If the goal is to sequester CO2 then its not enough to just grow trees, you also need to prevent them from being cut down and burned. One thing that I would argue in favor of using iceland and other areas around the arctic circle is that people has demonstrated in the last several decades that it isn't economical worth to grow and farm the land for anything, which include trees. That might then mean that people will leave the trees alone for a long enough time that the sequester of CO2 matters.