United States

Department of

Agriculture

Forest

Service

Alaska

Region

State and

Private Forestry

Forest Health

Protection

2009
Hazard Trees in

Alaska

A Guide to the Identification and

Management of Hazard Trees

in Alaska

Lori Trummer, U.S. Forest Service Pathologist

Paul Hennon, U.S. Forest Service Pathologist

Publication No. R10–TP–142

February 2009
I n t r o d u c t i o n

C

h

a

p

t

e

r

5

Insects, diseases, decays, and other forms of 

tree defect and mortality are important parts of a 

healthy, functioning forest ecosystem. They play 

many ecological roles in forests such as altering 

plant succession and providing wildlife habitat. 

In forest settings, trees die and fall to the ground 

where they become recycled. Although dying 

and falling trees are important in the development of forests, they are not welcome in highuse recreation areas where they risk human life 

and property.

People enjoy recreating in Alaska’s forests, 

particularly forests with large old trees. Unfortunately, these are the very trees that often provide 

the greatest risk in terms of hazard. Large trees 

are more likely to contain significant amounts 

of internal wood decay and other defects. Also, 

because of their size, they can cause more damage when they fall. As long as people want to be 

around standing trees, there will be some level 

of tree failure risk. Thus, recreation managers 

are often faced with an apparent paradox—how 

to maintain safety for visitors while providing an 

aesthetic environment with large trees.

The goal of vegetation management in developed sites is to sustain the forest in an aesthetically pleasing condition while reducing unacceptable risk to visitors. A hazard 

tree program is an important part 

of vegetation management. However, simply identifying and treating 

hazard trees is only a short-term approach. Careful vegetation management is the key to sustainable healthy 

forest conditions that will provide 

desired benefits and produce a minimum of hazard trees in the future.
Introduction

6

Introduction

7

This book was designed to provide managers 

with basic information about hazard trees. We 

present the information with a logical flow from 

hazard tree concepts to recognition, evaluation, 

and lastly prevention. Hazard profiles of Alas-

kan trees are included that describe the common 

defects and a general failure potential for the 

various tree species. A chapter is included on 

safe backcountry travel principles around hazard 

trees. References are included for further infor-

mation on this topic. 

What is hazard?

Hazard is the exposure to the possibility of loss 

or harm. With regard to trees, it is the potential 

that a tree or part of a tree will fail and cause 

injury or damage property. All standing trees 

of sufficient size, alive or dead, present some 

hazard. All trees will eventually come down. But 

high potential for tree failure by itself does not 

automatically mean a tree is hazardous. Hazard 

exists when a tree is within striking distance of 

an object of any value.

Hazard is the po-

tential that a tree or 

tree part will fail and 

cause injury or dam-

age property. 

The forests of Alaska encompass 51.4 million hectares and stretch from the coastal forests of Southeast Alaska through the boreal forests of Interior Alaska. 
Introduction

8

Introduction

9

Why be concerned about hazardous 

trees?

Moral obligations

Few visitors to recreations areas are experts on 

trees. Managers cannot assume that visitors will 

be able to evaluate hazardous trees and avoid 

them. Visitors will be either oblivious to the 

possibility of hazard trees or they will assume 

that managers have eliminated such trees. Thus, 

managers have some moral obligation to provide 

a relatively safe environment for visitors.

Legal obligations

Visitors assume some level of risk when they 

recreate on public lands, but managers who 

create and maintain designated recreation areas 

are responsible for ensuring visitor safety for 

reasonably foreseeable hazards. The Federal 

Tort Claims Act generally holds 

the federal government liable in 

the same way as a private party 

for negligent acts committed by 

federal employees in the course 

of their employment. Failure to 

inspect and treat known hazard 

trees in developed recreation areas 

may be considered negligent. 

Informing visitors of potential 

hazards (e.g. by signs) does not 

always eliminate the danger to 

visitors or the risk of liability. It is 

the responsibility of managers to 

inspect and correct the most serious threats or foreseeable dangerous conditions in order to minimize 

the potential for injury to visitors or 

damage to property.

Thus, a tree is considered potentially hazardous 

if:

• it has defects which predispose all or part 

of the tree to failure, and

• it is located so that the failure poses a 

threat to people or property.

Hazardous trees may 

occur in both the 

urban or wild land 

setting.

Managers of designated recreation 

areas are responsible to inspect 

and treat known hazard trees.
Introduction

10

C o m p o n e n t s o f Tr e e H a z a r d A n a l y s i s

C

h

a

p

t

e

r

11

Generally, liability in cases that involve injuries 

or damage resulting from hazard trees is based 

on what a reasonable professional in the situation would have done. If a manager knew, or 

should have known, of a hazard but failed to take 

reasonable actions to alleviate the hazard, the 

federal government may be liable for negligence. 

The individual manager may also be personally 

liable if such inaction is considered beyond the 

scope of his/her employment. A hazard tree program is an important tool to ensure the safety of 

visitors and help avoid liability. At a minimum, a 

hazard tree program consists of trained individuals inspecting trees on a regular basis, taking 

mitigating measures on trees that are judged 

to be the highest risk hazards, and documenting actions taken to mitigate hazards. The level 

of training provided to tree inspectors and the 

forms used to demonstrate inspection are critical 

components of establishing that the agency took 

proper actions to alleviate hazards. Thus, training and documentation are essential components 

of a hazard tree program.

Training of hazard tree inspectors is an essential 

component of a hazard tree program. 

 

Hazard increases with four factors:

A. Potential for tree failure

B. Potential for striking a target

C. Potential for serious damage of the target

D. Value of target.

These factors are treated below and later used in 

an example of a quantified hazard rating system.

A. Potential for Tree Failure.

Estimating the potential for tree failure is challenging because of many interacting variables. 

Failure potential is estimated by examining a 

tree, determining defects that contribute to weakening or failure, and estimating the potential for 

failure before the next inspection period.

The location of a defect can be critical factor 

when determining the hazard level of an individual tree. Tree stems have a zone called the 

failure zone which receives greater strain than 

wood above or below it. The failure zone occurs 

from about four feet above the groundline up to 

the lowest living branch. If defects occur within 

this zone, failure potential is increased.

The failure zone occurs 

from about four feet above 

ground, up to the first living branch.
Components of Tree Hazard Analysis

12

Components of Tree Hazard Analysis

13

B. Potential for Striking a Target

A hazardous situation requires

• a defective tree and

• a potential target.

Trees or tree parts that could fall on a potential 

target need to be inspected regularly.

The potential for a tree or tree part to strike a 

target is determined by evaluating where failed 

trees will likely land and whether the strike zone 

is occupied by a target at the time of failure. 

Moving targets are less likely to be struck than 

stationary targets.

Variables to consider include:

• Is the tree in striking distance of 

parking areas, tent pads, picnic

 areas, fire rings, restrooms, or

 children’s play areas?

• Timing of probable failure and use 

of the area. Are some targets absent 

(people) when failure is likely 

(winter)?

• Is the tree leaning towards the

 target? Strong winds and other 

factors, however, may alter the 

direction of fall.

Needs inspection

Tree could fall on 

potential target.

Does not need inspection

Courtesy of MN DNR

Tree would not reach the 

potential target.

Trees in striking distance of 

picnic tables require regular 

inspection.

C. Potential for Serious Damage

The potential for serious damage depends upon 

size of failed portion of tree (e.g., limbs, or size 

of entire tree in complete tree failure). Consider 

that some structures far from a hazard tree may 

not be seriously damaged if the top of a tree 

strikes, but a structure close to a hazard tree may 

be demolished if the bole strikes.

