Legal Question in Real Estate Law in California

Here's a summary. Ownership is covered in Civil Code sections 833 and 834; trees belong to the landowner upon whose property the trunks stand, regardless of roots, but trees are owned in common if the trunks are on the property line to any degree.

Overhanging limbs and intruding roots are considered trespasses by the tree owner. However, they are (by court-made "decisional" law) a special category of trespass insofar as the rules of self-help and damages go. A victim of "tree trespass" is allowed to resort to self-help to this extent: he may trim the limbs or roots back to the property line without a court order, but must do so in a "non-negligent manner" - generally meaning in a way that doesn't inflict permanent injury on the tree or create a hazard, i.e. make the tree likely to topple in a windstorm.

A neighbor resorting to self-help is generally not entitled to money damages or to reimbursement for the cost of pruning. A court could award damages and require reimbursement if the matter is handled by lawsuit rather than self-help, but as I understand it, they usually don't. An exception would be if roots have damaged the invaded property by bucking pavement, disrupting pipes, etc.

Finally, doing deliberate harm to another's trees is considered serious enough to warrant punitive damages in many cases. See Civil Code section 3346.