Appendix 5 Hazard-tree-field-guide.pdf

Field Guide for Tree Risk Assessments and Hazard Tree Mitigation on Developed Recreation Sites, Worksites, and Road Systems in Oregon and Washington Forests

FS-1273 | December 2025

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Field Guide for Tree Risk Assessments and Hazard Tree Mitigation on Developed Recreation Sites, Worksites, and Road Systems in Oregon and Washington Forests

Editors: Kristen L. Chadwick, Betsy A. Goodrich, and Holly S.J. Kearns

Authors (alphabetical order):

Josh Bronson, Plant Pathologist, U.S.

Department of Agriculture (USDA), Forest Service, Medford, OR

Rachel Brooks, Forest Pathologist, Washington Department of Natural Resources, Olympia, WA

Kristen L. Chadwick, Plant Pathologist, USDA Forest Service, Sandy, OR

Brennan Ferguson, Plant Pathologist, USDA Forest Service, Wenatchee, WA

Betsy A. Goodrich, Plant Pathologist, USDA Forest Service, Wenatchee, WA

Yiqiang (Kevin) Gu, Assistant Regional Transportation Engineer, USDA Forest Service, Portland, OR

Dave Hays, Regional Developed Recreation Program Manager (Retired), USDA Forest Service, Klamath Falls, OR

Holly S.J. Kearns, Plant Pathologist, USDA Forest Service, Sandy, OR

I. Blakey Lockman, Regional Plant Pathologist, USDA Forest Service (Retired), Portland, OR

Brent Oblinger, Plant Pathologist, USDA Forest Service, Bend, OR

Dan Omdal, Forest Pathologist, Washington Department of Natural Resources, Olympia, WA

Aaron Pedersen, Regional Saw Program Manager, USDA Forest Service, Portland, OR

Cameron Stauder, Geneticist, USDA Forest Service, Bend, OR

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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, 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 State or local Agency that administers the program or contact USDA through the Telecommunications Relay Service at 711 (voice and TTY). Additionally, program information may be made available in languages other than English.

To file a program discrimination complaint, complete the USDA Program Discrimination Complaint Form, AD-3027, found online at How to File a Program Discrimination Complaint 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, Mail Stop 9410, 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.

Neither the U.S. Department of Agriculture, Forest Service nor the authors accept any responsibility, explicit or implied, for liability, loss, or damage arising from the manner in which the materials presented in this guide are used.

All photographs and figures in this guide were taken or designed by the authors or other USDA Forest Service personnel unless otherwise stated.

Citation: Chadwick, K.L.; Goodrich, B.A.; Kearns, H.S.J., eds. 2025. Field guide for tree risk assessments and hazard tree mitigation on developed recreation sites, worksites, and road systems in Oregon and Washington Forests. FS-1273. Washington, DC: U.S.

Department of Agriculture, Forest Service. 204 p.

Cover photo: A Douglas-fir in a campground that failed from laminated root rot.

Title page photo: A mitigated hazard tree with extensive internal decay and a conk of Phaeolus schweinitzii.

https://www.usda.gov/oascr/how-to-file-a-program-discrimination-complaint https://www.usda.gov/oascr/how-to-file-a-program-discrimination-complaint mailto:program.intake%40usda.gov?subject=

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Dr. Greg Filip sharing his hazard tree expertise.

The editors dedicate this guide to Gregory Filip for his leadership in the field of hazard tree management.

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This field guide outlines the steps for a tree risk assessment program. Some information, particularly in chapter 4, is specific to forests in Oregon and Washington; the appendixes also contain tree and pathogen profiles for species and diseases common to that region. The remainder of the guide—including the prioritization process for determining where and when to conduct tree risk assessments, two tiers of surveys, identifying potential failure zone and impact potential, methods for documenting survey results, recommended mitigation options, and guidance on large-scale disturbances—can be applied elsewhere. These recommendations are based on the best available science, previous field guides, and the collective expertise and experience of the authors.

When adopting any part of the tree risk assessment program outlined within this guide, use locally or regionally specific tree defects and associated failure potentials.

Acknowledgments The authors thank Gregory Filip for leading the effort to write the previous guides for hazard trees (Filip et al. 2013) and danger trees (Filip et al. 2016) for Oregon and Washington which served as the foundation for this guide. We also thank Bob Harvey and Paul Hessburg for the 1992 guide that served as a template to the Filip guides and provided some content for this guidebook. The authors thank Craig Leech, Alan Kanaskie, and Paul Ries for developing a tiered survey and training system for Oregon State Parks that inspired parts of this guide.

