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.