Multiple things removed Scotlands trees. Initially to clear land for farming and to use timber for building and fuel. Then for making charcoal for iron smelting in places like Glen Etive before large scale coal mining replaced charcoal as the fuel of choice.
A lot of timber was used in the Royal Navy and Merchant Navy as the British Expanded. God alone knows how many Oak trees went into a ship like HMS Victory, but ships were used from the 1600’s up till late 1800’s. Then huge fishing fleets all used huge quantities of scots pine, larch and oak. Most of the little harbours all the way around the Scottish coast had huge quantities of fishing boats all fishing for cod and herring, all along with potatoes were exported to low countries in wooden barrel transported in wooden ships (and all the Dutch half round roof tiles on old east coast houses came back as ballast on the same boats).
As for the clearances. This was a mix of political and economic. Post Jacobite rebellion the likes of the Duke of Sutherland and Duke of Argyl thought that large scale sheep farming would be far more profitable that having lots of little tenant farmers growing crops and cattle. They also had a dire for labour in the new colonies in the Americas as well as in Industrial Mills in Central belt and further south. So pretty much the tenants were given their marching orders and the offer of new land in the New World or the prospects of work in the mills. Some went willingly, some with a little persuasion. Some ended up as indentured labour for the rest of their lives, sone prospered, many died.
The sheep project didn’t last very long. Sheep are really a dryland animal. Most sheep in the British Isles were on the Eastern downlands and fens - hence the very large churches built by all the wool barrons in the middle ages.
Scotland was cattle country - think Rob Roy etc with Scottish cattle being exported down to England and the continent by Drovers who walked them down.
By the mid 1800’s Sheep in Scotland were producing the wool to make the landowners wealthy. Wool production went to Australia and cheap cotton from India and Americas in many ways replaced wool.
Fortunately Queen Victoria had married the German Prince Albert who rather enjoyed to sport of stalking deer and chamois in the Alps. British Gentlemen hunted, but on horseback with hounds after foxes and deer.
But stalking became the height of fashion. Mang of Scottish Lairds - now pretty much broke with little in the way of rents from tenants (whom they had kicked off the land) nor income from sheep, were only too happy to lease or sell of parcels of land to wealthy industrialists and aristocrsts all of whom wanted to emulate Prince Albert and Queen Victoria.
The opening of the railways made Scotland very accessible. Indeed the likes of Earl of Caithness and Duke of Sutherland funded the railway from Inverness up the Helmsdale, across the flow country to Thurso. At the same time selling of little estates (each of several thousand acres) as sporting estates with river frontage for Salmon and hill ground for Deer.
And hence the Scottish Sporting estate was born.