Actually I'm going to come to the defence of trees growing on barrows as well. Barrows have a long history in the landscape both as markers, think of the romans aiming their roads for them, and even using them to bury their dead as well, as did the saxons. Celtic monks wandering the old roman highways would have probably stood on top of them to declaim the heathen ways of the local populace... beech trees on the Overton barrows may be 18th garden design but could also provide shelter for animals. so trees whether naturally or artifically planted on barrows are part of their ongoing history - its impossible to freeze frame time in prehistory.....
Modern use of barrow; on Overton barrow someone has put up a stone with a memorial to a young man,(must have been a couple of years ago) last time I looked the wind was blowing the paper (encased in plastic) into the field, and all that was being left was a few plastic flowers (mmmm) but before we all get carried away in moral indignation or whatever - time will also erase the flowers as well as the trees....