HazardTreeGuidelines_v4-FINAL.pdf

Forest Health Protection

Pacific Southwest Region

April 2012 (Report # RO-12-01)

Hazard Tree Guidelines For Forest Service Facilities and Roads in the Pacific Southwest Region

Peter A. Angwin, Daniel R. Cluck, Paul J. Zambino, Brent W. Oblinger and William C. Woodruff

These hazard tree guidelines provide a means to identify and abate hazard from trees that are likely to fail and cause injury to either people or property on Forest Service system roads or at Forest Service facilities (i.e. campgrounds, boat ramps, trailhead parking, summer home tracts, administrative sites, kiosks, information centers, etc.) in California. They are intended to provide consistent direction for hazard tree identification and abatement and their use is highly encouraged and fully supported by Forest Health Protection (FHP) staff.

It must be recognized from the outset that even under the best of circumstances and with the highest standard of care, our ability to predict tree failure is not infallible. Simply put, we are limited in our ability to reasonably foresee all tree failures all the time. However, by exercising good professional judgment and using a systematic approach such as the one suggested in these guidelines, it is possible to significantly reduce (but not totally eliminate) the risk of injury to people and damage to property (Figure 1) .

The specific objectives of these guidelines are to:

1. Describe the need for hazard tree management and review current policy direction for the National Forest System.

2. Explain hazard tree evaluation procedures and describe a hazard tree rating system appropriate for:

Figure 1. Distribution of risk in a tree population before (left) and after (right) hazard tree inspection and high-priority action (Dunster and Associates Environmental Consultants Ltd.)

a. recreation, administration, high-use areas, and

b. other areas with at-risk targets, including Forest Service road systems.

3. Describe ways to mitigate hazard from trees.

Assistance with hazard tree management is available from FHP staff in the Regional or Shared Service Area offices. Available assistance includes:

1. Training in identification, mitigation and management of hazardous trees, as well as in the use of the International Tree Failure Database. Training may range from formal instruction to informal site visits.

2. On-site consultation to address specific hazard tree situations.

3. Helping to draft NEPA documents in support of hazard tree management.

4. Assistance with vegetation management planning for recreation areas.

5. Financial assistance to treat specific insect and pathogen-related problems, including the development of associated vegetation management plans and NEPA documents.

Hazard/Danger Tree Definition

Tree hazards include dead or dying trees, dead parts of live trees, or unstable live trees (due to structural defects or other factors) that are within striking distance of people or property (a target). Hazard trees have the potential to cause property damage, personal injury or fatality in the event of a failure.

It has often been common practice to refer to such trees as either “hazard trees” or “danger trees” according to the different settings in which they are found; “hazard trees” near structures or in recreation areas versus “danger trees” along roads. Properly speaking, hazard trees and danger trees are synonymous terms, referring to trees that have the potential to cause death, injury or property damage if they fail.

This document uses the two terms interchangeably.

Additional Definitions

When rating trees for hazard or danger, one attempts to determine whether a loss from failure of a tree could be reasonably expected to occur before the next inspection. Loss is defined as property damage or personal injury and may be expressed in dollars. Failure is the mechanical breakage of a tree or tree part. Failures often result from the interaction of defects, weather factors, ice or snow loading or exposure to wind. Defects are flaws in a tree that reduce its structural strength. Trees may have single or multiple defects which may or may not be detectable. Failures result in accidents only if they strike a target.

Targets can be stationary, such as buildings, or mobile, such as vehicles or people. In this discussion we will not consider other trees as targets, but will only address people, property, and structures. A hazard tree rating system is principally concerned with recognizing, assessing and recording detectable defects where actions can be taken to reduce or eliminate the hazard.

Responsibility and Forest Service Policy

The Chief of the Forest Service and the Regional Forester have stressed that the safety of the public and our employees is our central concern. In developed recreation areas and within the transportation corridors, hazard tree management is vital to everyone’s safety. In the USDA Forest Service Strategic Plan for 2007-2012, Objective 4.1 is to “Improve the quality and availability of outdoor recreation experiences.”

Two of the means and strategies that are listed to accomplish this goal are to “Provide tools, guidance, and resource management to provide safe recreation use.” and “Efficiently and effectively manage and maintain recreational opportunity infrastructure while protecting public health and safety.” Identification and mitigation of hazard trees on Forest Service land is clearly part of the Agency’s mission.

Line officers have the responsibility for annual inspection and management of hazard trees on campgrounds and other heavily used recreation areas. Forest Service Manual, FSM 2332 states that health and safety-related items must be given highest priority in the operation and maintenance of recreation sites. FSM

2332.1 states that “To the extent practicable, eliminate safety hazards from developed recreation sites.

Inspect each public recreation site annually before the beginning of the managed-use season. Maintain a record of the inspections and corrective actions taken with a copy of the operation and maintenance plan. Immediately correct high-priority hazards that develop or are identified during the operating season or close the site.” FSM 2332.11 requires that, “Consistent with preserving the recreation resource, remove trees or tree limbs identified as hazardous at developed recreation sites. Obtain assistance from timber management, forest pest management, and recreation specialists, as necessary.”