Special thanks go to Nicole Lagioia, Rithy Bein, Barbara Garcia, Ken Kitrell, and Lisa Ball for their insightful contributions in developing this field guide and the program it outlines. We would like to thank the following people for their feedback and review of this guide relevant to their program or specialty: Andrea Lyons, Joe Doerr, Barbara Webb, and Aiden Forsi.

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Contents

Assessment”?

Assessments 7 Step 1—Develop

Programs of Work

and Recommendations

Program of Work

Type and Frequency

Considerations for Dead Trees, and Maximum Risk Rating

20 Field Supplies

Potential Failure Zone

Impact Potential Rating

Failure Potential 36 Systematic Tree

Examinations in Surveys

Failure Potential

Synergistic Effects 41 Leaning, Root-Sprung, or Hung-Up Trees 43 Exposed, Undermined, or

Severed Root Systems

Limbs or Tops

Mistletoe Cankers, or Other Fungal Cankers

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50 Visible Evidence of Decay and Fungal Conks

Tier 1 Basic Surveys 57 Rating Tree Defects for Tier

2 Advanced Surveys 57 Sound Rind Thickness and

Determining the Extent of Decay

for Dead Trees

Rating and Mitigation Options

Determine Mitigation Priorities 82 Mitigation Options for

Tree Risk Reduction

Considerations

Established Variables

Site Development

Large-Scale Disturbances 103 An Approach to Large-Scale

Disturbance Events

the Priorities, and Define Mitigation Options

105 Select a Failure Zone Distance and Delineate the Survey Boundaries

106 Identify the Failure Potentials Requiring Mitigation

106 Additional Considerations in Selecting Maximum Acceptable Failure Potentials

107 Writing the Prescription for Implementation

Identification

Rot (White Mottled Rot) 122 Heterobasidion Root Disease and Butt/Stem Decay 125 Laminated Root Rot and

Cedar Laminated Root Rot

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Rot of Hardwoods

169 Ramorum Canker and Sudden Oak Death

Species and Groups in Oregon and Washington Forests

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Tables 15 Table 1.1—Road maintenance levels used by the Forest Service

31 Table 3.1—Impact potential rating examples for developed recreation sites, infrastructure, and administrative facilities, including roads within these sites

32 Table 3.2—Impact potential rating examples for roads and worksites outside of developed recreation and administrative sites

55 Table 4.1—Tier 1 basic survey failure potential (FP) ratings based on defects

63 Table 4.2—Tier 2 advanced survey failure potential (FP) ratings based on defects

70 Table 4.3—Thresholds for failure potential (FP) ratings based on diameter inside bark (DIB) and average sound rind for trees without open wounds

71 Table 4.4—Thresholds for failure potential (FP) ratings based on diameter inside bark (DIB) and average sound rind for trees with open wounds

74 Table 4.5—Alternative failure potential ratings for dead trees

136 Table A1.1—Frequency of occurrence by host species of root and butt diseases in Oregon and Washington

179 Table A2.1—Resinous and nonresinous tree species groups in Oregon and Washington forests

180 Table A2.2—Distribution of failures by position of defect and tree species in Pacific Northwest recreation sites

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Introduction Trees contribute to the beauty, enjoyment, and ecosystem functions of recreation sites, forest scenic routes, and viewpoints. Exposure to pathogens, insects, weather, fire, and a changing climate all influence forest conditions;

however, these same factors can cause structural instability, damage, and mortality of forest trees. Structurally unstable trees or tree parts may fail and cause injury to people or damage to property if located within developed sites, along roadways, or near active worksites.

Recreating and working in forested areas exposes people and property to natural forest conditions that can potentially cause harm. This field guide provides a comprehensive methodology to assess the potential risks that trees can pose and recommends risk mitigation options to reduce the hazards with an emphasis on native forest trees in Oregon and Washington.

This field guide presents a tree risk assessment program that minimizes risk from potential tree failures. It is intended for forest recreation and resource managers, concessionaires, consultants, road maintenance engineers, incident management teams, and other specialists who assist in the management of developed sites, roads, and worksites located within forested areas. It is important to note that parts of this field guide are informed by the International Society of Arboriculture’s (ISA) “Tree Risk Assessment Manual” (Dunster et al.

2017). Professional, qualified arborists use a similar risk rating system developed by ISA to evaluate tree risk in areas that are generally more urban and developed than natural forests. Professional arborists may not customarily work in natural forest settings and may not be as familiar with the diseases and natural processes that occur there.

