Open Journal of Forestry 2011. Vol.1, No.1, 6-9 Copyright © 2011 SciRes. DOI:10.4236/ojf.2011.11002 Carbon Stock Assessment at a Regional Level, on Pure and Mixed Stands, Based on Inventory Data: A Case Study * Teresa Fidalgo Fonseca, Carlos Pacheco Marques Department of Forest Science and Landscape Architecture, University of Trás-os-Montes e Alto Douro, Vila Real, Portugal. Email: tfonseca@utad.pt Received July 4 th , 2011; revised July 19 th , 2011; accepted July 22 th , 2011. Following Kyoto protocol considerable research efforts have been made on carbon sequestration assessment of forest stands. The aim of the present study was to test the adequacy of two documented methodologies to obtain stand level dry biomass and carbon estimates, considering its application at regional scale, for pure and mixed stands. The procedures refer to: 1) the estimation of dry biomass through the use of tree biomass models and 2) the use of biomass conversion and expansion factors (BCEF) to convert stem volume raw data into aboveground dry biomass. Using the dry biomass values, carbon stock is easily estimated using conversion factors, being 0.50 a generally accepted value. Case study reports to a pilot zone with pure and mixed stands of Eucalyptus globulus Labill and Pinus pinas- ter Ait. in Portugal. Procedure 1) gives the most reliable results when the database includes detailed information at tree level, both for pure or mixed stands. When individual tree data is not accessible, option 2) is an interest- ing option to consider for pure stands. The use of BCEF-based procedure in mixed stands is not advocated at least when considering the direct use of a single conversion factor, as the value of the factor strongly depends on the forest composition. Keywords: Biomass, BCEF, Greenhouse Inventory, Carbon Monitoring Introduction The assessment of carbon in the forest areas is a timely sub- ject because carbon stock is an important criterion of Sustain- able Forest Management and at the same time it is required for greenhouse gas inventories needed in the LULUCF sector (Land Use, Land Use Change and Forestry) for the United Na- tions Framework Convention on Climate Change (UNFCCC) reporting. Two major methodologies have been proposed and accepted for use to assess the carbon stock of the tree component using ground level inventory data. One is based on tree biomass and carbon concentration in the biomass (g of carbon per g of bio- mass). Under this approach, tree dry biomass is estimated from tree characteristics using allometric equations that relate tree biomass with tree size (usually, diameter or diameter and height). For general forest inventory purposes this approach seems to be the most reliable provided that there are biomass models available for the inventoried species. This seems not to be of great concern as there is a wealth of literature about the subject for both whole tree biomass and the biomass of differ- ent components. Just to mention some biomes, Zianis et al. (2005) present a database on biomass equations for various tree species growing in Europe. A compilation for North-American and for Canadian tree species can be found in Ter-Mikaelian and Korzukhin (1997) and Lambert et al. (2005), respectively. However, it is not unusual a lack of recorded values on indi- vidual tree diameters, namely for evaluations made at sub-regional and at regional scale. In such cases, alternative methods based on inventory data need to be considered. One of the most practical approaches makes use of stand volume data (see Brown et al. 1989; Brown 1997). Biomass (B) is estimated from volume (v) data using the general formula B = Ev, where  represents wood density and E goes for an expansion factor used to expand values from one tree component, usually the stem, to other tree components (e.g. crown) to get aboveground estimates. Two main options are offered under this approach. One option, not discussed here, refers to the use of the individ- ual values of tree volume and wood density. Dry mass values are calculated using the volume of the trees (measured or esti- mated) and the wood density values for the species (a large database of wood density values is available from the World Agroforestry Centre website). This is a helpful option when is required a roughly estimate of carbon and there is no informa- tion of proper biomass equations for the existing species. Other option requires the use of biomass conversion (BCF) and biomass expansion factors (BEF). Those combined factors (BCEF) allow converting stem volume raw data per unit area into stem dry biomass and then to expand stem biomass values to other components of the trees, such as crown and roots. This method is one of the methods currently suggested in the Inter- governmental Panel on Climate Change IPCC guidelines based on National Forest Inventories for estimating biomass gains and losses (IPCC 2006, 4.11). The approach presents a great value for monitoring purposes using data from past forest in- ventories. Criticism has been made to the use of this approach: biomass factors used are, in most cases, not representative, because they are based on local studies (Somogy et al. 2007); forest inventories often report volumes to different standards, e.g., to minimum diameters greater than 10 cm, thus particular care is needed to unify the volume data (Brown, 1997), this being quite relevant either for monitoring as well as for com- parison purposes between regions or countries. Using the dry biomass values, carbon stock of a tree or of a