Bill, Stonehenge is on the top of a hill in an area with few trees. I don't know how long that part of Salisbury Plain has been like that, but I imagine they would have chopped down any close ones in line of sight, if only to keep themselves warm during the festivals. (Another complaint English Heritage have against the new age travellers is their use of any remaining scrub on their fires).
A lsit of Stonehenge web links is available at http://scivis.net/pages/stonehenge.html along with some rather interesting homework assignments!

Rod