This field guide integrates and updates the “Field Guide for Hazard-Tree Identification and Mitigation on Developed Sites in Oregon and Washington Forests” (Filip et al. 2013) and the “Field Guide for Danger-Tree Identification and Response along Forest Roads and Work Sites in Oregon and Washington” (Filip et al. 2016), which were based on the 1992 guide “Long-Range Planning for Developed Sites in the Pacific Northwest: The Context of Hazard Tree Management” by Harvey and Hessburg (1992).

Campground and swimming area with dead lodgepole pines killed by mountain pine beetles.

This updated edition introduces a prioritization process to identify where tree risk assessments should be conducted, presents two tiers of survey options, details the steps for performing tree risk assessments, describes methods for documenting survey results, and offers recommendations for mitigation strategies. The objectives of this field guide are to present:

should be surveyed.

• A prioritization system for determining when to survey and treat road systems, developed sites, administrative facilities, and worksites.

• A field aid for accurate identification of diseases, defects, and potential tree failure.

beetle outbreaks.

• Considerations for long-term vegetation management plans for developed sites.

What Is a “Tree Risk Assessment”?

A tree risk assessment evaluates both the potential for failure by examining a tree’s structural defects and the potential for impact by considering the amount of risk that tree failure may pose to people or property. Identification of a hazard does not necessarily mean there is a risk; it must threaten a specific target for a hazardous condition to exist. Risk arises when a tree or tree part has an impact potential and is within striking distance of people or property. Risk increases with the severity of tree defect (i.e., failure potential) and the potential for impact (i.e., impact potential) due to increased exposure or value of the target.

Managers take action after they decide to reduce risk, especially when they deem the hazards unacceptable.

Tree risk assessments require a unique skill set that combines scientific knowledge, keen observational skills, risk management, and a thorough documentation process. A working knowledge of tree species and forest pathology, regular monitoring, and an intimate knowledge of local site conditions, coupled with a thorough, consistent, and scientifically based evaluation process, all contribute to a high-quality tree risk assessment process and hazard tree management program.

Definitions For the purposes of this field guide the following definitions apply. The glossary also defines these and other terms.

Designated site—An agency-identified location where the public is directed to go, which may or may not be developed with facilities. Examples include designated dispersed camping areas and designated backcountry or wilderness campsites.

Developed site—A place that concentrates use and has facilities provided for visitors or employees. The term “facilities” may be used in either a broad or narrow context (e.g., administrative facilities, telecommunication sites, ranger stations, and recreation sites such as campgrounds, day-use sites, trailheads, and boat launches).

Exposure—The state of being vulnerable to damage or harm, regardless of outcome, by virtue of being in proximity to a potentially hazardous tree.

The duration and frequency of exposure are used in determining the impact potential.

Failure potential—The likelihood of a tree or its parts breaking, falling, or collapsing. (Ratings are described in chapter 4.)

Failure zone—The area within which a tree or its parts will likely land in the event of failure.

Forest road—A transportation facility intended to support motor vehicle traffic wholly or partly within or adjacent to and serving public lands with jurisdiction by a federal or local government entity that is necessary for the protection, administration, and utilization of public lands and the use and development of its resources.

Hazard tree—A tree or its parts that pose a risk of injury or damage to people or property and exceeds the risk tolerance of the responsible manager. Hazard trees are sometimes referred to as “danger trees” in policy, Occupational Safety and Health Administration (OSHA) documents, and other field guides. For the purposes of this document, the two terms are interchangeable.

Hazard tree management—The reduction of risk posed by hazard trees with a program that includes prioritization, assessments, documentation, monitoring, and mitigation, while balancing risk with the benefits trees provide.

Impact potential—The likelihood that a tree or tree part could strike a target and the resulting damage that may occur. Impact potential is determined by evaluating both the level of exposure and the severity of possible damage or loss (consequences). (Ratings are described in chapter 3.)

Mitigation—The action taken to reduce risk of damage or injury, such as closing sites, closing roads, moving targets, removing the defective tree or parts, etc.

Occupancy—The frequency that a site is used by people for the intended or managed purpose.

Risk—The probability that harm or loss will occur if exposed to a hazard. In the context of hazard trees, risk is the combination of the probability of tree failure (failure potential) and the level of exposure and the severity of possible damage or loss (impact potential).

Target—People, property, or infrastructure that could be injured or damaged by failure of a tree or its parts.

Tree risk assessment—A systematic process used to identify and evaluate the threat a tree may pose to a given target.

Worksite—An area in the forest where work is actively occurring, generally temporary in nature, infrequent, and related to activities such as road construction, logging, planting, surveys, or infrastructure repair and maintenance.

