Modelling the vegetation of the Earth

Sammanfattning: This thesis describes the development and application of an equilibrium terrestrial biosphere model, BIOME3. The model simulates vegetation distribution and biogeochemistry, and couples vegetation distribution directly to biogeochemistry. A photosynthesis scheme was developed with maximum photosynthetic rates derived using a theory based on optimal nitrogen allocation; the scheme has the important feature of predicting optimal photosynthesis rates that are proportional to absorbed photosynthetically active radiation. Inputs to BIOME3 consist of latitude, soil texture class, absolute minimum temperature and monthly climate (temperature, precipitation and sunshine). A minimal set of ecophysiological constraints is first applied to determine which plant functional types (PFTs) may potentially occur. A coupled carbon and water flux model is then used to calculate, for each PFT, the leaf area index (LAI) that maximizes net primary production (NPP), subject to the constraint that NPP must be sufficient to maintain this LAI. Competition between PFTs is simulated by using the opitimal NPP of each PFT as an index of competitiveness, with additional rules to approximate the dynamic equilibrium between natural disturbance and succession driven by light competition. A two-layer soil hydrology scheme allows simulation of the competitive balance between grass and woody vegetation, including the effects of soil texture. Model output consists of a quantitative vegetation state description in terms of the dominant PFT, secondary PFTs present and the total LAI and NPP for the ecosystem. A two-way coupling of the carbon and water fluxes through canopy conductance allows BIOME3 to simulate the response of photosynthesis, canopy conductance and leaf area to environmental factors including atmospheric carbon dioxide concentration. Comparison with the mapped distribution of global vegetation shows that the model successfully reproduces the broad scale patterns in potential natural vegetation distribution. Comparison with NPP measurements and a fractional absorbed photosynthetically active radiation (FPAR) climatology based on remotely sensed greenness measurements provide further checks on the models internal logic. BIOME3 is used to examine the response of vegetation to changes in carbon dioxide and climate since the Last Glacial Maximum; this application of the model demonstraters that the lowered atmospheric carbon dioxide concentration had a potentially large impact on patterns in vegetation distribution. The model is also used to simulate the combined effects of climate and carbon dioxide concentration of global patterns of both vegetation distribution and NPP under a doubled greenhouse gas climate.