An acre of forest sequesters somewhere in the region of 30,000 lbs of carbon dioxide. Individual trees die and are reborn, but the forest as a whole, if it does not shrink, retains that carbon.
But it's even more complicated than that ...
We are reforesting ~20 acres of woodland just outside of San Francisco and we are not, as you say, thinking about individual trees - we are thinking of the woods as a whole ...
... which means clearing out a tremendous amount of sapling / deadwood / litter material that would quickly combust in a wildfire and destroy the entire forest.
So while we are on track for planting >200 redwood and douglas fir trees, we have also had to cut down >100 marginal trees to reduce fuel load. We're thinking on the scale of the entire forest and the entire forest needs to survive a burn ... including our relatively new trees that aren't as fire resistant as the old growth redwoods that were originally logged out of here.
Generally speaking:
The complexity of carbon mathematics is fraught and complex and I don't think it's reasonable to expect end users like consumers to navigate it.
And frankly it’s not something any organization can promise. Governments change and clear forest that was previously protected. Brazil provides a current and unfortunate example of this.
Contrast to an oil deposit which, left undisturbed, might never release its carbon.