Pan-European delta super(13)C values of air and organic matter from forest ecosystems

We present carbon stable isotope, delta super(13)C, 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 delta super(13)C 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 delta super(13)C values. In this framework, we also tested the potential of delta super(13)C in canopy air and plant organic matter to record regional-scale ecophysiological patterns. Our network estimates for the mean delta super(13)C of ecosystem respired CO sub(2) and the related 'discrimination' of ecosystem respiration, delta sub(er) and Delta sub(er), respectively, were -25.6 plus or minus 1.9ppt and 17.8 plus or minus 2.0ppt in 2001 and -26.6 plus or minus 1.5ppt and 19.0 plus or minus 1.6ppt in 2002. The results were in close agreement with delta super(13)C 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 delta super(13)C signals over Europe, but may be limited in capturing some of the interannual variations. In 2001, but less so in 2002, there were discernable longitudinal and seasonal trends in delta sub(er). From west to east, across the network, there was a general enrichment in super(13)C ( similar to 3ppt and similar to 1ppt for the 2 years, respectively) consistent with increasing Gorczynski continentality index for warmer and drier conditions. In 2001 only, seasonal super(13)C enrichment between July and September, followed by depletion in November (from about -26.0ppt to -24.5ppt to -30.0ppt), was also observed. In 2001, July and August delta sub(er) values across the network were significantly related to average daytime vapor pressure deficit (VPD), relative humidity (RH), and, to a lesser degree, air temperature (T sub(a)), but not significantly with monthly average precipitation (P sub(m)). In contrast, in 2002 (a much wetter peak season), delta sub(er) was significantly related with T sub(a), but not significantly with VPD and RH. The important role of plant physiological processes on delta sub(er) in 2001