D. Value of Target

The value of a target depends upon the maxi-

mum extent of loss if the target is struck by a 

failed tree. Values are typically expressed in rela-

tive terms such as low, medium, and high. Obvi-

ously, target value is at a maximum when human 

life is at risk. Examples of low value targets may 

be garbage cans, and information boards when 

people are not present.

Other Factors Affecting Hazard Level

Additional factors to consider when assessing 

hazard level include

• site factors

• tree age and size

• tree species

Site Factors

Site factors to consider include exposure to 

wind, especially prevailing wind during storm 

seasons, slope, soil conditions, and history of 

tree failure. Trees that have lived most of their 

lives in an exposed condition are usually welladapted to wind. A higher potential for failure 

exists when stands are opened through thinning, 

other forms of management, or natural causes 

(wind events, fire, etc.). Trees with a high height 

to diameter ratio (e.g. tall trees with skinny 

boles) are more susceptible to wind damage.
Components of Tree Hazard Analysis

14

Components of Tree Hazard Analysis

15

Recent research in old-growth stands in southeast Alaska, indicates slope has a strong influence on tree failure direction. In most cases, 

trees fall downslope. While this seems obvious, 

it is an important factor in evaluating whether 

a tree has the potential for striking a target in 

the event of failure. Targets located upslope of 

hazardous trees have a lower potential for being 

struck than those located on the downslope side.

Tree Age and Size

Large trees present a greater hazard because 

they can strike targets at considerable distance 

and cause more damage when they fail. Defect, 

especially heart rot, is highly associated with 

tree age.

Tree species have a characteristic lifespan and 

the risk of tree failure increases as they reach 

maturity. For example, paper birch and aspen 

trees less than 60 years old are generally free of 

stem and root decay and thus have a low failure 

Polar diagram of tree failures for trees with broken boles and 

uprooted trees. Note that the direction of tree failure was predominantly downslope, indicating that targets upslope have a lower 

potential of being struck. Of course, trees can fall in any direction 

and targets upslope from a tree are not necessarily safe.

potential. However, birch and aspen over 90 

years old routinely have extensive heart and root 

rot. Thus, older birch and aspen trees constitute 

a higher failure potential than younger trees. 

Similar relationships of higher failure potential 

have also been noted for older conifer trees in 

Southeast Alaska.

The amount of decay in trees is highly correlated 

with tree age. Note in the preceeding figure how 

the percentage of conifer trees with any decay 

increases sharply after trees are about 100 years 

old. Very few trees over 500 years old are completely free of decay. Note also the differences 

by tree species.

Relationship 

of tree age to 

the percent of 

trees with decay. 

(Graph modified 

from Kimmey 

1956.)
Components of Tree Hazard Analysis

16

Components of Tree Hazard Analysis

17

Also, the graphs depict that the amount of decay 

increases sharply in trees over 400 years old. 

Western hemlock has more decay than Sitka 

spruce across all tree ages. Western redcedar 

is even more defective than either hemlock or 

spruce.

Tree longevity is an important factor to consider 

both when evaluating tree hazards in developed 

recreation sites and when selecting locations for 

new sites. Short-lived species, such as aspen and 

paper birch, should be avoided unless plans are 

made to regenerate or replace these species as 

they reach maturity.

Tree Species

Each tree species has its respective defects that 

affect the failure potential of the tree or sections of the tree. From this general information, 

preliminary hazard profiles of Alaska tree species have been developed (see Chapter 6). For 

each tree, profiles include their general potential 

for failure and the specific types of defect or 

injury that indicate failure potential. As more 

information becomes available through research 

or monitoring of failures at recreation sites, the 

profiles will be refined.

Birch trees have higher 

defect at younger ages 

than late successional 

species such as spruce.

Large trees are a 

greater hazard because they can strike 

distant targets and 

cause more damage 

when they fail.

Relationship of 

tree age to the 

percent of cubic 

foot cull in the 

stem. (Graph 

modified from 

Kimmey 1956.)
Components of Tree Hazard Analysis

18

Tr e e D e f e c t s t h a t I n f l u e n c e H a z a r d

C

h

a

p

t

e

r

19

Common indications of defect in Alaska trees include: cracks, lean, root damage, top damage, internal decay, scars and stem cankers, dead trees 

and branches, and insect boring dust at the base 

of the tree. The unique symptoms and associated 

probabilities for failure of the defect categories 

are presented below.

1. Cracks

Cracks are the number one hazardous defect 

because they indicate the tree is already failing. 

Cracks need to be evaluated very carefully. They 

may or may not indicate substantial amounts of 

internal wood decay. Sometimes, cracks form 

as a consequence of massive internal decay as 

the tree buckles in the wind. Cracks near major 

branch unions can also indicate that trees have 

begun to fail, sometimes without associated 

heart rot. In other cases, bark cracks are caused 

by frost or some weather extreme and may or 

may not indicate decay or immediate failure. 

Careful evaluation using an increment borer or 

drill will help indicate extent of any associated 

decay.

Hemlock stem crack. Birch frost crack.
Tree Defects that Influence Hazard

20

Tree Defects that Influence Hazard

21

2. Lean

Leaning trees or large branches do not always 

indicate high failure potential. If the lean is a 

recent change, the potential for failure is much 

higher than if the tree is a “long-term leaner.” 

A tree that has been leaning for a long time will 

try to compensate for the lean by straightening 

the growth at the top of the tree. This corrected 

lean is a sign of a strong root system, indicating 

the tree may be fairly stable. A tree without corHigh Failure Potential

• Freshly leaning tree with recent rootlifting, soil movement or mounding 

near base. 

• Inadequate root support, greater than 

half of the roots severed inside drip 

line.

• Lean associated with unstable soils or 

cracks in the tree.

• Leaning over a target with greater 

than 45° angle to the lean.

• Tree with other defects leaning over 

target.

Trees with greater 

than a 10 degree lean 

may be unstable and 

should be monitored 

for potential failure. 

High Failure Potential

• A crack goes completely through the 

stem or branch.

• Stem has two cracks on the same segment with a cavity or extensive decay 

on the inside.

• Stem has a crack in contact with 

another defect (canker, decay, weak 

branch union, leaning).

• Tree has a single crack with in-rolled 

bark and cavity or decay are inside.

rected lean suggests the lean occurred 

recently—a potentially more hazardous 

situation. The direction of lean can give 

a strong indication of where the tree 

will probably fall. This knowledge can 

help evaluate the likelihood of a tree 

striking a target. 

3. Root damage, including root 

disease

Because of cold soils and sometimes 

excessive moisture, many trees in 

Alaska have shallow root systems. 

Their root systems are easily damaged 

or killed. Trees with damaged or dead 

roots are more likely to uproot causing the whole tree to fail during wind 

storms. Trees in forests that are opened 

up due to thinning, the removal of adjacent 

hazard trees, or natural causes (wind events, fire, 

etc.) are more susceptible to windthrow. Exposed roots can be wounded in the same manner 

as tree boles and lead to the same problems of 

invasion by wood decay fungi. Undercut root 

systems are often encountered along streams and 

areas of road building.

Uprooting due to tomentosus 

root disease caused by Inonotus 

tomentosus.

Adequate root support 

may be compromised 

on trees with extensive 

exposed root systems. 
Tree Defects that Influence Hazard

22

Tree Defects that Influence Hazard

23

For more information on rot and butt rot fungi 

refer to Insects and Diseases of Alaskan Forests.

