It sounds like you might be making a stock vs flow error here. Or I could be misunderstanding you.
> And even then, it's not obvious that those new trees would sequester more carbon than the previous trees would have had they been allowed to continue growing.
1. Trees slow as they grow (after a certain point).
2. Commercial tree farms exist to grow trees. If they weren't growing trees for consumption, something else would be done with that land.
Yes, for example let the natural vegetation take over, increase biodiversity, and sequester more carbon thanks to the increased biodiversity and better soil than fucking tree farms.
- A lot of the time plantations are planted on less productive farmland. - Not all plantations will naturally regenerate quickly back to native vegetation.
- Those trees were planted for a reason and a lot of plantations are planted on degraded farmland or scrubland.
It's a very safe assumption
If X amount of carbon was in the trees, X amount of carbon is now in the building materials (at best). There's no further removal of carbon from the atmosphere by this alone. And all of the future carbon the trees would have sequestered is now not going to be sequestered by those trees. So more likely carbon positive than negative.
> The space the trees were in will now have more trees grown in them.
That's an assumption. And even if it were true, those trees would take years to begin to sequester the same amount of carbon that the previously existing trees did. And even then, it's not obvious that those new trees would sequester more carbon than the previous trees would have had they been allowed to continue growing.
Plus if you're going to say that building with blocks is carbon negative, you're going to have to talk about how those trees get turned into blocks, and how the overall process including that, is somehow carbon negative.