Why trees fail

With all of the wild weather we’ve had lately it’s not surprising that so many trees have fallen or been badly damaged. As upsetting as it is to loose our large and mature trees it is ultimately a natural process that will allow the next generation of large, mature and beautiful specimens to grow into the space opened up between the canopies. But, why is it that these trees fall in the first place? 

As with all living organisms, when trees age they become less able to defend themselves against infection. Infections may come in the form of fungi that can attack the root system of a tree. As these fungi develop in a root system they may decay the heavier anchor or buttress roots causing them to be brittle and less able to stabilise the tree in those strong winds. The picture below shows a mature Elm that had serious root decay caused by Honey Fungus (Armillaria).DSC02691.JPG

Another common reason for trees falling over is their exposure to wind that had not been present before. Confused? Trees will generally grow to suit their environment and this is most evident in the amount of root they lay down in response to their exposure to the local wind. If a tree is sheltered by other trees or structures then it will display a less substantial root system than a tree stood in the open. The problem here is that once the surrounding trees have been cut or blown down, the once sheltered tree becomes very susceptible to fall due to it’s inadequate anchorage. We find that once a ‘wind break’ has been removed the adjacent trees will gradually be damaged or blown over before they have time to adapt.

Fig 2. Included Bark

Fungal and bacterial infections may also cause decay in a trees trunk or major branch unions, rendering them very weak and unable to stand up to strong winds, but a more common cause of tree failure above ground is something know as ‘included bark’. Included bark occurs when upright branches or stems increase in girth and press against one another. The area or bark that is pressed together will often decay, but more importantly the diameter of the branches will not increase in proportion to the weight of the branches making the branch union less able to support it’s own weight, so again, once that wind increases the branch or stem can snap, often bringing down a large portion of the crown and, in some cases, rendering the tree unsafe and a candidate for removal. The image to the left shows a Horse Chestnut with included bark in the main fork. This may be alleviated by reducing the weight of the stem on the left, but this could have been avoided, as in every case, by simple formative pruning whilst the tree was younger.


If you are concerned about the safety of any of your trees then please get in touch via the contact page. We offer a full range of tree management services including tree safety inspections and tree surveys that will include detailed advice on how to reduce the risk of your trees failing in the future.

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Bring on the summer. Ben.