The first part of this thesis describes the further development of a dynamic global vegetation model, LPJ, and its application to selected scientific questions. LPJ has been extended to include isotopic fractionation of 13C at the leaf level during assimilation and includes a full isotopic terrestrial carbon cycle. Hence, it simulates the isotopic signature of the heterotrophic respiration fluxes. The model thus allows a quantitative analysis of the net biosphere exchange of CO2 and 13CO2 with the atmosphere as a function of changes in climate, land cover, atmospheric CO2, and the isotope ratio of CO2.

The extended version of LPJ has been used to study the response of the global vegetation distribution to an abrupt climate change event (Younger Dryas) and the thereby incurred changes in the terrestrial carbon pools and fluxes and their isotopic 13C/12C ratio. Climate data from a 850-year-long coupled ocean-atmosphere general circulation model (ECHAM3/LSG) is used for these simulations. The comparison of the modelled vegetation distribution and shifts during this idealized Younger Dryas event with reconstructed
vegetation maps for North America and Europe based on pollen records shows a reasonable agreement. The impact of the terrestrial carbon release during the Younger Dryas on the atmospheric CO2 and delta 13C is analyzed using a simplified ocean model and compared to ice core measurements. In the standard case the simulation exhibits a significant change in global total terrestrial carbon stocks of about 180 Pg C leading to an atmospheric CO2 increase of app. 28 ppmv as a consequence of the climate change event. The robustness of the terrestrial signal during the Younger Dryas is studied by several sensitivity experiments concerning the initial values of the carbon pool sizes as well as the CO2 fertilization effect and the temperature dependency of the carbon decomposition rates.

The isotope version of LPJ has also been used to study the effects of climate variability, fire, and changes in land use on the exchange fluxes of
CO2 and 13CO2 between the terrestrial biosphere and atmosphere for the last 100 years in greater detail. A transient, spatially explicit dataset of C4 crops and tropical C4 pastures has been compiled which, in combination with a land use scheme, allows the analysis of the impact of land use and C4 cultivation on the terrestrial stable isotope composition. LPJ simulates a global mean isotopic fractionation of 17.7 permil at the leaf level with interannual variations of ca. 0.3 permil in the case without land use for the years 1950 to 1998. In this case, interannual variability in the net 13CO2 flux between atmosphere and terrestrial biosphere is of the order of 15 Pg C permil per year. It is reduced to 4 Pg C permil per year if the leaf-level fractionation factor is held constant at the long term mean. Depending on the chosen land use scheme modelled values of leaf discrimination vary between 17.9 permil and 17.0 permil with results
from the experiment specifying C4 crops and C4 pastures being the lowest. Modelled values of isotopic disequilibrium similarly depend on the
amount of prescribed C4 vegetation.

The second part of the thesis describes the construction and application of a terrestrial Carbon Cycle Data Assimilation System (CCDAS). In the assimilation step parameters of a terrestrial biosphere model, BETHY, are constrained subject to observations. The technique is demonstrated by using atmospheric CO2 concentration observations from 1979 to 1999 and satellite remote sensing data (identifying vegetation activity) for the years 1989 and 1990 to optimize the tunable
parameters in the model (some spatially explicit, some global) and also give an estimate of their uncertainties. Quantities (global and spatially explicit carbon fluxes) derived from the prognostic step of CCDAS are then analyzed with respect to climate anomalies. Processes responsible for the mean terrestrial fluxes and their variability are identified. A highly significant correlation between El Nino/Southern
Oscillation and terrestrial CO2 fluxes, with CO2 lagging by a few months was found. Net CO2 outgasing during El Nino events is caused mainly by a reduction of photosynthesis in large parts of the tropics, notably the Amazon basin and Central Africa. The most important deviation of this pattern is found after the eruption of Mount Pinatubo in 1991.