I presume sometimes barrows simply reverted to woodland if they were useless for any sort of farming. Silbury has always been useful for sheep maybe so is clear, but if so how come Thornborough's Northern henge is thickly wooded?
I suspect barrows were also planted with trees to provide coverts for foxhunting. I believe "copse-studded open landscapes" were considered ideal for it. Not my favourite activity but on the other hand John Aubrey was at it when he discovered Avebury...