Treatment to prevent the future development of hazard trees by Heterobasidion root disease is also specified. Region 5 FSM Supplement 2303.14 states: “To perpetuate the forest environment in and around developed recreation sites, treat all freshly cut coniferous stumps to prevent introduction and spread of Fomes annosus.” (causal agent of Heterobasidion root disease). The newly revised Forest Service Handbook, FSH R5 Supplement 3409.11, Chapter 60 (effective February 9, 2010) states: Because of the high value of residual trees in developed recreation areas, and especially the importance of minimizing the development of hazard trees in these areas, it is recommended that in developed recreation areas all conifer stumps greater than 3” across (outside bark diameter) receive borax at the time the stump is created. …The same directions shall apply to other high value areas, such as progeny test sites, seed orchards, and other areas where there are high value trees.” It is anticipated that R5 FSM Supplement 2303.14 will be revised to match the FSH 3” borax treatment recommendation.

Forest Supervisors have a similar responsibility for the safe operation and management of roads and must “…to the extent permitted by funding levels, systematically provide for elimination of identified hazards.” (FSM 7733.04c). Forest Service Handbook direction, (7709.59 Chapter 40, effective 02/05/2009) contains more specialized guidance pertaining to hazard trees. FSH 7709.59_40.3 policy states that:

1. Safety is the predominant consideration in road operation and maintenance and takes priority over biological or other considerations.

2. Roadways must be managed for safe passage by road users. This includes management of hazards associated with roadside vegetation, including identification and mitigation of danger trees.

3. Identification of danger trees must be performed by qualified persons.

4. When high priority hazards to road users are identified on National Forest System roads and those hazards cannot be immediately mitigated, the roads must be closed.

FSH 7709.59_41.6 also affirms that “Road maintenance includes removing danger trees that threaten safe use of the transportation system.” In addition, FSH 7709.59_41.7 states that “Roads that are open should have a condition survey at least annually. Roads that have been closed should be checked for obvious hazards prior to being opened. Roads open to travel should be checked following major storms or similar events that could significantly affect their condition, result in changes in their traffic service level, or have created new safety hazards.” This section also states that “Danger tree hazards on roads will be prioritized by high, medium and low categories.”; that “Roads or segments thereof identified as high priority constitute a considerable adverse effect on public safety and thus require prompt action.”;

that work to eliminate danger trees in areas of highest exposure should be scheduled first; that road segments should be closed if the hazards cannot be mitigated; and finally, that roads identified as medium to low priority tree hazard are not time-sensitive, but “should be monitored for increases in hazard due to ongoing tree deterioration.” This direction also makes it clear that “Road maintenance, including treatment of danger trees, may be categorically excluded from analysis and documentation in an environmental assessment or environmental impact statement under certain circumstances. See FSH 1909.15, chapter 30 for guidance concerning categorical exclusions and specifically section 31.12(4).

Please refer to FSH 7709.59_40 for more information regarding hazard trees and road maintenance.”

Legal Aspects

Anyone who is injured or suffers property damage from a tree failure may sue the landowner, including governments. During such a suit, the agency may be held negligent and liable if the claimant can prove that:

1. The agency had a legal duty to the claimant; and

2. The agency breached its duty; and

3. The agency’s breach was the legal cause of the claimant’s injury or property damage.

If the Forest Service is negligent in attempting to carry out its duties, it may be held liable for resulting damages. One of the questions that is typically asked following an accident is, “Do you have a regular program of tree inspection and maintenance?” If so, the plaintiffs will want to know how thorough it is, how the inspectors are trained, the methodology and equipment used in the inspection, what kind of record keeping is used, etc. Then the questions turn to the tree that failed. When was it last inspected? What were the results? Was there any defect or indicator that might have suggested to a trained inspector that this particular tree was hazardous? Risks from hazard trees can never be totally removed, if it is desired to maintain recreation sites and roadways in forested conditions. Still, the Forest Service is duty-bound to reduce those risks to an acceptable level. A proactive hazard tree management and inspection program can not only protect lives and property, it can ensure that managers and Forest Health Protection staff have confidence in their ability to answer the above questions in case of a lawsuit.

Hazard Tree Management Programs

A hazard tree management program includes: 1) designating responsible individuals, 2) identifying and prioritizing the sites to be examined, 3) performing and documenting the inspection, 4) performing the necessary actions to reduce the hazards, 5) maintaining the records of inspection and actions taken, and 6) recording tree failures. The program should be compatible with available resources (personnel and funding).

