength remains at a point of interest such as the tree’s base or an opening. As 

with minimum sound wood, the concept of minimum sound wood shell thickness is more useful in conifers than in 

hardwoods. Conifers often have relatively evenly proportioned decay in cross-section, are more likely than 

hardwoods to retain their full height when mature (have not experienced and survived a break part-way up the bole), 

and boles have an even taper and are less irregular than hardwoods. 

Table 2 shows cut-off values for inches of sound wood shell to assign trees with different diameters their Failure 

Potential values as shown in Table 1. The leftmost column (Inches for 15% Sound Wood Shell) is for trees without 

openings or other defects in addition to decay. Other columns are used for trees with decay plus additional defects 

such as openings, lean, and cracks that require trees to have more strength and sound wood shell to withstand 

destructive forces (see Table 2). The Failure Potential table (Table 1) and Multiple Defects section of Chapter 4 

identify the common combined effects and present cut-off ratios between Failure Potential values. 

The two most commonly encountered situations using sound wood shell measurements are trees with decay and no 

additional defects, and decay with an opening. The cut-off ratio between FP3 and FP4 for conifers that lack surface 

openings is a sound wood shell thickness of 0.15x tree diameter, displayed as “15% sound shell”. For conifers with 

open cavities with decay visible at the bole surface, the cutoff is 0.2x diameter inside tree bark, displayed as “20% 

sound wood shell”. When the average thickness of the sound wood shell is less than the values given for the tree’s 

diameter in Table 2, the Failure Potential (FP) is the higher FP choice. 

Measurement of Sound Wood Shell Thickness 

These examinations are done by coring or drilling, but should not be routine; they should only be used to determine 

critical differences in Failure Potential. For soft-wooded species such as cedar, spruce, or alder, incipient decay may 

be more difficult to detect, so coring with an increment borer may be more useful than drilling. However, holes made 

with either tool damage trees and can increase decay, and is not normally needed. Drilling may be warranted when a 

high incidence of hidden decay is suspected or when nearby trees have fallen from decay not visible from the 

outside. For example, sites with tomentosus root and butt rot may need closer examination. Drilling or coring does 

not need to go any deeper than the depth of the required amount of sound wood needed for the tree diameter. For 

13 
 

 

 

 

 

 

 

 

 

 

 

 

 

example, a 10 in. diameter tree only needs 2 in. of sound wood (Table 2). Drilling deeper unnecessarily damages the 

tree and may spread decay. 

Table 2. Reference sound wood shell thicknesses1 for determining Failure Potential values of trees with different 

within-bark tree diameters, based on Sound Wood Shell Ratios for single or combined defects listed in Table 1 and 

Chapter 4. 

Within-Bark Inches for Inches for Inches for Inches for 

Tree Diameter 15% Sound 20% Sound 25% Sound 30% Sound 

in Inches2 Wood Shell Wood Shell Wood Shell Wood Shell 

4 

6 

8 

10 

12 

14 

16 

18 

20 

22 

24 

26 

28 

30 

32 

34 

36 

38 

40 

42 

44 

46 

48 

50 

0.6 

0.9 

1.2 

1.5 

1.8 

2.1 

2.4 

2.7 

3 

3.3 

3.6 

3.9 

4.2 

4.5 

4.8 

5.1 

5.4 

5.7 

6 

6.3 

6.6 

6.9 

7.2 

7.5 

0.8 

1.2 

1.6 

2 

2.4 

2.8 

3.2 

3.6 

4 

4.4 

4.8 

5.2 

5.6 

6 

6.4 

6.8 

7.2 

7.6 

8 

8.4 

8.8 

9.2 

9.6 

10 

1 

1.5 

2 

2.5 

3 

3.5 

4 

4.5 

5 

5.5 

6 

6.5 

7 

7.5 

8 

8.5 

9 

9.5 

10 

10.5 

11 

11.5 

12 

12.5 

1.2 

1.8 

2.4 

3 

3.6 

4.2 

4.8 

5.4 

6 

6.6 

7.2 

7.8 

8.4 

9 

9.6 

10.2 

10.8 

11.4 

12 

12.6 

13.2 

13.8 

14.4 

15 

1 Compare average of bole measurements at bole height of greatest weakness or defect to values in column; if tree

measurement is less, record Failure Potential as the higher Failure Potential value. 

2 Values for most diameters not shown can be calculated by halving, doubling, or averaging. 

3 Consult Table 1 for the application of the 15-30% rules. 

Proper Drilling Technique 

To measure sound wood shell thickness with a cordless electric drill, use a long, narrow-diameter bit and, while 

drilling, note the depth where resistance or wood color changes. First, drill directly into a wound, canker, or flattened 

area. If decay is found, up to three more drillings can be made, one on the opposite side of the bole and one each at 

90 degrees to the first drill spot using a minimum of three drillings to calculate the average sound wood shell 

thickness. Note thickness of bark and of sound wood shell for each drill spot. Tree species that have buttress roots or 

fluted butts (e.g., western hemlock, western redcedar) may require more sampling since the distal portions of fluted 

areas are often thicker. Estimate sound wood shell thickness by averaging all measurements (Table 2). Measuring 

sound wood shell thickness with an increment borer or drill can be part of the baseline survey and then repeated no 

more often than every 5 years to minimize wounding and spreading decay. 

Example Determination of Sound Wood Shell 

A tree with an outside diameter of 32 in. at its base and an open cavity near its base is being examined. Bark 

thickness is roughly estimated at 2 in. Twice the bark thickness is subtracted from the diameter to get within-bark 

diameter of 28 in. For this diameter, the value for minimum sound wood shell thickness at the opening is found in the 

“20% sound wood shell” column in Table 2 (5.6 in.) or manually calculated (0.20 x the 28 in. diameter = 5.6 in.). 

Sound wood remaining may be visually less than this value, so coring is not needed. Or, an increment borer may be 

used at three points at the base, and average sound wood shell thickness determined to be only 4 in. In either case, 

the tree would be assigned a high Failure Potential value (FP4). 

14 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

Documentation 

A record of inspection provides evidence that a survey was completed for a site. When a formal report submitted to a 

responsible official includes a record of trees with elevated hazard, protocols used to determine the hazards, and 

suggested recommendations, it can provide support for the recommendations and aid the official in making 

management decisions. Data from inspections can be recorded on the tree hazard evaluation form shown in the 

Appendix or similar forms. Evaluators may choose whether or not to use such forms to record information from 

inspections of healthy-appearing trees that would have inconsequential Hazard Tree Ratings. 

Recording results of trees with elevated ratings in developed site evaluations is helpful for several additional reasons: 

1) The assessment of current hazards provides a foundation for future vegetation management activities. 

2) Recording principal defects at a site can make future surveys easier. 

3) It provides documentation that can be a baseline for future hazard management and monitoring efforts. 

A baseline survey and a permanent database can simplify subsequent evaluations. Hazard and monitoring 

information can be entered into a database so that information can be made available every year, prior to inspection, 

listing the trees that have been identified as in need of monitoring. Files can provide the locations of specific trees 

relative to reference points, their species, size, and type of defect, their prior defect severity, the appropriate 

monitoring interval, prior hazard and risk ratings, and recommended treatments. Maps showing tree locations can be 

generated using GPS (Global Positioning System) reference points. Maps can aid tree removal by contractors, reevaluations of trees being monitored, determination of recurring reasons for tree removals, and long-range planning. 

For some developed sites, it may be more useful to use individual tree forms instead of a form that records one tree 

per line. Single-tree forms can aid the evaluator in monitoring trees with disease or defects. Individual tree records 

can be created during a baseline survey and updated during re-evaluations. 

Mitigation 

The terms “mitigate” and “mitigation,” as used in this guide, refer to actions that may be taken to reduce, minimize, 

or eliminate the potential risk posed by a tree hazard. 

Trees in developed recreation sites have inherent value and are difficult to replace. Removal decisions can be based 

on careful hazard evaluations and can consider other hazard mitigation alternatives, such as seasonal closures and 

moving the location of potential targets. For example, areas with moderate numbers of moderate defects and high 

visitor occupancy may be repurposed for limited-use interpretive trails. Hazard of individual high-value trees could be 

mitigated by approaches other than removal, such as limiting visitor access to outside of the PIZ. 

15 
 

 

 

 

 

 

 

 

 

 

 

Table 3. Simplified Failure Potential table for quick reference during examination. This table is used with the Region 1 

and 4 Tree Hazard Evaluation Form (see Appendix). 

Code 

DECAY AND CAVITY Sound wood shell (SHELL) 

excludes bark and may be measured on trees with 

suspected significant decay. Severity of decay values can 

be adjusted based on tree species. Use for decay in whole 

tree or tree parts! 

Code LEAN/SWEEP Growth at the very top of the tree may 

provide some indication of how long a tree has been leaning 

1D None 1L Lower bole is vertical, mid to upper bole has sweep from 

factors affecting growth 

2D Some decay without conks OR with but still openings has 

average SHELL >30% diameter 2L Lean of any amount where recent top growth is fully vertical (“corrected”) 

3D 

If no bole opening, SHELL is 15-30% of diameter OR if 

bole has an opening, SHELL is >20% of diameter OR if 

decay can’t be measured in upper bole, estimated decay is 

< SHELL thresholds 

3L Lean ≥15° AND recent growth not fully vertical (not 

“corrected”) AND soil is intact 

4D 

Large conks visible OR if no bole opening, SHELL 

<15% diameter OR if bole has an opening, SHELL <20% 

diameter OR if decay can’t be measured in upper bole 

there are multiple or large conks/ fruiting bodies present 

4L 

Tree has evidence of failure by soil cracks, movement, or 

sprung roots OR uncorrected lean ≥15° is associated with 

decay, cracks, or canker 

STEM DAMAGE AND WOUNDS (cracks, 

mechanical, animal, human, fire) 

BRANCHES AND TOPS (dead or forked, dwarf mistletoe 

and other witches brooms, higher FP codes for Populus and 

non-resinous conifers) 

1W Cracks/wounds of any size are well-calloused or 

superficial and lack decay 1B Small dead/hanging branches or tops <3" diameter OR small live witches brooms 

2W 

Minor structural damage, scraped bark, OR fire scar is 

<10% circumference OR small, shallow at least partially 

calloused cracks 

2B 

V-shaped forks or multiple tops without trapped bark OR 

rust killed tops or branches OR dead tops in resinous species 

of any size OR non-decayed tops <3”diam. in non- resinous 

species OR Large live DF brooms, small live brooms in 

other species 

3W 

Some structural damage and associated decay but <50% 

circumference OR large fire scars leaving >50% wood 

OR significant cracks without movement 

3B 

V-forks with embedded bark OR live cracked tops or limbs 

>3” OR dead tops in hardwoods or non-resinous species >3” 

diam. OR DF with small dead brooms OR other species 

with large dead brooms 

4W 

Multiple wounds and associated decay OR wound >50% 

circumference OR significant cracks with evidence of 

movement OR <50% sound wood remaining after fire 

4B 

V-shaped fork with trapped bark and cracks, or decay OR 

detached branches or tops OR dead tops or branches ≥3” in 

Populus, Salix or non-resinous conifers OR dead tops ≥3” 

with decay or cracks in resinous conifers OR DF with large 

dead brooms and any species with brooms on cracked 

branches 

BOLE CANKERS (blister rust, gall rust, dwarf 

mistletoe, etc.) ROOT DAMAGE, BUTT ROT, ROOT DISEASE 

1C No cankers, but a burl of any size 1R A few roots are exposed and these lack decay and are not 

root-sprung 

2C <25% of bole girdled 2R 

Exposed small roots have minor damage or decay but are 

not root sprung and there is no evidence of root disease on 

tree or its neighbors 

3C 25-50% of bole girdled 3R 

Some roots damaged but <50% root system intact OR root 

disease suspected from symptoms or diseased neighbors, but 

disease is not obvious or confirmed 

4C >50% of bole girdled OR bole with less girdling has 

another defect 4R 

Roots exposed, decayed or substantial damage on >50% of 

root system OR root disease is obvious from thinning/fading 

crown, basal resin or stain 

16 
 

 

 

Hazard Tree Rating and Treatment Priority Classes 

The numerical scores for Failure Potential, Damage Potential, and Target Value are added to determine the Hazard 

Tree Rating. Priorities for mitigating hazard range from low to very high based on these numerical values. A Hazard 

Tree Ratings matrix (Fig. 7) indicates Treatment Priority Classes (low, moderate, high, and severe). Note that even 

highly defective trees with high Failure Potential may have low treatment priority if scores for Damage Potential and 

Target Value are minimal. For sites administered by public agencies, the deciding official determines which 

Treatment Priority Classes and particular trees are to be treated or monitored. Also, there may be reasons why rating 

of certain trees within Treatment Priority Classes might be modified, such as whether a tree has a protected vs. open 

position in a stand, and whether a tree recently became more exposed to wind. 

Figure 7. Hazard Potential Matrix. Hazard Tree Ratings (numbers in the matrix) and Treatment Priority Classes (colors) increase in bands from 

the matrix’s top left to bottom right. For trees in developed sites in Intermountain and Northern Rocky Mountain Regions, Hazard Potential and 

Treatment Priority is considered Low if 3-7 (Green background), Moderate if 8 (Yellow), High if 9 (Red), and Severe if 10 (Purple). 

17 
 

 

 

 

 

Chapter 4. Identification of Defects and Diseases 

Dead Trees 

The longer a tree has been dead, the more likely it is to fail (Figs. 8 & 9). Fungi decay 

roots, boles, and tops of dead trees making them less stable over time. Decay is much 

faster in sapwood than in heartwood of dead trees because sapwood has nutrients that 

accelerate the growth of all wood-colonizing fungi, but heartwood has fewer nutrients 

and has compounds that inhibit decay by some sap-rotting fungi. Smaller dead trees 

with more sapwood decay faster than larger dead trees or tree parts with more 

heartwood. 

Dead trees of any species, size, or age have a high Failure Potential. They are given a 

high priority for mitigation at developed sites if they have sufficient Target Value and 

Damage Potential. 

FAILURE 

POTENTIAL 

(FP) RATING 

FP4: Dead trees. 

Figure 8. Douglas-fir trees killed by bark beetles (FP4). 

