After reading POPPOP's post, I now realize we're talking about a
double row of trees. I realized the OP had also planted rows of
trees, but I didn't realize that one row was along the same line as
the neighbor.

So, there is one line of trees 2 feet inside the neighbors property
and another 10 feet inside the OP's. I still don't see the problem.
Now you have a double row of trees only 12 feet apart forming a
privacy screen. These trees can grow to 60ft by 15ft. They will
overlap, forming one continuous privacy screen. Why would anyone want
or care about mowing between them, trimming etc.

Before I wasted money on lawyers, I'd do an internet search. It won't
be hard to find plenty of info that will validate the right to do
trimming over your airspace, but at your own cost.