Douglas-fir near power lines: height, fall zone and fire risk
The clearance distance reaches only a Douglas-fir standing inside it. The species grows past the width of any right of way and fails at the base from decay seeded by an old fire scar, while the crown stays green. The tree that started the Dixie Fire was 65 feet tall and stood about 40 feet from the pole.
At 6:48 on the morning of July 13, 2021, a Douglas-fir fell across a 12 kV distribution span in the Feather River canyon. The base of that tree stood about 40 feet from Pole 100403909. In the High Fire-Threat District, the clearance PG&E owed between that conductor and any vegetation was four feet.
The tree was 65 feet tall. Four feet governed the prune. Sixty-five feet governed the outcome.
That is the whole of the Douglas-fir problem stated once. Three questions follow from it, and the clearance table answers a different one:
whether the tree can reach the conductor from where it stands
whether the wood at the base is still holding the tree up
whether anyone will look at that wood before the next cycle closes
The Dixie Fire burned 963,309 acres. PG&E had ranked that circuit 11th of 3,535 for wildfire risk. The Commission's investigation report, dated October 9, 2023, runs 50 pages, and most of what it establishes about the species is contained in one sentence: PG&E's Inspection Record Details going back to 2016 do not mention the Subject Tree.
The tree owed four feet and stood sixty-five
Safety and Enforcement Division opened incident W20210713-01 on the Bucks Creek 1101 Circuit, at coordinates 39.874608, -121.378855 near Cresta Dam in Butte County. The circuit sits in Tier 2 of the High Fire-Threat District, and PG&E's own Equipment Risk Model ranked it 11 out of 3,535 circuits for wildfire risk, which SED reads as the top one percent. Its report quotes CAL FIRE's causation finding:
The fire ignited as a result of a 65' tall, damaged and decayed Douglas fir tree when it fell and contacted conductors at approximately 6:48 AM. Two of the three fuses blew (opened) upon initial contact with the conductors, but the third fuse remained closed and kept a line energized. The tree being in contact with energized conductors and the ground created a high impedance fault. The high impedance fault energized the tree, which caused heat and arcing to ignite a dry and receptive fuel bed over the course of 10 hours.
SED walked the site and recorded the geometry. The root cluster left behind after CAL FIRE removed the stump was "estimated to be 40 feet from Pole 100403909," and the root photographed in Figure 14 "had a significant amount of rot at the center."
Now set that against what the corridor was being measured to. SED states the standard in one line:
The MDRs from vegetation to distribution assets, such as conductors, are set by GO 95, Rule 35 at 1.5 feet generally and four feet for vegetation in High Fire Threat Districts (HFTDs).
What Rule 35 requires and who it binds is a separate subject, as is the radial clearance the same span owes CAL FIRE. Both of them are distances measured from the conductor outward. A tree standing 40 feet away is compliant with both on the morning it falls.
The provision that does reach it is the reach test, and the guide puts reach ahead of defect: a tree that cannot strike the utility infrastructure under reasonably foreseeable conditions is not hazardous to it. Read the other way around, a 65-foot tree at 40 feet passes the reach test with 25 feet to spare. Everything after that is a question about the condition of the trunk.
What the species grows to, and which California circuits carry it
Richard K. Hermann and Denis P. Lavender wrote the Douglas-fir chapter of Silvics of North America, Agriculture Handbook 654, published December 1, 1990. On size:
Trees 150 to 180 cm (60 to 72 in) in diameter and 76 m (250 ft) in height are common in old-growth forests (22). The tallest tree on record, found near Little Rock, WA, was 100.5 m (330 ft) tall and had a diameter of 182 cm (71.6 in).
The Dixie tree at 65 feet was a small one. That is the point worth carrying: 65 feet is roughly a quarter of what the species reaches, and it still crossed the 40 feet to the pole with 25 feet of stem to spare.
