The Caledonian Forest is the name given to the former (ancient old-growth) temperate rainforest of Scotland. The known extent of the Roman occupation suggests that it was north of the Clyde and west of the Tay.
The Scots pines of the Caledonian Forest are directly descended from the first pines to arrive in Scotland following the ice-age; arriving about 7,000 BC. The forest reached its maximum extent about 5,000 BC after which the Scottish climate became wetter and windier. This changed climate reduced the extent of the forest significantly by 2,000 BC. From that date, human actions (including the grazing effects of sheep and deer) reduced it to its current extent.
Today, that forest exist as 35 remnants covering about 180 square kilometres (44,000 acres). The Scots pines of these remnants are, by definition, directly descended from the first pines to arrive in Scotland following the ice-age. These remnants have adapted genetically to different Scottish environments, and as such, are globally unique; their ecological characteristics form an unbroken, 9,000-year chain of natural evolution with a distinct variety of soils, vegetation, and animals.
To a great extent the remnants survived on land that was either too steep, too rocky, or too remote to be agriculturally useful. The largest remnants are in Strathspey and Strath Dee on highly acidic freely drained glacial deposits that are of little value for cultivation and domestic stock.
It’s also amazingly beautiful. I can happily drag myself out of bed at 3am and drive several hours north to arrive at the forest in time to watch the sun rise: with its ancient history, the scent of the heather, watching mist flowing around the old pine tree-tops catching the morning light, there’s nowhere more gorgeous on Earth.