I'm with you on this one fitz. I've long been considering the possibility of avenues cut through forests radiating from monuments.
It's easy to forget that in order to see a distant peak you don't have to cut an avenue right the way to its base. Depending on the height of the trees you would probably only have to go a couple of hundred yards at the most. If done in such a way it could make any peak appear to sit on top of the trees at the end of the avenue, thus bringing the peak nearer.
The rate at which communities such as those at the Ceide Fields must have used wood would easily take care of a few avenues of trees a couple of hundred yards long. IIt's also important to remember that not every monument was in use at the same time.
Perhaps the cursus-type monuments were constructed to mark where these tree-cut avenues once stood, but no longer existed after further deforestation.