Hazard trees: what CAL FIRE's field guide actually says
Updated: Sep 21
CAL FIRE's California Power Line Fire Prevention Field Guide never defines the term hazard tree. Its 2021 Hazard Trees section, pages 41 through 52, sets a two part inquiry instead: whether the tree can strike utility infrastructure under reasonably foreseeable conditions, and how likely it is to fail. Reach is decided first.
A 70 foot white fir with a dead top stands 90 feet from the nearest phase of a 12 kV span in a State Responsibility Area. The dead top is the least ambiguous defect in either edition of CAL FIRE's field guide. The November 2008 edition says dead tops in white and California red fir "must be considered hazardous." The 2021 edition says dead tops in non-resinous conifer species "should be abated."
The tree is still not a hazard tree to that span, and the guide disposes of it in one sentence. Move the same tree to 45 feet and nothing about the defect has changed, but the answer has. A hazard tree is not a condition of a tree. It is a relationship between a tree, a target and a weather assumption, and each of the three has to appear in the record for the finding to survive being read a year later.
The guide names the section and never defines the term
The 2021 California Power Line Fire Prevention Field Guide gives this twelve printed pages, 41 through 52, under the heading "Hazard Trees/Vegetation Clearance." In the whole 132 page document the phrase "hazard tree" occurs fourteen times: once in the main table of contents, once as the section heading, and twelve times in the running header printed at the top of the section's pages. It occurs in no sentence. There is no glossary entry, no definition paragraph and no criteria list carrying the term.
What the section defines is failure, at page 43:
Tree failure is the structural failure of the root system, stem (trunk), or branches. Structural failures may occur when the forces acting on a tree exceed the strength of the tree structure or the soil supporting the tree. Even a structurally strong tree may fail when a load or force is applied that exceeds the strength of one or more of its parts.
The November 2008 edition, whose section is headed "12. Hazardous Tree Identification," comes nearer to a definition in its opening paragraph, though what it gives is a description rather than a test:
Except in unusual circumstances (extreme weather conditions, etc.), healthy live trees seldom cause such problems. Yet trees are subject to injury, disease, insect and fungus attacks and ultimately death. When so significantly afflicted, they may become hazardous to power lines or any other improvements.
That is as close as either edition gets. Note the wording of the older one. The 2008 guide never uses the phrase "hazard tree" at all. It uses "hazardous tree," three times, two of them in the section heading. The regulation that gives the term its legal weight, 14 CCR 1257(b), puts "Hazard Trees" in quotation marks as though quoting a defined term and points at that November 2008 guide for its content.
And 14 CCR 1251, which defines eight terms for the whole of Article 4, from "Conductor" through "Tree wire," defines neither "hazard tree" nor "hazardous tree." What that incorporation by reference does to an exemption, and where the page range it names goes wrong, is that section's own subject. What is left for everybody working under it is a term with a legal consequence and no definitional sentence anywhere in the stack.
The first question is whether the tree can reach
One sentence at page 43 disposes of more field cases than the rest of the section combined:
If a tree cannot strike the utility infrastructure during reasonably foreseeable conditions, it is not hazardous to it.
Read the last two words. To it. The white fir in the opening paragraph may be hazardous to a road, a structure or a crew working beneath it, and none of that makes it a hazard tree to the span. Hazard in this guide is relational, and the relation has to be established before any defect matters.
The 2008 edition put the same principle in the limb section, at 16.2, and stated it as a test with two required elements:
A basic principle in tree hazard control is that a potential hazard exists only if there is a likelihood of injury or damage should a tree, or part of a tree, fail. Before a situation can be said to be potentially hazardous from tree limbs, two basic elements must be present: (a) defective limbs larger than a specified diameter and length (see 16.5), and (b) limbs located so that in falling they have a likelihood of striking power lines.
Both editions therefore put reach ahead of defect, which means the most common gap in a hazard tree file is not a missing defect description. It is a missing reason why a visibly defective tree was left standing.
