Legal Question in Real Estate Law in California

On the safety issue, if the trees threaten only your backyard and not your house and its occupants, my personal feeling is not to be too aggressive about it ......... people love their trees and the neighbor would be very upset about being forced to remove or prune them sufficiently to benefit you. Ask the neighbor if he has insurance (he probably does) to cover damage done by a blow-over. Also, you might hire a registered arborist (tree expert) to look at them and give you a written opinion as to the trees' health, safety, etc.

The legal remedy, if you needed to use it, is damages and an injunction through a suit for "private nuisance." You might win such a suit if you can show through expert (arborist) testimony that the trees are dangerous, or that the fallen leaves are a major problem interfering with your enjoyment and use of your property. Filing and prosecuting a suit would be costly, and the outcome uncertain.

Fallen leaves from a neighbor's tree can also be addressed as a trespass, and if I were filing a lawsuit, I'd allege trespass as well as private nuisance, but the damages recoverable would be tiny and there would be less likellihood of an injunction.

A court is unlikely to order the removal of the trees. An additional reason is that certain communities restrict the right to remove healthy native-species trees by ordinance.