You and your neighbour can create a 'boundary agreement' to record: the boundary between 2 properties. who's responsible for maintaining a hedge, wall, tree or fence between 2 properties.
The Boundary Line Agreement (BL AGR) provides a procedure for County review whenever a point or line determining the boundary between two or more parcels of real property cannot be identified from the existing public record, monuments, and landmarks or is in dispute. RCW 58.04.
Under such an agreement, the parties acknowledge the true boundary line between the properties, and the encroaching owner releases any claim to the strip of land encroached upon. In return, the “encroached upon” owner allows the encroachment to continue for so long as he/she is not adversely affected.
California law recognizes the right of two adjoining landowners to agree on a specific line or marker to act as the property lines between the two parcels, notwithstanding the legal description in each parcel's deed.
Boundary lines help determine the extent of a property and its legal ownership. Specifically, these lines define a property's physical limits and help prevent conflicts between neighboring property owners.
The boundary line is where the property ends. An easement is aright to cross over your property. Most easements are for utilities or access to a property. Both are pretty much written in stone and beyond your ability to control.
If the property line goes through the middle of the tree trunk, the tree is jointly owned by you and your neighbor. Since you jointly own such boundary trees, you are allowed to trim them. But if your trimming kills the tree or you cut down the tree entirely, you could be liable for timber trespass.