The guide also concedes, in the same section, that the term has no content without a stated weather envelope. Page 42:
For the purposes of this section of the Guide, the scope is limited to reasonably foreseeable and observable field conditions that are not extreme. The reason for this limitation on scope is practical. As explained below, trees fail when force(s) exerted upon them are stronger than the trees. If the scope of this Guide were all-encompassing, intended to apply to all possible field conditions, then it becomes clear that every tree can be hazardous once the forces become strong enough (i.e. during extreme conditions) to cause it to fail.
Page 50 supplies the envelope, and it is not a fixed one. "Normal weather" is defined there as a meteorological term describing a location's average temperature, wind and precipitation for the previous 30 years, with extreme and abnormally extreme storms as two further categories the guide admits are "less clearly defined." Then the operative instruction: if a region is prone to strong storms or heavy snowfall and such events are likely to occur before the next inspection interval, the inspector should consider the likelihood of failure during such events.
So the envelope is the 30 year normal, stretched to cover whatever is reasonably expected to arrive before somebody looks at the tree again. On a corridor where the next look is scheduled fourteen months out, that envelope is wider than on one inspected each spring, and the file should say which interval the judgment was made against.
Height, path, lean and weight
Page 43 names four factors that decide reach, and the section then takes each in turn.
Height is measured from the hinge, not from the ground. The guide says a tree is very unlikely to strike in two cases: when it is shorter than the utility facilities, or when the distance to the conductor is more than the height of the tree or the part most likely to fail. It then adds that the inspector "should consider slope during this portion of the evaluation and should evaluate the tree from the point it will 'hinge,' which may not be ground level."
The 70 foot white fir at 90 feet on level ground cannot reach the phase. The same fir on a cut bank whose root plate sits 15 feet above conductor elevation, hinging at the top of that bank, is a different arithmetic problem. A record that captured only "90 feet from conductor" did not capture the one that was actually solved.
Path is what stands in between. Trees, buildings or other objects between the subject tree and the infrastructure "may shield the infrastructure from contact. If strong and numerous enough, the trees or objects may intercept the subject tree or branch and prevent it or direct it away from striking facilities." The qualifier is doing real work. A single 20 foot ornamental plum under a 70 foot fir is neither strong nor numerous, and a claim of interception that rests on it will not hold.
Lean is the only place in the section where a condition of the tree, by itself, takes it out of scope. Page 44: "Trees with more than a slight lean away from utility infrastructure are unlikely to strike the infrastructure, regardless of their weight distribution. Within reasonably foreseeable field conditions, such trees are generally not hazardous to infrastructure." Everything else about lean is a matter of reading the ground.
The guide separates corrected lean, which is a hardwood growing decurrently or a conifer that has straightened after snow load and "may not constitute a structural weakness," from uncorrected lean caused by wind, soil conditions, severed roots, construction or the impact of another falling tree. Humps and soil mounding on the side opposite the lean are named as indicators of broken or loosened roots.
And then the guide's only instruction to act on the spot: "Cracks in the bole and roots are often signs of a failure in progress, and abatement may be required right away." What authority exists to act on that observation before anyone is notified is a separate question with its own answer.
Lean is also where the guide and the statute diverge in emphasis. PRC 4293's trailing paragraph puts trees leaning toward the line inside its scope by name, subject to their own reach condition, and requires them to be felled, cut or trimmed. The guide treats lean as one input to a likelihood judgment. A tree leaning toward a conductor is in the statute's scope whatever the guide's likelihood analysis concludes, and the file has to answer both.
Weight gets a single sentence, and it is fair to say so rather than to pad it: "The inspector should evaluate weight distribution within the tree, particularly while assessing limb failure."
Around those four sits the target itself. Page 42 defines "utility infrastructure" as the overhead electrical facilities plus the communications infrastructure that frequently occupies the same poles, with a footnote observing that communication lines pose a risk of pulling on or snapping and thereby pulling down or contacting power lines.
Single use communications pole lines are treated differently: for fire prevention purposes they are "generally not an ignition source because the energy within communications infrastructure is usually insufficient to ignite a fire." Which attachments on a shared pole change the answer is worth settling before a corridor is walked.
The target zone is then defined three ways in one paragraph on page 43, "in terms of distance from conductors, distance from the edge of a maintained corridor, or distance from the centerline of a right-of-way (ROW)." Those are three different baselines that produce three different numbers on the same span, so a distance recorded without its baseline is not a measurement.
