The role of vegetation dynamics in the control of atmospheric CO2 content

Abstract: This thesis contains a description of the Lund-Potsdam-Jena Dynamic Global Vegetation Model (LPJ-DGVM) and its application to infer the role of vegetation dynamics on atmospheric CO2 content at different time-scales. The model combines vegetation dynamics and biogeochemistry in a modular framework. Individual modules describe ecosystems processes, including vegetation resource competition and production, tissue turnover, growth, fire and mortality, soil and litter biogeochemistry, including the effects of CO2 on these processes. The model simulates realistic post-disturbance succession in different environments. Seasonal exchange of H2O and CO2 between the terrestrial biosphere and the atmosphere is modelled in reasonable agreement with observation. Global estimates of carbon stocks in soil, litter and vegetation are within their acceptable ranges and the model captures the present-day patterns in vegetation. Fire return intervals are simulated correctly in most regions. Results emphasise the important role of the terrestrial biosphere in both the seasonal cycle and in the inter-annual variability in the growth rate of atmospheric CO2. LPJ successfully reproduced both the amplitude and phase of the seasonal cycle of atmospheric CO2 content as measured at a global network of monitoring stations. The model predicted a small net terrestrial biosphere uptake of CO2 during the 1980's with a strong CO2 fertilisation effect, which enhances plant production, reduced by the effects of climate and land use change. Historical land use change and CO2 fertilisation have been the dominant, albeit opposing factors governing the response of the terrestrial biosphere with respect to carbon storage during the 20th century. LPJ is run using one future climate and atmospheric CO2 scenario until 2200. Enhanced production due to the CO2 fertilisation effect eventually reaches an asymptote, and consequently the ability of the terrestrial biosphere to store additional carbon diminishes. At the end of the forcing period, vegetation is no longer in equilibrium, justifying the need for dynamic models in global change research. LPJ predicts the abundance of woody vegetation to increase everywhere, except in regions facing large reductions in precipitation. The tree-line extends northwards and temperate woody plants expand into the present-day boreal forest.