Analysis of carbon and water fluxes from the NOPEX boreal forest: Comparison of measurements with FOREST-BGC simulations

The ecosystem process model, FOREST-BGC, was applied on a stand in the NOPEX region in central Sweden. It was compared with measured data of net ecosystem carbon flux (F(n)) and transpiration (E(Q)) on a daily basis. Using the parameterized model, yearly budgets of carbon and water were constructed. F(n) was obtained from eddy correlation measurements on a tower at heights of 35 and 100 m. E(Q) was obtained from sap flow measurements using a heat balance method. The model predictions were generally good, considering the relatively low requirements for input parameters. The explained variability of E(Q) was high (95%), particularly relative to the presence of large water deficit conditions on the site. The explained variability of F(n) was lower: it was 50% and 66% when compared to the measurements at 35 and 100 m, respectively. These results reflect the large spatial variability of F(n) and the quantitative differences of measured F(n) at the two heights over a patchy forest consisting of small stands of different age, density and pine/spruce composition (the validation was made prior to a detailed footprint analysis). The model performed differently for various periods during a year, which demonstrates the value of long-term measurements for model validations. The simulated yearly net carbon ecosystem uptake for the 50-year-old stand with a high leaf area index was 1.99 t ha^-1, with a range of 0.55-2.04 t ha^-1 for leaf area index of 3-6 observed at the NOPEX site. The model analysis of controls for mass fluxes showed that soil water shortage was the main limiting factor on the NOPEX site in the year studied. The comparative model run for the northern BOREAS site in central Canada indicated that a high atmospheric drought and plant resistance to water flow frequently limited fluxes there. A more maritime climate of NOPEX site permits a larger gross production; however, larger respiration and decomposition rates reduce the quantitative differences of net ecosystem carbon uptake relative to the BOREAS site with a continental climate.