1. DESIGNATING RESPONSIBLE INDIVIDUALS:

Designation of responsible individuals should occur prior to initiating a hazard tree program. As mentioned above, each line officer is responsible for preparing operations and maintenance plans and implementing them for their public use areas (FSM 2330.43). One employee should be responsible for the hazard tree management program at each Ranger District. This will ensure that the program functions are completed, including periodic surveys, required mitigation actions, and records maintenance, and that there is continuity from year-to-year. The individual should have the authority to supervise crews and the background and training to run a hazard tree program. This includes knowledge of the public use areas, the ability to identify and rate defects as to their potential to cause failure, and the ability to prescribe the proper actions to reduce the tree hazard. Knowledge of past failures in an area is an added benefit because of the information it provides on the types of defects and tree species involved in such failures. The inspection crews, if any are formed in addition to the responsible individual, also need to have adequate training in recognizing and rating tree defects. They should have familiarity with the recreation sites to determine occupancy and the likelihood of a target being present. Similarly, knowledge of the local weather conditions would be useful since many failures occur during storms and unusual weather conditions.

While the above discussion specifically addresses the qualifications of the lead person and crew responsible for inspections in developed recreation areas, the same attributes apply for the people involved with performing hazard tree inspections along Forest Service roads.

2. IDENTIFYING AND PRIORITIZING SITES TO BE EXAMINED:

Developed Recreation Areas, Administration Sites and Other High Value Sites

Annual safety inspections are required for every public National Forest recreation site (FSM 2332.1). This includes hazard tree inspections. Inspections should normally be done prior to the primary use season, with sufficient time allowed for corrective actions. Inspections may also be needed following severe storms during the use season. While Region 5 FHP generally recommends inspecting once a year with supplemental inspections after storms, local conditions (including level of use of the recreation area, tree species and unusually good or poor tree health or defect levels) may necessitate additional inspections or allow for fewer inspections. While some areas may need to be inspected twice a year, in other areas it may be reasonable to only fully inspect every two or three years, with a more cursory inspection in the intervening years.

However, if more cursory inspections are done, close attention should be given to any trees that were noted as suspect in previous inspections. In any case, the reasons for changing the inspection interval should be well documented and supported by local conditions.

A systematic inspection is preferable. A map of an area will simplify the planning of a survey route and will aid in the recording of tree locations during the survey. In recreation sites, areas can be divided into tree hazard risk zones (High, Medium and Low) prior to inspection (Figure 2 ).

Inspection intensity should vary directly with the risk level. All trees that have a target within striking distance should be visually examined. On level ground, this striking distance is generally one to one and a half times the tree height. On sloped ground, additional striking distance should Figure 2. Recreation areas can be divided into tree hazard risk zones which determine the intensity of evaluation.

be added on the downhill side to take into account the potential for a falling tree or tree part to slide or roll. At the very least, all trees within areas of intensive public use should be evaluated. Special attention should be given to trees >8” diameter at breast height (dbh), since two-thirds of reported failures occur in trees of this size.

The High Risk Zone includes high use areas with many people, parked vehicles and permanent structures.

This zone is the highest priority for regular inspection and treatment. The Medium Risk Zone includes 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 treatments in this zone have low priority.

Forest Service Roads

As was stated above, the Forest Service Handbook stipulates that roads that are open should have a condition survey at least annually, that roads which have been closed should be checked for obvious hazards prior to being opened and similar checks should be made following major storms. Because it is not feasible to intensively survey all roads every year, a system to prioritize roads for intensive and more cursory inspection is recommended. When establishing priorities for roadside hazard tree inspection and mitigation, land managers should consider the Road Maintenance Level (1-5), the level of public use and the potential for trees to strike the road. The intensity and frequency of hazard tree inspections will also depend on past failure history, the species of tree and impacts from insects, diseases, weather and fire. While some roads may only require an informal inspection every few years, others may require a more intensive inspection program similar to what would be required in a developed recreation area.

Again, the rationale for the prioritization decisions should be well documented and supported by local conditions. The table below provides a suggested prioritization scheme for the inspection of roads:

Table 1. Road inspection priority

Road Segment Exposure Duration

Inspection Priority

Areas where people stop and congregate such as:

• Trailheads & parking areas

• Active projects/contracts along the road where work is stationary such as culvert replacement and bridge construction

• Intersections along operational maintenance level 3-5 roads, scenic vistas, geologic points of interest, anywhere along roads where people are encouraged to stop

Areas along roads with higher traffic volumes such as:

• Operational maintenance level 3-5 roads not within intersections

• Haul routes during commercial use – Operational maintenance levels 2-5 Level 2 roads are included because of elevated traffic volume during commercial use

Long

Short

Intermittent but High frequency

Higher

Lower

Areas with low traffic volumes, such as:

• Operational maintenance level 2 roads All Low

3. PERFORMING THE INSPECTION

All trees with a target need to be identified and rated for hazard potential using an accepted rating system.

