Net ecosystem exchange (NEE), as measured with eddy covariance was compared for two European beech stands for the years 1996-1999: a young beech forest (32 year-old) growing in east France, and a mature beech stand (80 year-old) located in Denmark. Those sites are included in the Carboeuroflux/Carbodata European networks. Except for some short-term differences (1-5 days), the temporal variation of NEE followed similar patterns in both sites. This similarity followed from similar values of. (i) dates of bud break and of leaf fall; (ii) ecosystem respiration rates during winter; (iii) diurnal NEE during the growing season. Related to this close parallelism was the similarity of temporal variation in temperature and in global radiation range and variations. Nevertheless, different rainfall distribution within the years, especially in 1997, induced different water stress intensities at the two sites, explaining carbon uptake being significantly lower at the Danish than at the French site. If it exists, an age effect could not be detected in this comparison. (C) 2002 Elsevier Science B.V. All rights reserved.