Hazard Tree Management Land managers and employers have responsibilities defined by their respective agencies, as well as Federal and State policies, to assess and reduce risks to visitors and employees. Appropriate agency policies should be followed while developing a tree risk assessment program.

Hazard tree management should focus primarily on providing safe access and use by reducing the risk of injury to people and damage to property. A secondary focus should be to enhance the long-term health of forest stands and the ecosystem services they provide. The key here is to strike a balance between minimizing risks in developed areas while maintaining a forest structure that provides an aesthetically pleasing user experience with the need to maintain diverse forest stands and resilient ecosystems that benefit the natural environment. Vegetation management planning (chapter 7) provides strategies to meet these long-term goals and desired future conditions.

However, even under the best of circumstances, and with the highest standard of care, tree failure predictions are imperfect. A manager’s ability to predict tree failure is limited, and even more so when trying to predict the timing of failure. Additionally, in unusual situations, such as extreme weather events, it is possible for trees without significant defects to fail. In general, it is impossible to manage for all situations in which trees or their parts may fail. However, by using a systematic approach it is possible to effectively and efficiently use limited agency resources to significantly reduce the risk of injury to people and damage to property (fig. 1).

Figure 1—Proportion of trees in different risk categories before and after mitigation.

Six Steps for Tree Risk Assessments A tree risk assessment program involves a systematic approach that includes a prioritization process for surveys and mitigation, assessing the likelihood of a tree or its parts striking a target, evaluating the exposure and potential for damage associated with specific targets, inspecting and rating a tree’s structural defects and determining its failure potential, determining the risk rating of the tree, and documenting the risk assessment. This field guide incorporates these elements into six steps, outlined below and detailed in chapters 1–6.

Step 1—Develop the program of work by prioritizing where to survey and determining what type of survey to conduct—Office exercise (chapter 1)

Step 2—Determine potential failure zone of trees or tree parts (chapter 2)

Step 3—Determine the type of exposure and assign impact potential (chapter 3)

Step 4—Determine tree defects and assign a failure potential; reevaluate failure zone if defect is just part of a tree (chapter 4)

Step 5—Determine the tree’s risk rating and mitigation options (chapter 5)

Step 6—Document the risk assessment and provide recommendations to the manager (chapter 6)

Step 1—Develop Programs of Work A successful tree risk assessment program requires a systematic approach for prioritizing areas where tree risk assessments will be conducted and for determining the appropriate survey to conduct: a Tier 1 or Tier 2 survey (defined in chapter 1). As it is rarely possible to conduct comprehensive tree risk assessments across a broad landscape due to limited time and resources, the first step includes an office exercise with resource managers and tree risk assessors.

When prioritizing where to complete tree risk assessments, decision makers should consider workloads, budgets, values at risk, visitation, use, seasonality of use, duration of exposure, site history (including history of tree failures), specific local hazards, and timing of possible mitigation activities. These factors will determine the type and frequency of tree risk assessments. Coordinate with other specialists, such as wildlife and fisheries biologists, especially in areas with known sensitive species or restrictions (e.g., designated critical habitat for listed species or late-successional reserves), during step 1 (and step 6). Land managers should document their prioritization process.

Steps 2–5—Assign Risk Ratings Once prioritization and survey type plans are confirmed and tree risk assessors begin to conduct risk assessments at prioritized sites, each tree in the area will be given a standard tree risk assessment incorporating two components:

• Impact potential is determined by evaluating both the level of exposure and the consequences of possible damage or loss. This portion of the rating is based on what is around the tree. This must incorporate the likelihood that a failure will strike a target (people or property), the likelihood of damage, and an estimated value of the target(s).

• Failure potential addresses the potential for tree or tree part failure within a specified time period, such as between inspection periods. This portion of the rating is based on the observed tree conditions.

The risk rating for each individual tree is determined by combining the values from the impact potential (scored 1–5) and failure potential (scored 1–5) components of the rating system. Thus, 9 risk ratings ranging from 2 to 10 are possible. Mitigation priorities are then based on the level of risk a tree may pose if left untreated. Maximum risk ratings and mitigation actions will vary by project based on how targets are evaluated for impact potential.

Step 6—Documentation and Recommendations Documentation of tree risk assessments is key to communicating the level of risk a tree may pose and if mitigation is recommended. Record keeping will help to identify and communicate potential hazards before a failure occurs.

Record keeping also demonstrates an agency’s ongoing process of evaluating and managing the risk of injury to people and damage to property. Additionally, documenting where and when surveys have occurred will help prioritize where surveys should occur in the future.

Tree risk assessors collecting data.