Following rating, action is prescribed to correct or monitor the situation. Examples of rating systems include those developed by the state of Washington (Mills and Russell 1980), the province of British Columbia (Wallis, Morrison and Ross 1980), and Regions 2 and 6 of the Forest Service (Johnson 1981;

Harvey and Hessburg 1992; Filip et al 2012), and the systems for evaluation of roadside danger trees in use in Region 6 (Schmitt 2004; Toupin et al 2008). These systems are described in the publications listed at the end of this guide. While all of these systems are appropriate for assessing and documenting tree hazards, a modified version of the “7-Point System” of Mills and Russell, that has been in use the longest in Region 5, is described in this document and recommended for continued use.

General Procedures

The hazard tree inspection form in Appendix 1 and tatum guide in Appendix 2 will help inspectors to record all of the data needed to determine hazard ratings. While performing an assessment, the area should be covered in a logical sequence, such as by campsite number or road segment. From year-to-year, try to approach sites/roads from different directions to get a better perspective of the trees. Always try to survey with optimal lighting. Record tree number, species, dbh, and mapping information as appropriate.

The hazard tree inspection process involves four steps:

1. Assess the target for each tree. If the tree would not hit a road, major trail, structure, parking area, campsite or other target, you can ignore it. Do not overlook any desirable sleeping areas near campsites that are used by campers. A line tape and clinometer are useful in determining target zones, as are forestry laser measuring devices.

2. Assess the tree for defects. When first learning to evaluate hazard trees, it helps to consider all possible defects and decide if any apply. Look at the tree from 2-3 perspectives, close and far and all around. Diameter tapes, hand lenses, binoculars, sounding mallet and field identification guides are all helpful tools. Based on indicators and experience, it may be necessary to follow up initial visual observations with more invasive methods using a pulaski, hand axe, increment borer or drill to look inside the tree. Because wounding can lead to further damage by decay organisms, do not drill or cut into a tree unless other indicators or your experience leads you to believe that additional useful information will be gained. In any case, be sure to document all of your tests and observations, both positive and negative. The various kinds of defect one may likely encounter are outlined later in this guide.

3. Calculate the hazard rating score. The hazard tree evaluation system that is provided combines target and defect into an overall hazard rating for the tree. In some cases, you may feel the rating does not accurately reflect the hazard potential of the tree. Remember, tree inspection and hazard rating are a combination of science and experience. Rate each tree according to what experience and sound judgment tell you is right, and make notes in the space provided on the hazard tree form to document your assessment. If necessary, use a supplemental notebook. Even if the rating seems reasonable, use the notes column to record details on the defects, such as the identity of the root pathogen, canker, conk, etc.

4. Determine a corrective action. Corrective actions may address either the defective tree or the target. Depending on the circumstances, these may include moving the target or removing the tree.

Assessing Targets and Defects

The two most important items to evaluate in a hazard tree inspection are targets and defects. Following are important considerations. Keep in mind that the target and defect categories presented here are guidelines only. Local conditions, history, observations, and experience are far more important to consider when rating targets and defect than strict adherence to guidelines. However, changes should be well documented and consultation with your local Forest Health Protection specialist is encouraged:

1. Assessing Targets

Target potential (sometimes referred to as “failure impact” or “damage potential”) incorporates the potential for a tree or tree part to strike a target (“potential failure zone”), the potential for damaging a target, and the value of the target. Moving targets are less likely to be struck than stationary targets.

When a tree or tree part fails, it may strike other trees or debris on the ground and fling material a considerable distance. For this reason, the potential failure zone of a tree on level ground is generally (depending on local conditions and judgment) one to one and a half times the height of the tree (Figure 3). Similarly, when only a branch, top or other tree part fails, the potential failure zone may be one to one and a half times the length of the tree part that becomes dislodged (Figure 4). On sloping ground where the tree or dislodged part may slide or roll downhill, the failure zone may need to be

Figure 4. Potential failure zone associated with top failure with no slope or lean.

Figure 3. Potential failure zone associated with total tree failure with no slope or lean.

extended on the downhill side for whatever distance is necessary to protect people or property (Figures 5 and 6). Potential failure zones should only be established to the extent that a tree or tree part may reasonably strike a target, and should not be arbitrarily expanded as a pretext to capture timber volume. Inappropriate assignment of target zones in this manner could lead to the loss of the categorical exclusions that allow the routine treatment of hazard trees (FSH 1909.15, Chapter 31.12 (3, 4 and 5).

Once the potential failure zone is determined, the tree is assigned a “Failure Impact Rating” (sometimes referred to as a “Target Rating”) that represents both impact potential and target value.

Generally, a target that is rated high has a high value (person or property) with a high likelihood of being impacted in the event of a failure (long exposure duration, such as a permanent or overnight stationary target). In roadside evaluations, impact potential and target value is rated according to “Exposure Duration” (Long, Short or Intermittent With High Frequency), which reflect such factors as road maintenance level, traffic volume, and degree to which people stop and congregate. Specific examples for both developed recreation area and roadside assessments are in the description of the 7-Point Rating System later in this guide.

