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.
Sources of tree damage
The causes of tree damage and abnormalities can conveniently be divided into either biotic (from living sources) or abiotic (from non-living sources).
Abiotic sources include lightning, vehicles impacts, construction activities, drought, waterlogging, frost, winds, chemicals in the soil and air and soil nutrient deficiencies. Construction activities can involve any of a number of damage types, including grade changes or compaction that prevent aeration to roots, spills involving toxic chemicals such as cement or petroleum products, or severing of branches or roots. Trees with thinner bark such as birch and American sycamore are more sensitive to such damage.
Any of these damage sources and the natural ageing of trees may result in trees or parts of them failing prematurely. The term "hazard trees" is commonly used by arborists/arboriculturists, and industry groups such as power line operators, for trees that, due to disease or other factors, are more susceptible to falling in windstorms, or having parts of the tree fall. Damage may also disfigure amenity trees, create unacceptable risks to people, reduce the safe useful life of trees or reduce the value of commercial timber.
Fallen logs of white spruce and trembling aspen at various stages of decomposition were sampled from undisturbed and 1, 14, and 28-year-old post-fire and post-harvest sites in northern Alberta, and studied for differences in the associated microfungus communities (Lumley et al. 2001). Wood samples were plated directly onto each of 6 different media and from these fungal species were identified and enumerated over a 24-month period. Approximately 10 000 isolates were obtained, representing 292 species of filamentous microfungi, including 41 ascomycetes, 29 zygomycetes, and 222 mitosporic fungi. The most commonly isolated species were Trichoderma viride, Rhinocladiella atrovirens, Penicillium pinophilum and Mortierella ramanniana. Cluster analysis and ordination of microfungus communities in logs showed that the tree species of the log had the greatest influence on the species composition of communities. Fungus community composition was also correlated with the stage of decomposition. Species richness was highest in logs from undisturbed sites, and lowest in logs from the most recently disturbed sites. Species diversity (Shannon-Weaver) was only slightly higher at undisturbed sites than at disturbed sites. The most significant environmental factor was log moisture, which increased proportionately with stage of decomposition and was significantly correlated with climatic factors.
Wounds inflicted on residual trees during partial cutting often provide portals for decay fungi. Affected trees are prone to blowdown and breakage at the wound site, and even if they survive to rotation age their value is reduced by staining and decay in the wood. The influence of temperature on microbial diversity in wounds in white and black spruces was investigated by Dumas and McLaughlin (2003). Samples were taken from trees wounded during manual or feller-buncher partial cutting and skidding operations in the Black Sturgeon Forest, 120 km northeast of Thunder Bay, Ontario. The samples were taken from 76 trees in early October when the mean aerial temperature exceeded 0 °C and 23 trees in late October/early November when the mean aerial temperature was below 0 °C, to serve as the pre-freeze-up and post-freeze-up groups, respectively. The wounds were sampled and cultured. The number and ratio of bacteria, actinomycetes, and fungi on one-week-old wounds varied between pre- and post- freeze-up wounds, wound locations, and media. However, random samples of the different classes of microbes isolated from the 2 spruce species did not differ significantly, indicating no relationship between tree species and microbe. Wounds were more common on stems (94) than on roots (64) or butts (33). Wounds on roots averaged 2 and 3 times the area of those on stems and butts, respectively. More bacteria than fungi were isolated from the pre-freeze-up wounds than from the post-freeze-up wounds, while fungi were more plentiful than bacteria on the post freeze-up wounds.
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,[clarification needed] 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:
- Review of the construction plans
- Development of the related tree inventory
- Application of standard construction tree management protocols
- Assessment of potential for expected tree damages
- Development of a tree protection plan (providing for pre-, concurrent, and post construction damage prevention and remediation steps)
- Development of a tree protection plan
- Development of a remediation plan
- Implementation of tree protection zones (TPZ)
- Assessment of construction tree damage, post-construction
- Implementation of the remediation plan
- Strouts, R. G. and Winter, T. G., Diagnosis of ill-health in trees, Research for Amenity Trees No. 2 1994
- Wiseman, P. Eric, Integrated Pest Management Tactics, Continuing Education Unit, International Arboricultural Society Vol 17, Unit 1, February 2008
- "Top 5 ways to keep your trees healthy". devostree.ca. Feb 12, 2018. Retrieved June 6, 2018.
- Lumley, T.C.; Gignac, L.D.; Currah, R.S. 2001. Microfungus communities of white spruce and trembling aspen logs at different stages of decay in disturbed and undisturbed sites in the boreal mixedwood region of Alberta. Can. J. Bot. 79:76–92.
- Dumas, M.T.; McLaughlin, J.A. 2003. Microbes inhabiting Picea wounds and their antagonism to Haematostereum sanguinolentum p. 181–193 in Laflamme, G.; Bérubé, J.A.; Bussières, G. (Eds.). Root and Butt Rots of Forest Trees. Proc. 10th Internat. Conf. on Root and Butt Rots, IUFRO Working Party 7.02.01, Quebec QC, Sept. 2001. Nat. Resour. Can., Can. For. Serv., Inf. Rep. LAU-X-126. 450 p.
- Evaluation of Hazard Trees in Urban Areas, Matheny, Nelda. P., Clark , James R, International Society of Arboriculture Books, 1994
- Ellison, M. J. Quantified Tree Risk Assessment Used in the Management of Amenity Trees. Journal Arboric. International Society of Arboriculture, Savoy, Illinois. 31:2 57-65, 2005
- Many sources e.g. Qualitative Tree Risk Assessment, Smiley, Matheny and Lilly, Arborist News, February 2012
- Principles of Tree Hazard Assessment and Management, Lonsdale David, Research for Amenity Trees No. 7, 1999
- Schoeneweiss, D. F., Prevention and treatment of construction damage. Journal of Arborculture 8: 169
- BS5837 (2012): Trees in relation to design, demolition and construction - Recommendations, British Standards Institute, 2012