Two Major Types of Root Problems.

• Undermined or Undercut Roots.

These conditions are caused by 

mechanical damage/root severing or 

soil erosion. Undercut root systems 

are often encountered along streams 

and areas of road building. Trees with 

undermined roots have a high potential for failure.

• Root and Butt Diseases. Root and 

butt rots are caused by a number of 

fungi and are responsible for one of 

the most serious defects affecting 

trees in recreational areas. Learn to 

recognize the leading root disease 

fungi, especially Phaeolus schweinitzii and Inonotus tomentosus.

High Failure Potential.

• Inadequate root support; tree has 

more than 1/2 of the root system decayed or severed inside the drip-line.

• Freshly leaning trees with evidence 

of recent root-lifting, soil movement, 

mounding near base of tree, or broken/decayed roots.

4. Top damage

This defect is usually fairly simple to recognize. 

Dead tops can result from various forms of 

damage, including root disease, stem cankers, 

animal feeding and soil problems. Forked tops 

need to be evaluated carefully because they 

may or may not be hazardous. If the tree forked 

because its top was broken out or killed, it may 

have internal wood decay around the fork. Look 

for cracks, conks, or other signs that the fork is 

beginning to fail.

High Failure Potential.

• Broken tops with adjacent branches 

unhealthy.

• Large forked tops.

• Dead large tops and branches,

 especially if broken and lodged in 

other branches.

• Heavy U-shaped branches formed 

when side branches turn up to become 

leaders.

Forked and broken top. Broken top. Dead spike top.
Tree Defects that Influence Hazard

24

Tree Defects that Influence Hazard

25

5. Internal decay

Decay is a leading cause of tree failure, but is 

difficult to observe in some trees. Hemlocks, 

cottonwood, birch, and aspen are all very prone 

to high levels of heart rot, but heart rot is common in all tree species of Alaska. Decaying 

trees can be prone to failure, but the presence of 

decay, by itself, does not indicate that a tree is 

hazardous. Thus, when heart rot is 

discovered in a tree, it is important to determine its extent and the 

amount of sound wood. Regarding 

failure potential, experience suggests using the One Third Rule. In 

most cases, visible indicators can 

be used to identify trees with heart 

rot. The most reliable indicators of 

decay are conks and mushrooms. 

Other indicators include large 

exposed wounds, broken or dead 

tops, and cracks.

High Failure Potential.

• Less than 1

/3 of the tree’s radius (or 

diameter) is sound. This is the One 

Third Rule for evaluating failure 

potential.

• Rot in a tree’s failure zone; the 

area from about four feet above the 

groundline up to the lowest branch.

• Cavity, decay or fruiting body

 associated with an open crack or a 

weak branch union.

Internal decay can be difficult to 

observe but is a leading cause of tree 

failure.

One Third Rule

Trees have a high rate of failure 

when less than 1/3 of its radius 

(or diameter) is sound.

Thus, if a tree with an 18" diameter (9" radius) 

has a column of wood decay in its center, then, 

at a minimum 3" of sound wood should be present in the outer wood for the tree to pass the One 

Third Rule.

 Minimum Sound 

 Tree Diameter Wood Needed 

 9" 1.5"

 18" 3"

 30" 5"

 48" 8"

This Rule needs to be modified when an exposed 

scar is present; an additional 25% sound wood 

is needed to provide more support. For example, 

an 18" diameter tree with a scar would need a 

minimum of 4" of sound wood to pass the One 

Third Rule.

Relatively safe.

More than 1/3 radius 

or diameter is sound.

Marginal.

About 1/3 radius or 

diameter is sound.

Likely to fail.

Less than 1/3 radius or 

diameter is sound.
Tree Defects that Influence Hazard

26

Tree Defects that Influence Hazard

27

Decay Indicators:

6. Scars and Stem Cankers

Unless a tree is scarred all the way around the 

circumference of its bole, scarring alone does 

not usually kill trees. Invasion by decay fungi 

is often the most serious aspect to a scar. Trees 

with scars, especially old, large scars should be 

evaluated very carefully for the extent of internal wood decay. Trees with exposed scars need 

more sound wood than the One Third Rule to be 

considered anything but a high hazard potential. 

Use an increment borer to determine the extent 

of decay.

Cankers caused by fungi can look similar to exposed scars. Cankers are localized dead areas in 

the bark of stems and branches. The presence of 

Conks on base

of tree.

Large exposed 

wound.

Broken top. Crack.

High Failure Potential.

• Scar present with associated fungi 

fruiting bodies

• Scar or canker present with associated 

internal decay

• Multiple scars or cankers

 affecting more than 1

/2 of the tree’s 

circumference

Scar from moose 

damage.

a canker increases the chances of stem breakage 

near the canker. A tree with a canker that encompasses more than half of the tree’s circumference 

may be hazardous even if exposed wood appears 

sound. Thin barked and easily wounded, trembling aspen and paper birch are highly susceptible to invasion by canker fungi.

7. Dead branches

Depending upon the location of targets, dead 

branches may or may not present a hazard. 

Hanging dead branches are likely to fail soon 

and should be treated immediately if a target is 

nearby.

High Failure Potential.

• Any dead branch.

• A broken branch that is hanging or 

lodged in the crown.

Canker caused by Ceratocystis fimbriata on aspen.

Canker caused by Nectria 

galligena on paper birch.
Tree Defects that Influence Hazard

28

H a z a r d T r e e R a t i n g

C

h

a

p

t

e

r

29

8. Dead trees

Dead trees are simple to recognize and among 

the most likely to fail. Dead trees should typically be removed immediately, because once a 

tree dies, decay organisms invade and structurally weaken the stem. Large limbs and the top 

often break out of the crown before the entire 

tree fails. All Alaska trees other than western 

redcedar and Alaska yellow-cedar decay rapidly after death. Spruce killed by bark beetles in 

particular are rapidly invaded by saprots which 

quickly compromise the structural stability of 

the tree.

9. Carpenter Ants

Live trees with heartrot may be invaded by 

carpenter ants, further compromising the structural integrity of a tree. The ants specifically 

select softened wood to tunnel into for shelter 

and brood raising, but they do not eat the wood. 

Presence of carpenter ants can be detected by 

piles of boring dust at the base of a tree. Suspect 

trees should be increment 

cored or drilled to determine 

sound wood thickness and 

whether the tree passes the 

One Third Rule.

High Failure Potential.

The stability and structural soundness of 

dead trees is severely compromised. Dead 

trees or sections of dead trees could fail at 

any time.

Carpenter ant boring 

dust at tree base.

Dead trees are extremely 

hazardous and have a 

high failure potential. 

Which Trees Should Be Evaluated?

Trees are not considered hazardous and need 

not be evaluated if they are not within striking 

distance of a target. This can be determined by 

measuring the height of the tree and distance 

from the base of the tree to any potential target. 

Some experts suggest systematic evaluations be 

made at least annually.

In recreation sites, areas can be stratified into 

tree hazard risk zones (High, Medium, Low) 

prior to inspection. Inspection intensity should 

vary directly with risk level. All trees within 

falling distance of targets (structures, vehicles, 

or people) should be inspected. The height of 

hazardous trees projected to the ground determines the width of the hazard zone. All trees 

within areas of intensive public use should be 

evaluated. Special attention should be given to 

trees eight inches or greater in diameter at breast 

height, since two-thirds of reported failures occur in trees of this size.

Every tree will ultimately fail unless removed. The 

task in rating hazard is to estimate the probability of 

failure during a specific period.