2. Assessing Defects

Common indicators of defect are cracks, weak branch unions, stem or branch decay, cankers, dead tree, top or branches, bark and wood boring beetles, root damage and root disease, lean and poor architecture. Multiple and connected defects increase failure potential. Information on the biology, Figure 5. Potential failure zone associated with total tree failure with slope and lean.

Figure 6. Potential failure zone associated with top failure with slope and lean.

identification and management of many of the biotic and abiotic agents that can contribute to tree defect is detailed in the 2009 USFS/CALFIRE California Insect and Disease Training Manual on the internet at http://www.fs.fed.us/r5/spf/publications/foresthealth/manual_090507.pdf. The unique symptoms and probabilities of failure for the above defects are presented below:

a. Cracks. Vertical cracks in the trunk, often accompanied by callus that may or may not be ruptured, may indicate that tree failure has already begun. Some consider them the number one indicator of hazard when a target is present. Cracks most commonly arise from improper wound closure, but can also be from short-term mechanical injury.

Cracks need to be evaluated very carefully. They may or may not indicate substantial amounts of internal wood decay. Some cracks form as a consequence of massive internal decay, as a 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 other weather extremes such as lightning and may or may not indicate decay or immediate failure. Careful evaluation using an increment borer or other tools (such as a resistograph, carbide tipped drill bit or tomograph) will help indicate the extent of any associated decay.

Cracks that indicate high failure potential include:

• A crack that goes completely through the stem or branch.

• A stem that has two cracks on the same segment with a cavity or extensive decay on the inside.

• A stem that has a crack in contact with another defect (canker, decay, weak branch union) or is at the base of a leaning tree.

• A branch (4” or larger) with any crack.

• A conifer stem with a single crack with inrolled bark and a cavity or decay inside.

Cracks that indicate medium failure potential include:

• A hardwood stem with a single crack with a cavity or decay inside (decay should also be evaluated based on the criteria listed below).

Cracks that indicate low failure potential include:

• Trees with a single frost or lightning crack and no internal decay.

Trees with cracks that are determined to have medium or low failure potential that are not removed should be monitored on a regular schedule and after significant weather events.

b. Branch unions and forked tops. A branch union is a fork in the stem or a place where two or more branches join the stem together. Strength of the union is determined by whether the wood is well connected between the branches, or has entrapped (“included”) bark. If bark becomes entrapped by the expanding wood of the branches at the union, it acts as a wedge, preventing the wood of the branches at the union from fusing together.

As the branches continue to grow, more and more bark becomes included in the bark wedge inside the tree, while pressure of the expanding wood against the bark wedge increases. Eventually, the remaining woody connections between the branches are no longer able to support the weight of the branch, and the weak union fails. Bark that curves inwardly into a branch union (“inrolled bark”) indicates that included bark is present within the branch union.

Strong branch unions are characterized by a U-shaped connection with branch angles approaching 90 degrees. Bark at this type of union is usually raised, with a roughened bark ridge over a slightly raised patch of “stress wood” from the branch union. Weak unions have an acute (V-shaped) angle with inrolled bark in the junction between the stems. V-shaped unions with a raised bark ridge at the connection (and no other defect) are generally strong.

Narrow branch angles are more likely to have inrolled and included bark, particularly when one branch is not clearly larger than the other (as with a forked top).

Forked tops need to be evaluated carefully because they may or may not have high failure potential. Forked tops may be weak because they contain included bark as described above.

Forked tops may also form when two or more of the lateral branches take over as a new leader when the old leader is broken or killed. When one lateral branch takes over, the old top is often referred to as a “sucker limb”. Because they are associated with broken or dead tops, these new leaders, which are usually U-shaped, often have internal decay near the point of attachment with the old top. These new leaders become prone to breakage as they become heavier or are exposed to high winds, ice or snow. However, this breakage is less common in pines and incense cedar.

Epicormic branches (also called water sprouts) are new branches that replace injured, pruned, or declining branches. They typically form in response to injury or environmental stress. By their very nature, epicormic branches form weak unions because they are shallowly attached instead of being attached all the way to the center of the stem. They grow and become heavy very quickly. Old, large epicormic branches growing on decaying stems and branches are likely to fail.

Branch unions (including forked tops) that indicate high failure potential include:

• A weak branch union (V-shaped with inrolled bark) that is also cracked, cankered, decayed or streaming pitch. Strong branch unions with these defects, except as listed below, should be assessed on the basis of the associated defects.

• Heavy U-shaped branches of all species except for pines and incense cedar that form when side branches turn up to become leaders. Pines and incense cedar receive a high rating only if an associated defect (cracked, cankered, decayed or streaming pitch) is also present at the branch union.

• Large epicormic branches on decaying stems and branches.

Branch unions (including forked tops) that indicate medium failure potential include:

• A weak (V-shaped) union with inrolled bark.

Branch unions (including forked tops) that indicate low failure potential include:

• Heavy U-shaped branches of pines and incense cedar that form when side branches turn up to become leaders (no additional defect in the branch union).

c. Decay in stem or branches. Decay is a leading cause of tree failure, but is difficult to observe in some trees because the health and vigor of the crown is not a reliable indicator of internal decay. Decay is usually concealed inside the bole, roots and/or branches.