Figure 9. Dead ponderosa pine trees. The dead tree on the left with 

brown and green needles was killed by bark beetles a year before 

the photo; the dead tree on the right died five years earlier and lacks 

needles and small branches (FP4). 

18 
 

 

 

 

 

 

 

 

 

 

 

 

 

Wounded Trees 

Tree wounds are injuries that break through the bark of the stem, branch, 

or root. Tree wounds are caused by a variety of factors: vehicles, people, 

falling trees, weather, fire, animals, or insect attack. A wound is considered 

open if the sapwood is exposed (Fig. 10). Fresh wounds on Douglas-fir or 

ponderosa pine are often covered with resin. New wounds on living trees 

(Fig. 11), especially large, deep wounds, can be entry points for stem decay 

or canker-causing fungi. Wounds can also activate dormant decay fungi, 

such as the Indian paint fungus. Wounds on non-resinous tree species, such 

as true firs (Abies spp.), generally result in more decay than do wounds on 

resinous species, such as pines. In some tree species, wounds occurring on 

roots and root collars can result in root disease. 

After a tree is wounded, decay fungi may be confined to compartments 

within the tree through a process called compartmentalization, but the 

ability to compartmentalize varies by tree species. Over time, the tree may 

seal the wound with new wood resulting in a callus scar. The rate of wound 

sealing is a function of tree growth rate and vigor. When 

compartmentalization occurs, new wood formed after the time of 

FAILURE POTENTIAL (FP) 

RATING 

FP4: >50% bole 

circumference affected OR 

<20% sound wood shell. 

FP3: <50% bole 

circumference affected AND 

20-30% sound wood shell. 

FP2: <50% bole 

circumference affected AND 

>30% sound wood shell. 

wounding will remain relatively free of decay unless another wound occurs. Compartmentalized wood decay may 

eventually become a decayed or hollow cylinder surrounded by healthy-appearing wood. Non-decayed wood 

external to either a hollow or decay is referred to as the sound wood shell, and its thickness determines the Failure 

Potential of the affected tree (Table 2). A tree with an open wound and a sound wood shell thickness <20% of the 

tree’s diameter inside bark has high Failure Potential (Table 2). 

Figure 10. This Jeffrey pine had a large basal wound affecting 

slightly less than 30% of the stem’s circumference and 4-6 in. 

deep, lengthwise cracks in the exposed wood. 

Figure 11. The many small insect- and animal-caused 

wounds in these aspens have little short-term failure 

potential but are infection sites for canker fungi that can 

cause tree failures. 

19 
 

 

 

 

 

 

 

 

 

 

Aggressive Root Diseases and Root and Butt Rots 

Aggressive root disease can rapidly kill and consume roots without causing 

butt rot. Trees often fail at the roots without bole breakage (Fig. 12). Root 

and butt rots can kill roots but can cause extensive internal decay in lower 

boles and within roots (Fig. 13). Trees can fail either by breaking at the 

lower bole or at the roots. It is extremely important to document root 

diseases since they contribute to ongoing problems in developed sites. 

Some root diseases will continue to cause mortality and structural failure 

of all sizes and ages of host trees, while others cause failures primarily in 

very mature trees. And for some root diseases, types of root mortality, 

decay, and failure can depend both on tree species and how the disease 

entered the tree, through wounds vs. root-to-root contact. Determining 

how a root disease could affect tree stability in a hazard tree setting 

involves identifying the disease, understanding its potential for damage 

for all tree species present on a site, ages of trees present, and considering 

the damage already done by the pathogen in and near the site. 

The Failure Potential (FP) ratings in the text box at right are general, and 

can be used for any root disease and root and butt rots. But FP ratings 

specific to particular root diseases should be used if they can be identified. 

General symptoms and indicators: 

• Mixture of dead, dying, and declining trees within discrete areas 

or pockets. 

• Entire live crown can be in decline, characterized by fading or 

yellowing foliage, dying branches, premature shedding of older 

needles, and reduced shoot growth (Fig. 14). 

• Basal resin flow and/or bark staining may be present. 

FAILURE POTENTIAL (FP) 

RATING 

FP4: Tree with obvious crown or 

basal symptoms of aggressive 

root disease OR extensive butt 

decay: <15% sound wood shell 

(Table 2) OR decay in >50% of 

the structural roots. 

FP3: Aggressive root disease 

suspected based on proximity 

OR moderate butt decay: 15-

25% sound wood shell OR decay 

in <50% of the structural roots. 

FP2: Aggressive root disease not 

suspected in proximity; little or 

no butt decay: >25% sound 

wood shell and no decay in 

structural roots. 

• Wind-thrown trees failed at roots or in bole at or near ground level and have characteristic decay. 

• Bark beetle attacks on individual trees when bark beetles are not at outbreak levels. 

• Distress cone or seed crops. 

• Mushrooms or conks on root collar or in stumps. 

• Presence of characteristic mycelia of the fungal pathogens on, in, or under the host bark in the roots and root 

collar area. 

• Some root and butt rots, in particular Schweinitzii and tomentosus root and butt rots cause little or no 

detectable symptoms until wind throw occurs. 

20 
 

Figure 12. Many lodgepole pine and Engelmann spruce in this root Figure 13. This grand fir has Heterobasidion root disease present 

disease pocket were wind-thrown or broke at the lower bole. More both as a butt rot, shown by central decay column with conks, 

than one root disease was present. and as an aggressive root disease, shown by irregular stained and 

decayed patches outside the central butt rot. 

Figure 14. Thin, upward-receding crowns and dead trees can be observed near the 

edge of this pine root disease infection center. 

21 
 

 

 

 

 

 

 

Conifers: Heterobasidion (Annosus) Root Disease 

Caused by two species of fungi, Heterobasidion occidentale and H. 

irregulare 

Hosts: Root and Butt Rot: Primarily true firs, Douglas-fir, hemlocks, and 

spruces (H. occidentale). Root disease: pines and junipers (H. 

irregulare) 

Identification: Look for nearby symptomatic trees or stumps. This disease 

occurs in trees of all ages. 

• Crown symptoms: None or foliage fading and thinning, needle 

chlorosis, growth reduction, and appearance of scattered dead 

branches in the crown (Fig. 14). 

• Bole symptoms: Central columns of decay or basal resin flow (Fig. 

15). 

• Decay: Incipient decay is a light-brown to reddish stain in the 

outer heartwood. Moderate decay is laminated with elongated 

pits on only one side of the laminations. Advanced decay is white 

and stringy and often has black flecks (Fig. 16). 

• Fruiting bodies: Perennial conks (upper surface woody or leathery 

black to chestnut-brown, undersurface creamy-white) found 

FAILURE POTENTIAL (FP) 

RATING 

FP4: Extensive butt decay (<15% 

sound wood shell) OR any decay 

in >50% of structural roots. 

FP3: Susceptible trees adjacent 

to Heterobasidion-caused tree 

failure OR moderate butt decay 

(15-25% sound wood shell) OR 

any decay (<50%) of the 

structural roots. 

FP2: Little or no butt decay 

(>25% sound wood shell) AND 

NO decay in structural roots. 

within hollows and decay of old stumps or in root crotches of living trees, or on portions of roots in the duff 

layer or upper layers of the soil (Fig. 17). 

Damage: Root and/or butt rot (the latter common in true firs and western hemlock) that predisposes trees to wind 

throw or breakage, or can kill the host by killing the cambium around all or most of the root collar. 

Figure 15. Heterobasidion (annosus) root disease causes 

reddish-brown incipient decay staining (just below axe 

handle) and central butt-rot in some conifers. 

Figure 17. Heterobasidion (annosus) root disease is indicated by conks 

with white edges and laminated decay in this old hollow stump. Nearby 

trees may also be affected. 

Figure 16. Decay from Heterobasidion (annosus) root 

disease showing laminated decay with black flecks. 

22 
 

 

 

 

 

 

 

Conifers: Armillaria Root Disease 

Caused by various Armillaria spp. Armillaria ostoyae causes an aggressive root disease of conifers. 

Hosts: All conifers are susceptible to A. ostoyae, particularly when less 

than 30 years old. This is the most common root disease of conifers in the 

northern Rocky Mountain region. Douglas-fir and true firs are most 

susceptible and are damaged at all ages. Engelmann spruce is a common 

host in southern Idaho, Utah, and Wyoming. 

FP4: Trees with symptoms or 

Identification: Wind throw is common in infected stands. signs are near wind-thrown 

trees with root disease. • Crown symptoms: Foliage fading and thinning, needle yellowing, 

growth reduction, and appearance of scattered dead branches in FP3: Trees with symptoms or 

the crown. signs but not adjacent to wind- • Bole symptoms: None, basal resin flow (Fig. 18), basal stain. thrown trees with root disease. • Decay: White or yellowish, becoming stringy, with black lines 

between patches of decay. FP2: Uninfected trees near root 

• Fruiting bodies and other fungal signs: Yellow to tan colored disease centers. 

mushrooms may be produced at the base of infected trees in late 

summer or early autumn. The diagnostic thick, fan-shaped mats of 

white mycelium under the bark of roots and root crown are the most readily identified indication throughout 

the year (Fig. 19). 

Damage: Armillaria root disease often extends further in a stand than is apparent from symptomatic trees. Trees in 

and around root disease centers vary in Failure Potential depending on tree species and condition. Live, symptomatic 

trees on sites with wind throw may be considered to have high Failure Potential. 

FAILURE POTENTIAL (FP) 

RATING 

Figure 18. Armillaria root disease often causes resinosis at the 

base of the tree. Chopping into bark with basal resinosis often 

reveals mycelial fans. 

Figure 19. Armillaria ostoyae often produces mycelial fans under the bark 

of infected trees. 

23 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Conifers: Laminated Root Rot; Cedar Laminated Butt Rot 

Caused by Phellinus sulphurascens; Phellinus weirii 

Hosts: Laminated root rot: Douglas-fir and grand fir are highly 

susceptible; western hemlock and subalpine fir are less susceptible. 

Larch, spruce, and western hemlock are generally much more 

tolerant of the disease, but may develop substantial butt rot over 

time. Cedar laminated butt rot: western redcedar. 

Identification: It is found in northern Idaho and northwestern 

Montana. Mortality can occur in small groups or large disease 

centers. Infected trees are often attacked by bark beetles. 

• Crown symptoms: Foliage fading and thinning, needle 

yellowing, growth reduction, and appearance of scattered 

dead branches in the crown. 

• Bole symptoms: Occasional basal resinosis or stain. 

• Decay: Decayed wood separates easily along annual rings 

and is extensively pitted on both sides of ring with small 

white- or cinnamon-colored pockets (Fig. 20). 

• Fruiting bodies and other fungal structures: The most 

common diagnostic structure of the fungus is a thin layer 

of cream- to dark yellow-colored ‘ectotrophic mycelium’ 

covering the outer bark of infected roots (Fig. 21) and setal 

(reddish-brown, thick-walled, and pointed) hyphae in 

advanced decay. Additionally, fuzzy, cinnamon-colored 

mycelium often occurs in bark crevices along with the 

cream-colored mycelium. Conks are rare, but when 

observed on roots they are thin and flat along the root 

surface, leathery when fresh and crumbly with age, light 

weight, yellow to cinnamon brown with small pores. 

FAILURE POTENTIAL 

(FP) RATING 

FP4: Signs or symptoms present. 

Western redcedar use sound wood shell 

values. 

FP3: Douglas-fir, mountain hemlock, or 

true fir with signs and symptoms absent 

AND ≤25’ from an infected tree or stump

(check for butt rot). Western larch, 

Engelmann spruce, or western hemlock 

with signs and symptoms absent AND 

same-species tree or stump is infected 

somewhere on the site AND ≤25’ from 

any infected tree or stump. 

FP2: Douglas-fir, mountain hemlock, or 

true fir with signs and symptoms absent 

AND >25’ from an infected tree or 

stump. Western larch, Engelmann 

spruce, or western hemlock with signs 

and symptoms absent AND same-species 

tree or stump is infected somewhere on 

the site AND >25’ from any infected tree 

or stump. 

Damage: Wind throw of green, otherwise asymptomatic trees frequently occurs among highly susceptible hosts. This 

makes laminated root rot particularly dangerous in a developed site. More tolerant hosts that develop butt rot may 

break at the butt or lower stem as decay advances with age. 

Figure 21. Close-up of setal hyphae of laminated root disease (Phellinus 

sulphurascens) on grand fir. 

Figure 20. Laminated decay caused by Phellinus 

sulphurascens. 

24 
 

 

 

 

Conifers: Schweinitzii Root and Butt Rot 

Caused by Phaeolus schweinitzii 

Hosts: Douglas-fir; all conifers are susceptible. 

Identification: Generally causes tree failures in old (>150 years) conifers. 

Douglas-fir is most disease-prone. Conks (Fig. 22) are often rare on drier sites, 

so Schweinitzii root and butt rot is often identified by observing stumps with 

central brown cubical decay surrounded by a solid outer wood shell, or by FP4: Extensive butt decay 

uprooted trees with stubbed roots and brown cubical decay. On mesic and (<15% sound wood shell) OR 

wetter sites conks are more abundant, but not always present. any decay in >50% of 

structural roots OR conks on • Crown symptoms: Uncommon. Crowns of extensively infected roots of Douglas-fir. Douglas-fir are sometimes thin and show poor shoot growth and 

some branch dieback. FP3: Moderate butt decay 

• Bole symptoms: Butt swell, brown cubical decay, shake or cracking of (15-25% sound wood shell) 

butt, and ant or wood borer activity in the butt may be indicators. OR any decay (<50%) of the 

• Decay: Decay is dry and yellow at first, becoming brown and cubical structural roots. 

in advanced stages. Thin, resinous felts are often present in cracks FP2: Little or no butt decay within advanced decay. Brown, cubical, advanced decay is easily (>25% sound wood shell) crushed to a fine powder (Fig. 23). AND NO decay in structural • Fruiting bodies: Conks are occasionally produced on the ground near roots. infected trees or rarely, on infected trees or stumps (Fig. 22). Conks 

are annual and spongy, with short stems and large, irregular pores on 

the undersurface. They can persist and can be identified for several years. Caps are usually five to 10 in. in 

diameter, brown and velvety on top, greenish on the undersides when fresh, and the whole conk becomes 

brown and brittle. They can resemble cow pies or pieces of bark. 

