[downWidOutaFite]

From a kid's level of understanding you could also deduce that the carbon must end up in the plant matter, and that a forest's plant matter density doesn't increase forever. Old forests reach a carbon-neutral equilibrium (decaying plants release carbon). New forests suck up carbon until they reach that equilibrium.

Another idea you could deduce from this is that if we remove plant matter from the forest, and lock that carbon up so it doesn't go back into the atmosphere, it could become a long term carbon-sink. Even better if the plant matter can be useful. So vast lumber farms with the fastest growing trees you can get could be a good carbon capture technology.

[gtsop]

I think more information is needed as input here. In pwrticular, what is the ratio of consumed/stored co2 in the photosynthesis process. My super basic understanding is that co2 is being used as fuel to convert energy into tree-food. By that logic i would expect at least a portion of the co2 to be actually transformed to other elements rather than stored as-is into the tree.

[downWidOutaFite]

Your intuition is correct. The basic photosynthesis chemical formula is

  6CO2 + 6H2O + (light) ---> C6H12O6 + 6O2

The carbon goes from CO2 to C6H12O6, which are carbohydrates. The carbs are used by the plants for both energy and plant matter. The carbs that are used for energy are released back into the atmosphere as CO2 via cellular respiration. For carbon sequestration what matters is that the carbon in the plant matter stays fixed and doesn't get converted back into CO2 and released.