Courtesy of MN DNR
Hazard Tree Rating

30

Hazard Tree Rating

31

The High Risk Zone includes high use areas 

with many people, parked vehicles and permanent structures. This zone is the highest priority 

for regular inspections and treatment. The Medium Risk Zone are areas with intermittent use 

by people and moving vehicles. The priority for 

inspections in this zone is based on amount and 

type of use. The Low Risk Zone includes areas 

lacking vehicles or structures with low visitor 

use. Regular inspections and treatment in this 

zone have low priority.

The purpose of a hazard tree evaluation is NOT to remove every tree 

that has defects; rather, the goal is 

to preserve the greatest number of 

trees in recreation areas consistent 

with safety. Removal of too many 

trees in an area can destroy the 

aesthetic qualities for which the site 

was selected. Also, stand stability 

may be affected and the probability 

of wind-throw increased.

Courtesy of U.S. Forest Service—Region 2

Camping sites are a high use area 

and a high priority for regular 

inspections.

Recreation areas can be divided into 

hazard risk zones which determine 

the intensity of evaluation.

Hazard Rating System

All hazard tree programs need to have some 

method of objectively evaluating hazard. A 

quantified hazard rating system is recommended. 

Below is a rating method that has been developed in Region 6 of the USFS. Other systems 

can be used as long as they combine values of 

the two necessary components: A) potential for 

tree failure and B) potential and seriousness of 

damage in the case of a tree failure.

A. Potential for Tree Failure. This component addresses the potential for tree failure in a 

specific time frame. Ratings are on a scale of 1 

to 4 in order of increasing failure potential.

1 = Very low failure potential. Sound trees 

that lack indicators of failure that are not 

leaning or not exposed to wind or snow 

load.

2 = Low failure potential. Trees with only 

minor defects, including internal decay 

that does not approach or exceed the One 

Third Rule and are not leaning or not exposed to wind or snow load.

3 = Medium failure potential. Trees with 

moderate defects (e.g., at or near the 

threshold of acceptable sound wood thickness) or that are growing in shallow soil 

or exposed to a high water table, or highly 

defective trees in areas well-sheltered from 

weather and wind extremes; or highly 

defective trees in areas exposed to weather 

extremes (e.g., heavy snow loads) only in 

the off-season.

Tree Hazard Rating Score =

Potential for Failure + Potential for Damage
Hazard Tree Rating

32

Hazard Tree Rating

33

4 = High failure potential. Highly defective 

trees in unsheltered areas; trees with root 

anchorage limited by erosion; dead trees; 

trees with obvious root disease.

B. Potential for Damage. Damage potential 

incorporates the potential for striking a target, 

potential for damaging a target, and target value. 

Damage potential is rated on a scale of one to 

four in order of increasing potential.

1 = No damage. Target impact will only 

involve very small trees or parts of trees; or 

there is no chance tree will cause damage 

to target.

2 = Minor damage. Failure of only small 

trees or parts of trees; damage is likely to 

occur when target is not occupied; target 

value is low.

3 = Medium damage. Failure involves 

small trees or medium-sized parts of trees; 

impacts will likely occur in areas with 

targets; impacts will be direct; and damage will likely be moderate; target value is 

moderate.

4 = Extensive damage. Failure involves 

medium to large tree parts or entire trees; 

impacts will be direct in areas with targets; 

target value is high; damage to property 

will likely be severe; or serious personal 

injury or death is the likely result.

Calculating the Tree Hazard Rating Score

The hazard score for individual trees is determined by adding the values from the two parts 

on the rating system. Each tree would yield a 

score from 2 to 8.

The next step is to evaluate the tree rating scores 

for a recreation area, which can then be helpful 

to prioritize which trees need treatment and in 

which order when resources are not available for 

treating all higher risk trees.

Recording an Evaluation

Documentation is a key part of a hazard tree 

management program to provide evidence that a 

tree was examined. Two examples of hazard tree 

inspection forms are included in the following 

pages. The forms can be used as is or modified 

as needed.

Recording the results of hazard tree evaluations 

has many advantages.

• Provides an assessment of current hazards 

and a framework for future vegetation 

management activities.

• Provides a database for future monitoring 

and treatment efforts.

• Provides a record of performance in the 

event of litigation (tort claims).

Hazard Rating Score

2–5

6

7

8

Treatment Priority

Low

Moderate

High

Very High
Hazard Tree Rating

34

Hazard Tree Rating

35
Hazard Tree Rating

36

P r e v e n t i o n a n d M a i n t e n a n c e

C

h

a

p

t

e

r

37

Increment Borer

or Drill with long

bit

Binoculars

Hatchet or Axe

Diameter Tape

Clinometer

Tape Measure & 

Compass

Camera

Data Forms

for boring into tree to test for amount of 

sound wood or decayed wood; increment 

borers extract a small core of wood

approximately 1⁄

2

" in diameter.

to check the top of the tree for defects

to check for root rot or to tap tree to 

judge for hollowness

to measure diameter at breast height 

(DBH) of tree. DBH = 4.5 ft. above 

ground

to measure height of tree

to determine location of tree for future 

inspection

to document tree defects, hazard tree situations

to record pertinent information regarding 

the hazard tree

Tools Used in Hazard Tree Evaluation Corrective Actions for Hazardous Trees

Numerous corrective actions are available for 

managers including moving the target, closure of 

recreation areas, pruning part of the tree, topping 

the tree, adding cabling or pole supports, and 

tree removal. 

Move the target

Moving the target to a new location or to a safe 

distance is a good option when the value of the 

tree is high. A thorough examination of trees in 

the target’s new location is then necessary.

Closing the recreation area

Temporary or permanent closure of a recreation area (or portion thereof) is another option, 

especially if the area is concentrated with severe 

hazard trees and another more suitable site can 

be found. Closing recreation areas 

during severe storms can be an effective means of reducing injuries to 

people and property. 

Prune part of the tree

When branches or multiple tops 

are the main form of failure potential, then pruning can be an effective method of action. Pruning can 

reduce failure potential and maintain 

the tree. If done correctly, pruning 

can improve the health of the tree. 

Improper pruning can lead to an architecture that has a high probability 

of failure.

Top the tree

Removing the top of the tree is the 

best option under some circumstancCareful tree and area assessment 

will help determine the appropriate corrective action for hazardous 

trees.
Prevention and Maintenance

38

Prevention and Maintenance

39

es. It may reduce the height of the tree to the 

point that the tree can no longer reach a target if 

the tree fails. Topping the tree can greatly reduce 

the weight high in the tree and lower the “sail 

area” of the crown impacted by wind. This may 

curtail the potential for failure. Topping a tree 

can produce a more natural appearing structure 

than a stump, particularly if the cut top has a jagged shape. Topped trees often have considerable 

wildlife use. Topped trees should be carefully 

evaluated into the future to monitor for increased 

failure potential because of the long-term effects 

of decay that enters in the cut surface. Trees 

topped below the live crown will die, but this 

may be acceptable if reducing the height mitigates the chance of hitting a target.

Cabling and pole supports

Using cable supports on leaning or defective 

trees or poles for supporting large lateral limbs 

is usually not applicable for large conifers and 

would be used only as a last resort.

Remove the tree

Tree removal may be necessary to adequately 

reduce hazard potential. Care should be taken 

during tree falling to avoid wounding neighboring trees. If the stumps are viewed as unsightly, 

they can be treated with a stump grinder which 

reduces their height to about ground level. Careful analysis of the hazard potential should be 

made before recommending tree removal; credibility is lost when a specialist suggests removal 

because of internal defect but the tree is found 

to be largely sound when it is felled. The use of 

an increment borer or drill in several locations of 

the tree’s bole is often the best method of determining how much sound wood exists in a tree.