• Roots: Girdled roots develop central brown-rot decay and can become club-like or knobby when overgrown 

by living bark (Fig. 24). 

Damage: Severely weakened Douglas-fir may have little indication of advanced decay, so presence of conks is an 

immediate concern. This disease may be slow to weaken trees of other species to hazardous levels. In species where 

anchoring roots are less affected, suspected trees may be retained and monitored each year or every other year, 

depending on extent of defect and decay. 

FAILURE POTENTIAL (FP) 

RATING 

Figure 22. Fresh, rust-colored (foreground) and old, dark-colored 

conks (background) of Phaeolus schweinitzii have grown from 

infected Douglas-fir roots. Figure 23. Extensive brown 

cubical butt rot from Schweinitzii 

root and butt rot. 

Figure 24. Schweinitzii root and butt 

rot causes clubbed or greatly reduced 

root systems, as in this wind-thrown 

Douglas-fir. 

25 
 

 

 

 

 

 

 

Conifers: Tomentosus Root and Butt Rot 

Caused by Onnia tomentosa 

Hosts: Engelmann spruce is the primary host. Lodgepole pine, Douglas-fir, subalpine fir, western white pine, and 

Colorado blue spruce are occasional hosts. 

Identification: Honeycomb-like decay in cut stumps or roots of wind-thrown 

trees is the best indicator of this disease (Fig. 25). Coring or cutting into roots of 

a stump can help determine if decay is more extensive above ground or in the 

roots, and eliminate other causes of decay. If decay is much more extensive in 

roots, then there is an increased likelihood of tomentosus root and butt rot 

disease as opposed to a stem decay in the stump, such as red ring rot. 

• Crown symptoms: Uncommon, but can produce crown thinning, 

yellowing, and dieback when decay is extensive. Infected trees with 

healthy-looking crowns commonly fail. 

• Bole symptoms: None. 

• Decay: Early decay produces a red-brown or brown stain, often 

resinous, in the heartwood of roots and butts. Advanced decay is a 

white pocket rot which forms empty, spindle-shaped pockets with firm 

wood between the pockets. This decay has a distinctive honeycombed 

appearance in cross-section. 

• Fruiting bodies: Uncommon, but can be found in late summer or fall 

during some years. They are small (usually 2 to 5 in. in diameter) yellow 

to cinnamon colored, and leathery annual conks with short stems. The 

lower surface is porous and cream colored when fresh, but darkening 

with age. 

• Root symptoms: Resin production at the root surfaces and reddish 

brown staining in the roots (Fig. 26). 

Damage: Host trees (usually spruce) in root disease centers can have high or 

medium Failure Potential, depending on extent of butt decay. In developed 

FAILURE POTENTIAL (FP) 

RATING 

FP4: Extensive butt decay 

(<15% sound wood shell) OR 

any decay in >50% of 

structural roots. 

FP3: Susceptible trees 

adjacent to tomentosuscaused tree failure OR 

moderate butt decay (15-

25% sound wood shell) OR 

any decay (<50%) of the 

structural roots. 

FP2: Susceptible trees NOT 

adjacent to tomentosuscaused tree failure with little 

or no butt decay (>25% 

sound wood shell) AND NO 

decay in structural roots. 

recreation sites dominated by old, similar-aged Engelmann spruce, the onset of tree failure from tomentosus root 

and butt rot can indicate that many more trees will soon fail. Many trees can be lost within a decade or two. Spruce 

with Potential Impact Zone affecting targets may be checked with a drill or borer to detect decay, despite lack of 

symptoms. Engelmann spruce are relatively shallow-rooted and changing wind dynamics caused by loss of root 

diseased trees could result in wind throw of otherwise healthy spruce. 

Figure 25. Tomentosus root and butt rot disease (Onnia tomentosa) 

stages of decay on Engelmann spruce include reddish stain and early 

decay (left) and advanced decay with a “honeycomb” appearance (right). 

26 

Figure 26. Tomentosus root and butt rot disease (Onnia 

tomentosa) caused surface resin and internal stain and 

decay on these Engelmann spruce roots. 
 

 

 

 

 

 

 

 

Conifers: Yellow Root Rot or Stringy Butt Rot 

Caused by Perenniporia subacida 

Hosts: Suppressed or stressed conifers, especially Douglas-fir, true firs, and 

western hemlock as well as many hardwoods. 

Identification: Because signs and symptoms are below ground, the disease is 

difficult to detect in live trees. 

• Crown symptoms: May show no symptoms or general crown stress 

symptoms (sparse crown with poor growth). 

• Bole symptoms: Uncommon. Butt swell or cracking and ant or wood 

borer activity in the butt may be indicators. 

• Decay: Early decay is a light-brown stain that resembles wetwood. 

Advanced decay is composed of irregularly-shaped pockets of 

decayed spring wood that coalesce into masses of stringy fibers with 

large black flecks (Fig. 27). Annual rings may separate into laminated 

decay. 

• Fruiting bodies and other fungal signs: Conks are white, crust-like or 

leathery and flattened against the wood or bark on the undersides of 

roots, logs, fallen trees, or exposed roots (Fig. 28). Yellow-white 

mycelial felts may form between layers of decayed wood. 

Damage: Affected trees may be easily wind-thrown. 

FAILURE POTENTIAL (FP) 

RATING 

FP4: Extensive butt decay 

(<15% sound wood shell) OR 

any decay in >50% of structural 

roots. 

FP3: Moderate butt decay (15-

25% sound wood shell) OR any 

decay (<50%) of the structural 

roots. 

FP2: little or no butt decay 

(>25% sound wood shell) AND 

NO decay in structural roots. 

Figure 27. Advanced root rot caused by Perenniporia 

subacida is typically soft and wet with a white-mottled 

yellow color. Large black flecks may also be present. 

Photo credit Blakey Lockman. 

Figure 28. Fruiting body of Perenniporia subacida. Photo credit Blakey Lockman. 

27 
 

 

 

 

 

 

 

 

 

 

 

Ganoderma White Mottled Rot (Artist’s Conk) and Hemlock Varnish-Shelf Conk 

Fungus species are Ganoderma applanatum and G. oregonense, respectively. 

Hosts: Live or dead hardwoods, particularly cottonwoods and aspen, and dead 

or partially dead conifers (G. applanatum); conifers only (G. 

oregonense) 

FAILURE POTENTIAL (FP) 

RATING 

Identification: Conks are the only useful indicator of decay or infection in live, 

standing trees. Advanced decay may produce conks before trees fall over. The 

species affecting aspen is most common on mesic aspen sites. 

• Crown symptoms: None. Crown damage from other causes or agents, 

such as wind or snow breakage or canker fungi. 

• Bole symptoms: None. Basal wounds are infection sites; cavities rarely 

develop. 

• Decay: Both species produce a white-mottled rot, white spongy rot with 

black specks. In G. applanatum, the decay alternates between small 

areas of white and light tan. Black zone lines may develop. 

• Fruiting bodies: G. applanatum conks are shelf-like and occur at the 

bases of trees, usually within a foot of the ground (Figs. 29 & 30). They 

are perennial, leathery to woody, with an irregular, brown to gray upper 

surface. The margin and undersurface are white to creamy with small 

pores and are easily bruised when fresh. Bruising is permanent, allowing 

artists to “draw” on the undersurface, giving this fungus a common 

name of “artist’s conk”. 

FP4: Trees with conks OR 

extensive butt decay (<15% 

sound wood shell) OR any 

decay in >50% of structural 

roots. 

FP3: Susceptible trees 

adjacent to trees with conks 

or tree failure OR moderate 

butt decay (15-25% sound 

wood shell) OR any decay 

(<50%) of the structural 

roots. 

FP2: Susceptible trees with 

little or no butt decay (>25% 

sound wood shell) AND NO 

decay in structural roots. 

G. oregonense conks are annual, stalked, reddish-brown, and shiny, as if lacquered or varnished. 

Damage: When root and butt rot caused by a Ganoderma species is diagnosed in developed sites, host trees can 

have FP3-4 depending on the extent of butt decay. 

Figure 29. The wind-thrown trees in a Ganoderma root and butt 

rot pocket in aspen have little to no attached root system. A conk 

is visible on the left side of the standing stem. 

Figure 30. This young Ganoderma conk has an upper surface of tan, 

concentric zones and a white undersurface. If undersurface is scratched, 

a dark brown interior would be easily seen. 

28 
 

 

 

Other Root Problems 

Other important root defects include undermined roots that result from 

erosion, severed roots caused by heavy equipment, and roots loosened, 

cracked, or broken during high winds. Failure Potential is determined based on 

the proportion of structural roots remaining in the ground (Table 1). 

FAILURE POTENTIAL (FP) 

RATING 

Undermined Roots 

FP4: <50% of the structural These can result from bank erosion near roads or trails in and around roots remaining intact and developed sites. They are also observed where there is seasonally high water at in the ground. water’s edge by lakes, streams, or rivers (Fig. 31). Severe undermining without 

compensating root growth causes tree failure from poor root anchoring. FP3: 50 – 75% of the 

structural roots remaining 

intact and in the ground. 

FP2: >75% of the structural 

roots remaining intact and 

in the ground. 

Severed roots 

Severed roots reduce anchoring and are entry points for root and butt rot and decay fungi. Roots can be severed 

installing utilities and building roads, trails, tent pads, or water control ditches (Fig. 32). 

Figure 32. Excavation to install a 

utility line severed many of this 

tree’s roots. Compaction and 

burying the base of the tree (see 

soil line on trunks) may have 

further weakened the root system. 

Figure 31. Undermined root systems: At left, trees were felled because of undermining and 

failure. Failures were away from the undermined side. At right, undermined roots of a western 

redcedar show ice damage common to trees along streams with winter freezing. 

29 
 

 

Loosened, Cracked, or Broken Roots; Leaning Trees FAILURE POTENTIAL Wind events, saturated soils, and soil disturbances occurring singly or in 

combination often lead to loosening, cracking, or breakage of roots. Trees with (FP) RATING 

such damage often fail under less stress than caused the initial damage. Soil 

saturation is a leading factor in wind throw of shallow-rooted species and for any 

FP4: Soil cracked or lifted species growing at high density or in shallow soil. Indicators of root damage 

near tree or root breakage. include: trees with newly developed or recently increased lean (Fig. 33); a gap 

between the base of the tree and soil or litter on the side away from the lean; and FP3: Soil looks intact, 

recent soil mounds, heaves, ridges or cracks near major lateral roots. uncorrected lean ≥15o 

. 

Root-sprung trees are failing because their roots are compromised. Leaning trees FP2: Soil looks intact, 

(≥15 degrees) can result from root and butt decay and from high winds that cause uncorrected lean <15o OR 

root wrenching (Fig. 34). All leaning trees with potential targets can be examined corrected lean. for evidence of root and butt rot. 

Trees with recent leans (<5 years) may have a conspicuous gap between the base 

of the tree and disturbed soil and litter on the opposite side of the tree from the lean. Cracks, mounds, or ridges of 

recently heaved soil may be found adjacent to major lateral roots of leaning trees. Trees with newly developed lean 

are tilted over their entire length, and trees that are uprooted or broken but supported by other trees have High 

Failure Potential (Table 1). 

Persistent leaning trees are those that have been leaning for a considerable time and have grown upwards towards 

vertical (corrected lean) (Fig. 35). If they don’t fall, leaning trees develop tension and compression wood at stress 

points to aid in their support. They also often develop a reinforced root system to compensate for prior stress or 

damage. Unless roots are disturbed or decay is present, persistent leaning trees with vertically “corrected” growth 

have FP1-FP2. 

Figure 33. Broken roots and recent lean on a 

ponderosa pine. This failure was severe and 

obvious, with the soil crack opening and closing 

several inches in even moderate wind. 

Figure 34. Three western redcedars at left with 

failed roots and severe lean are supported by a 

grand fir with a wound on its upper bole caused 

by years of contact with one of the redcedars. 

Figure 35. Leaning Douglas-fir with 

vertically “corrected” top growth. 

30 
 

 

 

 

 

 

Heart Rots 

In this guide, heart rots are defined as types of decay confined to the 

interior wood of living trees. When conducting hazard tree assessments, 

we include true heart rots that are usually confined within a growth ring 

when an injury occurred, and canker rots that initially decay heartwood, 

but encroach on, kill, and then decay sapwood of living tree boles. 

FAILURE POTENTIAL (FP) 

RATING 

All heart rots are caused by fungi. Most enter wounds but some colonize FP4: Conks visible OR <15% through branch stubs. Wounds, branch stubs, and dead branches are sound wood shell without places to look for indicators of decay. Heart rots are more commonly openings or wounds; <20% if seen in older trees. In conifers, heart rot mostly occurs in trees greater openings, wounds, or cracks than 150 years old, but decay-prone species such as true firs and western 

present. hemlock may have substantial decay at younger ages. The sound wood 

shell guidelines (Table 2) can be applied to any tree with heart rot. The FP3: Significant decay present, failure potential guidelines for each specific heart rot given below are but >15% sound wood shell designed to supplement the guidelines in Table 2. 

without openings, wounds, or 

cracks OR >20% with an Indicators of potential heart rot, from most to least reliable are: 

opening, wound, or crack. • Conks, mushrooms sprouting from wounds, and swollen areas of 

sterile fungus at knots. However, conks and mushrooms from FP2: Some decay present, some decays may be rare and are less common on drier sites (Fig. sound wood quantity over 36). thresholds (see Table 2). • Decayed wood in broken trees and branches (Fig. 37). 

• Large, old wounds or branch stubs. 

• Flat or sunken area on the bole with dead bark, indicating hidden damage or disease that has killed bark. 

• Evidence of older broken tops on living trees. 

How to detect heart rot in trees and assess heart rot severity: 

1. Strike (“sound”) tree trunk with suspected basal heart rot with a rubber mallet or the butt of an axe and 

listen for hollowness. Thick-bark species are often difficult to sound. 

2. Bore into tree with suspected basal heart rot decay with an increment borer or a cordless drill at the point of 

defect or as near as possible and determine if thickness of remaining sound wood shell is acceptable using 

methods described in Chapter 3 (Table 2). 