The growth curve is what makes the height field go stale. On a medium site at low elevation, height growth "averages 61 cm (24 in) annually at age 30, continues at a rate of 15 cm (6 in) per year at age 100, and 9 cm (3.6 in) at age 120." Coastal Douglas-fir is very long lived, with ages over 500 years not uncommon.
So a Douglas-fir recorded at 90 feet keeps adding height through the second century of its life, long after a hardwood on the same span has settled at its mature height. The growth assumption the cycle is built on governs the clearance prune. The fall radius answers to a different curve, and it gains six inches a year on a tree standing well outside prune range.
The California range is specific:
Nearly pure stands of Douglas-fir continue south from their northern limit on Vancouver Island through western Washington, Oregon, and the Klamath and Coast Ranges of northern California as far as the Santa Cruz Mountains. In the Sierra Nevada, Douglas-fir is a common part of the mixed conifer forest as far south as the Yosemite region.
That is the North Coast, the Klamath, the Marin and Napa hills, the Santa Cruz Mountains, and the west slope of the Sierra down to Yosemite. On those circuits the fall zone is a band whose half-width is the tree height. Do the arithmetic on a worked example: a 30-foot maintained corridor with 140-foot firs standing along it.
The corridor is 30 feet wide. The band of ground a 140-foot tree can reach the conductor from is 280 feet wide, roughly nine times as much. The corridor edge and the easement boundary are already two different lines, and the strike band is a third one, wider than either.
The crown stays green while the base goes
The failure mode is basal. The inspection looks at crowns, from a helicopter or from the ground under the wire. The look and the failure sit at opposite ends of the tree.
PG&E produced a photograph of the Dixie tree taken in 2019, two years before it fell. SED records what it showed: "in which the Subject Tree's canopy appeared green." A footnote adds the reason that mattered, that the color of the canopy is a key factor in vegetation management inspectors' determinations of tree health. The consulting utility forester who patrolled the circuit on December 3, 2020 was shown the same photograph and said the tree appeared to be a healthy fir. That forester described the method:
The CUF would have performed a Level One assessment of the tree by walking underneath the powerlines in the right-of-way and observing those trees for signs of disease distress or structural or compliance problems
And then the sentence that closes the gap: "The CUF also stated that inspecting a tree's root structure is generally outside of the scope of inspections performed since the roots are buried." The senior forester who audited that patrol did not audit the stretch near the incident location, flew the January 14, 2021 second patrol by helicopter, and said it was unlikely the tree would have been identified as requiring trimming or removal.
The CAL FIRE arborist, writing for McNeil Arboriculture Consultants LLC on October 10, 2021, disagreed about what was there to be seen:
In my opinion the degraded condition of the base of the Douglas fir was the primary cause of failure of the tree and that defect would have been visible without extraordinary discovery effort, from under the conductors. It would have been visible as a catface, an open injury on the trunk. A cursory visual inspection around the base of the tree would have revealed the poor mechanical condition of the tree. A pre-inspector who was close to the tree for a brief visual inspection for any reason should have discovered the decay.
Read those two accounts together and the disagreement is narrow. The forester walked under the wire and looked up at crowns. The arborist says the defect was at eye level on the trunk and visible from that same ground. Nobody was looking at the part of the tree that failed. Which assessment level a look is, and what each one commits the program to is the machinery that decides whether that changes.
The same split shows up in the forest health record. The 2024 California Forest Pest Conditions report describes Siskiyou County fieldwork where "bark peeled from multiple fading Douglas-fir trees revealed few flatheaded fir borer galleries," with lingering drought stress and long-term sapwood damage identified as the earlier cause.
The crown fades after the wood has already gone. And at Humboldt Redwoods State Park the report records a windthrown Douglas-fir over 60 inches in diameter whose "root system of the fallen tree ... was completely decayed by the root-rotting fungus Phaeolus schweinitzii." A 60-inch tree came over on a decayed root plate. That defect announces itself on the day it fails.