Why the corridor edge and the easement boundary are not the same line matters here more than anywhere else in the guide. And for critical infrastructure the zone stops being a distance at all: for substations and high voltage transmission lines, "the target zone may include any tree tall enough to strike the target," which is the same logic FAC-003 applies on a transmission right of way.
Likelihood of failure is a list, not a threshold
The second half of the inquiry starts at page 45 with a list the guide labels non-exhaustive:
standing dead trees and dead parts of trees
broken or hanging branches
cracks
weakly attached branches or codominant stems
decayed or missing wood
unusual tree architecture, meaning lean, balance, branch distribution or lack of taper
loss of root support
shallow soils
insect infestation
diseases
suppressed or intermediate stems within a forest stand
fire damage
Twelve items, no thresholds, and an express instruction that defects can be rated in aggregate. The guide's own worked example: "the likelihood of failure of a specific dead branch might be rated as possible, while the likelihood of failure of any one of several dead branches in a tree might be rated as likely."
The heart rot material shows what the section is doing most clearly. Page 46 gives fourteen things an inspector's assessment should include but not be limited to. Among them:
open wounds showing visible rot
conks anywhere on the bole
wildlife cavities
carpenter ants or termites
the number, size and distribution of fungal fruiting bodies
the amount of solid radial wood remaining where visible
poor live crown ratio and poor diameter to height ratio
The 2008 edition covered the same ground with seven lettered items at 14.1 and a separate five factor list at 14.2 headed "Degree of Hazard from Heart Rot or Butt Rot." The newer guide roughly doubled the number of things to look at and removed the section that told you what the observations added up to.
One sentence at page 47 has a consequence most programs do not build for: "An inspector may need to conduct an invasive inspection to determine the extent of decay where detected. They may also need to sound supporting wood in the decayed area.
If an inspector elects to abate a tree, further invasive inspection techniques may not be required." Deciding to abate closes the investigation. Deciding to leave the tree standing does not, and a file recording a hollow-sounding trunk with no bore, no sounding and no abatement has left the guide's own next step undone.
Page 49 is the sentence to hand any inspector working from photographs: "Inspectors should not confuse tree health and tree stability. High-risk trees can appear healthy in that they can have a dense, green canopy."
Adventitious rooting or sufficient vascular transport in the sapwood can keep a crown full while the structure that holds it up is inadequate. A green crown in an image is not a defense, and neither is a poor crown a finding on its own, since the guide says on the same page that trees in poor health may still be structurally stable.
The same page directs utility vegetation managers to monitor tree failure incidents and build experience-based failure profiles for the species actually present along their lines, and names the Western Tree Failure Database, formerly the California Tree Failure Reporting Program, as a source. It then limits what a profile can do. Failure profiles "are an aid to analysis, but they do not substitute for working through the steps of the inspection process." All portions of a tree should be considered rather than only those matching the species' common failure pattern.
The numbers the 2021 edition removed
The two editions differ in kind, not only in wording. The 2008 guide told an inspector what to measure and what the measurement meant. The 2021 guide tells an inspector what to look at.
Condition | November 2008 edition | 2021 edition |
Dead tops and limbs, conifers | Remove when defects exist, or when limb size exceeds 3 inches in diameter and 6 feet in length (16.5.A) | No dimension. Species-specific limb failure information "can support abatement decisions" (p. 48) |
Dead tops and limbs, true oak and madrone | Remove when limb size exceeds 2 inches in diameter and 4 feet in length (16.5.C) | No dimension |
Root exposure and burial | 25 percent or more root exposure; grade altered so 25 percent or more of the root system or the trunk is buried (12.1.A, 12.1.B) | No percentage; physical and biological root problems listed by example (p. 46) |
Dwarf mistletoe and rust cankers | Breakage likely when the dead face approaches half the circumference of the swelling; rust canker a concern where the depression is deep and sixteen feet or more above the base (15.1) | No fraction and no height (p. 47) |
Broken out conifer tops | "are not considered to be hazardous" (17.2) | "usually not considered to be hazardous," and the inspector should examine the ratio of decayed wood to sound wood (pp. 48 to 49) |
Volunteer tops in conifers | "not considered hazardous so long as such tops remain live" (17.2) | "may be hazardous because as the tree regrows, it adds weight to wood that may be decaying" (p. 49) |
Dead tops, white and California red fir | "must be considered hazardous, and such tops should be removed as soon as possible" (17.1) | "Dead tops in non-resinous conifer species should be abated" (p. 48) |
Every categorical statement in the older edition became conditional, and every number disappeared. For a program written to the current guide that is manageable, because it is guidance and the judgment is the deliverable. For one place it is not manageable, because the older numbers are still the operative ones.
