Is there any evidence on the flora of the area around the time the dolmen was erected...what if it had originally been surrounded by trees(in a copse for example)???
Local author Philip Henwood (Prehistoric East Cornwall 2007) says:-
At the time of its construction, probably in the first half of the fourth millennium, Trethevy Quoit would have been surrounded by forest and a clearing made for its erection. To make it a more impressive focal point, avenues were most likely cut through the forest, terminating at the site.
I would go along with most of that regarding the tree cover but the site area itself may have been devoid of substantial trees because of the thin layer of topsoil present. But who really knows?
Isn't it the case that the reason there is no or poor topsoil because the felling of trees without replanting allowed the topsoil to be washed away as is the case in many of the heathlands and commons especially in the south of England(Reigate and Frenches Commons being prime examples)? Whether this felling happened pre or post the quoit's erection....I have no idea....(over to an expert)