Indicators of internal decay include wounds, broken or dead tops, cracks, wildlife cavities, conks, mushrooms and the presence of carpenter ants or termites. “Sounding” a tree by hitting it with a mallet or the back end of a hand axe and listening for a hollow sound may also give an initial indication of internal decay.

Decaying trees can be prone to failure, but the presence of decay alone does not indicate that a tree is highly defective. 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 decayed without greatly reducing the strength. Thus, when decay is discovered in a tree, it is important to determine its extent and the amount of remaining sound wood. Drilling into the tree or taking core samples with an increment borer can help assess the amounts of sound and decayed wood. High-tech tools, such as a resistograph or tomograph, can also help detect and assess internal decay.

The One Third Rule: Trees are rated in the highest failure potential category if the remaining undecayed, sound wood shell is less than one third of the radius (radius is one-half of the diameter, as measured with a diameter tape). This is known as the “One Third Rule”. For example, if a tree has a 18-inch diameter (9-inch radius), then a minimum of 3-inches of sound wood should be present around the whole circumference to pass the One Third Rule.

Keep in mind that the One Third Rule is only a general guide and there are many exceptions.

Local knowledge, conditions and experience must always be considered. For example, if the tree is leaning, or if the decay column is not in the center of the tree or is in a horizontal branch, or if there is a cavity or crack that opens to the outside, then additional sound wood is needed to support the tree or tree part and the amount of sound wood needed to pass the One Third Rule must be increased. Failure potential is also considered high when there is canker-rot in the main stem or the decay is associated with a weak branch union or open crack.

Decay conditions in stems or branches that indicate high failure potential include:

• Less than ⅓ of the tree’s radius (or diameter) is sound. This is the One-Third Rule for evaluating failure potential. Additional sound wood is needed if the tree is leaning, if the decay is off-center in the bole or present between four feet above the groundline and the lowest live branch, or is associated with an open cavity.

• A cavity, decay or fruiting body is associated with an open crack or weak branch union.

• Decay in a horizontal branch.

Decay conditions in stems or branches that indicate medium failure potential:

• Failure potential of trees with greater than ⅓ of the tree’s radius in sound wood may or may not have medium failure potential, depending on the extent of the decay, species of decay fungus and position within the tree. At the very least, trees with identified decay should be closely monitored on a regular schedule and after significant weather events.

Note: If decay is detected in a tree that has a low failure impact (target) rating, such as one located along a low-use road, but the extent of the decay (whether or not it meets the One Third Rule) cannot be determined because of limited time, access and/or proper equipment, then use the following as a general guideline:

• True fir and hardwoods with known, but unmeasured decay, especially if a cavity is open to the outside, should be considered as having a high failure potential.

• Douglas-fir, incense cedar and pine species with known, but unmeasured decay, es-pecially if a cavity is open to the outside, should be considered as having a medium failure potential.

All trees that have moderate or high failure impact ratings should be thoroughly evaluated with the proper equipment and/or mitigated regardless of species.

Fungal Fruiting Bodies: Fungal fruiting bodies (conks) may or may not indicate high failure potential, depending on the species, size and number of conks. When fruiting bodies are present, it is best to assess associated decay.

Examples of fungal fruiting bodies that indicate high failure potential:

• Phaeolus schweinitzii conks (also known as Schweinitzii root and butt rot, velvet top or cow pie fungus) associated with butt swell on Douglas-fir.

• One or more Indian paint fungus (Echinodontium tinctorium) conk on true fir or hemlock.

• Five or more red ring rot (Porodaedalia (Phellinus) pini) conks on Douglas-fir, ponderosa pine, Jeffrey pine, lodgepole pine, or more than one on true fir or hemlock.

• One or more quinine (Fomitopsis officinalis) conks on Douglas-fir, pines, western larch, spruce or hemlock.

• One or more sulfur fungus (Laetiporus sulphureus) conks (also known as “chicken of the woods”) on a wide range of conifers and hardwoods, including Douglas-fir, true firs, pines, hemlocks, spruces, larch, western redcedar, oaks, maples, birch and willow.

Examples of fungal fruiting bodies (conks) that indicate medium failure potential:

• Phaeolus schweinitzii conks without associated butt swell on Douglas-fir.

• Fewer than five red ring rot (Porodaedalea (Phellinus) pini) conks on Douglas-fir, pon-derosa pine, Jeffrey pine, lodgepole pine, or one on true fir or hemlock.

• Incense cedar pecky rot (Oligoporus amarus) conks on incense cedar greater than

150 years old.

d. Cankers. A canker is an area of exposed sapwood or dead cambium underneath the bark. Cankers often have decayed wood underneath. Cankers can be caused by fungi, insects, weather, fire or other mechanical damage. Stems or branches often fracture at or near cankers. Cankers caused by fungi can look similar to exposed scars, but depending on the fungal species, fungal cankers can expand over time. Invasion of underlying wood by decay fungi is often the most serious consequence of a canker. However, depending on the tree species, a canker that encompasses more than one third or one half of the circumference of the tree is often hazardous, even if the exposed wood appears sound.

