Legal Question in Real Estate Law in California

The liability, if any, depends upon negligence principles. The truck owner would have to show that the tree owner breached the standard of care in inspecting and maintaining the tree. In most urban settings, a tree owner is going to be responsible for toppling trees that were fairly easy to observe and care for.

If the properties were way out in the woods, it might be different. It's also possible there is no provable negligence. At law, an accident is an incident or occurrence that causes harm, but where there is no fault such as negligence or intentional misconduct. Accidents do happen, and when they do, there is no one to sue (successfully, at least). If the tree showed no signs of poor health, was appropriate for the site, etc., there may be no negligence and no liability.

(Note that some activities are so inherently dangerous that anyone who carries them on is "liable per se" without the necessity to show negligence. This would be things like raising lions and tigers, manufacturing explosives, or maintaining an earthen dam on your property. Having trees would rarely fall into this category).

In most cases the tree owner will have a homeowner's insurance policy and the insurance company should be notified and in most cases will settle with the neighbor without the necessity of legal action on either neighbor's part.