Sudden Limb Shear in Trees

What is the major cause of limb shear in trees?
There is no single cause. Limb shear occurs because of a combination of different factors. These include environmental conditions, the age of the tree and species variation. The variation may be within species as well as between different species. You can see a whole paddock of River Red Gums and 48 out of the 50 will be fine and two of them will have fallen limbs all round them. Certainly some species of trees are particularly prone to limb shear.
However the majority of cases are caused by some defect in the union between the limb and the trunk, or some section of the limb adjacent to the trunk. Very often we can see evidence of decay. This may have been caused by something as simple as a cockatoo or possum eating away at the bark, allowing decay to set in. Or perhaps the tree has been subject to environmental limitations, such as drought or compaction, and its ability to repel decay may have been reduced. Sometimes the reasons may go back 10 or 15 years. For example, people forget that 10 years ago, a new water main was put in and half the tree’s roots were severed. Trees are amazing survivors but such things take their toll. Limb shear may be the consequence of actions from years ago.
Having said all that there are still occasions when arborists can find no explanation at all for why limbs fail.
Which species are most vulnerable to limb shear?
Some eucalypts are particularly vulnerable. River Red Gum (Eucalyptus camaldulensis) (see top right), Narrow-leafed Peppermint (E. nicholii), Sugar Gum (E. cladocalyx), Lemon-scented Gum (Corymbia citriodora), Spotted Gum (Corymbia maculata) and Swamp Mahogany (Eucalyptus botryoides) are trees to be watched in particular. Most species of Elm are also prone to limb shear. Ulmus procera and U. x hollandica seem equally likely to do it.
The problem with Swamp Mahogany in the urban situation may go right back to the original genetic stock used for propagation, or the lack of care taken in the nursery to remove problem branches. The problem seems to stem from the morphology of the tree. Because of a condition called "included bark", the branches are typically not very well attached to the tree.
I think it’s important to understand that the eucalypts in the urban situation are quite different to the eucalypts in the forest. Eucalypts in the forest tend to grow straight up as they compete with each other for light. In the urban situation, the trees have more light and consequently, a more extensive canopy and branch structure.
We are currently engaged in research to test the hypothesis that shading of lower branches, or over pruning of branches, may have an influence on their tendency to drop.
What can be done to minimise the danger?
I think it is important to realise it is difficult to predict when a limb will fall. However there are steps that can be taken that will minimise the risk. As trees age, the likelihood of limb shear increases. Old or vulnerable trees should be examined regularly by a professional arborist who needs to climb the tree and examine the limbs closely. Some limbs may need to be removed, or it may be that the whole tree needs removing. Where a limb is aesthetically important, cabling and bracing can be employed, so that if the limb does fail, it will be less likely cause an injury.
About Leigh Stone: Leigh Stone is a full-time arborist, a part time lecturer at Burnley College and is completing his Masters thesis on "Sudden Limb Failure in Trees".