On the contrary. A mature 300 foot conifer has already fixed nearly all of the CO2 that it is going to in its life, and respires only as much as necessary to maintain its foliage and root functions. By contrast, a young stand can double or triple its carbon inventory in a decade or less. We should be focused more on developing young stands than on stagnating old ones. A good rule of thumb: if the average tree in a stand has a pointy top, that stand is vigorously growing, and if the average tree in a stand is a flat-topped wagon wheel, the stand is no longer putting on volume.
EDIT: I should also add that if you cut a tree and use the lumber to build a house, that carbon remains fixed throughout the lifetime of that house and beyond, through the actions of decomposers and the complex food webs that depend on them. It is only released as CO2 through combustion.