Where two equal stems meet with bark caught between them, the fork is held by less than wood.
Codominant stems are two stems of nearly the same size growing from one union, and where they grow tight together, bark can be caught between them in place of wood. The USDA Forest Service says that ingrown bark "does not have the structural strength of wood," and its pruning guide says included bark often causes a crack below where the stems meet.
General field guidance, not a diagnosis from a distance. A tree can hide problems from the fence line and hide strengths too.
Two stems from one point are not a trunk and a branch
A codominant stem, in NC State Extension's definition, is "a stem or branch growing at about the same rate and nearly of the same diameter as another stem or branch; both branches originate from the same union." Extension adds that it "does not have the same structure as a true branch." A true branch is held by interlocking branch and trunk tissue, the gardener handbook says, and Extension's pruning guide notes that codominant stems typically lack the collar that wraps a branch's base around the trunk wood. From the ground, that looks like two trunks of similar thickness rising from one point.
Bark caught inside the union is not wood, and it can act as a wedge
NC State Extension's gardener handbook explains how included bark forms: on a narrow attachment, branches "eventually run out of room to grow" as they thicken, the branch bark is surrounded by woody tissue, and "The union is weak and likely to split." The USDA guide describes two or more similarly sized, usually upright stems growing so close that bark grows between them, and says the included bark may also "act as a wedge and force the branch union to split apart."
A seam of bark and a crack in the wood are two different readings
A seam at the fork can be the included bark itself, "a wedge of inward-rolled bark" in How to Prune Trees' words, pressed into the fork. A crack, in the USDA guide's definition, is "a deep split through the bark, extending into the wood of the tree," and the guide calls cracks in general "extremely dangerous because they indicate that the tree is already failing." How to Prune Trees links the two: included bark often causes "a crack at the point below where the branches meet." A seam shows bark where wood would be; a crack below the fork shows the split itself. The guide's action list for weak unions includes "A weak branch union is cracked."
Storms are often when a fork with included bark gives way
NC State Extension's gardener handbook says trees with several trunks "often develop included bark in the crotch that eventually causes one of the trunks to split from the rest of the tree during a storm." Why narrow forks break under ice, and why Extension singles out Bradford pear, get their own treatment from ice storms and Piedmont trees.
Young trees get structural pruning; older forks get a decision
For a young tree, the published answer is structural pruning: NC State Extension's gardener handbook says that when a young tree has two competing leaders, the weaker comes out, and the USDA's How to Prune Trees adds another route, trimming some side branches from one stem so the other can become dominant.
For a grown tree, the sources offer several answers. The USDA guide prunes where a weak branch union exists and one of the branches can be removed; the NC Forest Service's urban forestry specifications reduce the length or height of codominant stems with included bark, while NC State Extension discourages reducing the crown of older trees; removal is the guide's final option. Which fits a fork is settled at the tree, and the cutting is done as tree trimming and pruning or tree removal.
Worth a call is the check's reading of a crack at the fork
The home page's fence-line check gives a crack at the fork its Worth a call label. That is the check's general reading; the USDA guide's stronger words on cracks are its own. The hazard tree field guide places the sign among the others.
A walk-through of a forked tree looks at where the seam or crack starts and whether it runs down the trunk below the union, whether the two stems are close to the same size, which way each leans and what sits under each, and whether a cavity or decay sits near the union.
Show where the seam starts and which way each stem leans.
Questions people actually ask.
Is every V-shaped fork a weak one?
No. NC State Extension says relative branch size matters more than the angle of attachment, and calls a V-shaped union one that may be prone to breakage, not one certain to fail. The USDA guide's weak union is two similar-sized stems with bark between them.
Which trees are more likely to form weak forks?
NC State Extension's Before the Cut says decurrent trees, with a rounded or spreading crown and no central leader, such as maple, ash and lacebark elm, may be more likely to develop weak unions and bark inclusions; excurrent trees like sweet gum have a central leader.
Does it matter whether the fork is in the trunk or out in a branch?
Yes. The USDA Forest Service's hazard guide lists a weak branch union on the main stem, and for branches it names a cracked branch of sufficient size to cause injury. A fork in the trunk has the whole crown above it.
What should I note about a fork before calling?
Where the seam or crack starts and how far down it runs, whether the two stems are about the same size, which way each one leans, and what sits under each.