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Step 1: Develop the Program of Work Before implementing a tree risk assessment program, land managers should develop a plan to determine how to best allocate limited resources. The first step is to define the area where staff will perform tree risk assessments. After identifying the area(s), a manager must prioritize the sites and resources within it for survey and mitigation and select the appropriate survey type (Tier 1 Basic or Tier 2 Advanced). The program of work is completed within the scope of individual resource types (roads, developed recreation sites, or administrative sites) since the use patterns and frequency of surveys for these resources are intrinsically different—prioritize recreation sites with other recreation sites and roads with other roads. Coordinate with specialists (e.g., recreation, engineering, wildlife, aquatics, silviculture, timber, heritage, and fire) during this process as survey and mitigation options may have timing constraints, especially in project areas with federally listed species where consultation with regulatory agencies is required.

Developing the Program of Work for Tree Risk Assessments

target types and the associated mitigation strategies

Prioritizing Survey Areas Two factors drive the prioritization of surveys: the value of the infrastructure at the site and the amount of time people are exposed to potential tree hazards at that site. Local knowledge of site history and other management goals are also important, but in general:

• If knowledge of site history and forest type indicates an increased likelihood of tree failure at a site, survey priority increases.

• As use constraints increase, so does the survey priority (fig. 1.1).

In many instances agencies direct use patterns of the public (e.g., developed campgrounds with designated tent pads, backcountry sites where people are directed to camp within a certain distance of a post that designates a campsite, or signage and fencing that directs use to a specific area). In these cases, the survey priority would be higher than if use patterns were not being directed or “constrained.”

Local knowledge of a site’s history and forest composition can influence survey prioritization. Priority will increase if a site has a history of tree failures, which may be related to past disturbances and the age and composition of the forest. There may also be other goals that overlap with tree risk assessments, such as silviculture activities in surrounding areas, fuels treatments, wildlife habitat improvements, etc. Ultimately, site prioritization is highly dependent on the knowledge, experience, Figure 1.1—Considerations for prioritization of sites for tree risk assessments.

and goals of the specialists developing the program of work. Prioritizing sites helps determine the most suitable survey type and the timing and frequency of tree risk assessments while aligning with available resources and acceptable risk levels. Consider developed sites, roadways, and worksites as separate groups and use criteria specific to each group during prioritization.

Developed Sites Developed sites include administrative and recreation facilities. The level of development can vary widely—from a large campus of facilities that includes water and electric utilities to a campsite with a single signpost or marker.

Survey prioritization depends on the type of infrastructure or development, the exposure time, the seasonality of operations, use patterns, site hazard tree history, and local unit objectives.

Sites With Overnight Occupancy Campgrounds and other recreation and administrative sites designed for overnight occupancy (fig. 1.2) typically have the highest potential exposure and therefore are prioritized above other recreational and administrative sites.

Figure 1.2—A campsite designed for overnight recreational use.

Other Sites and Facilities Generally, sites with higher levels of infrastructure, sites designed for extended occupancy, or sites with high occupancy rates should be prioritized over sites with lower development or occupancy or those designed for short-duration uses. Depending on resource availability and other site-specific or programmatic variables, not all developed sites may be surveyed every year. The prioritization discussion allows specialists and managers to develop a systematic approach and document the decision.

Prioritization hierarchy example for developed sites and facilities:

A. Administrative facilities, recreation sites with overnight occupancy, and snow parks where people are exposed in extreme weather

Use (high, moderate, low) can be quantified for more explicit prioritization.

Ensure ongoing vegetation conditions and stand health are considered in addition to the above hierarchy.

Roads It is impracticable to conduct surveys and mitigate hazard trees on the thousands of miles of roads that travel through forests. Tree risk assessments and mitigation should prioritize areas with the highest risk. Areas and road segments with the highest volume of use, the greatest exposure times, and largest number of defective trees should have the highest priority. Forest Service roads may be prioritized by their operational maintenance level (table 1.1).

Developed roads with higher traffic volume and speed of use (typically State routes or county arterial roads) may be given the highest priority for tree risk assessments, while roads that are restricted to high-clearance vehicles and have a low traffic volume may be lower in priority. Areas with increased tree mortality from disturbance events such as insect outbreaks, fires, or extreme weather should be given higher priority than areas with lower mortality or unchanged conditions. Other priorities may include roads of local concern or with unique circumstances, such as roads that provide access to rural communities or infrastructure (emergency egress routes, roads to power lines, access to fire lookouts and communication sites, etc.).