Trees with cankers should be evaluated for internal wood decay. When applying the One Third Rule, cankered trees require more sound wood to be stable. In addition, the potential for failure is higher if a canker is accompanied by decay or if it is connected to another defect, such as a lean or crack.

Ramorum canker (also known as sudden oak death) is caused by the fungus-like water mold, Phytophthora ramorum. Millions of trees have been killed by ramorum canker in California and Oregon since the mid-1990’s. Affected species include tanoak, California black oak, Shreve’s oak, coast live oak and canyon live oak. Although many other plants are infected by P. ramorum, tree-killing bole cankers are only produced in tanoak and the oaks listed above. Trees infected by P. ramorum are often rapidly colonized by ambrosia (wood boring) beetles, bark beetles and decay fungi, making them prone to breakage, even when they are still alive.

In fire-injured trees, cankers are caused by lethal heating of tissues resulting in the death of the cambium. Dead cambium is often indicated by deep charring of the bole, where all of the bark is blackened and the bark characteristics are no longer discernible. When assessing defect in an area with deeply-charred trees, the consistency of the relationship between deep char and underlying dead cambium needs to be checked by cutting into the affected area of the bole. If deep char reliably indicates underlying dead cambium, then all areas with dead char may be assumed to be cankered. However, if it does not, then all deeply charred areas have to be checked on all trees that are assessed.

Cankers that indicate high failure potential include:

• Cankers with associated fruiting bodies of decay fungi.

• Cankers with associated internal decay.

• Cankers physically connected to a crack or other defect.

• Single or multiple cankers without decay that affect more than ½ of the tree’s circum-ference, particularly if the cankers are located between four feet above the groundline and the lowest live branch.

• Basal cankers in true fir that affect more than ⅓ of the bole circumference.

• Cankers in oak or tanoak caused by Phytophthora ramorum (ramorum canker or sud-den oak death) that affect more than ⅓ of the bole circumference, or have associated decay fungi, ambrosia beetles or bark beetles.

• Deep charring in true fir over more than ⅓ of the bole circumference when the relation-ship between deep char and cambial mortality has been confirmed.

• Deep charring in sugar pine, ponderosa pine, Jeffrey pine, incense cedar or Douglas-fir over ½ of the bole circumference when the relationship between deep char and cambial mortality has been confirmed.

Cankers that indicate medium failure potential include:

• For all species other than true fir, single or multiple cankers without decay that affect less than ½ of the tree’s circumference (including fire-caused cankers).

• For true fir, single or multiple cankers without decay that affect less than ⅓ of the bole circumference (including fire-caused cankers).

• For oak or tanoak, cankers caused by Phytophthora ramorum (ramorum canker or sudden oak death) without associated decay, ambrosia beetles or bark beetles that affect less than ⅓ of the bole circumference.

• A large old wound or canker with no decay at the base of a leaning tree.

Cankers that indicate low failure potential include:

• True fir with bole swelling from dwarf mistletoe infection, but with no bark sloughing or evidence of decay.

• Lodgepole pine or ponderosa pine with basal (hip) cankers from western gall rust with no bark sloughing or evidence of decay.

• Lodgepole pine or ponderosa pine with elongated stalactiform rust cankers covering less than ⅓ of the bole circumference.

e. Dead tree, top or branches. Dead trees are simple to identify and among the most likely to fail. If a target exists, dead trees should be removed immediately. Snag trees for wildlife habitat should not be left if they are tall enough to hit a target. Dead tops and limbs are likewise easy to identify, and should be checked closely for evidence of rot or instability (e.g. exhibiting indicators of decay, previous breakage, wood destruction by woodpeckers or, in some species, sloughing bark). The dead portion of the tree must have the potential to reach a target. Dead tops of true firs tend to decay very quickly and usually require removal. New dead tops in Douglas-fir, spruce, hemlock, and hardwoods are highly susceptible to attack by decay fungi, and their failure potential is higher than other conifer species on the same sites. However, old spike tops in pine, incense cedar, juniper and Douglas-fir that give evidence of long-term persistence are usually dry and resin-impregnated, and are generally not any more hazardous than living portions of the tree. Large dead branches (larger than two inches in diameter) have an obvious potential for failure and should be removed, especially from hardwoods. Broken branches that are hanging or lodged in the crown should be treated immediately if a target is nearby. Note that healthy trees with dead limbs may be pruned rather than removed. Large dead dwarf mistletoe brooms should likewise be removed, while live ones should be removed if they have associated decay or other defect.

Dead tree, top or branch conditions that indicate high failure potential include:

• Any dead tree.

• Dead tops greater than ten feet long or smaller ones with associated decay or other defect (note that old dead tops of pine, incense cedar, juniper or Douglas-fir may not have high failure potential).

