I believe that in most localities, this issue is not specifically addressed. There are periodic news articles describing law suits where the "victim' might be otherwise judged as the villain:
http://articles.latimes.com/2000/sep...state/re-25961
https://www.google.com/search?hl=en&...=us&authuser=0
I believe that you
SHOULD BE allowed to trim those roots, but it would be so much better if you could talk to the neighbor and get his buy-in before you do anything.
There is so much case law discussing these issues that I fear that you will not get a useful, authoritative answer until you wind up in court. Even then, a decision might be the luck of the draw, depending on the judge and circumstances, if there is no definitive regulation on the books.
Jim