When a trunk's wood cannot be trusted, the crane carries the pieces instead of the tree.
With a dead or rotten trunk, the crane holds each piece from above before it is cut, so the piece's weight goes up the crane's line instead of into ropes anchored in wood that may not hold. The crane does not make the tree sound: the climber still ties in to the trunk, and the sling still needs wood that grips.
A roped takedown makes the tree carry its own pieces
In a roped takedown the climber ties in to the tree, the lowering rope runs over one of its crotches or around its trunk, and every piece cut free loads that wood on the way down. Michigan's FACE program puts it plainly: "Once a piece is cut and begins to fall, it will create a dynamic load on the rigging line." That is ordinary tree removal on ropes.
OSHA's hazard bulletin on tree care falls describes a large hickory whose trunk snapped in half while a piece was being roped down, and concludes that "A thorough preliminary examination would have shown that the tree could not support the forces resulting from rigging and roping down cut tree sections." The bulletin does not call that hickory dead; rot just makes the question harder to answer from the ground.
Decay usually starts inside, behind wood that looks sound
The USDA Forest Service's guide to hazardous tree defects says "A tree usually decays from the inside out, eventually forming a cavity, but sound wood is also added to the outside of the tree as it grows," and warns that "green foliage in the crown does not ensure that a tree is safe." NC State Extension adds that damage from insects or disease "can weaken the wood."
In one New Jersey fatality investigation, the wood cut from the top was "solid, good wood," yet "the tree fractured because the tree stump was rotted." The investigators note that trees "can decay in any area from roots to crown."
Dead wood can give way somewhere other than the cut
The Forest Service guide calls dead wood "often dry and brittle" and says it "cannot bend in the wind like a living tree or branch." OSHA's hurricane guidance for tree workers warns: "If you have to cut a dead tree, be careful: the top could break off."
A NIOSH alert on tree trimming deaths shows what that can mean in a leaning, rotted trunk: as the top piece was cut away, "the tree bent with it," and as it sprang back, "the backlash was strong enough to fracture the trunk" below the cut, where the climber was tied in.
What the crane takes off a rotten trunk: its own weight
Crane-assisted tree removal moves each piece's weight off the tree before the wood is separated. The piece is slung and the crane's line is put under tension first, the order one Massachusetts crew followed in a FACE report and each pick of a crane day sets out. A piece that rides out on the crane puts no weight on a rope anchored in the tree and does not fall away and pull the trunk over, the two loads behind the snapped hickory and the backlash fracture above.
What the crane cannot fix: the tie-in and the sling
The person making the cuts still works in the tree, and Massachusetts FACE investigators recommend "a minimum of two means of being tied in the tree at all times." Unless a sound tree stands close enough to use, those tie-ins go around the one coming down. The sling has to grip the piece as well, and the Forest Service describes advanced decay as wood that is "soft, punky, or crumbly."
The crane also needs a place to stand. New Jersey's FACE investigators listed "the use of cranes, tying in to nearby trees, or dropping the tree" as alternate methods for trees too hazardous to climb, and in that case the employer felt "the back yard was too small and inaccessible to use a crane." The setup is its own subject: how much room a tree crane needs.
Rotten pines in Millers Creek, in a customer's words
One customer review of this crew, from a job in Millers Creek, tells it from the homeowner's side: two of the five drops "involved extremely rotten pines" that the customer had expected could be dropped from the top and kept off a neighbor's yard. "Because they were so 'spongy' it could not happen safely." The review does not describe how those pines came down. It shows the pattern: the plan that looks simplest from the ground can be the one rotten wood rules out.
Some decay still leaves climbing the right call
Decay alone does not settle the method. The Forest Service guide says "the presence of decay, by itself, does not indicate that the tree is hazardous," and OSHA's tree-care guidance treats a tree not damaged or decayed to the point of making climbing unsafe as normally climbable, the ordinary case set out in crane or climber. Dead wood's effect on the number is item five of the tree removal cost drivers.
Whether rot changes the method gets settled at the tree
No photograph shows the inside of a trunk. NIOSH's alert on tree trimming deaths tells workers to "Inspect trees and limbs for structural weakness before climbing or cutting." Arbor Aves quotes a job after looking at the tree in person. A tree already lying on a roof is a separate case: a tree on a house, piece by piece.
A trunk nobody trusts? Have it looked at before anyone climbs it.
Questions people actually ask.
Does a green, leafy crown mean the trunk under it is sound?
No. The USDA Forest Service's hazard guide says "green foliage in the crown does not ensure that a tree is safe," because a trunk can be badly defective under a full crown.
Can tapping on a trunk show whether a tree is rotten inside?
Not always. NIOSH's alert on tree trimming deaths describes a crew removing dead trees that tapped each trunk to check for rot and still missed wood wasps in the upper trunk, a sign of rot.
Does a crane make a rotten tree safe to climb?
No. A crane takes each piece's weight off the tree, but the person making the cuts works in the tree and ties in to it, so the wood at the tie-in has to hold. NIOSH tells tree workers to inspect trees and limbs for structural weakness before climbing.
Does any decay in a trunk mean the tree needs a crane?
No. The USDA Forest Service's hazard guide says decay by itself does not indicate that a tree is hazardous. Where the wood still holds a climber and a rope, a roped takedown stays the ordinary method; cranes are one of the methods New Jersey FACE investigators list for trees too hazardous to climb.