3. Use binoculars to examine the upper crown for decay conks or other evidence of decay. Estimate Failure 

Potential from numbers and sizes of conks and defects using Table 1. 

Figure 36. A stem decay conk on living 

grand fir. 

Figure 37. Severe stem decay was the main cause 

of failure in this western redcedar. 

31 
 

 

Figure 38. Fruiting body of Phellinus tremulae causing aspen trunkrot. 

Figure 39. Stem failure due to aspen trunk-rot. 

Hardwoods: Aspen Trunk Rot or Aspen Heart Rot 

Caused by Phellinus tremulae 

Host: Aspen 

Identification: 

• Decay: Early heartwood decay is a yellow-white zone with a yellowgreen to brown margin. Advanced decay is soft and yellow-white with 

fine, black zone lines. 

• Fruiting bodies: Hard, woody, conks that are generally triangleshaped in longitudinal section; brown under-surface with small, 

regular pores (Fig. 38). 

• Breakage: Nearby trees may have wind breakage in the bole (Fig. 39). 

Damage: A single conk generally indicates considerable internal decay; such 

trees have FP3. Coring may determine more extensive decay which could 

indicate FP4. 

FAILURE POTENTIAL 

(FP) RATING 

FP4: ≥1 conk AND open 

cracks or exposed rot. 

FP3 or FP2: ≥1 conk without 

cracks or exposed rot, 

depending on sound wood 

shell. 

FP2: ≥1 conk without cracks 

or exposed rot. 

32 
 

 

 

FAILURE POTENTIAL 

Hardwoods: Brown Cubical Rot of Birch (Birch Polypore) 

Caused by a fungus, Piptoporus betulinus 

Host: Birch 

Identification: 

• Decay: Yellow-brown decay breaks into cubes separated by thin 

white layers of mycelium. Advanced decay easily crumbles to 

powder. 

FP4: >1 conk. 

FP3: 1 conk. 

(FP) RATING 

• Fruiting bodies: Annual, extremely tough and persistent conks have short, sturdy stalks. The conk develops 

below the level where the stalk emerges. The underside is initially smooth and white, becoming light brown 

and irregular at maturity (Fig. 40, left). Upper surface is white to tan and very smooth, fading to white. The 

upper surface curves over the margin to form a ridge all around the pore surface (Fig. 40, right). 

Damage: A single conk generally indicates considerable internal decay, and such trees have FP3. 

Figure 40. Fruiting bodies of birch polypore, Piptoporus betulinus. On the left, the spore producing surface is shown; on the right is the upper 

surface. From: http://wildflowers.perverdonk.com. Used with permission. 

33 
 

 

 

 

 

Hardwoods: White Spongy Trunk Rot (Tinder Conk) 

Caused by Fomes fomentarius 

Hosts: Dead trees or dead portions of living hardwoods 

Identification: 

• Decay: Early decay is brown and firm. Advanced decay is soft and 

spongy, yellow-white, with dark to black zone lines. 

• Fruiting bodies: Hoof-shaped conk with a concentrically zoned, 

smooth, gray to gray-black upper surface. Underside is brown with 

small, regular-shaped pores (Fig. 41). 

Damage: Trees with multiple conks indicating extensive decay have FP4. 

Figure 41. Fomes fomentarius on birch. 

FAILURE POTENTIAL (FP) 

RATING 

FP4: Dead trees OR live trees 

with ≥3 conks.

FP3: Live trees with 2 conks. 

FP2: Live trees with 1 conk. 

34 
 

 

 

 

 

 

Hardwoods: Hardwood Trunk Rot (False Tinder Fungus) 

Caused by Phellinus igniarius 

Hosts: Hardwoods 

Identification: 

• Decay: Early decay is seen as a yellow-white portion of the 

heartwood that is usually surrounded by a yellow-green to brown 

margin. Advanced decay is soft and yellow-white with fine black 

zone lines. 

• Fruiting bodies: Woody and generally hoofed-shaped conks with 

lower surfaces nearly horizontal. Conks are easy to knock off bole. 

Upper surface is gray-black to black and rough when old. 

Underside is brown with small and regular pores (Fig. 42). 

FAILURE POTENTIAL (FP) 

RATING 

FP4: Multiple conks and less 

than 15% sound wood shell. 

FP3: 1 conk. 

FP2: No conks; symptomatic 

decay present, sound wood 

shell over thresholds. 

Damage: A single conk generally indicates considerable internal decay; such trees have at least FP3. More than 1 

conk may have higher Failure Potential, but the relationship between number of conks and amount of decay is not 

well-established. 

Figure 42. False tinder conk, Phellinus igniarius, on birch. 

35 
 

 

 

 

 

FAILURE POTENTIAL (FP) 

Hardwoods and Conifers: Brown Cubical Rot (Sulfur Shelf or “Chicken of the Woods”) 

Caused in western North America by two species of fungi, Laetiporus 

conifericola (conifers) and L. gilbertsonii (hardwoods) (both formerly L. 

sulphureus) 

Hosts: Conifers and hardwoods 

Identification: Conks are the most reliable indications of decay. 

• Decay: Early decay is a light-brown stain. Advanced decay is redbrown, crumbly, and cubical with interspersed white, felt-like layers 

of mycelium. 

FP4: >1 conk. 

FP3: 1 conk. 

RATING 

• Fruiting bodies: Multiple horizontal conks are produced in fall, attached directly to decayed wood or 

knotholes, without stems. The annual conks are initially soft and yellow to bright orange (Fig. 43), becoming 

very tough, and fading to persistent chalky-white, broken structures that may have black surface molds. 

Damage: Decay is usually well-advanced before conks develop, so trees with >1 conk have FP=4. 

Figure 43. Fruiting body of Laetiporus conifericola on Douglas-fir. Photo credit Gregg DeNitto. 

36 
 

 

 

 

 

 

FAILURE POTENTIAL 

(FP) RATING 

Conifers: Brown Top-rot (“Rose-colored Conk”) 

Caused by Fomitopsis cajanderi 

Hosts: Conifers; usually living trees with past top damage from wind and ice 

or snow loading. 

Identification: Wood strength may be moderately affected before 

discoloration or texture change is evident. 

• Decay: Early decay is a faint brownish or yellow-brown stain, 

sometimes marked by greenish-brown zone lines. Advanced decay 

is yellowish to reddish brown, soft, and with irregular cubes. Thin, 

white to faintly rose-colored mycelial felts may develop in the 

cracks between the cubes. 

FP2-4: Follow general 

guidelines in Tables 1 and 2. 

• Fruiting bodies: Relatively small, woody, bracket-like to hoof-shaped conks with pink to rose-colored 

undersurfaces and inner tissue; upper surface can be brown to black and is usually cracked and rough. Often 

appear stacked in a shelf-like arrangement (Fig. 44). 

Damage: Boles and replacement leaders of trees with one or more conks at the base of a replacement leader have a 

FP3. Replacement leaders and boles on trees with top-breaks but no conks would have FP2. Monitoring for future 

conks is advisable. 

Figure 44. Rose-colored conk of Fomitopsis cajanderi, usually found decaying damaged Douglas-fir 

upper boles. 

37 
 

 

 

 

 

 

 

 

 

 

FAILURE POTENTIAL 

Conifers: Brown Trunk Rot (Quinine Conk or “Chalky Fungus”) 

Caused by Fomitopsis officinalis 

Hosts: Conifers 

Identification: 

• Decay: Early stain is yellow-green to brownish-green and could be 

confused with Schweinitzii root and butt rot. Advanced decay is 

brown, crumbly, and cubical (Fig. 24) with large, thick bitter mycelial felts in cracks. 

FP4: ≥1 conk(s).

(FP) RATING 

• Fruiting bodies: Large, hoof-shaped or columnar conk (Fig. 45) with a chalky-white upper surface and a white 

pore surface. Interior is usually soft and crumbly. Develops at branch stubs, over old wounds and especially 

at old broken tops. 

• Punk knots: May be observed at large, older branch stubs that have usually rotted and fallen off. Punk knots 

may have a yellowish-brown exudate that stains the bark below. 

Damage: Severe stem decay occurring either as a top rot when it has entered a broken top, or as a heart rot of the 

main stem when the site of the old broken top is much lower in the bole and no longer visible. This fungus also enters 

through basal fire scars. Even one conk indicates FP4. 

Figure 45. Fomitopsis officinalis conks are typically cylindrical, and many specimens 

are more massive and longer than this example on an old bole wound. 

38 
 

 

 

 

Conifer: Juniper Pocket Rot 

Caused by Pyrofomes demidoffii 

Hosts: Juniper 

Identification: 

• Decay: Early decay is a light-yellow color. Advanced decay is a white 

rot with abundant buff-colored mycelial felts in the decayed wood 

(Fig. 46). Decay typically forms discrete pockets in infected wood. 

• Fruiting bodies: Hoof-shaped conk with a brown to black upper 

surface with a buff to black rim. The undersurface is buff-colored 

with round pores (Fig. 47). 

Damage: Trees with >1 conks have considerable decay but rarely fail and 

therefore have FP2. 

FAILURE POTENTIAL 

(FP) RATING 

FP4: Dead trees with conks. 

FP3: Trees with 1+ conk(s) 

and signs of other structural 

problems such as “v” forks. 

FP2: Trees with 1+ conk(s). 

Figure 46. Typical decay of Juniper Pocket Rot (Pyrofomes demidoffii). 

Photo credit Gail Durham. 

Figure 47. Fruiting body of Juniper Pocket Rot (Pyrofomes 

demidoffii). Photo credit Gail Durham. 

39 
 

 

 

 

 

 

FAILURE POTENTIAL (FP) 

Conifers: Cedar Brown Pocket Rot or Redcedar Pencil Rot 

Caused by Postia sericeomollis 

Host: Western redcedar 

Identification: 

• Bole symptoms: Trees with a conspicuous bole flattening at the 

butt called a “dry side” or “dry face” from other damage, including sun exposure, logging wounds, or 

Armillaria infection, may have significant decay. Trees with evidence of a dry side can be sounded with an 

axe or mallet and/or drilled to determine the extent of decay and the sound wood shell thickness. 

FP4: 1 conk. 

RATING 

• Decay: Minor decay appears as long, thin, tapered cylinders (“pencils”) of brown cubical decay and are round 

to irregular in cross-section. Advanced decay is a brown, cubical pocket rot (Fig. 48). 

• Fruiting bodies: Rarely seen, when present they are thin, flat, white crusts (Fig. 49). 

Damage: Stem decay and butt rot; decay is usually confined to the butt (first 40 ft.) of affected trees. Failure 

potential depends on the amount of sound wood shell thickness (Table 2). 

Figure 48. Decay caused by cedar brown pocket rot shown in lengthwise- (left) and 

cross-sections (right). 

Figure 49. Crust-like fruiting of Postia sericeomollis on 

western redcedar. 

40 
 

 

 

 

 

Conifers: Red Heart Rot (Bleeding Stereum) 

Caused by Stereum sanguinolentum 

Hosts: Conifers 

Identification: 

• Decay: Early decay appears as a red-brown heartwood stain. 

Advanced decay is light-brown and soft, and thin mycelial felts 

may be present. 

• Fruiting bodies: Numerous, leathery, gray to light-brown, with a 

lower surface that is wrinkled and turns red when bruised. Found 

on infected wounds of live trees and on the bark of dead trees 

(Fig. 50). 

Figure 50. Red staining caused by bruising the conk of a bleeding stereum, Stereum 

sanguinolentum. 

FAILURE POTENTIAL (FP) 

RATING 

FP4: Any tree species: ≥1

conks WITH open cracks or 

exposed rot. 

FP3: True fir, hemlock, 

spruce, hardwoods: ≥1

conks WITHOUT open cracks 

or exposed rot. Douglas-fir, 

pine, redcedar, or larch: ≥3

conks ≥6” wide within a 3’

portion of bole WITHOUT 

open cracks or exposed rot. 

FP2: Douglas-fir, pine, cedar, 

larch: <3 conks. 

41 
 

 

 

 

Conifers: Red Ring Rot or White Speck (Pini Conk) 

Caused by Porodaedalea (Phellinus) pini 

Hosts: Conifers: Douglas-fir, western larch, Engelmann spruce, and pines are the most common hosts. 

Identification: FAILURE POTENTIAL (FP) 

RATING • Decay: White pocket rot (Fig. 51) that occurs in rings separated by 

Figure 51. White pocket rot caused by Porodaedalea pini. Figure 52. Porodaedalea pini producing conks on conifer branch stubs. 

sound wood until decay becomes very advanced (very soft and 

crumbly). 

• Fruiting bodies: Irregular hoof-shaped conks are often formed at 

knots or below branch stubs (Fig. 52). Conk interior and pore 

surfaces on underside are cinnamon-brown to tan with irregular 

pores. Also commonly forms “punk knots”: lumps of cinnamonbrown fungal tissue. Punk knots and conks have equal value as 

indicators of decay. 

Damage: Failure Potential depends on tree species, number and size of 

conks per tree, and associated indications of extensive decay such as open 

cracks or weeping frost cracks (Table 1). If sound wood shell is being 

determined, core at location of conks when possible. 

FP4: ≥2 conks ≥6” wide 

WITHOUT visible cracks or 

decay; OR any conk ≥6” wide 

or ≥2 conks <6” AND visible

cracks or decay. 

FP3: Only 1 conk, ≥6” wide or

≥2 conks <6” WITHOUT 

visible cracks or decay. 

FP2: Only 1 conk, <6” 

WITHOUT visible cracks or 

decay. 

42 
 

 

 

 

Figure 54. Indian paint fungus conk, Echinodontium tinctorium, emerges beneath branch stubs 

and indicates severe decay in grand fir. Its natural appearance is shown at left, and its red 

internal color is shown in a cross section at right. 

Conifers: Rust-red Stringy Rot (Indian Paint Fungus) 

Caused by Echinodontium tinctorium 

Hosts: True firs and hemlocks, rarely western larch 

Identification: 

• Decay: Tan and water-soaked at first, becoming a rust-red stringy 

rot that may result in nearly hollow stems (Fig. 53). 