Fire is the cause, and then it is the consequence
The Silvics chapter carries the mechanism in two sentences that sit a paragraph apart. First, on how the species survives fire:
Crown fires, when they occur, destroy stands of all ages. The thick bark of older Douglas-firs, however, makes them fairly resistant to ground fires.
Then on what the survivors carry afterward:
Of the many heart rot fungi (more than 300) attacking Douglas-fir, the most damaging and widespread is red ring rot (Phellinus pini). Knots and scars resulting from fire, lightning, and falling trees are the main courts of infection. Losses from this heart rot far exceed those from any other decay.
Thick bark is why the tree is still standing after a ground fire. The scar that bark took is the court of infection. The tree that survives the fire is the tree that starts decaying, and the decay runs for years behind a wound at the base.
Put dates on it. SED records that the arborist traced the Dixie tree's damage "as a result from 2008 Butte County Fire, and an unidentified event between 2015 and 2016." The arborist's description of the wound: "the south side of the lower trunk was burned and killed over more than half its circumference. This would have formed a wound with no bark cover, visible as exposed and decaying wood." The fire that made the wound was thirteen years before the fire the tree started.
Thirteen years is longer than any inspection interval and longer than most systems of record keep a circuit-level event visible. What the stack requires on a burned circuit is written for the season after the burn, and its clock runs out with that season.
A look at the surviving firs in year ten is something a program has to schedule for itself. The trigger has to be built: a burn perimeter and a date, attached to the circuit in whatever system holds the record, and re-read on a schedule somebody chose.
Three phases, two fuses, ten hours
A Douglas-fir landing on a three-phase tap can leave one phase energized against the trunk. That is the part of the fire risk that is electrical rather than botanical, and the Dixie sequence is the clearest published account of it.
SED explains the circuit behavior plainly:
Typically, distribution circuits have three phases. Each phase carries electricity, and normally each phase has a fuse. In some cases, a fault can affect only one or two of the phases, causing one or two of the fuses to operate. If two fuses operate to de-energize two of the three phases, the third phase could remain energized. This is called single phasing, which can damage customer equipment designed to use all three phases.
The tree fell at 0648. Two fuses operated. The third conductor stayed energized against the trunk, drawing through wood and into the ground at a current too low to operate the remaining fuse. That is a high impedance fault, and it heated and arced for ten hours.
What the utility saw during those ten hours was an outage. A PG&E Distribution Operator attested that fuses often operate, so "a 'blown fuse' alone is not a reason to deenergize the line." The troubleman reached Cresta Dam at 1218 and could not determine why the fuses had blown.
He reached Pole 100403908 at 1650, found two of three cutouts open, opened the third, and saw a fire 60 to 80 yards downhill running 600 to 800 square feet. PG&E de-energized at Switch 941 at 2030 hours, at CAL FIRE's request.
The weather was ordinary. No Red Flag Warning was issued, no wind event was forecast, and PSPS criteria were not met. The nearest PG&E weather station recorded 84.7 degrees, 19.91 percent relative humidity and 11.2 miles per hour at 1700 hours. The day supplied the fuel moisture and none of the force.
SED charged that gap twice. One finding is under GO 95 Rule 31.1, for failing to identify the tree on the line, which was observable from Cresta Dam. The second is under Public Utilities Code section 451, for failing to adequately consider the hazard of the circuit in its response to the outage.
PG&E's own position, as SED records it, is that "PG&E considers a tree on a line a safety hazard requiring prompt action" and that a troubleman finding one would normally de-energize immediately. The finding is about the ten hours before anybody knew there was a tree. What has to be reported when a facility may be linked to an ignition and which body grades which failure are their own subjects.
Where the rule stops and the specification starts
Take a different tree: a sound, vertical, 140-foot Douglas-fir standing 100 feet from a 12 kV span in Tier 3, with a full green crown and no wound at the base. It can reach the conductor by 40 feet. Here is what each provision does with it.
