I there anything to exclude from GI that might help render times? If I have a dozen trees in my scene do they really need to be contributing to GI? Would excluding them save enough time to matter? I would think I would want them to receive GI, but not contribute. Any thoughts?
its a tricky situation. You would want them to recieve for sure. In some particular cases they need not cast gi, for example in day lit scene they probably are not going to make much difference. The thing to watch for is, if they are too close to each other, they may case gi shadows from one tree to another and that may look strange without correct bounce.