• Fruiting bodies: Woody conks with a dark brown or black upper 

surface and a gray, toothed underside (Fig. 54). Conk interior is 

rusty-red to bright orange-red (Fig. 54). Conks typically develop 

under dead branches, stubs, or knotholes. 

• Punk knots: Filled with brick-red tissue. 

Damage: One conk often indicates enough decay for FP4. If trees are cored, amount of sound wood shell will only be 

accurate at the location of conks or for trees with no conks, at wounds or where closely-spaced branch whorls 

indicate suppressed growth during tree development. 

FP4: ≥1 conk(s). 

FP3: ≥1 conk(s) but still 

retaining sound wood shell 

over guidelines (Table2). 

FAILURE POTENTIAL (FP) 

RATING 

Figure 53. Stringy decay from 

Echinodontium tinctorium in a broken 

grand fir bole. 

43 
 

 

 

 

 

 

FAILURE POTENTIAL (FP) 

RATING 

Figure 55. Sap rot on standing live tree with a 

section of dead wood. 

Figure 56. Decayed sapwood, cross section. 

Sap Rots of Conifers 

Sap rots are decays that occur in the sapwood. Most sap-rotting fungi 

cause rapid decay of dead sapwood only. In living trees, sap rots occur on 

wood tissue killed by other agents, most often bark beetles and 

mechanical or weather damage (Fig. 55). On dead trees, especially those 

killed by root diseases and/or bark beetles, sap rot is sure to occur, and 

the rate of sapwood decay can be rapid (Fig. 56). FP4: Dead trees with conks. 

FP3: Live trees with conks 

and/or other indicators of sap 

rot on >25% of the stem. 

FP2: Live trees with only 1 or 

2 conks and/or other 

indicators of sap rot on <25% 

of the stem. 

44 
 

 

 

 

 

Conifers: Brown Crumbly Rot (Red-Belt Fungus) 

Caused by Fomitopsis pinicola 

Hosts: Conifers and hardwoods 

Identification: 

• Decay: Early decay is a faint yellow-brown to brown stain. 

Advanced decay is red-brown and cubical with white mycelial felts 

between the cracks. 

• Fruiting bodies: Woody bracket-shaped conk with a dark-gray to 

black upper surface and a conspicuous red band near the margin, 

white under surface, (Figs. 57 & 58). Young conks appear white 

and round. 

Damage: Causes sapwood and heartwood decay of dead trees but 

sometimes found in butt of very old living conifers, particularly spruce. 

Live trees with >1 conks may have FP2 or FP3 depending on the extent of 

decay (Table 1). Dead trees have FP4. 

FAILURE POTENTIAL (FP) 

RATING 

FP4: >1 conks. 

FP3: 1 conk, decay extensive. 

FP2: 1 conk, decay less 

extensive, sound wood shell 

over thresholds (Table 2). 

Figure 57. Conk of the red belt fungus, Fomitopsis pinicola, in a 

dead part of a living tree. 

Figure 58. Brown rot with fungus mycelium layers caused by red belt 

fungus; conk is at lower right. 

45 
 

 

 

 

Conifers: Gray-brown Sap Rot (Pouch Fungus) 

Caused by Cryptoporus volvatus 

Hosts: Conifers killed or attacked by insects 

Identification: 

• Decay: Gray areas develop in the sapwood beneath the conks. 

Advanced decay may completely destroy the sapwood in dead 

parts of trees. 

• Fruiting bodies: Small, rounded, leathery conk with pore layer in 

an internal chamber initially covered by a thin membrane that 

develops a hole over time. Conks develop in trees that were 

attacked the previous year, are tan their first year, fading to white 

(Fig. 59). 

Damage: Rapid decay of sapwood in bark beetle-attacked trees. Trees 

attacked by bark beetles with more than 50% of the bole circumference 

attacked should be considered dead. Dead trees have FP4. Trees with 

FAILURE POTENTIAL (FP) 

RATING 

FP4: Dead trees with sap-rot 

conks. 

FP3: Strip attacked trees with 

25-50% of the circumference 

damage AND substantial 

decay. 

FP2: Trees with narrow (less 

than 25%) of the 

circumference killed. 

strip attacks on less than 50% of the bole may be FP2 or FP3 depending on the portion of the stem killed (Table 1). 

Figure 59. Pouch fungus (Cryptoporus volvatus) indicates successful insect attack of conifers and causes sapwood 

decay. On the grand fir at left, past strip attacks are shown by dark vertical ridges. At right, the tan color indicates 

fresh pouch fungus conks and attack by bark beetles the previous year. Also, the split conk shows the fruiting 

bodies’ internal structure and fungal colonization in the sapwood near visible insect galleries. 

46 
 

 

 

 

 

 

Canker Diseases, General Information 

Canker diseases, also called stem cankers, kill bark and cambium and result 

FAILURE POTENTIAL in dead bark patches. The fungi that cause canker diseases often enter 

through various stem wounds or insect damage. Most canker fungi do not (FP) RATING 

penetrate far into wood, but the dead patches they cause can serve as an 

entry for stem decays or wood boring insects. Aggressive canker diseases FP4: If the tree is killed by 

can rapidly kill stems and have a flat appearance, while cankers that the canker. 

colonize host tissue slowly can be surrounded by ridges of callous wood 

laid down as a host defense, which can cause stems to bend or distort. FP3: For most canker 

Both decay and distortion create weak spots where stems and branches diseases, if more than 1/2 

can fail. Some canker diseases are more prevalent and damaging when a of the circumference of the 

host tree species is under environmental stress. The end result of this stem is killed. dynamic interaction between host and pathogen is a spectrum of damage 

ranging from tree death to relatively small wounds on the stem. FP2: If less than 1/2 of the 

stem circumference is killed 

General symptoms and indicators: by the canker. 

• Localized, flat to sunken, sometimes discolored areas on the bark 

of twigs, branches, or trunks. 

• If the disease kills the cambium all the way around the stem, the stem is girdled, resulting in death above the 

canker. 

Cankers come in 3 categories, with some overlap between categories: 

Annual: Damage is associated with wounds, damage limited to the year of infection. 

Target: Damage is over several years and degree of damage depends on the defensive reactions of the host which 

limit canker expansion. Pattern of annual growth of the pathogen and callus production by the host result in 

concentric ridges (Fig. 60). 

Diffuse: Damage is rapid, in some cases killing the host by girdling within a few years (Fig. 61). 

Figure 60. A slowly expanding (“target”-type) 

canker on aspen. The pathogen and the host 

tree have interacted many times over decades 

forming multiple ridges of callous tissue. 

Photo Credit Tom Zegler. 

Figure 61. A quickly expanding (diffuse-type) canker on aspen. The entire canker 

developed within 1 year. Photo Credit Tom Zegler. 

47 
 

 

 

 

 

Hardwoods: Sooty Bark Canker 

Caused by Encoelia pruinosa 

Hosts: Primarily aspen, but found in cottonwood 

Identification: Cankers are uncommon in stands under 60 years of age. 

Cankers develop rapidly, extending as much as 3 feet in length and 1 foot in 

width in a year. Young cankers first appear on the bark as sunken oval areas. 

The bark killed by the fungus will begin to slough after 2 or 3 years, exposing 

blackened inner bark. This dead inner bark is easily crumbled in the hand to 

a soot-like residue that is the origin of the common name of this canker (Fig. 

62). 

• Crown symptoms: Usually none, but near-girdling of stems by 

cankers can lead to dieback and decline of crowns. 

• Bole symptom 1: Death of the cambium with blackened bark (Fig. 

62); in aspen the canker begins as a small, sunken, oval patch of 

dead bark. 

• Bole symptom 2: Each year’s canker growth typically has an 

alternation of light and dark, mottled zones. 

FAILURE POTENTIAL 

(FP) RATING 

FP4: If the tree is killed by 

the canker. 

FP3: If more than 1/2 of the 

circumference of the stem 

is killed. 

FP2: If less than 1/2 of the 

stem circumference is killed 

by the canker. 

• Bole Symptom 3: On older cankers, you can often see the pattern of canker expansion as alternating black 

and white stripes. 

Damage: This disease is the most aggressive tree-killing canker of aspen. 

Figure 62. Sooty bark canker (Encoelia pruinosa) with characteristic mottling on aspen. Trees may be killed quickly (left) or 

survive as defective trees (right). Photo credit Tom Zegler. 

48 
 

 

 

 

 

 

 

 

 

 

 

Hardwoods: Snake (Cryptosphaeria) Canker-rot 

Caused by the organism Cryptosphaeria populina on aspen and C. 

pullmanesis on cottonwoods. 

Hosts: Primarily aspen and other species in the tree genus Populus. 

Identification: This canker disease causes branch, sprout, and sapling 

mortality, trunk cankers, and discoloration and decay of aspen stems. This 

disease is called a canker-rot because it is capable of both causing a canker 

by killing the cambium, and decaying the wood like a stem decay. It’s also 

commonly associated with other decay organisms like white trunk rot and is 

commonly associated and confused with Cytospora canker, especially on 

young trees. Snake canker is commonly long and narrow, spiraling around 

the tree like a snake, hence its common name (Fig. 63). Bark near canker 

edges can become discolored light brown to orange. The dead, black, stringy, 

soot-like bark adheres tightly to the sapwood and contains small, scattered, 

lens-shaped, light-colored areas. 

• Crown symptoms: Usually none, but near-girdling cankers can lead 

to dieback and decline of crowns. 

• Bole symptoms: Often will form a narrow canker visible on the 

surface which kills a narrow strip of dead cambium and twists 

around the tree (Fig. 63). Internally, this canker-rot causes decay, 

necrosis and wood staining. 

FAILURE POTENTIAL 

(FP) RATING 

FP4: Less than 20% sound 

shell OR less than 50% of the 

circumference alive. 

FP3: Tree with substantial 

canker(s), but greater than 

20% sound wood shell and 

over 50% live cambium. 

FP2: If less than 1/3 of the 

stem circumference is killed 

by the canker and greater 

than 25% sound wood shell 

is present. 

Damage: Extensive decay and discoloration in the wood is associated with external cankers. 

Figure 63. Cryptosphaeria (Snake) canker rot on aspen. 

49 
 

 

 

 

 

 

 

Hardwoods: Hypoxylon Canker 

Caused by Entoleuca (Hypoxylon) mammata. 

Hosts: Primarily aspen, but alders, cottonwoods and willows can become 

infected. 

Identification: Hypoxylon causes cankers on all tree size classes, typically 

when trees are stressed, but is fairly rare in Northern Rocky Mountain and 

Intermountain forests. The canker surface is mottled, and outer bark is raised 

in blister-like patches containing fungal fruiting bodies (Fig. 64), and sloughs 

off exposing the blackened crumbling cortex. Old cankers may be several feet 

long and are rough and blackened at the center (Fig. 65) and yellowishorange at the newly invaded margins. 

• Crown symptoms: Usually none, but larger cankers lead to dieback 

and decline of crowns. 

• Bole symptoms: Death of the cambium with discolored bark, in aspen 

the bark can turn orange as the cambium dies. Dead bark typically 

has a mottled checkerboard type pattern. 

Damage: The disease is often found associated with wounds and other damage. 

FAILURE POTENTIAL 

(FP) RATING 

FP4: If the tree is killed by 

the canker. 

FP3: If more than 1/2 of the 

circumference of the stem 

is killed by the canker. 

FP2: If less than 1/2 of the 

stem circumference is killed 

by the canker. 

Figure 64. Typical fruiting bodies of Hypoxylon canker (Hypoxylon 

mammatum) on aspen. Photo credit James Worrall. 

Figure 65. Typical Hypoxylon canker damage on aspen. Photo 

credit James Worrall. 

50 
 

 

 

 

 

 

 

Hardwoods: Black canker 

Caused by Ceratocyctis fimbriata. 

Hosts: Primarily aspen. 

Identification: This pathogen is fairly slow growing and colonizes host 

cambium fairly slowly, so the host tree has time to react to colonization by 

forming callous tissue around canker edges. The result is often an elaborate 

target-type canker with highly deformed, black callous ridges surrounding 

the dead center (Figs. 66 & 67). 

• Crown symptoms: Usually none, but larger cankers can lead to 

dieback and decline of crowns. 

• Bole symptoms: Death of the cambium and tree death from girdling 

can occur. More commonly, living trees have highly deformed 

cankers with margins that blacken over time. 

Damage: The most common damage associated with this disease is stem 

breakage due to stem weakening caused by the highly deformed area 

caused by host reaction to the disease. 

FAILURE POTENTIAL 

(FP) RATING 

FP4: If the tree is killed by 

the canker. 

FP3: If more than 1/2 of the 

circumference of the stem 

is killed. 

FP2: If less than 1/2 of the 

stem circumference is killed 

by the canker. 

Figure 66. Defects caused by multiple black cankers of 

(Ceratocystis) of aspen. Photo Credit Tom Zegler. 

Figure 67. Close-up showing target-like rings of canker 

development for black canker of aspen. Photo Credit Tom 

Zegler. 

51 
 

 

 

 

 

 

Conifers: Atropellis Canker 

Caused by Atropellis piniphila and A. pinicola. 

Hosts: A. piniphila: lodgepole pine; and A. pinicola: a number of species, 

predominantly white pines. 

Identification: Both species cause a vertically elongated canker on all size 

classes of trees. This canker does not commonly girdle trees, but can lead to 

a structural weak spot. Damage is more severe on small trees. 

• Crown symptoms: Usually none, but larger cankers can lead to 

dieback and decline of crowns. 

• Bole symptoms: A vertically elongated canker, typically sunken and 

resinous. The wood under the canker typically is stained blue to 

black (Fig. 68). 

• Fruiting bodies: Tiny (1/32 in. to 1/16 in. diameter), black, cupshaped fruiting bodies are usually present on older cankers. 

Damage: Can cause a structural weak spot at the canker, and can rarely 

girdle or nearly girdle trees. Infected trees can be evaluated by 

circumference damage guidelines. 

FAILURE POTENTIAL (FP) 

RATING 

FP4: If the tree is killed by the 

canker. 

FP3: If more than 1/2 of the 

circumference of the stem is 

killed. 

FP2: If less than 1/2 of the stem 

circumference is killed by the 

canker. 