Table 1.1—Road maintenance levels used by the Forest Service

Maintenance level Traffic type

1 In a period of storage, closed to highway-legal vehicles; open for nonmotorized uses; may be managed or designated as a motorized trail

Maintained for high-clearance vehicles; low traffic volume and low speed;

may not be passable in periods of inclement weather; traffic consists of administrative, permitted, dispersed recreation, or other specialized uses

3 Open for standard passenger cars during normal season of use; low traffic volumes and speeds; local, commercial, and recreational use; aggregate surface

Moderate traffic volumes and speeds; typically two lanes of traffic and aggregate surface, but may be paved

5 High traffic volumes and speeds; generally paved; typically connect to county or State roads

Prioritization hierarchy example for roads:

A. Areas and road segments with constant exposure and where extensive damage to targets may occur, such as viewpoints, pullouts, or other places where people are encouraged to park their vehicle or any other place where vehicles or people are exposed to hazard trees for a long duration. Additionally, places where a work activity could occur for a long duration of time, such as a bridge replacement or other road reconstruction activity.

B. Short-duration exposure areas (e.g., intersections, stop signs, or emergency pull-outs).

C. Areas with intermittent but high-frequency exposure (e.g., high-traffic roads, timber haul routes, or areas with limited site distance (sharp corners).

D. Areas with low traffic volumes.

Ensure ongoing vegetation conditions and stand health are considered in addition to the above hierarchy.

Worksites The type of work activity and the length of exposure determine when, where, and what type of tree risk assessments to conduct. Work activities present various levels of risk associated with the potential for inducing a tree failure.

For instance, a worksite with heavy machinery presents a higher likelihood of influencing a hazardous tree to fail compared to one with only hand tools.

Work activities that involve direct tree contact, such as hand felling, vibration from earth-moving equipment, or rotor wash associated with helicopter operations, may warrant higher prioritization.

Survey Types and Frequency Tree Risk Assessment Tiers Two tiers of tree risk assessments, or surveys, can be performed—a simpler Tier 1 Basic survey or a more thorough and lengthier Tier 2 Advanced survey. The choice between them is dependent on factors that include visitor use, presence of targets, length of exposure, development type, current site conditions (including forest health and disturbance conditions), survey crew availability, and budgetary constraints (fig 1.3). The length of time between surveys and the survey type may change over time based on changing priorities and conditions.

Regardless of survey type, all trees within striking distance of targets—1 to

1.5 times the height of the tree—need to be assessed (refer to chapter 2, “Determining Potential Failure Zone”).

Figure 1.3—Suggested survey level and frequency based on resource and activity type.

Tier 1 Basic Survey A Tier 1 survey is a visual assessment of a tree or population of trees for the purpose of identifying obvious defects. While a basic survey entails only a visual assessment of the tree, it requires a thorough and systematic examination of a tree on all sides from its base to its crown (described in chapter 4). The Tier 1 survey focuses on identifying trees with obvious defects that contribute to high or very high failure potential.

Trained individuals conduct this visual assessment on foot. A Tier 1 assessment can efficiently cover a large area and identify dead trees, hung-up or root-sprung trees, broken or hung-up tops and large branches, or trees with cracks, evident decay, or other obvious visible defects that may contribute to tree failure. Tier 1 surveys may be initiated from a slow-moving vehicle with good visibility to all possible hazard trees, so long as the person doing the survey is not operating the vehicle. A Tier 1 survey from a slow-moving vehicle can be a first step to evaluate what tree defects may be present. However, it does not constitute a complete survey and is not appropriate for roads that have parked cars, picnic areas, parking areas, long-term occupancy, or high year-round use.

A Tier 1 survey will help determine if a particular tree or a given site requires a more in-depth examination to properly identify the risk of tree failure (i.e., initiate a more advanced Tier 2 survey). For example: A Tier 1 survey in a developed site identified conks of an unknown fungus on several trees. The extent of decay present and failure potential could not be easily determined so further examination was warranted.

Tier 2 Advanced Survey A Tier 2 survey begins with a thorough visual inspection that evaluates the butt, stem, and crown of the tree on all sides. Advanced surveys require in-depth knowledge of the structural properties of different tree species and identification of less obvious defects associated with potential tree failure, such as heartwood decay and root disease. When signs and symptoms indicate tree damage that presents a risk of failure, a more thorough examination is advisable to determine the extent to which the damage has compromised structural integrity.

Tier 2 surveys often include drilling trees when decay is suspected or excavating roots to assess for root disease. On a tree that requires additional investigation to determine its failure potential, assessors may use tools such as sounding mallets, increment borers or drills, binoculars, hand lenses, hatchets, and Pulaskis. Tier 2 surveys also involve inspecting the area in the immediate vicinity of each tree, looking for both obvious and subtle patterns of underlying forest health issues or site conditions that may lead to tree failure. Advanced surveys cannot be completed from a vehicle and always require a full inspection of all sides of a tree.

