Legal Question in Real Estate Law in California

Trees are owned by the party upon whose property their trunks stand. Trimming trees is the responsibility of the tree owner, but if branches overhang the property line, they become a trespass. Ordinarily, one cannot resort to self-help to remove a trespass, but in the case of tree limbs and roots, the law makes an exception and allows the non-owner a limited right to resort to self-help to remove the trespassing objects. The landowner may trim the neighbor's trees up to the property line at his own expense and without a court order, but the trimming must be done without negligence.......i.e., in a way that won't kill the tree or make it a hazard to topple over in a windstorm, for example.

Yes, this is something that can be sued over, and it sometimes happens.

I usually recommend that anyone thinking of using self-help to trim the neighbor's tree first consult with a licensed arborist and get a written report on what may be done without injury to the tree or creation of a hazard.

Anybody can sue anybody for anything. From there, your actual answer is the the owner is responsible for maintenance and damage caused.