Legal Question in Real Estate Law in California

Re: How much responsibility?

Probably not. Unless he can prove that you were negligent in your maintainence of the tree or root system or fence and that this negligence was the cause of the damage. This is an act of god for which homeowners carry homeowners insurance.

Re: How much responsibility?

This is kind of a tough question, especially without seeing the sites involved or knowing the history of the planting of the trees or what was there when the hand of man first began to reshape nature at this location.

Nevertheless, I think the neighbor is over-reaching and asking for more than he's entitled to. Neighbors owe each other duties of lateral and subjacent support, to be sure. However, one cannot reasonably expect support against natural runoff and gradual erosion connected with the undisturbed contours of the earth. A duty to provide lateral support arises when the downhill neighbor excavates, for example; at most, this obliges you to fill the hole created by removal of the tree roots.

If anything, I think the uphill neighbor has a duty to protect you from mud and earth slides off his property. You might try telling him he might have to build the wall at HIS expense to protect you. In real life, however, I don't think a court would give either of you any relief against the other, unless the conditions are a lot more extreme than I visualize from your description.