Provision | What it reaches | The sound 140 foot fir at 100 feet |
GO 95 Rule 35, clearance | Vegetation inside the minimum distance from the conductor | Not in scope |
GO 95 Rule 35, overhang provision | Dead, rotten or diseased trees and portions that overhang or lean toward and may fall into a span, on actual knowledge | Not in scope |
PRC 4293, clearance | Vegetation inside the radial distance for the voltage band | Not in scope |
PRC 4293, trailing paragraph | Four categories of tree, three of them conditions and the fourth a lean toward the line | Not in scope |
GO 95 Rule 31.1 | Maintenance in accordance with accepted good practice for known local conditions | The only provision that reaches it |
Four of the five are written in conditions the tree does not have or distances it is nowhere near. The fifth has no number in it at all, and the Commission decides after the fact what accepted good practice was. That is the honest answer on a tall sound tree in the fall zone: the four provisions with tests in them stop short, and Rule 31.1 leaves the judgment with the utility.
What fills the gap is the utility's own standard, which is why PG&E's definition reads as a rule. SED quotes the Distribution Vegetation Management Standard:
Trees that are dead, show signs of disease, decay or ground or root disturbance, which may fall into or otherwise impact the conductors, towers or guy wires before the next inspection cycle
Two things are doing work there. The trigger is condition, the same kind of test the statutes apply, so a sound tall tree sits outside this definition too. And the reach test is bounded by time, not by distance: before the next inspection cycle. A twelve-month cycle and a six-month cycle produce different answers on the same tree, and the specification leaves the interval to the program.
The corollary runs the other way as well. Once a removal is on the books, PG&E's re-inspection procedure allows a TRAQ-qualified inspector to close it with a reason of "NO STRIKE POTENTIAL," and what that reversal requires in the record is a documented judgment with a credential behind it. Strike potential is therefore already a field on the form. Height and distance are what populate it, and on this species they are the two numbers least likely to have been re-measured.
The standing dead arrive faster than the patrol
The fall-zone population turns over between cycles. The 2024 California Forest Pest Conditions report, published by the California Forest Pest Council, gives the swing in one line:
Douglas-fir mortality caused by flatheaded fir borer (Phaenops drummondi) decreased substantially in 2024 to an estimated 55,000 dead trees across 16,000 acres, compared to an estimated 800,000 dead trees across 93,000 acres in 2023.
Eight hundred thousand dead Douglas-fir in one year, then 55,000 the next. The report attributes the drop to the wet winter of 2022 to 2023 and the near normal winter that followed. It adds that most areas carrying high mortality in Butte and Shasta Counties over the previous two to three years had little to no mortality in 2024. Butte County is where the Dixie Fire started.
Two consequences for a corridor. A circuit walked once in a good year can accumulate standing dead firs inside the fall zone faster than the next annual patrol will find them. The trees that die are the stressed ones, which puts them in pockets rather than scattered evenly along the line. And a dead fir keeps working on the span.
The same report records Douglas-fir beetle infesting large windthrown trees, so the first failure supplies the brood material for the next. What a patrol is required to detect and what a detailed inspection adds is the choice that decides whether any of this is visible before it is on the wire.
What a Douglas-fir record has to carry
Start with what the published failure record measures. The Western Tree Failure Database lists Pseudotsuga menziesii at 81 reports as of January 10, 2026, seventeenth of the 24 species with 50 or more. That is a real number drawn from the wrong population. The reporters are urban tree care professionals, and what those counts measure is abundance in the areas reports come from. Douglas-fir near a California distribution circuit is a forest tree on a slope, and the 81 reports were filed where tree care professionals work.
So the species record a program can actually build is its own, and it needs five fields beyond the ones a clearance record carries.
