Before any neighbour opts to trim any portion of a tree, he should first find out who legally owns the tree. Trees that rest on the boundary line between two properties are known as "boundary trees." Both neighbours have equal responsibility of maintaining this type of tree. If the majority of the trunk rests on one side of the property line, that neighbour has sole ownership.
Neighbours have the legal right to trim any hanging tree limb that crosses over their property line. However, neighbours may only cut the limb back to the property line and may not enter their neighbour's property for further cutting without permission.
Act of God
If one neighbour complains to the other that a tree limb is hanging over the property line and the tree-owning neighbour refuses to trim it, that neighbour can be held liable if the limb falls and causes property damage. However, if a tree limb crosses the property line during a natural disaster, such as a storm, and causes damage, the tree-owning neighbour will not be held liable. This is known as an Act of God.
In the event that one neighbour does trim limbs that hang over her property line and, as a result, causes significant damage to the tree's health, the tree-owning neighbour may sue for damages. Courts may issue a settlement for up to three times the value of the tree. If the tree is historical or an ornamental tree, the settlement can be even higher.