Northern peatlands contain up to 25% of the world's soil carbon (C) and have an estimated annual exchange of CO 2 -C with the atmosphere of 0.1-0.5 Pg yr À1 and of CH 4 -C of 10-25 Tg yr À1 . Despite this overall importance to the global C cycle, there have been few, if any, complete multiyear annual C balances for these ecosystems. We report a 6-year balance computed from continuous net ecosystem CO 2 exchange (NEE), regular instantaneous measurements of methane (CH 4 ) emissions, and export of dissolved organic C (DOC) from a northern ombrotrophic bog. From these observations, we have constructed complete seasonal and annual C balances, examined their seasonal and interannual variability, and compared the mean 6-year contemporary C exchange with the apparent C accumulation for the last 3000 years obtained from C density and agedepth profiles from two peat cores. The 6-year mean NEE-C and CH 4 -C exchange, and net DOC loss are À40.2 AE 40.5 (AE 1 SD), 3.7 AE 0.5, and 14.9 AE 3.1 g m À2 yr À1 , giving a 6-year mean balance of À21.5 AE 39.0 g m À2 yr À1 (where positive exchange is a loss of C from the ecosystem). NEE had the largest magnitude and variability of the components of the C balance, but DOC and CH 4 had similar proportional variabilities and their inclusion is essential to resolve the C balance. There are large interseasonal and interannual ranges to the exchanges due to variations in climatic conditions. We estimate from the largest and smallest seasonal exchanges, quasi-maximum limits of the annual C balance between 50 and À105 g m À2 yr À1 . The net C accumulation rate obtained from the two peatland cores for the interval 400-3000 BP (samples from the anoxic layer only) were 21.9 AE 2.8 and 14.0 AE 37.6 g m À2 yr À1 , which are not significantly different from the 6-year mean contemporary exchange.

[1] Accurately simulating gross primary productivity (GPP) in terrestrial ecosystem models is critical because errors in simulated GPP propagate through the model to introduce additional errors in simulated biomass and other fluxes. We evaluated simulated, daily average GPP from 26 models against estimated GPP at 39 eddy covariance flux tower sites across the United States and Canada. None of the models in this study match estimated GPP within observed uncertainty. On average, models overestimate GPP in winter, spring, and fall, and underestimate GPP in summer. Models overpredicted GPP under dry conditions and for temperatures below 0 C. Improvements in simulated soil moisture and ecosystem response to drought or humidity stress will improve simulated GPP under dry conditions. Adding a low-temperature response to shut down GPP for temperatures below 0 C will reduce the positive bias in winter, spring, and fall and improve simulated phenology. The negative bias in summer and poor overall performance resulted from mismatches between simulated and observed light use efficiency (LUE). Improving simulated GPP requires better leaf-to-canopy scaling and better values of model parameters that control the maximum potential GPP, such as ɛ max (LUE), V cmax (unstressed Rubisco catalytic capacity) or J max (the maximum electron transport rate).

Many wetland ecosystems such as peatlands and wet tundra hold large amounts of organic carbon (C) in their soils, and are thus important in the terrestrial C cycle. We have synthesized data on the carbon dioxide (CO 2 ) exchange obtained from eddy covariance measurements from 12 wetland sites, covering 1-7 years at each site, across Europe and North America, ranging from ombrotrophic and minerotrophic peatlands to wet tundra ecosystems, spanning temperate to arctic climate zones. The average summertime net ecosystem exchange of CO 2 (NEE) was highly variable between sites. However, all sites with complete annual datasets, seven in total, acted as annual net sinks for atmospheric CO 2 . To evaluate the influence of gross primary production (GPP) and ecosystem respiration (R eco ) on NEE, we first removed the artificial correlation emanating from the method of partitioning NEE into GPP and R eco . After this correction neither R eco (P 5 0.162) nor GPP (P 5 0.110) correlated significantly with NEE on an annual basis. Spatial variation in annual and summertime R eco was associated with growing season period, air temperature, growing degree days, normalized difference vegetation index and vapour pressure deficit. GPP showed weaker correlations with environmental variables as compared with R eco , the exception being leaf area index (LAI), which correlated with both GPP and NEE, but not with R eco . Length of growing season period was found to be the most important variable describing the spatial variation in summertime GPP and R eco ; global warming will thus cause these components to increase. Annual GPP and NEE correlated significantly with LAI and pH, thus, in order to predict wetland C exchange, differences in ecosystem structure such as leaf area and biomass as well as nutritional status must be taken into account.

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