Trees would be replanted as they die, and the dead trees would be used for products. It's a constant process or cycle, not a one time thing. Also, by the time you plant enough trees to cover the entire landmass hundreds of years will have already passed and by then we're sequestering co2 with technology and firing it into space or using it for something else, with large reforestation projects ongoing because it's beautiful to have trees.

From a climate concern point of view, trees as carbon sinks work.