Dry mass is typically used because it's consistent. Trees fluctuate over the year, being drier in some seasons than other. But they'll have the same dry mass.

Dry mass is typically used because it's consistent. Trees fluctuate over the year, being drier in some seasons than other. But they'll have the same dry mass. Trees are mostly C6 H10 O5, coming from CO2 and H20. Based on atomic weight about 70% of the mass of the dry wood comes from CO2. Fresh wood will have extra water content, taken from the soil (though that water came from the air too). There is carbon in the soil, but most plants don't take carbon up from their roots meaningfully. Nitrogen, phosphorus, potassium, and other trace minerals. Decomposers like fungi, bacteria, and insects will eat the carbon in the soil and release CO2.