My simplistic model for the carbon cycle was that plants (or trees in particular) photosynthesise and transpire. The former takes up $\small\mathsf{CO_2}$ and $\small\mathsf{H_2O}$ and the latter does the reverse but obviously not netting out. The carbon for the cellular material (in particular wood) comes entirely from atmospheric $\small\mathsf{CO_2}$ (?).
The mass of trees comes mainly from cellulose which is roughly 50% carbon. Therefore the mass of sequestered $\small\mathsf{CO_2}$ is simply related to the mass of trees, with the proportion equal to the ratio of the molecular masses of cellulose and $\small\mathsf{CO_2}$ with balanced carbon content. Wiki says cellulose is some sort of sugar type thing $\small\mathsf{(C_6H_{10}O_5)n}$ and carbon dioxide is obviously $\small\mathsf{CO_2}$ so the ratio is something like: for each kilo of tree/wood grows 1.62 kg of $\small\mathsf{CO_2}$ is sequestered?
Is this completely wide of the mark?