Placement of New Recreation Sites

A thorough hazard tree evaluation should be 

conducted before new recreation sites are established. Some forest stands are better suited 

for the development of new recreation sites than 

others. The stage of forest development should 

be considered—younger forests generally have 

less decay and fewer tree defects than older forests. Also, vegetation can be manipulated more 

flexibly in some forest ecosystems or at some 

forest development stages. In the development 

of new recreation sites, potential targets can be 

established away from large valuable trees or 

in areas of forest stands that have relatively few 

defect problems.

Minimizing Future Hazard

• Educate visitors about the importance of 

not wounding trees in recreation areas.

• Minimize bole wounding and damage to 

roots systems during management activities such as tree removal.

• Maintain trees in a vigorous condition.

• Develop a vegetation plan that directs efforts towards maintaining and enhancing 

tree health.

Tree Failure Reporting

Using a reporting process on tree failure is an 

essential way to learn about the effectiveness of 

hazard evaluations. This involves documenting 

some details on how a tree failed, what defect 

factors were present, and how the tree had been 

evaluated during the most recent inspection.

If most trees that fail are in a high risk category 

then you know that the hazard evaluation is 

effective. If trees with low scores for failure 

potential are failing, then adjustments need to 
Prevention and Maintenance

40

Hazard Profiles of Alaskan Tree Species

C

h

a

p

t

e

r

41

be made in evaluating hazard. Thus, tree failure 

reporting can provide a useful check for a hazard 

tree program.

Reporting tree failure can also be an effective 

way to build information on the characteristics 

of each of our Alaska tree species that may 

make them hazardous. For example, we might 

determine that the failure rate is so high for large 

cottonwood trees that management plans should 

describe methods to discourage them and favor 

tree species with fewer problems. Or by another 

example, we might learn that scars on conifers 

are more stable and constitute a lower hazard 

than scars on hardwood trees.

Tree failure databases can be developed at the 

local level or managers can utilize the International Tree Failure Database (ITFD). This 

comprehensive database has an Internet based 

form for collecting important information about 

trees that have failed structurally. Reports can be 

generated from the database to reveal characteristics of trees that fail and improve predictions of 

future failures. Training is required to report tree 

failures in ITFD. The ITFD application can be 

accessed at http://svinetfc2.fs.fed.us/natfdb/.

Hazard Profiles of Alaskan Tree Species

Preliminary hazard profiles of Alaskan tree species have been developed. As more information 

becomes available through research or monitoring of failures at recreation sites, these profiles 

will be refined.

Distribution and Habitat

White spruce is widespread and the most common conifer across Southcentral and Interior 

Alaska. The tree generally occupies sites with 

well-drained, permafrost-free soils. Pure stands 

of white spruce are largely confined to stream 

bottoms, river floodplains and terraces, and 

warm south-facing upland sites.

Common Defects

White spruce generally has a modest level of 

defect caused by heart, butt, and root rot fungi 

that predispose the tree to failure. The heart rot 

fungus Phellinus pini, and several brown 

rot fungi cause most of the stem defect in 

white spruce. Bole breakage is the most 

common failure of trees with heart or butt 

rot.

Stem wounds and top breakage often lead 

to internal wood decay of white spruce, but 

many trees with decay lack such indicators. Conks and mushrooms, when present, 

indicate substantial decay. Spruce trees 

with stem wounds should be evaluated 

carefully with a drill or increment borer White spruce.
Hazard Profiles of Alaskan Tree Species

42

Hazard Profiles of Alaskan Tree Species

43

to ensure trees are not unnecessarily removed. The root rot fungus 

Inonotus tomentosus causes a slow 

decline and death of white spruce 

of all ages. This fungus structurally 

weakens the root system of infected 

trees, increasing the probability of 

failure from uprooting, bole breakage, or outright mortality.

Dense perennial witches’ brooms, caused by the 

rust fungus Chrysomyxa arctostaphyli, are common in white spruce. Rust brooms, unless very 

large or dead, are not generally hazardous.

Outbreaks of the spruce bark beetle, Dendroctonus rufipennis, have resulted in widespread 

mortality of white spruce across Southcentral 

Alaska. White spruce snags killed by bark 

beetles generally do not stand for long. The 

fungus Fomitopsis pinicola causes a great deal 

of the wood decomposition in dead trees. Dead 

spruce typically fail suddenly by snapping near 

the base.

General Failure Potential

Stem breakage and uprooting are the most 

common forms of failure in white spruce. Dead 

standing spruce snags deteriorate and will fail. 

Due to the unpredictability of timing or location of failure, dead trees are considered a high 

hazard in recreation areas when they occur near 

targets and should be promptly removed. White 

spruce trees with shallow root systems on poorly 

drained soils are not generally windfirm. Damage to the root system will increase the probability of root failure.

Uprooted spruce due to root disease.

The most common 

failure zone of a spruce 

bark beetle-killed tree is 

within the first four feet 

of the base.

Distribution and Habitat

Cottonwood is a fast growing tree found 

throughout Interior, Southcentral and 

Southeast Alaska, generally occurring 

along streams, river floodplains, and sandbars. 

Common Defects

Older cottonwood trees typically develop 

wide spreading crowns with large dead or 

rotten branches. Large, old cottonwoods are 

often defective. Open wounds and poorly 

healed branch stubs provide entrance courts 

for wood decay fungi. One fungus,

Ganoderma applanatum, causes the most 

defect in cottonwood.

General Failure Potential

The major hazard with this species is in the 

upper portion of the tree with top breakage 

and branch shedding of defective and dead 

limbs. Large limbs with weak unions or 

defective limbs should be removed to reduce this hazard. Older cottonwood stands 

usually contain many defective trees. Careful inspection of trees for conks or mushrooms of wood decay fungi is important. 

Use of a drill or increment borer may be 

necessary to determine the extent of decay 

in the stem and lower bole.

Cottonwood.

Breakage of tops and large 

limbs are the most common 

defects in cottonwoods.

Ganoderma applanatum

conk.

Eagle Eye Photo © Glenn M. Oliver
Hazard Profiles of Alaskan Tree Species

44

Hazard Profiles of Alaskan Tree Species

45

Distribution and Habitat

Quaking aspen occurs extensively 

throughout the Interior of Alaska, is 

common in Southcentral Alaska, and 

only occurs in the extreme northern part 

of Southeast Alaska near Haines and 

Skagway. Aspen is a fast growing tree 

common on slopes with a southern ex-

posure, in well-drained benches lacking 

permafrost, and creek bottoms.

Common Defects

Due to its thin bark, aspen is highly 

susceptible to trunk injuries, particularly 

in recreation areas. Wounds and poorly 

healed branch stubs often lead to infection by 

wood decay and canker-causing fungi on the 

stem. Perennial cankers gradually enlarge over 

time and may girdle the tree. The presence of 

conks on the trunk indicates extensive wood de-

cay. Armillaria root disease predisposes 

aspen to root failure. Most older aspen, 

>100 years old, have significant levels of 

wood decay and root disease.

General Failure Potential

Stem breakage and uprooting are the 

most common forms of failure for aspen 

trees. Cankers structurally weaken the 

tree if they are large or infected by decay 

fungi. Four canker fungi are common in 

Alaska; of these, the sooty bark canker 

Phellinus tremulae is the most 

common decay agent of aspen.

Aspen stand.

