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Carbon exchange between the terrestrial biosphere and the atmosphere is one of the key processes that need to be assessed in the context of the Kyoto Protocol. Several studies suggest that the terrestrial biosphere is gaining carbon, but these estimates are obtained primarily by indirect methods, and the factors that control terrestrial carbon exchange, its magnitude and primary locations, are under debate. Here we present data of net ecosystem carbon exchange, collected between 1996 and 1998 from 15 European forests, which confirm that many European forest ecosystems act as carbon sinks. The annual carbon balances range from an uptake of 6.6 tonnes of carbon per hectare per year to a release of nearly 1 t C ha(-1) yr(-1), with a large variability between forests. The data show a significant increase of carbon uptake with decreasing latitude, whereas the gross primary production seems to be largely independent of latitude. Our observations indicate that, in general, ecosystem respiration determines net ecosystem carbon exchange. Also, for an accurate assessment of the carbon balance in a particular forest ecosystem, remote sensing of the normalized difference vegetation index or estimates based on forest inventories may not be sufficient.

Summary This paper presents CO2 flux data from 18 forest ecosystems, studied in the European Union funded EUROFLUX project. Overall, mean annual gross primary productivity (GPP, the total amount of carbon (C) fixed during photosynthesis) of these forests was 1380 ± 330 gC m−2 y−1 (mean ±SD). On average, 80% of GPP was respired by autotrophs and heterotrophs and released back into the atmosphere (total ecosystem respiration, TER = 1100 ± 260 gC m−2 y−1). Mean annual soil respiration (SR) was 760 ± 340 gC m−2 y−1 (55% of GPP and 69% of TER). Among the investigated forests, large differences were observed in annual SR and TER that were not correlated with mean annual temperature. However, a significant correlation was observed between annual SR and TER and GPP among the relatively undisturbed forests. On the assumption that (i) root respiration is constrained by the allocation of photosynthates to the roots, which is coupled to productivity, and that (ii) the largest fraction of heterotrophic soil respiration originates from decomposition of young organic matter (leaves, fine roots), whose availability also depends on primary productivity, it is hypothesized that differences in SR among forests are likely to depend more on productivity than on temperature. At sites where soil disturbance has occurred (e.g. ploughing, drainage), soil espiration was a larger component of the ecosystem C budget and deviated from the relationship between annual SR (and TER) and GPP observed among the less‐disturbed forests. At one particular forest, carbon losses from the soil were so large, that in some years the site became a net source of carbon to the atmosphere. Excluding the disturbed sites from the present analysis reduced mean SR to 660 ± 290 gC m−2 y−1, representing 49% of GPP and 63% of TER in the relatively undisturbed forest ecosystems.

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Comparisons were made among Douglas-fir forest, aspen (broad leaf deciduous) forest and wheatgrass (C 3 ) grassland for ecosystem-level water-use efficiency (WUE). WUE was defined as the ratio of photosynthetic CO 2 assimilation rate and evapotranspiration (ET) rate. The ET data measured by eddy covariance were screened so that they overwhelmingly represented transpiration. The three sites used in this comparison spanned a range of vegetation (plant functional) types and environmental conditions within western Canada. When compared in the relative order Douglas-fir (located on Vancouver Island, BC), aspen (northern Saskatchewan), grassland (southern Alberta), the sites demonstrated a progressive decline in precipitation and a general increase in maximum air temperature and atmospheric saturation deficit (D max ) during the mid-summer. The average ( AE SD) WUE at the grassland site was 2.6 AE 0.7 mmol mol À1 , which was much lower than the average values observed for the two other sites (aspen: 5.4 AE 2.3, Douglasfir: 8.1 AE 2.4). The differences in WUE among sites were primarily because of variation in ET. The highest maximum ET rates were approximately 5, 3.2 and 2.7 mm day À1 for the grassland, aspen and Douglas-fir sites, respectively. There was a strong negative correlation between WUE and D max for all sites. We also made seasonal measurements of the carbon isotope ratio of ecosystem respired CO 2 (d R ) in order to test for the expected correlation between shifts in environmental conditions and changes to the ecosystemintegrated ratio of leaf intercellular to ambient CO 2 concentration (c i /c a ). There was a consistent increase in d R values in the grassland, aspen forest and Douglas-fir forest associated with a seasonal reduction in soil moisture. Comparisons were made between WUE measured using eddy covariance with that calculated based on D and d R measurements. There was excellent agreement between WUE values calculated using the two techniques. Our d R measurements indicated that c i /c a values were quite similar among the Douglas-fir, aspen and grassland sites, despite large variation in environmental conditions among sites. This implied that the shorter-lived grass species had relatively high c i /c a values for the D of their habitat. By contrast, the longer-lived Douglas-fir trees were more conservative in water-use with lower c i /c a values relative to their habitat D. This illustrates the interaction between biological and environmental characteristics influencing ecosystem-level WUE. The strong correlation we observed between the two independent measurements of WUE, indicates that the stable isotope composition of respired CO 2 is a useful ecosystem-scale tool to help study constraints to photosynthesis and acclimation of ecosystems to environmental stress.