Determining Survey Type and Frequency

Surveying Developed Sites For developed sites with overnight use, Tier 1 surveys should be conducted annually before seasonal opening and after the severe weather season(s) has passed. This frequently occurs in the spring as winter weather conditions often result in tree damage. Damage caused by winter storms or wind events brings attention to the most defective trees or limbs and may help to identify the portions of stands with root disease or stem decay. For developed sites without overnight use, Tier 1 survey frequency should be based on site occupancy, exposure, infrastructure, and resource availability. In the event of a changed condition, such as a flood or windthrow event, additional Tier 1 surveys may be justified between the normal inspection cycle.

In high-use developed recreation sites, such as campgrounds, Tier 2 surveys provide a more comprehensive assessment and may identify recurring forest health issues that require regular monitoring. Tier 2 surveys should be completed at regular intervals as deemed feasible and appropriate based on the management agency’s standards (e.g., every 5 years) and after major disturbances such as fire, insect outbreaks, or on sites with chronic damage from root disease or stem decay. Between Tier 2 surveys, an annual Tier 1 survey should be adequate to capture changes from one year to the next.

Surveying Road Systems Unlike developed recreation areas, surveys along road systems are generally not conducted at regular intervals. Tier 1 surveys will likely only occur on the highest priority road systems, and survey frequency will change based on recent disturbances, priorities, and funding. Outside of the highest priority sections of roads identified on a unit, road surveys are often triggered by large-scale disturbance events (see chapter 8 for additional considerations).

Tier 2 surveys may occur along forest roads under unique circumstances;

however, these are typically limited in scope and frequency to areas with high-value trees, high use, or sensitive resources (e.g. critical habitat, areas with heritage trees).

Surveying Worksites At worksites, a Tier 1 survey can be useful in determining the risk posed to field-going personnel. However, many temporary work sites with limited exposure and infrequent use, such as those associated with field surveys, tree planting, or short-term road maintenance projects (linear grading, brushing, culvert maintenance) (fig. 1.4) may not require an explicit Tier 1 survey. Personnel can use the information in this field guide to identify potential hazards at these temporary work sites.

A Tier 2 survey may be more appropriate on worksites where there is long-term activity, concentrated use, or the activity may disrupt a tree’s stability (e.g., landings on logging units or road reconstruction activities, such as rock crushing operations and culvert or bridge replacement projects).

Figure 1.4—Dead trees surrounding a temporary worksite.

Determine Failure Zone Size, Considerations for Dead Trees, and Maximum Risk Rating After identifying, prioritizing, and assigning survey types to areas for assessment, several additional decisions must be made. Selecting a standard for failure zone size, which will define the survey area, should be decided and documented prior to conducting surveys. It is best practice to remove dead trees in developed recreation sites; outside of developed recreation sites retention of dead trees may be considered where exposure is low. The maximum risk rating for different project areas will be different and based on the level of exposure and target value. That is, not all project areas have a maximum risk rating of 10; for example, some forest roads may only have a maximum rating of eight.

Thus the maximum possible risk rating should be determined and then an appropriate maximum acceptable risk rating above which mitigation will occur should be established for each project area individually.

Field Supplies Before heading to the field, identify and gather the necessary maps, survey records, forms, and equipment to conduct thorough tree risk assessments.

Useful field equipment may include binoculars, sounding mallet, increment borer, battery-powered drill, detailed site maps, appropriate tree risk assessment forms or applications for mobile data collection, hand lens, Pulaski, hatchet, laser/clinometer, logger’s tape, diameter tape, flagging, and appropriate field guides. A device with Global Positioning System (GPS) capabilities is helpful to record the location of trees.

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Step 2: Determining Potential Failure Zone The potential that a tree or tree part may strike a target is determined by where the tree or its parts will likely land in the event of a failure (fig. 2.1), known as the potential failure zone. A tree with no target in the potential failure zone poses no risk and does not need a tree risk assessment.

Figure 2.1— Picnic table damaged by failure of a hazard tree.

The potential failure zone is the area that any part of a failed tree might reach.

Variables for calculating the potential failure zone include the height of the tree or length of the defective tree part, the direction and degree of slope, and the direction and degree of tree lean. Decide and document the standards for potential failure zone area prior to conducting surveys, which will help define the survey area and provide a justification if a project or individual tree removal is challenged. Additionally, documentation of site conditions at a specific area or tree will provide justification for changes to that area’s or tree’s potential failure zone, if necessary. Recommendations included here do not supersede guidance provided by relevant regulatory agencies or agency policies.

