Summary• Stem and branch respiration of 30-yr-old Fagus sylvatica trees was measured in a temperate forest for 1 yr to estimate the annual flux at the stand level.• The seasonal response of respiration to air temperature was determined using infra-red gas analysis (IRGA) systems. Annual respiration was derived from half-hourly temperature recording and allometric relations established for the same forest.• The basal respiration rate at 15 ° C (R15) increased greatly during the growing season. On a volume basis, monthly means of R 15 were higher for branches than for stems. For stems, Q 10 was relatively constant throughout the year, with an annual average of 1.7. Estimated annual respiration was approx. 325 g C m -2 ground surface area yr -1 with 50% of this amount attributed to growth respiration.• Stem and branch respiration played a major role in the annual carbon balance of the beech stand. It represented approx. one third of the ecosystem-level carbon loss from respiration. The magnitude of crown respiration makes it obvious that information on branch respiration characteristics is required for reliable estimations at the stand level.

Summary1. We present measurements of CO 2 fluxes over 2 years above and within a young Beech stand in the east of France. This site is part of the Euroflux network set up to monitor fluxes over representative European forests. 2. The net ecosystem carbon (C) exchange was derived from continuous eddy flux measurements. Major components of the total flux (i.e. soil and above-ground biomass respiration and assimilation of leafy branches) were measured independently using chambers. The main C stocks (i.e. root, stem and branch biomass) were also quantified. 3. Daily minima of CO 2 flux were typically around -20 µ mol CO 2 m -2 s -1 during the period of full leaf expansion, while night-time ecosystem respiration varied between 5 and 15 µ mol CO 2 m -2 s -1. The seasonal pattern of net ecosystem assimilation was very close to that of net assimilation at the single branch scale. The seasonal variation of net ecosystem exchange was closely related to leaf expansion and soil water content during the dry year of 1996. 4. Measurements of ecosystem respiration (eddy flux) were corrected for CO 2 storage within the stand. This C flux showed a seasonal pattern, the maximum rates (4-7 g C m -2 day -1 ) occurring in spring and summer, and appeared to be correlated with soil temperature. Temporal variation of soil respiration showed the same pattern, and effects of both temperature and soil drying were found. Annual soil respiration was ≈ 70% of ecosystem respiration. Root respiration was 60% of the total below-ground respiration. 5. Annual net C exchange was -218 and -257 g C m -2 in 1996 and 1997, respectively, corresponding to net C uptake by the forest. These values are much lower than the annual biomass increment (stems and large roots) of the stand: 427 and 471 g C m -2 year -1 , respectively. The difference may be explained by a release of CO 2 from the decomposition of woody debris. 6. Ecosystem C loss by respiration was 800-1000 g C m -2 year -1 . Gross C gain was 1000-1300 g C m -2 year -1 . Ecosystem respiration therefore played a major role in the annual C balance of this forest.

-This study is part of a larger project aimed at quantifying the biomass and biomass increment of an experimental beech stand aged 30 years, and comparing the carbon sequestration in trees to carbon fluxes. The below ground part of trees is expected to play an important role in carbon sequestration. A method has been developed to estimate the biomass and biomass increment of coarse, small and fine roots of trees from root system excavations of sampled trees of different crown classes. The biomass and biomass increment of broken root ends during excavation was estimated from the diameter of the roots at the broken points. Equations were then established relating the biomass and biomass increment of the different root categories to tree DBH. These equations were then used to estimate the root biomass and biomass increment of the experimental stand from stem inventory, for the different root categories. Trees from dominant and codominant crown classes contribute for more than 80% to below ground biomass and biomass increment of the stand. root system / biomass / biomass increment / biomass distribution / Fagus sylvatica Résumé -Biomasse et accroissement en biomasse du système racinaire dans un peuplement de hêtre du Nord-Est de la France. Cette étude fait partie d'un projet plus vaste ayant pour objectif l'estimation de la biomasse aérienne et souterraine d'un peuplement expérimental de hêtre de 30 ans et de son accroissement, pour comparer la quantité de carbone séquestrée annuellement dans les arbres aux flux de carbone mesurés par les échanges gazeux. La partie souterraine des arbres paraît jouer un rôle important dans la séquestration du carbone. Une méthode a été développée pour estimer la biomasse et l'accroissement en biomasse des racines de différentes catégories de grosseur grâce à l'extraction du sol de systèmes racinaires d'un échantillon d'arbres représentatif des diffé-rentes classes de statut social du peuplement. La biomasse et l'accroissement en biomasse des parties de racines cassées lors de l'extraction du sol ont pu être estimés, pour chaque catégorie de racine, à partir du diamètre des racines au niveau de la cassure. Des relations ont ensuite été établies entre la biomasse racinaire et son accroissement, pour chaque catégorie de racines, et le diamètre de l'arbre à 1,30 m. Ces équations ont été utilisées pour estimer les biomasses au niveau du peuplement à partir de l'inventaire des arbres. Les arbres dominants et codominants du peuplement contribuent pour plus de 80 % à la biomasse souterraine du peuplement et à son accroissement. système racinaire / biomasse / accroissement de la biomasse / répartition de la biomasse / Fagus sylvatica

-The objectives of this study were 1) to scale photosynthesis from leaf to crown and to tree scale, 2) to determine the proportion of assimilated carbon used for wood construction and whether the fraction of assimilation used for production varies among social classes and 3) to validate the approach by comparing assimilation estimates with independent measurements provided by the eddy covariance technique (EC). Measurements (growth and gas exchange) were performed in a 30-year-old Fagus sylvatica stand during the 1997 growing season on five sample trees of different crown classes (dominant, codominant and intermediate trees). A nonlinear relationship between net CO 2 assimilation and photosynthetically active radiation (PAR) was found for each sample trees. Canopy net CO 2 assimilation was then modelled over a period of non limiting soil water soil water content. Simulated gross assimilation scaled to stand level was in good agreement with stand measurements performed by EC. growth / carbon balance / photosynthesis / crown class / Fagus sylvatica L Résumé -Bilan de carbone et croissance dans un jeune peuplement de Fagus sylvatica. Les objectifs de cette étude étaient 1) d'estimer la photosynthèse à l'échelle de la couronne, puis à l'échelle de l'arbre à partir de mesures foliaires, 2) de déterminer la proportion du carbone assimilé utilisée pour la construction de bois et sa variation en fonction du statut social de l'arbre, 3) de valider l'estimation de l'assimilation en la comparant à une mesure indépendante (technique des corrélations turbulentes, EC). Les mesures (croissance et échanges gazeux) ont été effectuées pendant la saison de végétation 1997 sur cinq hêtres de 30 ans de statuts sociaux différents (dominant, codominant et dominé). Une relation non linéaire entre l'assimilation nette de CO 2 et le rayonnement photosynthetiquement actif (PAR) a été établie pour chaque arbre échantillon. L'assimilation nette de CO 2 à l'échelle de la couronne a ensuite été estimée pour une période sans stress hydrique. L'estimation de l'assimilation brute à l'échelle du peuplement à partir de mesures foliaires est en bon accord avec des mesures effectuées à l'échelle du peuplement par EC. croissance / bilan de carbone / photosynthèse / statut social / Fagus sylvatica L

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