Trees in urban landscape settings are often subject to disturbances, whether human or natural, both above and below ground. They may require care to improve their chances of survival following damage from either biotic or abiotic causes.
Arborists can provide appropriate solutions, such as pruning trees for health and good structure, for aesthetic reasons, and to permit people to walk under them (a technique often referred to as “crown raising”), or to keep them away from wires, fences and buildings (a technique referred to as “crown reduction”). Timing and methods of treatment depend on the species of tree and the purpose of the work. To determine the best practices, a thorough knowledge of local species and environments is essential.
There can be a vast difference between the techniques and practices of professional arborists and those of inadequately trained tree workers who simply “trim trees”. Some commonly offered “services” are considered unacceptable by modern arboricultural standards and may seriously damage, disfigure, weaken, or even kill trees.
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One such example is tree topping, lopping, or “hat-racking”, where entire tops of trees or main stems are removed, generally by cross-cutting the main stem(s) or leaders, leaving large unsightly stubs. Trees that manage to survive such treatment are left prone to a spectrum of detrimental effects, including vigorous but weakly attached regrowth, pest susceptibility, pathogen intrusion, and internal decay.
Pruning should only be done with a specific purpose in mind. Every cut is a wound, and every leaf lost is removal of some photosynthetic potential. Proper pruning can be helpful in many ways, but should always be done with the minimum amount of live tissue removed.
In recent years, research has proven that wound dressings such as paint, tar or other coverings are unnecessary and may harm trees. The coverings may encourage growth of decay-causing fungi. Proper pruning, by cutting through branches at the right location, can do more to limit decay than wound dressing.
Chemicals can be applied to trees for insect or disease control through soil application, stem injections or spraying. Compacted or disturbed soils can be improved in various ways.
Arborists can also assess trees to determine the health, structure, safety or feasibility within a landscape and in proximity to humans. Modern arboriculture has progressed in technology and sophistication from practices of the past. Many current practices are based on knowledge gained through recent research, including that of the late Alex Shigo, considered one “father” of modern arboriculture.
Source: Wikipedia. http://en.wikipedia.org/wiki/Arborist