How is coming onto someone else's property and cutting down their trees legal possession? Sounds a little like cattle rustling.
I agree with JL Sargent if you've got a stamped surveyed parcel map and your paying the taxes its your property.
I have not faced a situation like this yet but the parcel next to me just sold. I guess I will see what kind of new neighbors I get.
I think I would start with the Sheriff, if he says you need a lawyer so be it.