Forest Health Protection: Hazard Tree Management

Downloads & Links

  • Hazard Tree Evaluation Form (PDF). The paper form for inspection of trees in developed forest sites. The form was reorganized and reformatted for greater efficiency and consistency with the new DDF and Hazard Tree Database (below).

    • An optional sketch-mapping guide for locating trees in campsites, available in Word format and in GIF format. To download the gif file in Windows, right click and choose 'save link as'. Its size may need to be adjusted prior to printing, depending on software.

  • Hazard Tree Data Dictionary File & User Guide (zip file). The zip file contains a User's Guide and DDF for Trimble dataloggers. The DDF is an electronic "Hazard Tree Evaluation Form" for the inspection of trees in developed forest sites that can be run from Trimble Dataloggers. This electronic form can be used instead of the paper form. The file can be edited and uploaded to a Trimble Datalogger using the program GPS Pathfinder Office.

  • Hazard Tree Database (zip file). This application is used to input, import, edit, export, and analyze hazard tree data collected with paper form or dataloggers. Requires Microsoft Access. It contains the database application, user's guide, and sample import file.

  • A comprehensive and informative Hazard Tree Web Page (hosted by USFS Northeastern Area, St. Paul)


Outline of this Page

  1. Introduction
    1. Hazard Components
  2. Hazard Tree Inspection: Program and Procedure
    1. Planning
    2. Documentation
    3. Doing it - Field Procedures
  3. Hazard Prevention
    1. New or Reconstructed Site
    2. Avoiding Wounding in Established Sites
    3. Wound Dressings
    4. Vegetation Management
  4. Tree Failure Reporting

1. Introduction

Tree hazard means the likelihood of property damage or personal injury from tree failure. For the U.S. Forest Service, it is an issue primarily in developed recreation areas, especially as recreation becomes a more important focus. A program of hazard tree management is necessary to reduce the risk of property damage, injuries and fatalities due to tree failures in developed recreation areas.


Hazard Components

Hazard incorporates not just the condition of the tree, but also the potential target. So hazard is:

  • potential for tree failure, plus
  • potential for serious damage should the tree fail, which includes:
    • potential for hitting target
    • value of target

 2. Hazard Tree Inspection: Program and Procedures


Hazard inspection is an attempt to determine tree hazard in order to determine if a tree should be removed or otherwise treated. Every tree is potentially hazardous, even if it looks perfect. Realistically, however, risk analysis or cost-benefit analysis is implicit in hazard tree management. One funny knot or scar or even limited amount of decay is not enough to decide to take a tree down. Systems and guidelines have been developed for inspection and decision-making, but knowledge, judgment, common sense and experience are an important part of the process.

In developing a hazard tree inspection program, a number of decisions need to be made. Forest Health Management is available to assist in planning and developing a program and in training field personnel in suitable inspection procedures and criteria. Here are some decisions that need to be made:


Documentation is important to provide legal evidence should an accident occur, to track the progress of the program, and to detect trends in disease and hazard development in the recreation sites. Training of inspectors should be documented. Inspection procedures and criteria (including decisions made on the issues listed) above should be described in writing. Inspection data should be saved, as well as documentation of treatment actions.

A Microsoft Access application is designed to store and summarize data from hazard tree inspections. It was developed here in R2 to accommodate the Region hazard tree rating system. It is used on a local computer (C drive) . Users can import data from field data recorders, spreadsheets, or enter/edit data directly. Useful summary reports and tree lists can be quickly and automatically generated to support decision-making and implementation of treatment plans. Since all USFS computers already have Microsoft Access, no software purchase is necessary. See download links above.

Doing it - Field Procedures

A Hazard Tree Inspection form recommended for use in the Rocky Mountain Region can be downloaded (see links at head of article). It guides the user in recording all needed data and determining hazard ratings. It is 2-sided, with instructions and tips on the back. Once you are familiar with it, you can copy the form itself 2-sided for greater efficiency. An optional mapping guide is also available (see above). It can be copied onto the back of the inspection form and used for mapping sites (e.g., campsites) individually.

Work the area in a logical, consistent sequence. Don't jump around. Following campsite numbering is a good idea. Record tree number, species, dbh, and mapping info as appropriate.

Assess the target for each tree. If the tree would not hit a road, major trail, structure, parking area or campsite, you can probably ignore it. If so, rate the target 2 or 1 (see form).