14 CCR 1257(b) withholds every exemption in that section from Hazard Trees "as identified and explained" on named pages of the Department's November 2008 guide. It names the edition by date.
A regulation does not update because a department publishes a new edition, and the 2021 foreword's instruction to discontinue prior editions is an instruction to field personnel, not an amendment to Title 14. So the identification criteria that decide whether an exemption survives are the 2008 ones, including the three inch by six foot conifer limb and the 25 percent root exposure.
Meanwhile the same utility's felling program under PRC 4293 is being measured against a species profile and an inspector's judgment. One white fir, two documents, and a file that quotes only the current guide has answered only one of them. Which agency reads which of the two decides which answer gets asked for first.
Imminent, likely, possible, unlikely, and the words the guide changed
Page 51 gives the only rating scale in the section:
The likelihood of failure within a specified time frame can be categorized using the following guidelines: Imminent: Failure has started or is most likely to occur very soon. Immediate action may be required. Likely: Failure may be expected under normal conditions. Possible: Failure may be expected in extreme conditions, but it is unlikely during normal conditions. Unlikely: The tree or branch is not likely to fail during normal conditions and may not fail in extreme conditions.
Three of those four labels sit one word away from the categories used in the International Society of Arboriculture's tree risk assessment best management practices, the companion publication to ANSI A300 Part 9, which rates likelihood of failure as Improbable, Possible, Probable and Imminent. CAL FIRE renamed Probable to Likely and Improbable to Unlikely, which is cosmetic, and made one substantive change: the ISA definitions each end in the phrase within one year, and none of the guide's four does.
What replaced the fixed year is the utility's own interval. The scale is introduced as "the likelihood of failure within a specified time frame." Page 42 says where that time frame comes from: "A time frame until the next inspection (inspection frequency) should be specified; this is usually the inspection interval determined by the utility and agencies accordingly." The consequence is arithmetic.
A codominant stem with included bark rated against a twelve month window and the same stem rated against a five year window are not the same rating, and neither inspector is wrong. The rating is only readable if the interval it was made against is recorded beside it, which is why the GO 165 intervals and the growth and cycle assumptions behind a vegetation program belong in the same file as the tree.
The A300 connection is worth stating exactly, because it is often described as a requirement and it is not. Page 51: "When determining their approach to tree inspections along their facilities, utilities have the option of following the American National Standards Institute (ANSI) A-300 tree risk assessment standard per their field conditions and needs." A footnote names ANSI A300 (Part 9) 2017 Tree Risk Assessment. An option, chosen per field conditions and needs.
That leaves a gap the guide does not close. It gives a likelihood of failure scale on page 51. On page 52 it gives an instruction about consequences, and the potential consequences of failure include:
damage to human life and property
regulatory compliance enforcement actions
service interruption
facility damage and repair
fire with associated losses
They "vary in severity and should be rated according to that severity." The guide never combines the two into a risk rating. A300 Part 9 and the ISA practices supply that matrix; the guide leaves it out and makes the standard optional.
Two utilities can both follow the field guide faithfully and reach different dispositions on the same white fir. Where the guide is silent the method is the utility's own choice, and the only thing that makes the choice auditable is naming it in the procedure and applying it consistently.
What the guide is, and which document creates the duty
The foreword at page 1 sets the guide's authority twice, and both statements matter to a hazard tree finding. First: "These procedures are considered minimum guidelines. Field conditions may warrant more stringent procedures." Second:
Regardless of varying interpretations of Guide language, the law must be obeyed. Thus, if there is any conflict between any statement in this Guide and any applicable statute, regulation or order, the statute, regulation or order shall take precedence.
