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Free lessonDomain 8: Tree Risk Management · lesson 36 of 48

Defects and conditions that raise the likelihood of failure

The tree defects and conditions that make failure more likely, how to recognize them from the ground, and how to judge their significance.

Key points

DefectsDead trees and branches, hangers, cracks, decay and cavities, weak unions (codominant, included bark), cankers, root problems.
Signs of decayConks and mushrooms, cavities, soft or hollow-sounding wood (mallet), bleeding and swellings.
ArchitectureOverextended, end-heavy or lion-tailed limbs; sudden load or exposure changes; previous failures and poor pruning.
SignificanceSize and location, remaining sound wood, response growth, load on the defect, species failure patterns.
PracticeRecord each defect with its location, size and likely failure mode.

Summary

DefectWhat to look for
Dead wood, hangersNo buds, brittle, lodged branches
CracksSplits, often at unions or decay
Decay, cavitiesConks, mushrooms, hollow sound, openings
Weak unionsCodominant stems, included bark
Root problemsRoot decay, severed or girdling roots, heaving, recent lean
ArchitectureOverextended, end-heavy or lion-tailed limbs

Quick check

A broken branch lodged in the crown is called a:
  1. Hanger
  2. Canker
  3. Conk
  4. Knuckle
Answer: A. Hangers can fall without warning.
Conks on a trunk are a sign of:
  1. Healthy woundwood
  2. Wood decay fungi
  3. Insect feeding
  4. Nutrient deficiency
Answer: B. Conks are the fruiting bodies of decay fungi.
Why are codominant stems with included bark a concern?
  1. They always die within a year
  2. They cause root rot
  3. They prevent flowering
  4. The union is prone to splitting
Answer: D. Included bark prevents a strong wood connection.

On the job

  • Sound three trunks with a mallet and compare the results.
  • Photograph and record every defect on one tree with its location and size.
  • Learn the typical failure patterns of the three commonest species you assess.

Video transcript

Most failures show warning signs beforehand. In this lesson: the defects and conditions that raise the likelihood of failure, and how to judge how serious they are.

Watch for these defects. Dead trees and branches are brittle, and hangers are broken branches lodged in the crown. Cracks are splits in stems and branches, often at unions or where there's decay. Decay and cavities reduce wood strength; conks and mushrooms are signs of decay fungi. Weak unions, like codominant stems with included bark, are prone to splitting. Cankers are dead areas of bark and cambium that create weak points. And root problems: decay, severed or girdling roots, heaving soil or a recent lean.

Tree architecture matters too. Long, overextended limbs with heavy foliage at the ends carry high loads, as do lion-tailed branches. A sudden change in load or exposure, like neighboring trees removed, raises the likelihood of failure. And previous failures, poor pruning cuts and topping all leave weak points and decay.

A defect's significance depends on context. Consider its size and location relative to the stem or branch. Estimate how much sound wood remains around decay or a cavity. Look for response growth, such as ribs or swellings, showing the tree has compensated. Consider the load on the defect, from wind, weight and exposure. And know species failure patterns, because some species fail in characteristic ways.

This is one of 48 lessons in ArboristPass

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