If he damaged community property to put the trees in, I'd suggest paying a visit to the local Committee offices. Your local president might have something to say about it. The trees are obviously not on his land…
Why muck about poisoning them? If they're not on his land, just take them out and put them into his garden. Whatever you do, confrontation looms.
I'd start with a letter to the committee, then wade in with the napalm if that fails.