How trees fail: what the guidance describes

This page describes what four published sources say about the ways trees and tree parts fail. It does not assess any tree and says nothing about whether a given tree will fail. Each source is written for its own readers, from owners of single trees to managers of whole woodlands, and they do not use one set of terms, so the table keeps each source's wording attached to it.

Three places a failure starts

Part	What the sources describe
Branch	The Forest Service brochure says dead wood is dry and brittle and cannot bend in the wind like living wood. It says bark growing inside a union between similar-sized upright stems is weaker than wood and can act as a wedge that splits the union. The Florida publication says codominant stems of similar size raise the likelihood of branch failure in strong storms and hurricanes, and that a sharp V-shaped union is considered more likely to fail than a smooth U-shaped one. The ISA brochure says branches or stems with weight concentrated near the end are more prone to failure.
Stem	The Forest Service brochure describes a crack as a deep split through the bark into the wood and calls cracks an indication that a tree is already failing. It says a canker raises the chance of the stem breaking near it. It says a tree usually decays from the inside out while sound wood is added outside, so what matters is the ratio of sound to decayed wood. The Florida publication says decay can greatly reduce the strength of the part it affects and can be hard to detect.
Root	The Forest Service brochure says trees with root problems may blow over in wind storms and may even fall without warning in summer under the weight of their leaves. The ISA brochure says cracks or separations in the soil may indicate soil heaving from excessive root movement. The Florida publication says leaning trees on steep slopes or saturated soils have an increased likelihood of failure.

What the sources link to each kind of failure

Dead and broken wood: the Florida publication says dead branches typically break off during storms but may also drop in less severe weather, and that broken or partly attached branches are already in a state of failure and can drop in calm weather.
Roots hidden below ground: the National Tree Safety Group says roots are often damaged unintentionally by building work, trenching and compaction, that the damage tends to be progressive, and that the first visible signs in the crown can appear only years or decades later.
Signs above ground that point to roots: the Forest Service brochure names soil mounding, twig dieback, dead wood in the crown and off-color or smaller leaves as symptoms often associated with root problems. The National Tree Safety Group lists root plate lifting among the signs that may indicate a structural defect.
Lean: the Florida publication says a lean that formed quickly, or that keeps increasing, differs from a long-standing lean in which the tree has made extra wood at the base to compensate. The page on lean in trees sets out the sources on this.

Where the sources differ

The Forest Service brochure treats decay and cavities as defects to examine, and says a tree with a sound outer shell depends on the ratio of sound to decayed wood. The National Tree Safety Group says hollowing does not necessarily weaken old trees if their height-to-diameter ratio is low enough and their adaptive growth keeps compensating, and notes that old hollow trees are often found still standing after storms while nearby younger solid-stemmed trees are uprooted. The first is written for owners checking a tree and the second for managers of veteran trees, and they put the weight in different places. The National Tree Safety Group also says deformities do not necessarily imply weakness and can be an adaptive response to stress.

For the defects the sources name, see structural defects in trees, root damage and root plate lifting and white, brown and soft rot compared. For how a look at a tree is organised in the guidance, see visual tree assessment.

How trees fail: what the guidance describes

Three places a failure starts

What the sources link to each kind of failure

Where the sources differ

Sources