• Dead branches greater than two inches in diameter, branches that are hanging or lodged in the crown, large dead dwarf mistletoe brooms and large live dwarf mistletoe brooms with associated decay or defect.

Dead tree, top or branch conditions that indicate medium failure potential include:

• Dead tops less than ten feet long with no associated decay or other defect (note that old dead tops of pine, incense cedar, juniper or Douglas-fir may have lower than me-dium failure potential).

• Any branch greater than two inches in diameter and more than ⅔ dead (remove the entire branch).

• Live dwarf mistletoe brooms with no associated decay or other defect (monitor closely).

Dead tree, top or branch conditions that indicate low failure potential include:

• Old spike tops in pine, incense cedar, juniper or Douglas-fir that give evidence of long-term persistence.

f. Trees attacked by bark and/or wood-boring beetles. Bark beetles have the ability to attack and rapidly kill affected trees. Wood boring beetles typically attack heavily stressed, dying or dead trees. They can introduce wood decay fungi and hasten the rate of decay and failure. Important bark and wood boring beetles in California include mountain pine beetle (Dendroctonus ponderosae), Jeffrey pine beetle (Dendroctonus jeffreyi), western pine beetle (Dendroctonus brevicomis), Douglas-fir beetle (Dendroctonus pseudotsugae), fir engraver beetle (Scolytis ventralis), pine engraver beetle (Ips spp.), and several species of flatheaded (Buprestidae) and roundheaded (Cerambicidae) borers, including the goldspotted oak borer (Agrilus auroguttatus) on California black oak, coast live oak and canyon live oak in southern California. Red turpentine beetle (Dendroctonus valens) is also important, but rarely kills trees on its own. Trees that are mass-attacked by bark beetles are usually under stress from other physical conditions or diseases, including drought, root disease or dwarf mistletoe cankers. Trees in advanced stages of attack cannot be cured, and will die within six months to a year. After that, they will decay and become highly likely to fail. Because the progression from advanced infestation to failure can be rapid, it is important to identify and remove heavily infested trees with targets, or mitigate the hazard by moving the targets.

Green trees that have evidence of significant bark and/or wood boring beetle activity are considered dead and indicate high failure potential if they possess any combination of the following factors over at least one third of the bole circumference:

• Pitch tubes with pink or reddish boring dust associated with them (in contrast to clear pitch streamers that indicate successful defense against attack).

• Pouch fungus conks and/or current woodpecker activity (holes into the sapwood and/ or bark flaking, specifically excluding injury caused by sapsucker feeding).

• Boring dust or frass (in bark crevices, webbing along the bole, or that accumulates at the base of trees).

Note: This specifically excludes basal attacks by the red turpentine beetle (large pitch tubes associated with coarse boring dust generally restricted to the lower two to three feet of the bole or woodpecker activity restricted to this area) and when the above indicators are only associated with wounds, old fire scars, etc. (Smith and Cluck 2011).

Green trees are also considered dead and indicate high failure potential if they have the following indicator of significant bark and/or wood boring beetle activity:

• 50% or more of the foliage-bearing crown actively fading, as indicated by a uniform change in color over that part of the crown. Dead tops that have no foliage are not part of the living crown and do not count toward this 50%. This also does not include drought-induced needle cast (non-uniform fading restricted to the older needles) or branch mortality (“flagging”) caused by dwarf mistletoe/Cytospora infections in true fir.

g. Root damage and root disease. Root problems are generally difficult to find and assess since tree roots are underground and out of sight. The two major kinds of root problems are physical and biological. Physical problems include undermined, severed, loosened, cracked, broken, exposed, and stem-girdling roots. A variety of activities can cause these root problems, including soil compaction, erosion, flooding or saturation, construction activities, prolonged heavy equipment or foot traffic, etc. Biological problems are generally caused by root disease and decay fungi. Root problems often only become apparent when tree crowns begin to show symptoms such as chlorosis, thinning, terminal growth loss and production of distress cones, or when signs of root failure become obvious, such as soil cracking or mounding, root lifting or breaking, partial windthrow or increased lean. Additional symptoms or signs of root disease include basal resinosis (patches of resin-impregnated wood and/or resin flow at the base of the tree, not associated with insect attack); for some root diseases, unusual swelling of the butt; root decay or butt rot;

bark beetle mass attack; the presence of nearby fading, standing dead, windthrown or shattered trees; and the production of certain characteristic mushrooms or conks at root collars and in nearby decayed stumps.

Heterobasidion (annosus) root disease is the most widespread and damaging root disease in California, affecting many species of conifers and a few hardwoods. Pines, incense cedar, western juniper, true firs and giant sequoia are highly susceptible hosts, and failure of these trees in root disease infection centers is common. More complete information on Heterobasidion root disease is in the newly revised Forest Service Handbook R5 Supplement 3409.11, Chapter 60 (effective February 9, 2010).