Tree health

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.

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.

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.

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