DYNAMIC MODELLING OF BIOGEOCHEMICAL PROCESSES IN FOREST ECOSYSTEMS

Sammanfattning: Forests in Sweden are undergoing rapid and considerable changes regarding both management methods and environmental conditions. To understand the effects of these changes and to be able to predict impending changes in forest ecosystems, a holistic understanding of the processes governing the ecosystem is needed. To gain such an understanding, the ForSAFE model of the biogeochemical cycles in a forest ecosystem has been developed. ForSAFE is a dynamic model which closes the physical cycles between the living biomass, the soil organic matter, the soil hydrology and the soil chemistry. The model was complemented with a supplementary module, called VEG, to include the response of the composition of the ground vegetation to the conditions at the sites. This thesis describes ForSAFE-VEG and its application to forest ecosystems in Sweden. The model was applied to a number of coniferous Swedish sites with a wide geographical distribution to investigate the responses of the forest ecosystems to changes in atmospheric deposition and harvest methods. The model output was validated on different levels, including the biomass, the soil organic matter and the soil chemistry. The results of the simulations show that there is a high risk of nitrogen leaching, particularly in the south and centre of Sweden. The leaching of nitrogen is found to be triggered by harvesting, but the magnitude of the leaching depends on the historical accumulation of N from deposition. The simulations also predict an increase in the number of sites with acidified soils in the future, contrary to what has been predicted by previous models. The reason behind this increase was found to be the high uptake rates of base cations associated with high biomass growth rates, which in turn are the result of the high nitrogen deposition. Finally, the results of the ForSAFE-VEG were used to differentiate between the different causes of change in the ground vegetation diversity. The results indicate that the changes in the composition of the ground vegetation are triggered by clear-cutting, but that the direction of these changes is governed by the histories of the sites and the intensity of atmospheric deposition.