An arborist (formerly called a tree surgeon) is a professional who manages and maintains trees (generally in an urban environment). This includes planting, pruning, structural support, the treatment of disease, insect, or abiotic disorders, lightning protection, and tree removal. It also can include planning, consulting, report writing and even legal testimony. Because trees provide many benefits to the landscape and to humans, but they are also very large, heavy, and complex organisms, they require monitoring and care to ensure survival and safety in the human landscape.

Cultural practices

Trees may require pruning to keep them away from wires, fences and buildings, to improve long-term health and/or structure, for aesthetic reasons, and to permit people to walk and sit under them. They might also require other care to improve their chances of survival and longevity, or treatments in response to damage from biotic or abiotic factors. Trees in urban landscape settings are often subject to human disturbances above and below ground, as well as natural disturbances. Timing or methods depend on the species of tree and the purpose of the work. A thorough knowledge of local species and environments is necessary to determine the best practices.

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 a great deal in technology and sophistication from practices of the past, and more of the practices are based on knowledge gained through recent research.

How to select an arborist

The American Society of Consulting Arborists maintains an online list of Registered Consulting Arborists. This site can be accessed at ASCA

The International Society of Arboriculture, a non-profit organization, maintains a list of ISA Certified Arborists who have passed a written exam and demonstrated a basic level of knowledge in arboriculture. Ther are also certification classifications for Certified Tree Workers, and the highest level, the Board Certified Master Arborist. The list of ISA Certified Arborists can be found on their web site at ISA

The most common abuse of a tree is a practice called "topping" in which the outer part of the branches is cut off. This has several bad effects. It deprives the tree of leaves, starving it and making it more susceptible to insects and fungi. Since most of a branch is intact, the sap continues to flow to the end of the branch, encouraging new growth to be small, weak bushy sprouts at the end of the cut branches. The new weak branches shade each other and are ugly.

Pruning should only be done with a specific purpose in mind. Thinning of a tree is often unnecessary. Removal of small internal branches can be detrimental to the tree’s health, by causing remaining branches to grow long and narrow, without much taper And making them more susceptible to breakage.

In recent years, research has proven that wound dressings such as paint, tar or other coverings are unnecessary and may be harmful to the tree. The coverings may actually encourage the growth of decay-causing fungi. Proper pruning, by cutting the branches at the right location, can do more to limit decay than any wound dressing yet devised. Trees do not respond to wounds as animals do, and treatment by covering the wounds, as with people, is not a sound arboricultural treatment.

A professional arborist will not leave branches on the ground, to be a safety hazard or nuisance. If he cuts down a tree, he will know how to remove stumps. He will also have a truck with a chipper, and clean up after himself.

Lastly, be sure to ask if the arborist carries insurance. In some jurisdictions, unless you make other arrangements, as his employer you will be responsible for his care if he has an accident while climbing your tree. If he is going to remove a tree near a building, be sure he is bonded.

Legal issues

Depending on legal jurisdiction, there are a number of legal issues surrounding the practices of arborists and of urban tree management in general:

• ownership of trees on or near boundaries - neighbours may have legal rights regarding trees which adjoin or overhang their property.
• "right to light" - some jurisdictions grant property owners rights to enjoy a "reasonable" amount of sunlight, and neighbouring trees which deny this may be subject to trimming or felling as a consequence.
• structural impact - the growth of tree roots (or their removal) may affect the stability of nearby walls or building foundations. Equally, unstable, diseased or dead trees may fall, causing structural damage or personal injury.
• control of disease - in an attempt to control epidemics of tree diseases or agricultural pests, many jurisdictions require property owners to ensure their trees are healthy and that rot and disease are controlled.
• conservation - in many locations, certain trees are protected (often on a basis of species or size), requiring a specific permission be obtained before they are felled or radically trimmed.
• safety - property owners are generally liable for injuries arising from unsafe trees or tree branches, and may also be liable for the safety of arborists working on their trees