Another theory that was mooted for the planting of trees on barrows and the like was to "defeminise" them to prevent them looking like lewd iconography....Seriously you'd have to be pretty frustrated to get find such bumps in the ground as sexual turn-ons...but hey each to his (or her) own. I was told by an archaeologist in this area that the accepted theory was nothing more than asthetics. Another wild idea (wilder than my idea that they were the progeny of grave goods...) is that the trees were planted as a tribute to (Christian) god to take away or disguise the "paganism" of the sites. Maybe not so far fetched as it was taught that anyone without religion (at least the religion of the day) was a savage and wouldn't have had the mental capacity and logical wherewithal to venerate their dead or whatever sites were built to venerate.