Look at the tree from 2-3 perspectives, close and far and all around. When you are learning, it will help to go through each defect on the form and decide if it applies. Inspectors should be able to recognize defects and understand their implications.

Based on indicators and experience, it may be necessary to check for root disease, usually using a pulaski. Examples of indicators would be thinning crown, dieback, reduced height growth, basal resinosis, exposed decay in butt or roots.

Also based on indicators, you may need to increment core or drill the tree to assess internal stem decay. Such indicators include wounds and bird cavities. Trees are rated in the highest hazard cavity if the remaining, undecayed, sound shell is less than 1/3 of the radius (use dbh/2 to get radius, not the core).

Calculate the rating. In some cases, you may feel the rating does not accurately reflect the hazard of the tree. Tree inspection and hazard rating are a combination of science and art: do what experience and sound judgment tell you is right, but make notes in the space provided about what you do. If necessary use a supplemental notebook. Even if the rating seems reasonable, use the notes column to record details on the defects, such as identity of the root pathogen, canker, conk, etc.

Here, in summary, are things to look for in a typical inspection:


  1. Target.
  2. Tree species. Some species are more prone to failure from particular defects.
  3. Tree age/size.
  4. General condition.
  5. Defects. If defects are connected that is worse:
    1. Cracks (vertical cracks in the trunk, often accompanied by callus that may or may not be ruptured) are taken pretty seriously. They are an indication that tree failure has already begun. Some consider them the number one indicator of hazard. They arise from short-term mechanical injury or, most commonly, by improper wound closure.

      Callus forms at the margin of injured cambium and grows outward to cover the wound. If all is well, the callus margins will meet and seal over the wound. Sometimes, the callus tends to curl inward, or inroll, during growth. This is sometimes called am's-horning. If this happens, the callus never meets and seals properly. Instead, the bark covered surfaces of the callus rolls meet. As growth rings are added, the callus sides push against one another, leading to formation of cracks. Also, tension generated in the stem can lead to formation of secondary cracks elsewhere. Inrolled callus might might be expected where the wound surface is concave. The callus margin follows the concavity as it grows, increasing the likelihood that the callus surfaces will meet before the margins.

    2. Weak branch unions. A branch union is a fork in the stem or a place where two or more branches join the stem together. If bark is included in the union, the branches are unlikely to fuse. That is bad. How do you tell? If there is good fusion, there will usually be a raised, roughened bark ridge between the two. If not, the union will be a cleft with bark going down (included) into the union. Narrow branch angles are bad, and doubly so when they represent a forking trunk, that is, one is not clearly larger than the other. The two stems continue diameter growth, pressing against each other, pushing them apart. A split or crack is a common result. Wind helps.
    3. Decay in stem or branches. Some decay, especially in the interior, is tolerable. Most of the strength of a tree, like a pipe, is on the outside, so interior wood can be removed without great effect on strength. A rule of thumb is that if the thinnest layer of sound, outer wood is more than 30-35% of the overall stem radius, most of the strength is retained. Failure potential is considered high when there is canker-rot in the main stem or decay assoicated with weak branch union or open crack. Any decayed branch also has a high failure potential.
    4. Cankers. A canker (or scar or cavity) that involves more than 120of the circumference is serious. We generally recommend removal if half or more of the circumference is occupied by an individual wound or canker. The potential for failure is higher if a canker is accompanied by decay or if it is connected to another defect.
    5. Dead branches or whole tree. Large dead branches have an obvious potential for failure and should be removed. The same goes for any dead tree. The mantra is, "Dead wood is non-negotiable." If there are conflicts with wildlife or other benefits of dead trees, a decision must be made whether to remove the tree or move/close the target.
    6. Root problems. Road construction, severing for utilities, soil erosion, trampling damage, partial windthrow, and root disease are the considerations here.
    7. Poor tree architecture. These may be caused by poor pruning in the past. Sharp bends or crooks in the branches are naturally weak, liable to failure. If a tree is topped, multiple branches may come out just below, they are usually weakly attached. Failure potential is considered high when tree leans more than 45or when it leans and has another defect in mains stem. A significant lean that appears to have occurred recently and has not been corrected by negatively geotropic top growth ("unnatural lean") should be taken seriously.

3. Hazard Prevention

New or Reconstructed Recreation Sites

A critical step in avoiding hazard tree problems is during design and planning for new or reconstructed recreation sites. Consultation with Forest Health Management at this stage can help in choosing sites where vegetation can be managed safely. Also, individual trees may be selected for removal or retention at this stage to ensure that the most desirable, healthy trees are left on site.

