You take a large tree with a large surface area that's transpiring lots of carbon and replace it with a sapling with a small surface area. Yes that sapling is,

You take a large tree with a large surface area that's transpiring lots of carbon and replace it with a sapling with a small surface area. Yes that sapling is, over time, turning carbon into tree trunk, but that time scale for that is decades Actually the growth peaks in a young forest as soon as there is at least one leaf or needle on most possible paths of a sunray. This is called canopy closure, and as soon as it has been achieved the growth starts to decline. This has been studied extensively, here is one example: https://www.nrs.fs.fed.us/pubs/2397 https://www.nrs.fs.fed.us/pubs/2397 The reason in a nutshell is that when trees start to compete on the available sunlight by out-growing and overshadowing each other, they need to work harder to pull water up to the top where most of the photosynthesis happens. The leaves or needles in the shadow produce almost nothing but still consume resources for respiration.