Figure 68. Elongated, sunken Atropellis canker (Atropellis 

piniphila) with diagnostic black wood stain on lodgepole 

pine. 

52 
 

 

 

 

 

 

 

 

 

 

Conifers: Western Gall Rust 

Caused by Endocronartium harknessii and occurs west-wide. 

Hosts: Lodgepole, Jeffrey and ponderosa pines 

Identification: The disease causes galling on branches and small stems and 

slowly expanding cankers on larger stems. Sporulation typically occurs in 

June from gold-colored spore-producing surfaces under the bark at the 

edges of galls or cankers (Fig. 68). 

• Crown symptoms: Branch flagging, bole breakage, top-kill, and 

mortality of young trees. 

• Bole and branch symptoms: The galls are rounded swellings on 

branches and boles. “Hip cankers” form when the main stem 

flattens and broadens as it grows at bole infections (Fig. 69). 

Damage: “Hip cankers” that develop on trees with main-stem galls 

increase trees’ Failure Potential as more of the stems’ circumference is 

killed or flattened. 

FAILURE POTENTIAL 

(FP) RATING 

FP4: If the tree is killed by the 

canker. 

FP3: If more than 1/2 of the 

circumference of the stem is 

killed. 

FP2: If less than 1/2 of the 

stem circumference is killed 

by the canker. 

Other Stem Rust Diseases: Other stem rust diseases (e.g. white pine blister rust) can cause branch or stem cankers, 

bole girdling, and tree death. Infections are most apparent when branches or boles have been girdled and red 

needles are retained, or when resin is abundant on the tree surface. Infections can also cause stem deformations like 

galls or swollen irregular cankers that act as structurally weak areas of affected stems. For trees with top-kill, Failure 

Potential of killed parts depends on bole diameter at the site of girdling. Tree mortality caused by rust has FP4. 

Figure 69. Western Gall rust caused a flattened “hip 

canker” on lodgepole pine. Stem flattening and 

decay at bole cankers increase Failure Potential. The 

production of orange spores and swelling at right is 

typical of gall rust. 

53 
 

 

 

 

 

 

All Trees, Bole Cracks and Splits 

Cause: Load from wind, snow, and any defects in structural form (lean, FAILURE POTENTIAL 

bends and twists, eccentric canopy, high height to diameter ratio); (FP) RATING 

temperature changes with freezing; and lightning strikes. Variants are 

listed below. These openings to the wood can allow entry of decay fungi 

and increase failure potential. FP4: Structural failure cracks 

OR if <20% sound wood shell. 

Tension and Compression Failure, Limbs 

Identification: Vertical or horizontal cracks in the bole, often where decay FP3: Significant cracks 

is extensive, where fibers of the wood appear folded on one side of the without movement AND 20-

failure, and pulled straight apart by tension on the other (Figs. 70 & 71). 30% sound wood shell. 

FP2: Small shallow cracks OR 

partially callus sealed cracks 

(e.g. frost cracks) AND >30% 

sound wood shell. 

Figure 70. Tension and compression branch 

failure. On the right side of this branch, fibers 

are folded from compression; at left, tension 

caused fibers to pull apart and splinter. 

Figure 71. Tension and compression failure 

of a large whitebark pine branch. The 

branch to the right side of the picture was 

heavily loaded with snow when the branch 

failure occurred. 

Lightning Strikes 

Cause: Electric current seeking the path of least resistance from a strike point in the crown to the ground, often by a 

mix of conduction on wet bark and through the moist cambium and sapwood. Vaporized water expands and creates 

cracks (Fig. 72). 

Factors contributing to failure: High elevation and exposed ridges. 

Identification: 

• Shallow spiraling furrows that just penetrate the bark. 

• Cracks and blown-out strips that may be several inches wide and penetrate deeply into the wood. 

• Occasionally, entire trees or portions will be shattered, severely cracked, or split. 

Cracks from Wind and Wind Shake 

Cause: Stress from high wind affecting the bole. Frequent high winds contribute to development of wind shake 

(separation of wood along growth rings) in the lowest section of the butt. 

Factors contributing to failure: Frequent winds, trees growing at higher elevations or exposed locations. 

Identification: Surface cracks, sometimes hidden by resin in resinous species. For shake, twisting of the tree in the 

wind causes separations to develop, initially along growth rings, later extending radially outward, breaching the bark 

and becoming visible. Surface cracks from shake (Fig. 73) are difficult to distinguish from other surface cracks. 

54 
 

 

Frost Cracks 

Cause: Wood at different depths expands and shrinks at different rates as temperatures change during extreme cold 

weather (Fig. 74). 

Factors contributing to failure: High elevation and cold air drainages. 

Identification: 

• Frost cracks appear as long, raised, nearly vertical callus lines that may extend to the ground. 

• Frost cracks often begin from an old wound, and seldom go higher than 15 feet up the bole (Fig. 74). 

• Older frost cracks develop “frost ribs”, a series of raised vertical ridges of callus tissue parallel to the frost 

cracks themselves. 

Figure 72. Shallow, spiral scar caused by 

lightning on the upper bole of a Douglas-fir. 

Figure 73. Ridges of old resin indicated four 

wind shake cracks that reached the surface in 

this leaning Douglas-fir. 

Figure 74. Bark has grown over a surface 

frost crack in this grand fir. 

55 
 

 

 

 

 

 

 

 

Structural Failure Cracks, Main Bole 

A structural failure crack is defined as a deep crack indicating that critical wood supporting a tree or branch has failed 

or is about to fail. These cracks are typically deep, and may be associated with other wood failure indicators such as 

compressed or separated wood fibers (Figs. 70 & 71, 75), cracks that go across instead of along the grain, excessive 

lean, and wood on the opposite sides of the crack moving independantly of each other by slipping past each other or 

the crack widening and compressing in wind (Figs. 75 & 76). Cracks can be associated with internal decay which has 

weakened the stems, but can also be found on non-defective wood that has experienced overwhelming physical load 

from gale force winds or sudden physical shock from another tree. Recent structural failure cracks are indicated by 

the lack of ridges of callus “wound response” wood (Figs. 75-77). 

Figure 75. Recent trunk crack after high wind. The 

crack is deep, and the lengthwise split and some 

cross-grain splintering at the recent bend indicate 

structural failure (FP4). 

Figure 76. Western redcedar with cracks, cause 

unknown. 

Figure 77. The sides of this deep crack move 

independently past each other, indicating high failure 

potential (FP 4). 

56 
 

 

 

 

 

 

 

 

 

 

 

Forked Multiple Tops or Trunks (Conifers) 

Cause: In conifers, loss of a leader, leader bud, or tree top often causes 

more than one side branch to grow upward and vie for dominance. In 

hardwoods, multiple tops develop from growth of adventitious buds after 

loss of leaders or branches, or in some fork-prone species, from multiple, 

closely spaced buds at the tips of the leaders. 

Factors contributing to failure: Forked tops or trunks with crotches that are 

tightly V-shaped— having a narrow angle between stems— can split and 

break from foliage and branch weight, snow or ice loads, or wind. Internal 

decay at the fork increases the potential for failure under load. 

Identification: 

• Forked tops or trunks with crotches that are tightly V-shaped (Figs. 

78 & 79). 

• Multiple tops originating from a broken top with 3 or more stems, 

some with tight branch angles (Fig. 80). 

FAILURE POTENTIAL 

(FP) RATING 

FP4: V-shaped forks WITH 

embedded bark AND open 

cracks OR cracks on 

opposite sides, OR v- forks 

with substantial decay, or a 

conk. 

FP3: V-shaped forks WITH 

embedded bark AND at 

most a minor crack but NO 

visible decay or conks. 

FP2: Forks without 

embedded bark and NO 

visible vertical cracks, 

decay, or conks. 

Figure 78. Broken-out stem from a “V-fork” in a Jeffrey 

pine. Old sap and decay indicate a crack was present for 

years. 

Figure 80. Multiple tops that develop after a 

top-break can be weak due to their off-center 

structure or to a column of decay starting at the 

break. 

Figure 79. The remaining stem of the failed “V-fork” in Fig. 78. 

Failure potential of the surviving top remains high from decay, 

outward bend, and flattened cross-section where the failure 

occurred. 

57 
 

 

 

 

 

 

 

Dead Tops (Conifers) 

Dead tops in non-resinous conifers such as grand fir should be regarded to 

have nearly as high a Failure Potential as dead trees, but resinous conifers 

such as pines and cedars can retain dead tops for a considerable period, 

depending upon associated defects. Due to the spreading nature of 

hardwood crowns, Failure Potential for dead hardwood tops can be 

assessed using criteria for defective branches. 

Cause: Dead tops can have many causes, including the top-killing insects fir 

engraver beetle and Ips (engraver) beetles (Fig. 81), dwarf mistletoe, canker 

diseases, rust diseases, drought stress, and combinations of factors (Fig. 

82). 

Factors contributing to failure: Decay or signs of structural failure (cracks, 

decay conks, leaning) near the base of the dead section. 

Identification: 

• Open cracks or other signs of structural weakening at the base of 

the dead top. 

• Decay conks at the base of the dead top. 

FAILURE POTENTIAL 

(FP) RATING 

FP4: Tops ≥3” diam. with 

signs of decay or other 

defects, or if already 

detached. 

FP3: Dead tops in aspen, 

cottonwoods and large 

birches or non-resinous 

conifers >3” diam. without 

decay or other defects. 

FP2: Dead tops <3” diam., 

or >3” diam. in resinous 

species. 

• New branches from sites where the original top died, particularly those with cracks or decay signs. 

Figure 81. Approximately 30 foot tall dead top in a 

ponderosa pine. 

Figure 82. Dead top on living Douglas-fir (Approximately 10 feet 

tall). 

58 
 

 

 

 

 

 

 

 

 

 

 

Dead or Defective Branches 

Defective branches: 

Many hardwood species, especially poplars, cottonwoods, maples, and 

alders are more susceptible to branch failure than most conifers due to 

hardwoods’ lower resistance of branches to decay and frequent 

occurrence of weak, narrow angles between branches. In contrast, most 

conifer branches are resinous and nearly at right angles from the boles. 

Also, long hardwood branches can be heavily weighted with foliage and 

fruit during the growing season. Early and late season wet snows, when 

foliage is present, also weigh down branches and tops causing breakage. 

Once damaged, poplars, cottonwoods, and birches are particularly likely 

to develop weaknesses and breakage because decay in these species can 

move outward and consume living sapwood (see description of canker 

rots in section on decay), rather than being restricted to within a sound 

wood shell as in most other hardwoods. 

Identification: 

• Open cracks or splits. 

• Resin/sap flow from the defective branch base. 

• Embedded bark (bark inclusions). 

• New branches formed at the sites of old branch failures. 

• Evidence of decay fungi. 

FAILURE POTENTIAL (FP) 

RATING 

FP4: Branches ≥3” diam.

that are dead OR live with 

indicators of past breaks 

and decay. 

FP3: Live branches ≥3”

diam. with evidence of past 

breakage but no decay 

indicators. 

FP2: Live branches ≥3”

diam. without breakage or 

decay indicators. 

59 
 

 

 

Branch Brooms (Conifers) 

Branch brooms, or “witches brooms”, are defined as dense branching, and 

are usually caused by infection. They are mostly caused by dwarf 

mistletoes, broom rusts, or Elytroderma needle disease. In areas with high 

snow loading or severe winter storms, large brooms can break off in winter 

storms due to snow and ice load and wind. Breakage from dwarf mistletoe 

occurs most commonly in Douglas-firs and sometimes on lodgepole and 

ponderosa pines. Brooms from broom rusts are on true firs and spruce. 

Elytroderma needle disease is mostly on ponderosa pine. 

Cause: Parasitic plants and fungi cause branches of some tree species to 

produce many more branches than normal. 

Factors contributing to failure: Snow and ice loading. 

Identification: Deformed branches with unusually dense branching (Fig. 83). 

Figure 83. Living dwarf mistletoe brooms in Douglas-fir. 

FAILURE POTENTIAL 

(FP) RATING 

FP4: Douglas-fir with large 

(≥10’ diameter) dead 

brooms OR any species 

brooms with cracked 

branches. 

FP3: Douglas-fir with small 

(<10’ diameter) dead 

brooms OR other tree 

species with large, dead 

brooms. 

FP2: Douglas-fir with large, 

live brooms OR other tree 

species with dead brooms 

<10’ diameter. 

60 
 

 

 

Fire-Caused Damage 

Fire may have consumed sufficient wood of tree boles, limbs, or roots to reduce strength and predispose them to 

break (Figs. 84 & 85). 

Cause: Fire that has consumed wood or killed cambium. 

Factors contributing to failure: Root disease or heart rot, bark beetles. 

Identification: 

• Loss of wood at base of bole or roots consumed. 

• Wood char. 

Dead cambium encircling the base of recently fire-damaged trees indicates a 

dead tree, even if crown still appears green. Methods for assessing post-fire 

survival for individual tree species can differ from the failure potential 

assessments presented here and are beyond the scope of this document. 

Figure 84. Repeated fire damage to a 

ponderosa pine has caused little wood loss. If 

there is no decay in the interior wood, Failure 

Potential this tree would be FP2. 

Figure 85. This ponderosa pine has lost >50% of its 

interior wood from repeated fires, and Failure 

Potential is FP4. 

FAILURE POTENTIAL 

(FP) RATING 

FP4: Remaining sound wood 

of cross section is <50% of 

original OR structural roots 

damaged in >1 quadrant, or 

with significant decay 

present (see Table 2). 

FP3: Species except cedar, 

ponderosa pine, and larch: 

remaining sound wood of 

cross section is 50-75% OR 

structural roots damaged in 

1 quadrant. 

FP2: Cedar, ponderosa 

pine, and larch: bole cross 

section >50% sound wood; 

other species: sound wood 

remaining >75%. 

61 
 

 

 

 

 

FAILURE POTENTIAL 

Height to Diameter Ratios 

Live trees with a high height-to-diameter ratio (an index of slenderness) may affect a tree’s Failure Potential rating. 

Cause: Stem bending or breaking of tall, thin trees is most likely to occur 

after densely stocked stands or groups of trees are thinned. 

