Legal Question in Real Estate Law in California

The most effective response to such a situation is to threaten, and then if necessary, to file, a lawsuit for trespass and private nuisance. There is a self-help remedy - invasive limbs and roots can be trimmed at the property line - but the offended party has to bear the cost and receives no damages, and your right to trim is limited by a requirement that you do it in a "non-negligent manner," which has been held to mean so the tree doesn't die, or become a danger to topple in a windstorm. Probably better to have a court place the burden of paying for past damage and taking action to prevent future damage directly on the tree owner, though it may cost you some fairly significant fees if he decides to litigate.