(caused by the fungus Encoela pruinosa) 

is considered the most lethal, capable of 

girdling and killing an aspen in 3–10 years. 

Stems that have been structurally weakened 

by wood decay fungi are prone to breakage. 

Aspen suspected to have wood decay may be 

bored to determine the decay extent.

Distribution and Habitat

Paper birch is common throughout Interior and 

Southcentral Alaska on rolling benchlands and 

lower foothill slopes. Paper birch de-

velops best on warm slopes with moist 

porous soils but is common on cold north 

slopes and poorly drained lowlands.

Common Defects

The thin fragile bark of paper birch 

is highly susceptible to injury. Trunk 

wounds, cracks, basal injuries, and bro-

ken branches provide entrance courts for 

wood decay and canker fungi. Both the 

false tinder conk (Phellinus igniarius), a 

stem decay fungus, and the cinder conk (Inonotus obliquus), a canker rot fungus, 

Paper birch.

Sooty bark canker is the most 

lethal canker of aspen.
Hazard Profiles of Alaskan Tree Species

46

Hazard Profiles of Alaskan Tree Species

47

are common in older birch stands. Incidence of root and stem decay increases 

with tree age; older stands (>90 years) 

typically have high levels of defect. The 

presence of conks or mushrooms indicates 

extensive decay. Root rot fungi, such as 

Armillaria spp., are particularly common 

in older birch trees and trees that are under 

stress. Older birch stands that have been 

heavily thinned or recently opened up may 

suffer from a progressive dieback or decline of twigs and branches from the upper 

crown downwards. 

General Failure Potential

Stem and branch breakage are the most 

common types of failure of birch trees. 

Bole breakage is typically associated with 

large stem cankers and structural weakening 

of the stem and root system from wood decay 

fungi. Trees are also easily fractured at the site 

of canker rot infections. Decay extent can be 

determined by boring trees, however, boring 

should be done only when necessary, as these 

practices create wounds which may serve as infection courts for canker and decay fungi. Root 

diseases structurally weaken the root system 

of infected trees, increasing the probability of 

failure. Cracks at the root collar of birch indicate 

a potential for breakage. Death and breakage of 

branches and tops are common in older trees, 

often resulting in lodged or hanging branches 

with a high failure potential.

Cinder conk, Inonotus 

obliquus, a common canker 

rot fungus of birch.

Distribution and Habitat

Sitka spruce is a large tree that grows in coastal 

Alaska from the southern portion of southeast Alaska to Kodiak and the adjacent Alaska 

Peninsula. On favorable sites, Sitka spruce is 

fast growing and can reach impressive sizes. It 

is particularly common along beaches and rivers where many developed recreation sites are 

located.

Common Defects

Sitka spruce is less defective than western hemlock. Most of the internal wood decay of live 

spruce is brown rot. One such fungus, Phaeolus schweinitzii, causes root and butt rot that 

can predispose spruce to failure in recreation 

areas.

Much of the internal wood decay in Sitka 

spruce is associated with bole wounds and top 

breakage. Bole wounds are common on campground trees as people seem preoccupied with 

injuring trees with hatchets and axes. Wood 

decay in bole wounds progresses slowly 

in Sitka spruce; it often does not penetrate 

deeply into the bole. Spruce trees with bole 

wounds should be evaluated carefully with a 

drill or increment borer to ensure trees are not 

removed unnecessarily.

Although not particularly defective as a live 

tree, the wood of Sitka spruce is not decay 

resistant. Snags do not stand for long. The 

Sitka spruce.

Susan McDougall © USDA—NRCS PLANTS Database
Hazard Profiles of Alaskan Tree Species

48

Hazard Profiles of Alaskan Tree Species

49

fungus Fomitopsis pinicola causes a great deal 

of the wood decomposition in dead trees. Dead 

Sitka spruce trees may fall piece by piece as 

the top and branches deteriorate, or fail near the 

base. Dead branches can be large and are shed 

from live trees as a normal part of shade pruning. Dead hanging branches can be a hazard if 

targets are located underneath these trees.

General Failure Potential

Stem breakage, branch falling, and windthrow 

are the most common forms of failure in Sitka 

spruce. Dead standing trees deteriorate and will 

fail. Sitka spruce are susceptible to windthrow, 

particularly in wind-exposed locations (southfacing slopes), or when their roots are weakened.

Western hemlock. Mountain hemlock. Charles Webber © California Academy of Sciences

Eagle Eye Photo © Glenn M. Oliver

Distribution and Habitat

Western hemlock is a large tree that grows in 

coastal Alaska from the southern portion of 

Southeast Alaska to Prince William Sound. The 

smaller mountain hemlock tree occurs at higher 

elevations or on poor soils in coastal Alaska and 

on the Kenai Peninsula. Western hemlock is 

exceedingly common in Southeast Alaska where 

it is the predominant tree species.

Common Defects

Both western and mountain hemlock have high levels of heart rot 

especially in older trees. These 

decays are caused by a large number of fungi for western hemlock; 

whereas two fungi (Phellinus pini

and Echinodontium tinctorium) 

cause most of the defect in mountain hemlock.

Most internal wood decay in western hemlock is 

associated with bole wounds and top breakage. 

Wood decay in bole wounds progresses rapidly 

in hemlock compared to other tree species. 

Along with wounding, defect levels are highly 

correlated with tree age. Unwounded western 

hemlocks have little internal wood decay in trees 

up to a hundred years old. The percentage of 

trees with decay increases rapidly with tree age 

to 50% in 300-year-old trees, and essentially all 

trees over 500 years old are infected.

The wood of hemlock is not decay resistant and 

snags do not stand for long. Dead hemlock trees 

may fall piece by piece as the top and branches 

deteriorate, or fail suddenly by snapping near 

the base.

Phellinus pini conks.
Hazard Profiles of Alaskan Tree Species

50

Hazard Profiles of Alaskan Tree Species

51

General Failure Potential

Stem breakage and windthrow are the most 

common forms of failure in both western and 

mountain hemlock. Large old trees can be 

assumed to have at least some internal wood 

decay. Exposed wounds, top abnormalities, 

and bole seams (cracks) can be reliable indicators of wood decay. Dwarf mistletoe brooms 

break out of canopies and fall to the forest floor. 

Dead standing trees deteriorate and fail, either 

in pieces or suddenly through bole breakage. 

Dead standing hemlock, especially those dead 

for more than a few years (e.g., fine twigs gone, 

bark slaughing off) have a greater risk of failure 

in the near future than other coastal forest trees.

Distribution and Habitat

Two species of cedar grow in 

coastal Alaska. Yellow-cedar 

occurs throughout Southeast 

Alaska and in small populations 

in the Prince William Sound. 

Western redcedar is restricted to 

Southeast Alaska south of Petersburg. Generally, neither tree 

is as common as Sitka spruce or 

western hemlock in developed 

recreation areas.

Yellow-cedar.

Western

redcedar.

Susan McDougall © USDA—NRCS PLANTS Database

Common Defects

Both cedar species can have high levels of 

internal wood decay. Western redcedar generally 

has higher rates of heart rot, particularly in older 

trees. Most of the causal fungi do not produce 

large fruiting bodies, however. Therefore, much 

of the wood decay of live trees is hidden and not 

easily seen. The dead spike tops that are so common on western redcedar trees are not reliable 

indicators of decay.

Both cedars have thin bark and are easily wounded. Wounds may be infected 

by decay fungi, but these tree species 

are adept at compartmentalizing, or 

partitioning, wood decay.

