Not quite so easy, nothing on the host has any idea about trees.
HiTech
Could you just shoot a ray-cast at a possible target and see if anything would collide before the target? If clear, Fire!
[Edit]
Nevermind. You said the server doesn't see the trees.
Interesting... Could the player client determine if the emplacement could shoot at them?
Like the AI tank sends a request to the client "I'd like to shoot at you please. May I?"
The client checks a ray cast back to the requesting AI position and sends back, "You have a clear line of fire! Go ahead!"
Boom.