Dynamic modelling of the forest ecosystem: Incorporation of the phosphorus cycle

Sammanfattning: The forest ecosystem provides and regulates many important ecosystem services, such as soil and water protection, timber production, and climate regulation. It has been influenced by changes induced by humans, for example, our increasing demand for timber and bioenergy, the increase in nitrogen (N) deposition and CO2 concentrations due to industrialization, and climate change which has caused an increase in global temperature and extreme events such as storms. It is important to know how forests will be affected by all these changes, and dynamic modeling provides a powerful tool with which this can be studied. However, some important processes are missing or oversimplified in present-day models, which will lead to unreliable predictions about the future of forests. For example, the lack of the phosphorus (P) cycle has been identified as a possible problem in many models.In this thesis, I first combined the dynamic forest biogeochemistry model, ForSAFE, with empirical monitoring data, to study the effects of management intensification and storm disturbances on two Swedish forest sites. I then included the phosphorus cycle in the ForSAFE model and evaluated the model at one of the sites.The model results indicated that the three measures of management intensification (residue removal, intensification of thinning, and shorter rotation time) and storm disturbances would both increase the biomass loss.The carbon (C) storage in the soil, and the leaching of N and dissolved organic C would decrease in management intensification but increase in storm disturbances. Shortening the rotation time appears to lead to soil acidification due to the loss of base cations. Storms lead to short-term soil acidification, but there seems to be no negative effect on acidification in the long term. However, measures should be taken to avoid acidification after both management intensification and storms, to prevent the loss of base cations from the soil.The results from the new ForSAFE, including the P cycle, showed that, in southwestern Sweden, P mineralization contributes about 80% to the plant P uptake in the forest, while the inputs—deposition and weathering—account for the remaining 20%. Phosphorus weathering should be highlighted in the forest due to its important contribution to plant uptake and its dominant role as a P input. Both model results and measurements indicated that the study site showed signs of P limitation at the present time, but the model predicts that it may return to be N-limited in the future if N deposition declined strongly.In conclusion, the combination of forest dynamic modeling and empirical data has proved to be a powerful approach to study the processes and ecosystem services of the forest, and could be useful in the future to support forest management and policy-making. The inclusion of the P cycle in ForSAFE could be valuable for other terrestrial models for the same task. Some important findings in this thesis could be significative and constructive not only in forest management, but also in regional or global C and nutrient cycling, thus potentially contributing to future climate regulation or biodiversity conservation.