When on slopes, tree failures typically result in the tree or its parts sliding or rolling distances well beyond what would normally be calculated for a failure zone on flat ground. In these situations, the rolling material may strike other trees, rocks, or debris on the ground and fling material a considerable distance.

This is especially true when failures occur in stands of dead trees or on slopes devoid of vegetation.

Total Tree Failure On level ground, the recommended potential failure zone is generally equal to the height of an individual tree, though that may increase to 1.5 times the height of the tree for certain situations, depending on local conditions and regulatory policies. Hence, the potential failure zone around the base of the tree is a circle defined by a radius that is 1 to 1.5 times the height of the tree (fig. 2.2). A 100-foot-tall tree’s potential failure zone has a radius of 100–150 ft.

On sloped ground, the failure zone downhill of the tree should be extended to whatever distance is necessary to protect people or property if the tree slides or rolls (fig. 2.3). For targets uphill from the tree, the total tree height should be adequate to calculate failure zone.

Figure 2.2—Potential failure zone for a tree without a lean on a site without slope.

Figure 2.3—Potential failure zone for a tree without a lean on a site with a slope.

Leaning Trees It is important to differentiate between slightly leaning trees and trees with significant leans (15 degrees or more). For trees leaning 15 degrees or more, the failure zone is an area—the same radius as the 1 to 1.5 times the height of the tree— beginning at the base of the tree and extending toward the direction of the lean and out 90 degrees on either side of the tree from the direction of the lean (fig. 2.4). The area behind the lean is not within the failure zone but allowance for backlash should be made. If the tree has other structural defects in addition to a significant lean, the direction of potential failure is unpredictable and a radius equal to 1 to 1.5 times the height of the tree should be used on all sides.

Figure 2.4—Potential failure zone for a tree with a lean 15 degrees or more.

Tree Part Failure For tree parts, such as tops, forks, or branches, the recommended potential failure zone is 1.5 times the length of the tree part that would become dislodged, even on flat ground. Often treetops or parts dislodge when it is windy and can be carried farther than just the length of the tree part, as has been documented on fork failures in ponderosa pine in central Oregon (Oblinger 2016). On level or sloped ground where the tree has no discernable lean, determine the length of the part that could be dislodged; for forks or codominant stems, the section often includes some distance below where the forks could separate. The failure zone forms a circle around the tree with a radius equal to 1.5 times the length of the defective part (fig. 2.5). For instance, if a dead top is 10 ft long, the potential failure zone has a radius of 15 ft from the base of the tree. On sloped ground where the dislodged part may slide or roll downhill, the failure zone should be extended on the downhill side (fig. 2.6).

Figure 2.5—Potential failure zone on a site with no slope when only the top or a portion of the tree may fail.

Figure 2.6—Potential failure zone on a site with slope when only the top or a portion of the tree may fail.

APTER3

Step 3: Determining Impact Potential Rating Once a tree’s potential failure zone has been determined, an assessment of the targets that exist within that area needs to be made and an impact potential rating assigned. Impact potential incorporates the probability of occupancy (exposure) and extent of injury or damage to property (consequences) that may result if tree failure occurs. The consequences of a tree failure are estimated by determining the maximum extent of loss if a target is struck.

Exposure and the consequences of failure are expressed in relative terms and are used to determine the impact potential rating on a scale of 1 to 5 (tables 3.1 and 3.2).

On some occasions the size of the tree or its failed part may be considered in evaluating impact potential. For example, only larger trees may be considered for work activities where the operator of heavy machinery is in a protected cab that can withstand some impact from smaller trees. In other situations, such as tent camping or working without overhead protection, even smaller trees or their parts may cause serious injury or death.

Local knowledge of the site will help inform the impact potential rating.

Variables to consider when determining the rating include:

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• Type of target and occupancy (e.g., designated tent sites, restrooms, overnight parking areas, picnic tables, information boards, scenic viewpoints)

• Work activity and duration (e.g., tree planting, trail construction, culvert replacement)

• Probable timing of tree failure (e.g., failures tend to be more common during storms and when soils are saturated)

A streamlined approach may be applied in a project area where impact potential remains constant, such as a right of way or a transmission line or corridor. In this situation all trees will have the same impact potential rating, which allows tree risk assessment and mitigation decisions to focus on the failure potential ratings.

Rating Impact Potential Refer to tables 3.1 and 3.2 for examples.