The heartwood of both cedars is protected by natural fungicides and is decay resistant. This allows dead trees to persist 

standing for long periods of time, even 

up to a century. Enough wood decay finally develops near the ground line that 

old snags then break.

General Failure Potential

Cedars can succumb to windthrow when they 

have shallow roots where they grow in wet soils 

or when their roots are compromised by physical 

damage. Stem breakage is common in trees that 

have extensive heart rot. The extreme decay resistance of the wood in both cedar species allows 

dead standing trees to persist without failure for 

much longer than other tree species.

Alaska yellow-cedar snags.
Hazard Profiles of Alaskan Tree Species

52

Hazard Profiles of Alaskan Tree Species

53

This overview of diseases and decays of Alaskan trees is adapted from 

the book Insects and Diseases of Alaskan Forests. We recommend 

using this reference book for identification of, and learning more about, 

the disease agents that affect the hazard profiles of Alaskan trees.

Aspen, Quaking (Populus tremuloides)

 Stem and Branch Diseases

 Cytospora canker (Cytospora chrysosperma)

 Sooty bark canker (Encoelia pruinosa)

 Ceratocystis canker (Ceratocystis fimbriata)

 Cryptosphaeria canker (Cryptosphaeria lignyota)

 Root Diseases

 Armillaria root disease (Armillaria spp.)

 Wood Decays

 Artist’s conk (Ganoderma applanatum)

 Yellow cap mushroom (Pholiota spp.)

 Oyster mushroom (Pleurotus ostreatus)

 False tinder conk (Phellinus tremulae) 

Birch, Paper (Betula papyrifera)

 Stem and Branch Diseases

 Nectria canker (Nectria galligena)

 Root Diseases

 Armillaria root disease (Armillaria spp.)

 Wood Decays

 Red belt fungus (Fomitopsis pinicola)

 Artist’s conk (Ganoderma applanatum)

 Yellow cap mushroom (Pholiota spp.)

 Oyster mushroom (Pleurotus ostreatus)

 Tinder conk (Fomes fomentarius)

 False tinder conk (Phellinus igniarius)

 Cinder conk (Inonotus obliquus)

 Birch conk (Piptoporus betulinus)

 Rainbow conk (Trametes versicolor)

Cedar, Yellow- (Chamaecyparis nootkatensis)

 Root Diseases

 Armillaria root disease (Armillaria spp.)

 Decline Syndrome

 Yellow-cedar decline

Cedar, Western Red (Thuja plicata)

 Root Diseases

 Yellow ring rot (Phellinus weirii)

 Wood Decays

 Red ring rot (Phellinus pini)

 Red belt fungus (Fomitopsis pinicola)

 Redcedar white ring rot (Ceriporiopsis rivulosa)

Cottonwood, Black (Populus trichocarpa)

 Stem and Branch Diseases

 Cytospora canker (Cytospora chrysosperma)

 Cryptosphaeria canker (Cryptosphaeria lignyota)

 Root Diseases

 Armillaria root disease (Armillaria spp.)

 Wood Decays

 Artist’s conk (Ganoderma applanatum)

 Yellow cap mushroom (Pholiota spp.)

 Oyster mushroom (Pleurotus ostreatus)

 False tinder conk (Phellinus igniarius)

 Cinder conk (Inonotus obliquus)

 Rainbow conk (Trametes versicolor)
Hazard Profiles of Alaskan Tree Species

54

Hazard Profiles of Alaskan Tree Species

55

Hemlock, Mountain (Tsuga mertensiana)

 Root Diseases

 Armillaria root disease (Armillaria spp.)

 Wood Decays

 Red ring rot (Phellinus pini)

 Red belt fungus (Fomitopsis pinicola)

 Chicken of the woods (Laetiporus sulphureus)

 Indian paint fungus (Echinodontium tinctorium)

 Hartig’s conk (Phellinus hartigii)

 Borealis conk (Climacocystis borealis)

 Coniophora brown rot (Coniophora spp.)

 

Hemlock, Western (Tsuga heterophylla)

 Stem and Branch Diseases

 Hemlock cankers (Xenomeris abietis, Discocania treleasii)

 Root Diseases

 Annosus root and butt rot (Heterobasidion annosum)

 Armillaria root disease (Armillaria spp.)

 Wood Decays

 Red ring rot (Phellinus pini)

 Red belt fungus (Fomitopsis pinicola)

 Chicken of the woods (Laetiporus sulphureus)

 Velvet top fungus (Phaeolus schweinitzii)

 Artist’s conk (Ganoderma applanatum)

 Lacquer conk (Ganoderma tsugae)

 Quinine conk (Fomitopsis officinalis)

 Hartig’s conk (Phellinus hartigii)

 Purple conk (Trichaptum abietinum)

 Yellow cap mushroom (Pholiota spp.)

 Coniophora brown rot (Coniophora spp.)

 Coral fungus (Hericium abietis)

 

Poplar, Balsam (Populus balsamifera)

 Stem and Branch Diseases

 Cytospora canker (Cytospora chrysosperma)

 Sooty bark canker (Encoelia pruinosa)

 Cryptosphaeria canker (Cryptosphaeria lignyota)

 Wood Decays

 Artist’s conk (Ganoderma applanatum)

 Root Diseases

 Armillaria root disease (Armillaria spp.)

Spruce, Sitka (Picea sitchensis)

 Root Diseases

 Annosus root and butt rot (Heterobasidium annosum)

 Armillaria root disease (Armillaria sp.)

 Tomentosus root rot (Inonotus tomentosus)

 Wood Decays

 Red ring rot (Phellinus pini)

 Red belt fungus (Fomitopsis pinicola)

 Chicken of the woods (Laetiporus sulphureus)

 Velvet top fungus (Phaeolus schweinitzii)

 Artist’s conk (Ganoderma applanatum)

 Quinine conk (Fomitopsis officinalis)

 Borealis conk (Climacocystis borealis)

 Purple conk (Trichaptum abietinum)

 Coniophora brown rot (Coniophora spp.)

 Coral fungus (Hericium abietis)
Hazard Profiles of Alaskan Tree Species

56

Backcountr y Travel and Hazard Trees

C

h

a

p

t

e

r

57

Spruce, White (Picea glauca)

 Root Diseases

 Armillaria root disease (Armillaria sp.)

 Tomentosus root rot (Inonotus tomentosus)

 Wood Decays

 Pini conk (Phellinus pini)

 Pinicola conk (Fomitopsis pinicola)

 Velvet top fungus (Phaeolus schweinitzii)

 Artist’s conk (Ganoderma applanatum)

 Quinine conk (Fomitopsis officinalis)

 Purple conk (Trichaptum abietinum)

 Coniophora brown rot (Coniophora spp.)

Recreationists may 

encounter a wide 

variety of standing or 

elevated tree hazards.

Alaska is known as one of the world’s premier 

destinations for backcountry adventure. But with 

adventure comes the risk inherent in backcountry travel. Risks posed by hazard trees in remote 

areas are often overlooked or underestimated. 

Backcountry travelers assume responsibility for 

their safety and need to learn to identify tree 

hazards in settings where hazard tree evaluations and treatments are not possible. 

Managers can use the concepts in 

this chapter to advise visitors of risks 

associated with camping, backcountry travel, and hazard trees.

Trees are the dominant feature of 

forested ecosystems. Every tree will 

ultimately die, decay, and be recycled into the ecosystem to provide 

nutrients for future forests. While 

these processes are natural, they can 

pose a threat to backcountry travelers.

