[PeterisP]

Well, because to the first approximation pretty much all land is used - any land thats suitable to be a forest but is not already a forest is only that way because it's used for grazing or farming, otherwise it would overgrow naturally (though not as efficiently as with planting). If you want to increase the amount of woodland, you have to decrease the amount of pastures or farmland, so you have to either buy that land from the previous owner (because they won't be able to graze or plant there anymore) or take it from them by force.

[ed_balls]

I pretty sure that are that 10 mln km^2 around the word that is not use (Canada, Alaska, Syberia and patches of land around the world)

[PeterisP]

As I said, I'm talking about "any land thats suitable to be a forest but is not already a forest". These large uninhabited areas in Canada, Alaska and Siberia don't have such land - if any spot there is suitable for trees, then trees already have filled that area for hundreds of years (I mean, these areas already have massive forests), and in the areas where trees aren't growing naturally, it's for a reason, planting won't make a difference.

If you want to convert not-woodland into woodland, then that limits you to farmland or pastures - because there's no such thing as "unused natural potential woodland", any potential woodland that's not used and left alone becomes actual woodland; any potential woodland that's not woodland only became that way when we cut down the trees and cleared the land because we wanted to use it otherwise.

[ed_balls]

I agree that my estimate could be too big, but I do think there is a potential to increase the forestation in the north. Because of the warmer climate, Iceland can grow aspen. The government is planning to grow tree in a large part of the island, but the have a problem with free roaming sheeps. The land the can only sustain shrubs and moss could grow trees now.

Some countries are increasing the forestation as well https://www.citylab.com/environment/2018/01/northern-forest-...