It's really hard to deal with unreasonable people because the usual tactics don't work.
If it were me, I'd approach them and tell them I'm hiring a land surveyor just in case *I* was misunderstanding the property lines. Then I'd ask if they'd be willing to hold off clearing any more trees...
Get a surveyor to establish the property lines ASAP and build a fence. Cameras are great idea, as well.
Do you think going over to the neighbor and saying you got off on the wrong foot would help? Explain that you'll hire a surveyor to determine the lot lines so no one has to guess who's on...