If you plant a forest where there was no forest before, then while individual trees will die, the entire forest will capture carbon dioxide proportional to its

If you plant a forest where there was no forest before, then while individual trees will die, the entire forest will capture carbon dioxide proportional to its biomass. Forests will typically grow for years-decades, and absorb carbon dioxide throughout, till their biomass (per km^2) reaches an equilibrium point. At that point the forest will go from a carbon sink to carbon neutral (averaged over several years to account for forest fires and subsequent regrowth).