Avoid Wounding during Construction

Existing trees are often left, to the extent possible, when new recreation sites or buildings are constructed. Here are some things that can go wrong at this stage.

  • Stem wounds: Construction equipment bangs into the trees. The trees are used to hold tools, to tie cables onto, etc. This creates infection courts and weakens trees.
  • Root wounds: Excavation too close to the tree severs and damages major roots, reducing tolerance to stress and creating infection courts for root-rot fungi.
  • Fill: To follow the design of the site, fill is often placed over the soil and even up against the base of the tree. Insufficient oxygen, water molds, root and butt rot can result.

Here are some steps to avoid such damage:

  • A competent person should select trees on the site that are in the best condition and have the greatest likelihood of survival and safety for some time. Sometimes it is best to forget the mature trees and save the 4-6trees.
  • Design and construction should be carefully planned to avoid tree damage. Install bumpers if possible to protect trees in vulnerable situations and during construction.
  • For campsites and parking areas, permanent bumpers should be installed to protect trees from vehicle damage.
  • Utilities should be routed to avoid trees.
  • Roof runoff should be planned so that it does not hit rooting soil. This tends to greatly increase compaction.


Avoiding Wounding in Established Sites

One of the major sources of wounding and resultant disease and hazard in campgrounds is campers. Axe throwing, chopping, hanging lanterns, dumping hot ashes at tree bases, etc. cause permanent damage that lead to decay and hazard. Educate campground hosts to encourage campers to care for trees. A signing and public education campaign may also be helpful in preventing such damage.

Maintain bumpers to protect trees from vehicles.

Wound dressings

Most authorities believe that wound dressings or sealants have little value in most cases and can be detrimental. Most studies have shown that some actually slow the growth of callus or increase the growth of decay fungi. Some studies only consider rate of callus growth, but other studies have shown that that is not necessarily correlated with decay. We do not recommend wound dressings. However, it must be said that there is not much research on the question, and there may be some sealants that are actually beneficial.

The only exception is with elms and oaks. In areas where the two wilt diseases, Dutch elm disease and Oak wilt, are of concern, fresh wounds should be covered with some kind of dressing to prevent access of the vectors to the wound area.


Vegetation Management

If we are to maintain developed recreation sites indefinitely, it is important to develop a vegetation management plan for each site. Such a document should take into account current conditions of the vegetation (including canopy, regeneration, shrubs, etc.), management objectives such as aesthetics and screening, potential pathways of stand development in the future, current and potential development of insects and diseases that may affect those pathways, and alternative approaches to meeting the objectives in the future. We know that the trees that make our campgrounds so beautiful will, at some point in the future, either die, fall, or be removed because we can no longer tolerate the risk that they present. There is no better time than now to plan for replacement of those trees.

The Rocky Mountain Region produced a guidebook to help managers of recreation and administrative sites develop and implement vegetation management plans. A new appendix considers detailed issues involved in revegetation of campgrounds following mortality due to bark beetles:

  • Anonymous. 2003. Vegetation Management Planning Guide: Planning and Implementation for Developed Sites in Region 2. USDA Forest Service, Lakewood CO. 37 pp.  Available in PDF format - 6 MB.
  • Anonymous. 2009. Vegetation Management Planning Guide: Planning and Implementation for Developed Sites in Region 2. Appendix E: Revegetation of Campgrounds after Mortality of Pines due to Bark Beetles in the Rocky Mountain Region. USDA Forest Service, Lakewood CO. 22 pp. Available in PDF format - 950 KB.

Printed copies of the guide are also available from Renewable Resources in the Regional Office and from Forest Health Management Service Centers.


4. Tree Failure Reporting

FSM R2 Supplement No. 2300-94-6 (2332.1) indicates that the Region formerly had a tree failure reporting system. This system provided information on failed hazard trees that was useful in improving criteria for detecting hazard trees and also provided an indication of particular problems in the Region that needed to be addressed.

An international tree failure reporting system was later established, the International Tree Failure Database (ITFD). It was initiated by USDA Forest Service in cooperation with the International Society of Arboriculture.  It is now offline, so no tree failure reporting system is currently available.  However, it is strongly recommended that districts record and maintain records of tree failures in all developed sites.  If the ITFD comes back online, those records could be entered.