Legal Question in Real Estate Law in California

If title has not transferred to you, you have no rights. If you want to go forward with the purchase, but retain rights of action against the neighbor, the proper method would be to include an assignment of the right to sue the neighbor in the transaction, and then file suit after close of escrow. If something needs to be done to stop the neighbor before you close escrow, however, you should get the current owner to do so.

I have a different opinion. While I don't dispute that getting an assignment of the seller's right to sue is a darn good idea, I believe you have the right to sue in your own name IF you presently have a contract to purchase the property which you could enforce against the seller.

This is because of a doctrine called "equitable conversion" which says that once buyer and seller are in contract, and any contingencies under which the seller could refuse to convey (other than receipt of the balance due) are removed, equitable title passes to the buyer. At that point, the seller holds legal title as collateral for buyer's payment of the remaining purchase price and many of the aspects of ownership vest in the buyer.

There is an old California Supreme Court case holding that the owner of equitable title has the right to sue third parties for trespass - and damage to trees is a species of trespass. The case is McDonald v. Bear River and Auburn Water & Mining Co., decided in 1859 and published at 13 Cal. 220.

Also note that California statutes provide for double or even triple damages for injury to trees under certain conditions. See Civil Code section 3346 and Code of Civil Procedure sections 733 and 734.