Trees can live for a long time but eventually die, either from natural causes or by being cut down 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. Tree owners and 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). Biotic sources include insects (e.g. that bore into the tree), mammals (e.g. deer that rub bark off), fungi, birds, nematodes, bacteria and viroids. 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. One of the most common naturally occurring hazards in large trees is weakness in the union between trunk and branch (or between co-dominant substems). V-shaped unions may create weakness and increase failure risk; in some situations this can be reduced by tree cabling, which limits how far the union can flex in strong winds or other loads.. 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. Trees can withstand large amounts of some types of damage and survive, but even small amounts of other traumas can result in death, disfiguration or hazards. Established trees will normally not tolerate any appreciable disturbance of the root system. Without arboricultural advice, lay people and construction professionals may not be aware how easily or indirectly a tree can be killed.
Decay studies 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
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