The same page makes the guide a moving target. It is now "a living document," the online version "will have portions updated as necessary" rather than at whole-edition intervals, and CAL FIRE maintains a Power Line Fire Prevention Field Guide Update Tracker Log for recent, ongoing and pending changes. A program document that quotes the guide should therefore record the edition and the date the text was taken, because the section a procedure was written against can change without a rulemaking and without a new cover date.
The duties themselves live in four provisions, and no two use the same vocabulary:
Provision | The words it uses | What triggers the duty |
Dead, old decadent or rotten, weakened by decay or disease, and leaning toward the line | The condition itself, with a reach condition written in | |
Dead, rotten or diseased trees, or such portions of otherwise healthy trees, that overhang or lean toward and may fall into a span | The company's actual knowledge | |
Hazardous, dead, rotten, diseased, or structurally defective live trees | Access to abate, after notice and an opportunity to be heard | |
"Hazard Trees," in quotation marks | A finding withdraws every exemption in the section |
Only the last of the four uses the term this page is about, and it supplies its content by pointing outside the regulation. So a field note reading "hazard tree" and nothing else has not told a later reader which of the four columns the crew was standing in, and the three statutory columns do not overlap cleanly enough for the ambiguity to be harmless.
One absolute does exist elsewhere in the guide, and it is easy to miss because it sits in a different section. Page 12, under "Tree Connections": conductors may be attached to trees only in accordance with 14 CCR 1257 and 1258, "However, in no case are conductors of any kind to be mounted to snags or dead trees." No likelihood analysis, no target zone, no exception.
A finding of not hazardous is dated
Page 42 contains the sentence a program should quote in its own procedure, because it is the one that makes a clean inspection defensible after an ignition rather than indefensible:
A tree failure inspection is time-dependent; the inspection and conclusions are drawn at the time of the inspection, and conditions may change before the next inspection is made. Usually, in the utility context, inspections happen at regular intervals. Making the determination that a tree does not require abatement work means that at the time of the inspection, the inspector determined that the tree was not hazardous.
The finding is dated and bounded. It is not a warranty about the tree, and the guide says so in its own voice. What that requires from the record is the date, the conditions observed on it, and the interval the conclusion was scoped to. A no-abatement entry carrying only a checkmark and a tree number has none of the three.
The same page describes who makes it, and names no credential:
Persons conducting tree failure inspections should be properly equipped, trained and possess the knowledge and experience necessary to draw conclusions and make decisions about whether a tree requires abatement at the time of the review. Training should be documented.
Judgment "can be guided by applying knowledge and experience, examining local species failure profiles, literature and site conditions," and the guide adds that the person's knowledge should include review of recent publications about factors affecting tree health and sustainability, "like climate change." Trained, equipped, experienced, documented, and no certification named anywhere in the section.
Set that against the same tree's other file. 14 CCR 1257(a)(3) requires a Certified Arborist or a Registered Professional Forester to confirm that a tree qualifies as an Exempt Tree, by name, for a finding about six inches of clearance.
The hazard tree finding that cancels that exemption under 1257(b) has no named credential attached to it at all. The asymmetry is real and it is in the regulation, and the practical answer is that the training record behind a hazard tree finding is the only thing standing where a license number would otherwise be.
What the record has to name
The guide gives its own instruction for a tree that needs work, at page 51:
When a tree needs abatement, it must be identified for those who will do the work and for follow-up inspection. The tree should be marked with flagging, timber-marking paint or other means like GPS coordinates and/or electronic record. It should also be located by map, sketch, or bearing and distance from an identifiable object. In some cases, one or more photographs would be helpful.
And at page 52, in one line: "The decision to abate a tree should be properly documented."
That is the floor. A finding that will survive being read by somebody who was not there carries more, and every field below is either in the guide's own two part inquiry or in one of the four provisions that create the duty:
The tree. Identifier, species, diameter at breast height, total height, and the circuit and structure it sits against.
The target and the reach. What the target is, conductor, energized hardware, a communications attachment on the same pole, or a substation. The distance, and the baseline it was measured from, conductor, corridor edge or right of way centerline. The hinge point used, and the slope, where those are not ground level and flat.
