I'm sure this question will make me sound like a dimwit, but why are clumps of trees often found on barrows? I heard they were sometimes planted to provide route markers along trackways, but surely they would be long dead by now?
It was done at Coldrum and Little Kits Coty by 13th Century religious zealots, plus probably more locally. It supposedly helped to cleanse the place of non-Christian overtones, and in many cases helps to undermine the foundations and leads to eventual collapse.
Is this the wrong time to mention our cute and cudddly bunny friends? Lambourn seven barrows is riddled with scrapes and the bunny equivalent of high rise housing...
As for trees, in my local area, they tend to self seed on a barrow (thats beech trees on the rudge).