And pretty damn useless for most purposes too, regardless of temperature.

the low and middle boreal forests are rather dense, so to get any farming done you'd need to fairly much clearcut that, which would require decidedly nontrivial amounts of time, money, and energy.

the high boreal thins out and would probably work for farming.

then you get into the permafrost, which will become a muddy mess when it thaws, and barren rock.  it doesn't matter how warm it gets, you ARE NOT going to be farming on either of those.