Forest biomass and its change over time have been measured at both local and large scales, an example for the latter being forest greenhouse gas inventories. Currently used methodologies to obtain stock change estimates for large forest areas are mostly based on forest inventory information as well as various factors, referred to as biomass factors, or biomass equations, which transform diameter, height or volume data into biomass estimates. However, while forest inventories usually apply statistically sound sampling and can provide representative estimates for large forest areas, the biomass factors or equations used are, in most cases, not representative, because they are based on local studies. Moreover, their application is controversial due to the inconsistent or inappropriate use of definitions involved. There is no standardized terminology of the various factors, and the use of terms and definitions is often confusing. The present contribution aims at systematically summarizing the main types of biomass factors (BF) and biomass equations (BE) and providing guidance on how to proceed when selecting, developing and applying proper factors or equations to be used in forest biomass estimation. The contribution builds on the guidance given by the IPCC (Good practice guidance for land use, land-use change and forestry, 2003) and suggests that proper application and reporting of biomass factors and equations and transparent and consistent reporting of forest carbon inventories are needed in both scientific literature and the greenhouse gas inventory reports of countries.

Since biomass is one of the key variables in ecosystem studies, widespread effort has aimed to facilitating its estimation. Numerous stand-specific volume and biomass equations are available, but these cannot be used for scaling up biomass to the regional level where several age-classes and structural types of stands coexist. Therefore simplified generalized volume and biomass equations are needed. In the present study, generalized biomass and volume regression equations were developed for the main tree species in Europe. These equations were based on data compiled from several published studies and are syntheses of the published equations. The results show that these generalized equations explain 64-99% of the variation in values predicted by the original published equations, with higher values for stem than for crown components.

-Comparable regional scale estimates for the carbon balance of forests are needed for scientific and political purposes. We developed a method for deriving these estimates from readily available forest inventory data by using statistical biomass models and dynamic modelling of litter and soil. Here, we demonstrate this method and apply it to Finland's forests between 1922 and 2004. The method was reliable, since the results obtained were comparable to independent data. The amount of carbon stored in the forests increased by 29%, 79% of which was found in the biomass and 21% in the litter and soil. The carbon balance varied annually, depending on the climate and level of harvesting, with each of these factors having effects on the biomass differing from those on the litter and soil. Our results demonstrate the importance of accounting for all forest carbon pools to avoid misleading pictures of short-and long-term forest carbon balance.carbon inventory / forest biomass / greenhouse gas inventory / litter / soil modelling Résumé -Accumulation de carbone dans les forêts finlandaises entre 1922 et 2004, une estimation obtenue en combinant les données de l'inventaire forestier avec une modélisation de la biomasse de la litière et du sol. Une estimation comparable à l'échelle régionale du bilan de carbone des forêts était nécessaire pour des objectifs scientifiques et politiques. Nous avons développé une méthode pour déduire ces estimations de données facilement disponibles de l'inventaire forestier en utilisant des modèles statistique de la biomasse et une modélisation dynamique de la litière et du sol. Ici nous présentons cette méthode et l'appliquons aux forêts de Finlande entre 1922 et 2004. La méthode a été fiable, puisque les résultats obtenus ont été comparables à des données indépendantes. La quantité de carbone accumulée dans les forêts s'est accrue de 29 %,79 % de ce qui a été trouvé dans la biomasse et 21 % dans la litière et le sol. Le bilan de carbone varie annuellement, selon le climat et l'importance de la récolte, chacun de ces facteurs ayant des effets sur la biomasse différents de ceux qui agissent sur la litière et sur le sol. Nos résultats démontrent l'importance de comptabiliser tous les réservoirs de carbone en forêt pour éviter des images trompeuses du bilan de carbone des forêts à court et moyen terme.inventaire du carbone / biomasse forestière / inventaire des gaz à effet de serre / litière / sol

Summary 1.Norway spruce ( Picea abies ), one of the dominant tree species in Eurasia, has spread slowly westward in northern Europe, invading eastern Finland about 6500 calibrated years ago (cal. years BP), eastern central Sweden about 2700 cal. years BP and southern Norway about 1000 cal. years BP. Its spread is the most recent and best constrained invasion of a main tree species in northern Europe and allows an assessment of colonization patterns and associated competitive replacement processes. 2. We analysed five selected high-resolution pollen accumulation rate (PAR) -records along a 700-km long transect in the direction of P. abies invasion from eastern Finland to central Sweden across the present P. abies -and Pinus sylvestris-dominated southern boreal zone. 3. Our results show that the P. abies population increased in size from the time of the initial expansion to levels comparable with the modern in 100-550 years. At each site P. abies invaded a dense, intact Pinus -Betula -Alnus forest, mixed with temperate deciduous taxa, particularly Tilia cordata and Corylus avellana . The resident mixed forest provided no or weak resistance to the colonization of P. abies , and the variable population growth rate was therefore not caused by compositional differences in the resident forest but by other, possibly local edaphic factors. 4. Of the taxa that formed the resident forest, T . cordata responded most strongly to the invasion of P. abies . This suggests that the mid-Holocene T. cordata population decline was not directly climateinduced but resulted from competitive replacement due to overlapping ecological niches with P. abies , a stronger competitor. 5. Synthesis . The rise to dominance of P. abies was caused not only by its rapid population growth but by associated competitive suppression of other taxa, leading to a major ecosystem change from a mixed conifer-deciduous forest to the modern P. abies -and P. sylvestris -dominated boreal conifer forest in central Fennoscandia. This competitive suppression by P. abies is still reflected in the scattered occurrence and generally weak performance of T. cordata in the boreal zone of Europe and may influence its distribution and abundance patterns under predicted future climate scenarios.

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