Why would rising sea levels lead to deforestation in the Orkneys? Rising sea levels could flood some low lying coastal areas and stream valleys, but I would think forests would not be affected even a few feet above the shoreline. Is the whole water table so low that a few feet would make that much of a difference on the spread of salinity in groundwater?
There used to be more extensive land, which was forested, but is now covered by the ocean. The rest of the island's forests got used up, maybe as recently as the medieval period, when the Vikings swarmed in. The presence of the remains of a submerged forest is interesting because the same thing is found in places off Wales, and the Sinking Lands that used to bridge Britain with Ireland shows up in the Welsh Mabinogeon tales I think. And off the end of Cornwall was (supposedly) the land of Lyonnesse.