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 NOPE X region in central Sweden. It was compared with measured data of net ecosy stem 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 how measurements using a heat bala nce method. The model predictions were generally good, considering the rela tively low requirements for input parameters. The explained variability of E-Q was high (95%), particularly relative to the presence of large water de ficit 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, respe ctively. 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 analysi s). The model performed differently for various periods during a year, whic h demonstrates the value of long-term measurements for model validations. T he simulated yearly net carbon ecosystem uptake for the 50-year-old stand w ith a high leaf area index was 1.99 t ha(-1), with a range of 0.55-2.04 t h a(-1) for leaf area index of 3-6 observed at the NOPEX site. The model anal ysis of controls for mass fluxes showed that soil water shortage was the ma in limiting factor on the NOPEX site in the year studied. The comparative m odel run for the northern BOREAS site in central Canada indicated that a hi gh atmospheric drought and plant resistance to water how 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 BO REAS site with a continental climate. (C) 1998 Elsevier Science B.V. All ri ghts reserved.