Funny how I interpreted the question differently than the person above. I interpreted the question to be asking how to estimate the amount of carbon dioxide sequestered from the atmosphere by a growing forest, without having to do labwork.

Obviously the labwork will give a more accurate estimate, but you can get a rough estimate by the following procedure:

CO2 sequestered (kg / hectare / year) = (a) annual forest growth (m^3 / ha / yr) x (b) bone-dry wood density (kg / m^3) x (c) proportion of carbon in bone-dry wood (unitless or kg / kg) x (d) ratio of molecular mass of CO2 to C (unitless or Dalton / Dalton)

If the forest is a single-species, even-age plantation forest, any forester in your area can give you an estimate of (a), based on site class. Natural forests are tricker, but a good forester in your country should have an estimate.

(b) mostly depends on tree species, but may be somewhat variable within species. For many species you can look up an estimate of bone-dry density. Try google, or ask a forester/wood products person. Fonseca (2005) lists several. For mixed-species stands, this will get tricky.

(c) is about 0.5 (also variable, but that's a very rough rule-of-thumb)

(d) is 44/12 or 3.67

A slightly more accurate way to estimate (c), but still just a rough estimate, is described in IPCC (2003). We used this approach in Frey et al. (2010) to estimate carbon sequestered by hypothetical forest and agroforest plantations:

IPCC (2003) Equation 3.2.3 is the formula for estimating total forest biomass from merchantable biomass. Annex 3A.1 gives international default conversion factors based on scientific estimates. Adapted Equation 3.2.3: C = [MBM x BEF2] x (1 + R) x CF, where C is total carbon in biomass (metric tons C), MBM is merchantable bone-dry biomass (metric tons), BEF2 is biomass expansion factor for conversion of merchantable volume to aboveground tree biomass (dimensionless), R is root-to-shoot ratio (dimensionless), and CF is carbon fraction.

The default values used for BEF2 (from IPCC 2003, Table 3A.1.10) are: hardwoods, 1.4; pine, 1.3. The default values used for R (from IPCC 2003, Table 3A.1.8) are: oak, 0.35; other hardwoods, 0.26; pine, 0.23. The default value used for CF is 0.5 (from IPCC 2003, p. 3.25).

The mass of CO2 sequestered from the atmosphere is greater than the mass of the carbon (C) alone, because C is stored and oxygen (O2) is reemitted to the atmosphere. The atomic mass of C is 12, and the molecular mass of CO2 is 44.

Therefore, for every 12 metric tons of C stored, 44 metric tons of CO2 have been sequestered. The conversion factor from C to CO2 is 44/12 = 3.67.

References:

Fonseca, MA. 2005. The measurement of roundwood: Methodologies and conversion ratios. CABI Publishing, Cambridge, MA. 246 p.

Frey, GE, DE Mercer, FW Cubbage, RC Abt. 2010. Economic Potential of Agroforestry and Forestry in the Lower Mississippi Alluvial Valley with Incentive Programs and Carbon Payments. SOUTHERN JOURNAL OF APPLIED FORESTRY 34(4): 176-185.

INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE (IPCC). 2003. Good practice guidance for land use, land-use change and forestry. Penman, J., M. Gytarsky, T. Hiraishi, T. Krug, D. Kruger, R. Pipatti, L. Buendia, K. Miwa, T. Ngara, K. Tanabe, and F. Wagner, eds. Institute for Global Environmental Strategies. Available online at www.ipcc-nggip.iges.or.jp/public/gpglulucf/gpglulucf_contents.html; last accessed Jan. 23, 2010.