I understand Lewis and Orkney, which are now completely without trees except for plantations (e.g. woodland near Stornoway) was due to excessive farming from the neolithic onwards.
"Excessive" farming! Brilliant!
The moorland of Dartmoor that we see today was the result of exactly that. The only bit that isn't a result of man's environmental impact is Wistman's Wood and even that has changed somewhat in terms of species.