We present carbon stable isotope, d13C, results from air and organic matter samples collected during 98 individual field campaigns across a network of Carboeuroflux forest sites in 2001 (14 sites) and 2002 (16 sites). Using these data, we tested the hypothesis that d13C values derived from large-scale atmospheric measurements and models, which are routinely used to partition carbon fluxes between land and ocean, and potentially between respiration and photosynthesis on land, are consistent with directly measured ecosystem-scale d13C values. In this framework, we also tested the potential of d13C in canopy air and plant organic matter to record regional-scale ecophysiological patterns. Our network estimates for the mean d13C of ecosystem respired CO2 and the related 'discrimination' of ecosystem respiration, der and Der, respectively, were ! 25.6" 1.9% and 17.8 " 2.0% in 2001 and ! 26.6" 1.5% and 19.0" 1.6% in 2002. The results were in close agreement with d13C values derived from regional-scale atmospheric measurement programs for 2001, but less so in 2002, which had an unusual precipitation pattern. This suggests that regional-scale atmospheric sampling programs generally capture ecosystem d13C signals over Europe, but may be limited in capturing some of the interannual variations.