Accurate estimates of below-ground biomass of trees are important when quantifying the amount of carbon sequestered in forests. Allometric single-tree below-ground biomass functions were developed for Pinus sylvestris , Picea abies and Betula pendula and Betula pubescens in Sweden. The idea was to calibrate an existing comprehensive data set of about 600 trees that only covered the stump and coarse roots against a new data set that covered roots down to 2 mm diameter. The new data set consisted of about 80 trees acquired using the same design as for the existing set, but complemented with a detailed inventory of the fine root fractions remaining in the ground. Checks were made to determine whether the density properties of the two data sets were comparable and it was concluded that they were. This was a prerequisite for calibrating the older data against the new information and further for merging the two data sets. The merged data set was used to derive regression functions for below-ground biomass. For all functions the adjusted R 2 values were always higher than 0.95 and the root mean square errors were always lower than 36% for P. sylvestris and P. abies. Below-ground biomass predicted with the new functions was approximately 11% higher than the values obtained using the existing biomass functions.

-Differences and uncertainties of alternative methods applicable to estimation of biomass in national greenhouse gas inventories are evaluated. The alternative methods employed to obtain biomass estimates of trees are (1) aggregated stand-level volume estimates multiplied by biomass expansion factors (BEF), and (2) biomass equations applied to tree-wise data of a national forest inventory. In comparison to the reference value obtained using tree-wise biomass equations, the age-dependent BEFs for the whole of Sweden resulted in a 6.7% lower aboveground biomass estimate. The estimates were the closest for conifer-dominated forests in central Sweden, and the largest discrepancies were for spruce in southern Sweden. This result indicates that these age-dependent BEFs cannot be applied to conditions where stand development deviates from the conditions under which the BEFs were developed. The degree of uncertainty in both methods was highest in the young age-classes. At the regional level, the relative standard errors of the BEF-based biomass estimates were in the range of 4-13%.biomass function / carbon stock of trees / forest inventory / greenhouse gas inventory / uncertainty Résumé -Estimation de la biomasse sur pied en Suède : comparaison des équations de biomasses avec les facteurs d'expansion de la biomasse (BEF) liés à l'âge. Nous avons évalué les différences et les incertitudes des méthodes alternatives d'estimation utilisable dans les inventaires nationaux de GES. Ces méthodes sont (1) la multiplication des estimations du volume au niveau des peuplements avec les facteurs d'expansion de la biomasse (BEF) et (2) l'application des équations de biomasses sur les données par arbres de l'IFN. La méthode des BEF a donné, pour la forêt suédoise, une estimation de la biomasse inférieure de 6,7 % par rapport à la méthode de référence (2). Les différences entre estimations étaient minimales dans les forêts de conifères du centre du pays et maximales dans les forêts d'épicéas du sud. Cela indiquerait que les BEF liés à l'âge ne sont pas applicables dans des conditions s'éloignant du développement des peuplements boréaux. Le degré d'incertitude était le plus élevé pour les classes d'âge jeunes. Au niveau régional, l'erreur relative standard des estimations se basant sur les BEF se situait entre 4 et 13 %.équation de biomasses / stock de carbone sur pied / inventaire forestier / inventaire des gaz à effet de serre / degré d'incertitude

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For estimating the amount of carbon (C) in dead wood, conversion factors from raw volume per decay class to dry weight were developed using three different classification systems for the species Norway spruce (Picea abies L. Karst), Scots pine (Pinus sylvestris L.) and birch (Betula pendula Roth and B. pubescens Ehrh) in Sweden. Also the C concentration in dead wood (dry weight) was studied. About 2500 discs were collected from logs in managed forests located on 289 temporary National Forest Inventory (NFI) sample plots and in 11 strips located in preserved forests. The conversion factors were based on an extensive data compilation with a wide representation of different site-, stand-, species-and dead wood properties and were assumed to represent the population of fallen dead wood in Sweden. The density decreased significantly by decay class and the range in density for decay classes was widest for the NFI decay classification system, suggesting this to be the most suitable. The C concentration in dead wood biomass increased with increasing decay class and in average Norway spruce (Picea abies) showed a lower C concentration than Scots pine (Pinus sylvestris). The average dead wood C store of Swedish forests was estimated to 0.85 Mg C/ha.