Why do some trees fail?
November 5, 2014
Some trees that fail have had very noticeable defects and failure could have been prevented if they had only been identified. Below I have written a generalised overview for the reasons as to why we see tree failure.
Major causes of failure are:
1. Mechanical damage
This type of damage can be caused by vehicles, either by collision with the trunk or compaction of the roots when cars have driven over or parked close to the tree. Compaction of the roots can be very serious and in turn can cause die back and eventual death of the tree. Root damage is never obvious straight away, it can take a couple of years before leaf development is affected. The importance of root protection barriers on building sites is an area Treesure is involved in. We undertake BS5837 Tree Surveys in order for the trees to be protected whilst building work is carried out. Digging, compaction from vehicles, damage to limbs from heavy machinery are all detrimental to trees and we work hard to ensure a harmonious balance between new builds and existing trees.
2. Weak forks
Trees that share two main limbs (co-dominant stems) can sometimes undergo weak fork failure. Although some trees such as the Lombardy Poplar have acute forks as a typical feature, but their upright and compact branching help to restrict excessive movement at the forks. Other trees such as the Horse Chestnut frequently fail at weak forks as they tend to grow long horizontal branches, these sway in winds causing breakage. Through personal experience I have found that other trees such as Beech are prone to failure when they have an acute fork from two main leaders with considerable weight on the main branches. Reducing excessive weight by pruning always helps to lower the risk of failure as the long branches no longer act as a ‘sail’ or carry as much weight.
3. Climatic factors
In the UK during the winter of 2012 we found that we were called out to deal with many trees that had failed due to snow damage. In the North West the snow came from the south yet, our prevailing wind usually comes from the west. This caught out the trees at their most vulnerable position. The trees that were mostly affected were Conifers and Pines. The reason that these were primarily affected was because they carry their foliage all year round unlike the common broadleaf deciduous trees which had already lost their leaves in the autumn.
The trees that we found to be mainly affected were Cedar of Lebanon, Macrocarpa cypressus, Scots Pine and Black Pine. It became evident that the evergreen trees with the most horizontal branches were the ones that suffered the most damage and in many cases complete removal was the only solution.
Wind, temperature, salt and sunshine also play an important role in the health of the tree. Some trees grow better in certain areas than others. Certain species of Poplar especially the taller varieties are not suitable in areas that are susceptible to strong winds. Poplars have low wood density meaning that their wood is weaker and more easily broken. Macrocarpa cypressus and Corsican pine are prevalent in the south of England and in coastal resorts as they prefer warmer temperatures, loamy soil and are more tolerant of salt than other species.
Decay is one of the most important causes of hazard. Deterioration of the wood affects the tensile strength of the tree. Decay can have a different effect on the wood depending on the species. Fungi can be recognized by its fruiting body in the form of brackets, fronds or toadstools. They produce spores which travel through the air and land on other hosts. Fungus usually enters the tree through wounds. Other types of fungus such as honey fungus can spread underground from tree to tree. I have seen many trees that have had to be felled due to the presence of honey fungus, if decay is extensive in the root system there is a high chance of wind throw. Over the years I have seen the result of Beech trees affected by Meripilus giganteus which causes root plate failure. If large fruiting bodies are seen close to or at the foot of a large Beech tree there is a high chance of windfall.
5. Pests and Disease
There are many types of disease that can affect trees. Insect damage can cause weakness in the structural strength of the tree. The Pine shoot moth affects the leading shoots of young trees causing co-dominant stems. The Horse Chestnut Leaf Miner is now moving north through Britain. It causes loss of photosynthesis and is most detrimental to stressed trees that may have had repeat infestations. The Grey squirrel can also affect the health of the tree by bark stripping which in turn impacts on growth rate. Grey squirrels can also damage woodland structure by preventing young saplings from developing crowns.
Fireblight is a well established bacterial infection that affects trees with apple like fruit (Rosaceae) such as Cotoneaster, Malus, Sorbus and others. Branches, whole shrubs or trees can be killed by this pathogen which causes die back. One of the more well known diseases that has devastated populations of Elms in the U.K is Dutch Elm disease. The pathogen attacked in the 1960’s and over 80% of the Elm population were killed within 2 decades. A type of Beetle (the Scolytid beetle) can carry the disease but also the disease can travel along root systems as Elms in woodlands are very often cloned trees and have developed the same suckering roots system.
Image copyright: Kara Brugman