Fenton’s strategy for the uplands of West Scotland is a wilderness one: freeing up natural systems to evolve without interference.
He is concerned about how sitka plantations here are freely seeding into open moorland, changing the natural ecology. He is also concerned about the proliferation of tracks, which cause the spreading of species not normally found here eg gorse.
Interestingly he lays out a case for not planting native trees up here, saying that it is wrong to equate the Western Highlands with the landscapes of Norway, where there is extensive snow cover in winter, resulting in less browsing and a far more extensive shrub layer.
Also, the land on the western side of Scotland receives more rainfall than Norway and is lower in altitude. He attests that the land is meant to be blanket bog and should be left as such, not planted with either conifers or broadleaves.
The image in this post is of the inland part of the Kintyre peninsula, only a couple of miles from the sea. In the distance some sitka plantations are visible, but the rest of the area is moorland, with virtually no trees. I had always put this down to overgrazing and the lack of a seed source, but Fenton has made me wonder if it is a natural system, adapted over millennia to the herbivores that roamed the land.
bioregioning 
Very interesting Ed. What level is Fenton suggesting in terms of a historic tree line. Thought there may be carbon dating and geological analysis of other areas that might provide an indication?
Fenton’s analysis is based on the presence of trees – many centuries, even millenia old, buried in the peat. But this is on areas of blanket bog, not hillsides. I have not come across any pollen analysis of such sites: there is a lowland site near Ferry Wood, “one of the best in the country” – but I don’t know of any hill sites which explore the very question you raise. Maybe you could do some research on this?