Legal Question in Real Estate Law in California

In the 12th Century, kings ruled by Divine right and could do no wrong. You couldn't sue the king in his own court. By the time of the Magna Carta this ironclad rule was eroding, but the government retained some immunities until quite recently, and even today most suits against the state, any California municipality or government agency requires following a certain procedure set forth in the Government Claims Act.

The procedure starts with presentation of a claim containing specified information and presented in a certain way within certain time limits. The governmental unit may respond by honoring (paying) the claim, rejecting it formally, or ignoring it. After a claim is rejected or not responded to within the allowed time, the complaining party is then able to sue.

Even so, there are still some suits that cannot be won against a government. For example, a private citizen cannot succeed in a suit to quiet title to public land by adverse possession.

With specific reference to the tree situation, I do not know if the city is correct. I rather doubt it has categorical immunity, but it may either be bluffing by rejecting properly-brought GCA claims, or perhaps the claims did not comply with the GCA. Or, there may be some other ground for a claim of no liability, such as a covenant in the private owners' titles.

This is a common problem in cities in California. My guess is your problem trees are liquidambers, which were planted wholesale by CA cities some years ago, and are renown for their invasive shallow roots. Regardless of the type of tree, however, the city is probably right. The remaining power of that old rule you can't sue the king, is that you still can only sue the government for what the government has agreed to be sued for. Generally you can sue the government for contracts, negligence, and civil rights. Not much more. You mention that people have been injured and sued the homeowners, and that when the homeowners have tried to go after the city they have failed. Considering that the homeowners were probably represented by insurance lawyers with plenty of legal power behind them, and they failed, my guess is that the city has covered it's backside as relates to these trees and you are not going to succeed in going after them.