Investigations of the mechanisms behind the carbon loss in a Swedish pine & spruce forest, with the ecosystem model LPJ-GUESS

Abstract: Vegetation on land is globally taking up about 30% of the CO2 that is emitted by human activities (oceans take up 30%, while 40% remains in the atmosphere). This ability to store carbon in the vegetation and in the soil is very important to mitigate the climate changes, but all land based ecosystems are not carbon sinks. Ecosystems that are losing carbon needs to be further investigated, so we can predict if more ecosystems are going to become carbon sources in the future and to find better management practices for the forests and farmlands. The forest stand in Norunda, about 30 km north of Uppsala, Sweden, is one of few forest ecosystems with a healthy production and long term continuous CO2 flux measurements that is losing carbon. Pine and spruce are the dominating plant species in Norunda and the soilis spatially heterogeneous with a fraction of about 10% peatland and 90% moderately moist land. The management history of the site includes e.g. drainage in the year 1890 and clear-cut in the year 1900. Measurements suggest that the soil respiration is unusually high in Norunda and this is probably a contributing factor to the loss of carbon. The ecosystem model LPJ-GUESS is the tool used to test different hypotheses for the carbon source in Norunda. The model parameters were calibrated such that the model result for soil moisture, evapotranspiration, photosynthetic carbon assimilation, biomass increment and soil carbon content, had a good fit to in-situ measurements. The gaps between model result and measurements for soil respiration and net ecosystem exchange was left remained with purpose, because the idea was to test if the different simulated hypotheses could decrease these gaps. The model was modified to test the hypotheses for drainage, peatland, clear-cut and thinning and the 20th century temperature increase. The result for the drainage and peatland modification suggests that these factors contributes with 1% - 47% of the observed carbon loss in Norunda. The simulation of clear-cut and thinning indicates that this slightly decreased the loss of carbon in Norunda 100 years after the clear-cut. A simulation with detrended temperature was performed, to test the effect of the 20th century temperature increase and the result suggests that the increase in temperature increased the carbon loss in Norunda with 17% over the period 1995-2003. Other hypotheses for the carbon loss in Norunda might need further investigation, such as the possibility of net inflow of carbon from surrounding areas by the groundwater or by horizontal advection, which is common during calm nights.