Factors contributing to failure: Thin trees can break or bend permanently 

after trees with snow or ice load are exposed to wind, especially if dwarf 

mistletoe brooms are present. High wind causes sway in thin trees, especially 

those with green crowns concentrated at the tops of their boles. Thin trees 

with low growth increment have a low amount of sapwood—the portion of 

the wood that has the greatest strength to tension forces—and are more 

prone to break at mid-bole. 

Identification: Determine height-to-diameter ratio: estimate the total tree 

height in feet and divide by the DBH measured in feet. For example, a tree 100 foot tall and 1 foot (12 in.). DBH 

would have a height-to-diameter ratio of 100. 

FP4: H:D ratio >100. 

FP3: H:D ratio 80 to 100. 

FP2: H:D ratio 60 to 80. 

(FP) RATING 

62 
 

 Figure 87. White ambrosia beetle frass. Figure 88. Carpenter ants in decayed 

wood. 

Insect-caused Damage 

Forest insects can weaken roots, stems, tops, or branches and allow entry of fungal spores that results in wood decay 

• Carpenter ants and termites colonize and further weaken trees that are already decayed (Fig. 88). 

• Wood borers primarily colonize dead trees, but can attack weak living trees. Their boring into sapwood or 

heartwood can significantly lower tree’s structural integrity even prior to fungus colonization. 

Figure 86. Red bark beetle boring dust. 

. 

and physical degradation. 

Cause: Insect attacks. 

Factors contributing to failure: Wood decay and physical wood 

degradation. 

Identification: 

• Of insects, bark beetles cause most conifer mortality. 

Successful attacks are usually on trees with root disease, bole 

damage, defoliation, or water stress from drought or high 

stand density, but healthy trees can succumb during outbreaks. 

Successful attacks produce red boring dust (frass) or pitch with 

brown boring dust (Fig. 86) and later, galleries under the bark, 

along with pouch fungus, fading or red crowns, dead tops, and 

group mortality. Pitch streaming and white pitch tubes with 

beetles stuck in the pitch can mean unsuccessful attacks, called 

“pitching out”. 

• Ambrosia beetles in conifers colonize mostly dead wood, 

produce white frass, and can indicate internal decay (Fig. 87). 

• Large globs of pitch indicate larvae of sequoia pitch moth 

feeding in the inner bark and cambium, often at branch whorls 

or galls. This feeding rarely causes permanent damage or 

mortality so such pitch globs do not indicate increased failure 

potential. 

FAILURE POTENTIAL (FP) 

RATING 

FP4: If >50% of the bole 

circumference has red boring dust 

or pitch, tree should be 

considered dead regardless of 

foliage condition. 

FP3: If <50% of the bole 

circumference is affected, the tree 

is considered to have a “strip 

attack” and needs further 

monitoring. 

FP2: Bark beetles only successfully 

attacked at one or no points OR 

tree only has sequoia pitch moth 

attacks. 

63 
 

 

 

 

 

 

Interacting Indicators (Multiple Defects) 

The potential for tree failure increases dramatically with the combined effects of 

multiple defects. 

In the case of multiple indicators, one condition (indicator) often contributes to 

Failure Potential of another; one weakness enhances another. 

Some examples of multiple defects that indicate increased potential for failure 

are: 

• V-forks with decay (Fig. 89). 

• Heart rot and cankers, stem injury, or saprot, often with an opening (Fig. 

90). 

• Root rot in a leaning tree. 

• Butt rot in a tree with ring separation (shake). 

• Leaning trees with external hollows and internal decay (Fig. 91). 

FAILURE POTENTIAL (FP) 

RATING 

FP4: Two or more interacting 

medium-FP (FP3) indicators. 

FP3: Two or more interacting 

low-FP (FP2) indicators. 

FP2: Two or more interacting 

very low-FP (FP1) indicators. 

Figure 89. In this aspen, weakness from 

crack and decay (indicated by conks) at a “Vfork” interact, so Failure Potential is higher 

than for the separate defects. 

Figure 90. Multiple defects: Including a 

large wound and visible decay. 

Figure 91. Multiple defects. Lean, decay, and 

crack all interact to weaken this tree. 

64 
Glossary 

Abiotic – non-living parts of the ecosystem such as soil particles, bedrock, fire, air, or water. 

Azimuth – a compass reading of 0 to 3590

, usually adjusted for declination, i.e. variation from magnetic north. 

Bark beetles – beetles that bore and feed between bark and sapwood as larvae and as adults. 

Basal wound – a wound at the base of a tree. 

Biotic – pertaining to the living parts of the ecosystem such as animals, plants, fungi, or insects. 

Bole – a trunk or main stem of a tree. 

Branch flagging – a dead branch on a live tree with dying or dead foliage attached. 

Branch stub – the remnant of a tree branch after it breaks off near the trunk. 

Branch whorl – a set of branches that developed from one node on a tree. 

Branchlet – a small branch or twig. 

Broom or witches’ broom – an abnormally dense cluster of branches associated with infection by a dwarf mistletoe, 

rust fungus, genetic aberration, or other insect or disease. 

Butt – the base and widest part of a tree stem or bole. 

Butt rot – decay developing in and confined to the butt; originating at basal wounds or soil line or entering through 

roots. 

Callus – tissue (wound wood) produced at the edge of wound sites on bole or branches in response to injury; callus 

may or may not grow to seal an injured area. 

Canker – an area of diseased tissue, typically longer than wider and often sunken, on a living stem, branch or root. 

Check – a longitudinal fissure in wood resulting from stresses that caused wood fibers to separate along the grain. 

Chlorosis/Chlorotic – an abnormal yellowing of foliage. 

Codominant stems – two tree stems of equal size that are joined at the base or partway up the bole. 

Conk – a shelf-like, usually tough-textured reproductive structure formed by many wood decay fungi; also called a 

sporophore or fruiting body. 

Crook – an abrupt bend in a tree or log. 

Crotch – an area in a tree where stems or larger branches diverge. 

Crown – the upper part of any tree carrying the main branches and foliage. 

Cubical decay – decayed wood that breaks into distinct brown, crumbly cubes. 

DBH – diameter of a tree at breast height; a standard at 4.5 feet above the ground on the uphill side of any tree. 

65 
Damage potential -- the potential that property damage, injury, or death may result from a tree or tree part failure. 

Decay – degradation or decomposition of wood by fungi and other microorganisms resulting in the progressive loss 

of integrity and strength of affected parts; degree is described as incipient (not yet apparent except by staining), early 

or advanced. 

Defect – any feature, fault, or flaw that lowers the strength, integrity, or utility of an affected part. 

Developed recreation site – According to Forest Service Manual, a recreation site that has a development scale of 3, 

4, or 5. The numbers refer to degree of development with 5 being most developed. 

Disease – a prolonged disturbance of the normal form or function of a tree or its parts. 

Distal – away from the point of attachment (e.g. tip of a branch). 

Distress cone crops – production of an unusually high number of cones by trees stressed in the current or previous 

year. The most common stressing agents associated with this condition are drought and root disease. Stress cone 

crops occasionally add so much weight to a canopy that branches or upper stems break in even mild winds. 

Drip line – the maximum radial extent of a tree crown projected to the ground. 

Duff – partially decomposed organic material on the forest floor beneath the litter of freshly fallen twigs, needles, 

and leaves. 

Dwarf mistletoe – a parasitic flowering plant that develops extensive absorption systems in the xylem tissue of 

conifers and derives most of its water and nourishment from the host. 

Ectotrophic mycelium – fungal layer, usually white to cream-colored, produced by certain root pathogens on the 

bark surface of living roots, to cause disease (laminated root disease, Heterobasidion (annosus) root disease, 

tomentosus root disease). 

Embedded bark (bark inclusions) – bark that has formed between parallel, appressed branches or trunk stems in a 

tree fork. Embedded bark has wood on two sides, and may be interrupted at its edges by at least partial wood 

grafting of the adjacent stems or branches. As the tree continues growing, the bark inclusion will be a weak spot 

where splitting can start, and often, will act as a wedge as more wood and bark is deposited. 

Failure – partial or total breakage or collapse of a tree or tree part. 

Failure potential - the potential that a tree or tree part may fail within the time period before reassessment. 

Frost crack – splitting of the outer bark and sapwood that occurs in the trunks of trees subjected to extreme cold. 

Such fissures follow the grain and are usually superficial and may be called bleeding or weeping cracks if exudates are 

apparent. 

Fruiting body – a conk, mushroom, or other fungal reproductive structure. 

Gall – a pronounced swelling or tumor-like structure a tree may produce when parasitized by certain fungi or 

bacteria, infested by gall-forming insects, or in response to other environmental stimuli. 

Hazard – the recognized potential that a tree or tree part may fail and cause injury or damage by striking a target. 

66 
Heart rot – decay usually restricted to the heartwood in living trees. 

Heartwood – the inner, nonliving part of a tree stem that has been altered to be less prone to decay as a result of 

normal, physiological processes and which provides structural support, especially for compression forces. 

Incipient decay – very early stages of wood decay. 

Laminated decay – wood that has separated into sheets along annual rings (delaminated); a typical symptom of 

some forms of root and butt rot disease (laminated root disease, Heterobasidion (annosus) root disease). 

Mitigation – actions that may be taken to reduce, minimize, or eliminate the potential risk posed by a tree hazard. 

Mycelial fan – a mass of fungal tissue that grows under the bark of infected roots and butts and has a fan-like shape; 

a typical sign of Armillaria root disease. 

Mycelial felt – a dense and expansive sheet of fungal tissue formed by some decay fungi between layers of decayed 

wood. 

Necrosis – death of a plant or a plant part; usually referring to localized death of living tissues of a host. 

Pitch tubes – tubular masses of resin that form on the surface of the bark at the entrance holes of bark beetles. 

Potential Impact Zone (PIZ) – the area that a tree or its parts could normally reach after failure. 

Pruning – the removal of live or dead side branches or removal of codominant or multiple leaders to restore a single 

dominant leader; preferred method for pruning live branches is to make a final cut just beyond the branch collar to 

speed sealing of open wound by callus. 

Punk knot or swollen knot – a protruding knot of a tree with heart rot in which abundant, non-sporulating fungus 

tissue has kept the surface from becoming fully encased (“sealed over”) by bark. Punk knots can resembles either 

normal bark or the interior tissue of normal spore-producing conks of the colonizing fungus. 

Resin – aromatic secretions, sticky when fresh, of certain trees, especially conifers; substance generally associated 

with tree resistance to fungi and insects; also called pitch. 

Resinosis – reaction of a tree to invasion by pathogens, insects, or abiotic injury which results in flow of resin on 

outer bark or accumulation of resin within or under bark. 

Risk – the possibility, exposure, or chance of damage, injury or loss; for trees, relative risk is estimated as a 

combination of its potential for failure, and potentials that a target will be present and be damaged if failure occurs. 

Root-sprung – the condition of a tree with a root system that has failed or is failing, as shown by portions of the root 

wad that are physically lifted above their normal plane; due to major roots breaking, slipping through water-logged 

soil, or having decayed. Roots that have pulled out may appear to have “sprung out of the ground”—are “sprung”. 

Rust or rust fungus – term used for a particular group of diseases or the fungi that cause them, which often have one 

or more visible, orange-to-red colored spore types; those important for tree hazards include gall and stem rusts. 

Saprophyte – an organism that lives on dead organic matter. 

Sap rot – rapid wood decay of dead sapwood portions of a tree or branch by decay fungi. 

67 
Sapwood – the outer wood layers beneath the bark that conduct water up living trees and provide strength for a tree 

to resist tension forces. 

Setal hyphae – thick-walled, reddish-brown fungal filaments that can be seen microscopically to taper to stubble-like 

points, but are visible to the eye as a fine growth between layers of advanced laminated decay caused by laminated 

root rot. 

Shake – internal separation either along growth rings or radially, originating deep in the heartwood; a physical defect 

of trees caused by exposure to high winds or in western hemlock, from years of poor growth and wetwood, 

especially in trees over 100 years old. Shake is of most concern when deep longitudinal radial fissures erupt 

externally in the butt log portion of a tree. Growth ring separations do not commonly show external fissures. 

Snag – a standing dead tree that is more than 20 feet tall. 

Sound wood –the remaining wood, not affected by any wood deterioration or damage. 

Sound wood shell - the remaining wood surrounding internal wood deterioration or damage 

Stem – the main trunk or central stalk of a plant; also called a bole in trees. 

Structural roots – major tree roots that provide significant support for a standing tree. 

Symptoms (symptomatic) – the outward manifestations of disease in a host such as chlorotic foliage, dead branches 

or tops, or dead trees. 

Target – person or object within striking distance (the Potential Impact Zone) of a tree or its parts. 

Target value – the value given to a tree based on its proximity to people and property. 

Topping – practice in which large branches are pruned between nodes to remove a tree’s upper crown and reduce its 

height; practice promotes decay and poor architecture and is not recommended. 

Undermined roots – roots that are no longer firmly anchored due to removal or loss of their surrounding soil. 

Wind shake – a separation along the grain in a tree stem caused by wind stress. 

Wind throw – a tree that has been uprooted by wind at the roots, soil line, or butt; though the term attributes failure 

to excessive wind, roots and bole of wind-thrown trees may include contributing damage or decay, as from 

construction, heart rot, or root and butt disease. 

Wind shatter – a tree that has fallen to the ground as a result of a break above the butt due to excessive wind or 

perhaps due to a decayed stem. 

Witches’ broom – see broom. 

Wood boring insects – insects that feed in the sap wood and heart wood of trees. Common insects are in the 

Families: Cerambycidae (rounded borers, longhorned beetles), Buprestidae (flatheaded borers, metalic wood 

beetles) and Siricidae (wood wasps) and Curculionidae: Scolytinae and Platypodinae (Ambrosia beetles). 

Wound – an injury that usually breaks the bark of branches, stems, or roots of a tree and serves as a possible entry 

point for fungi. Old wounds may become sealed with new bark and eventually become hidden. Scars are wounds that 

have been sealed by callus (wound wood). 

68 
 

 

 

Appendix. Field Form and Instructions 

Data Collection Fields 

The list below shows types of data used for determining hazard and recording management decisions and 

completion. These could be collected using a paper form or by direct entry into an electronic database format. 