Land management agencies cannot remove all 

hazard trees in the vast public lands in Alaska. 

Typically they only attempt to inspect and treat 

hazard trees in developed areas with high public 

use. Forest visitors, therefore, need to recognize 

the dangers of hazard trees and take precautions, 

especially in backcountry settings. 
Backcountry Travel and Hazard Trees

58

Backcountry Travel and Hazard Trees

59

Backcountry travelers engage in 

a variety of activities that may 

be influenced by hazard trees 

including hiking, picnicking 

and camping around trees. The 

exposure time to hazard trees 

varies based on the amount of 

time visitors are in one location. 

Hikers spend relatively little 

time in one location, thus their 

exposure to hazard trees is limited. Picnickers have a moderate exposure time to hazard 

trees, typically spending one to several hours 

in a location. Campers spend the longest time 

in a single location, thus they have the highest 

potential to encounter a hazard tree. 

One of the most important decisions that backcountry travelers can make is where to camp 

for the night. Avoiding potentially dangerous 

situations around hazard trees takes knowledge, 

awareness and good judgment. 

Suggested messages from managers to visitors:

What you can do to reduce risk

Be observant! Examine trees in your camping or rest area for evidence 

of hazard or failure potential. Take action by moving yourself and your 

belongings to a safe area if you suspect a hazard. 

1. Avoid all standing dead trees, hazardous trees, and the danger 

zone in which they may fail. Never picnic or camp within at least 

 one tree length of a dead tree. It could fall at any time

 without warning. Make a thorough assessment of

 the potential camping area to determine if it is a

 safe location to spend the night.

 2. Never underestimate the danger posed by small trees

 and dead limbs. Both live and dead tree parts 6" in cross-

 section have fallen and hurt hikers and campers. A small

 tent offers little protection from falling trees and limbs.

3. Avoid trees with excessive lean (10° or greater) or

 evidence of defect. 

4. Be especially cautious of hazard trees in 

strong winds. Trees or tree parts may dislodge 

in strong winds. Select a safe place to spend 

the night. 

5. Avoid chopping or banging into dead trees. 

The entire tree, top, or branches may break 

out and fall on you. 

Recognizing a Hazard Tree

First look up! Many hazards occur above our heads. Be sure to inspect 

trees carefully and systematically on all sides. Examine all parts of the 

tree, including roots, root or trunk flare, main 

stem, branches and branch unions. Some of the 

most common “watch out” hazard tree situations in the backcountry include: 

1. Dead trees are unpredictable, very dangerous, and can break or fall at any time. 

Dead wood is often dry and brittle and 

cannot bend in the wind like a living tree. 

 

Dead trees.
Backcountry Travel and Hazard Trees

60

2. Hanging branches and tree tops that have 

already broken off are especially dangerous. These defects are called “hangers” or 

“widow-makers.” Inspect the tops of trees 

carefully for these defects.

3. Signs of disease such 

as mushrooms or conks 

(fungal fruiting bodies) 

may occur on roots, stems 

and branches. These are 

evidence of advanced 

decay and weakened tree 

structure. Avoid trees with 

signs of disease.

4. Leaning trees at greater 

than 10° from vertical 

should be inspected carefully. A freshly leaning 

tree with recent root 

lifting, soil movement or 

mounding near the base 

indicates a high failure 

potential.

Broken branches.

Signs of disease.

Trees leaning more

than 10°.

The three basics for safety 

around hazard trees in the 

backcountry are knowledge, 

awareness and good judgment.

Acknowledgments

In compiling ideas, text, and pictures for this book, considerable use 

was made of multiple sources. The book Long Range Planning for 

Developed Recreation Sites in the Pacific Northwest was invaluable. 

We thank the authors, Bob Harvey and Paul Hessburg, for considerably 

advancing the thought process on this topic.

The book How to Detect, Assess and Correct Hazard Trees in Recreational Areas was also extensively used. We thank the State of Minnesota, Department of Natural Resources for allowing adaptation of 

failure potential guidelines and utilization of various line drawings and 

photographs.

We are especially indebted to David Allen, graphic designer with the 

Chugach Design Group, Chugach National Forest, for his creative and 

insightful design of this publication. We thank Jim Worrall and Marcus 

Jackson for their careful review of this book.

We thank the California Academy of Sciences for permission to use the 

western hemlock photograph on page 48. We thank Susan McDougall 

@ USDA—NRCS PLANTS Database for the Sitka spruce photograph 

on page 47 and the western redcedar photograph on page 50. We thank 

Eagle Eye Photo © Glenn M. Oliver for the mountain hemlock photo 

on page 48 and the cottonwood photo on page 43. All other photos 

were taken by Forest Health Protection Staff, Region 10, Alaska.

References

Albers, J.; Hayes, E. 1993. How to detect, assess and correct hazard 

trees in recreational areas. State of Minnesota, Department of Natural 

Resources- Forestry. 63p.

Farr, W.A.; LaBau, V.J.; Laurent, T.H. 1976. Estimation of decay in 

old-growth western hemlock and Sitka spruce in Southeast Alaska. 

USDA Forest Service Research Paper PNW-204. Pacific Northwest 

Forest and Range Experiment Station. 24p.

61
Harvey, R.D.; Hessburg, P.F. 1992. Long range planning for developed recreation sites in the Pacific Northwest. USDA Forest Service, 

Portland, OR. FPM-TP 039-92. 120p.

Holsten E.; Hennon, P.; Trummer, L.; Kruse, J.; Schultz, M.;

Lundquist, J. 2008. Insects and Diseases of Alaskan Forests. USDA 

Forest Service, Alaska Region, Technical Report, R10–TP–140. 245 p.

Johnson, D.W. 1981. Tree hazards: recognition and reduction in recreation areas. USDA Forest Service, Rocky Mountain Region. Lakewood, CO. Technical Report R2-1. 17p.

Kimmey, J.W. 1956. Cull factors for Sitka spruce, western hemlock, 

and western red cedar in Southeast Alaska. Alaska Forest Research 

Center, Station Paper No. 6. USDA Forest Service. 31p.

Paine, L.A. 1971. Accident hazard evaluation and control decisions 

on forest recreation sites. USDA Forest Service, Pacific Southwest 

Forest and Range Experiment Station, Berkeley, CA. Research Paper 

PSW-68. 10p.

Wagener, W.W. 1963. Judging hazard from native trees in California 

recreation areas; a guide for professional foresters. USDA Forest 

Service Research paper PSW-P1. 29p.

For additional references regarding hazard trees, the website 

http://www.fs.fed.us/r10/spf/fhp/hazard/index.htm

has an extensive alphabetical listing of hazard tree references compiled 

by Mike Schomaker and Bob Mathiasen.

62

The U.S. Department of Agriculture (USDA) prohibits discrimination 

in all its programs and activities on the basis of race, color, national 

origin, age, disability, and where applicable, sex, marital status, 

familial status, parental status, religion, sexual orientation, genetic 

information, political beliefs, reprisal, or because all or part of an 

individual’s income is derived from any public assistance program. 

(Not all prohibited bases apply to all programs.) Persons with 

disabilities who require alternative means for communication of 

program information (Braille, large print, audiotape, etc.) should 

contact USDA’s TARGET Center at (202) 720–2600 (voice and 

TDD). To file a complaint of discrimination, write to USDA, Director, 

Office of Civil Rights, 1400 Independence Avenue, S.W., Washington, 

DC 20250–9410, or call (800) 795–3272 (voice) or (202) 720–6382 

(TDD). USDA is an equal opportunity provider and employer.