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Trees can live for a long time but eventually die, either from natural causes or killed by man. Ill-health of trees can be diagnosed, and early treatment, pruning or felling to prevent the spread may result in timber stocks and amenity trees being saved. Arborists/arboriculturists need to be aware of the risk posed by hazardous trees. Construction projects sometimes avoidably damage trees.
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Tree risk assessment
Evaluating the danger a tree presents, whether by its state of health or by its situation, to people and/or property is called Tree Risk Assessment. Techniques have emerged based on Matheny & Clark's matrix of three factors which contribute to the degree of risk namely (i) failure potential (ii) size of defective part and (iii) target rating (how often something or someone is present to be harmed or damaged). Subsequently a Quantified Tree Risk Assessment ("QTRA") system has been developed by others that calculates the risk numerically with reference to cost implications of tree damage and published societal norms of acceptable, tolerable and unacceptable risk. The International Society of Abroriculture updated its approach in 2012 with a qualitative (words based) matrix known Qualitative Tree Risk Assessment ("TRAQ")
One of the most common naturally occurring hazard within large trees is the union between trunk and branch. 'V'-shaped unions may create weakness and increase failure risk; this can be reduced by tree cabling, which reduces how far the union can flex in strong winds or other loads.. However, there are many types of defect, some of which cannot be remedied and for which the risk cannot be reduced to an acceptable level without major pruning or felling. It is important that tree inspections are carried out by a competent person and that their recommendations are implemented
Trees can withstand large amounts of some types of damage and survive, but even small amounts of other traumas can result in death. Arborists/arboriculturists are very aware that established trees will normally not tolerate any appreciable disturbance of the root system. However, lay people and construction professionals seldom recognise how easily or indirectly a tree can be killed.
Construction and tree protection
Assessment of the damaging effect of construction activities on a tree can be based on three factors: severity, extent and duration. Fundamentally activity should avoid the crown of the tree and the volume of rooting required by the tree for ongoing vitality. Severity is related to the degree of intrusion into the rooting area and resultant root loss. Extent is related to a percentage of a factor such as canopy, roots or bark, and duration is based on the length of time that the activity interferes with the tree's normal functions.
Various organisations, such as the International Society of Arboriculture, the British Standards Institute and the Tree Industry Association (formerly the National Arborist Association), have long recognised the sensitivity of tree health to construction activities. The effects are important because they can result in monetary or amenity losses due to tree damage and resultant remediation or replacement costs, as well as violation of government ordinances (in the UK, planning laws, regulations and policies) or community or subdivision restrictions.
In the US, protocols for tree management prior to, during and after construction activities are well established, tested and refined. These basic steps are involved: