I have read that the very small increase in carbon dioxide in the atmosphere has caused an overall increase in the green on the planet as a whole.  Unfortunately I don't have a reference for the data.  If biomass is increasing it should contribute to carbon dioxide drawdown.  Another thing to consider is that back during the ice age most of Alaska was grassland supporting such megafauna as mamoth and giant bison as attested to by frozen carcuses in places like the circle mining district.  Perhaps it is the continuation of a process that has been going on for ten thousand years.