In order to estimate the total carbon pool in the region/area, it is necessary to know the no. of individual /sp., but in order to estimate the efficiency from the genetic angle, it is important to express in the unit of Kg C/cubic metre. Infact Carbon flux in vegetation is regulated by RUBISCO enzyme.
Dear friend, it doesn't depends upon the number of species, but it more depends upon the age structure, basal area and height.
We can't say that if their are more no. of species and carbon will be more, but it is not like that, but it depends on the age structure and girth class also.
As rightly pointed in the above answers and as is evident from the fact that the carbon sequestration overall depends upon the age and structure of the forest stand which can be well defined by calculating the basal area. However, when species specificity is considered, if you look at some forest patches say for example a Dipterocarpus rich forest patch in the Western Ghats forest, you can well say that, that forest stand in most probability will have a higher basal area and ultimately high carbon sequestration. Hence, in some cases the presence of particular species, genera or family can be taken as a indicator for the forest structure and condition and hence, as indicator of its sequestration potential.