The carbon balance of a young beech forest

International audienceWe present measurements of CO2 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. 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. Daily minima of CO2 flux were typically around -20 µmol CO2 m-2 s-1 during the period of full leaf expansion, while night-time ecosystem respiration varied between 5 and 15 µmol CO2 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. Measurements of ecosystem respiration (eddy flux) were corrected for CO2 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. 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 CO2 from the decomposition of woody debris. 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 fores