The carbon balance of a young Beech forest

1. We present measurements of CO2 fluxes over 3 years above and within a yo ung Beech stand in the east of France. This site is part of the Euroflux ne twork set up to monitor fluxes over representative European forests. 2, The net ecosystem carbon (C) exchange was derived from continuous eddy f lux measurements. Major components of the total flux (i.e. soil and above-g round biomass respiration and assimilation of leafy branches) were measured independently using chambers. The main C stocks (i.e. root, stem and branc h biomass) were also quantified. 3, Daily minima of CO2 flux were typically around -20 mu mol CO2 m(-2) s(-1 ) during the period of full leaf expansion, while night-time ecosystem resp iration varied between 5 and 15 mu mol CO2 m(-2) s(-1). The seasonal patter n of net ecosystem assimilation was very close to that of net assimilation at the single branch scale. The seasonal variation of net ecosystem exchang e was closely related to leaf expansion and soil water content during the d ry year of 1996. 4, Measurements of ecosystem respiration teddy flux) were corrected for CO2 storage within the stand. This C flux showed a seasonal pattern, the maxim um rates (4-7 g C m(-2) day(-1)) occurring in spring and summer, and appear ed to be correlated with soil temperature. Temporal variation of soil respi ration showed the same pattern, and effects of both temperature and soil dr ying were found. Annual soil respiration was approximate to 70% of ecosyste m respiration. Root respiration was 60% of the total below-ground respirati on. 5, Annual net C exchange was -218 and -257 g C m(-2) in 1996 and 1997, resp ectively, corresponding to net C uptake by the forest. These values are muc h lower than the annual biomass increment (stems and large roots) of the st and: 427 and 471 g C m(-2) year(-1), respectively. The difference may be ex plained by a release of CO2 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 pla yed a major role in the annual C balance of this forest.