Field | What goes in it |
Total height | Feet, with the method named: measured, estimated, or LiDAR derived |
Distance to nearest conductor | Feet, with the baseline named |
Hinge elevation | Feet above or below conductor elevation at the root plate |
Base condition | Catface, basal scar, conks, exposed decayed wood, or none seen |
Date at the trunk | The date somebody stood at the base of this tree, or blank |
The last field is the one the Dixie record would have failed. A patrol performed from under the conductors, audited on a stretch that excluded the incident location, and followed by a helicopter pass produces three inspection events and no visit to the trunk.
The index map for the routine patrol was signed November 11, 2020 for work SED found was done December 3, 2020, and PG&E did not know why. What a clearance record has to show and how long it has to survive is settled ground. A fall-zone record for this species is a different document, and a blank in the last row is the finding.
Further reading
GO 95 Rule 35 clearances: The clearance duty, the overhang provision, and the actual-knowledge trigger that decides when the second one starts.
What makes a tree a hazard tree: Reach before defect, and the four factors the guide says decide whether a tree can get to the wire.
Hazard trees after a fire: What the stack requires on a burned circuit, and why it does not send anybody back down it a decade later.
Frequently asked questions
Does any California rule require removing a sound Douglas-fir that is tall enough to reach the line?
No rule reaches it on height alone. The clearance halves of GO 95 Rule 35 and PRC 4293 are written as distances from the conductor, and a tree standing 100 feet away owes nothing under either. The overhang provision in Rule 35 reaches dead, rotten or diseased trees and portions, and PRC 4293's trailing paragraph reaches dead, old decadent or rotten trees, trees weakened by decay or disease, and trees leaning toward the line. A sound, vertical fir is in none of those categories. What reaches it is GO 95 Rule 31.1, which requires maintenance in accordance with accepted good practice, and the Commission decides after the fact what that was.
How tall does Douglas-fir get, and where in California?
The USDA Forest Service Silvics chapter puts old-growth trees at 60 to 72 inches in diameter and 250 feet in height, with the record tree at 330 feet near Little Rock, Washington. Height growth on a medium site averages 24 inches a year at age 30 and is still running 6 inches a year at age 100, and ages over 500 years are not uncommon. The California range is nearly pure stands through the Klamath and Coast Ranges as far south as the Santa Cruz Mountains, plus mixed conifer forest on the Sierra Nevada west slope as far south as the Yosemite region.
What did the CPUC find in the Dixie Fire investigation?
Safety and Enforcement Division's report, dated October 9, 2023, lists seven violations against PG&E: three of GO 95 Rule 18.B on records and corrective-action deadlines, three of GO 95 Rule 31.1, and one of Public Utilities Code section 451. One of the Rule 31.1 findings is that PG&E failed to maintain its 12 kV overhead conductors safely and properly by failing to identify a hazardous tree condition and not taking appropriate steps to prevent the Subject Tree from striking the overhead conductors.
Why does a decayed Douglas-fir still look healthy from the right of way?
Because the decay is at the base and the crown is 60 feet above it. PG&E produced a 2019 photograph of the Dixie tree in which the canopy appeared green, and the consulting utility forester who inspected the circuit on December 3, 2020 said it appeared to be a healthy fir. The CAL FIRE arborist found the defect was visible as a catface without extraordinary discovery effort, from under the conductors. The 2024 California Forest Pest Conditions report records the same split in Siskiyou County, where bark peeled from fading Douglas-fir revealed the crown was lagging the trunk.
Is there a published failure profile for Douglas-fir?
There is a count, not a profile. The Western Tree Failure Database lists Pseudotsuga menziesii at 81 reports as of January 10, 2026, seventeenth of the 24 species with 50 or more. The database says its numbers reflect the relative abundance of a species in the areas reports come from rather than how often it fails, and its reporters are urban tree care professionals. Douglas-fir on a distribution circuit is mostly a forest tree, so the 81 reports are not the population the corridor is exposed to.
