The carbon and water balances of two temperate forest ecosystems: A process based modeling approach

In the context of climatic changes and their impact on ecosystems, several methods to measure and model carbon and water fluxes between ecosystems and the atmosphere were developed during the last decades. The overall objective of this thesis was the further development of two existing forest ecosystem models (FORUG and ANAFORE) as useful research tools. At the same time, some specific research questions were addressed by applying these models on temperate forest stands. (1) A multi-year analysis (with FORUG) indicated that interannual variability of gross primary production (GPP) of the Hesse beech (Fagus sylvatica L.) forest is not only determined by direct climatic effects. (2) A new carbon allocation module was developed and integrated into the FORUG model. The allocation pattern differed between years with and without drought, although total net primary production decreased only in case of severe drought. (3) The sensitivity analysis showed that the overall output uncertainty of the FORUG model is largely determined by accounting for the uncertainty of only a limited number of key parameters. The Monte Carlo simulation technique was found to be a useful tool to rank the different parameters for their contribution to the output uncertainty. (4) The incorporation of a water flow model in ANAFORE resulted in accurate model predictions of diurnal patterns and one seasonal pattern of sap flow for Scots pine (Pinus sylvestris L.) in Brasschaat. The combination of ANAFORE with sap flow measurements gave interesting possibilities to calculate water storage of individual trees. Storage water use in Scots pine was found to vary among trees and over time (up to 44% of daily transpiration). (5) Relations between water fluxes, storage water use and climatic drivers were analysed in detail. As expected, solar radiation was the main driver of storage water use. Nevertheless, the day-to-day patterns of measured sap flow and storage water use were quite different, with vapour pressure deficit being the main driver of these differences.; This work has contributed to the further development of the FORUG and the ANAFORE models, especially regarding carbon allocation and the role of internal water in the tree water balance.