The path. What stands between the tree and the target, or an explicit statement that nothing does. An interception claim names the objects.
The defect, as an observation rather than a conclusion. "Conk at four feet on the north face, 18 inch open basal wound, hollow response across roughly 40 percent of the diameter on sounding" is an observation. "Decayed" is a conclusion, and a conclusion cannot be re-examined by a reader who disagrees with it.
The lean. Direction relative to the line, corrected or uncorrected, and the ground evidence, including soil mounding or bole and root cracks where present.
The rating and its window. The likelihood of failure category in the guide's own four words, the time frame it was rated against, and whether ANSI A300 Part 9 was applied. If a consequences rating was made, the method that combined the two.
The disposition and its authority. Abate or leave, and which provision the finding was made under, named as PRC 4293's trailing paragraph, GO 95 Rule 35's overhang provision, PRC 4295.5's abatement list, or 14 CCR 1257(b). For an abatement, the marking method used and the date the work was completed.
The person and the date. Name, the training record behind the judgment, and the inspection date, with the next scheduled inspection date for anything left standing.
Findings also arrive from the other direction. A fire agency writes its own up on CAL FIRE's form LE-38a and sends a copy to the utility, and page 16 holds those reports to a standard of specificity aimed at both the utility and a court. How that notice is issued and what it obliges is set out elsewhere. What it means here is that a utility entry answering an LE-38a should name at least as much as the notice did.
What a complete clearance record holds around these entries and how long each part has to be kept are their own subjects. The part the hazard tree material contributes is narrower and easy to state: the guide will not tell an auditor whether a tree was a hazard tree. It will only tell them whether the person who decided asked the two questions the guide asks, in the order it asks them, and wrote down enough to be checked.
Further reading
14 CCR 1257: which trees are exempt, and the records that prove it: The one operative provision in the stack that uses the term, and what a hazard tree finding costs the utility that claimed an exemption.
PRC 4293: radial clearance in State Responsibility Areas: The felling duty a hazard tree finding usually feeds, and the four tree conditions the statute names instead.
CPUC GO 95 Rule 35: the vegetation clearance rule, explained: The Commission's overhang provision, its actual knowledge trigger, and its own vocabulary for a defective tree.
Frequently asked questions
Does CAL FIRE's Power Line Fire Prevention Field Guide define a hazard tree?
No. In the 2021 edition the phrase hazard tree appears fourteen times, and every one of them is a table of contents entry, the section heading, or a running header. It appears in no sentence. The guide defines structural tree failure at page 43 and sets a two part inquiry, likelihood of impact and likelihood of failure, without ever defining the term in its own section title.
What is the test for whether a tree is hazardous to a power line?
Reach first, then failure. Page 43 of the 2021 guide states that if a tree cannot strike the utility infrastructure during reasonably foreseeable conditions, it is not hazardous to it. The November 2008 edition made the same point at section 16.2, requiring two basic elements: a defect of specified size, and a position from which the failed part has a likelihood of striking power lines.
Which edition of the field guide governs a hazard tree finding?
For guidance purposes the current edition governs, because the 2021 foreword discontinues prior editions except as historical copies. For the one place the term carries regulatory force, 14 CCR 1257(b), the regulation names the Department's November 2008 guide by date, and a regulation does not change because a department publishes a new edition.
What are the field guide's likelihood of failure categories?
Page 51 of the 2021 guide gives four: Imminent, failure has started or is most likely to occur very soon, and immediate action may be required; Likely, failure may be expected under normal conditions; Possible, failure may be expected in extreme conditions but is unlikely during normal conditions; and Unlikely, the tree or branch is not likely to fail during normal conditions and may not fail in extreme conditions.
Does a hazard tree finding have to be made by a Certified Arborist?
The field guide names no credential. It says at page 42 that persons conducting tree failure inspections should be properly equipped, trained and possess the knowledge and experience necessary to draw conclusions, and that training should be documented. The named credential requirement in Article 4 sits elsewhere: 14 CCR 1257(a)(3) requires a Certified Arborist or a Registered Professional Forester to confirm an Exempt Tree.