1. Site name 

2. Date of survey 

3. Name of recorder(s)/examiner(s) 

4. Tree tag number 

5. Tree species 

6. Tree DBH 

7. Tree height 

8. Height to defect(s) or conk(s) (if present) 

9. Symptoms and signs of root or butt disease (resinosis, conks, thin crown, cracks) 

10. Thickness of remaining sound wood shell 

11. Cause of root disease or root and butt rot if known (Phellinus sulphurascens, Armillaria spp., Phaeolus 

schweinitzii, etc.) 

12. Stem defects (conks, decay, mechanical injury, fire damage, frost crack, dead top, etc.) 

13. Stem decay cause if known (Porodaedalea pini, Fomitopsis officinalis, etc.) 

14. Scores for Failure Potential, Damage Potential, and Target Value 

15. Recommended action 

16. Date action was accomplished 

Recommended Tools 

1. Forms and instructions 

2. Tatum, pencils 

3. Pulaski and hatchet 

4. Binoculars 

5. Cordless drill with 1/16 in. to 1⁄4 in. drill bits 

6. Loggers tape 

7. Clinometer or other means of measuring height (laser range finder) 

8. Field knife 

9. Increment borer 

10. Compass 

11. GPS 

Instructions for Completing Region 1 and 4 Tree Hazard Evaluation Form 

Location: location of site being evaluated 

Site name: name of site being evaluated 

Evaluated by: name(s) of person(s) conducting the evaluation 

Date: date of evaluation 

Page: indicate page number for sites with enough trees to fill multiple page numbers 

Possible Site Factors: factors that may influence tree hazard rating. These include shallow, compacted, or poor soil; 

poor drainage, high water table, recent flooding; recent changes near roots; recent stand opening; wind patterns and 

exposure; loss agents. 

Unit #: unit or section number 

Map #: map number 

Tree #: tree number 

Tree species: tree species being evaluated 

69 
DBH: diameter at breast height, in inches 

Sound Wood SHELL: a trunk’s average thickness of wood with no decay, excluding bark. This is not a routine 

measurement. Drilling may be warranted when a high incidence of hidden decay is suspected or when nearby trees 

have fallen from decay not visible from the outside. If decayed tree is tested, record average inches sound wood 

shell; Used to determine if shell is less than 15, 20, 25, 30% diameter for some Failure Potential valuations. 

Failure Potential: potential for a tree or tree part to fail before the next inspection period. General Failure Potential 

severity ratings are: 

1 = very low; live trees without visible defects 

2 = low; live trees with only minor defects such as wounds that don’t impair tree structure; trees with slight decay 

3 = medium; live trees with moderate defects; decayed trees where sound wood shell thickness is at or near the 

minimum preferred amounts 

4 = high; dead trees; live trees with major defects; decayed or burned trees with less than acceptable remaining 

sound wood 

Per Defect: record all applicable severity codes here. Use simplified Failure Potential table on the back of the 

datasheet to determine specific severity codes for each tree defect. Dead trees are always rated FP4. Consult 

Table 1 (Failure Potential Ranking Guide) and Chapter 4, Identification of Defects and Diseases, as needed. 

Combined: record Failure Potential severity rating of the most severe defect OR if 3 or more defects of the 

same severity are identified for a single tree (i.e. 3D, 3W, 3L), the overall Failure Potential for the tree may be 

increased one level. 

Failed: record an “F” here if the tree is dead or has evidence it has failed, e.g. cracked roots, broken hanging top. 

Damage Potential: estimation of extent or severity of damage or injury that could result from tree failure. Damage 

Potential takes account of tree or tree part size and the height at which a defect occurs. Damage Potential Ratings 

are: 

1 = minor; failure of a small tree part by direct or indirect impact could cause light damage 

2 = medium; failure of a small tree or a medium-sized tree part and direct impact could cause moderate damage 

3 = extensive; failure of a medium to large tree part or an entire tree and direct impact could cause severe damage to 

property or serious personal injury or death 

Target Value: score assigned to a tree based on how long and how often the Potential Impact Zone (PIZ; area that a 

tree or its parts could normally reach) will be occupied by persons or structures or other valued property. Target 

Value Ratings are: 

1 = low; tree’s potential impact zone includes, at most, features with limited use and short use length such as 

garbage cans, campground signs, or fences; may also be used for trees with moderate-use features just outside 

estimated impact zones. 

2 = moderate; tree’s potential impact zone includes, at most, features with moderate use and short use length such 

as water sources or waste disposal stations. 

3 = high; tree’s potential impact zone often has people or valuable property present for long periods. This score is 

used for trees around developed tent sites, toilets, parking spurs, and other high-occupancy sites. 

Total Rating: sum values for Failure Potential (A), Damage Potential (B), and Target Value (C) to determine Hazard 

Tree Rating. Hazard tree Rating may need to be modified based on possible site factors as indicated at the top of the 

page. Hazard Tree Rating ranges from 3 to 10. Priorities for mitigating hazard range from low to very high based on 

these numerical values (10 = Severe; 9 = High; 8 = Medium; 7 or less = Low). 

Recommended ACTION: record recommended action for tree (none; monitor; prune; remove; other) or site (none; 

restrict access to treat; close; repurpose) 

Comments: record additional identifying information such as tree location, target, specific defects, part size, details 

of recommended actions, etc. 

Date Action Taken: record date action was taken 

70 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Region 1 and 4 Tree Hazard Evaluation Form 

Location: _________________________ Site Name: _________________________________________________________________________ 

Evaluated by: ______________________________________________________________________ Date: _____________ Page___________ 

Possible Site Factors (See Below): ________________________________________________________________________________________ 

Record Appropriate Codes for Tree Defect / Failure Potential 1-4 (A), Damage Potential 1-3 (B), & Target Value 1-3 (C) based on guidance below and on reverse. 

UNIT 

# 

MAP 

# 

Tree 

# 

Tree 

Species

DBH inches

Sound 

Wood 

SHELL* 

Failure Potential (FP) 

Per Defect | Combined 

(A) 

Failed (

F)*

Damage 

Potential 

(B) 

Target 

Value 

(C) 

Total 

Rating 

(A+B+C) 

Recommended 

ACTION 

COMMENTS (Such as tree location, target, 

defects, part size, recommendation details, etc.) 

Date 

Action 

Taken 

Possible Site Factors: Shallow, compacted, or poor soil; poor drainage, high water table, recent 

flooding; recent changes near roots; recent stand opening; wind patterns and exposure; loss agents. 

Damage Potential (B): 1 = Minor (small tree or tree part could cause light damage); 2 = 

Medium (small to medium tree or tree part, moderate damage); 3 = Extensive (medium or 

large tree or tree part, severe damage). 

*SHELL: if decayed tree is tested, record AVG inches sound wood shell; Used to determine if shell 

is less than 15, 20, 25, 30% diameter for some Failure Potential valuations. 

Target Value (C): 1 = Occasional use trails and signs, vehicles not present; 2 = Intermittent 

use such as picnic and day-use areas or busy trails; 3 = Frequent use and long occupation 

areas such as camp sites, parking areas, and heavily-used permanent structures. 

Failure Potential (A): 1 = Very low; 2 = Low; 3 = Medium; 4 = High. See reverse or Table 1 and 

Chapter 4 for specific FP Severity Codes. Left column: enter FP codes for each Defect. 

Right column: for tree’s Combined FP, choose most severe FP OR if 2 or more defects of the same 

severity are interacting (e.g. 3D, 3W), record next higher value in column at right (e.g., 4). 

DEAD TREES are always FP4. 

* Failed = F: Tree is dead or has evidence it has failed, e.g., cracked roots, broken hanging top, etc. 

Total Rating (A+B+C) is the Hazard Tree Rating and Treatment Priority Class: 

10 = Severe; 9 = High; 8 = Medium; 7 or less = Low. 

Recommended Actions for TREE: None; monitor; prune; remove; (other). 

Recommended Actions for UNIT or SITE: None; restrict access to treat; close; repurpose. 
 

 

Table of Failure Potential (FP) Codes: Descriptions and values may not fit all situations, as these descriptions are provided as aids, not definitions, for determining FP of 

defects. Consult Table 1 (Failure Potential Rating) and Chapter 4 as needed for more detailed descriptions of individual tree defects and FP severity codes. 

Code 

DECAY AND CAVITY Sound wood shell (SHELL) excludes bark and may be 

measured on trees with suspected significant decay. Severity of decay values can be 

adjusted based on tree species. Use for decay in whole tree or tree parts! 

Code LEAN/SWEEP Growth at the very top of the tree may provide some indication of 

how long a tree has been leaning 

1D None 1L Lower bole is vertical, mid to upper bole has sweep from factors affecting growth 

2D Some decay without conks OR with but still openings has average SHELL >30% 

diameter 2L Lean of any amount where recent top growth is fully vertical (“corrected”) 

3D 

If no bole opening, SHELL is 15-30% of diameter OR if bole has an opening, 

SHELL is >20% of diameter OR if decay can’t be measured in upper bole, estimated 

decay is < SHELL thresholds 

3L Lean ≥15° AND recent growth not fully vertical (not “corrected”) AND soil is intact 

4D 

Large conks visible OR if no bole opening, SHELL <15% diameter OR if bole has an 

opening, SHELL <20% diameter OR if decay can’t be measured in upper bole there 

are multiple or large conks/ fruiting bodies present 

4L Tree has evidence of failure by soil cracks, movement, or sprung roots OR 

uncorrected lean ≥15° is associated with decay, cracks, or canker 

STEM DAMAGE AND WOUNDS (cracks, mechanical, animal, human, fire) BRANCHES AND TOPS (dead or forked, dwarf mistletoe and other witches 

brooms, higher FP codes for Populus and non-resinous conifers) 

1W Cracks/wounds of any size are well-calloused or superficial and lack decay 1B Small dead/hanging branches or tops <3" diameter OR small live witches brooms 

2W Minor structural damage, scraped bark, OR fire scar is <10% circumference OR 

small, shallow at least partially calloused cracks 2B 

V-shaped forks or multiple tops without trapped bark OR rust killed tops or branches 

OR dead tops in resinous species of any size OR non-decayed tops <3”diam. in nonresinous species OR Large live DF brooms, small live brooms in other species 

3W Some structural damage and associated decay but <50% circumference OR large fire 

scars leaving >50% wood OR significant cracks without movement 3B 

V-forks with embedded bark OR live cracked tops or limbs >3” OR dead tops in 

hardwoods or non-resinous species >3” diam. OR DF with small dead brooms OR 

other species with large dead brooms 

4W 

Multiple wounds and associated decay OR wound >50% circumference OR 

significant cracks with evidence of movement OR <50% sound wood remaining after 

fire 

4B 

V-shaped fork with trapped bark and cracks, or decay OR detached branches or tops 

OR dead tops or branches ≥3” in Populus, Salix or non-resinous conifers OR dead 

tops ≥3” with decay or cracks in resinous conifers OR DF with large dead brooms and 

any species with brooms on cracked branches 

BOLE CANKERS (blister rust, gall rust, dwarf mistletoe, etc.) ROOT DAMAGE, BUTT ROT, ROOT DISEASE 

1C No cankers, but a burl of any size 1R A few roots are exposed and these lack decay and are not root-sprung 

2C <25% of bole girdled 2R Exposed small roots have minor damage or decay but are not root sprung and there is 

no evidence of root disease on tree or its neighbors 

3C 25-50% of bole girdled 3R Some roots damaged but <50% root system intact OR root disease suspected from 

symptoms or diseased neighbors, but disease is not obvious or confirmed 

4C >50% of bole girdled OR bole with less girdling has another defect 4R Roots exposed, decayed or substantial damage on >50% of root system OR root 

disease is obvious from thinning/fading crown, basal resin or stain 
 

 

In accordance with Federal civil rights law and U.S. Department of Agriculture (USDA) civil rights regulations 

and policies, the USDA, its Agencies, offices, and employees, and institutions participating in or 

administering USDA programs are prohibited from discriminating based on race, color, national origin, 

religion, sex, gender identity (including gender expression), sexual orientation, disability, age, marital status, 

family/parental status, income derived from a public assistance program, political beliefs, or reprisal or 

retaliation for prior civil rights activity, in any program or activity conducted or funded by USDA (not all 

bases apply to all programs). Remedies and complaint filing deadlines vary by program or incident. 

Persons with disabilities who require alternative means of communication for program information (e.g., 

Braille, large print, audiotape, American Sign Language, etc.) should contact the responsible Agency or 

USDA’s TARGET Center at (202) 720-2600 (voice and TTY) or contact USDA through the Federal Relay Service 

at (800) 877-8339. 

To file a program discrimination complaint, complete the USDA Program Discrimination Complaint Form, 

AD-3027, found online at http://www.ascr.usda.gov/complaint_filing_cust.html and at any USDA office or 

write a letter addressed to USDA and provide in the letter all of the information requested in the form. To 

request a copy of the complaint form, call (866) 632-9992. Submit your completed form or letter to USDA by: 

(1) mail: U.S. Department of Agriculture, Office of the Assistant Secretary for Civil Rights, 1400 

Independence Avenue, SW, Washington, D.C. 20250-9410; (2) fax: (202) 690-7442; or (3) email: 

program.intake@usda.gov. 

USDA is an equal opportunity provider, employer, and lender. 

73 
 

 

Sound Wood Shell Thickness 

Values for Stem Diameters 

WithinBark Tree 

Diameter in 

Inches 

Inches for 

15% Sound 

Wood 

Shell 

Inches for 

20% Sound 

Wood 

Shell 

4 0.6 0.8 

6 0.9 1.2 

8 1.2 1.6 

10 1.5 2 

12 1.8 2.4 

14 2.1 2.8 

16 2.4 3.2 

18 2.7 3.6 

20 3.0 4.0 

22 3.3 4.4 

24 3.6 4.8 

26 3.9 5.2 

28 4.2 5.6 

30 4.5 6.0 

32 4.8 6.4 

34 5.1 6.8 

36 5.4 7.2 

38 5.7 7.6 

40 6.0 8.0 

42 6.3 8.4 

44 6.6 8.8 

46 6.9 9.2 

48 7.2 9.6 

50 7.5 10